diff --git a/.changeset/curvy-streams-watch.md b/.changeset/curvy-streams-watch.md
new file mode 100644
index 0000000..17d5e16
--- /dev/null
+++ b/.changeset/curvy-streams-watch.md
@@ -0,0 +1,5 @@
+---
+"@openrouter/agent": patch
+---
+
+Detect streamed Responses API results by readable stream behavior instead of constructor names or unsupported adapters.
diff --git a/packages/agent/src/lib/model-result.ts b/packages/agent/src/lib/model-result.ts
index c187f72..e706fe0 100644
--- a/packages/agent/src/lib/model-result.ts
+++ b/packages/agent/src/lib/model-result.ts
@@ -92,25 +92,22 @@ function isRecord(value: unknown): value is Record<string, unknown> {
 }
 
 /**
- * Type guard for stream event with toReadableStream method
- * Checks constructor name, prototype, and method availability
+ * Type guard for stream event responses
+ * Checks constructor name and readable stream behavior
  */
 function isEventStream(value: unknown): value is EventStream<models.StreamEvents> {
   if (value === null || typeof value !== 'object') {
     return false;
   }
 
-  // Check constructor name for EventStream
-  const constructorName = Object.getPrototypeOf(value)?.constructor?.name;
-  if (constructorName === 'EventStream') {
+  if (typeof ReadableStream !== 'undefined' && value instanceof ReadableStream) {
     return true;
   }
 
-  // Fallback: check for toReadableStream method (may be on prototype)
   const maybeStream = value as {
-    toReadableStream?: unknown;
+    getReader?: unknown;
   };
-  return typeof maybeStream.toReadableStream === 'function';
+  return typeof maybeStream.getReader === 'function';
 }
 
 export interface GetResponseOptions<
@@ -432,14 +429,11 @@ export class ModelResult<
 
   /**
    * Type guard to check if a value is a non-streaming response
-   * Only requires 'output' field and absence of 'toReadableStream' method
+   * Only requires 'output' field and absence of readable stream behavior
    */
   private isNonStreamingResponse(value: unknown): value is models.OpenResponsesResult {
     return (
-      value !== null &&
-      typeof value === 'object' &&
-      'output' in value &&
-      !('toReadableStream' in value)
+      value !== null && typeof value === 'object' && 'output' in value && !isEventStream(value)
     );
   }
 
diff --git a/packages/agent/tests/unit/model-result-stream-detection.test.ts b/packages/agent/tests/unit/model-result-stream-detection.test.ts
new file mode 100644
index 0000000..ab43938
--- /dev/null
+++ b/packages/agent/tests/unit/model-result-stream-detection.test.ts
@@ -0,0 +1,111 @@
+import type { OpenRouterCore } from '@openrouter/sdk/core';
+import type * as models from '@openrouter/sdk/models';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const mockBetaResponsesSend = vi.hoisted(() => vi.fn());
+
+vi.mock('@openrouter/sdk/funcs/betaResponsesSend', () => ({
+  betaResponsesSend: mockBetaResponsesSend,
+}));
+
+import { ModelResult } from '../../src/lib/model-result.js';
+
+function makeResponse(): models.OpenResponsesResult {
+  return {
+    id: 'resp_test_stream_detection',
+    object: 'response',
+    createdAt: 0,
+    model: 'test-model',
+    status: 'completed',
+    completedAt: 0,
+    output: [
+      {
+        id: 'msg_test_stream_detection',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [
+          {
+            type: 'output_text',
+            text: 'ok',
+            annotations: [],
+          },
+        ],
+      },
+    ],
+    error: null,
+    incompleteDetails: null,
+    temperature: null,
+    topP: null,
+    presencePenalty: null,
+    frequencyPenalty: null,
+    metadata: null,
+    instructions: null,
+    tools: [],
+    toolChoice: 'auto',
+    parallelToolCalls: false,
+  } as models.OpenResponsesResult;
+}
+
+function makeCompletedStream(
+  response: models.OpenResponsesResult,
+): ReadableStream<models.StreamEvents> {
+  return new ReadableStream<models.StreamEvents>({
+    start(controller) {
+      controller.enqueue({
+        type: 'response.completed',
+        response,
+        sequenceNumber: 0,
+      } as models.StreamEvents);
+      controller.close();
+    },
+  });
+}
+
+function makeResult(): ModelResult<[]> {
+  return new ModelResult({
+    request: {
+      model: 'test-model',
+      input: 'hello',
+    },
+    client: {} as OpenRouterCore,
+    tools: [],
+  });
+}
+
+describe('ModelResult stream detection', () => {
+  beforeEach(() => {
+    mockBetaResponsesSend.mockReset();
+  });
+
+  it('accepts ReadableStream responses whose constructor name is not EventStream', async () => {
+    const response = makeResponse();
+    const stream = makeCompletedStream(response);
+
+    expect(Object.getPrototypeOf(stream)?.constructor?.name).not.toBe('EventStream');
+    expect(stream).toBeInstanceOf(ReadableStream);
+
+    mockBetaResponsesSend.mockResolvedValue({
+      ok: true,
+      value: stream,
+    });
+
+    await expect(makeResult().getResponse()).resolves.toEqual(response);
+  });
+
+  it('rejects toReadableStream-only adapters before ReusableReadableStream reads them', async () => {
+    const response = makeResponse();
+    const adapter = {
+      toReadableStream: () => makeCompletedStream(response),
+    };
+
+    expect('getReader' in adapter).toBe(false);
+
+    mockBetaResponsesSend.mockResolvedValue({
+      ok: true,
+      value: adapter,
+    });
+
+    await expect(makeResult().getResponse()).rejects.toThrow('Unexpected response type from API');
+  });
+});