diff --git a/e2e/responses.test.ts b/e2e/responses.test.ts
index 968b002..b13d210 100644
--- a/e2e/responses.test.ts
+++ b/e2e/responses.test.ts
@@ -239,6 +239,30 @@ describe("openai-responses adapter through runInference", () => {
     expect(error.data.error.message).toContain("backend exploded");
   });
 
+  test("response.incomplete surfaces its reason as a protocol_mismatch inference.error", async () => {
+    harness = setupHarness({ adapters: registry });
+    const stream = harness.scenario.createStream();
+    harness.scenario.whenRequestMatches(() => true, stream);
+    stream.enqueueAll(
+      [
+        sse({
+          type: "response.incomplete",
+          response: {
+            status: "incomplete",
+            incomplete_details: { reason: "max_output_tokens" },
+            usage: { input_tokens: 10, output_tokens: 100 },
+          },
+        }),
+      ],
+      { startAt: 1 },
+    );
+    const events = await collect(harness, [userTurn("hi")]);
+    expect(events.some((e) => e.type === "inference.done")).toBe(false);
+    const error = errorEvent(events);
+    expect(error.data.error.category).toBe("protocol_mismatch");
+    expect(error.data.error.message).toContain("max_output_tokens");
+  });
+
   test("a prior signed reasoning turn replays as a reasoning item ahead of its function_call", async () => {
     harness = setupHarness({ adapters: registry });
     const stream = harness.scenario.createStream();
diff --git a/src/protocol/iterator.ts b/src/protocol/iterator.ts
index 19246ec..7e607c3 100644
--- a/src/protocol/iterator.ts
+++ b/src/protocol/iterator.ts
@@ -70,6 +70,9 @@ const CompletedEvent = type({ response: { "usage?": ResponsesUsage } });
 const FailedEvent = type({
   "response?": { "error?": { "message?": "string" } },
 });
+const IncompleteEvent = type({
+  response: { "incomplete_details?": { "reason?": "string" } },
+});
 const ErrorEvent = type({ "message?": "string" });
 
 // Maps the Responses API's usage object onto the internal TokenUsage,
@@ -459,6 +462,24 @@ export function parseResponse(
       const message = validated.response?.error?.message ?? "response failed";
       throw new ProtocolMismatchError(`${provider}: ${message}`, parsed);
     }
+    // Throws like parseJSONResponse does for status "incomplete". The
+    // harness drops the events of a batch that throws, so this event's usage
+    // cannot also be reported.
+    case "response.incomplete": {
+      const validated = IncompleteEvent(parsed);
+      if (validated instanceof type.errors) {
+        throw protocolMismatch(
+          provider,
+          `response.incomplete failed schema validation: ${validated.summary}`,
+          parsed,
+        );
+      }
+      throw protocolMismatch(
+        provider,
+        `response status is "incomplete": ${validated.response.incomplete_details?.reason ?? "no reason given"}`,
+        parsed,
+      );
+    }
     case "error": {
       const validated = ErrorEvent(parsed);
       if (validated instanceof type.errors) {
@@ -475,7 +496,7 @@ export function parseResponse(
     }
     default:
       // Lifecycle envelopes (response.created, response.in_progress,
-      // content_part.*, *_text.done, response.incomplete) carry no
+      // content_part.*, *_text.done) carry no
       // incremental payload a caller needs; ignore them. Unknown event
       // types are protocol-legal — the vocabulary is expected to grow.
       return events;