fix: convert Python-safe field names (async_, continue_) to CLI format (#239)

Fixes critical bug where hook outputs using `async_` and `continue_`
(Python-safe names avoiding keyword conflicts) were not being converted
to `async` and `continue` as expected by the CLI. This caused hook
control fields like `{"decision": "block"}` or `{"continue_": False}` to
be silently ignored.

Changes:
- Add _convert_hook_output_for_cli() to handle field name conversion
- Apply conversion in hook callback handling
- Update type documentation to clarify field name usage
- Add comprehensive test coverage for field name conversion
- Update existing tests to verify conversion occurs correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: ashwin-ant

src/claude_agent_sdk/_internal/query.py

@@ -31,6 +31,25 @@
 logger = logging.getLogger(__name__)
 
 
+def _convert_hook_output_for_cli(hook_output: dict[str, Any]) -> dict[str, Any]:
+    """Convert Python-safe field names to CLI-expected field names.
+
+    The Python SDK uses `async_` and `continue_` to avoid keyword conflicts,
+    but the CLI expects `async` and `continue`. This function performs the
+    necessary conversion.
+    """
+    converted = {}
+    for key, value in hook_output.items():
+        # Convert Python-safe names to JavaScript names
+        if key == "async_":
+            converted["async"] = value
+        elif key == "continue_":
+            converted["continue"] = value
+        else:
+            converted[key] = value
+    return converted
+
+
 class Query:
     """Handles bidirectional control protocol on top of Transport.
 
@@ -244,11 +263,13 @@ async def _handle_control_request(self, request: SDKControlRequest) -> None:
                 if not callback:
                     raise Exception(f"No hook callback found for ID: {callback_id}")
 
-                response_data = await callback(
+                hook_output = await callback(
                     request_data.get("input"),
                     request_data.get("tool_use_id"),
                     {"signal": None},  # TODO: Add abort signal support
                 )
+                # Convert Python-safe field names (async_, continue_) to CLI-expected names (async, continue)
+                response_data = _convert_hook_output_for_cli(hook_output)
 
             elif subtype == "mcp_message":
                 # Handle SDK MCP request

src/claude_agent_sdk/types.py

@@ -198,18 +198,56 @@ class SessionStartHookSpecificOutput(TypedDict):
 
 # See https://docs.anthropic.com/en/docs/claude-code/hooks#advanced%3A-json-output
 # for documentation of the output types.
+#
+# IMPORTANT: The Python SDK uses `async_` and `continue_` (with underscores) to avoid
+# Python keyword conflicts. These fields are automatically converted to `async` and
+# `continue` when sent to the CLI. You should use the underscore versions in your
+# Python code.
 class AsyncHookJSONOutput(TypedDict):
-    """Async hook output that defers hook execution."""
+    """Async hook output that defers hook execution.
 
-    async_: Literal[True]  # Using async_ to avoid Python keyword
+    Fields:
+        async_: Set to True to defer hook execution. Note: This is converted to
+            "async" when sent to the CLI - use "async_" in your Python code.
+        asyncTimeout: Optional timeout in milliseconds for the async operation.
+    """
+
+    async_: Literal[
+        True
+    ]  # Using async_ to avoid Python keyword (converted to "async" for CLI)
     asyncTimeout: NotRequired[int]
 
 
 class SyncHookJSONOutput(TypedDict):
-    """Synchronous hook output with control and decision fields."""
+    """Synchronous hook output with control and decision fields.
+
+    This defines the structure for hook callbacks to control execution and provide
+    feedback to Claude.
+
+    Common Control Fields:
+        continue_: Whether Claude should proceed after hook execution (default: True).
+            Note: This is converted to "continue" when sent to the CLI.
+        suppressOutput: Hide stdout from transcript mode (default: False).
+        stopReason: Message shown when continue is False.
+
+    Decision Fields:
+        decision: Set to "block" to indicate blocking behavior.
+        systemMessage: Warning message displayed to the user.
+        reason: Feedback message for Claude about the decision.
+
+    Hook-Specific Output:
+        hookSpecificOutput: Event-specific controls (e.g., permissionDecision for
+            PreToolUse, additionalContext for PostToolUse).
+
+    Note: The CLI documentation shows field names without underscores ("async", "continue"),
+    but Python code should use the underscore versions ("async_", "continue_") as they
+    are automatically converted.
+    """
 
