| `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 vetoes or takes over ([semantics](cordis-primer.md#cordis-waterfall-semantics)).
A **session** is append-only. An ordinary **turn** claims one queued `send()` item; injection claims none. A successor awaits its predecessor's checkpoint but may share its `running` interval ([decision](../.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.md)). A turn ends when model or plugins stop it; a **step** is one model request plus tools. Quotes in the [sequence below](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 failures emit `agent-loop/config-start-failed`; teardown is silent.
Each step assembles ordered stable system sections, cache-safe dynamic contexts, 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)).
Admission-time and active-turn `inject()` stage for the next step; post-tool `additionalContexts` settles after results. Steering shares that staging boundary and requests another step. The default loop closes its optional `trySteer()` acceptance immediately before the final steering drain; ordinary `steer()` keeps its best-effort routing semantics. Idle `inject()` appends immediately without changing turn numbers; persistence drains eagerly.
Pruning precedes summaries; overflow retries require durable progress. `agent/request-error` may authorize one retry turn between failed-step and turn close; cancellation wins. Adapter-owned `retryPolicy` makes normal mode bounded; always mode delegates specialized recovery before retrying until success or cancellation ([compaction](../.agents/notes/implemented/architecture/2026-07-10-after-call-compaction-pressure-and-overflow-recovery.md), [retry foundation](../.agents/notes/implemented/architecture/2026-06-21-bounded-llm-request-recovery.md), [provider policy](../.agents/notes/implemented/feature/2026-07-24-provider-retry-policies.md)).
Adapter failures close their step before `agent/request-error` receives the exact `Error`, normalized `LlmFailure`, and signal. A handled failure closes its turn and opens a retry turn from durable history without an idle notification; exhaustion leaves terminal `turn/end`. Failed chunks commit neither messages nor tool calls.
Other failures use `agent/error`. Cancellation and disposal beat recovery. Before request-header commit, the turn signal cancels asynchronous model-capability preparation; undispatched tools get synthetic `tool/call`/`ABORTED_BEFORE_DISPATCH` pairs. Effective `cancel(cause)` emits its cause before queue clearing and abort; observers cannot veto; idle calls emit nothing. Durability records user or parent cancellation as `aborted`, teardown as `disposed`; teardown awaits quiescence. The cause affects reporting, not late result-context handling ([decision](../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md)).
Turn and step events are turn-enclosed. Idle `user/message` and standalone `compact/* { turn: null }` consume no turn; their lock-time markers may interleave with injection. 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](core-data-structures/session.md#why-a-turn-ended-turnendreasonmap).
`ctx.agents` owns agents, returning `AgentHandle { agent, dispose() }`. Plugins use `send()` or `followup()`, `steer()`, optional `trySteer()`, and `inject()` presets; [`reserveTurnAdmission()`](../packages/core/agent/README.md#agent-interface-typests) synchronously reserves idle for durable work without changing queued prompt identity. The default loop's `trySteer()` atomically rejects after the current step's final steering drain begins, while ordinary `steer()` retains best-effort routing. `cancel()` and `whenIdle()` control lifecycle. Awaited disposal owns teardown.
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**: before `step/start`, the loop appends the full current runtime-context snapshot as a sourced `user/message`, then snapshots derived messages. Those messages and the folded `request/header` reconstruct each request. The header marks adapter defaults so later proposals discard them and re-resolve the route without losing explicit settings. `dsh-agent-loop/invariant` asserts this through `ctx.invariants` ([reconstructability](../.agents/notes/implemented/architecture/2026-07-05-reconstructable-requests.md)).
Durability is a plugin concern. Backends eagerly drain synchronous `session/event` notifications. `session/flush` barriers precede each request and top-level tool dispatch, then follow `turn/end` before another queued turn or idle observation. `SessionPersistence` stores `SessionEvent` directly and metadata in `SessionHeader`; JSONL defaults to checksummed Zstandard, while SQLite shares the contract ([decision](../.agents/notes/implemented/bug-fix/2026-07-21-semantic-session-checkpoints.md)).
Between turns, owners append log-only events through `Session`, flushing only for durability. `session/title` needs eager persistence and lifecycle drains; manual compaction flushes its bracket before releasing admission. Title work never delays responses; latest wins with provenance. 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](core-data-structures/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, successful provenance, and replay state. Remote adapters use per-read idle watchdogs. Replay crosses routes only through a shared adapter instance ([contract](core-data-structures/llm-streaming.md)).
A swappable capability usually has **interface / implementation / consumer** layers: service/events, backend, and model-facing tools/prompts. Bash is the reference; the [capability graph](capability-seams.md) maps each family.
Exceptions combine LLM interface/consumer, filesystem policy, web registries, and named skill/subagent providers. Subagents spawn fresh, fork a completed-turn prefix, or use ACP children ([subagent.md](core-data-structures/subagent.md)).
`dsh-workspace-context` injects baseline at the first `agent/step` and appends `ctx.fs`-discovered changes through `tools/post-execute`; its [decision](../.agents/notes/implemented/feature/2026-06-24-workspace-context.md) records isolation. `dsh-paths` owns shared paths.
`dsh-agent-spine-demo` bundles a spine and optional goals. App packages own TUI, CLI, ACP automation, and JSON-RPC front doors ([README](../packages/examples/agent-spine-demo/README.md), [acp/](../packages/acp/README.md), [ui/](../packages/ui/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).
