feat: preserve thoughtSignature for Gemini 3 Pro function calling

- Add thoughtSignature field to ToolUse TypedDict as optional field
- Add thoughtSignature to ContentBlockStartToolUse TypedDict
- Preserve thoughtSignature during streaming event processing
- Fixes compatibility with Gemini 3 Pro thinking mode

This change enables proper multi-turn function calling with Gemini 3 Pro,
which requires thought_signature to be passed back in subsequent requests.

Resolves: Gemini 3 Pro 400 error for missing thought_signature
See: https://ai.google.dev/gemini-api/docs/thought-signatures

Author: dpolistwm

src/strands/event_loop/streaming.py

@@ -185,6 +185,9 @@ def handle_content_block_start(event: ContentBlockStartEvent) -> dict[str, Any]:
         current_tool_use["toolUseId"] = tool_use_data["toolUseId"]
         current_tool_use["name"] = tool_use_data["name"]
         current_tool_use["input"] = ""
+        # Preserve thoughtSignature if present (required for Gemini 3 Pro)
+        if "thoughtSignature" in tool_use_data:
+            current_tool_use["thoughtSignature"] = tool_use_data["thoughtSignature"]
 
     return current_tool_use
 
@@ -285,6 +288,11 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
             name=tool_use_name,
             input=current_tool_use["input"],
         )
+
+        # Preserve thoughtSignature if present (required for Gemini 3 Pro)
+        if "thoughtSignature" in current_tool_use:
+            tool_use["thoughtSignature"] = current_tool_use["thoughtSignature"]
+
         content.append({"toolUse": tool_use})
         state["current_tool_use"] = {}
 

src/strands/types/content.py

@@ -123,16 +123,18 @@ class DeltaContent(TypedDict, total=False):
     toolUse: Dict[Literal["input"], str]
 
 
-class ContentBlockStartToolUse(TypedDict):
+class ContentBlockStartToolUse(TypedDict, total=False):
     """The start of a tool use block.
 
     Attributes:
         name: The name of the tool that the model is requesting to use.
         toolUseId: The ID for the tool request.
+        thoughtSignature: Optional encrypted token from Gemini for multi-turn reasoning.
     """
 
     name: str
     toolUseId: str
+    thoughtSignature: NotRequired[str]
 
 
 class ContentBlockStart(TypedDict, total=False):

src/strands/types/tools.py

@@ -52,19 +52,24 @@ class Tool(TypedDict):
     toolSpec: ToolSpec
 
 
-class ToolUse(TypedDict):
+class ToolUse(TypedDict, total=False):
     """A request from the model to use a specific tool with the provided input.
 
     Attributes:
         input: The input parameters for the tool.
             Can be any JSON-serializable type.
         name: The name of the tool to invoke.
         toolUseId: A unique identifier for this specific tool use request.
+        thoughtSignature: Optional encrypted token from Gemini that preserves
+            the model's internal reasoning process for multi-turn conversations.
+            Required for Gemini 3 Pro when using function calling.
+            See: https://ai.google.dev/gemini-api/docs/thought-signatures
     """
 
     input: Any
     name: str
     toolUseId: str
+    thoughtSignature: NotRequired[str]
 
 
 class ToolResultContent(TypedDict, total=False):