Answer

Fix: 9router appends data: [DONE] to a non-streaming chat.completion

Component: 9Router central gateway (decolua/9router), OpenAI-compatible POST /v1/chat/completions (the ds/ DeepSeek lane). Symptom: with stream omitted, a 200 response is a valid chat.completion JSON body immediately followed by the SSE sentinel:

...\"system_fingerprint\":\"aeb56401ca74e127821c4f9126dcb669\"}data: [DONE]\n\n

---

Root cause

9Router treats an omitted stream as streaming. In open-sse/handlers/chatCore.js (v0.5.95, a99cf57):

const providerRequiresStreaming = PROVIDERS[provider]?.forceStream === true;
let stream = providerRequiresStreaming ? true : (body.stream !== false);   // <-- BUG

body.stream !== false is true when the key is absent — backwards from the OpenAI spec (omitted ⇒ non-streaming). deepseek/ds (open-sse/providers/registry/deepseek.js) does not set forceStream, so a stream-less request is routed down the SSE path.

The upstream then returns a plain application/json body (no SSE framing). The passthrough transform in open-sse/utils/stream.js flushes the leftover buffer verbatim, then unconditionally terminates the “SSE” body with the OpenAI sentinel:

if (mode === STREAM_MODE.PASSTHROUGH) {
  if (buffer) {                                   // whole JSON body, no trailing \n
    controller.enqueue(sharedEncoder.encode(buffer));   // emitted with no `data:` prefix
  }
  if (!streamDoneSent && !isGeminiFamily) {
    controller.enqueue(sharedEncoder.encode("data: [DONE]\n\n"));  // <-- sentinel
  }
}

Response is sent as Content-Type: text/event-stream, so the client sees <json>data: [DONE]\n\n. The correct fix is the routing decision: a client that did not ask for SSE must not enter the SSE path.

open-sse/utils/bypassHandler.js has the same wrong default for the Claude-CLI naming/warmup path:

const stream = body.stream !== false;   // <-- BUG (same default)

---

Exact fix

Canonical upstream fix (PR #4427), two one-line changes:

--- a/open-sse/handlers/chatCore.js
+++ b/open-sse/handlers/chatCore.js
@@ -133,7 +133,7 @@
   const clientRequestedStreaming = body.stream === true || sourceFormat === FORMATS.ANTIGRAVITY || sourceFormat === FORMATS.GEMINI || sourceFormat === FORMATS.GEMINI_CLI;
   const providerRequiresStreaming = PROVIDERS[provider]?.forceStream === true;
-  let stream = providerRequiresStreaming ? true : (body.stream !== false);
+  let stream = providerRequiresStreaming ? true : (body.stream === true);

--- a/open-sse/utils/bypassHandler.js
+++ b/open-sse/utils/bypassHandler.js
@@ -73,7 +73,7 @@
   if (!shouldBypass) return null;

   const sourceFormat = detectFormat(body);
-  const stream = body.stream !== false;
+  const stream = body.stream === true;

Apply from the repo root:

git apply /workspace/nonstream-default.patch   # patch file also written to the sandbox

Semantics after the change (all OpenAI-spec compliant):

stream in request	provider forceStream	behaviour
omitted	false	application/json (fixed)
omitted	true	text/event-stream (unchanged)
false	false	application/json (unchanged)
true	any	text/event-stream + data: [DONE] (unchanged)

Optional defense-in-depth

Not required once the default is fixed, but if a non-stream path ever receives SSE it can strip the sentinel:

const json = JSON.stringify(restoreToolNames(translatedResponse, toolNameMap));
const clean = json.endsWith("data: [DONE]") ? json.slice(0, json.lastIndexOf("data: [DONE]")).trimEnd() : json;
return { success: true, response: new Response(clean, { headers: { "Content-Type": "application/json", ... } }) };

---

Verification

Repository https://github.com/decolua/9router @ a99cf57 (v0.5.95), Vitest. Install under tests/ with pnpm add vitest@^4 undici@^7 uuid@^13, plus a root node_modules symlink so open-sse/… resolves undici/uuid.

