feat(capabilities): add extensions field for SEP-1724#643

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Merged
alexhancock merged 1 commit into from
Feb 4, 2026
Merged

feat(capabilities): add extensions field for SEP-1724 #643

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163 changes: 153 additions & 10 deletions crates/rmcp/src/model/capabilities.rs
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Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
#[serde(skip_serializing_if = "Option::is_none")]
pub experimental: Option<ExperimentalCapabilities>,
/// Optional MCP extensions that the client supports (SEP-1724).
/// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
/// values are per-extension settings objects. An empty object indicates
/// support with no settings.
#[serde(skip_serializing_if = "Option::is_none")]
pub extensions: Option<ExtensionCapabilities>,
#[serde(skip_serializing_if = "Option::is_none")]
pub roots: Option<RootsCapabilities>,
#[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
#[serde(skip_serializing_if = "Option::is_none")]
pub experimental: Option<ExperimentalCapabilities>,
/// Optional MCP extensions that the server supports (SEP-1724).
/// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
/// values are per-extension settings objects. An empty object indicates
/// support with no settings.
#[serde(skip_serializing_if = "Option::is_none")]
pub extensions: Option<ExtensionCapabilities>,
#[serde(skip_serializing_if = "Option::is_none")]
pub logging: Option<JsonObject>,
#[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
ServerCapabilities {
experimental: ExperimentalCapabilities,
extensions: ExtensionCapabilities,
logging: JsonObject,
completions: JsonObject,
prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
}
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
const E: bool,
const EXT: bool,
const L: bool,
const C: bool,
const P: bool,
const R: bool,
const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
pub fn enable_tool_list_changed(mut self) -> Self {
if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
}
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
const E: bool,
const EXT: bool,
const L: bool,
const C: bool,
const R: bool,
const T: bool,
const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
pub fn enable_prompts_list_changed(mut self) -> Self {
if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
}
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
const E: bool,
const EXT: bool,
const L: bool,
const C: bool,
const P: bool,
const T: bool,
const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
pub fn enable_resources_list_changed(mut self) -> Self {
if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
ClientCapabilities{
experimental: ExperimentalCapabilities,
extensions: ExtensionCapabilities,
roots: RootsCapabilities,
sampling: JsonObject,
elicitation: ElicitationCapability,
tasks: TasksCapability,
}
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
pub fn enable_roots_list_changed(mut self) -> Self {
if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
/// Enable JSON Schema validation for elicitation responses.
/// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
assert_eq!(json["cancel"], serde_json::json!({}));
assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
}

#[test]
fn test_client_extensions_capability() {
// Test building ClientCapabilities with extensions (MCP Apps support)
let mut extensions = ExtensionCapabilities::new();
extensions.insert(
"io.modelcontextprotocol/ui".to_string(),
serde_json::from_value(serde_json::json!({
"mimeTypes": ["text/html;profile=mcp-app"]
}))
.unwrap(),
);

let capabilities = ClientCapabilities::builder()
.enable_extensions_with(extensions)
.enable_sampling()
.build();

// Verify serialization matches MCP Apps spec format
let json = serde_json::to_value(&capabilities).unwrap();
assert_eq!(
json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
serde_json::json!(["text/html;profile=mcp-app"])
);
assert!(json["sampling"].is_object());
}

#[test]
fn test_server_extensions_capability() {
// Test building ServerCapabilities with extensions
let mut extensions = ExtensionCapabilities::new();
extensions.insert(
"io.modelcontextprotocol/apps".to_string(),
serde_json::from_value(serde_json::json!({})).unwrap(),
);

let capabilities = ServerCapabilities::builder()
.enable_extensions_with(extensions)
.enable_tools()
.build();

// Verify serialization
let json = serde_json::to_value(&capabilities).unwrap();
assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
assert!(json["tools"].is_object());
}

#[test]
fn test_extensions_deserialization() {
// Test deserializing capabilities with extensions from JSON
let json = serde_json::json!({
"extensions": {
"io.modelcontextprotocol/ui": {
"mimeTypes": ["text/html;profile=mcp-app"]
}
},
"sampling": {}
});

let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
assert!(capabilities.extensions.is_some());
let extensions = capabilities.extensions.unwrap();
assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
assert!(ui_ext.contains_key("mimeTypes"));
}

