Harnesses are [Cordis](cordis-primer.md) contexts; packages contribute services, typed events, and disposable registrations. `packages/core/` groups the default flow; capabilities remain plugins.
| `ctx.agentDefaultModel` | [`dsh-agent-default-model`](../packages/core/agent-default-model/README.md) | Settings-backed model selection shared by Agent front doors |
| `ctx.credentials` | [`credentials/`](../packages/credentials/README.md) | named secret references resolved per operation, never inlined in configuration |
Waterfalls are around-middleware: listeners delegate with `next()`; returning without it short-circuits or takes over ([semantics](cordis-primer.md#cordis-waterfall-semantics)).
A **session** is append-only. A **turn** claims one queued follow-up, waits for its predecessor's checkpoint, and may share its `running` interval ([decision](../.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.md)); injection claims none. A **step** is one model request plus tools. Fresh creation and persisted resume first acquire an exact unpublished `SessionPreparation`; Agent and session publication happen only after private setup against that Session is ready ([decision](../.agents/notes/implemented/architecture/2026-08-05-session-preparation.md)). Quotes in the [sequence](agent-lifecycle.md) mark durable events.
Creation without an id mints `<config-id>-session-<uuid>`; `sessionId` resumes or creates, while `resumeSessionId` requires history. Resume restores lineage and delegation depth before publication; setup failure emits `agent-loop/config-start-failed`.
Each step assembles ordered prompt sections, tool schemas, and variables; unknown references fail the turn. `dsh-system-prompt` owns identity and persona; the loop supplies `provider`, `model`, and `cwd` ([prompt ownership](../.agents/notes/implemented/architecture/2026-07-05-prompt-variables-and-tool-guidance-ownership.md)).
`inject()` queues non-waking `next-step` context; an idle driver leaves it pending until `followup()` or `steer()` wakes the driver. Post-tool `additionalContexts` use the same inbox. The `agent/pre-step` payload carries the exclusive claimed batch and the upcoming turn, step, and signal. Reject opens no step; enter supplies the complete batch appended after `step/start`. Empty tool continuations still traverse the waterfall, whose final value settles all rewrites.
Other failures use `agent/error`; cancellation and disposal beat recovery. Before request-header commit, the turn signal cancels capability preparation; undispatched tools get synthetic `tool/call`/`ABORTED_BEFORE_DISPATCH` pairs. Effective `cancel(cause)` reports its cause before clearing and aborting; idle calls emit nothing. Waking input that lands after the abort fires but before convergence runs at the driver's convergence boundary, while a `disposed` cancel leaves it parked ([cancel-convergence wake latch](../.agents/notes/implemented/bug-fix/2026-08-07-cancel-convergence-wake-latch.md)). Durability distinguishes `aborted` cancellation from `disposed` teardown, which awaits quiescence ([decision](../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md)).
Turn and step events are turn-enclosed; the loop appends `user/message` events only from entered batches inside a turn. A turn opens before the initial claim and pre-step, so rejection, empty input, cancellation, or failure closes a durable turn without any step events. Standalone `compact/* { turn: null }` events consume no turn, and their lock-time markers may interleave with inbox splices. Reload synthesizes interrupted turn ends; `session/end-seed` distinguishes stale compaction orphans from live locks. After close, only `agent/error` reports failures. Each turn has one [TurnEndReason](subsystems/session.md#why-a-turn-ended-turnendreasonmap).
`ctx.agents` owns agents and returns `AgentHandle { agent, dispose() }`. Plugins use `send()` or its `followup()`, `steer()`, and `inject()` presets. `cancel()` and `whenIdle()` control lifecycle, while awaited disposal owns teardown. A follow-up `MessageId` follows durable inbox insertion, claiming, and discard notifications, not prompt output or turn ending; only an owner of a whole activity interval may summarize it as a run result ([decision](../.agents/notes/implemented/architecture/2026-07-30-followup-enqueue-and-owned-runs.md)).
The session log is authoritative. `deriveMessages()` projects model history; raw `assistant/chunk` events preserve replay and UI fidelity. Fork, resume, transcript rendering, telemetry, and persistence derive from this stream.
