Add schema validation to lowlevel server #1005
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ihrpr
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Jun 24, 2025
- Add jsonschema dependency for schema validation - Implement tool definition cache in Server class that gets refreshed when list_tools is called - Add _validate_tool_arguments helper method to validate tool arguments against inputSchema - Update call_tool handler to validate arguments before execution - Log warning and skip validation for tools not found in cache - Add comprehensive tests for validation scenarios This ensures tool arguments are validated against their JSON schemas before execution, providing better error messages and preventing invalid tool calls from reaching handlers.
- Refactor code to extract _get_tool_definition helper and simplify validation - Update call_tool to support three return types: content only, dict only, or both - Add outputSchema validation that checks structured content matches the schema - Serialize dict-only results to JSON text content - Factor error result construction into _make_error_result helper - Add comprehensive tests for all output validation scenarios The server now validates tool outputs against their defined schemas, providing better error messages and ensuring tool responses match their contracts.
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Lowlevel server
inputSchema@server.call_tool(validate_input=False)can be used to disable input validationoutputSchema, if definedMotivation and Context
Previously lowlevel server did no schema validation for tool calls.
How Has This Been Tested?
New tests added, old tests (which now run lowlevel tool calls through schema validation) succeed without change.
Breaking Changes
Possible that some tools defined in the lowlevel server "just worked" against invalid incoming arguments, these will now fail validation.
@server.call_tool(validate_input=False)can be used in this situation.Types of changes
Checklist
Additional context
In addition to adding validation to the lowlevel server,
FastMCPdelegates to the lowlevel server and will depend on validation being performed here.