Method compactWithReason

pkg/runtime/runtime.go:1848–1877

compactWithReason runs a session compaction with the supplied reason and emits a TokenUsageEvent so the UI immediately reflects the new context pressure. reason is reported to BeforeCompaction / AfterCompaction hooks as CompactionReason. Use [compactionReasonThreshold] for proactive 90%-of-context

(ctx context.Context, sess *session.Session, additionalPrompt, reason string, events EventSink)

// hooks then fire inside [LocalRuntime.doCompact] with [Input.CompactionReason]
// set to the canonical reason; they may veto or supply a custom summary.
func (r *LocalRuntime) compactWithReason(ctx context.Context, sess *session.Session, additionalPrompt, reason string, events EventSink) {
	// Stamp the session ID on ctx so the compaction LLM call carries
	// `X-Cagent-Session-Id` to the gateway. Manual compaction
	// (via `Summarize` from the App) bypasses `runStreamLoop`'s seed;
	// internal callers (proactive threshold, overflow recovery) already
	// run with a stamped ctx, but re-stamping is idempotent.
	ctx = httpclient.ContextWithSessionID(ctx, sess.ID)
	a := r.resolveSessionAgent(sess)

	source := preCompactSourceFor(reason)
	skip, msg, extraPrompt := r.executePreCompactHooks(ctx, sess, a, source, events)
	if skip {
		slog.WarnContext(ctx, "pre_compact hook signalled skip",
			"agent", a.Name(), "session_id", sess.ID, "source", source, "reason", msg)
		if msg != "" {
			events.Emit(Warning(msg, a.Name()))
		}
		return
	}
	additionalPrompt = joinPrompts(additionalPrompt, extraPrompt)

	r.doCompact(ctx, sess, a, additionalPrompt, reason, events)

	// Emit a TokenUsageEvent so the sidebar immediately reflects the
	// compaction: tokens drop to the summary size, context % drops, and
	// cost increases by the summary generation cost.
	modelID := r.getEffectiveModelID(ctx, a)
	contextLimit := r.effectiveContextLimit(ctx, a, r.resolveContextLimit(ctx, a.Model(ctx), modelID))
	events.Emit(NewTokenUsageEvent(sess.ID, a.Name(), SessionUsage(sess, contextLimit)))
}

// preCompactSourceFor maps the canonical compaction reason
// ([compactionReasonThreshold] / [compactionReasonOverflow] /