# RFC: Core-data-structures catalog and the `ts type-equiv` drift gate

Status: implemented

## Problem

A reader trying to understand the harness could find its *behavior* in [architecture.md](../../../architecture.md) (the service map, the session/turn/step lifecycle, the event taxonomy) but had no single place describing its *vocabulary* — the data structures that behavior moves around. The type shapes lived only in source, scattered across `packages/*/src/types.ts`, so understanding "what is a `Message`, a `SessionEvent`, a `StreamChunk`" meant reading the declarations directly. A prose catalog would help, but a catalog that paraphrases or paste-copies type definitions rots the instant a field changes — and an out-of-sync type doc is worse than none, because a reader trusts it.

So the work had two intertwined questions: **what belongs in such a catalog** (the scoping problem — a harness has dozens of cross-package types and dumping all of them helps no one), and **how to keep pasted type definitions from drifting** (the durability problem). This RFC records both decisions. Its sibling, [the generated cordis events + services catalog](2026-06-20-generated-cordis-catalog.md), is the *wiring*-axis complement: this one catalogs the data structures, that one the events and services that move them.

## Decision

A new `docs/core-data-structures/` folder catalogs the vocabulary, with a new `verify-type-equiv` doc-sync gate that keeps every pasted type declaration and its JSDoc synchronized with source.

### What counts as "core" — the spine-vs-seam line

The scoping line was not picked top-down; it was discovered by testing candidate definitions against concrete borderline types until one rule survived every case. The decisive test was `BashExecRequest`/`BashExecSpec`/`BashRunResult`: bash is a capability *seam*, not part of the agent-loop spine, so if those are "core" then "core" means *all cross-package vocabulary* and the catalog is a flat dump; if they are not, "core" means *the central spine* and bash vocabulary belongs on a sub-page. The latter won, which set the whole structure: a **tiered folder**, not a flat document.

The rule that settled the remaining cases: ***the type you write, hold, or receive is core; the machinery that types it, renders it, or persists it is a sub-page detail.*** Worked through:

- A data structure is **core** if it flows through the agent-loop spine — the loop holds, derives, streams, or logs it on every turn regardless of which plugins load (`Message`, `StreamChunk`, `SessionEvent`, the `Agent` handle) — **or** it is the single headline type a plugin author writes against a pipeline (`ToolDefinition`).
- `ToolDefinition` is core (it is what every tool author writes) **even though the loop never holds one** — authoring-importance overrides the strict flows-through-spine rule for this one headline type. But its typing machinery — the `SchemaSpec`/`InferArgs` DSL — is a sub-page detail (you write a `ToolDefinition`; the type-level machinery that types it you do not). That is the spine-vs-seam line made sharp.
- `ToolSchema` is core (it is a field of `GenerateOptions`, the model request that flows through every step) even though it is conceptually part of the tool pipeline — *flows through the spine* wins over *conceptual home* when they conflict.
- The tool-presentation vocabulary (`ToolCallView`/`ToolResultView`, …), the `SessionPersistence` durability seam, and bash vocabulary are sub-pages.

`core.md` is a **self-contained spine doc**: it states the exact type definition of each spine structure with minimal prose and links to sub-pages for the per-seam detail. The sub-pages are `llm-streaming.md`, `session.md`, `persistence.md` (split from session along the in-memory-model vs. durability-seam line), `tools.md`, and `bash.md`.

### The `ts type-equiv` mechanism — literal AND drift-proof

The durability requirement was specific: the doc shows the **literal** current type declaration and original JSDoc (so a reader sees the real shape and source contract, not a paraphrase) **and** is mechanically guaranteed to match source. The repo already compiles fenced ` ```ts ` blocks (`doc-typecheck`), but a real typechecked block needs import noise and proves only *assignability* — a renamed field or changed JSDoc can pass. So:

- Complete type declarations and their JSDoc are pasted verbatim into a dedicated ` ```ts type-equiv ` fence. A concise ` ```ts public-api ` fence carries the source-equivalent ambient projection for a class whose implementation bodies do not belong in the catalog. `doc-typecheck` recognizes both and skips them (the bare declarations are not standalone-compilable), and **excludes them from the opt-out ratio** — they are a separately-checked category, not unchecked sketches.
- A new `scripts/verify-type-equiv.ts` extracts each block via the TypeScript parser and asserts that its declaration structure and every JSDoc comment match the declared symbol, ignoring only formatting whitespace and non-JSDoc comments. Ordinary blocks retain the complete declaration. A `public-api` projection retains a class's public fields, constructor, accessors, and methods with their original JSDoc while removing implementation bodies and private or protected members. This is chosen over a compiled `_Check` assertion because source names and documentation identity, not assignability, are the properties the catalog preserves.
- Provenance lives in a central `scripts/type-equiv.manifest.json` (`{ doc, symbol, source }` entries), **not** in directive comments in the prose. The script enforces a **1:1 correspondence**: every type-equiv block has exactly one manifest entry and vice versa, so a block can never be silently unchecked and an entry can never rot.
- Wired into `doc-sync`, so it runs in the same lefthook pre-push and CI paths as the other doc gates.

### Maintenance is the author's job, with a gate backstop

`verify-type-equiv` catches a *drifted paste* of an already-documented type, but it cannot tell you a brand-new core type went undocumented. So AGENTS.md and the `dsh-code-review` skill were updated to require keeping the catalog in sync when a change adds or reshapes a documented type — the gate handles drift, the human handles new surface.

## Alternatives considered

- **A flat dump of all cross-package vocabulary** — the `BashExecRequest` test case killed it: if seam vocabulary is "core", the catalog helps no one; the tiered spine-vs-seam structure won.
- **A compiled `_Check` assignability assertion** instead of the source match — rejected because assignability does not preserve names or JSDoc: a renamed field with the same type or a changed contract comment would pass.
- **Provenance as directive comments in the prose** — rejected for the central manifest, whose enforced 1:1 correspondence means a block can never be silently unchecked and an entry can never rot.

## Verification lesson

The spine-vs-seam rule was tested against `BashExecRequest`, tool schemas and definitions, the schema DSL, presentation types, and the session/persistence split before adoption.

`verify-type-equiv` must scan the complete Markdown scope, not only manifest-named documents. Otherwise an unmanifested `type-equiv` block escapes the claimed one-to-one check. The gate therefore reports such blocks as orphans. This RFC records that fail-closed scan rule together with the spine-vs-seam and verbatim-match decisions; the generated Cordis catalog has the symmetric design record in [its RFC](2026-06-20-generated-cordis-catalog.md).

## Consequences

- The vocabulary now has a single home that **cannot silently drift**: a field or public class-member change in source fails `verify-type-equiv` in the pre-push hook and CI until the paste is refreshed. Cordis service methods remain owned by the generated services catalog rather than being duplicated here.
- The spine-vs-seam line is a reusable scoping tool, not a one-off: the same "the thing you write/hold/receive is core; the machinery that types/renders/persists it is a detail" rule is what later scoped the events/services catalog's harness-vs-inherited tiering.
- The `ts type-equiv` fence is a third doc-block category alongside ` ```ts ` (compiled) and ` ```ts ignore-check ` (sketch). A later sibling added a fourth, ` ```ts cordis-catalog ` (generated signature), reusing the same skip-and-exclude treatment.
- Adding or reshaping a core type now carries a documentation obligation the author must honor (the gate cannot detect a missing *new* type), backstopped by the `dsh-code-review` checklist.
