Add validate_output option for OpenAPI tools

[jlowin]

When backends don't conform to their own OpenAPI response schemas, the MCP SDK's output validation rejects the response and the tool fails with "Output validation error." Users have no way to opt out — they're stuck if the backend is wrong.

This adds validate_output (default True) to OpenAPIProvider and FastMCP.from_openapi(). When set to False, extracted output schemas are replaced with a permissive {"type": "object", "additionalProperties": true} that accepts any response. Structured JSON still flows to clients normally; only the strict schema checking is disabled. The x-fastmcp-wrap-result flag is preserved so non-object responses (arrays, primitives) continue to be wrapped correctly.

mcp = FastMCP.from_openapi(
    openapi_spec=spec,
    client=client,
    validate_output=False,  # accept any response shape
)

For per-tool control, mcp_component_fn can be used to selectively disable validation on specific endpoints.

Closes #2472

[coderabbitai]

Walkthrough

Adds a new optional parameter validate_output: bool = True to FastMCP.from_openapi and OpenAPIProvider.__init__. The provider stores this flag and, when validate_output is False and an OpenAPI operation has an output schema, replaces that schema with a permissive object schema (preserving x-fastmcp-wrap-result if present). Also ensures tool run results are dicts by wrapping non-dict structured outputs as {"result": ...}. Several documentation files update inline anchors and the from_openapi docs to document the new parameter.

Possibly related PRs

• Remove OpenAPI timeout parameter, make client optional, surface timeout errors #3067: Modifies the OpenAPI provider and FastMCP.from_openapi signatures, touching the same constructors that now accept validate_output.
• Fix unhandled exceptions in OpenAPI POST tool calls #3133: Changes runtime behavior in the OpenAPI components' tool run and output wrapping/validation logic, overlapping with the added dict-wrapping and validation adjustments.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	The description provides clear context and example usage, but the contributors checklist items are not marked, and the self-review checklist is missing required sign-offs.	Complete the contributors and review checklists by marking the appropriate checkboxes to confirm testing, documentation updates, and self-review completion.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding a validate_output option for OpenAPI tools.
Linked Issues check	✅ Passed	The PR successfully addresses issue #2472 by implementing permissive output validation that accepts responses matching any valid object/array structure without strict schema enforcement.
Out of Scope Changes check	✅ Passed	All changes are directly related to adding the validate_output feature; documentation updates reflect the new parameter and line number adjustments are necessary for accuracy.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/openapi-validate-output

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[jlowin]

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[coderabbitai]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve wrapping when output validation is disabled

With validate_output=False, this replaces the extracted schema with a plain object schema and only carries x-fastmcp-wrap-result forward when it already existed. For endpoints whose spec says type: object, a backend that actually returns an array/primitive will not be wrapped, so OpenAPITool.run passes a non-dict structured_content and ToolResult raises structured_content must be a dict; the tool still fails instead of acting as a validation opt-out for response shape mismatches.

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