fix: omit toolConfig when tool_choice="none" in BedrockChatClient (#4535)

Bedrock's Converse API only accepts "auto", "any", or "tool" as valid
toolChoice keys. The previous code mapped tool_choice="none" to
{"none": {}}, which causes a botocore.exceptions.ParamValidationError.

When tool_choice="none" (set by FunctionInvocationLayer after exhausting
max iterations), the fix now omits toolConfig entirely so the model
won't attempt tool calls.

Added tests for tool_choice="none", "auto", and "required" modes.

Fixes #4529

Co-authored-by: Eduard van Valkenburg <eavanvalkenburg@users.noreply.github.com>

python/packages/bedrock/agent_framework_bedrock/_chat_client.py

@@ -405,11 +405,16 @@ class BedrockChatClient(
 
         tool_config = self._prepare_tools(options.get("tools"))
         if tool_mode := validate_tool_mode(options.get("tool_choice")):
-            tool_config = tool_config or {}
             match tool_mode.get("mode"):
-                case "auto" | "none":
-                    tool_config["toolChoice"] = {tool_mode.get("mode"): {}}
+                case "none":
+                    # Bedrock doesn't support toolChoice "none".
+                    # Omit toolConfig entirely so the model won't attempt tool calls.
+                    tool_config = None
+                case "auto":
+                    tool_config = tool_config or {}
+                    tool_config["toolChoice"] = {"auto": {}}
                 case "required":
+                    tool_config = tool_config or {}
                     if required_name := tool_mode.get("required_function_name"):
                         tool_config["toolChoice"] = {"tool": {"name": required_name}}
                     else:

python/packages/bedrock/tests/test_bedrock_client.py

@@ -31,6 +31,15 @@ class _StubBedrockRuntime:
         }
 
 
+def _make_client() -> BedrockChatClient:
+    """Create a BedrockChatClient with a stub runtime for unit tests."""
+    return BedrockChatClient(
+        model_id="amazon.titan-text",
+        region="us-west-2",
+        client=_StubBedrockRuntime(),
+    )
+
+
 async def test_get_response_invokes_bedrock_runtime() -> None:
     stub = _StubBedrockRuntime()
     client = BedrockChatClient(
@@ -65,3 +74,66 @@ def test_build_request_requires_non_system_messages() -> None:
 
     with pytest.raises(ValueError):
         client._prepare_options(messages, {})
+
+
+def test_prepare_options_tool_choice_none_omits_tool_config() -> None:
+    """When tool_choice='none', toolConfig must be omitted entirely.
+
+    Bedrock's Converse API only accepts 'auto', 'any', or 'tool' as valid
+    toolChoice keys. Sending {"none": {}} causes a ParamValidationError.
+    The fix omits toolConfig so the model won't attempt tool calls.
+
+    Fixes #4529.
+    """
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    # Even when tools are provided, tool_choice="none" should strip toolConfig
+    options: dict[str, Any] = {
+        "tool_choice": "none",
+        "tools": [
+            {"toolSpec": {"name": "get_weather", "description": "Get weather", "inputSchema": {"json": {}}}},
+        ],
+    }
+
+    request = client._prepare_options(messages, options)
+
+    assert "toolConfig" not in request, (
+        f"toolConfig should be omitted when tool_choice='none', got: {request.get('toolConfig')}"
+    )
+
+
+def test_prepare_options_tool_choice_auto_includes_tool_config() -> None:
+    """When tool_choice='auto', toolConfig.toolChoice should be {'auto': {}}."""
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    options: dict[str, Any] = {
+        "tool_choice": "auto",
+        "tools": [
+            {"toolSpec": {"name": "get_weather", "description": "Get weather", "inputSchema": {"json": {}}}},
+        ],
+    }
+
+    request = client._prepare_options(messages, options)
+
+    assert "toolConfig" in request
+    assert request["toolConfig"]["toolChoice"] == {"auto": {}}
+
+
+def test_prepare_options_tool_choice_required_includes_any() -> None:
+    """When tool_choice='required' (no specific function), toolChoice should be {'any': {}}."""
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    options: dict[str, Any] = {
+        "tool_choice": "required",
+        "tools": [
+            {"toolSpec": {"name": "get_weather", "description": "Get weather", "inputSchema": {"json": {}}}},
+        ],
+    }
+
+    request = client._prepare_options(messages, options)
+
+    assert "toolConfig" in request
+    assert request["toolConfig"]["toolChoice"] == {"any": {}}