feat: add optional outputSchema support for tool specifications

- Add outputSchema as optional field in ToolSpec using NotRequired[JSONSchema]
- Filter outputSchema unconditionally in streaming.py for compatibility
- Add TODO comment referencing issue #780 for future improvements

This allows tools to define their output schema while maintaining full
backward compatibility. The outputSchema field is filtered out before
sending to model providers to prevent validation errors until proper
model-specific behavior handling is implemented.

Closes #787

Author: Vamil Gandhi

src/strands/event_loop/streaming.py

@@ -2,7 +2,7 @@
 
 import json
 import logging
-from typing import Any, AsyncGenerator, AsyncIterable, Optional
+from typing import Any, AsyncGenerator, AsyncIterable, Optional, cast
 
 from ..models.model import Model
 from ..types._events import (
@@ -337,7 +337,17 @@ async def stream_messages(
     logger.debug("model=<%s> | streaming messages", model)
 
     messages = remove_blank_messages_content_text(messages)
-    chunks = model.stream(messages, tool_specs if tool_specs else None, system_prompt)
+
+    # TODO(#780): Remove outputSchema filtering once model-specific behavior handling is implemented.
+    # For now, we filter out outputSchema from all tool specs to ensure compatibility with all model providers.
+    # Some providers (e.g., Bedrock) will throw validation errors if they receive unknown fields.
+    filtered_tool_specs = None
+    if tool_specs:
+        filtered_tool_specs = cast(
+            list[ToolSpec], [{k: v for k, v in spec.items() if k != "outputSchema"} for spec in tool_specs]
+        )
+
+    chunks = model.stream(messages, filtered_tool_specs, system_prompt)
 
     async for event in process_stream(chunks):
         yield event

src/strands/types/tools.py

@@ -9,7 +9,7 @@
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, AsyncGenerator, Awaitable, Callable, Literal, Protocol, Union
 
-from typing_extensions import TypedDict
+from typing_extensions import NotRequired, TypedDict
 
 from .media import DocumentContent, ImageContent
 
@@ -27,11 +27,15 @@ class ToolSpec(TypedDict):
         description: A human-readable description of what the tool does.
         inputSchema: JSON Schema defining the expected input parameters.
         name: The unique name of the tool.
+        outputSchema: Optional JSON Schema defining the expected output format.
+            Note: Not all model providers support this field. Providers that don't
+            support it should filter it out before sending to their API.
     """
 
     description: str
     inputSchema: JSONSchema
     name: str
+    outputSchema: NotRequired[JSONSchema]
 
 
 class Tool(TypedDict):