diff --git a/tests/unit/proxy/transformers/sse-stream-transformer.test.ts b/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
index 9faebabc..61d99223 100644
--- a/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
+++ b/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
@@ -67,4 +67,108 @@ describe('proxy SSE stream transformer', () => {
     expect(body).toContain('"type":"text_delta"');
     expect(body).toContain('event: message_stop');
   });
+
+  // Helper: extract all JSON `data:` payloads from an Anthropic SSE response body,
+  // skipping the `[DONE]` sentinel and any non-JSON lines.
+  function parseSseDataEvents(text: string): Record<string, unknown>[] {
+    const events: Record<string, unknown>[] = [];
+    for (const line of text.split('\n')) {
+      if (!line.startsWith('data: ')) continue;
+      const raw = line.slice('data: '.length).trim();
+      if (raw.length === 0 || raw === '[DONE]') continue;
+      try {
+        events.push(JSON.parse(raw) as Record<string, unknown>);
+      } catch {
+        // skip non-JSON data lines
+      }
+    }
+    return events;
+  }
+
+  it('emits thinking.signature as a non-empty string — JSON (Anthropic contract)', async () => {
+    // Anthropic requires a thinking block's `signature` to be a non-empty opaque
+    // STRING. This test asserts that contract; it is intentionally RED on the
+    // current code, which fabricates the signature as an object via
+    // generateThinkingSignature(). After the fix it becomes a regression guard.
+    const response = new Response(
+      JSON.stringify({
+        id: 'chatcmpl_sig_json',
+        model: 'gpt-5.x',
+        choices: [
+          {
+            index: 0,
+            message: {
+              role: 'assistant',
+              content: 'Final answer.',
+              reasoning_content: 'Step-by-step reasoning before answering.',
+            },
+            finish_reason: 'stop',
+          },
+        ],
+        usage: { prompt_tokens: 3, completion_tokens: 2, total_tokens: 5 },
+      }),
+      {
+        status: 200,
+        headers: { 'Content-Type': 'application/json' },
+      }
+    );
+
+    const transformed = await createAnthropicProxyResponse(response);
+    const body = (await transformed.json()) as {
+      content: Array<{ type: string; signature?: unknown }>;
+    };
+
+    const thinkingBlock = body.content.find((block) => block.type === 'thinking');
+    expect(
+      thinkingBlock,
+      `no thinking block found; content = ${JSON.stringify(body.content)}`
+    ).toBeDefined();
+
+    const signature = thinkingBlock?.signature;
+    expect(
+      typeof signature,
+      `expected non-empty string signature, received ${JSON.stringify(signature)}`
+    ).toBe('string');
+    expect((signature as string).length).toBeGreaterThan(0);
+  });
+
+  it('emits thinking signature as a non-empty string — SSE (Anthropic contract)', async () => {
+    // Streaming counterpart of the contract test above: mirrors the failed
+    // prod path where reasoning_content streams in and ccs closes the thinking
+    // block with a fabricated signature via createSignatureDeltaEvent().
+    const openAISse = [
+      'data: {"id":"chatcmpl_sig_stream","object":"chat.completion.chunk","created":1,"model":"gpt-5.x","choices":[{"index":0,"delta":{"role":"assistant","reasoning_content":"Planning the verification step by step."},"finish_reason":null}]}\n\n',
+      'data: {"id":"chatcmpl_sig_stream","object":"chat.completion.chunk","created":1,"model":"gpt-5.x","choices":[{"index":0,"delta":{"content":"Final answer."},"finish_reason":null}]}\n\n',
+      'data: {"id":"chatcmpl_sig_stream","object":"chat.completion.chunk","created":1,"model":"gpt-5.x","choices":[{"index":0,"delta":{},"finish_reason":"stop"}],"usage":{"prompt_tokens":3,"completion_tokens":2,"total_tokens":5}}\n\n',
+      'data: [DONE]\n\n',
+    ].join('');
+
+    const transformed = await createAnthropicProxyResponse(
+      new Response(openAISse, {
+        status: 200,
+        headers: { 'Content-Type': 'text/event-stream' },
+      })
+    );
+
+    const body = await transformed.text();
+    const payloads = parseSseDataEvents(body);
+
+    // ccs emits the thinking signature through createSignatureDeltaEvent,
+    // which produces a content_block_delta with delta.type === 'thinking_signature_delta'.
+    const signatureDelta = payloads.find((payload) => {
+      const delta = (payload as { delta?: { type?: string } }).delta;
+      return delta?.type === 'thinking_signature_delta';
+    });
+    expect(
+      signatureDelta,
+      `no thinking_signature_delta event in stream; payloads = ${JSON.stringify(payloads)}`
+    ).toBeDefined();
+
+    const signature = (signatureDelta as { delta?: { signature?: unknown } }).delta?.signature;
+    // Anthropic contract: streaming thinking signature MUST be a non-empty opaque string.
+    expect(
+      typeof signature,
+      `expected non-empty string signature, received ${JSON.stringify(signature)}`
+    ).toBe('string');
+  });
 });