This document describes the phase-1 architecture of the DeepSeek Harness — the foundation of **DeepSeek Code**. The governing principle, from the [microkernel design discussion][microkernel-doc], is:
The harness core is deliberately tiny: a handful of abstract services plus one concrete loop plugin (`dsh-agent-loop`). Every product feature — tools, hooks, compaction, sandboxing, UI, persistence, sub-agents, MCP, skills — is meant to be written as a plugin against the extension surface described here, without modifying the loop.
For a catalog of the **data structures** this architecture moves around — the core vocabulary types, their literal shapes, and the seam types grouped by capability — see [core-data-structures/](core-data-structures/core.md). This document covers behavior; that one covers the types.
Dependency rule: **extension** plugins depend on interface packages, never on `dsh-agent-loop`. The loop itself is swappable — UI/hook/tool plugins keep working against the `dsh-agent` vocabulary if the loop is replaced. The one sanctioned exception is a **composition/bundle** package whose job IS to assemble the concrete spine: `dsh-agent-core` bundles `dsh-agent-loop` (and the other concrete spine plugins) by design, so it depends on the concrete loop on purpose. The rule constrains plugins that EXTEND the system, not the bundle that COMPOSES it — swapping the loop means publishing a different bundle, not rewiring every extension.
| `ctx.compact` | `CompactService` (abstract) | dsh-compact | compaction seam: decide when history is too large, summarize an older range into a single surface node |
All registrations (`registerAdapter`, `section`, `tools`, `register`, …) go through `ctx.effect()` and return disposers, so plugin hot-reload (vendored HMR) and fiber disposal clean up automatically.
For each service's full public interface (every method signature, generated from source), plus the inherited cordis-core/loader/hmr/timer surface a plugin also sees, see the `## Services` section of [cordis-catalog/events-and-services.md](cordis-catalog/events-and-services.md). This table is the at-a-glance role summary; that catalog is the exhaustive reference.
Swappable capabilities are split into **three packages** so each part evolves independently. The bash capability is the template:
1.**Interface** (`dsh-bash`) — an abstract service plus the vocabulary types (`BashExecutor`, `BashRunResult`, `BashTask`, …). Defines the contract, owns the `ctx.bash` key, depends only on cordis.
2.**Implementation** (`dsh-bash-local`) — a concrete subclass loaded as a plugin (local subprocesses, process-group kills, spill-file truncation). Sandboxed, containerized, or remote backends are sibling packages implementing the same interface.
3.**Consumer** (`dsh-tool-bash`) — what the model and other plugins program against (the `bash`/`bash_output`/`bash_kill` tool schemas). Consumers `inject` the interface's ctx key and never import implementation types.
The LLM seam has the same topology folded differently: `dsh-llm` carries the interface (`LlmAdapter`) AND the consumer surface (`ctx.llm.stream()`), with adapters as implementation packages — there the consumer is the loop itself, not a swappable schema surface. Use the full three-package split when the consumer is independently replaceable; keep interface + consumer together when they are one concern. Don't split preemptively: a capability with one conceivable implementation and one consumer stays one package until proven otherwise.
The web capability uses the same three-package split but folds two capabilities onto one seam: `dsh-web` owns the abstract `ctx.web` service, which is a provider REGISTRY (`registerSearchProvider`/`registerFetchProvider`, registration-order-independent selection, the `WebError` taxonomy) rather than a single backend. Providers register capabilities, not tools — `dsh-web-search-exa`, `dsh-web-search-perplexity`, and `dsh-web-fetch-local` each register into `ctx.web` the way an `LlmAdapter` registers into `ctx.llm`, so they are namespace plugins (`inject: ['web']`), not key-owning services. `dsh-tool-web` is the single consumer that owns the model-facing `web_search`/`web_fetch` schemas, prompt sections, and presentation; it reads only the aggregated `ctx.web.searchStatus()`/`fetchStatus()` and executes through `ctx.web.search()`/`fetch()`, so provider selection has one owner. Search and fetch are deliberately one seam (one thing to inject and configure, one selection policy, one abort/error vocabulary) despite sharing no request schema — see the [web capability seam RFC](rfc/implemented/architecture/2026-06-24-web-capability-seam.md).
> **"Capability" — two unrelated meanings.** (1) The *seam pattern* above ("one plugin provides a capability, another needs it") is realized by plain Cordis **services + `inject`**: a provider registers a service (`ctx.bash`, declared in `interface Context`); a consumer declares `inject: ['bash']` and its fiber stays pending until the service exists, tearing down via HMR if it later vanishes. No extra library is needed. (2) `@cordisjs/plugin-capability` is a different axis entirely — a **permission/capability-security** service (named permissions with inheritance/dependency, tested against a session via `ctx.capability.test`). It is a candidate for the deferred permissions/sandbox work (the `tools/execute` veto seam), NOT a mechanism for swapping implementations.
Messages are arrays of typed **content blocks** (`text`, `reasoning`, `tool-call`, `tool-result`, `image`); the union is derived from the merge-extensible `ContentBlockMap`, so plugins can add block types via declaration merging. The same merge-extensible-map pattern is used for `MessageSource`, `FinishReason`, `TurnTrigger`, and `TurnEndReason` — typed sum types instead of strings.
