vtcode-exec-events 0.159.0

Structured execution telemetry event schema used across VT Code crates.
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# vtcode-exec-events
[Root AGENTS.md](../AGENTS.md) | Authoritative `ThreadEvent` contract. All runtime events flow through this crate.

## Key Types

`ThreadEvent` enum — the single event type (serde-tagged) | `VersionedThreadEvent` wrapper with schema version | `EventEmitter` trait | `Usage` token accounting | `ThreadItem` + `ThreadItemDetails` item taxonomy | `EVENT_SCHEMA_VERSION` semver string

## ThreadEvent Variants

`thread.started` | `thread.completed` | `thread.compact_boundary` | `context.reset` | `turn.started` | `turn.completed` | `turn.failed` | `turn.blocked` | `item.started` | `item.updated` | `item.completed` | `plan.delta` | `plan.approval.requested` | `plan.approval.resolved` | `error`

## Rules

- **Do not invent parallel event types.** Extend `ThreadEvent` and `ThreadItemDetails` enums.
- `EVENT_SCHEMA_VERSION` must be bumped when the serialized contract changes.
- `EventEmitter` trait has a blanket `FnMut(&ThreadEvent)` impl.
- Feature-gated emitters: `telemetry-log` (LogEmitter), `telemetry-tracing` (TracingEmitter), `schema-export` (JSON Schema), `serde-json` (JSON helpers).
- `atif/` module exports ATIF (Agent Trace Interchange Format).
- `trace/` module implements Agent Trace spec for AI code attribution.
- **Keep `ThreadEvent` compact**: large sparse payloads must be `Box`ed (see `thread_event_stays_compact` size-guard test; ≤80 bytes). `Box<T>` is serde/schema transparent.

## Gotchas

- `vtcode-core::exec::events` re-exports these types — consumers should use that path, not depend on this crate directly.
- Plan approval state is represented by `PlanApprovalRequestedEvent` and
  `PlanApprovalResolvedEvent`; keep `PlanApprovalDecision` stable because it is
  consumed by headless clients and Open Responses adapters. Bounded failure
  explanations use the existing `ReasoningItem` with stage `"diagnosis"`; do not add a parallel event variant.
- `HarnessEventItem` uses `HarnessEventKind` enum — adding variants requires schema version bump.
- Schema `0.12.0` adds blocked-handoff resolution metadata; keep legacy payloads readable and ATIF output stable.
- Schema `0.14.0` adds `SessionToolLimitIncreased` and `ToolLoopLimitIncreased` harness kinds for same-turn limit grants; schema `0.13.0` added `turn.blocked` plus `TurnBlocked`/`BlockedRecoveryStarted`/`BlockedRecoveryFinished`, emitted alongside `turn.failed` with fuse counters for UI subscribers.