Handling tool errors and partial results in an agent loop

What it is

Three outcomes need three different results. A protocol error (unknown tool, invalid arguments, server unreachable) is a failure of the call itself; the Model Context Protocol reports these as JSON-RPC errors. A tool execution error (the upstream API returned 500, the file does not exist, the query timed out) is a valid result that says the operation failed; MCP returns it in the result with isError: true, and the Claude tool-use documentation describes the equivalent is_error: true flag on a tool_result block, after which the model incorporates the error into its next step. A partial result (7 of 10 files processed, the first page of a search, a batch with two rejected rows) is a success whose content must say what is missing.

Why it matters

An agent acts on what the tool result says. An exception that never reaches the model produces a confident answer built on nothing. An error without detail produces blind retries of the same call. A partial result reported as complete produces a task marked done with rows silently lost.

How to apply

• Never let an exception escape the tool; catch it and return an error result with a stable error type, the message and, where known, whether a retry can help (not for a 404, yes for a timeout).
• For partial results, return the successful part plus an explicit list of what failed and why, and a cursor or identifier for continuing.
• Keep error text short and factual; no stack traces or upstream output that could carry injected instructions.
• Enforce retry limits in the loop, not by instruction: the same tool with the same arguments after an error is allowed a fixed number of times, then the loop returns control with a summary.
• Make write tools idempotent or give them an idempotency key, so that a retry after an ambiguous failure does not duplicate the effect.
• Distinguish "no results" from "error": an empty search is a valid, complete result.
• Log every error result with the run ID; the pattern of errors is the tool's usability report.

Pitfalls

Returning null or an empty string on failure. Mapping every failure to one generic message. Letting the model decide how many times to retry. Raising a protocol error for a business condition ("order not found" is a result, not a malformed call). Surfacing partial results only in a log the model never sees.