/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 proposed via PreCompact hook | | Memory updates | Optional auto-proposal of [LEARN] entries | Always proposes [LEARN] entries — distillation is exactly the moment when generalizable lessons surface |

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 hook triggered. The hook can call /compress-session automatically if the user has wired it in.

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

Read the most recent session log under quality_reports/session_logs/. If none exists, treat the current conversation as the source.

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

Step 2: Distil into structured sections

Produce a note with these sections:

markdown
# 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 (or personal-memory.md if machine-specific). 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 — that's /promote-memory's job.

Pairing with PreCompact hook

Forkers who want automatic compression can wire /compress-session into the PreCompact hook:

json
{
  "hooks": {
    "PreCompact": [{
      "hooks": [{
        "type": "command",
        "command": "echo 'Run /compress-session before compacting' && exit 2",
        "timeout": 5
      }]
    }]
  }
}

The hook surfaces a reminder; the user runs /compress-session manually. 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.