sentry-javascript Replay tunnel fails due to empty envelope

Is there an existing issue for this?

[x] I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
[x] I have reviewed the documentation https://docs.sentry.io/
[x] I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/sveltekit

SDK Version

8.26.0

Framework Version

Sveltekit 2.5.18

Link to Sentry event

No response

Reproduction Example/SDK Setup

Using the example provided from the documentation does not work with replay requests. The docs show using an array buffer. This works well with traces, however for replays the code error on parsing piece. In this case the envelopeBytes is 0 and envelope empty.

https://docs.sentry.io/platforms/javascript/troubleshooting/#dealing-with-ad-blockers

const SENTRY_HOST = "oXXXXXX.ingest.sentry.io";
const SENTRY_PROJECT_IDS = ["123456"];

export const POST: RequestHandler = async ({ request }) => {
  try {
    const envelopeBytes = await request.arrayBuffer();
    const envelope = new TextDecoder().decode(envelopeBytes);
    console.log(envelopeBytes, envelope);
    const piece = envelope.split('\n')[0];
    const header = JSON.parse(piece);
    const dsn = new URL(header['dsn']);
    const project_id = dsn.pathname?.replace('/', '');

    if (dsn.hostname !== SENTRY_HOST) {
      throw new Error(`Invalid sentry hostname: ${dsn.hostname}`);
    }

    if (!project_id || !SENTRY_PROJECT_IDS.has(project_id)) {
      throw new Error(`Invalid sentry project id: ${project_id}`);
    }

    const upstream_sentry_url = `https://${SENTRY_HOST}/api/${project_id}/envelope/`;
    await fetch(upstream_sentry_url, {
      method: 'POST',
      body: envelopeBytes,
    });

    return json({}, { status: 200 });
  } catch (error) {
    console.error('error tunneling to sentry', error);
    return json({ error: 'error tunneling to sentry' }, { status: 500 });
  }
};

Steps to Reproduce

Create an API route, eg /api/errors/+server.ts.
Export a POST api.
Configure sentry client to tunnel through /api/errors.

Expected Result

The replays get sent to sentry successfully.

Actual Result

Traces are tunneled. Replays result in this error:

// console.log(envelopeBytes, envelope);
ArrayBuffer { [Uint8Contents]: <>, byteLength: 0 }

error tunneling to sentry SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at POST (/src/routes/api/errors/+server.ts:17:25)
    at async Module.render_endpoint (/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/endpoint.js:49:18)
    at async resolve (/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:454:17)
    at async documentPolicy (/src/lib/hooks/document-policy.ts:6:20)
    at async /node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:140:21
    at async instrumentHandle (/node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:128:27)
    at async Module.respond (/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:330:20)
    at async file:///node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:524:22

Aug 23 '24 22:08 johnathanludwig

Hello, thanks for writing in.

The envelopes should not be empty for replays. You could add a guard to exit out early if envelopeBytes is empty, but I'm curious why you're getting empty envelopes to begin with.

Please set debug: true in Sentry.init and paste your logs.

Aug 26 '24 07:08 andreiborza

Yes the body of the request is there so I'm not sure why I can't get anything out of it on the server side.

Client side debug: https://gist.github.com/johnathanludwig/1bcefb76f2609a2b44eb3820dbdea26e

Server side:

Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: GET /(unprotected)/login
  ID: 4e95db40cdd3ecc2
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "GET /(unprotected)/login" with ID 4e95db40cdd3ecc2
Sentry Logger [log]: SpanExporter exported 1 spans, 0 unsent spans remaining
Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: POST /api/errors
  ID: 4d39a562acc53f42
Sentry Logger [log]: [Tracing] Inheriting parent's sampled decision for HTTP POST: true
Sentry Logger [log]: [Tracing] Starting sampled span
  op: < unknown op >
  name: HTTP POST
  ID: b8cf5def993c2314
  parent ID: 4d39a562acc53f42
  root ID: 4d39a562acc53f42
  root op: http.server
  root description: POST /api/errors
Sentry Logger [log]: [Tracing] Finishing "< unknown op >" span "HTTP POST" with ID b8cf5def993c2314
Sentry Logger [log]: SpanExporter has 1 unsent spans remaining
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "POST /api/errors" with ID 4d39a562acc53f42
Sentry Logger [log]: SpanExporter exported 2 spans, 0 unsent spans remaining
Sentry Logger [log]: Flushing client reports based on interval.
Sentry Logger [log]: Flushing outcomes...
Sentry Logger [log]: No outcomes to send
Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: POST /api/errors
  ID: 3bc95f07de7ced89
error tunneling to sentry SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at POST (/Users/johnathanludwig/git/frontend/apps/web/src/routes/api/errors/+server.ts:16:25)
    at async Module.render_endpoint (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/endpoint.js:49:18)
    at async resolve (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:454:17)
    at async /Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:140:21
    at async instrumentHandle (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:128:27)
    at async Module.respond (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:330:20)
    at async file:///Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:524:22
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "POST /api/errors" with ID 3bc95f07de7ced89
Sentry Logger [log]: SpanExporter exported 1 spans, 0 unsent spans remaining

