Yichen Jiang
18fe174897
A preset is a directory holding one `agent.cordis.yml`. Mounting it under an agent's scope context during `setup(agentCtx)` gives that one session its own tools and prompt sections while every other live session keeps its own. No registry gains a tier. `dsh-tools` and `dsh-system-prompt` already file registrations into the calling context's scope layer, and entry contexts chain to the context a subtree was plugged into, so a composition mounted under `agent.ctx` is that agent's alone and unwinds with it. The mount audits itself because a directly-plugged subtree is absent from `ctx.loader.entries()` and no boot audit covers it. It rejects an unscoped target, a row that never became usable, and a row that published a service into the root service realm — that last one is process-global rather than per-session, and its collision with the next session surfaces as an unhandled rejection `setup` never observes, leaving a half-composed agent that looks healthy. The package invariant re-checks that rule on every service notification, since a row publishing from a timer would escape a one-shot audit. Raises the `packages/README.md` word ceiling from 920 to 980: the group table must enumerate every group, and the new `preset/` row is necessary content. Design: .agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md
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: stringThe 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 todefaultId. 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.
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.
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.
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.
isolaterealms cannot be expressed across rows withoutcordis:group— an entry-local realm works on a single row, but grouping a provider with its consumers under one shared realm needs the group builtin, whichdsh-app-bootdoes not register.- Root scans are not watched — every read hits the filesystem instead, which keeps the roster fresh but puts one
readdirper root on eachlist().