Host runtime assembly for dsh: bootHost composes the core plugin spine (LLM service + DeepSeek adapter, sessions with JSONL persistence and immediate fallback titles, optional first-message model summaries, system prompt, tools, agents, agent loop, workspace instructions, local bash, and the provider-neutral user-interaction service), createApiProxy implements the dsh-host-apiproxy contract over that composition, and startHost is the one-step shell seam returning { api, handler, defaults, ctx, dispose }.

Which plugins mount and with what defaults is decided only here — shells must not ctx.plugin to alter the assembly. RunningHost.ctx is a formal seam with exactly two sanctioned uses: mounting protocol front-door plugins (e.g. a future dsh acp) and headless session-event subscription; consuming clients must not bypass api through it.

Configuration

Key	Default	Contract
`persistenceRoot`	(required)	Root directory for JSONL session persistence.
`workspaceContext`	(required)	`AGENTS.md`/`CLAUDE.md` loader config with an explicit `maxBytes`, or `false` to disable it.
`provider`	`'deepseek'`	Default provider route injected as agentOptions on create/resume and reported by `host.describe`.
`model`	`'deepseek-v4-flash'`	Default model id, same single source as `provider`.
`cwd`	`process.cwd()`	Default project directory for a session whose create request omits `cwd`.
`sessionTitle`	5 words / 40 fallback bytes / 80 accepted bytes	Deterministic fallback and accepted-title limits.
`sessionTitleLlm`	disabled	`true` enables the 5-word / 10-CJK-character, 4,096-input-byte, 64-output-token, 60-second first-message policy; an explicit config overrides it. An omitted route inherits the logged main-request provider and model.

ApiProxy implementation notes

Unary methods take the narrow RpcRequest<P> and echo request.rpcId; a prompt's rpcId rides MessageSource into the user/message event so clients can promote optimistic echoes. history/prompt on a cold session implicitly resume it, deduplicating concurrent calls through an in-flight table; history paginates backwards on message boundaries (never mid-message). The mux stream replays a session/subscribed baseline per attached session and every still-pending question with its original rpcId. Question responses, including blank per-item answers, are validated against the owning session and exact request before an atomic first-wins claim; answer, whole-request cancellation, owner abort, and provider disposal broadcast question/resolved. The host stream carries session lifecycle, running flips, and agent/error as the only outlet for live failures with no turn position.

The Web front door installs the shared installAgentLlmTarget coupling for every created or resumed agent. A session starts from the latest logged request/header route when one exists, otherwise from the Host default. session.models discovers every registered provider concurrently, keeps successful groups when another provider fails, and inserts the current target as an unlisted row when its provider omits it. A selection changes the mutable session target immediately; prompt assembly snapshots it atomically with request routing, so a change during a running step first applies to the next assembled step.

Model Experience

Indirectly, through the non-blocking first-message title request owned by dsh-session-title-llm when sessionTitleLlm is enabled, the provider/model defaults injected into created and resumed agents, the other model-facing plugins bootHost mounts, and the logged workspace-instruction prefix when workspaceContext is enabled.

KV Cache effect

No main-request invalidation; when enabled, the auxiliary title request has its own cache behavior and leaves the conversation prefix unchanged.

Known Limitations and Deferred Work

Question waits are process-memory state — browser reconnects recover them, but a host process restart aborts the owning tool call instead of restoring the wait from persistence.
host.describe.version is a placeholder — it does not yet report the apps/cli package version.
The assembly is fixed — per-deployment plugin selection (user profile, log sinks, alternative persistence) has a documented home here but no configuration surface yet.