deepseek-harness/packages/preset/agent-presets

dsh-agent-presets

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Per-session agent composition. A preset is a directory holding one agent.cordis.yml; mounting it under an agent's scope context gives that one session its own tools, prompt sections, and other model-facing contributions, while every other live session keeps its own.

The mechanism is entirely Cordis: entry contexts chain to the context a subtree was plugged into, and both dsh-tools and dsh-system-prompt file registrations into the calling context's scope layer. Mounting a composition under agent.ctx therefore makes it that agent's alone, and unwinds it with the agent, without any new layering in those registries.

Service: AgentPresets (ctx key: agentPresets)

Discovery is unmemoized: list() and resolve() re-read the roots on every call, so a preset authored while the process runs is visible immediately and a deleted one disappears from the next read.

• ctx.agentPresets.defaultId: string The preset id mounted when a caller names none.
• ctx.agentPresets.list(): Promise<AgentPreset[]> Every preset the configured roots currently supply, earlier root winning a duplicate id.
• ctx.agentPresets.resolve(id?): Promise<AgentPreset> One preset by id, defaulting to defaultId. Throws naming the available ids when no root supplies it.
• ctx.agentPresets.mount(agentCtx, id?): Promise<AgentPreset> Compose one agent from a preset and return the preset that was mounted, for the caller to record.

AgentPreset carries id (the directory name), trust (system or user, from the root it was found under), and path (the absolute composition file).

Where to call mount()

The agent factory's setup(agentCtx) hook is the one supported call site. Only there is the composition installed while the agent is still unpublished, so a rejected mount rolls the whole creation back rather than leaving a half-composed session. The subtree is owned by agentCtx's fiber, so it unwinds with the agent and the caller receives no disposer.

Which preset a session runs

The creation header names the preset a session STARTED with; resolveSessionPreset(session) names the one it RUNS. They differ whenever a blank session switched, so every reconstruction path — the summary a picker reads, a resume, a fork — resolves rather than reading the header.

The header stays frozen because it is a creation fact. A switch is an agent-preset/selected session event appended after the swap commits, which is what the model-visible ⟺ logged rule requires: the preset decides the tool schemas and prompt sections the model sees, so it has to be reconstructable from the log. Reading the header alone would rebuild a switched session under the composition it was created with, replaying history the new tool set cannot act on — the exact hazard the blank-only lock exists to prevent.

Config

Field	Default	Meaning
default	required	Preset id mounted when a caller names none
roots	[]	Scanned directories in precedence order; each supplies path (a leading ~ expands) and trust (defaults to user)

An absent root supplies no presets rather than failing: the user root does not exist until the first locally authored preset, and naming a default no root supplies already fails loud at resolution.

The default preset is a user setting

When a settings provider is composed, this plugin registers the agent-presets namespace with config.default as its composition base, so the user document layers over the deployment's engineering default:

agent-presets:
  default: core-web

The value is read per resolution rather than snapshotted, so a hot-reloaded document takes effect on the next session created and every running session stays on the preset it was composed from. Clearing the user field re-inherits the composition default. A default naming a preset no root supplies is stored without complaint and fails at the next resolve() — the roster is a live directory, so a name absent now may exist by the time a session asks for it.

What a mount rejects

A directly-plugged subtree is absent from ctx.loader.entries(), so no boot audit covers it. mount() therefore proves the result usable itself, and rejects three things.

An unscoped target. Mounting into a context that carries no agent scope would register the preset's tools globally, for every agent in the process.

A row that never became usable. The loader already rejects a row whose module failed to import or whose plugin threw; what remains is a row still waiting for a service the composition never supplies, which the audit names.

A row that published a service into the root realm. Such a service is process-global rather than per-session, so the second session mounting the same preset collides with the first. A preset that genuinely owns a service puts it behind an isolate realm — entry-local for one session's private instance, or a shared label when several sessions should share one — or the service belongs in the host composition instead.

The package invariant re-checks that last rule on every service notification, because a row that publishes from a timer or an asynchronous continuation would escape the one-shot audit.

A preset file is an input, never a persistence target

The Loader writes a tree back to its source file whenever it decides the config changed, and a row disposing its own fiber is enough to decide that: the entry is marked disabled and the tree is written. Inherited, that would burn one session's runtime state into a file every session shares — comments stripped by the YAML round trip, and a writeFile rejection inside a setTimeout for a read-only shipped preset.

The mounted subtree therefore overrides write() as a no-op. Nothing in this package writes a composition; authoring one is a separate, explicit operation.

Trust

Presets are compositions, so a preset is exactly as privileged as the plugins it names. A user preset — authored by a person or by an agent — carries the same trust as shell access; the trust field exists so consumers can present that difference, not to enforce it.

Model Experience

Indirectly, through the plugins a mounted composition registers, which own every tool schema and prompt section the preset makes visible to its one agent.

KV Cache effect

Prefix-stable for the life of an agent: a composition is installed once, before the agent is published and therefore before its first request, and is never re-read while the agent runs. Choosing a different preset for a new session establishes a different prefix for that session alone and cannot invalidate reuse for any session already running.

Known Limitations and Deferred Work

• A preset cannot be changed on a live agent — the mount happens once during creation, so switching a running session's composition would mean unwinding its subtree mid-turn, dropping tools the model may already have called. Changing the default affects only sessions created afterwards.
• Display names are the directory id — a preset carries no manifest, so pickers and settings surfaces show the id until a consumer needs richer metadata.
• Root scans are not watched — every read hits the filesystem instead, which keeps the roster fresh but puts one readdir per root on each list().