Aug 26 '24 14:08 johnathanludwig

Could you please amend the server side debug log with the logs that occur on app startup?

Also, does this happen if you disable compression?

Sentry.replayIntegration({
  useCompression: false,
});

Aug 26 '24 14:08 andreiborza

Disabling compression does resolve the issue. Thank you for the help with that.

Here are the server startup logs when compression was still enabled:

Sentry Logger [log]: Initializing Sentry: process: 19324, thread: main.
Sentry Logger [log]: Integration installed: InboundFilters
Sentry Logger [log]: Integration installed: FunctionToString
Sentry Logger [log]: Integration installed: LinkedErrors
Sentry Logger [log]: Integration installed: RequestData
Sentry Logger [log]: Integration installed: Console
Sentry Logger [log]: Integration installed: Http
Sentry Logger [log]: Integration installed: NodeFetch
Sentry Logger [log]: Integration installed: OnUncaughtException
Sentry Logger [log]: Integration installed: OnUnhandledRejection
Sentry Logger [log]: Integration installed: ContextLines
Sentry Logger [log]: Integration installed: LocalVariablesAsync
Sentry Logger [log]: Integration installed: Context
Sentry Logger [log]: Integration installed: Modules
Sentry Logger [log]: Integration installed: Express
Sentry Logger [log]: Integration installed: Fastify
Sentry Logger [log]: Integration installed: Graphql
Sentry Logger [log]: Integration installed: Mongo
Sentry Logger [log]: Integration installed: Mongoose
Sentry Logger [log]: Integration installed: Mysql
Sentry Logger [log]: Integration installed: Mysql2
Sentry Logger [log]: Integration installed: Redis
Sentry Logger [log]: Integration installed: Postgres
Sentry Logger [log]: Integration installed: Nest
Sentry Logger [log]: Integration installed: Hapi
Sentry Logger [log]: Integration installed: Koa
Sentry Logger [log]: Integration installed: Connect
Sentry Logger [log]: Integration installed: RewriteFrames
Sentry Logger [log]: Integration installed: ProfilingIntegration
Sentry Logger [log]: Running in CommonJS mode.
Sentry Logger [debug]: @opentelemetry/api: Registered a global for diag v1.9.0.
Sentry Logger [debug]: @opentelemetry/api: Registered a global for trace v1.9.0.
Sentry Logger [debug]: @opentelemetry/api: Registered a global for context v1.9.0.
Sentry Logger [debug]: @opentelemetry/api: Registered a global for propagation v1.9.0.
Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: GET /(unprotected)/login
  ID: 6cd4b34ad8046c2e
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "GET /(unprotected)/login" with ID 6cd4b34ad8046c2e
Sentry Logger [log]: SpanExporter exported 1 spans, 0 unsent spans remaining
Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: POST /api/errors
  ID: d587828578ab7951
Sentry Logger [log]: [Tracing] Inheriting parent's sampled decision for HTTP POST: true
Sentry Logger [log]: [Tracing] Starting sampled span
  op: < unknown op >
  name: HTTP POST
  ID: 51e6c9757e8effd3
  parent ID: d587828578ab7951
  root ID: d587828578ab7951
  root op: http.server
  root description: POST /api/errors
Sentry Logger [log]: [Tracing] Finishing "< unknown op >" span "HTTP POST" with ID 51e6c9757e8effd3
Sentry Logger [log]: SpanExporter has 1 unsent spans remaining
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "POST /api/errors" with ID d587828578ab7951
Sentry Logger [log]: SpanExporter exported 2 spans, 0 unsent spans remaining
Sentry Logger [log]: [Tracing] Starting sampled root span
  op: http.server
  name: POST /api/errors
  ID: 4592d3baeb001a91
error tunneling to sentry SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at POST (/Users/johnathanludwig/git/frontend/apps/web/src/routes/api/errors/+server.ts:16:25)
    at async Module.render_endpoint (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/endpoint.js:49:18)
    at async resolve (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:454:17)
    at async /Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:140:21
    at async instrumentHandle (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected]_@[email protected]_@opentelemetry+a_gixmxtpq6bgq6uyfpgkkbh5rvy/node_modules/@sentry/sveltekit/build/cjs/server/handle.js:128:27)
    at async Module.respond (/Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/runtime/server/respond.js:330:20)
    at async file:///Users/johnathanludwig/git/frontend/node_modules/.pnpm/@[email protected]_@[email protected][email protected][email protected]_@types+node@_l5lnugt2i4tpym4dqtslohvg24/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:524:22
Sentry Logger [log]: [Tracing] Finishing "http.server" root span "POST /api/errors" with ID 4592d3baeb001a91
Sentry Logger [log]: SpanExporter exported 1 spans, 0 unsent spans remaining

