# Your retry logic has never seen a real 429. Here is how to test it with a local fault-injecting mock

> Source: <https://dev.to/renev3408/your-retry-logic-has-never-seen-a-real-429-here-is-how-to-test-it-with-a-local-fault-injecting-mock-2pi2>
> Published: 2026-10-17 03:16:59+00:00

Most LLM client code has a retry branch that has never actually run. The provider returns 200 in every test, so the `429`, `500` and timeout paths sit untested until production hits them at 3am.

The fix is cheap: point your client at a local server that fails on a schedule you control.

Run a tiny HTTP server on localhost that mimics a chat-completions endpoint. Give it a failure schedule, for example:

```
[
  {"status": 429, "retry_after": 2},
  {"status": 500},
  {"status": "timeout"},
  {"status": 200}
]
```

Each incoming request consumes the next step. Your client sees a 429 with a `Retry-After` header, then a 500, then a hang, then a good response.

``` python
import json, sys, time
from http.server import BaseHTTPRequestHandler, HTTPServer

SCHEDULE = json.load(open(sys.argv[1]))
step = {"i": 0}

class H(BaseHTTPRequestHandler):
    def do_POST(self):
        s = SCHEDULE[min(step["i"], len(SCHEDULE) - 1)]
        step["i"] += 1
        if s["status"] == "timeout":
            time.sleep(60)
            return
        self.send_response(s["status"])
        if "retry_after" in s:
            self.send_header("Retry-After", str(s["retry_after"]))
        self.end_headers()
        self.wfile.write(b'{"choices":[{"message":{"content":"ok"}}]}')

HTTPServer(("127.0.0.1", 9797), H).serve_forever()
```

Point your client's base URL at `http://127.0.0.1:9797` and run your normal code path.

I bundled a slightly more complete version (schedule file, README, examples) as a pay-what-you-want download, minimum $1: [https://renevibe76.gumroad.com/l/sargxm](https://renevibe76.gumroad.com/l/sargxm)

The snippet above is enough to get started on your own, though. The point is to see your retry code run at least once before production does it for you.