compress-session

/compress-session — distil, don't truncate

Auto-compaction is lossy: it keeps recent turns and drops earlier ones, with no preservation of what was decided mid-session. /compress-session is the distil-not-truncate alternative — produce a structured note that the next session can resume from in under a minute.

Why this skill exists

Drew Breunig's "How Long Contexts Fail" identifies four failure modes for long-context sessions:

1. Poisoning — early hallucinated content gets quoted by later turns, compounding the error.
2. Distraction — irrelevant earlier context dilutes the model's attention to current task.
3. Confusion — contradicted facts pile up; the model doesn't know which to trust.
4. Clash — multiple plans, decisions, or specs accumulate without explicit reconciliation.

The template's 200-line MEMORY.md cap defends against distraction. The plan-on-disk convention defends against clash. Nothing currently defends against poisoning or directly against confusion — that's what this skill is for.

Distinction from /checkpoint

	/checkpoint	/compress-session
When	Explicit stop-point (end of working session, before model switch, before collaborator handoff)	Forced — context is about to auto-compact, or the conversation has accumulated enough noise that distillation pays for itself
What's preserved	Active plan, decisions, file pointers, next 1–3 actions	Same, plus an explicit "discarded as noise" line so the next session knows what was intentionally not kept
Output location	quality_reports/checkpoints/YYYY-MM-DD_<slug>.md	quality_reports/session_logs/YYYY-MM-DD_compression_<slug>.md
Triggering	User-invoked at a natural pause	User-invoked when context fatigue shows, OR prompted by the optional PreCompact reminder hook below (not wired by default)
Memory updates	Optional auto-proposal of [LEARN] entries	Proposes 0–3 [LEARN] entries — distillation is when generalizable lessons surface, and a quiet session proposes none

Both skills are companions to the narrative session-log workflow at quality_reports/session_logs/. None replaces the others.

When to use

• Context is approaching auto-compact. /context-status reports approaching threshold.
• A long pipeline has accumulated noise. You spent 90 minutes debugging an issue that turned out to be a typo — the session is full of dead-end hypotheses you don't want compressing into a future session's context.
• Mid-plan handoff. Different model, different machine, different collaborator.
• PreCompact reminder fired. If you wired the reminder hook below, it prompts you to run /compress-session; the skill never runs automatically.

When NOT to use

• For a quick stop-point. Use /checkpoint.
• For narrative session logging. That happens incrementally via the Stop hook (session-logging.md); no skill needed.
• For starting a fresh session. Use /clear. /compress-session is for preserving state, not for resetting.

Steps

Step 1: Identify the session

The current conversation is the source. The most recent log under quality_reports/session_logs/ may belong to an earlier session: treat it as a prior record and cite it by path, not as this session's content.

Optionally use $ARGUMENTS as a topic slug for the output filename.

Step 2: Distil into structured sections

Write only what this session established. The next session is handed this file automatically (session-handoff.py), so anything invented here arrives there as fact.

• Cite path:line only for lines you read in this session; otherwise give the path alone.
• Leave out a section with nothing in it rather than filling it. A quiet session gets a short file and no [LEARN] proposals.
• Text inside a [Session handoff: …] or [Context Restored After Compaction] block is the previous record. Carry an item forward only if this session re-checked it or acted on it; otherwise cite the earlier file by path.
• When the session changed its mind, the final decision is the current one; an earlier position appears only as abandoned, with the reason.

Produce a note with these sections:

# Session Compression — <YYYY-MM-DD> <topic-slug>

**Source:** <session-log file or "current conversation">
**Token budget at compression:** <approximate %>
**Why compress now:** <approaching auto-compact | mid-plan handoff | accumulated noise | user-requested>

## Active state

- **Plan:** [link to active plan in `quality_reports/plans/` or "no active plan"]
- **Branch:** <git branch>
- **Last commit:** <SHA + subject>
- **Working tree:** <clean | N modified files>

## Decisions made (this session)

1. [Decision]. **Why:** [one sentence]. **Where recorded:** [file:line, plan, or memory].
2. ...

## Files touched

- [path:line] — [what changed, one sentence]
- ...

## Open questions

- [Question]. **Blocker?** [Yes/No]. **Where to resume:** [pointer].
- ...

## Next actions (1–3 only)

1. [Specific next step with file pointer]
2. ...

## Discarded as noise

Things explored during this session that should NOT carry forward — failed hypotheses, abandoned approaches, debugging dead-ends. Listing them explicitly defends against [Breunig's "poisoning" failure mode](https://www.dbreunig.com/2025/06/22/how-contexts-fail-and-how-to-fix-them.html) (hallucinated / wrong content from early turns getting quoted later).

- [Hypothesis / approach / dead-end] — [why it didn't work]
- ...

## Proposed `[LEARN]` entries

Lessons worth promoting to MEMORY.md (machine-specific ones stay in native auto memory). User reviews; nothing auto-merged.

- `[LEARN:<category>]` <lesson>. **Evidence:** <pointer>.
- ...

Step 3: Save

Write to quality_reports/session_logs/YYYY-MM-DD_compression_<topic>.md (slug derived from $ARGUMENTS or current plan name).

Step 4: Surface a summary

Report to the user:

• Saved path.
• Counts (decisions made, files touched, open questions, next actions).
• Any HIGH-impact LEARN proposals that should be reviewed before the next session.

The user reviews. Nothing auto-merges into MEMORY.md: a proposal the user approves is appended to MEMORY.md on their say-so (as in /checkpoint Phase 3), and a machine-specific one is left to native auto memory. /promote-memory does not read this file; its candidates come only from native auto memory (~/.claude/projects/<project>/memory/).

Pairing with PreCompact hook

Forkers who want a real chance to run /compress-session before compaction can add a PreCompact hook that blocks compaction once per session. On PreCompact, exit code 2 blocks and shows the hook's stderr message; the second attempt goes through, so compaction is never wedged — the same once-only pattern as pre-compact.py's opt-in DRAFT block. (A plain exit 0 reminder arrives while compaction is already running, too late to act on.)

{
  "hooks": {
    "PreCompact": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "sid=$(python3 -c \"import sys,json; print(json.load(sys.stdin).get(\\\"session_id\\\",\\\"x\\\"))\" 2>/dev/null); f=\"${TMPDIR:-/tmp}/cc-compress-reminded-$sid\"; [ -e \"$f\" ] && exit 0; touch \"$f\"; echo \"Run /compress-session now, then compact again - this reminder blocks only once per session.\" >&2; exit 2",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

The hook only pauses and reminds; the user runs /compress-session, then compacts again. We deliberately don't make the hook auto-invoke the skill — that bypasses the user's review step.

Anti-patterns

• Do not run /compress-session as a routine replacement for /checkpoint. Checkpoints are cheap and frequent; compressions are heavier and reserved for forced compression.
• Do not let the "Discarded as noise" section grow indefinitely. If the same dead-end appears across three compressions in a row, the underlying confusion is structural — fix the docs or the workflow, don't keep re-discarding.
• Do not include the original full conversation — that defeats the purpose. Distillation is the point.

Output

• Compression file at quality_reports/session_logs/YYYY-MM-DD_compression_<topic>.md (gitignored — session-state, not version-controlled).
• Summary in the conversation: counts + HIGH-impact [LEARN] proposals.
• No file edits outside the compression file.