BedrockAgentCoreApp throws FastifyError (invalid payload type) when invocation handler returns async generator with Accept: text/event-stream

[stewartmoreland]

Describe the bug

When the invocation handler is an async generator (or a sync function that returns an async generator) and the client sends Accept: text/event-stream, the request completes with HTTP 500. The runtime calls reply.send(payload) with an object somewhere, and Fastify throws: "Attempted to send payload of invalid type 'object'. Expected a string or Buffer." (FST_ERR_REP_INVALID_PAYLOAD_TYPE).

To Reproduce

Steps to reproduce the behavior:

1. Install package: yarn add bedrock-agentcore zod (or npm equivalent).
2. Create a server using the code sample below and run it.
3. Send POST /invocations with header Accept: text/event-stream and body { "prompt": "hello" }.
4. See error (500 and FastifyError in logs).

Expected behavior

The runtime should stream SSE events from the async generator without calling reply.send() with an object. The client should receive text/event-stream chunks from reply.sse.send(...).

Code Sample

import { BedrockAgentCoreApp } from 'bedrock-agentcore/runtime';
import { z } from 'zod';

const app = new BedrockAgentCoreApp({
  invocationHandler: {
    requestSchema: z.object({ prompt: z.string() }),
    process(request, ctx) {
      async function* stream() {
        yield JSON.stringify({ type: 'meta', sessionId: ctx.sessionId });
      }
      return stream();
    },
  },
});

app.run();

Error Output

FastifyError: Attempted to send payload of invalid type 'object'. Expected a string or Buffer.
    at onSendEnd (node_modules/fastify/lib/reply.js:652:11)
    at onSendHook (node_modules/fastify/lib/reply.js:539:5)
    at Reply.send (node_modules/fastify/lib/reply.js:210:3)
    at BedrockAgentCoreApp._handleInvocation (node_modules/bedrock-agentcore/src/runtime/app.ts:403:31)
    at Object.sseHandler (node_modules/@fastify/sse/index.js:437:37)
    ...
"msg":"Promise errored, but reply.sent = true was set"

Environment (please complete the following information):

• OS: macOS 15.7.3 (24G419)
• Node.js version: Tested with 22.15.1 and 24.13.0 — same result on both
• Package version: bedrock-agentcore@0.2.0
• AWS SDK version: N/A (runtime uses Fastify 5.7.4 and @fastify/sse 0.2.0 via SDK)

Additional context

• Yielding only strings from the generator does not fix it, so the failure is likely not from the first chunk type.
• Using a sync process that returns the generator (so the runtime receives an AsyncGenerator, not a Promise) still reproduces the issue.
• Stack trace points to _handleInvocation (app.ts:403), so the invalid reply.send(object) is inside the runtime.
• Likely causes: (1) non-streaming branch taken and reply.send(result) is called with the generator object, or (2) catch block calls reply.status(500).send({ error }) after SSE has started, and the object is not serialized.

Possible Solution

1. Streaming branch: When the handler returns an async generator and the request has SSE, never call reply.send(result); only use reply.sse.send(...).
2. Chunk type in _handleStreamingResponse: Before reply.sse.send(chunk), coerce to string or Buffer: typeof chunk === 'string' ? chunk : Buffer.isBuffer(chunk) ? chunk : JSON.stringify(chunk).
3. Catch block: If reply.raw?.headersSent, do not call reply.status(500).send(...); optionally send an SSE error event or only log. If sending a JSON error body, call reply.type('application/json') first so Fastify serializes the object.