# DeepSeek Harness Architecture

This document describes the architecture of the DeepSeek Harness — the foundation of **DeepSeek Code**. The governing principle: **everything is a plugin**. The core is deliberately tiny — a handful of abstract services plus one concrete loop plugin (`dsh-agent-loop`) — and every product feature is a plugin against the extension surface described here, without modifying the loop. The stack is three tiers: plugins (the loop itself, seam implementations, model-facing tools, bridges) over interface/service packages (each owning one `ctx` key and its vocabulary) over the vendored Cordis kernel (`vendor/`).

This document covers **behavior**; type shapes live in [core-data-structures/](core-data-structures/core.md), the per-event/service reference in the generated [events](cordis-catalog/events.md) / [services](cordis-catalog/services.md) catalogs, visual relationship maps in the [documentation graph index](graph-atlas.md), and per-package contracts in the package READMEs ([map](../packages/README.md)).

## Service map

The spine — the product API under `packages/core/`:

| ctx key | Package | Role |
|---|---|---|
| `ctx.sessions` | dsh-session | creates/holds event-sourced `Session`s |
| `ctx.systemPrompt` | dsh-system-prompt | ordered sections + tool schemas → `assemble()` |
| `ctx.tools` | dsh-tools | tool definitions; `execute()` through waterfall |
| `ctx.skills` | dsh-skill | discovers user/project/system skills and adds request-time skill guidance |
| `ctx.agents` | dsh-agent | live `Agent` handles + create/resume factory (returns `AgentHandle { agent, dispose() }`) |
| `ctx.agentLoop` | dsh-agent-loop | THE concrete loop plugin: creates and drives `ReactLoopAgent`s |

The swappable capability seams:

| ctx key | Package | Role |
|---|---|---|
| `ctx.llm` | dsh-llm | adapter registry; `stream()` |
| `ctx.sessionPersistence` | dsh-session-persistence | durable persistence: create/append/load/list |
| `ctx.bash` | dsh-bash | bash execution: foreground runs + background tasks |
| `ctx.fs` | dsh-fs | filesystem provider: read/stream, atomic writes/edits; owns the `fs/*` policy events |
| `ctx.compact` | dsh-compact | compaction: detect pressure, summarize an older range |
| `ctx.web` | dsh-web | search/fetch provider registries + `WebError` taxonomy |
| `ctx.subagents` | dsh-subagent | named provider registry for delegating to child agents |

Dependency rule: plugins depend on these interfaces, never on `dsh-agent-loop` — the loop is swappable; the sanctioned exception is the composition bundle `dsh-agent-core`, whose job is assembling the concrete spine ([full rule + generated graph](../packages/README.md#dependencies)).

All registrations go through `ctx.effect()` and return disposers, so hot-reload and fiber disposal clean up automatically (full service interfaces: the generated [services catalog](cordis-catalog/services.md)).

## Cordis waterfall semantics

`ctx.waterfall` is **around-middleware**, not a value reducer. Each listener receives `(...args, next)`:

- call `next()` to delegate to later listeners (and ultimately the core behavior), possibly wrapping it;
- return a value **without** calling `next()` to short-circuit (veto);
- listeners run in registration order; `prepend: true` jumps the queue.

Composition caveat: values propagate through `next()`'s **return value** — a listener that returns a *new* object makes earlier listeners' mutations invisible downstream. Prefer mutate-then-`next()` for cooperative middleware; return a replacement only to take over the result.

## Capability seams: interface / implementation / consumer

Swappable capabilities split into three packages — **interface** (abstract service + vocabulary, owns the ctx key), **implementation** (a concrete subclass loaded as a plugin), **consumer** (what the model and plugins program against) — so each evolves independently; the bash trio is the template ([capability seams RFC](rfc/implemented/architecture/2026-06-13-capability-seams.md)). Keep interface + consumer together when they are one concern (the LLM seam: `dsh-llm` carries both, adapters implement); don't split preemptively.

Two seams bend the template deliberately:

- **Filesystem** adds a policy layer as an **event gate**, not a method service: `dsh-tool-fs` (the `read`/`write`/`edit` tools AND executor) dispatches `fs/*` intent events that `dsh-fs-policy` decides, so dropping the policy plugin degrades to the bare provider instead of breaking an injection ([event-gate RFC](rfc/implemented/architecture/2026-06-26-file-context-as-event-gate.md)). Paths resolve against the caller's session cwd, matching bash ([per-session cwd RFC](rfc/implemented/architecture/2026-07-02-fs-per-session-cwd.md)).
- **Web** folds search and fetch onto one seam: `ctx.web` is a provider REGISTRY (`registerSearchProvider`/`registerFetchProvider`, registration-order-independent selection); providers register like LLM adapters, and `dsh-tool-web` is the single consumer owning the tool schemas ([web seam RFC](rfc/implemented/architecture/2026-06-24-web-capability-seam.md)).

> The seam pattern is plain Cordis services + `inject` (a consumer's fiber stays pending until the service exists). Despite the name, `@cordisjs/plugin-capability` is unrelated — a permission-security service (a candidate for the deferred permissions work), not a mechanism for swapping implementations.