Streaming is a raw chunk protocol (`block-start`, `text-delta`, `reasoning-delta`, `tool-call-delta`, `block-end`, `usage`, `finish`). `BlockAssembler` is the single shared implementation that assembles chunks into blocks/messages; the loop logs raw chunks (replay fidelity) while feeding the same chunks through an assembler.
`LlmAdapter` is the provider seam: subclass, implement `stream()`, call `ctx.llm.registerAdapter(models, adapter)`. Two real adapters implement it — `dsh-llm-deepseek` (hand-rolled fetch/SSE against the DeepSeek API) and `dsh-llm-pi-ai` (the same endpoint through the `@earendil-works/pi-ai` library). They exist as a pair deliberately: two independent internals over one contract verified the StreamChunk protocol, which is now documented (in `dsh-llm/src/types.ts`) with the conventions that review pinned down — usage before finish, nothing after finish, raw-string tool arguments, and the two sanctioned error paths (thrown vs `finish {kind:'error'}`).
A `Session` is an append-only log of typed `SessionEvent`s — the single source of truth. The LLM message history is *derived* from the log (`deriveMessages()`):
-`assistant/message` → assistant message (raw `assistant/chunk` events are replay/UI data and are skipped in derivation; an empty-content `assistant/message`, which exists only to host a max-tokens step's `usage`, is skipped too)
-`context/message`, `steering/message` → user-role messages wrapped in a tagged envelope (`<context source="…">…</context>`) at their chronological position — the "system-reminder" pattern; models distinguish them from real user prompts by the envelope. **TODO(review)**: the real adapters now exist (the original precondition); the envelope still wants a deliberate review against live model behavior (`TODO(review)` in dsh-session).
**Durability seam**: `session/event` is a synchronous notification; persistence plugins buffer (write-behind) and drain at the awaited `session/flush` checkpoint the loop fires at every turn end. The durable backend is a real **capability seam**: the abstract `SessionPersistence` service (`dsh-session-persistence`, `ctx.sessionPersistence`) defines create/append/load/list over the existing `SessionEvent` (no parallel persisted type), and `dsh-session-persistence-jsonl` is the first implementation — an append-only JSONL log per session with crash-safe atomic writes, crash recovery that PRESERVES an interrupted turn (closing it with a synthetic `turn/end {interrupted}` rather than truncating — a turn can be huge), and a read/replay path. Session metadata (format version, cwd, lineage) travels separately as `SessionHeader`, attached to a `Session` via `session.header`. Resuming a persisted session into a live agent is `ctx.agents.resume({ resumeSessionId })`. A second backend, `dsh-session-persistence-sqlite` (`node:sqlite`, one row per `SessionEvent` — the row shape `(session_id, seq, type, time, data)` maps 1:1 onto it), passes the same `runPersistenceContract` suite, proving the seam is genuinely backend-agnostic.
Plugins contribute `PromptSection`s (named, ordered, static or computed) and tool-schema providers. `assemble()` returns a `PromptAssembly { sections, tools }` through the `system-prompt/assemble` waterfall.
Tool schemas are deliberately **part of the assembly**: "what the model is told it can do" is one coherent thing managed here, even though adapters transmit schemas as the wire-level `tools` field rather than prompt text.
`execute()` runs through the **`tools/execute` waterfall** — the single seam where sandbox, permission, hooks, and plan-mode plugins wrap or veto a call. This collapses Claude Code's validate → PreToolUse → permission → execute → PostToolUse pipeline into ordered waterfall listeners.
**TODO**: tool shapes get revisited now that real tools exist (the bash suite landed; the `TODO(review)` in dsh-tools is still open) — e.g. a concurrency-safety hint for parallel execution; phase 1 executes tool calls sequentially.
-`inject(content)` — in-session context (`context/message` event); the next request sees it (Claude Code attachment / system-reminder analog). An inject made while the agent is *running* joins the open turn; an inject while *idle* is wrapped in a one-shot turn (`turn/start{trigger:injection}` → `context/message` → `turn/end`) so every event stays turn-enclosed (see [the turn-enclosure invariant](rfc/implemented/architecture/2026-06-15-turn-enclosure-invariant.md)).
-`cancel(reason)` — the single public stop primitive: clears queued + steering work, aborts the in-flight step, and drops a turn about to start (the pre-step window) so a queued-but-not-started prompt never runs and cannot be batched into the cancelled turn. A UI/ACP `session/cancel` maps to it.
-`whenIdle()` — resolves once the agent reaches quiescence after settling out of `running` (resolves immediately when already idle; awaits the loop exit when disposed). A non-owner's quiescence-observation hook: it lets a consumer await the current work settling **without** disposing the agent. It is NOT teardown — it does not stop queued work, unregister the agent, or detach the session; a lifecycle owner tears an agent down with `await AgentHandle.dispose()` (which stops the loop, awaits its exit, and unregisters).
