diff --git a/src/glmt/pipeline/response-builder.ts b/src/glmt/pipeline/response-builder.ts
index 50862703..6426e288 100644
--- a/src/glmt/pipeline/response-builder.ts
+++ b/src/glmt/pipeline/response-builder.ts
@@ -177,17 +177,16 @@ export class ResponseBuilder {
 
   /**
    * Generate thinking signature for Claude Code UI
+   *
+   * Anthropic requires a thinking block's `signature` to be a non-empty
+   * opaque STRING. Emit a deterministic base64 token (no Date.now()) so the
+   * thinking block stays valid against reasoning backends; returning an object
+   * here produces an invalid thinking block and breaks the stream.
    */
   generateThinkingSignature(thinking: string): ThinkingSignature {
-    // Generate signature hash
     const hash = crypto.createHash('sha256').update(thinking).digest('hex').substring(0, 16);
 
-    return {
-      type: 'thinking_signature',
-      hash: hash,
-      length: thinking.length,
-      timestamp: Date.now(),
-    };
+    return Buffer.from(`ccs-thinking:${hash}:${thinking.length}`).toString('base64');
   }
 
   /**
diff --git a/src/glmt/pipeline/types.ts b/src/glmt/pipeline/types.ts
index 27f68749..e18629ca 100644
--- a/src/glmt/pipeline/types.ts
+++ b/src/glmt/pipeline/types.ts
@@ -7,7 +7,7 @@ export interface ContentBlock {
   type: string;
   text?: string;
   thinking?: string;
-  signature?: ThinkingSignature;
+  signature?: string;
   id?: string;
   name?: string;
   input?: Record<string, unknown>;
@@ -95,12 +95,9 @@ export interface TransformResult {
   error?: string;
 }
 
-export interface ThinkingSignature {
-  type: string;
-  hash: string;
-  length: number;
-  timestamp: number;
-}
+// Anthropic contract: a thinking block's signature is a non-empty opaque
+// string. Kept as a named alias so existing imports stay intact.
+export type ThinkingSignature = string;
 
 // OpenAI response types
 export interface OpenAIChoice {
diff --git a/tests/unit/glmt/glmt-transformer.test.js b/tests/unit/glmt/glmt-transformer.test.js
index 05950ba7..930fbb7e 100644
--- a/tests/unit/glmt/glmt-transformer.test.js
+++ b/tests/unit/glmt/glmt-transformer.test.js
@@ -161,16 +161,17 @@ describe('GlmtTransformer', () => {
   });
 
   describe('Thinking signature', () => {
-    it('generates thinking signature', () => {
+    it('generates thinking signature as a non-empty opaque string', () => {
       const transformer = new GlmtTransformer();
       const thinking = 'This is my reasoning process';
       const signature = transformer.generateThinkingSignature(thinking);
 
-      assert.strictEqual(signature.type, 'thinking_signature');
-      assert.ok(signature.hash);
-      assert.strictEqual(signature.hash.length, 16);
-      assert.strictEqual(signature.length, thinking.length);
-      assert.ok(signature.timestamp);
+      // Anthropic contract: a thinking block's signature is a non-empty
+      // opaque string, not an object.
+      assert.strictEqual(typeof signature, 'string');
+      assert.ok(signature.length > 0);
+      // Deterministic: same reasoning yields the same signature (no Date.now()).
+      assert.strictEqual(signature, transformer.generateThinkingSignature(thinking));
     });
   });
 
diff --git a/tests/unit/glmt/test-thinking-signature-race.js b/tests/unit/glmt/test-thinking-signature-race.js
index c9a7b40f..1537ceb5 100755
--- a/tests/unit/glmt/test-thinking-signature-race.js
+++ b/tests/unit/glmt/test-thinking-signature-race.js
@@ -12,6 +12,9 @@
 
 const GlmtTransformer = require('../../../dist/glmt/glmt-transformer').default;
 const { DeltaAccumulator } = require('../../../dist/glmt/delta-accumulator');
+const { ResponseBuilder } = require('../../../dist/glmt/pipeline');
+
+const responseBuilder = new ResponseBuilder(false);
 
