atproto-lexicon

AT Protocol Lexicon + XRPC + Records

An AT Protocol lexicon is a JSON document that declares the shape of one record type or one XRPC method. Every record's $type, every XRPC path, and every ref between them ties back to an NSID. The lexicon is the protocol's type system; validation against it is the contract between clients, servers, and the repository layer.

This skill routes to per-language guides for Rust, TypeScript, and Go, sitting on top of a language-neutral spec in shared/.

Defaults

• Lexicon doc — {lexicon: 1, id: <nsid>, revision?, defs: {...}}. Primary types (record, query, procedure, subscription) live under main. Secondary types can use any name.
• NSID — reversed-DNS authority + name segment. ASCII only, 317 bytes max, name segment allows no hyphens. See shared/nsid.md.
• $type — required on every record. Bare NSID implies #main. Used for union dispatch. Missing $type = invalid record.
• AT-URIs in records — DIDs strongly preferred over handles for the authority. Query strings forbidden. See shared/at-uri.md.
• strongRef — {uri, cid} where cid is a plain string, not a cid-link. Common bug source.
• Blob refs (modern) — {$type:"blob", ref:{$link:<cid>}, mimeType, size}. Legacy {cid, mimeType} only accepted with an opt-in lenient flag.
• XRPC wire — /xrpc/<nsid> path. GET for queries, POST for procedures, WebSocket for subscriptions. Errors as {error, message}. Subscription frames are two concatenated DAG-CBOR objects per WS binary message.
• Validation — strict by default on write, lenient on read. ALLOW_LEGACY_BLOB and ALLOW_LENIENT_DATETIME are the typical lenient opt-ins.

Full normative rules: shared/lexicon-spec.md, shared/nsid.md, shared/at-uri.md, shared/record-model.md, shared/xrpc-wire.md, shared/backward-compat.md. Fixtures: shared/test-vectors.md. Cross-language differences: shared/divergence-matrix.md.

Language detection

Before generating or reviewing any lexicon / XRPC / record code, determine the target language from project files or the file being edited:

• Cargo.toml, *.rs, any mention of atproto-lexicon / atproto-client / atproto-record / atproto-jetstream → Rust — read from rust/.
• package.json, tsconfig.json, *.ts, *.tsx, imports of @atproto/lexicon / @atproto/xrpc / @atproto/xrpc-server / @atproto/api / @atproto/lex-cli → TypeScript — read from typescript/. Also *.js/*.jsx when there is no .ts present.
• go.mod, *.go, imports of github.com/bluesky-social/indigo/atproto/lexicon / atproto/data / xrpc / api/atproto / api/agnostic / events → Go — read from go/.

Prefer the file being edited over the repo root when they disagree: a .go consumer inside a TypeScript monorepo still means Go for that task.

If multiple languages are present and the task does not point at one unambiguously, ask which one applies. Never mix lexicon libraries across languages in generated code.

If an unsupported language is detected (Python, Java, Swift, …), point the user at shared/lexicon-spec.md, shared/record-model.md, and shared/xrpc-wire.md for the wire format, and offer indigo/atproto/lexicon (Go) as the most complete reference implementation to transliterate from.

Reading guide

For every lexicon / records / XRPC task:

1. Read the relevant shared/*.md first. They are short and define the rules your code must enforce.
2. Read the relevant task file in the detected language directory:
   • Authoring a new lexicon, loading into the catalog, codegen → {lang}/authoring.md
   • Validating records and XRPC payloads → {lang}/validation.md
   • Invoking XRPC methods, consuming subscriptions, server handlers → {lang}/xrpc-client.md
   • AT-URIs, TIDs, strongRef, blob refs, typed $type dispatch → {lang}/records.md
   • Library setup, public API at a glance, idioms → {lang}/README.md
3. Consult shared/divergence-matrix.md whenever porting between languages or reviewing cross-stack interop. The legacy-blob shape, BlobRef-class-vs-plain-object, and the Go two-stack (data.Blob vs. lex/util.LexBlob) split are the highest-frequency traps.

Always prefer the official library over hand-rolling: atproto-lexicon + atproto-client in Rust, @atproto/lexicon + @atproto/xrpc in TypeScript, indigo/atproto/lexicon + indigo/xrpc in Go.

The conceptual stack

                       ┌──────────────────────────────┐
  Lexicon (JSON)  ───▶ │ {lexicon:1, id:<nsid>,       │
  authoring / resolve  │  revision, defs:{main,...}}  │
                       └──────────────────────────────┘
                                   │
                                   ▼
                       ┌──────────────────────────────┐
  Catalog  ──────────▶ │ Lexicons / BaseCatalog —     │
  (runtime)            │ resolves <nsid>[#def] → def  │
                       └──────────────────────────────┘
                                   │ validates ↓          ↓ schemas XRPC
                                   ▼                      ▼
                       ┌──────────────────────────────┐   ┌──────────────────────────────┐
  Record value ──────▶ │ {$type:<nsid>, ...fields...} │   │ XRPC transport               │
  (record def)         │ DAG-CBOR → CID stability     │   │ GET/POST/WS @ /xrpc/<nsid>  │
                       └──────────────────────────────┘   │ JSON in/out; frames on WS   │
                                                          └──────────────────────────────┘

Validation runs at both ends: on write (strict, reject unknowns) and on read (lenient, tolerate unknowns from older producers). The catalog is shared across both paths.

