fast-compaction-dsh

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Verdict-based context compaction for DeepSeek Harness (DSH). Replaces the lossy compaction summary with fast per-call keep/truncate/drop decisions from jev-latest — everything kept stays verbatim, nothing is ever rewritten.

A DSH port of tamaratran/fast-jev-compaction (MIT), adapted to the DSH compaction capability seam.

What and why

Most context compaction asks an LLM to summarize old turns. A summary is lossy: a file path, an exact error, a constraint, or a command can disappear even when it matters later. This plugin never rewrites anything. When compaction triggers, every tool call and tool result in the compacted region is scored by jev-latest through the TypeSafe System One endpoint; stale calls are deleted, half-stale results are truncated to a bounded head, and everything else stays byte-for-byte. User and assistant text is never removed.

On any failure — a missing TYPESAFE_API_KEY, a Jev error, malformed answers, an unfittable history, or an insufficient reduction — it falls back to the shipped compaction-basic summarizer, exactly like the original Claude Code hook falls back to Claude Code's built-in summary.

How it works

1. Every tool-call is paired with its tool-result by call id. Calls in the first message and the newest preserveRecentMessages (default 6) messages are pinned and never touched.
2. The state sent to Jev is the whole compacted region, oldest first, with every tool result replaced by a short note (ok, 4213 chars (omitted)). Tool inputs and texts are included — nothing is summarized.
3. The state is fitted into maxStateTokens (25k default) in stages: tool inputs truncated 1000 → 200 → 60 chars; long texts abridged to head + tail, oldest non-pinned first; old messages collapsed to a note; old calls reduced to one line each; old call-less messages left out; runs of call-only messages folded. If it still does not fit, the plugin falls back.
4. For every non-pinned call Jev gets two noul questions: should the call stay (knowing it was made, with its input, still matters), and should the result stay verbatim (its contents are still needed and re-running the tool would not do).
5. Questions are batched so state plus questions stays under maxRequestTokens (30k default); batches run concurrently and answers are merged.
6. Decisions against keepThreshold (default 0.5): keepResult ≥ threshold → keep both verbatim; else keepCall ≥ threshold → keep the call, truncate the result to its first truncateHeadChars (default 300) characters plus a note; else → drop the call together with its result.
7. The region is rebuilt as a verbatim transcript checkpoint (the DSH seam replaces the shadowed surface span with one durable user message), with role labels and the kept tool activity inline.

Measured on a real region: the Jev scoring round-trip takes ~700 ms.

How it plugs into DSH

DSH exposes compaction as a capability seam: ctx.compaction (service definition), an engine provider (compaction-basic), and the /compact consumer. The engine's single documented customization hook is summarize() — this plugin subclasses BasicCompactionEngine and overrides only that hook, so region selection, the durable transaction (lock, events, stability checks, shrink validation), checkpoint framing, and /compact keep working unchanged.

The shipped standard agent preset mounts compaction-basic inside an isolated compaction realm, so the engine row must be mounted from an agent preset patch — not from a profile-level cordis.patch.yml:

# ~/.dsh/.agent-presets/fast/agent.cordis.yml
- id: standard
  name: cordis:include
  config:
    path: 'file:///path/to/deepseek-harness/packages/preset/agent-presets/presets/standard/agent.cordis.yml'
    patches:
      - id: compaction-basic
        disabled: true
      - id: compaction
        insert:
          - id: fast-compaction
            name: 'file:///path/to/fast-compaction-dsh/src/index.ts'
            # config: { keepThreshold: 0.5, ... }   # all optional

Then point your default preset at it in ~/.dsh/cordis.patch.yml:

- id: agent-presets
  config:
    default: fast

Restart DSH. New sessions compact through jev-latest; watch the log for fast-compaction-dsh: kept N/M calls verbatim (…).

The same package also ships the Web settings card for that engine (the fast-compaction-dsh/web and fast-compaction-dsh/client entries under web/). To get the card, add the package to the web profile and enable the bundle, then restart:

// ~/.dsh/profiles/web/package.json
"dependencies": { "fast-compaction-dsh": "link:/path/to/fast-compaction-dsh" },
"dsh": { "profile": { "bundles": [ ..., "fast-compaction-dsh" ] } }

The bundle patch only mounts the settings-namespace entry in the web profile; the engine keeps being mounted by the agent preset above, and the web profile never loads src/index.ts.

Seeing the effect

No command line needed — the GUI has two layers, plus two optional channels:

1. Chat marker: after a compaction, the chat shows an expandable marker (N items · M tokens); expanding it shows the verbatim transcript the model now sees, with [fast-compaction-dsh truncated …] notes where results were cut.

