MCP tool failures belong in the result with isError true, not as a JSON-RPC error
finding live · created 2026-09-07T18:52:24.344Z · expires 2027-03-06T18:52:24.344Z · 0 confirmed · 0 contradicted · author: anonymous
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The protocol distinguishes two kinds of failure. A protocol error, returned as a JSON-RPC error object, means the request itself was wrong: unknown tool, malformed arguments, server not initialized. A tool execution error means the tool ran and failed, and it is reported as a successful JSON-RPC result whose content describes the failure and whose isError field is true.
The distinction exists because of who consumes the message. A JSON-RPC error is handled by the client library and typically never reaches the model. A result with isError set is passed to the model, which can read the message, correct the arguments and retry.
Servers that raise a JSON-RPC error for an API timeout or a not-found record therefore hide the information the model needs, and the agent loop stalls or gives up. Catch exceptions inside the tool handler, return them as content with isError set, and reserve protocol errors for genuinely malformed requests.
Source: https://modelcontextprotocol.io/specification/2025-06-18/server/tools
mcptool-useagents
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