Aug 26 '24 17:08 johnathanludwig

Right, so TextDecoder can't deal with compressed zlib data. We'll have to update our docs to show how to deal with that.

For now, I suggest to either turn compression off or adapt your tunnel route to deal with the compressed data. Here's an example from our integration tests.

Aug 27 '24 07:08 andreiborza

Great, thanks for the help!

Aug 27 '24 18:08 johnathanludwig

Let's keep this open, I added it to our backlog so we can work on it.

Aug 27 '24 19:08 andreiborza

Here's our sveltekit solution, using a proxy defined in the server hooks - kitql library's handleProxies function:

const SENTRY_UPSTREAM_URL = `https://${SENTRY_HOST}/api/${SENTRY_PRODUCT_ID}/envelope/`;
const proxies = handleProxies([
  [
    "/api/sentry-tunnel",
    {
      to: SENTRY_UPSTREAM_URL,
      requestHook: (event) => {
        const request = event.request.clone();
        request.headers.set(
          "X-Sentry-Auth",
          `Sentry sentry_key=${SENTRY_KEY}, sentry_version=7`,
        );
        request.headers.set("Content-Type", "application/x-sentry-envelope");
        return request;
      },
    },
  ],
]);

// your other hooks here

export const handle = sequence([..., proxies, ...]);

where the variables are set based on your DSN, which is formatted as https://<SENTRY_KEY>@<SENTRY_INGEST_ENDPOINT>/<SENTRY_PRODUCT_ID.

Mar 04 '25 21:03 fnimick

I tried this solution but I still had to set useCompression: false for it to work. Otherwise I would get a 400! Would be nice to get this into the docs as an alternative way!

Jun 04 '25 12:06 jkittner

@jkittner do you mean the kitql proxy solution (https://github.com/getsentry/sentry-javascript/issues/13453#issuecomment-2698929236)? If so, I'm not entirely sure how the library handles proxying, so chances are that somewhere in there the compressed body is consumed and an error occurs?

I agree that the tunneling documentation is far from great and we need better examples. (Ideally, we can provide a tunnelRoute option just like in the NextJS SDK in more meta framework SDKs and a base proxy building block for lower level SDKs) Unfortunately, we didn't get to this yet, see #14248

Jun 06 '25 08:06 Lms24

Yes, the kitql solution only works w/o compression. For now disabling it solved the problem for me. I subscribed to the other issue as well.

Jun 06 '25 09:06 jkittner

Going to close this issue as the improvements necessary to fix are tracked in other issues.

Aug 07 '25 07:08 Lms24

sentry-javascript sentry-javascript copied to clipboard

Replay tunnel fails due to empty envelope

Is there an existing issue for this?

How do you use Sentry?

Which SDK are you using?

SDK Version

Framework Version

Link to Sentry event

Reproduction Example/SDK Setup

Steps to Reproduce

Expected Result

Actual Result

sentry-javascript
sentry-javascript copied to clipboard