# Concept clusters (/docs/concept-clusters) This page is the concept map. Every other doc page is the deep dive on one of the concepts named here. The substrate organizes around clusters of three. One triplet covers the whole runtime — *session, turn, row* — and reduces to the same primitive used everywhere else: an append-only typed audit row keyed by `turnId`. Six more cluster triplets sit one click in. Each one is what the substrate uses to do something specific: run a turn, route a call, record a call, compose work across spawns, advance over time, or survive a restart. If you read the [glossary entry on the name](/docs/glossary#p): each cluster triplet is a way the lattice carries one weight a single `streamText` call cannot. Routing, audit, lifecycle, persistence — different forces, same hedge. ## The substrate-wide triplet [#the-substrate-wide-triplet] Everything else reduces to these. The homepage opens with this triplet; the docs index repeats it; every cluster below is built out of one of them. * **Session.** Long-lived state for one user × one product surface. Channels, storage, family-lock, and the event log all hang off it. Mint with `runtime.sessions.create()`; resume by id across processes and clients via version-vector sync. See [SessionRuntime](/docs/session-runtime). * **Turn.** One user-message-in / one-answer-out cycle. Walks the four-stage lattice — `anchor-plan` → `tool-loop` ⇄ `synthesize` → `post-turn` — under singleton-synthesize discipline so exactly one final answer fires per turn. See [Turn lifecycle](/docs/turn-lifecycle). * **AuditableCall row.** One append-only typed row per LLM call, keyed by `turnId`. Carries `tenantId`, `toolName`, `subagentDepth`, `modelId`, and `tokenUsage`. The grain every cost rollup, compliance query, and replay reads from. See [The AuditableCall row](/docs/auditable-call-row). ## The six cluster triplets [#the-six-cluster-triplets] Each cluster lives on a host page that walks the three concepts as parallel bullets. Sibling concept pages open with a one- paragraph pointer to the cluster framing. The clusters are ordered roughly by where they fire in a turn — graph schedules work, the routing cluster picks a model, the audit cluster records the call, the composing-agents cluster spawns child work, the runtime-lifecycle cluster advances and persists, and the state cluster carries everything across restarts. ### Execution-graph cluster [#execution-graph-cluster] The per-turn execution substrate. Lives *inside* the lattice. Full walk in [Architecture](/docs/architecture#the-execution-graph-cluster). * **Graph.** Declarative topology — nodes wired to channels, edges constrained to the four-stage lattice. See [Graph](/docs/graph). * **Node.** An async function plus typed `StateGraphNodeMetadata` declaring `stageId`, `acceptsSeam`, `subscribes`, and `writes`. See [Nodes](/docs/nodes). * **Channel.** Typed reactive state slot. Six kinds with defined concurrent-write semantics and a `checkpoint()` / `restore()` pair. See [Channels](/docs/channels). * **Engine.** The `engine/` superstep scheduler (`SuperstepRunner`) that *runs* the compiled graph — fires triggered nodes in parallel, accumulates their writes, and commits channel updates atomically per step. Deterministic and race-free by construction; also owns retry, abort composition, and graph↔session state sync. See [Architecture → execution-graph cluster](/docs/architecture#the-execution-graph-cluster). ### Routing cluster [#routing-cluster] Decides which model fires for which kind of call, and pins what stays locked across the session. Lives between the execution graph and the LLM transport. Full walk in [Family-locked routing](/docs/family-lock#the-routing-cluster). * **CallClass.** Four-bucket taxonomy every LLM call declares — `utility`, `reasoning`, `converse`, `synthesize`. See [Call classes](/docs/call-classes). * **Seam.** Per-call-class factory that builds a `ProviderSeam`. Four factories live in `src/graph/seams/`. See [Seams](/docs/seams). * **Family-lock.** Session-start lock on one `ProviderFamily` and one `Transport`. See [Family-locked routing](/docs/family-lock). ### Audit-ledger cluster [#audit-ledger-cluster] The write side. The row is the grain; the ledger is the write path; the chain is the after-the-fact-tamper detector. Full walk in [Audit ledger](/docs/audit-ledger#the-audit-ledger-cluster). * **AuditableCall row.** Typed row shape with five typed payload slots. See [The AuditableCall row](/docs/auditable-call-row). * **ProviderDecisionLedger.