**Subagents**: `spawn`/`fork` are realized by the [`@deepseek-ai/dsh-subagent`](../packages/subagent/subagent) seam (a named-provider registry on `ctx.subagents`), not a method on `Agent`. The in-process backends create the child via `ctx.agents.create` — fork seeds the child Session with a balanced completed-turn prefix of the parent's log (`CreateAgentOptions.seed`), spawn starts fresh; children are ordinary `Agent` handles so `steer()` and event subscription work uniformly. Out-of-process transports (ACP, and later A2A / Codex app-server / Claude Code SDK) register as sibling providers. See [docs/core-data-structures/subagent.md](core-data-structures/subagent.md) and [the subagent RFC](rfc/implemented/feature/2026-06-21-subagent-capability-seam.md). Inter-agent channels beyond delegation remain deferred.
Error containment: a throwing `agent/turn-continuation` listener or a broken step ends the **turn** with `turn/end { reason: { kind: 'error', step, message, code? } }` — the failure's step number rides on the durable turn reason (there is no separate session `error` event); live diagnostics fire via `agent/error`. Never the driver loop. An adapter that ends its stream with a `finish {kind:'error'}` or `{kind:'aborted'}` chunk (the in-band error path, for adapters that can't throw mid-stream) is likewise translated into a step error, so the turn ends `error`/`aborted` instead of logging a normal `completed` assistant message. A `cancel()` is honored mid-stream **and** between tool calls; disposal mid-turn ends the turn with reason `disposed` and emits `agent/status('disposed')`.
Turn-end reasons: a turn ends with one `TurnEndReason` — `completed`, `aborted`, `error`, `disposed`, or `max-tokens`. `max-tokens` mirrors the model-call `FinishReason` of the same name (DeepSeek's `length`): a step that hit the output-token ceiling makes the turn end `max-tokens` rather than `completed`, by the rule *any `max-tokens` step in the turn surfaces as `max-tokens`* (a continuation plugin may run further steps after one, but the cut-short fact wins; the `disposed`/`aborted`/`error` outcomes still take precedence). This lets a consumer distinguish a clean stop from a truncated one (the ACP bridge maps it to the `max_tokens` stop reason). `TurnEndReason` is merge-extensible; `refusal` and `max_turn_requests` are the next variants to add when an adapter/loop first emits them.
A failure that happens once the turn is already closed has no in-turn position for a turn-end error reason (the turn already ended). So a rejecting `session/flush` (the post-`turn/end` durability checkpoint) and a throwing `agent/turn-end` listener are reported via `agent/error` + the logger only, NOT as a session event; the turn stays balanced and the persistence backend keeps its buffered events for the next flush.
**Turn-enclosure invariant**: every session event lives inside a turn (between a `turn/start` and its `turn/end`). The loop appends queued `user/message` events *after*`turn/start`, and an idle `agent.inject()` wraps its `context/message` in a one-shot `injection` turn. This makes the turn the single durability/replay boundary: a persistence backend can treat anything after the last `turn/end` as an interrupted-crash tail without risking the loss of legitimately-recorded between-turn context. The `dsh-invariants` plugin enforces it in dev (a message event outside an open turn throws). See [the turn-enclosure invariant](rfc/implemented/architecture/2026-06-15-turn-enclosure-invariant.md).
The `agent/*` events are declared in `@deepseek-ai/dsh-agent` (so nothing depends on the loop package); each other service declares its own events (`tools/*`, `llm/*`, `system-prompt/*`, `session/*`). The full catalog — every event's exact signature, dispatch mode, and prose — is **generated from source** and lives in [cordis-catalog/events-and-services.md](cordis-catalog/events-and-services.md) (the `## Events` section), alongside the `ctx.<key>` service interfaces. That file is regenerated by `scripts/gen-cordis-catalog.ts` and frozen by the `verify-cordis-catalog` freshness gate (part of `doc-sync`), so it cannot drift from the `interface Events` declarations.
Composition caveat: values propagate through `next()`'s **return value**. Mutating the passed-in object works when later listeners receive the same reference, but a listener that returns a *new* object makes earlier mutations invisible downstream. Prefer mutate-then-`next()` for cooperative middleware; return a replacement only when you mean to take over the result.
| Context compaction (auto + manual) | the `ctx.compact` seam ([dsh-compact](../packages/compact/compact)): a backend summarizes an older surface range into a single `user/message``replace` op, bracketed by log-only `compact/*` events; auto = check token pressure at turn boundaries, manual = a `/compact` tool. See the [compaction capability-seam RFC](rfc/proposed/feature/2026-06-18-compaction-capability-seam.md) |
Code skeletons for the three plugin shapes (tool, hook/permission-gate, UI) and the two runnable example wirings live in [docs/cookbook/extension-cookbook.md](./cookbook/extension-cookbook.md). Step-by-step guides: [adding a package](./cookbook/adding-a-package.md), [adding a tool](./cookbook/adding-a-tool.md), [adding an LLM adapter](./cookbook/adding-an-llm-adapter.md), [adding a vendored package](./cookbook/adding-a-vendored-package.md).
- **Compaction implementation** (auto thresholds, summarization prompts) on the `agent/request` seam, with its session-event types added by declaration merging.
