fix: return JSON-RPC error for nonexistent tool calls#1514
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kaigritun wants to merge 2 commits intomodelcontextprotocol:mainfrom
Open
fix: return JSON-RPC error for nonexistent tool calls#1514kaigritun wants to merge 2 commits intomodelcontextprotocol:mainfrom
kaigritun wants to merge 2 commits intomodelcontextprotocol:mainfrom
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Per the MCP specification, calling a nonexistent tool should return a JSON-RPC error response, not a result with isError: true. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#error-handling Before this fix, calling a nonexistent tool would return: { jsonrpc: '2.0', id: 3, result: { content: [{ type: 'text', text: 'MCP error -32602: Tool x not found' }], isError: true } } After this fix, it correctly returns: { jsonrpc: '2.0', id: 3, error: { code: -32602, message: 'Tool x not found' } } Fixes modelcontextprotocol#1510
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Summary
Per the MCP specification, calling a nonexistent tool should return a JSON-RPC error response, not a result with
isError: true.Specification reference: https://modelcontextprotocol.io/specification/2025-11-25/server/tools#error-handling
Problem
When a client calls a tool that doesn't exist, the current behavior returns:
{ "jsonrpc": "2.0", "id": 3, "result": { "content": [{ "type": "text", "text": "MCP error -32602: Tool nonexistent_tool not found" }], "isError": true } }Solution
This PR updates the error handling to return a proper JSON-RPC error for tool-not-found cases:
{ "jsonrpc": "2.0", "id": 3, "error": { "code": -32602, "message": "Tool nonexistent_tool not found" } }Changes
callToolHandlerto re-throwProtocolErrorwhen the error indicates a tool was not foundisError: trueTesting
All 364 tests pass.
Fixes #1510