fix(sse): set X-OmniRoute-Selected-Connection-Id on successful combo dispatches (#11810) (#11986)

Co-authored-by: Markus Hartung <mail@hartmark.se>

changelog.d/fixes/11810-combo-success-connection-header.md

@@ -0,0 +1 @@
+- fix(sse): set X-OmniRoute-Selected-Connection-Id on successful combo dispatches so downstream consumers stop falling back to an empty connection id (#11810)

src/sse/handlers/chat.ts

@@ -1913,11 +1913,15 @@ async function handleSingleModelChat(
         }
         if (telemetry) telemetry.startPhase("finalize");
         if (telemetry) telemetry.endPhase();
+        const successResponse = withSelectedConnectionHeader(
+          result.response,
+          credentials?.connectionId
+        );
         if (requestBody.stream === true) {
-          return wrapResponseWithOAuthSessionRelease(result.response, releaseOAuthSession);
+          return wrapResponseWithOAuthSessionRelease(successResponse, releaseOAuthSession);
         }
         releaseOAuthSession();
-        return result.response;
+        return successResponse;
       }
 
       // A final hard-lease fence rejection is authoritative. It must never mutate

tests/unit/combo-success-selected-connection-header-11810.test.ts

@@ -0,0 +1,143 @@
+/**
+ * Regression test for #11810.
+ *
+ * On a successful provider dispatch, `handleSingleModelChat()`'s
+ * `result.success` branch in `src/sse/handlers/chat.ts` used to return
+ * `result.response` (non-streaming) or
+ * `wrapResponseWithOAuthSessionRelease(result.response, releaseOAuthSession)`
+ * (streaming) directly, without first calling `withSelectedConnectionHeader()`
+ * — unlike every failure exit in the same function, which does call it.
+ *
+ * As a result `X-OmniRoute-Selected-Connection-Id` was absent on every
+ * successful response for a dynamically-selected (unpinned) connection, so
+ * combo.ts's consumers (success-decay, provider cooldown recovery, webhook
+ * attribution, session stickiness, LKGP) fell back to the target's static
+ * `connectionId`, which is empty for provider-level combo targets like
+ * `openai/o3-mini`.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { createChatPipelineHarness } from "../integration/_chatPipelineHarness.ts";
+
+const harness = await createChatPipelineHarness("combo-success-sel-conn-11810");
+const { buildOpenAIResponse, buildRequest, combosDb, handleChat, resetStorage, seedConnection } =
+  harness;
+
+const textEncoder = new TextEncoder();
+
+function buildOpenAIStreamResponse(text = "hello streamed") {
+  return new Response(
+    new ReadableStream({
+      start(controller) {
+        controller.enqueue(
+          textEncoder.encode(
+            `data: ${JSON.stringify({
+              id: "chatcmpl_stream_11810",
+              object: "chat.completion.chunk",
+              created: 1,
+              model: "o3-mini",
+              choices: [{ index: 0, delta: { role: "assistant", content: text } }],
+            })}\n\n`
+          )
+        );
+        controller.enqueue(textEncoder.encode(`data: [DONE]\n\n`));
+        controller.close();
+      },
+    }),
+    {
+      status: 200,
+      headers: { "Content-Type": "text/event-stream" },
+    }
+  );
+}
+
+test.beforeEach(async () => {
+  await resetStorage();
+});
+
+test.afterEach(async () => {
+  await resetStorage();
+});
+
+test.after(async () => {
+  await harness.cleanup();
+});
+
+test("#11810 combo success response carries X-OmniRoute-Selected-Connection-Id for a dynamically selected connection", async () => {
+  const connection = await seedConnection("openai", {
+    apiKey: "sk-openai-11810",
+  });
+
+  await combosDb.createCombo({
+    name: "combo-11810-priority",
+    strategy: "priority",
+    config: { maxRetries: 0, retryDelayMs: 0 },
+    // Provider-level target, no pinned connectionId — the connection is selected
+    // dynamically at dispatch time.
+    models: ["openai/o3-mini"],
+  });
+
+  globalThis.fetch = async () => buildOpenAIResponse("hello from o3-mini", "o3-mini");
+
+  const response = await handleChat(
+    buildRequest({
+      body: {
+        model: "combo-11810-priority",
+        stream: false,
+        messages: [{ role: "user", content: "hi" }],
+      },
+    })
+  );
+
+  assert.equal(response.status, 200);
+  const body = (await response.json()) as { choices: Array<{ message: { content: string } }> };
+  assert.equal(body.choices[0].message.content, "hello from o3-mini");
+
+  const selectedConnectionId = response.headers.get("X-OmniRoute-Selected-Connection-Id");
+  assert.equal(
+    selectedConnectionId,
+    connection.id,
+    `expected the response to carry the dynamically selected connection id (${connection.id}), got ${selectedConnectionId}`
+  );
+});
+
+test("#11810 combo streaming success response carries X-OmniRoute-Selected-Connection-Id for a dynamically selected connection", async () => {
+  const connection = await seedConnection("openai", {
+    apiKey: "sk-openai-11810-stream",
+  });
+
+  await combosDb.createCombo({
+    name: "combo-11810-priority-stream",
+    strategy: "priority",
+    config: { maxRetries: 0, retryDelayMs: 0 },
+    // Provider-level target, no pinned connectionId — the connection is selected
+    // dynamically at dispatch time.
+    models: ["openai/o3-mini"],
+  });
+
+  globalThis.fetch = async () => buildOpenAIStreamResponse("hello streamed from o3-mini");
+
+  const response = await handleChat(
+    buildRequest({
+      body: {
+        model: "combo-11810-priority-stream",
+        stream: true,
+        messages: [{ role: "user", content: "hi" }],
+      },
+    })
+  );
+
+  assert.equal(response.status, 200);
+
+  const selectedConnectionId = response.headers.get("X-OmniRoute-Selected-Connection-Id");
+  assert.equal(
+    selectedConnectionId,
+    connection.id,
+    `expected the streaming response to carry the dynamically selected connection id (${connection.id}), got ${selectedConnectionId}`
+  );
+
+  // Drain the stream so the harness's fetch mock/db handles are released cleanly.
+  await response.text();
+});