# Lat Md

> Writing and maintaining lat.md documentation files — structured markdown that describes a project's architecture, design decisions, and test specs. Use when creating, editing, or reviewing files in the lat.md/ directory.

- Skill: `vercel-labs/lat-md` (Agent Skill)
- Install (CLI): `npx skillmds@latest add vercel-labs/lat-md`
- Raw SKILL.md: https://api.skillmd.com/api/skills/vercel-labs/lat-md/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Docs & Writing
- Author: Vercel Labs (https://skillmd.com/u/vercel-labs)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/vercel-labs/lat-md

---


# lat.md Authoring Guide

This skill covers the syntax, structure rules, and conventions for writing `lat.md/` files. Load it whenever you need to create or edit sections in the `lat.md/` directory.

Project-specific lat documentation belongs in `lat.md/`. Do not modify this generated skill or a generated `AGENTS.md` to record project guidance: both are owned by lat tooling and may be replaced by a later `lat init`.

## What belongs in lat.md

`lat.md/` files describe **what** the project does and **why** — domain concepts, key design decisions, business logic, and test specifications. They do NOT duplicate source code. Think of each section as an anchor that source code references back to.

Treat `lat.md/` as a focused snapshot of the current state of affairs or planned features. Do not use it as a journal or changelog, and do not grow it just to record insignificant implementation details.

Good candidates for sections:
- Architecture decisions and their rationale
- Domain concepts and business rules
- API contracts and protocols
- Test specifications (what is tested and why)
- Non-obvious constraints or invariants

Bad candidates:
- Step-by-step code walkthroughs (the code itself is the walkthrough)
- Auto-generated API docs (use tools for that)
- Journal/changelog entries for each change
- Temporary notes or TODOs
- Insignificant implementation details that do not change the current-state model

## Section structure

Every section **must** have a leading paragraph — at least one sentence immediately after the heading, before any child headings or other block content.

The first paragraph must be ≤250 characters (excluding `[[wiki link]]` content). This paragraph is the section's identity — it appears in search results, command output, and RAG context.

```markdown
# Good Section

Brief overview of what this section documents and why it matters.

More detail can go in subsequent paragraphs, code blocks, or lists.

## Child heading

Details about this child topic.
```

```markdown
# Bad Section

## Child heading

This is invalid — "Bad Section" has no leading paragraph.
```

`lat check` enforces this rule.

## Section IDs

Sections are addressed by file path and heading chain:

- **Full form**: `lat.md/path/to/file#Heading#SubHeading`
- **Short form**: `file#Heading#SubHeading` (when the file stem is unique)

Examples: `lat.md/tests/search#RAG Replay Tests`, `cli#init`, `parser#Wiki Links`.

Configured external sources use `[[handle:path/to/file.md#Heading]]` for
Markdown headings and `[[handle:path/to/file.ts#symbol]]` for supported code
symbols. Run `lat external show <handle>` before cloning an external source;
its output includes the pinned commit and safe checkout suggestions.

## Wiki links

Cross-reference other sections or source code with `[[target]]` or `[[target|alias]]`.

### Section links

Section links connect related concepts while keeping their stable graph identities explicit.

```markdown
See [[cli#init]] for setup details.
The parser validates [[parser#Wiki Links|wiki link syntax]].
```

### Repository path links

Reference any existing file or directory beneath the project root without a fragment:

```markdown
[[schema.sql]]
[[CHANGELOG]]
[[src/components]]
```

Unsupported formats validate as references but cannot be opened by Lat. A `#fragment` requires a `lat.md/` section or a supported source file.

### Source code links

Reference functions, classes, constants, and methods in source files:

```markdown
[[src/config.ts#getConfigDir]]          — function
[[src/server.ts#App#listen]]            — class method
[[lib/utils.py#parse_args]]             — Python function
[[src/lib.rs#Greeter#greet]]            — Rust impl method
[[src/app.go#Greeter#Greet]]            — Go method
[[src/app.h#Greeter]]                   — C struct
```

When prose names an implementation symbol or a behavior governed by one, link the symbol instead of using a bare code span or copying its literal value. Prefer `[[src/config.ts#DEFAULT_TIMEOUT]]` (or `[[src/config.ts#DEFAULT_TIMEOUT|the default timeout]]`) over a bare identifier or copied value. Keep the behavioral explanation in prose; the link makes its implementation binding navigable and validated.

`lat check` validates that all targets exist.

## Code refs

Tie source code back to `lat.md/` sections with `@lat:` comments:

```typescript
// @lat: [[cli#init]]
export function init() { ... }
```

```python
# @lat: [[cli#init]]
def init():
    ...
```

Supported comment styles: `//` (JS/TS/Rust/Go/C) and `#` (Python).

Place one `@lat:` comment per section, at the relevant code — not at the top of the file.

## Test specs

Describe tests as sections in `lat.md/` files. Add frontmatter to require that every leaf section has a matching `@lat:` comment in test code:

```markdown
---
lat:
  require-code-mention: true
---
# Tests

Authentication test specifications.

## User login

Verify credential validation and error handling.

### Rejects expired tokens

Tokens past their expiry timestamp are rejected with 401, even if otherwise valid.

### Handles missing password

Login request without a password field returns 400 with a descriptive error.
```

Each test references its spec:

```python
# @lat: [[tests#User login#Rejects expired tokens]]
def test_rejects_expired_tokens():
    ...
```

Rules:
- Every leaf section under `require-code-mention: true` must be referenced by exactly one `@lat:` comment
- Every section MUST have a description — at least one sentence explaining what the test verifies and why
- `lat check` flags unreferenced specs and dangling code refs

## Frontmatter

Optional YAML frontmatter at the top of `lat.md/` files:

```yaml
---
lat:
  require-code-mention: true
---
```

Currently the only supported field is `require-code-mention` for test spec enforcement.

## Validation

Always run `lat check` after editing `lat.md/` files. It validates:
- All wiki links point to existing sections, repository paths, or source code symbols
- All relative markdown links point to existing files
- Full and collapsed reference-style links have definitions
- All `@lat:` code refs point to existing sections
- Every section has a leading paragraph (≤250 chars)
- All `require-code-mention` leaf sections are referenced in code
- Every `lat.md/` directory has an accurate index