** Append-only write interface, one method, idempotent on `(sessionId, turnId, stageId, seqWithinTurn)`. See [Audit ledger](/docs/audit-ledger). * **Tamper-evident hash chain.** `prev_hash` + `row_hash` columns detect silent backfill, reorder, and removal. See [Hash chain](/docs/hash-chain). ### Composing-agents cluster [#composing-agents-cluster] How composable work nests inside a session. Names the unit of work, spawns the child runtime that executes it, records the rolling signal that decides whether to spawn it again. Full walk in [Agents](/docs/agents#the-composing-agents-cluster). * **Skill.** Markdown file with YAML frontmatter; `SkillLoader` walks three sources. See [Skills](/docs/skills). * **Subagent.** Child runtime spawned from a tool call against a resolved skill spec. `SUBAGENT_LIMITS.maxDepth = 3` caps the nesting. See [Subagents](/docs/subagents). * **Agent profile.** Persistent registry record keyed by `specName`. See [Agents](/docs/agents). ### Runtime-lifecycle cluster [#runtime-lifecycle-cluster] What happens over time and what gets persisted. Sessions outlive turns; turns outlive their stream frames; the event log outlives both. Full walk in [Session lifecycle](/docs/session-lifecycle#the-runtime-lifecycle-cluster). * **Session lifecycle.** `createSession` / `resumeSession` / `deleteSession` — the arc above the turn. See [Session lifecycle](/docs/session-lifecycle). * **Turn lifecycle.** The per-message arc between `executeMessage` and the final SSE frame. See [Turn lifecycle](/docs/turn-lifecycle). * **Event log.** Append-only typed stream both lifecycles write into. See [Event log](/docs/event-log). ### State-and-persistence cluster [#state-and-persistence-cluster] What makes a session survive a process restart, a fork or rewind, and a multi-client edit. Lives below the execution graph and above the schema bundle. Full walk in [Storage](/docs/storage#the-state-and-persistence-cluster). * **Storage adapter.** Pluggable surface reading and writing `SessionState`. Memory / IndexedDB / Supabase adapters all implement the same interface. See [Storage](/docs/storage). * **Checkpoint.** Per-channel snapshot at every stage boundary. `runtime.checkpoints.rollback` restores in place; `TimeTravelAPI.fork` branches into a new session. See [Checkpointing](/docs/checkpointing). * **Sync (version vector).** `Record` per session. `compareVectors` flags concurrent writes; `SyncCoordinator` surfaces conflicts through `sync.conflict`. See [Sync](/docs/sync). ## What lives outside the cluster pattern [#what-lives-outside-the-cluster-pattern] Eight sidebar groups don't carve cleanly into three-concept triplets. They aren't deficient — they're shapes the pattern doesn't fit. Three are surfaces (facets, plugins, tools); five are heterogeneous concept sets or pairs (safety, observability, frontend, prompts, cross-session state). * **Facets.** Typed accessors on `runtime.` — `runtime.sessions`, `runtime.events`, `runtime.checkpoints`, `runtime.tenant`, `runtime.spans`, and the rest. A surface pattern that groups related methods by domain so the runtime reads as a short list of named accessors instead of a flat method dump. Not a three-concept cluster. See [Facets](/docs/facets). * **Plugins.** The `HarnessPlugin` extension contract — \~40 optional hooks plus four structural invariants the substrate enforces against them. A bounded contract, not a triplet. See [Plugin contract](/docs/plugin-contract). * **Tools.** The `defineTool` contract, Zod-validated input / output, per-invocation context, and batching. One surface end to end — contract, dispatch, result-handling — not a three-concept cluster. See [Tools](/docs/tools). * **Prompts.** Composition of system prompts from typed contributions plus the budgeted composer. A pair ([prompts](/docs/prompts) + [prompt builder](/docs/prompt-builder)), not a triplet. * **Cross-session state.** [Plans](/docs/plans) span multiple turns; [memory](/docs/memory) spans multiple sessions. Two variable surfaces that outlive a single session, each with a distinct mechanism (revision history vs decay curve). Not a three-concept cluster. * **Safety & determinism.** `safety`, `scrubbers`, `fabrication-detection`, `determinism`, `fingerprint`. Five concepts, each with a distinct mechanism; no natural triplet. `safety.mdx` orients the reader directly. * **Observability.** `lineage`, `observability`, `otel-observability`, `runtime-inspector`. Distinct surfaces a reader usually arrives at already knowing which one they need. * **Frontend integration.** `react`, `server`, `api-routes`, `query`, `devtools`. Wiring surfaces, not concepts. ## Where to go next [#where-to-go-next]