 // Test runner
 class TestRunner {
@@ -59,7 +62,6 @@ function assert(condition, message) {
 
 // Test 1: Signature not generated for empty thinking block
 runner.test('Signature not generated for empty thinking block', () => {
-  const transformer = new GlmtTransformer({ verbose: false });
   const accumulator = new DeltaAccumulator({ thinking: true });
 
   // Start thinking block but add no content
@@ -67,7 +69,7 @@ runner.test('Signature not generated for empty thinking block', () => {
   const block = accumulator.startBlock('thinking');
 
   // Try to generate signature for empty block
-  const signatureEvent = transformer._createSignatureDeltaEvent(block);
+  const signatureEvent = responseBuilder.createSignatureDeltaEvent(block);
 
   // Should return null for empty block (fix for race condition)
   assert(signatureEvent === null, 'Expected null for empty thinking block');
@@ -75,7 +77,6 @@ runner.test('Signature not generated for empty thinking block', () => {
 
 // Test 2: Signature generated correctly after content accumulated
 runner.test('Signature generated correctly after content accumulated', () => {
-  const transformer = new GlmtTransformer({ verbose: false });
   const accumulator = new DeltaAccumulator({ thinking: true });
 
   // Start thinking block and add content
@@ -85,15 +86,14 @@ runner.test('Signature generated correctly after content accumulated', () => {
   accumulator.addDelta('Second thinking delta.');
 
   // Generate signature
-  const signatureEvent = transformer._createSignatureDeltaEvent(block);
+  const signatureEvent = responseBuilder.createSignatureDeltaEvent(block);
 
   // Should return valid signature event
   assert(signatureEvent !== null, 'Expected signature event for non-empty block');
   assert(signatureEvent.event === 'content_block_delta', 'Expected content_block_delta event');
   assert(signatureEvent.data.delta.type === 'thinking_signature_delta', 'Expected thinking_signature_delta type');
+  assert(typeof signatureEvent.data.delta.signature === 'string', 'Expected signature string');
   assert(signatureEvent.data.delta.signature.length > 0, 'Expected signature length > 0');
-  assert(signatureEvent.data.delta.signature.hash, 'Expected signature hash');
-  assert(signatureEvent.data.delta.signature.hash.length === 16, 'Expected 16-char hash');
 });
 
 // Test 3: transformDelta skips signature for empty thinking blocks
@@ -179,8 +179,8 @@ runner.test('transformDelta generates signature for non-empty thinking blocks',
 
   // Verify signature structure
   const sig = signatureEvents[0].data.delta.signature;
-  assert(sig.hash && sig.hash.length === 16, 'Expected valid 16-char hash');
-  assert(sig.length > 0, 'Expected content length > 0');
+  assert(typeof sig === 'string', 'Expected signature string');
+  assert(sig.length > 0, 'Expected signature length > 0');
 });
 
 // Test 5: Loop detection handles empty thinking blocks
@@ -217,7 +217,7 @@ runner.test('Loop detection handles empty thinking blocks without signature', ()
 
   const currentBlock = accumulator.getCurrentBlock();
   if (currentBlock && currentBlock.type === 'thinking') {
-    const signatureEvent = transformer._createSignatureDeltaEvent(currentBlock);
+    const signatureEvent = responseBuilder.createSignatureDeltaEvent(currentBlock);
     // Should return null for empty block
     assert(signatureEvent === null, 'Expected null signature for empty thinking block during loop detection');
   }
diff --git a/tests/unit/proxy/transformers/sse-stream-transformer.test.ts b/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
index 9faebabc..6a946d78 100644
--- a/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
+++ b/tests/unit/proxy/transformers/sse-stream-transformer.test.ts
@@ -67,4 +67,109 @@ 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');
+    expect((signature as string).length).toBeGreaterThan(0);
+  });
 });