#[test]
fn test_extensions_empty_settings() {
// Test that empty extension settings work (indicates support with no settings)
let mut extensions = ExtensionCapabilities::new();
extensions.insert(
"io.modelcontextprotocol/oauth-client-credentials".to_string(),
JsonObject::new(),
);

let capabilities = ClientCapabilities::builder()
.enable_extensions_with(extensions)
.build();

let json = serde_json::to_value(&capabilities).unwrap();
assert_eq!(
json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
serde_json::json!({})
);
}
}
11 changes: 11 additions & 0 deletions crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"roots": {
"anyOf": [
{
Expand Down
11 changes: 11 additions & 0 deletions crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"roots": {
"anyOf": [
{
Expand Down
11 changes: 11 additions & 0 deletions crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"logging": {
"type": [
"object",
Expand Down
11 changes: 11 additions & 0 deletions crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"logging": {
"type": [
"object",
Expand Down
, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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feat(capabilities): add extensions field for SEP-1724#643

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to your account

 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

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Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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feat(capabilities): add extensions field for SEP-1724#643

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alexhancock
merged 1 commit into

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Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
feat(capabilities): add extensions field for SEP-1724#643

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 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

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feat(capabilities): add extensions field for SEP-1724#643

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 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
feat(capabilities): add extensions field for SEP-1724#643

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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By clicking “Sign up for GitHub”, you agree to our terms of service and
privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
Sign in
to your account

 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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Failed to load files. 
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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feat(capabilities): add extensions field for SEP-1724#643

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 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

Skip to content
feat(capabilities): add extensions field for SEP-1724#643

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 Merged

alexhancock
merged 1 commit into

from

Feb 4, 2026
 Merged


feat(capabilities): add extensions field for SEP-1724
#643
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163 changes: 153 additions & 10 deletions
crates/rmcp/src/model/capabilities.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -6,6 +6,28 @@ use serde::{Deserialize, Serialize};
use super::JsonObject;
pub type ExperimentalCapabilities = BTreeMap<String, JsonObject>;

/// MCP extension capabilities map.
///
/// Keys are extension identifiers in the format `{vendor-prefix}/{extension-name}`
/// (e.g., `io.modelcontextprotocol/ui`, `io.modelcontextprotocol/oauth-client-credentials`).
/// Values are per-extension settings objects. An empty object indicates support with no settings.
///
/// # Example
///
/// ```rust
/// use rmcp::model::ExtensionCapabilities;
/// use serde_json::json;
///
/// let mut extensions = ExtensionCapabilities::new();
/// extensions.insert(
/// "io.modelcontextprotocol/ui".to_string(),
/// serde_json::from_value(json!({
/// "mimeTypes": ["text/html;profile=mcp-app"]
/// })).unwrap()
/// );
/// ```
pub type ExtensionCapabilities = BTreeMap<String, JsonObject>;

#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Default)]
#[serde(rename_all = "camelCase")]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
Expand DownExpand Up@@ -187,6 +209,12 @@ pub struct ElicitationCapability {
pub struct ClientCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the client supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub roots: Option<RootsCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -217,6 +245,12 @@ pub struct ClientCapabilities {
pub struct ServerCapabilities {
 #[serde(skip_serializing_if = "Option::is_none")]
 pub experimental: Option<ExperimentalCapabilities>,
 /// Optional MCP extensions that the server supports (SEP-1724).
 /// Keys are extension identifiers (e.g., `"io.modelcontextprotocol/apps"`),
 /// values are per-extension settings objects. An empty object indicates
 /// support with no settings.
 #[serde(skip_serializing_if = "Option::is_none")]
 pub extensions: Option<ExtensionCapabilities>,
 #[serde(skip_serializing_if = "Option::is_none")]
 pub logging: Option<JsonObject>,
 #[serde(skip_serializing_if = "Option::is_none")]
Expand DownExpand Up@@ -339,6 +373,7 @@ macro_rules! builder {
builder! {
 ServerCapabilities {
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 logging: JsonObject,
 completions: JsonObject,
 prompts: PromptsCapability,
Expand All@@ -348,8 +383,15 @@ builder! {
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, R, true, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const R: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, R, true, TASKS>>
{
 pub fn enable_tool_list_changed(mut self) -> Self {
 if let Some(c) = self.tools.as_mut() {
Expand All@@ -359,8 +401,15 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const R: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, true, R, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const R: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, true, R, T, TASKS>>
{
 pub fn enable_prompts_list_changed(mut self) -> Self {
 if let Some(c) = self.prompts.as_mut() {
Expand All@@ -370,8 +419,15 @@ impl<const E: bool, const L: bool, const C: bool, const R: bool, const T: bool,
 }
}

impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool, const TASKS: bool>
 ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, L, C, P, true, T, TASKS>>
impl<
 const E: bool,
 const EXT: bool,
 const L: bool,
 const C: bool,
 const P: bool,
 const T: bool,
 const TASKS: bool,
> ServerCapabilitiesBuilder<ServerCapabilitiesBuilderState<E, EXT, L, C, P, true, T, TASKS>>
{
 pub fn enable_resources_list_changed(mut self) -> Self {
 if let Some(c) = self.resources.as_mut() {
Expand All@@ -391,15 +447,16 @@ impl<const E: bool, const L: bool, const C: bool, const P: bool, const T: bool,
builder! {
 ClientCapabilities{
 experimental: ExperimentalCapabilities,
 extensions: ExtensionCapabilities,
 roots: RootsCapabilities,
 sampling: JsonObject,
 elicitation: ElicitationCapability,
 tasks: TasksCapability,
 }
}

impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, true, S, EL, TASKS>>
impl<const E: bool, const EXT: bool, const S: bool, const EL: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, true, S, EL, TASKS>>
{
 pub fn enable_roots_list_changed(mut self) -> Self {
 if let Some(c) = self.roots.as_mut() {
Expand All@@ -410,8 +467,8 @@ impl<const E: bool, const S: bool, const EL: bool, const TASKS: bool>
}

#[cfg(feature = "elicitation")]
impl<const E: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, R, S, true, TASKS>>
impl<const E: bool, const EXT: bool, const R: bool, const S: bool, const TASKS: bool>
 ClientCapabilitiesBuilder<ClientCapabilitiesBuilderState<E, EXT, R, S, true, TASKS>>
{
 /// Enable JSON Schema validation for elicitation responses.
 /// When enabled, the client will validate user input against the requested_schema
Expand DownExpand Up@@ -528,4 +585,90 @@ mod test {
 assert_eq!(json["cancel"], serde_json::json!({}));
 assert_eq!(json["requests"]["tools"]["call"], serde_json::json!({}));
 }

 #[test]
 fn test_client_extensions_capability() {
 // Test building ClientCapabilities with extensions (MCP Apps support)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/ui".to_string(),
 serde_json::from_value(serde_json::json!({
 "mimeTypes": ["text/html;profile=mcp-app"]
 }))
 .unwrap(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_sampling()
 .build();

 // Verify serialization matches MCP Apps spec format
 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/ui"]["mimeTypes"],
 serde_json::json!(["text/html;profile=mcp-app"])
 );
 assert!(json["sampling"].is_object());
 }

 #[test]
 fn test_server_extensions_capability() {
 // Test building ServerCapabilities with extensions
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/apps".to_string(),
 serde_json::from_value(serde_json::json!({})).unwrap(),
 );

 let capabilities = ServerCapabilities::builder()
 .enable_extensions_with(extensions)
 .enable_tools()
 .build();

 // Verify serialization
 let json = serde_json::to_value(&capabilities).unwrap();
 assert!(json["extensions"]["io.modelcontextprotocol/apps"].is_object());
 assert!(json["tools"].is_object());
 }

 #[test]
 fn test_extensions_deserialization() {
 // Test deserializing capabilities with extensions from JSON
 let json = serde_json::json!({
 "extensions": {
 "io.modelcontextprotocol/ui": {
 "mimeTypes": ["text/html;profile=mcp-app"]
 }
 },
 "sampling": {}
 });

 let capabilities: ClientCapabilities = serde_json::from_value(json).unwrap();
 assert!(capabilities.extensions.is_some());
 let extensions = capabilities.extensions.unwrap();
 assert!(extensions.contains_key("io.modelcontextprotocol/ui"));
 let ui_ext = extensions.get("io.modelcontextprotocol/ui").unwrap();
 assert!(ui_ext.contains_key("mimeTypes"));
 }

 #[test]
 fn test_extensions_empty_settings() {
 // Test that empty extension settings work (indicates support with no settings)
 let mut extensions = ExtensionCapabilities::new();
 extensions.insert(
 "io.modelcontextprotocol/oauth-client-credentials".to_string(),
 JsonObject::new(),
 );

 let capabilities = ClientCapabilities::builder()
 .enable_extensions_with(extensions)
 .build();

 let json = serde_json::to_value(&capabilities).unwrap();
 assert_eq!(
 json["extensions"]["io.modelcontextprotocol/oauth-client-credentials"],
 serde_json::json!({})
 );
 }
}

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -296,6 +296,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "roots": {
 "anyOf": [
 {
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

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Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down

11 changes: 11 additions & 0 deletions
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

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Original file line numberDiff line numberDiff line change
Expand Up@@ -2369,6 +2369,17 @@
 "additionalProperties": true
 }
 },
 "extensions": {
 "description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
 "type": [
 "object",
 "null"
 ],
 "additionalProperties": {
 "type": "object",
 "additionalProperties": true
 }
 },
 "logging": {
 "type": [
 "object",
Expand Down