# Todo 45.6 Mixed Validation and Handler Snapshot Plan

**Status:** done

## Result

Fallback requires every reported validation issue to be missing-only. Required plus substantive validation errors skip LLM calls, and arbitrary payload snapshot exceptions become `HANDLER_ERROR` across sync, async, ToolRegistry, and EventAgent paths.

## Goal

Close the final reviewed cost and error-isolation gaps: mixed validation failures must never trigger LLM fallback, and arbitrary exceptions while snapshotting a handler payload must become normalized handler failures.

## Scope

- Treat validation as fallback-eligible only when every reported top-level failure classifies as missing-only.
- Cover required fields combined with enum, type, additional-property, and composed-schema substantive errors.
- Catch any normal exception raised while producing a strict JSON snapshot of a handler payload in async and sync execution.
- Verify ToolRegistry and EventAgent receive normalized error payloads rather than exceptions.

## Boundaries

- Preserve fallback for pure missing-only input, including the unique viable composed branch from Todo 45.5.
- Do not catch `BaseException` subclasses such as cancellation or process-exit signals.
- Keep strict JSON rejection behavior; do not coerce unusual dictionary subclasses or values.

## Files

- Modify: `src/agent_lab/application/events/kernel.py`
- Modify: `tests/test_event_kernel.py`
- Modify: `tests/test_event_agent.py`

## Requirement Execution Loop

### Plan

1. Add RED tests for required plus enum/type/additionalProperties and required plus composed substantive errors; fallback count must remain zero.
2. Add RED tests whose handler returns a dictionary subclass raising `RuntimeError` during JSON serialization.
3. Require all validation issues to be missing-only before setting `missing_required=True`.
4. Normalize strict payload snapshot exceptions to `HANDLER_ERROR` in sync and async paths.
5. Verify the behavior through EventKernel, ToolRegistry, and EventAgent, then run full reviews.

### Act

- Assert mixed invalid input reports `INVALID_ARGUMENTS`, `used_fallback=False`, and zero fallback calls.
- Assert pure missing input still falls back exactly once.
- Assert exploding payload snapshots return `event handler returned non-JSON payload` without escaping from kernel, ToolRegistry, or EventAgent.

### Observe

Confirm fallback counts represent only missing information and every ordinary plugin payload failure remains isolated inside the event result pipeline.

### Update

- Re-run final Todo 45 specification and quality reviews.
- Mark Todos 45 through 45.6 done only after fresh verification.

## Verification

- `UV_CACHE_DIR=.uv-cache uv run pytest tests/test_event_kernel.py tests/test_event_agent.py -q`
- `UV_CACHE_DIR=.uv-cache uv run pytest`
- `git diff --check`