## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
Client-side output schema validation is mandatory in session.py: invalid schemas and unresolvable $ref raise RuntimeError instead of skipping validation.
## Steps
1. Reproduce with call_tool on the named tool and capture the full RuntimeError message.
2. On the server, validate the advertised outputSchema with a JSON Schema validator (check_schema) and ensure all $ref targets exist in the same document.
3. Remove or fix broken $ref, add missing $defs, or omit outputSchema if structured output is not required.
4. Re-list tools and retry call_tool; client compiles validators per tool when the schema changes.
## Expected result
call_tool completes without RuntimeError; structured_content validates against the fixed schema, or the server stops advertising an invalid outputSchema.
## Misleading approaches
- Assuming the server response is malformed when the failure is schema advertisement.
- Patching only the returned payload without fixing outputSchema on tools/list.
## Limitations
- Does not disable client validation; there is no supported flag to skip outputSchema checks in the cited client code.
- Validation errors on structured_content content (not schema shape) produce a different RuntimeError prefix ('Invalid structured content returned by tool').
## Unknowns
- Whether a specific server framework auto-generates broken outputSchema for a given tool.
## Evidence claims
- (documented_platform_behavior) Unresolvable $ref during structured_content validation raises RuntimeError Invalid schema for tool. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/src/mcp/client/session.py
- (documented_platform_behavior) outputSchema that fails jsonschema check_schema raises RuntimeError Invalid schema for tool. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/src/mcp/client/session.py
Proposed approach
- Problem id
- 25eb8cc2-8827-4594-bb08-cb1252160f77
- Proposed action
- On the MCP server, fix the tool outputSchema so it is valid JSON Schema for the negotiated dialect and self-contained (resolve $ref within the document or inline definitions). On the client, treat RuntimeError with this message as a server schema defect; optionally catch and report rather than retrying blindly.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); MCP Python SDK 2.x client (mcp package) calling tools that declare outputSchema; error appears during call_tool after the server responds, not during tools/list.; affected: MCP Python SDK 2.x (client); fix_status: PARTIAL
- Limitations
- State
- partial
- Text
- Does not disable client validation; there is no supported flag to skip outputSchema checks in the cited client code. Validation errors on structured_content content (not schema shape) produce a different RuntimeError prefix ('Invalid structured content returned by tool').
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); MCP Python SDK 2.x client
Reported outcomes
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