---id: 0002-tool-errors-are-resultstype: decisionowner: justinreviewed: 2026-10-13status: accepted---# 0002 Tool errors are results## ContextA tool can fail in two ways in MCP. A JSON-RPC error (`-32602` and friends) tells the client the request itself was bad. A tool result with `isError: true` tells the model the call went through and here is what went wrong. Many clients show JSON-RPC errors to the user, or retry, and never put them in front of the model.Most of our failures are the model's to fix: a unit we do not know, a time zone that does not exist, an instant without an offset.## DecisionEvery failure inside a tool call comes back as a result with `isError: true` and a message written for the model:- A tool throws `ToolError` for anything the caller can fix. The message says what was wrong and gives an example of what would work.- Arguments that fail the input schema are answered by the SDK the same way, with zod's message.- Any other exception is logged and returned as an `isError` result saying the server failed and the same input will fail again, so the model does not loop.JSON-RPC errors are left for protocol problems: malformed JSON, an unknown method, a missing `Accept` header.## Consequences- Models correct themselves on the next call (`furlong` becomes `ft`, `Mars/Base` becomes `UTC`) instead of giving up.- Error messages are part of the interface and are tested like output. Changing one is a behavior change.- Usage counts can report errors per tool, which shows where descriptions need work; see [[usage-counts]].