Session compaction parity

Session compaction has one control-plane authority: the streaming compactor’s per-session posture snapshot. Read surfaces serialize that snapshot; they do not re-run compaction logic. All surfaces share one vocabulary so operator-visible state is identical across them. The read surfaces are GET /v1/sessions/{id}/compaction/status, get_session_compaction_status, bmo_get_session_compaction_status, the describe_context agent tool under compaction.session, bmo session status, the TUI Context hub, and the app read model.

Posture states

The bounded posture state vocabulary (from internal/sessioncompaction/posture.go):

• unavailable
• disabled
• idle
• budget_warning
• summary_linked
• summary_missing
• streaming_mechanical
• streaming_semantic
• streaming_ready
• streaming_degraded

Posture resolution follows a fixed precedence so every surface reports the same state from the same snapshot:

1. A broken summary link (summary_missing / summary_link_missing) is surfaced before any streaming phase.
2. Then the streaming phase (streaming_mechanical, streaming_semantic, streaming_ready, streaming_degraded).
3. Then linked-summary state (summary_linked).
4. Then prompt-budget warn / exceeded / provider_context_error.

Bounded reason codes

The fixed reason-code vocabulary (from internal/compaction/streaming/retention.go). Reason codes are bounded metadata: they never carry raw transcript text, prompt bodies, model responses, or provider error bodies.

Missing load-bearing facts:

• missing_active_goal
• missing_user_constraint
• missing_permission_boundary
• missing_active_path
• missing_tool_outcome
• missing_provider_pressure
• missing_decision_state
• missing_validation_result
• summary_link_missing

Semantic structural:

• semantic_truncated
• semantic_token_count_invalid
• semantic_error
• semantic_timeout
• semantic_canceled

Observation / boundary:

• quality_unobserved
• hierarchy_required_deferred
• boundary_crossing

Quality-gate fields

Posture exposes these quality-gate fields, scoped to the mechanical compaction version so a later run cannot reuse a prior verdict:

• retention_ratio — bounded number
• validation_passed — bool
• validation_observed — bool
• reasons — list of the bounded codes above

Load-bearing fact classes

The retention shadow tracks nine load-bearing fact classes:

• active_goal
• user_constraint
• permission_boundary
• active_path
• tool_outcome
• provider_pressure
• decision_state
• validation_result
• summary_link

Retention applies a two-tier policy: active_goal and active_path are majority / paraphrase-tolerant (retained at >= 50%); all other classes are fail-closed.

Continuity evidence

Beyond posture state, the snapshot carries bounded continuity metadata:

• a continuity epoch
• the latest trigger / status / reason
• a small per-session event ring of recent compaction events

All of this is bounded metadata; none of it carries raw bodies.

Surfaces

Surface	How it reads posture
GET /v1/sessions/{id}/compaction/status	Serializes the shared snapshot for HTTP clients
get_session_compaction_status	Returns the same JSON snapshot for native agent automation
bmo_get_session_compaction_status	Returns the same JSON snapshot for MCP clients
describe_context	Serializes the snapshot under compaction.session
bmo session status	Renders the snapshot for headless operator inspection
TUI Context hub	Shows the snapshot in the Compaction row
App read model	Exposes the snapshot as SessionCompactionStatus

Related surfaces

• Session compaction — user-facing overview
• Configuration — posture and budget configuration