# Jacquard: AT Protocol Library for Rust

> Simple and powerful AT Protocol (Bluesky) client library emphasizing spec compliance, flexible string backing types, and minimal boilerplate.

## Core philosophy

**Flexible string backing, zero-copy when you want it.** All API types are parameterised on `S: BosStr = DefaultStr` (where `DefaultStr = SmolStr`). By default, types use `SmolStr` for inline storage of short strings and `Arc<str>` for longer ones. When zero-copy is needed, use `CowStr<'_>` or `&str` as the backing type. `DeserializeOwned` works out of the box for `SmolStr`-backed types.

**Validated types everywhere.** DIDs, handles, AT-URIs, NSIDs, TIDs, CIDs -- all have strongly-typed, validated wrappers. Invalid inputs fail at construction time, not deep in your application logic.

**Batteries included, but replaceable.** High-level `Agent` for convenience, or use stateless `XrpcCall` builder for full control. Mix and match as needed.

---

## Critical patterns to internalize

### String backing types

All validated types (Did, Handle, AtUri, Nsid, etc.) and all generated API types are generic over `S: BosStr`:

```rust
pub struct Did<S: BosStr = DefaultStr> { /* ... */ }
pub struct Post<S: BosStr = DefaultStr> { /* ... */ }
```

The `BosStr` trait combines `Bos<str> + AsRef<str> + FromStaticStr`. Types that implement it:

| Type | Allocates? | Use case |
|------|-----------|----------|
| `SmolStr` (default) | Inline <=23 bytes, Arc for longer | General-purpose owned strings |
| `&str` | Never | Zero-copy borrowed access |
| `CowStr<'a>` | Only when owned variant needed | Borrow-or-own flexibility |
| `String` | Always on heap | Standard owned strings |

**Rule**: Use default `SmolStr` for most code. Use `CowStr<'_>` or `&str` when zero-copy parsing matters. Use `String` when interfacing with APIs that require it.

### String type constructors

ALL validated string types (Did, Handle, AtUri, Nsid, etc.) follow this pattern:

```rust
// Wraps S directly (validates). S defaults to SmolStr.
let did = Did::new(SmolStr::new("did:plc:abc123"))?;

// Zero-allocation for static strings.
let nsid = Nsid::new_static("com.atproto.repo.getRecord")?;

// Validates and produces owned (SmolStr-backed by default).
let owned = Did::new_owned("did:plc:abc123")?;

// Cheap borrow: Did<SmolStr> -> Did<&str>.
let borrowed: Did<&str> = owned.borrow();

// Cross-type conversion: Did<SmolStr> -> Did<String>.
let string_did: Did<String> = owned.convert();

// FromStr always allocates into SmolStr.
let did: Did = "did:plc:abc123".parse()?;

// AVOID: Roundtripping through String.
let s = did.as_str().to_string();
let did2 = Did::new_owned(&s)?;  // Pointless allocation.
```

**Rule**: Use `new_static()` for string literals, `new_owned()` when validating from `&str`, `borrow()` for cheap temporary access, `convert()` for cross-type conversion.

### Response parsing: choose your backing type

XRPC responses wrap a `Bytes` buffer. You choose how to deserialize:

```rust
let response = agent.send(request).await?;

// Option 1: Owned SmolStr-backed (DeserializeOwned -- just works!)
let output: GetPostOutput = response.into_output()?;
// Can return from functions, store anywhere, no lifetime concerns.

// Option 2: Zero-copy with CowStr (borrows from response buffer)
let output: GetPostOutput<CowStr<'_>> = response.parse()?;
// output borrows from response, both must stay in scope.

// Option 3: Zero-copy with &str (borrows from response buffer)
let output: GetPostOutput<&str> = response.parse()?;
// Cheapest, but output borrows from response.
```

**Rule**: Use `.into_output()` for most code -- it returns `SmolStr`-backed owned types with no lifetime concerns. Use `.parse::<CowStr<'_>>()` or `.parse::<&str>()` when zero-copy performance is critical and you can keep the response alive.

