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OAuth discovery aborts after first malformed well-known URL candidate #544

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OAuth discovery aborts after first malformed well-known URL candidate#544

Description

@sorech
sorech
opened on Nov 17, 2025

Describe the bug
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.

To Reproduce

  1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
  2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
  3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.

Expected behavior
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.

Logs
Added global MCP server 'server-name'.
Detected OAuth support. Starting OAuth flow…
Error: Metadata error: Failed to parse metadata: error decoding response body

Additional context
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:

match self.fetch_authorization_metadata(&discovery_url).await {
Ok(Some(metadata)) => return Ok(Some(metadata)),
Ok(None) => continue,
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
}

Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
Either approach lets discovery fall through to the next candidate URL.

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try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
      Skip to content
      OAuth discovery aborts after first malformed well-known URL candidate #544
      Closed
      Closed
      OAuth discovery aborts after first malformed well-known URL candidate#544
      Description
      @sorech
      sorech
      opened on Nov 17, 2025
      Describe the bug
      
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
      To Reproduce
      1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
      2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
      3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
      Expected behavior
      
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
      Logs
      
Added global MCP server 'server-name'.
      
Detected OAuth support. Starting OAuth flow…
      
Error: Metadata error: Failed to parse metadata: error decoding response body
      Additional context
      
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
      match self.fetch_authorization_metadata(&discovery_url).await {
      
Ok(Some(metadata)) => return Ok(Some(metadata)),
      
Ok(None) => continue,
      
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
      
}
      Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
      
Either approach lets discovery fall through to the next candidate URL.
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, 'i');
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style.textContent = '
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color-scheme: dark light;
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
          Skip to content
          OAuth discovery aborts after first malformed well-known URL candidate #544
          Closed
          Closed
          OAuth discovery aborts after first malformed well-known URL candidate#544
          Description
          @sorech
          sorech
          opened on Nov 17, 2025
          Describe the bug
          
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
          To Reproduce
          1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
          2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
          3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
          Expected behavior
          
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
          Logs
          
Added global MCP server 'server-name'.
          
Detected OAuth support. Starting OAuth flow…
          
Error: Metadata error: Failed to parse metadata: error decoding response body
          Additional context
          
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
          match self.fetch_authorization_metadata(&discovery_url).await {
          
Ok(Some(metadata)) => return Ok(Some(metadata)),
          
Ok(None) => continue,
          
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
          
}
          Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
          
Either approach lets discovery fall through to the next candidate URL.
          Metadata
          Metadata
          Assignees
          No one assigned
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            No labels
            No labels
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            No type
            Projects
            No projects
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, 'i');
if (__m === '*' || __re.test(location.href)) {
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var ref = document.referrer;
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} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
              Skip to content
              OAuth discovery aborts after first malformed well-known URL candidate #544
              Closed
              Closed
              OAuth discovery aborts after first malformed well-known URL candidate#544
              Description
              @sorech
              sorech
              opened on Nov 17, 2025
              Describe the bug
              
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
              To Reproduce
              1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
              2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
              3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
              Expected behavior
              
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
              Logs
              
Added global MCP server 'server-name'.
              
Detected OAuth support. Starting OAuth flow…
              
Error: Metadata error: Failed to parse metadata: error decoding response body
              Additional context
              
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
              match self.fetch_authorization_metadata(&discovery_url).await {
              
Ok(Some(metadata)) => return Ok(Some(metadata)),
              
Ok(None) => continue,
              
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
              
}
              Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
              
Either approach lets discovery fall through to the next candidate URL.
              Metadata
              Metadata
              Assignees
              No one assigned
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                No labels
                No labels
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                No type
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
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var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
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} catch (e) {
return url;
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function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
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(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
                  Skip to content
                  OAuth discovery aborts after first malformed well-known URL candidate #544
                  Closed
                  Closed
                  OAuth discovery aborts after first malformed well-known URL candidate#544
                  Description
                  @sorech
                  sorech
                  opened on Nov 17, 2025
                  Describe the bug
                  
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
                  To Reproduce
                  1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
                  2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
                  3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
                  Expected behavior
                  
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
                  Logs
                  
Added global MCP server 'server-name'.
                  
Detected OAuth support. Starting OAuth flow…
                  
Error: Metadata error: Failed to parse metadata: error decoding response body
                  Additional context
                  
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
                  match self.fetch_authorization_metadata(&discovery_url).await {
                  
Ok(Some(metadata)) => return Ok(Some(metadata)),
                  
Ok(None) => continue,
                  
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
                  
}
                  Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
                  
Either approach lets discovery fall through to the next candidate URL.
                  Metadata
                  Metadata
                  Assignees
                  No one assigned
                    Labels
                    No labels
                    No labels
                    Type
                    No type
                    Projects
                    No projects
                      Milestone
                      No milestone
                      Relationships
                      None yet
                      Development
                      No branches or pull requests
                      Issue actions
                      
, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
                      Skip to content
                      OAuth discovery aborts after first malformed well-known URL candidate #544
                      Closed
                      Closed
                      OAuth discovery aborts after first malformed well-known URL candidate#544
                      Description
                      @sorech
                      sorech
                      opened on Nov 17, 2025
                      Describe the bug
                      
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
                      To Reproduce
                      1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
                      2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
                      3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
                      Expected behavior
                      
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
                      Logs
                      
Added global MCP server 'server-name'.
                      
