815bac7de9
SessionSummary (updatedAt/title/firstPrompt) and SessionPersistence.update() were dead state: zero production callers of update(), no production reader of updatedAt/firstPrompt, and ACP's title comes from a tool-call presenter, not storage. The live Session.header was already typed SessionHeader, so the summary only ever existed in the persistence layer, written and read by nothing but its own contract test. Delete it entirely (no SessionMeta alias — SessionMeta collapses to SessionHeader everywhere). This removes the JSONL .summary.json sidecar machinery, the SQLite title/first_prompt/updated_at columns and per-append updated_at bump, and the update() method from the abstract service and both backends. SQLite SCHEMA_VERSION goes 1->2 and openDatabase now rejects any non-current user_version (older or newer) — no migration, unreleased software. Net -400 lines, and it erases the JSONL-sidecar-vs-SQLite-column durability divergence that the upcoming write coordinator would otherwise have to model. Records the decision in docs/rfc/implemented/2026-06-19-drop-mutable-session-summary.md and migrates the 2026-06-14 session-persistence RFC's facts to current truth. Adds a standalone AGENTS.md section "Tests document behavior, not golden truth" (a passing test pins current behavior, not necessarily correct behavior) with the summary-drop as its worked example, and reinforces the no-migration pre-release stance.
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RFCs
One kind of design doc lives here. An RFC records a decision or proposal that shapes this codebase — the why and what we gave up, the parts code and docs can't carry. (Earlier this split into separate "ADR" and "RFC" trees; they were unified, since most ADRs were simply implemented RFCs.)
Layout and naming
Files are grouped by lifecycle into three folders, and an RFC moves between them as its status changes:
proposed/— proposals reviewed before implementation; not yet built (or only partly).implemented/— the decision shipped. The file records what was decided and what was rejected, and is kept current with what actually shipped: when the code later moves a file, renames a package, or changes a key/default, the RFC is updated in the same change to match (facts only — paths, names, structure — not the decision itself). See implemented/AGENTS.md.rejected/— the proposal was considered and declined. Kept for the record so the rejection isn't re-litigated.
Each file is named yyyy-mm-dd-topic-title.md, where the date is when the topic was first proposed (per git history). Cross-references between RFCs use relative markdown links ([topic](../implemented/2026-…-….md)) — never bare prose or numbers — so they are mechanically checkable and survive moves between folders.
When to write one
Write an RFC when a decision is durable (it shapes the codebase beyond a single function or package), contested (there was a real alternative a reasonable engineer might have chosen), and surprising (a future reader would otherwise ask "why on earth is it done this way?"). A proposal for substantial future work starts in proposed/; a decision already made starts in implemented/.
Do NOT write one for a mechanical or local choice (a variable name, a one-file refactor), for anything already enforced and explained by a gate or a convention in AGENTS.md, or for a still-provisional decision tagged TODO(...) in the code — record those as TODOs and promote to an RFC only once they settle. An RFC is never edited into a different decision: supersede it with a new one and cross-link. (Editing an implemented/ RFC to track where its already-made decision now lives — a moved file, a renamed package — is not a different decision and is required, not forbidden; see implemented/AGENTS.md.)
Proposed
| Title | First proposed |
|---|---|
| Mutation testing as the coverage counterweight | 2026-06-11 |
| Deterministic tests, the replay invariant fixture, and race stress | 2026-06-11 |
| Architectural conformance — dependency rules and the adapter kit | 2026-06-11 |
| API extractor reports | 2026-06-11 |
| Supply chain checks and vendor drift verification | 2026-06-11 |
| Agent Client Protocol (ACP) support for external editors | 2026-06-14 |
| Multiplex concurrent ACP sessions over one connection | 2026-06-14 |
| Optional Code Mode — model writes TypeScript against an SDK of all tools | 2026-06-15 |
| Runtime schemas for the event vocabulary (Zod vs the merge-extensible-map pattern) | 2026-06-16 |
| Agent lifecycle and ownership seams | 2026-06-18 |
| Shared persistence write coordinator | 2026-06-18 |
Implemented
Rejected
| Title | First proposed |
|---|---|
| Deep-readonly public surfaces | 2026-06-11 |