### Type system: GATs parameterised on S

Jacquard uses **Generic Associated Types** on `XrpcResp` parameterised on backing string type:

```rust
trait XrpcResp {
    type Output<S: BosStr>;  // GAT parameterised on S, not lifetime.
    type Err: Error + DeserializeOwned;  // Always owned, always SmolStr-backed.
}
```

**Why this matters**: `SmolStr`-backed types implement `DeserializeOwned`, so `.into_output()` just works. For zero-copy, callers pass `CowStr<'_>` or `&str` as `S` via `.parse()`. Error types are always owned since they are uncommon and don't need zero-copy.

**When implementing custom types**: Parameterise on `S: BosStr`, not on lifetimes. Use `#[serde(bound(deserialize = "S: Deserialize<'de> + BosStr"))]`.

### Backing type conversion

```rust
// borrow() -- cheap Did<SmolStr> -> Did<&str>
let borrowed: Did<&str> = my_did.borrow();

// convert() -- type-level conversion (e.g., SmolStr -> String)
let string_did: Did<String> = my_did.convert();

// IntoStatic -- converts CowStr<'_> -> SmolStr (or other owned)
use jacquard::common::IntoStatic;
let owned: Post = borrowed_post.into_static();

// as_str() -- get &str from any concrete S (e.g., SmolStr)
let s: &str = my_did.as_str();

// as_ref() -- get &str from generic S (via AsRef<str>)
fn generic<S: BosStr>(did: &Did<S>) -> &str {
    did.as_ref()
}
```

**Rule**: Use `.borrow()` for cheap `Type<&str>` access. Use `.convert()` for cross-type conversion. Use `.into_static()` when converting from borrowed (`CowStr<'_>`) to owned. Use `.as_str()` for concrete types, `.as_ref()` for generic `S`.

---

## Crate-by-crate guide

### jacquard (main crate)

**Primary entry point**: `Agent<A: AgentSession>`

```rust
use jacquard::client::{Agent, CredentialSession, MemorySessionStore};
use jacquard::identity::PublicResolver;

// App password auth.
let (session, _info) = CredentialSession::authenticated(
    "alice.bsky.social".into(),
    "app-password".into(),
    None,  // session_id
).await?;
let agent = Agent::from(session);

// Make typed XRPC calls.
use jacquard::api::app_bsky::feed::get_timeline::GetTimeline;
let response = agent.send(&GetTimeline::<&str>::new().limit(50).build()).await?;
let timeline = response.into_output()?;
```

**Auto-refresh**: Both `CredentialSession` and `OAuthSession` automatically refresh tokens on 401/expired errors. One retry per request.

**Typed record operations** (via `AgentSessionExt` trait):

```rust
use jacquard::api::app_bsky::feed::post::Post;

// Create.
let post = Post::builder()
    .text("Hello ATProto!")
    .created_at(Datetime::now())
    .build();
agent.create_record::<Post, _>(post, None).await?;

// Get (type-safe!).
let uri = Post::uri("at://did:plc:abc/app.bsky.feed.post/123")?;
let response = agent.get_record::<Post>(&uri).await?;
let post_output = response.into_output()?;

// Update with fetch-modify-put pattern.
agent.update_record::<Profile>(&uri, |profile| {
    profile.display_name = Some(SmolStr::new("New Name"));
}).await?;
```

**Key traits**:
- `AgentSession`: Common interface for both auth types
- `XrpcClient`: Stateful XRPC (has base URI, auth tokens)
- `HttpClient`: Low-level HTTP abstraction

**Common mistake**: Forgetting that `SmolStr`-backed types are already owned. You do not need to call `.into_static()` on types returned from `.into_output()`. The `.into_static()` method is only needed when converting from `CowStr<'_>`-backed or `&str`-backed types to owned.