**Model-visible ⟺ logged**: messages entering at `step/start` plus the folded `request/header` reconstruct every request. The header marks adapter defaults so later proposals discard them and re-resolve the route without losing explicit settings. `request/context` separately records registration-bound provider, model, and capacity metadata when the route changes; it does not participate in request reconstruction or header equality. `dsh-agent-loop/invariant` asserts reconstructability through `ctx.invariants` ([reconstructability](../.agents/notes/implemented/architecture/2026-07-05-reconstructable-requests.md)).
Durability is a plugin concern. Backends copy synchronous `session/event` notifications into fixed-window durable batches; `session/flush` bypasses the wait before requests and top-level tool dispatch, and after `turn/end` before another turn or idle. `SessionPersistence` stores events and header metadata; JSONL defaults to checksummed Zstandard and SQLite shares the contract ([checkpoint decision](../.agents/notes/implemented/bug-fix/2026-07-21-semantic-session-checkpoints.md), [batching decision](../.agents/notes/implemented/architecture/2026-08-08-bounded-session-persistence-write-batching.md)).
Between turns, owners append log-only events through `Session`, flushing only for durability. `session/title` relies on bounded background persistence and lifecycle drains; manual compaction flushes its bracket before the operation completes. Title work never delays responses; the latest title event wins, and it records the source message seqs and whether the user, fallback, or provider supplied it. Title records are inherited fork boundaries ([decision](../.agents/notes/implemented/feature/2026-07-21-log-backed-session-titles.md)).
Messages use typed blocks from merge-extensible `ContentBlockMap`; the pattern also types `MessageSource`, `FinishReason`, `TurnTrigger`, and `TurnEndReason`. New blocks coordinate adapters, UI, compaction, token metering, and persistence; replay measurements live in [token-meter.md](subsystems/token-meter.md).
Streaming uses raw chunks and `BlockAssembler`. Each `LlmAdapter.stream()` is one provider attempt; adapters report normalized failure facts, and a handling `agent/request-error` plugin returns a retry action. The loop logs chunks, the successful provider/model route, and replay state. Remote adapters use per-read idle watchdogs. Replay crosses routes only through a shared adapter instance ([contract](subsystems/llm-streaming.md)).
A **seam** is a swappable capability across three packages: a **Service Definition** (the Cordis `Service` owning its `ctx` key and vocabulary), a **Service provider** (the implementation plugin), and a **Consumer**. The Service Definition alone is the service, not the seam. Bash is the reference; the [capability graph](capability-seams.md) shows every family.
Filesystem and subprocess providers define one execution world; Bash, PTY, and LSP run there without provider forks.
Exceptions combine LLM interface/consumer, filesystem policy, web registries, and named skill/subagent providers. Subagents spawn fresh, fork a completed-turn prefix, use ACP children, or delegate one self-contained turn to a real product provider such as Codex ([subagent.md](subsystems/subagent.md)).
`dsh-workspace-context` composes its baseline on the first `agent/pre-step` and folds it into the final entering batch right after the claimed prompt, so it reaches the first request with the direct prompt; rejection keeps it in the next-step inbox. When compaction removes that baseline from the visible surface, the next entering pre-step composes the current baseline and carries it in the same request. Filesystem changes projected after tools are likewise folded into the next entering pre-step instead of creating a later context-only step ([decision](../.agents/notes/implemented/feature/2026-06-24-workspace-context.md)). `dsh-paths` owns shared paths.
`dsh-agent-spine-demo` bundles a spine and optional goals. App packages own CLI, ACP automation, and JSON-RPC front doors ([README](../packages/examples/agent-spine-demo/README.md), [acp/](../packages/acp/README.md), [interaction/](../packages/interaction/README.md)). `dsh-jsonrpc-agent` boots external `cordis.yml`; the Python SDK defaults when config is absent ([Python SDK](../python/README.md)). Thin deployments use swappable backends and optional tools ([examples/](../examples/AGENTS.md), [runnable wirings](cookbook/extension-cookbook.md#runnable-wirings), [graph atlas](graph-atlas.md)).
The [extension cookbook](cookbook/extension-cookbook.md) has plugin skeletons and the feature-to-seam map; guides cover [packages](cookbook/adding-a-package.md), [tools](cookbook/adding-a-tool.md), [LLM adapters](cookbook/adding-an-llm-adapter.md), and [vendored packages](cookbook/adding-a-vendored-package.md).