Regression test (red → green)

tests/unit/nonstream-default-regression.test.js drives the real handleChatCore with the DeepSeek (ds) provider, Accept: */*, and asserts the stream flag passed to the executor:

it("does not stream when stream is omitted", async () => {
  const { handleChatCore } = await import("../../open-sse/handlers/chatCore.js");
  await handleChatCore(deepseekOptions(undefined));      // body has no `stream`
  expect(executeMock).toHaveBeenCalledTimes(1);
  expect(executeMock.mock.calls[0][0].stream).toBe(false);
});

Before (unmodified):

❯ tests/unit/nonstream-default-regression.test.js (6 tests | 1 failed)
  × does not stream when stream is omitted
    AssertionError: expected true to be false // Object.is equality

After applying the patch (git apply --check clean, then apply):

✓ tests/unit/nonstream-default-regression.test.js (6 tests)
  Test Files  1 passed (1)
       Tests  6 passed (6)

Confirms forceStream providers (openai/codex/commandcode), explicit stream: true, and explicit stream: false are unaffected.

Live post-deploy check

# BEFORE: body ends with }data: [DONE]\n\n, Content-Type: text/event-stream
curl -s -D - -X POST http://master001:20128/v1/chat/completions \
  -H "Authorization: Bearer $NINEROUTER_KEY" -H 'Content-Type: application/json' \
  -d '{"model":"ds/deepseek-v4-flash","messages":[{"role":"user","content":"hi"}]}' \
  -o /tmp/body.bin
tail -c 80 /tmp/body.bin | od -c        # ... } d a t a :   [ D O N E ] \n \n

# AFTER (fixed)
curl -s -D /tmp/hdrs.txt -X POST http://master001:20128/v1/chat/completions \
  -H "Authorization: Bearer $NINEROUTER_KEY" -H 'Content-Type: application/json' \
  -d '{"model":"ds/deepseek-v4-flash","messages":[{"role":"user","content":"hi"}]}' \
  -o /tmp/body.bin
grep -i '^content-type' /tmp/hdrs.txt   # content-type: application/json
python3 -c "import json;json.load(open('/tmp/body.bin'));print('valid JSON')"

Expected: omitted/false ⇒ application/json valid JSON with no [DONE]; true ⇒ text/event-stream terminated by data: [DONE]\n\n.

---

Files changed

• open-sse/handlers/chatCore.js — line 135: body.stream !== false → body.stream === true
• open-sse/utils/bypassHandler.js — line 76: body.stream !== false → body.stream === true

Artifacts written to /workspace: solution.md, nonstream-default.patch, nonstream-default-regression.test.js.

Reference: decolua/9router PR #4427 — “fix: default to non-streaming response when stream unspecified”.

# Evidence
- Problem class: openai-compat-gateway-appends-data-done-to-nonstream-body
- Model: openrouter/deepseek/deepseek-v4.1-flash
- Solved: 2026-10-05T18:03:59.867Z
- Verification: solution produced by pi in sandbox; see signatures.json

{"description": "REPRODUCED 2026-10-05 (prior capture 2026-10-04, DF-CHIMERA-V2-63): POST /v1/chat/completions to http://master001:20128/v1 with model ds/deepseek-v4-flash, stream UNSET, returns HTTP 200 with a valid OpenAI chat.completion JSON body followed by a literal 'data: [DONE]' line after the closing brace. Byte-verified tail: ...\"system_fingerprint\":\"aeb56401ca74e127821c4f9126dcb669\"}data: [DONE]\\n\\n. A strict JSON client hits parse errors; chimera's lenient client tolerates it, but other OpenAI-compat consumers of the shared fleet gateway may not. Fix belongs to 9router's non-stream response path (strip the SSE sentinel when Content-Type is application/json).", "environment": "9router central gateway :20128, ds/ upstream lane", "language": "go/http", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "openai-compat-gateway-appends-data-done-to-nonstream-body", "provider": "openrouter", "solved_at": "2026-10-05T18:03:59.867Z", "version": ""}