fix: strip and validate tool outputSchema and inputSchema by DaleSeo · Pull Request #860 · modelcontextprotocol/rust-sdk

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo · 2026-05-28T21:14:40Z

Motivation and Context

PR #856 cleaned up inputSchema by stripping the unnecessary top-level title and description fields, since the MCP spec does not require them and they only add noise to the LLM context. The same noise applies to outputSchema, and the spec's official examples for both schemas omit those top-level fields in identical fashion. This PR extends the same cleanup to outputSchema for symmetry.

EDIT: While auditing the symmetry, I found two related gaps and also addressed them. First, schema_for_input didn't enforce the MCP spec's type: "object" constraint, which schema_for_output already does. This meant that corner cases like Parameters<String> could silently create tool definitions that violated the spec. Now, the function is validated and returns Result<Arc<JsonObject>, String>, matching schema_for_output, with clear panics at call sites that previously couldn't fail. Second, schema_for_empty_input uses LazyLock to avoid allocating memory with each call.

How Has This Been Tested?

Added some unit tests and updated the integration test.

Breaking Changes

No breaking change. schema_for_input was added as a public function in PR #856 but has not been released yet.

Behavior change worth noting: tools whose input parameter type does not produce a type: "object" schema (e.g. Parameters<String>, Parameters<i32>) will now panic at construction time with a descriptive message rather than silently producing a spec-violating tool definition. Two tools inside this repo's own test suite fit that pattern and have been updated.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

Copilot

Pull request overview

This PR tightens and harmonizes MCP tool schema generation by (1) stripping top-level title/description from both inputSchema and outputSchema, and (2) validating that generated tool schemas have a root JSON Schema type: "object" (as required by the MCP spec), failing fast with clearer panics at tool-construction call sites.

Changes:

Strip top-level title/description from tool outputSchema (matching the existing inputSchema cleanup) and add shared validation/stripping logic.
Make schema_for_input validate root type: "object" and return Result, updating call sites to unwrap with descriptive panic messages.
Update tests/fixtures to use structured request/response types (object schemas) and validate outputSchema serialization behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Updates documentation around schema generation (currently needs clarification; see PR comment).
crates/rmcp/src/handler/server/common.rs	Adds shared validate+strip helper; enforces object-root schemas for tool input/output; caches results; optimizes empty-input schema via `LazyLock`.
crates/rmcp/src/model/tool.rs	Updates `with_input_schema` to unwrap `Result` with a clearer panic (matching output behavior).
crates/rmcp/src/handler/server/router/tool/tool_traits.rs	Updates default `input_schema()` to unwrap `Result` with a clearer panic.
crates/rmcp/src/handler/server/router/tool.rs	Updates router builder paths to unwrap `schema_for_input` with clearer panics.
crates/rmcp-macros/src/tool.rs	Updates macro-generated tool metadata to unwrap `schema_for_input` failures with descriptive panics.
crates/rmcp/tests/test_structured_output.rs	Adjusts test tools to use object-typed parameter structs to comply with the new input-schema validation.
crates/rmcp/tests/test_list_tools_result.rs	Changes tool to return `Json<AddResult>` so an `outputSchema` is generated and test can validate it.
crates/rmcp/tests/test_list_tools_result/list_tools_result.json	Updates fixture to include `outputSchema` (without top-level `title`/`description`).

 ```

-The generated tool `inputSchema` is derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.
+The generated tool `inputSchema` and `outputSchema` are derived from the fields of `T`. The type name and documentation on `T` are ignored; only field names, field types, and field documentation are used.


Brings in upstream v1.6.1..266f870 (15 commits): protocol-version negotiation/init-mismatch fixes, input/outputSchema strip+validate (modelcontextprotocol#856/modelcontextprotocol#860), tool-argument errors as tool results (modelcontextprotocol#894), OAuth issuer validation + OIDC application_type + offline_access reauth, roots/ sampling/logging deprecation (SEP-2577), empty-2xx notification handling. Conflict resolutions (fork changes preserved): - handler/server/common.rs: kept both test blocks. Upstream's validate_and_strip / schema_for_input layer composes on top of the fork's ReplaceBoolSchemas normalization in schema_for_type. - handler/server/router/tool.rs: combined upstream's into_tool_argument_error dispatch with the anthropic-ext normalize_call_tool_result post-processing (result kept mutable). Restored .githooks/commit-msg, which upstream modelcontextprotocol#898 deleted incidentally in an unrelated "progress demo" change; the hook gates our conventional- commit workflow and skips merge commits.

DaleSeo added 2 commits May 28, 2026 17:12

fix: remove unnecessary fields from tools' outputSchema
7018860

fix: validate input schema root type per MCP spec
d8f81f4

DaleSeo self-assigned this May 28, 2026

github-actionsBot added T-documentation Documentation improvements T-test Testing related changes T-config Configuration file changes T-core Core library changes T-handler Handler implementation changes T-macros Macro changes T-model Model/data structure changes labels May 28, 2026

DaleSeo marked this pull request as ready for review May 28, 2026 21:41

DaleSeo requested a review from a team as a code owner May 28, 2026 21:41

jokemanfire requested a review from Copilot May 29, 2026 02:15

Copilotstarted reviewing on behalf of jokemanfire May 29, 2026 02:15 View session

CopilotAI reviewed May 29, 2026
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alexhancock approved these changes Jun 2, 2026
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alexhancock merged commit 254f04a into mainJun 2, 2026
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alexhancock deleted the fix/output-schema-strip-title-description branch June 2, 2026 16:01

github-actionsBot mentioned this pull request Jun 2, 2026
chore: release v1.8.0 #850
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branben mentioned this pull request Jun 6, 2026
feat: relax outputSchema to accept non-object JSON Schema types (SEP-2106) #895
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claudeBot mentioned this pull request Jun 23, 2026
chore(deps): update rust crate rmcp to v1.8.0 factorial-io/imap-mcp#84
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This was referenced Jun 23, 2026
chore: release v2.0.0 #920
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MCP upstream changes need template review dinglebear-ai/soma#56
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jmagar mentioned this pull request Jul 3, 2026
Migrate workspace from rmcp 1.7.0 → 2.1.0 dinglebear-ai/labby#184
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