Cross-language hazards to flag up front

High-frequency failure modes; full detail in shared/divergence-matrix.md:

• strongRef cid is a string, not a cid-link. Emitting {$link: ...} for strongRef.cid produces a different CID. See shared/record-model.md §3.
• Blob shape divergence. Modern {$type:"blob", ref:{$link}, mimeType, size} is the only shape for new writes. Legacy {cid, mimeType} is accepted on read only with ALLOW_LEGACY_BLOB / AllowLegacyBlob / equivalent.
• BlobRef is a class in TypeScript. Plain-object blobs fail assertValidRecord. Construct with new BlobRef(cid, mime, size).
• Go has two data-model stacks. atproto/data (modern) and lex/util (legacy, still emitted by generated code). Convert at the boundary between validator and generated code.
• xrpc.Client.Do arg order (Go). (ctx, kind, inpenc, method, params, body, out) — kind before method. Easy to invert.
• @atproto/xrpc Subscription export has moved. Regrep node_modules on version bumps.
• Closed vs. open unions. Adding a ref to a closed union is breaking; open unions tolerate unknown $type. Default to open.
• $type missing on a record. Strict validators reject; lenient validators have nothing to dispatch on. Never omit.
• Bluesky-domain records are out of scope. app.bsky.* facets, richtext, embeds, threadgates — point users at the Bluesky appview or @atproto/api README rather than trying to cover them here.

Tools you can call

Prefer these MCP tools when the goal is to compute or validate rather than teach an implementation how:

• lexicon-garden → describe_lexicon(nsid), validate_lexicon(doc), invoke_xrpc(method, params, input?), create_record_cid(record), transmogrify_record(record), check_compatibility(old, new).
• atpmcp → get_lexicon(nsid), validate_lexicon_schema(doc), validate_xrpc(method, params, input?), get_record(uri), invoke_xrpc(method, params, input?), generate_tid(), create_record_cid(record), parse_facets(text), transmogrify_record(record).

For record-side helpers (TIDs, AT-URI parsing), prefer in-language libraries — see the per-language records.md. For lexicon authoring or compatibility review, lexicon-garden is the fastest way to validate without spinning up a stack.

Directory layout

atproto-lexicon/
├── SKILL.md                          # this file — router
├── shared/
│   ├── lexicon-spec.md               # lexicon doc structure + validation rules
│   ├── nsid.md                       # NSID grammar and reserved prefixes
│   ├── at-uri.md                     # AT-URIs in records and refs
│   ├── record-model.md               # $type, strongRef, blob refs
│   ├── xrpc-wire.md                  # HTTP + WebSocket wire format
│   ├── backward-compat.md            # breaking-vs-non-breaking matrix
│   ├── test-vectors.md               # canonical fixtures
│   └── divergence-matrix.md          # cross-language differences
├── rust/
│   ├── README.md                     # atproto-lexicon / atproto-client setup
│   ├── authoring.md                  # BaseCatalog, load_directory, refs
│   ├── validation.md                 # validate_record, ValidateFlags, DataValue
│   ├── xrpc-client.md                # Auth::{None,DPoP,AppPassword}, atproto-jetstream
│   └── records.md                    # ATURI, Tid, strongRef, Blob, typed dispatch
├── typescript/
│   ├── README.md                     # @atproto/{lexicon,xrpc,xrpc-server,api,lex-cli}
│   ├── authoring.md                  # Lexicons, lex-cli gen-api
│   ├── validation.md                 # assertValidRecord, BlobRef class, ValidationError
│   ├── xrpc-client.md                # XrpcClient, AtpAgent, createServer, Subscription
│   └── records.md                    # AtUri, TID, BlobRef, strongRef
└── go/
    ├── README.md                     # indigo/atproto/lexicon + xrpc + events
    ├── authoring.md                  # BaseCatalog, LoadDirectory, lexgen
    ├── validation.md                 # ValidateRecord, ValidateFlags
    ├── xrpc-client.md                # xrpc.Client.Do, api/atproto, HandleRepoStream
    └── records.md                    # data.Blob vs lex/util.LexBlob, typed dispatch

References

All reachable from the tree above. Listed here for quick grep:

• shared/lexicon-spec.md, shared/nsid.md, shared/at-uri.md, shared/record-model.md, shared/xrpc-wire.md, shared/backward-compat.md, shared/test-vectors.md, shared/divergence-matrix.md
• rust/README.md, rust/authoring.md, rust/validation.md, rust/xrpc-client.md, rust/records.md
• typescript/README.md, typescript/authoring.md, typescript/validation.md, typescript/xrpc-client.md, typescript/records.md
• go/README.md, go/authoring.md, go/validation.md, go/xrpc-client.md, go/records.md