diff --git a/open-sse/utils/streamReadiness.ts b/open-sse/utils/streamReadiness.ts
index 3b73024921..b61d5a6f3b 100644
--- a/open-sse/utils/streamReadiness.ts
+++ b/open-sse/utils/streamReadiness.ts
@@ -28,6 +28,11 @@ function hasUsefulValue(value: unknown): boolean {
     "delta",
     "reasoning_content",
     "reasoning",
+    // Mistral/Magistral thinking arrays and StepFun/OpenRouter reasoning_details are
+    // valid model output — without these a reasoning-only stream was misclassified as
+    // "no useful content" and turned into a spurious 502 (#2520).
+    "thinking",
+    "reasoning_details",
     "partial_json",
     "arguments",
     "name",
diff --git a/tests/unit/stream-readiness.test.ts b/tests/unit/stream-readiness.test.ts
index f322bd1516..bcd36ab671 100644
--- a/tests/unit/stream-readiness.test.ts
+++ b/tests/unit/stream-readiness.test.ts
@@ -446,3 +446,21 @@ test("ensureStreamReadiness returns 502 when stream ends without useful content"
   assert.equal(result.ok, false);
   assert.equal(result.response.status, 502);
 });
+
+// Regression for #2520: a reasoning-only stream (Mistral `thinking` array / StepFun
+// `reasoning_details`) is real output and must NOT be classified as "no useful content"
+// (which produced a spurious 502).
+test("hasUsefulStreamContent detects thinking[] and reasoning_details (#2520)", () => {
+  assert.equal(
+    hasUsefulStreamContent(
+      `data: ${JSON.stringify({ choices: [{ delta: { content: [{ type: "thinking", thinking: [{ text: "reasoning..." }] }] }, index: 0 }] })}\n\n`
+    ),
+    true
+  );
+  assert.equal(
+    hasUsefulStreamContent(
+      `data: ${JSON.stringify({ choices: [{ delta: { reasoning_details: [{ type: "reasoning.text", text: "deliberating" }] }, index: 0 }] })}\n\n`
+    ),
+    true
+  );
+});