diff --git a/src/agents/models/chatcmpl_converter.py b/src/agents/models/chatcmpl_converter.py
index 96f02a5fe..0ece1664b 100644
--- a/src/agents/models/chatcmpl_converter.py
+++ b/src/agents/models/chatcmpl_converter.py
@@ -107,7 +107,7 @@ def message_to_output_items(cls, message: ChatCompletionMessage) -> list[TRespon
             if hasattr(message, "thinking_blocks") and message.thinking_blocks:
                 # Store thinking text in content and signature in encrypted_content
                 reasoning_item.content = []
-                signature = None
+                signatures: list[str] = []
                 for block in message.thinking_blocks:
                     if isinstance(block, dict):
                         thinking_text = block.get("thinking", "")
@@ -116,15 +116,12 @@ def message_to_output_items(cls, message: ChatCompletionMessage) -> list[TRespon
                                 Content(text=thinking_text, type="reasoning_text")
                             )
                         # Store the signature if present
-                        if block.get("signature"):
-                            signature = block.get("signature")
+                        if signature := block.get("signature"):
+                            signatures.append(signature)
 
-                # Store only the last signature in encrypted_content
-                # If there are multiple thinking blocks, this should be a problem.
-                # In practice, there should only be one signature for the entire reasoning step.
-                # Tested with: claude-sonnet-4-20250514
-                if signature:
-                    reasoning_item.encrypted_content = signature
+                # Store the signatures in encrypted_content with newline delimiter
+                if signatures:
+                    reasoning_item.encrypted_content = "\n".join(signatures)
 
             items.append(reasoning_item)
 
@@ -518,7 +515,8 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
             elif reasoning_item := cls.maybe_reasoning_message(item):
                 # Reconstruct thinking blocks from content (text) and encrypted_content (signature)
                 content_items = reasoning_item.get("content", [])
-                signature = reasoning_item.get("encrypted_content")
+                encrypted_content = reasoning_item.get("encrypted_content")
+                signatures = encrypted_content.split("\n") if encrypted_content else []
 
                 if content_items and preserve_thinking_blocks:
                     # Reconstruct thinking blocks from content and signature
@@ -532,9 +530,9 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
                                 "type": "thinking",
                                 "thinking": content_item.get("text", ""),
                             }
-                            # Add signature if available
-                            if signature:
-                                thinking_block["signature"] = signature
+                            # Add signatures if available
+                            if signatures:
+                                thinking_block["signature"] = signatures.pop(0)
                             pending_thinking_blocks.append(thinking_block)
 
             # 8) If we haven't recognized it => fail or ignore
diff --git a/tests/test_anthropic_thinking_blocks.py b/tests/test_anthropic_thinking_blocks.py
index 933be2c0e..35446efe4 100644
--- a/tests/test_anthropic_thinking_blocks.py
+++ b/tests/test_anthropic_thinking_blocks.py
@@ -125,7 +125,12 @@ def test_anthropic_thinking_blocks_with_tool_calls():
                     "Let me use the weather tool to get this information."
                 ),
                 "signature": "TestSignature123",
-            }
+            },
+            {
+                "type": "thinking",
+                "thinking": ("We should use the city Tokyo as the city."),
+                "signature": "TestSignature456",
+            },
         ],
         tool_calls=[
             ChatCompletionMessageToolCall(
@@ -143,7 +148,7 @@ def test_anthropic_thinking_blocks_with_tool_calls():
     reasoning_items = [
         item for item in output_items if hasattr(item, "type") and item.type == "reasoning"
     ]
-    assert len(reasoning_items) == 1, "Should have exactly one reasoning item"
+    assert len(reasoning_items) == 1, "Should have exactly two reasoning items"
 
     reasoning_item = reasoning_items[0]
 
@@ -159,7 +164,9 @@ def test_anthropic_thinking_blocks_with_tool_calls():
     assert hasattr(reasoning_item, "encrypted_content"), (
         "Reasoning item should have encrypted_content"
     )
-    assert reasoning_item.encrypted_content == "TestSignature123", "Signature should be preserved"
+    assert reasoning_item.encrypted_content == "TestSignature123\nTestSignature456", (
+        "Signature should be preserved"
+    )
 
     # Verify tool calls are present
     tool_call_items = [
@@ -210,6 +217,20 @@ def test_anthropic_thinking_blocks_with_tool_calls():
         "Signature should be preserved in thinking block"
     )
 
+    first_content = content[1]
+    assert first_content.get("type") == "thinking", (
+        f"Second content must be 'thinking' type for Anthropic compatibility, "
+        f"but got '{first_content.get('type')}'"
+    )
+    expected_thinking = "We should use the city Tokyo as the city."
+    assert first_content.get("thinking") == expected_thinking, (
+        "Thinking content should be preserved"
+    )
+    # Signature should also be preserved
+    assert first_content.get("signature") == "TestSignature456", (
+        "Signature should be preserved in thinking block"
+    )
+
     # Verify tool calls are preserved
     tool_calls = assistant_msg.get("tool_calls", [])
     assert len(cast(list[Any], tool_calls)) == 1, "Tool calls should be preserved"