Anthropic API Error overloaded_error: API temporarily overloaded (529)

Identify: does your log match?

anthropic.InternalServerError: Error code: 529 - {"type": "error", "error": {"type": "overloaded_error", "message": "Overloaded"}, "request_id": "req_011CUQ5bJ7xVwKp3oRz9TnYd"}

1. High

   Provider-wide capacity spike

   Demand for a model exceeds available capacity for a few seconds or minutes. It is not tied to your organization’s usage.

2. Medium

   Retries disabled or too few

   Clients created with maxRetries: 0, or wrapped in a job queue that fails fast, surface a transient 529 as a hard error.

3. Low

   Synchronous fan-out at peak times

   Batch jobs that fire hundreds of parallel calls during peak hours hit overloads more often than the same work spread out.

Fix

const client = new Anthropic({ maxRetries: 0 });
const msg = await client.messages.create(req);

const client = new Anthropic({ maxRetries: 5 }); // backoff on 408, 409, 429 and 5xx incl. 529
const msg = await client.messages.create(req);

Verify: reproduce it on purpose

$ node -e 'require("http").createServer((q,s)=>{s.writeHead(529,{"content-type":"application/json"});s.end(JSON.stringify({type:"error",error:{type:"overloaded_error",message:"Overloaded"}}))}).listen(8529)' &
ANTHROPIC_BASE_URL=http://localhost:8529 node your-script.js

Prevent

• Treat 529 like 503: retry, never alert on a single occurrence.
• Use the Message Batches API for work that does not need an immediate answer.
• Alert on the 529 rate over 5 minutes, not individual errors.