### jacquard-common (foundation)

**Core types**: `Did<S>`, `Handle<S>`, `AtUri<S>`, `Nsid<S>`, `Tid`, `Cid<S>`, `CowStr<'a>`, `SmolStr`, `Data<S>`, `RawData<'a>`

**String type traits** (ALL validated types implement these):
- `new(S)` - Validates, wraps S directly
- `new_static(&'static str)` - Validates, zero alloc
- `new_owned(impl AsRef<str>)` - Validates, produces owned S
- `borrow(&self) -> Type<&str>` - Cheap borrowed view
- `convert<B: BosStr + From<S>>(self) -> Type<B>` - Cross-type conversion
- `raw(&str)` - Panics on invalid (use when you KNOW it's valid)
- `unchecked(&str)` - Unsafe, no validation
- `as_str(&self) -> &str` - Get string reference
- `Display`, `FromStr`, `Serialize`, `Deserialize`, `IntoStatic`

**SmolStr**: The default backing type. Inline storage for strings <=23 bytes (covers most handles, DIDs, rkeys). Longer strings use `Arc<str>` for O(1) clone. Implements `DeserializeOwned`.

**CowStr<'a>**: Borrow-or-own string. Uses `SmolStr` for owned variant. This type still uses lifetimes -- it is the borrow-or-own primitive.

**XRPC layer**:

```rust
// Stateless XRPC (with any HttpClient).
use jacquard::common::xrpc::XrpcExt;
let http = reqwest::Client::new();
let response = http
    .xrpc(Uri::parse("https://bsky.social").map_err(|(e, _)| e)?)
    .auth(AuthorizationToken::Bearer(token))
    .proxy(did)
    .send(&request)
    .await?;

// Stateful XRPC (implement XrpcClient trait).
let response = agent.send(request).await?;
```

**Data<S> vs RawData<'a>**:
- `Data<S>`: Validated, type-inferred atproto values (strings parsed to Did/Handle/etc.), parameterised on `S: BosStr`
- `RawData<'a>`: Minimal validation, lifetime-based (intentionally stays lifetime-based), suitable for pass-through/relay use cases

```rust
// Convert typed -> untyped -> typed.
let post = Post::builder().text("test").build();
let data: Data = to_data(&post)?;
let post2: Post = from_data(&data)?;

// NEVER use serde_json::Value.
// let value: serde_json::Value = ...;
// let data: Data = ...;
```

**Streaming** (feature: `streaming`):
- `ByteStream` / `ByteSink`: Platform-agnostic (works on WASM via `n0-future`)
- `HttpClientExt::send_http_streaming()`: Stream response
- `HttpClientExt::send_http_bidirectional()`: Stream both request and response

**WebSocket** (feature: `websocket`):
- `WebSocketClient` trait, `WebSocketConnection`
- Native + WASM: tokio-tungstenite-wasm

**Collection trait** (for record types):

```rust
pub trait Collection {
    const NSID: &'static str;
    type Record: XrpcResp;  // Marker type for get_record().
}

// Enables typed record retrieval:
let response: Response<Post::Record> = agent.get_record(did, rkey).await?;
```

**CallOptions<'a>**: Still uses lifetimes (not parameterised on S). This is intentional for the options builder pattern.

### jacquard-api (generated bindings)

**764 lexicon schemas** across 52+ namespaces.

**Feature organization**:
- `minimal`: Core atproto only
- `bluesky`: Bluesky app + chat + ozone
- `other`: Curated third-party lexicons
- `lexicon_community`: Community extensions
- `ufos`: Experimental/niche

**Generated patterns**:
- All types: `Foo<S: BosStr = DefaultStr>` with `#[serde(bound(deserialize = "S: Deserialize<'de> + BosStr"))]`
- All implement `IntoStatic`, `Serialize`, `Deserialize`, `Clone`, `PartialEq`, `Eq`
- Builders on types with 2+ fields (some required)
- Open unions: `Unknown(Data<S>)` variant via `#[open_union]` macro
- Objects: `extra_data: BTreeMap<SmolStr, Data<S>>` via `#[lexicon]` macro
- Known-values enums: `Other(S)` catch-all

**Union collision detection**: When multiple namespaces define similar types, foreign refs get prefixed:
```rust
// If both app.bsky.embed.images and sh.custom.embed.images exist:
pub enum SomeUnion<S: BosStr = DefaultStr> {
    BskyImages(Box<app_bsky::embed::images::View<S>>),
    CustomImages(Box<sh_custom::embed::images::View<S>>),
}
```

