fix(streaming): honor body stream intent for early heartbeats (#10127)

tests/unit/early-sse-route-intent.test.ts

@@ -0,0 +1,42 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+
+import { resolveStreamFlag } from "../../open-sse/utils/aiSdkCompat.ts";
+
+const ROUTES = [
+  {
+    name: "responses",
+    sourceFormat: "openai-responses",
+    bodyExpression: "resolvedBody?.stream",
+    source: fs.readFileSync(path.join("src", "app", "api", "v1", "responses", "route.ts"), "utf8"),
+  },
+  {
+    name: "messages",
+    sourceFormat: "claude",
+    bodyExpression: "body?.stream",
+    source: fs.readFileSync(path.join("src", "app", "api", "v1", "messages", "route.ts"), "utf8"),
+  },
+] as const;
+
+for (const route of ROUTES) {
+  test(`${route.name} early-heartbeat gate uses the real stream resolver`, () => {
+    const escapedBodyExpression = route.bodyExpression.replace(/[?.]/g, "\\$&");
+    assert.match(
+      route.source,
+      new RegExp(
+        `resolveStreamFlag\\(\\s*${escapedBodyExpression},\\s*accept,\\s*"${route.sourceFormat}"\\s*\\)`
+      )
+    );
+    assert.match(route.source, /if \(wantsStreaming\) \{[\s\S]*withEarlyStreamKeepalive\(/);
+    assert.doesNotMatch(route.source, /accept\.includes\(["']text\/event-stream["']\)/);
+  });
+
+  test(`${route.name} stream intent covers body and Accept without overriding stream:false`, () => {
+    assert.equal(resolveStreamFlag(true, "application/json", route.sourceFormat), true);
+    assert.equal(resolveStreamFlag(false, "text/event-stream", route.sourceFormat), false);
+    assert.equal(resolveStreamFlag(undefined, "text/event-stream", route.sourceFormat), true);
+    assert.equal(resolveStreamFlag(undefined, "application/json", route.sourceFormat), false);
+  });
+}