| `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 turn ends when the model or plugins stop it; a **step** is one model request plus its tool calls. Agent and session publication happen only after private setup and resume state are ready. Quotes in the [sequence below](agent-lifecycle.md) mark durable events.
Each step assembles the prompt, tools, runtime context, adapter settings, and model history before recording its reconstruction boundary. Tool calls then run through the shared execution pipeline. `inject()` adds context without opening an idle turn; `steer()` targets a next-step admission window; queued input remains the source of ordinary turns. The generated [agent lifecycle](agent-lifecycle.md) owns exact event order, and the [agent-loop README](../packages/core/agent-loop/README.md) owns queue, steering, retry, and cancellation mechanics.
Adapter failures close their step before `agent/request-error` can authorize recovery from durable history. Other failures use `agent/error`; cancellation and disposal take precedence over recovery. Failed model attempts commit no assistant message or tool side effect. Turn closure is represented by one [TurnEndReason](core-data-structures/session.md#why-a-turn-ended-turnendreasonmap); the exact retry contract belongs to [LLM streaming](core-data-structures/llm-streaming.md).
`ctx.agents` owns agents and returns `AgentHandle { agent, dispose() }`. Plugins submit queued work, steering, or injected context through the [agent interface](../packages/core/agent/README.md#agent-interface-typests); cancellation, idleness, and teardown stay behind the same handle.
Each agent owns scoped `agent.ctx`; shared storage overlays its tools, prompts, and commands on global contributions while scoped listeners filter dispatch. Setup composes before publication and cleanup unwinds contributions. The [agent-scope decision](../.agents/notes/implemented/architecture/2026-07-08-agent-scope-contexts.md) owns the detailed lifecycle.
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 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; 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).
