deepseek-harness/docs/architecture.md

DeepSeek Harness Architecture

The DeepSeek Harness SDK is an SDK for building agent harnesses on the Cordis framework. The governing principle is simple: everything is a plugin. The shipped agent loop is one plugin in the default bundle, not a privileged kernel.

Read this page as the system map before changing packages/. It explains how the runtime is shaped, how the default loop moves work, where state lives, and where extensions attach. Type shapes live in core-data-structures/; exact event and service signatures live in the generated events and services catalogs; package contracts live in the package map; rationale lives in the RFCs. New to Cordis? Start with the Cordis primer.

System Shape

A running harness is one Cordis context. Packages contribute service keys, typed events, and disposable registrations to that context. Services are the stable call surfaces (ctx.llm, ctx.tools, ctx.sessions); events are interception and notification points (agent/request, tools/pre-execute, session/event); registrations install prompt sections, tool schemas, providers, adapters, and listeners.

The default distribution is a composition, not a hierarchy. packages/core/ is a repository grouping for the default agent spine; capability seams around it are equally first-class plugins.

Default Service Spine

ctx key	Package	Role
ctx.sessions	dsh-session	in-memory event-sourced sessions
ctx.systemPrompt	dsh-system-prompt	ordered prompt sections, tool schemas, and prompt variables
ctx.tools	dsh-tools	tool registry and execution pipeline
ctx.agents	dsh-agent	live agent registry, public Agent handle, agent/* vocabulary
ctx.agentLoop	dsh-agent-loop	shipped ReactLoopAgent driver

Capability Services

ctx key	Package family	Role
ctx.llm	llm/	adapter registry and streaming model calls
ctx.bash	bash/	foreground/background command execution
ctx.codeRuntime	code-runtime/	model-written program execution
ctx.fs	fs/	filesystem provider primitives and policy events
ctx.web	web/	search/fetch provider registries
ctx.compact	compact/	session-surface compaction
ctx.subagents	subagent/	named delegation providers
ctx.sessionPersistence	session-persistence/	durable storage for session logs

Event Surface

Events are the harness extension API. Each service owns the vocabulary for the behavior it controls, and the generated events catalog is the exhaustive reference. The producer/consumer map shows which packages emit or listen to each event.

Event Domains

Pick the event domain for new behavior:

• Session events are durable, replayable facts. Turn and step boundaries, user input, assistant output, tool calls, tool results, steering, compaction records, and tool-owned durable facts append to the session log and flow through session/event.
• Agent events are live runtime surfaces. They carry the live Agent handle for status, diagnostics, prompt admission, call-config shaping, result validation, and continuation policy.
• Capability events belong to the seam that owns the action. tools/*, llm/*, system-prompt/*, fs/*, and subagent/* let policy and adapters attach without importing the loop.

Interception Semantics

Waterfall events behave like around-middleware: a listener delegates by calling next(); returning without it vetoes or takes over. Full rule: Cordis waterfall semantics.

Default Loop Lifecycle

The shipped loop drains queued work, assembles a request, streams a model answer, executes tools, decides whether to continue, and checkpoints durable state. The important architecture is where it pauses: each pause is a documented service call or event seam other plugins program against.

A session is one agent's append-only event log. A turn drains one queued batch and runs until the model stops asking for tools and no plugin requests continuation. A step is one model request plus the tool executions caused by that response. In the flow below (sequence companion), quoted names are durable session events and event names are extension seams.

Turn Flow

create agent -> emit agent/session-start(source)
forever:
  wait for queued messages
  emit agent/status(running)
  TURN:
    'turn/start'
    each queued message -> agent/prompt-submit
      allowed prompt -> 'user/message' plus injected context
    every prompt blocked -> 'turn/end'(rejected)
    STEP loop:
      drain steering
      assemble system prompt and tool schemas
      agent/session-prefix (first step)
      agent/pre-step
      'step/start'
      snapshot the derived messages (the reconstruction boundary)
      agent/request (config only) -> log request/header -> llm/stream (frozen)
        'assistant/chunk'
      agent/step-result
      'assistant/message'
      each tool call:
        'tool/call'
        tools/pre-execute -> tools/execute -> tools/post-execute
        'tool/result'
      append post-tool context and steering
      'step/end'
      agent/turn-continuation
      stop unless tools or continuation policy ask for another step
    'turn/end'
    checkpoint persistence and notify idle/running status

Prompt assembly is single-path: renderPrompt(assemble({ agent })) IS the system prompt sent to the model. Plugins contribute ordered sections (static or computed from the per-call AssembleContext), tool schemas, and named variables interpolated as {{name}} at render — strictly, so an unknown or valueless reference fails the turn instead of shipping a hole. dsh-system-prompt owns the openers — the static harness:identity section (order −100) and the deployment's persona (order 0, its persona config, shared context-wide) — while the shipped loop registers the model/cwd variables; prompt-fact ownership is pinned by the prompt-variables RFC.

