fix: only set signature in message if signature was provided by the model

src/strands/event_loop/streaming.py

@@ -194,16 +194,18 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
         state["text"] = ""
 
     elif reasoning_text:
-        content.append(
-            {
-                "reasoningContent": {
-                    "reasoningText": {
-                        "text": state["reasoningText"],
-                        "signature": state["signature"],
-                    }
+        content_block: ContentBlock = {
+            "reasoningContent": {
+                "reasoningText": {
+                    "text": state["reasoningText"],
                 }
             }
-        )
+        }
+
+        if "signature" in state:
+            content_block["reasoningContent"]["reasoningText"]["signature"] = state["signature"]
+
+        content.append(content_block)
         state["reasoningText"] = ""
 
     return state
@@ -263,7 +265,6 @@ async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[d
         "text": "",
         "current_tool_use": {},
         "reasoningText": "",
-        "signature": "",
     }
     state["content"] = state["message"]["content"]
 

tests/strands/event_loop/test_streaming.py

@@ -216,6 +216,21 @@ def test_handle_content_block_delta(event: ContentBlockDeltaEvent, state, exp_up
                 "signature": "123",
             },
         ),
+        # Reasoning without signature
+        (
+            {
+                "content": [],
+                "current_tool_use": {},
+                "text": "",
+                "reasoningText": "test",
+            },
+            {
+                "content": [{"reasoningContent": {"reasoningText": {"text": "test"}}}],
+                "current_tool_use": {},
+                "text": "",
+                "reasoningText": "",
+            },
+        ),
         # Empty
         (
             {