## Content blocks and streaming (dsh-llm)

Messages are arrays of typed **content blocks** (`text`, `reasoning`, `tool-call`, `tool-result`); the union derives from the merge-extensible `ContentBlockMap`; the same pattern types `MessageSource`, `FinishReason`, `TurnTrigger`, `TurnEndReason`. The core set is limited to blocks every shipping path honors — multimodal content (images, audio, …) has no core block type; a feature that needs one adds it via the map in the same coordinated change that maps it in the adapters, surfaces it in the UI bridges, and prices it in compaction ([the drop-image RFC](rfc/implemented/simplification/2026-07-04-drop-image-content-block.md)). Streaming is a raw chunk protocol (`block-start` … `finish`) with `BlockAssembler` as the single shared chunk→block assembler; the loop logs raw chunks (replay fidelity) while assembling them. `LlmAdapter` is the provider seam: subclass, implement `stream()`, register via `ctx.llm.registerAdapter(models, adapter)`; `dsh-llm-deepseek` and `dsh-llm-pi-ai` implement the one contract as deliberate design twins ([twin RFC](rfc/implemented/architecture/2026-06-13-twin-llm-adapters.md)). The StreamChunk conventions (usage/finish ordering, raw-string tool arguments, the two sanctioned error paths) are pinned in `dsh-llm/src/types.ts` and [llm-streaming.md](core-data-structures/llm-streaming.md).

## Event-sourced sessions (dsh-session)

A `Session` is an append-only log of typed `SessionEvent`s — the single source of truth. The LLM message history is *derived* (`deriveMessages()`): user/assistant messages, tool results, and envelope-tagged context/steering messages come from their events in chronological order (raw `assistant/chunk` events are replay/UI data, skipped; the per-event mapping is in [session.md](core-data-structures/session.md)). Replay/fork = `ctx.sessions.create(id, { seed })`; trace/telemetry = listen to `session/event` ([event-sourcing RFC](rfc/implemented/architecture/2026-06-11-event-sourced-sessions.md)).

**Durability**: `session/event` is a synchronous notification; persistence backends buffer write-behind and drain at the awaited `session/flush` checkpoint at every turn end. The abstract `SessionPersistence` seam defines create/append/load/list over `SessionEvent` (no parallel persisted type); metadata travels as `SessionHeader`; crash recovery preserves an interrupted turn by closing it with a synthetic `turn/end {interrupted}`. Two backends (JSONL, SQLite) pass one shared contract suite ([persistence RFC](rfc/implemented/architecture/2026-06-14-session-persistence.md), [write coordinator RFC](rfc/implemented/architecture/2026-06-18-shared-persistence-write-coordinator.md)). Resume = `ctx.agents.resume({ resumeSessionId })`.

## Prompt assembly (dsh-system-prompt)

Plugins contribute `PromptSection`s (named, ordered, static or computed) and tool-schema providers; `assemble()` returns `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 — though adapters transmit them as the wire-level `tools` field ([RFC](rfc/implemented/architecture/2026-06-11-tool-schemas-in-prompt-assembly.md)).

## Tool pipeline (dsh-tools)

`ToolRegistry.register()` takes schema + `execute()`; schemas flow into the assembly automatically. `execute()` runs through a two-waterfall pipeline — `tools/pre-execute` (a `PreToolDecision`: allow/deny/ask) → core dispatch → `tools/post-execute` (a `PostToolDecision`: accept/block, replace content, attach context) — the seams where sandbox, permission, hook, and plan-mode plugins live. A thrown tool still reaches `post-execute` as an `isError` result.

## Agents (dsh-agent) and the loop (dsh-agent-loop)

`Agent` is the handle every plugin programs against: `send()` (queued), `steer()` (mid-turn injection, drained between steps), `inject()` (in-session context; a one-shot `injection` turn when idle), `cancel()` (the single public stop primitive: clears queued + steering work, aborts the in-flight step, drops a turn about to start), `whenIdle()` (quiescence observation, not teardown), plus `session`/`status`/`options`. A lifecycle owner tears down via `await AgentHandle.dispose()` — stop, await exit, unregister. Full semantics: [core.md](core-data-structures/core.md), [lifecycle RFC](rfc/implemented/architecture/2026-06-18-agent-lifecycle-and-ownership-seams.md).

**Subagents** are a seam, not a method on `Agent`: `ctx.subagents` is a named-provider registry (`spawn` starts fresh, `fork` seeds the child with the parent's completed-turn prefix, ACP drives an out-of-process child); children are ordinary `Agent`s. See [subagent.md](core-data-structures/subagent.md), [subagent RFC](rfc/implemented/feature/2026-06-21-subagent-capability-seam.md).

