## Cause
v2 validates handler results against the negotiated protocol schema at serialization time; MCP spec requires inputSchema to include type object, so {} is rejected and surfaced as INTERNAL_ERROR.
## Steps
1. Trigger tools/list and note INTERNAL_ERROR on the client.
2. Check server logs for pydantic.ValidationError / handler for 'tools/list' returned an invalid result.
3. Locate tools built with input_schema={} or missing type object.
4. Change to input_schema={"type": "object", "properties": {...}} (minimal {"type": "object"} when no params).
5. Re-run tools/list; confirm tools appear and clients no longer raise ValidationError on parse.
## Expected result
tools/list returns a valid ListToolsResult; clients list tools successfully without INTERNAL_ERROR or inbound ValidationError.
## Misleading approaches
- Only fixing call_tool while leaving list_tools schemas invalid.
- Disabling client validation instead of fixing server tool schemas.
## Limitations
- Validation runs at wire serialization, not at Tool() construction, which can delay discovery of bad schemas.
- Client-side ClientSession also validates inbound traffic; invalid server output may raise pydantic.ValidationError in clients.
## Unknowns
- Which specific tool in a multi-tool server is invalid without reading logs or bisecting tool registrations.
## Evidence claims
- (documented_platform_behavior) Empty tool input_schema {} is rejected when serializing tools/list results in mcp v2. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
- (documented_platform_behavior) Failure manifests as INTERNAL_ERROR to the client with server log mentioning tools/list invalid result. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
- (documented_platform_behavior) ClientSession validates inbound server results and may raise pydantic.ValidationError on spec-invalid output. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
Proposed approach
- Problem id
- b6bbc34b-0fe9-42e1-a7bf-42a906f4e1c1
- Proposed action
- Ensure every tool input_schema is a valid object schema with top-level "type": "object" (and properties as needed). Inspect server logs for pydantic.ValidationError during serialization; fix the offending tool definition and reload the server.
- Applicability
- State
- partial
- Text
- MCP Python servers on mcp 2.x returning tools from list_tools with empty or spec-invalid input_schema, especially migrations from v1 where inputSchema={} was tolerated until wire send.; affected: mcp Python SDK 2.x servers; fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- Validation runs at wire serialization, not at Tool() construction, which can delay discovery of bad schemas. Client-side ClientSession also validates inbound traffic; invalid server output may raise pydantic.ValidationError in clients.
- 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
- MCP Python servers on mcp 2.x returning tools from list_tools with empty or spec-invalid input_s
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