docs(adr): 0006 — Runtime contract (v1) [closes #214]

[EmersonBraun]

Summary

Sixth and final core-contract ADR of Phase 0. The Runtime is the conductor of the substrate — where Adapter, Tool, Memory, Retriever, and Skill compose into an actual agent loop.

14 invariants (RT1-RT14) covering construction, the loop bound, resolution rules, confirmation, memory atomicity, retrieval, observers, delegation, errors, and abort.

This PR closes #214 (the 6 core contract ADRs).

Builds on #267, #270, #271, #272, #273.

Merge order: #267 → #270 → #271 → #272 → #273 → #274 (rebase each on main as merged; conflicts only on the ADR README index).

The substrate is now formal

ADR	What	Role
0001	Adapter	LLM provider seam
0002	Tool	Function the model calls
0003	Memory	Chat history + vector store + embed
0004	Retriever	Context fetching
0005	Skill	Declarative persona
0006	Runtime	The loop that composes them all

Highlight invariants

• RT1 Construction is config only — createRuntime does no I/O, no surprise side effects on import
• RT2 One entry point — run(task, options). No start/init/step. Streaming via observers.
• RT3 Options override config, never silently merge — debuggable resolution order
• RT4 Hard maxSteps cap — non-negotiable. Every "infinite loop bug" in agent libs traces to a soft cap.
• RT5 Tool resolution well-defined — option > config > skill-derived; missing tool → error chunk, not throw
• RT6 Confirmation refuse-by-default — no auto-approve timeout (matches Tool T9)
• RT7 Memory atomic at run boundary — failed runs don't save (preserves CM4)
• RT8 Retrieval per-turn, not per-step — deliberate v1 simplification
• RT9 Observers read-only — mutating logic = wrapper runtime, not observer
• RT10 Delegation is a tool — unifies model's mental model; no special syntax
• RT11 Bounded delegation depth — behavioral cap (delegates not even offered at limit)
• RT13 Abort is prompt and clean — abort stream, skip save, reject with AbortError
• RT14 Errors are categorized — adapter/tool/confirmation/memory/programmer; enables fallbacks ([F2] #34 — Declarative adapter fallback chain #147), self-debug ([F3] #64 — Memory adapters (pgvector, Pinecone, Qdrant, etc.) #177), durable execution ([F2] #43 — Durable execution (Temporal-style) #156)

Forward-compat carve-outs in v1.x (no v2 needed)

• Durability checkpoints ([F2] #43 — Durable execution (Temporal-style) #156) — additive, wraps the loop
• Deterministic replay ([F2] #21 — Deterministic replay engine #134) — runtime + replay adapter pair
• Multi-agent topology helpers ([F2] #44 — Ready-made multi-agent topologies #157) — built on delegates config
• runStream method if real demand for streaming-as-return-type emerges

Trade-offs accepted

• No per-step retrieval in v1 (custom runtime or tool-shaped retriever for ReAct-style)
• No mutating middleware — wrap the runtime instead
• Delegation depth is global, not per-branch (simpler, occasionally wasteful)
• Tools don't receive abort signal (in Open Questions; v1.x add)

Unlocks

• Multi-agent topologies ([F2] #44 — Ready-made multi-agent topologies #157): configure delegates — supervisor/swarm/hierarchical/blackboard fall out
• Durable execution ([F2] #43 — Durable execution (Temporal-style) #156): wrapper runtime, same run() signature
• Deterministic replay ([F2] #21 — Deterministic replay engine #134): replay adapter + runtime asserts trace match
• Sandbox runtime ([F2] #50 — Rate limiting by user/IP/key #163): wraps tool execute, transparent to consumers
• Observability ([F2] #37 — Evals in CI via GitHub Action #150): trace viewer, Langfuse/PostHog plug in via observers
• Cost guard ([F1] #9 — Cost guard with per-session limit #122): budget enforcement via observer + abort

Test plan

• Doc renders on GitHub
• ADR index updated, all 6 contracts marked Accepted
• No code changes — pure documentation
• Reviewer: 14 invariants match packages/runtime/src/runner.ts and packages/core/src/agent-loop.ts?
• Reviewer: forward-compat carve-outs cover the upcoming features (durability, replay, topologies)?
• Reviewer: any invariant that would block a real runtime variant?

Closes #214
Refs #211