Detected OAuth support. Starting OAuth flow…
                      
Error: Metadata error: Failed to parse metadata: error decoding response body
                      Additional context
                      
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
                      match self.fetch_authorization_metadata(&discovery_url).await {
                      
Ok(Some(metadata)) => return Ok(Some(metadata)),
                      
Ok(None) => continue,
                      
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
                      
}
                      Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
                      
Either approach lets discovery fall through to the next candidate URL.
                      Metadata
                      Metadata
                      Assignees
                      No one assigned
                        Labels
                        No labels
                        No labels
                        Type
                        No type
                        Projects
                        No projects
                          Milestone
                          No milestone
                          Relationships
                          None yet
                          Development
                          No branches or pull requests
                          Issue actions
                          
, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
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el.style.top = 'auto';
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}
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observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
                          Skip to content
                          OAuth discovery aborts after first malformed well-known URL candidate #544
                          Closed
                          Closed
                          OAuth discovery aborts after first malformed well-known URL candidate#544
                          Description
                          @sorech
                          sorech
                          opened on Nov 17, 2025
                          Describe the bug
                          
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
                          To Reproduce
                          1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
                          2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
                          3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
                          Expected behavior
                          
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
                          Logs
                          
Added global MCP server 'server-name'.
                          
Detected OAuth support. Starting OAuth flow…
                          
Error: Metadata error: Failed to parse metadata: error decoding response body
                          Additional context
                          
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
                          match self.fetch_authorization_metadata(&discovery_url).await {
                          
Ok(Some(metadata)) => return Ok(Some(metadata)),
                          
Ok(None) => continue,
                          
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
                          
}
                          Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
                          
Either approach lets discovery fall through to the next candidate URL.
                          Metadata
                          Metadata
                          Assignees
                          No one assigned
                            Labels
                            No labels
                            No labels
                            Type
                            No type
                            Projects
                            No projects
                              Milestone
                              No milestone
                              Relationships
                              None yet
                              Development
                              No branches or pull requests
                              Issue actions
                              
, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
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var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
}
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::-webkit-scrollbar-thumb { background: #444 !important; }
::-webkit-scrollbar-thumb:hover { background: #555 !important; }
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OAuth discovery aborts after first malformed well-known URL candidate · Issue #544 · modelcontextprotocol/rust-sdk · GitHub
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                              OAuth discovery aborts after first malformed well-known URL candidate #544
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                              OAuth discovery aborts after first malformed well-known URL candidate#544
                              Description
                              @sorech
                              sorech
                              opened on Nov 17, 2025
                              Describe the bug
                              
When the RMCP client performs OAuth metadata discovery, it stops immediately if the first candidate /.well-known/oauth-authorization-server URL returns non-JSON content (e.g., an HTML login page). The JSON parse error is treated as fatal, so the client never tries the rest of the RFC 8414 fallback URLs.
                              To Reproduce
                              1. Register a streamable MCP server whose default well-known URL (e.g., https://example.com/.well-known/oauth-authorization-server/) returns HTML instead of JSON, but whose canonical fallback https://example.com/.well-known/oauth-authorization-server returns valid metadata.
                              2. Run codex mcp add --url https://example.com/mcp/default server-name with experimental_use_rmcp_client = true.
                              3. Observe that Codex reports Failed to parse metadata and bails out without attempting the fallback URL.
                              Expected behavior
                              
If a candidate well-known URL returns malformed JSON, the RMCP client should log the failure and continue trying the next fallback URL (per RFC 8414), eventually succeeding when it reaches the canonical JSON endpoint.
                              Logs
                              
Added global MCP server 'server-name'.
                              
Detected OAuth support. Starting OAuth flow…
                              
Error: Metadata error: Failed to parse metadata: error decoding response body
                              Additional context
                              
Minimal fix: treat parse failures as “no metadata here” so the loop keeps going:
                              match self.fetch_authorization_metadata(&discovery_url).await {
                              
Ok(Some(metadata)) => return Ok(Some(metadata)),
                              
Ok(None) => continue,
                              
Err(_) => continue, // <— new branch with swallow parse errors, try next candidate
                              
}
                              Alternatively, inside fetch_authorization_metadata, convert JSON parse errors into Ok(None) after logging the body.
                              
Either approach lets discovery fall through to the next candidate URL.
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