### Loop lifecycle (session / turn / step)

- **Session**: the whole event log of one agent.
- **Turn**: ≥1 queued message; steps run until the model stops requesting tools and no plugin requests continuation.
- **Step**: one model request + its tool executions.

```
create agent → emit agent/session-start(source)       ⟵ once, before turn 1 (startup|resume)
forever:
  wait for queued messages (idle)
  emit agent/status(running)
  TURN (error-contained — a throwing plugin ends the turn, never the loop):
    'turn/start'                                        ⟵ durable turn boundary (no agent/* mirror)
    each queued msg: waterfall agent/prompt-submit      ⟵ allow (rewrite/+context) | block
      allow → session('user/message'…); inject additionalContext
    every prompt blocked → 'turn/end'(rejected), 0 steps ⟵ zero-step turn, model never called
    STEP loop:
      drain steering (late steering from previous step's listeners)
      assembly = ctx.systemPrompt.assemble()          ⟵ waterfall system-prompt/assemble
      await ctx.serial('agent/pre-step')              ⟵ surface mutation (compaction) OUTSIDE the step
      session('step/start')                           ⟵ durable step boundary (no agent/* mirror)
      req = {model, system, tools, messages: session.deriveMessages(), signal}
      req = waterfall agent/request                   ⟵ hooks, model switch
      stream ctx.llm.stream(req)                      ⟵ waterfall llm/stream (raw chunks)
        session('assistant/chunk')
      if assembler.finish is error/aborted: throw      ⟵ adapter's in-band error path →
                                                         step error (turn ends error/aborted,
                                                         not a normal completed message)
      msg = waterfall agent/step-result               ⟵ runs BEFORE the log append, so the
      session('assistant/message' {content, usage?})     log records what tool dispatch uses
      each tool-call (sequential, abort-checked between calls):
        session('tool/call'); ctx.tools.execute()     ⟵ waterfall tools/pre-execute (allow/
          deny/ask gate) → dispatch → tools/post-execute (accept/block, replace, +context)
          tool execution may append tool-owned session events, e.g. `todo/write`
        session('tool/result')
      append buffered post-execute additionalContext → session('context/message')(s)
                                                         ⟵ after ALL tool/results (adjacency)
      drain steering → session('steering/message')
      session('step/end')                             ⟵ durable step boundary (no agent/* mirror)
      cont = waterfall agent/turn-continuation(default = {action: hadToolCalls||steered
        ? 'continue' : 'stop'}) → ContinuationDecision
      a continue's reason is recorded as next-step steering (same turn); steering pending
        also forces continue (continuation OR step/end listeners — the /goal pattern)
      if action==stop: break
  session('turn/end')                                 ⟵ durable turn boundary (no agent/* mirror)
  await ctx.parallel('session/flush', session)        ⟵ durability checkpoint (failure
                                                         reported via agent/error, not fatal)
  leftover steering re-enqueued as queued messages    ⟵ steering is never stranded
  emit agent/status(idle) unless more queued
```

Error containment: a throwing listener or broken step ends the **turn** (`turn/end { reason: { kind: 'error', step, … } }`), never the driver loop; live diagnostics fire via `agent/error`; an adapter's in-band error/aborted finish chunk becomes a step error. `cancel()` is honored mid-stream and between tool calls; disposal mid-turn ends the turn `disposed`. A post-`turn/end` failure (a rejecting `session/flush`) is reported via `agent/error` only — the turn stays balanced, the backend keeps its buffer.

A turn ends with one `TurnEndReason` — `completed`, `aborted`, `error`, `disposed`, `max-tokens`, `rejected`, or `interrupted`; per-variant semantics (and the max-tokens-wins rule) are in [session.md § TurnEndReasonMap](core-data-structures/session.md#why-a-turn-ended-turnendreasonmap).

**Turn-enclosure invariant**: every session event lives inside a turn, making the turn the single durability/replay boundary — anything after the last `turn/end` is an interrupted-crash tail. `dsh-invariants` enforces it in dev ([invariant RFC](rfc/implemented/architecture/2026-06-15-turn-enclosure-invariant.md)).

## Event taxonomy

The `agent/*` events are declared in `dsh-agent` (so nothing depends on the loop package); each other service declares its own (`tools/*`, `llm/*`, `system-prompt/*`, `session/*`). The full catalog — signatures, dispatch modes, prose — is generated from source and freshness-gated: [cordis-catalog/events.md](cordis-catalog/events.md). Domain semantics (session = the fact log, agent = the live surface): [the event-domain RFC](rfc/implemented/architecture/2026-06-30-event-domain-semantics.md).

## Extension guide

Plugin skeletons (tool, hook/permission gate, UI, protocol bridge) and the feature→mechanism map — which extension seam implements each product feature — live in [the extension cookbook](cookbook/extension-cookbook.md); step-by-step guides: [adding a package](cookbook/adding-a-package.md), [a tool](cookbook/adding-a-tool.md), [an LLM adapter](cookbook/adding-an-llm-adapter.md), [a vendored package](cookbook/adding-a-vendored-package.md).

## Deferred work (TODO)

Designed-for but not implemented: inter-agent channels beyond delegation (shared state, streaming output); the model-facing `/compact` consumer tool over `ctx.compact` ([compaction RFC](rfc/implemented/feature/2026-06-18-compaction-capability-seam.md)); parallel tool execution (concurrency-safety hints on `ToolDefinition`); session branching/tree if seed-based forking proves insufficient.
