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));
     });
   });