Building an MCP server that handles LLM API errors without stalling agents
An MCP server that calls the Anthropic API can fail in ways that confuse the agent using it, since a raw traceback looks the same whether a retry would help or not. The author recounts a 2026 overload incident where a burst of 529 errors caused the agent to abandon the tool mid-job. The proposed fix classifies errors by status code and body type, retries transient failures like 429 and 5xx with jittered backoff that respects server-requested delays, and fails fast on quota and authentication problems. Errors returned to the client are kept to a single line that includes a request ID. The guide also flags setup pitfalls, such as environment variables not being passed to stdio servers and a missing anthropic-version header causing misleading 400 errors.
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