Host runtime assembly for dsh: bootHost composes the core plugin spine (LLM service + DeepSeek adapter, sessions with JSONL persistence, a derived SQLite FTS session-query index, immediate fallback titles, optional first-message model summaries, system prompt, tool and agent registries, agent loop, workspace instructions, local bash, the generic tool-timeout and 50,000-byte spill policies, and the provider-neutral user-interaction service), and startHost is the one-step shell seam returning { api, handler, defaults, ctx, dispose } (its api comes from dsh-host-apiproxy's createApiProxy over that composition).

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 logs and the derived `session-query.db` SQLite FTS index.
`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.

Model Experience

Optional session-query consumer

What the model sees

The derived ctx.sessionQuery index is not model-facing. bootHost intentionally leaves the optional dsh-tool-session-query consumer unmounted, so main host agents receive neither its prior-history prompt section nor its five schemas by default.

Token effect

The index adds no prompt or schema tokens. A custom composition that mounts the consumer owns its added prompt, schemas, calls, and results.

KV Cache effect

The index alone does not change the reusable model-request prefix; mounting the optional consumer would add its stable prompt and schema prefix.

Workspace instructions

What the model sees

When workspaceContext is enabled, the model receives the logged workspace-instruction prefix and loaded file contents, bounded by that configuration's explicit maxBytes. Setting workspaceContext: false removes this surface.

Token effect

Disabled mode adds no tokens. Enabled mode adds the frozen data-dependent baseline to each request, up to the configured byte budget; later discovered, changed, or removed instructions append bounded history messages.

KV Cache effect

Prefix-stable within one loop instance because its baseline is frozen. A new or resumed instance recomposes the baseline, while touch-discovered changes during an instance append after the reusable prefix.

First-message title auxiliary request

What the model sees

When sessionTitleLlm is enabled, a separate first-message title model receives the shared title instruction and a JSON array containing only the first eligible human message. It uses an explicitly configured route or inherits the exact logged main-request route; this auxiliary request does not add text to the main model request or delay its response.

Token effect

Disabled by default, so it normally adds no model request. With sessionTitleLlm: true, a fresh non-fork session makes at most one automatic request capped at 4,096 input bytes and 64 output tokens; an explicit configuration supplies its own caps. The deterministic fallback remains when the auxiliary request fails.

KV Cache effect

The main conversation prefix is unchanged. The auxiliary request has independent, provider-specific cache behavior; the host does not promise cache reuse.

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.