vtcode-exec-events 0.166.0

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` | `background_subprocess_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/` exports ATIF trajectories; `trace/` implements Agent Trace 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 `PlanApprovalRequested/Resolved`; keep `PlanApprovalDecision` stable for headless/Open Responses clients. Bounded failures use `ReasoningItem` stage `"diagnosis"`; no parallel variant. `HarnessEventKind` additions need a schema bump.
- Schema history: `0.12.0` blocked-handoff metadata; `0.13.0` `turn.blocked` + fuse counters; `0.14.0` limit-grant harness kinds; `0.15.0` optional `turn.completed.in_progress_exec_sessions`; `0.16.0` background completion harness identity fields, which ATIF must preserve in step `extra`; keep legacy readable and ATIF stable.
- Schema `0.16.0` adds `background_subprocess_completed` with task/session/exec-session identity, terminal status, exit code, and bounded archive/transcript references; empty optional fields remain omitted.