dsh-capmark-gate

Hold a DeepSeek Harness agent to a capmark capability manifest: mask the tools it can see, and judge every call it makes.

What it does

Two seams, both verified against @deepseek-ai/dsh 0.1.0-rc.7 and 0.1.1-rc.2:

• tools.restrict() narrows what the agent can see. This is where the payload saving comes from — a tool absent from the view is absent from the request.
• tools/pre-execute judges every call. A returned decision short-circuits the waterfall; next() delegates, so an allow here can never force-allow a call another policy would deny.

The two overlap deliberately. A mask alone is not enough: a tool registered into the agent's own layer bypasses the admit check, and run_code is re-added to every non-native view after restrictions apply. Both stay callable, so both are judged at pre-execute.

What it does not do

It does not sandbox a plugin's own code. A DSH plugin's apply() runs in-process with full Node privileges before any tool call happens. A capability manifest governs what an agent may call. Refusing to install an over-reaching plugin is a separate and earlier decision, made where the overlay row is written.

Saying this plainly matters more than the feature list. A permission system that implies a boundary it does not hold is worse than none, because people stop reading the code.

Measured on a live harness

The gate masked a standard agent with a manifest granting fs:read and forbidding proc:spawn, then drove real calls through the harness's own waterfall:

tools visible: 25 -> 4        (read, glob, grep, read_image)
masked (21):   bash, write, edit, job_*, skill, web_search, subagent, ...

read         allow
grep         allow
bash         deny  - reader declares `never proc:spawn`, and `bash` is part of it
write        deny  - reader declares no capability covering `write`
web_search   deny  - reader declares no capability covering `web_search`

That capture is committed as a fixture, and a test asserts the policy still produces those verdicts.

Configure

Ships disabled. A gate with no manifest denies every call in strict mode, so installing it must not silently mute an agent.

- id: capmark-gate
  disabled: false
  config:
    manifest: |
      ---
      capmark: 0.1
      plugin: reader
      ---
      ```cap
      grant fs:read
      never proc:spawn
      ```

strict defaults to true: with no manifest, deny. Set it to false only on purpose — a gate that fails open is decoration.

License

MIT