# Agent Note: Subsystems catalog and the `ts type-equiv` drift gate

Status: implemented

English | [中文](2026-06-20-core-data-structures-catalog.zh.md)

## Problem

A reader trying to understand the harness could find its *behavior* in [architecture.md](../../../../docs/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 Agent Note records both decisions. Its historical sibling, [the archived generated Cordis events + services catalog decision](../../archived/process/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/subsystems/` 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-subsystem line

> **Superseded as the page-scoping rule** by [package-anchored subsystem pages](2026-08-03-package-anchored-subsystem-pages.md): each page now anchors to the package group that declares its vocabulary. The `ts type-equiv` mechanism below remains current.

The decisive test for the scoping line is `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 its own subsystem 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 subsystem-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 — `ValueSchemaSpec`, `ParameterSchemaSpec`, `InferValue`, and `InferArgs` — is a subsystem-page detail. That is the spine-vs-subsystem 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 belong on subsystem 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 sibling subsystem pages for package-owned detail; the folder's [README](../../../../docs/subsystems/README.md) indexes every page. The original subsystem 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.
- Each type block's document, symbol, and source file are recorded in `scripts/type-equiv.manifest.json` (`{ doc, symbol, source }` entries), **not** in directive comments in the prose. The script enforces a **1:1 correspondence** between each primary type-equiv block and one manifest entry, so a block can never be silently unchecked and an entry can never rot. A paired `.zh.md` block reuses the unsuffixed sibling's entry only when the complete tracked fence sequence matches in order, kind, and byte-exact body; otherwise the gate checks it independently, finds no manifest entry, and fails.
- Wired into `doc-sync`, so relevant documentation changes run it locally and CI runs it with the other documentation checks.

### 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 types.

## 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-subsystem 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.
- **Put each type block's source in a directive comment** — 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

`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 Agent Note records that fail-closed scan rule together with the spine-vs-subsystem and verbatim-match decisions; the generated Cordis catalog has the symmetric design record in [its archived Agent Note](../../archived/process/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 `doc-sync` 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-subsystem 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.
- Since 2026-07-27 the subsystem-page tier spans every service-bearing subsystem: nine lean pages (permission presets, plan mode, runtime invariants, the HTTP carrier, storage — owning both `ctx.storage` and `ctx.storageDomain` — TUI extensions, workspaces, client modules, telemetry) cover the ten `ctx` services that had none, so each harness service and event scope has exactly one owning subsystems page — the precondition for generating per-subsystem service/event reference into these pages instead of flat catalogs.
