ADR 0021: Governed Compaction

Status: Accepted

Date: 2026-06-18

Context

ADR 0020 added the context-pressure gauge without changing run behavior. CC-2 activates the next step: when a run opts in, Truss can compact its own projected history at a run-step boundary before an outer harness or provider-specific context manager is forced to intervene.

The durable journal is still the source of truth. Compaction must therefore be presentation-only: it may change the message history sent to the next model call, but it must never delete or rewrite the signed journal entries that produced the original context.

Decision

MemoryConfig now includes a layered CompactionPolicy with safe defaults:

• mode = "off" by default, so existing runs are unchanged.
• threshold = 0.70 is the default pressure trigger when a caller opts in.
• min_tail = 6 keeps recent messages verbatim.
• exempt_tiers defaults to L0 core, L0 self, L1 principal, and L3 goal.

The durable in-loop executor checks the policy only at the boundary after a model result has been journaled and context pressure has been measured. If pressure is at or above the configured threshold, it uses projection.rs to summarize the elided middle of the message history. L0/L1 remain in the stable system prefix and L3 goal ledger is injected outside the compacted history, so the existing exempt layers remain protected.

For summarize mode, the next request keeps:

• the original head message,
• one deterministic context-summary message,
• the configured verbatim tail.

For evict mode, Truss stores the same summary for audit and recall but omits the summary from the active context. Both modes write EntryKind::ContextSummary through the governed memory substrate. The entry is initially Unverified; promotion remains governed by the existing verification policy rather than relabeling deterministic summaries as verified without a verdict.

Every compaction appends a signed compaction_performed journal entry with:

• before and after token counts,
• the context-summary memory id,
• the inclusive elided journal range,
• the policy mode, threshold, min-tail, exempt tiers, and verification policy.

The before count comes from the already-observed CC-1 projected input-token count. The after count is a conservative projection estimate from the compacted message shape, avoiding a second tokenizer pass.

Consequences

Governed compaction is opt-in and boundary-only, preserving prompt-cache prefix stability and deterministic replay discipline. Disabled and below-threshold runs continue to send the same request shapes as before.

The journal remains authoritative and fully auditable. The elided range can be recovered from signed journal rows, and Store::verify_chain covers the compaction_performed entry like every other durable event.

CC-2 intentionally does not add anticipation, compaction_imminent, eval variants, or provider-side context_management interop. Those remain CC-3 and CC-4 work.