Post-tool context lands after all tool results so tool-call/result adjacency stays stable. Steering drains between steps; leftover steering after a turn is re-queued as ordinary input.

Failure Boundaries

The turn is the containment boundary. A throwing listener, adapter error finish, or failed step ends the current turn with an error reason and reports live diagnostics through agent/error; it does not kill the driver loop. cancel() clears queued and steering work, aborts the active model/tool boundary when possible, and records the appropriate turn end. Disposal stops the loop, awaits quiescence, unregisters the agent, and lets service disposers drain.

Every session event is turn-enclosed. Reloading a crashed session preserves the interrupted tail and closes it with a synthetic interrupted turn end. A failure after the durable turn has closed reports through agent/error only because no safe in-turn position remains. A turn ends with one TurnEndReason (completed, aborted, error, disposed, max-tokens, rejected, or interrupted); per-variant semantics are in session.md § TurnEndReasonMap.

Agent Handles

ctx.agents owns live agents and returns an AgentHandle { agent, dispose() }. Agent is the surface other plugins drive: send() queues work, steer() injects mid-turn content, inject() appends context and opens a one-shot injection turn when idle, cancel() is the public stop primitive, and whenIdle() observes quiescence. Lifecycle owners tear down with await dispose().

State And Model Surface

Session Log

The session log is the source of truth. deriveMessages() projects session events into the Message[] sent to the model; raw assistant/chunk events stay in the log for replay and UI fidelity. Replay, fork, resume, transcript rendering, telemetry, and persistence all derive from the same event stream.

Model-visible ⟺ logged: the log reconstructs every request — messages at step/start fronted by the header's session prefix, headers by folding request/header — and dev invariants assert this (reconstructability RFC).

Durability is a plugin concern. Persistence backends buffer synchronous session/event notifications and the loop awaits a turn-end checkpoint before moving on. The SessionPersistence seam stores SessionEvent directly, with metadata in SessionHeader; JSONL and SQLite share one contract suite.

Model Content

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, and TurnEndReason. New block types are coordinated across adapters, UI bridges, compaction pricing, and persistence, so block vocabulary remains a repo-wide contract.

Streaming is a raw chunk protocol (block-start through finish) with BlockAssembler as the shared chunk-to-block assembler. The loop logs raw chunks while assembling them for dispatch. LlmAdapter is the provider seam: subclass, implement stream(), and register with ctx.llm.registerAdapter(models, adapter). StreamChunk conventions live in llm-streaming.md.

Extension And Composition

Capability Pattern

A swappable capability usually splits into interface / implementation / consumer: the interface owns the ctx key and vocabulary; an implementation registers a backend; a consumer exposes model-facing behavior through ctx.tools or prompt assembly. The bash trio is the reference shape, and the capability seam graph shows the package families.

Some seams bend the template deliberately. LLM keeps interface and consumer vocabulary together because adapters are the implementations. Filesystem adds policy as event gates around provider primitives. Web is one service with search and fetch provider registries, so provider swaps do not rename model tools. Subagents use a named provider registry because multiple delegation backends can coexist; spawn starts fresh, fork seeds from the parent's completed-turn prefix, and ACP can drive an out-of-process child (subagent.md).

Bundles And Apps

dsh-agent-core is the default composition bundle: one plugin loading the providerless spine as code (README). App packages compose it with a front door and own the boot bin: dsh-stdio-agent for the terminal REPL, and dsh-acp-agent for ACP over JSON-RPC stdio with no stdout logger (ui/). A deployment is a thin cordis.yml leaf: swappable backends, one app entry, and optional product tools (examples/, runnable wirings, graph atlas).

Where New Behavior Goes

New behavior should attach to a documented seam; changing the shipped loop requires updating this map.

Goal	Mechanism
Add a model provider	register an adapter on ctx.llm
Add a model-facing capability	register a tool on ctx.tools; schemas flow into prompt assembly
Add command execution	implement and register a ctx.bash backend
Add filesystem access or policy	implement a ctx.fs provider or listen on fs/* policy events
Intercept prompts, requests, tool use, or continuation	listen on the relevant agent/* or tools/* waterfall
Add a session-stable request prefix outside history	compose it on agent/session-prefix, once per loop instance; logged on the request header
Add UI or editor integration	drive ctx.agents and render from session/event
Add durable session state	add a SessionEventMap member and render/replay from the log
Fork a live session	use ctx.sessions.fork(source, boundary?, childSessionId?)

The extension cookbook carries plugin skeletons and the feature-to-seam map; step-by-step guides cover packages, tools, LLM adapters, and vendored packages.