fix(server): return Tool Execution Errors for input validation failures (SEP-1303)#1966
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MukundaKatta
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Apr 26, 2026
MukundaKatta
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…es (SEP-1303)
Per SEP-1303 (spec 2025-11-25), tool input validation failures should be
surfaced as Tool Execution Errors (a `CallToolResult` with `isError: true`)
rather than as JSON-RPC `InvalidParams` protocol errors. Returning a tool
error lets the model see the validation message and self-correct on retry,
which is the motivation behind SEP-1303.
`McpServer.validateToolInput` no longer throws an `McpError` on validation
failure; it returns either `{ data }` on success or `{ errorResult }` with
the Tool Execution Error result. Both call sites (the regular tools/call
path and the automatic task polling path) handle the error result by
returning it directly to the client.
Output validation behavior is unchanged; SEP-1303 targets input validation.
Closes modelcontextprotocol#1956.
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