compress-session — Claude Code skill: distil a session, don't

---
name: compress-session
description: Distill the current conversation into a structured note (decisions made, open questions, file pointers with line numbers, next 1–3 actions) and save to `quality_reports/session_logs/` before auto-compression. Differs from `/checkpoint` (explicit stop-point snapshot) and from auto-compaction (which truncates rather than distills). Use when context is approaching auto-compact threshold, when a long pipeline has accumulated many decisions, or when the user says "compress", "distil this session", "before we hit auto-compact", "structured handoff before context resets".
argument-hint: "[optional-topic-slug]"
disable-model-invocation: true
allowed-tools: ["Read", "Write", "Glob", "Grep", "Bash"]
metadata:
  author: Claude Code Academic Workflow
  version: 1.0.0
---

# `/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"](https://www.dbreunig.com/2025/06/22/how-contexts-fail-and-how-to-fix-them.html) 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](../../rules/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`](../../hooks/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:

```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 (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.)

```json
{
  "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.