dsh-facet/README.md

dsh-facet

Scope an agent session to one facet of a system — the folder the conversation was started in, plus whatever you explicitly grant it.

A DeepSeek Harness bundle: one host row (the policy) and one agent preset (the enforcement).

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Why this exists

A multi-faceted system is built from modules that look alike. In the project this was written against there are fourteen controllers — the same idioms, the same naming, the same shape — and a conversation about one of them that can see all fourteen spends its first several steps working out which one it is looking at. That is a quality problem before it is a cost problem, and the cost is real too: in a long session the dominant expense is re-reading accumulated history, and most of that history is file content the agent did not need.

So this narrows the world to one module rather than trying to steer through all of them. It is the same idea as the focus argument in dsh-recall — which confines a search to one module — applied to the whole agent: what it may read, where it may search, and what its prompt says it is.

What it does

Three things, and they are deliberately in three different places:

	where	why there
the rule ("work inside the facet; ask to go outside")	system prompt	invariant for the session, so it does not invalidate the prompt cache
the binding (which folder, what is granted)	runtime-context snapshot	it changes when you grant something, and a changed system prompt costs a full-context cache miss
the guard	tools/pre-execute	the part that does not depend on the model agreeing

The scope is not configured — it is where you already are. A session's working directory is recorded immutably when the session is created, so starting a conversation in src/controllers/room/occupancy makes that controller the facet, and starting one at the project root simply leaves the session unscoped (which is the honest answer for a conversation that is genuinely about everything).

Going outside is a question, not a refusal. In that same project 41.6% of the code — shared types, services, infrastructure — sits outside every controller. A wall that cannot be opened would not confine the agent, it would only make it wrong, and an agent that cannot reach a shared type does not stop needing it. So an out-of-scope file call is put to you, and saying yes records the grant for the rest of the conversation.

The facet tool

action	effect
status	the root, the mode, the standing grants and this session's grants
add	asks you to approve an extra path, then records it for this session
remove	drops a session grant
reset	returns to the session's own folder

add is the only one that needs approval. It is also asked in deny mode, because otherwise there would be no way to grant anything at all.

Install

dsh plugin --profile web add "/path/to/dsh-facet"
dsh --profile web --dump-config | grep preset-facet    # expect the row

The bundle contributes two layers:

• cordis.patch.yml — the host row facet-policy, which owns the mode and the standing grants and publishes them as ctx.facetPolicy.
• presets/facet.patch.yml — the agent preset facet, which is the shipped standard composition plus the guard and a configured compaction row.

Then pick Facet as the preset for a session — in the Web UI that is the preset picker in the new-session surface, or Settings → General → Agent preset to make it the default for new sessions. The preset is fixed once a session's first turn has run, so choose it when you start the conversation.

Configure it in your profile, not in the package

The host row's config is where this deployment's facts belong — the profile's own cordis.patch.yml:

- id: facet-policy
  name: dsh-facet
  config:
    mode: ask              # ask | deny | off
    extraRoots:            # always in scope, in every session
      - ../../shared       # relative to each session's own directory

extraRoots is for code a module legitimately always depends on. Anything you grant by hand is per-conversation and is never persisted — "this session may also see src/shared" is a statement about one conversation, and writing it down would quietly widen every future one, which is the failure this plugin exists to prevent.

The compaction row

presets/facet.patch.yml also configures compaction-basic, because a scoped session and a budgeted context are two halves of one idea — scoping reduces context at the source, compaction is the ceiling:

thresholdRatio: 0.35    # a 350k ceiling, not the shipped 800k
retainTokens: 80000     # an absolute working set, not 16% of a 1M window
maxTokens: 16384        # the summarisation cap, stated rather than inherited

The shipped ratio is 0.80 of a 1,000,000-token window, i.e. 800k. Measured across this deployment's 40 recorded sessions, the deepest ever reached 787,668 pressure tokens — the trigger had never once fired, so every token below it was re-read on every subsequent step. maxTokens is worth stating explicitly: its default is not 8192 but headroomTokens, 65536.

If you want that budget on every session rather than only scoped ones, presets/standard-compaction.patch.yml is an opt-in overlay that applies the same row to the shipped standard preset. It is not in the bundle's default patch list, and the header of that file explains why: a preset declaration replaces the whole plugin list rather than merging into it, so the file is a frozen copy of standard that will not follow the harness when standard changes.

What it does not cover

Honest boundaries, because each one is a decision rather than an oversight:

• bash is not guarded. A shell command cannot be judged reliably from argv, so the facet is enforced on the file tools and bash remains a legitimate way out. The prompt tells the model this rather than pretending otherwise.
• Tools that name a path in a shape the table cannot read are not guessed at — including MCP tools. See PATH_ARGUMENTS in lib/scope.js; it is a list of tools whose path argument is a plain top-level string.
• This guards the tool pipeline, not the filesystem. There is no read seam in the harness — dsh-fs-sandbox states plainly that "reads pass through untouched: every mode permits reading" — so anything that reads through ctx.fs without going through tool dispatch (another plugin, the HTTP workspace-files endpoint) is unaffected. Fencing all readers would mean replacing ctx.fs with a stricter FileSystem subclass.
• A grant is not persisted, so it does not survive a process restart.
• Sessions at the project root are not scoped at all, which is deliberate.

What has been verified, and what has not

Verified:

• 42 tests (npm test) over the scope algebra, the policy state, and the guard — including that containment is decided on canonical targets (a symlink inside the facet pointing outside it is refused), that a prefix is not a parent (/a/room does not contain /a/roommate), that a malformed tool call never makes the guard throw, and that every facet result satisfies the tool's own declared output schema.
• The bundle's patch layers parse, merge and land correctly against the real profile: dsh --profile web --patch ./cordis.patch.yml --patch ./presets/facet.patch.yml --dump-config shows facet-policy, preset-facet, the facet-guard row and the compaction config, with preset-standard untouched. The optional standard-compaction overlay was checked the same way.

Not yet verified: a live session. The plugin has not been loaded by a running harness, because that needs the bundle installed. The first thing to check after installing is that preset-facet is not broken — a preset whose child row fails to resolve still appears on the roster, with its diagnostic, and cannot compose a session:

dsh --profile web --dump-config | grep -A 3 preset-facet

Development

node scripts/dev-links.mjs   # link the harness's cordis/schemastery/dsh-tools
npm test

dev-links.mjs links rather than installs those three packages, and for @deepseek-ai/cordis that is a correctness requirement rather than tidiness: a second copy of Cordis means a second Context class, two service registries in one process, and a plugin whose services are invisible to the harness that loaded it.

Layout

index.js                     host row: the policy, published as ctx.facetPolicy
lib/policy.js                the stateful half — scope resolution, grants
lib/scope.js                 the pure half — argument extraction, text, reasons
lib/guard.js                 preset row: rule, binding, tool, pre-execute guard
presets/facet.patch.yml      the `facet` agent preset
presets/standard-compaction.patch.yml   optional: the same budget for `standard`
test/                        42 tests

Also in this repository

• facet-preset.md — the design note this was built from, including the harness mechanisms it rests on.
• compaction-tuning.md — the investigation into why compaction had never fired. Its measurements reproduce exactly; note that its "The change" section prescribes $DSH_HOME/.agent-presets/, a directory nothing reads any more on the installed harness. Presets are bundle patch declarations now, which is what presets/facet.patch.yml is.