Cloudflare Tail Worker source-mapped stack traces investigation

cf-tail-worker-sourcemaps.md

Source-mapped stack traces in Tail Workers require unhandled exceptions

Summary

We're building error monitoring with a Tail Worker and want source-mapped stack traces. We found that only unhandled exceptions populate the exceptions array with source-mapped stacks - meaning you can't have both a custom error response AND source-mapped stacks in the Tail Worker.

What we want

1. Custom error responses to clients (not Cloudflare's generic error page)
2. Source-mapped stack traces in our Tail Worker for alerting

What we found

With Hono's onError handler:

app.onError((err) => {
  if (err instanceof HTTPException) {
    return err.getResponse(); // Custom response for expected errors
  }
  throw err; // Re-throw for source-mapped stacks
});

Scenario	Client Response	Tail outcome	Tail exceptions	Source-mapped?
Re-throw error	Cloudflare error page	"exception"	✅ Has error	✅ Yes
Return custom response	Custom JSON	"ok"	❌ Empty	N/A
console.error(e)	Custom JSON	"ok"	❌ Empty	❌ No (just string in logs)

Tail Worker event with unhandled exception

{
  "outcome": "exception",
  "exceptions": [{
    "name": "Error",
    "message": "Test error",
    "stack": "    at triggerTestError (src/worker/index.ts:33:9)\n    at src/worker/index.ts:43:3"
  }]
}

The stack is correctly source-mapped to original TypeScript files.

Tail Worker event when error is caught

{
  "outcome": "ok",
  "exceptions": [],
  "logs": []
}

No exception data available to the Tail Worker.

Question

Is there a way to return a custom error response to the client while still having the exception (with source-mapped stack) appear in the Tail Worker's exceptions array?

Environment

• Cloudflare Workers with Hono
• upload_source_maps: true in wrangler config
• Tail Worker attached via tail_consumers

Documentation consulted

Cloudflare:

• Tail Handler - TailEvent structure and handler API
• Tail Workers - How to set up tail consumers
• Source maps and stack traces - Enabling source map uploads
• Errors and exceptions - Error handling in Workers

Hono:

• HTTPException - Custom error responses with HTTPException