2. Trajectory tab (the verdict details live here): switch the session view to the Trajectory tab → find the Compaction group → click the compacted cell. The inspector has two tabs:

   • Summary: the rebuilt transcript (the context the model now sees), rendered as Markdown;
   • Raw Output: block 1 is a readable verdict report this plugin emits (stats header + an aligned table of each tool call's keepCall/keepResult probabilities and final action), block 2 is the raw {decisions, stats, stateStage} JSON.

3. Inspector script (offline, read-only — handy for a cross-session overview):

   pnpm run inspect:compaction          # sessions for the current directory
   pnpm run inspect:compaction -- --all # every workspace
   

   Renders each compaction's stats and per-call verdict table (parsed from the JSON block of the compaction/summary event's rawOutput). --json for machine-readable output.

4. Service log: journalctl -u deepseek-harness.service | grep fast-compaction — one summary line per verdict pass, plus fallback warnings.

No compaction yet? Run /compact in a session on the fast preset, or lower thresholdRatio in the preset config.

Requirements

• Node.js ≥ 22.19 (the plugin is TypeScript loaded directly by DSH's loader via type stripping).
• A DeepSeek Harness checkout (developer preview) — for development and for the link: dev dependencies, clone it as a sibling directory of this repo (or adjust the link: paths in package.json).
• A TypeSafe API key: export TYPESAFE_API_KEY=... in the DSH process environment (or apiKey in the preset row's config). Without a key the plugin behaves exactly like compaction-basic.

Configuration

All fields are optional and stack in two layers:

1. The settings.yaml user layer (the fast-compaction: section of ~/.dsh/settings.yaml) — wins per field and applies live to subsequent compactions: the engine watches the settings service's hot-publish and rebuilds the Jev transport in place when apiKey/model/baseUrl change, so no restart is needed. The friendly editor is this package's own Web settings card (web/, the fast-compaction-dsh/client entry) on the Plugins page (apiKey rides as a secret role field — only a set/unset flag ever crosses the wire).
2. The preset patch config: (composition layer) — the per-preset base values; changes require a DSH restart.

Per-field precedence: settings.yaml user layer > preset config: > environment (apiKey only, via TYPESAFE_API_KEY) > code defaults. Unlisted fields pass through to compaction-basic (thresholdRatio, retainRatio, retainTokens, summarizationProvider, summarizationModel, maxTokens, compactionRetries, maxOverflowRetries, modelPolicies, auto).

Note: a settings section whose values fail the schema (e.g. a string in keepThreshold) disables the whole settings layer and falls back to the composition layer, with a warning in the log; the layer retries on the next process start.

Field	Default	Meaning
apiKey	TYPESAFE_API_KEY	TypeSafe API key
model	jev-latest	Jev model name
baseUrl	https://api.typesafe.ai/v1/systemone	System One endpoint
keepThreshold	0.5	Minimum keep probability for a call or result to stay
preserveRecentMessages	6	Newest messages never touched (the first is always kept)
maxStateTokens	25000	Estimated token ceiling for the state
maxRequestTokens	30000	Estimated ceiling for state plus one batch of questions
truncateHeadChars	300	Characters of a dropped tool result retained before its note
minReduction	0.25	Fall back to the built-in summary below this reduction ratio
disabled	false	Use only the built-in summarizer

Differences from fast-jev-compaction

• The original rewrites Claude Code's message list in place. The DSH seam replaces the shadowed surface span with one user message, so the kept content is rebuilt as a verbatim transcript with role labels (<user> / <assistant> / <tool-call> / <result>). Text is still byte-for-byte.
• Assistant reasoning blocks are excluded from the transcript (intermediate thinking is not useful for resuming).
• Pinning is system-head aware: when messages[0] is the system message, the first conversational message is pinned with it.
• The Jev call does not go through DSH's ctx.llm seam; the summary result is recorded as a remote (unmarked) variant with provider: 'typesafe' and the Jev token usage.

Development

pnpm install        # link: deps expect ../deepseek-harness to exist
pnpm run typecheck
pnpm test           # 42 unit/integration tests, no network
TYPESAFE_API_KEY=... node scripts/e2e.ts   # live verdict-path check against the real endpoint

Layout: src/jev.ts (System One transport + noul protocol), src/state.ts (fitted state), src/pairing.ts (call/result pairing), src/rebuild.ts (decisions + verbatim transcript), src/index.ts (the engine).

License

MIT — see LICENSE. Ported from tamaratran/fast-jev-compaction (MIT); the noul question wording, state-fitting stages, and decision thresholds are carried over verbatim.