     # Common control fields
-    continue_: NotRequired[bool]  # Using continue_ to avoid Python keyword
+    continue_: NotRequired[
+        bool
+    ]  # Using continue_ to avoid Python keyword (converted to "continue" for CLI)
     suppressOutput: NotRequired[bool]
     stopReason: NotRequired[str]
 

tests/test_tool_callbacks.py

@@ -322,8 +322,11 @@ async def comprehensive_hook(
         # The hook result is nested at response.response
         result = response_data["response"]["response"]
 
-        # Verify control fields are present
-        assert result.get("continue_") is True or result.get("continue") is True
+        # Verify control fields are present and converted to CLI format
+        assert result.get("continue") is True, (
+            "continue_ should be converted to continue"
+        )
+        assert "continue_" not in result, "continue_ should not appear in CLI output"
         assert result.get("suppressOutput") is False
         assert result.get("stopReason") == "Test stop reason"
 
@@ -386,10 +389,72 @@ async def async_hook(
         # The hook result is nested at response.response
         result = response_data["response"]["response"]
 
-        # The SDK should preserve the async_ field (or convert to "async")
-        assert result.get("async_") is True or result.get("async") is True
+        # The SDK should convert async_ to "async" for CLI compatibility
+        assert result.get("async") is True, "async_ should be converted to async"
+        assert "async_" not in result, "async_ should not appear in CLI output"
         assert result.get("asyncTimeout") == 5000
 
+    @pytest.mark.asyncio
+    async def test_field_name_conversion(self):
+        """Test that Python-safe field names (async_, continue_) are converted to CLI format (async, continue)."""
+
+        async def conversion_test_hook(
+            input_data: dict, tool_use_id: str | None, context: HookContext
+        ) -> HookJSONOutput:
+            # Return both async_ and continue_ to test conversion
+            return {
+                "async_": True,
+                "asyncTimeout": 10000,
+                "continue_": False,
+                "stopReason": "Testing field conversion",
+                "systemMessage": "Fields should be converted",
+            }
+
+        transport = MockTransport()
+        hooks = {"PreToolUse": [{"matcher": None, "hooks": [conversion_test_hook]}]}
+
+        query = Query(
+            transport=transport, is_streaming_mode=True, can_use_tool=None, hooks=hooks
+        )
+
+        callback_id = "test_conversion"
+        query.hook_callbacks[callback_id] = conversion_test_hook
+
+        request = {
+            "type": "control_request",
+            "request_id": "test-conversion",
+            "request": {
+                "subtype": "hook_callback",
+                "callback_id": callback_id,
+                "input": {"test": "data"},
+                "tool_use_id": None,
+            },
+        }
+
+        await query._handle_control_request(request)
+
+        # Check response has converted field names
+        assert len(transport.written_messages) > 0
+        last_response = transport.written_messages[-1]
+
+        response_data = json.loads(last_response)
+        result = response_data["response"]["response"]
+
+        # Verify async_ was converted to async
+        assert result.get("async") is True, "async_ should be converted to async"
+        assert "async_" not in result, "async_ should not appear in output"
+
+        # Verify continue_ was converted to continue
+        assert result.get("continue") is False, (
+            "continue_ should be converted to continue"
+        )
+        assert "continue_" not in result, "continue_ should not appear in output"
+
+        # Verify other fields are unchanged
+        assert result.get("asyncTimeout") == 10000
+        assert result.get("stopReason") == "Testing field conversion"
+        assert result.get("systemMessage") == "Fields should be converted"
+
 
 class TestClaudeAgentOptionsIntegration:
     """Test that callbacks work through ClaudeAgentOptions."""