**For each collection (record type)**, generated code includes:
1. Main record struct (e.g., `Post<S: BosStr = DefaultStr>`)
2. `GetRecordOutput` wrapper (`PostGetRecordOutput<S>` with uri, cid, value)
3. Marker struct (`PostRecord`) implementing `XrpcResp`
4. `Collection` trait impl
5. Helper: `Post::uri()` for constructing typed URIs

**For each XRPC endpoint**:
1. Request struct with builder
2. Output struct
3. Error enum (open union, includes `Unknown` variant)
4. Response marker implementing `XrpcResp`
5. Request marker implementing `XrpcRequest`
6. Endpoint marker implementing `XrpcEndpoint` (server-side)

**Common mistakes**:
- Not handling `Unknown` variant in union matches (non-exhaustive!)
- Forgetting that when not using the builder pattern, or Default construction, you must supply `extra_data: BTreeMap::new()` in the constructor, in addition to explicitly named fields
- Calling `.into_static()` in tight loops (it is a no-op for SmolStr, but is unnecessary)

### jacquard-derive (macros)

**`#[lexicon]`**: Adds `extra_data` field to capture unknown fields during deserialization.

```rust
#[lexicon]
#[derive(Serialize, Deserialize)]
struct MyType<S: BosStr = DefaultStr> {
    known_field: S,
    // Macro adds: pub extra_data: BTreeMap<SmolStr, Data<S>>
}
```

**`#[open_union]`**: Adds `Unknown(Data<S>)` variant to enums.

```rust
#[open_union]
#[serde(tag = "$type")]
enum MyUnion<S: BosStr = DefaultStr> {
    KnownVariant(Foo<S>),
    // Macro adds: #[serde(untagged)] Unknown(Data<S>)
}
```

**`#[derive(IntoStatic)]`**: Generates conversion from any `S` to `SmolStr`-backed types.

```rust
#[derive(IntoStatic)]
struct Post<S: BosStr = DefaultStr> {
    text: S,
    likes: u32,
}

// Generates conversion so Post<CowStr<'_>> -> Post<SmolStr>, etc.
```

**`#[derive(XrpcRequest)]`**: Generates XRPC boilerplate for custom endpoints.

```rust
#[derive(Serialize, Deserialize, XrpcRequest)]
#[xrpc(
    nsid = "com.example.getThing",
    method = Query,
    output = GetThingOutput,
    error = GetThingError,  // Optional, defaults to GenericError.
    server  // Optional, generates XrpcEndpoint marker.
)]
struct GetThing {
    pub id: SmolStr,
}
```

**Critical**: Custom types used as XRPC outputs must derive `IntoStatic` if they will be used with `.parse()` and then converted. For `.into_output()`, they just need `DeserializeOwned` (which `SmolStr`-backed types get for free).

### jacquard-oauth (OAuth/DPoP)

**Session types parameterised on S** for flexible backing storage.

**OAuth flow**:

```rust
use jacquard::oauth::client::OAuthClient;
use jacquard::client::FileAuthStore;

let oauth = OAuthClient::with_default_config(
    FileAuthStore::new("./auth.json")
);

// Loopback flow (feature: loopback).
let session = oauth.login_with_local_server(
    "alice.bsky.social",
    Default::default(),
    LoopbackConfig::default(),
).await?;

let agent = Agent::from(session);
```


**OAuthMetadata<S>**: Parameterised so callers can borrow from stored metadata.

**Nonce handling**: Automatic retry on `use_dpop_nonce` errors (400 for auth server, 401 for PDS). Max one retry per request.

**Token refresh**: Automatic on `invalid_token` errors. Uses `SessionRegistry` with per-DID+session_id locks to prevent concurrent refresh races.

**private_key_jwt**: For non-loopback clients. Automatically used if server supports it.

### jacquard-identity (identity resolution)

**Resolution chains** (configurable fallback order):

**Handle -> DID**:
1. DNS TXT `_atproto.<handle>` (feature: `dns`, skipped on WASM)
2. HTTPS `https://<handle>/.well-known/atproto-did`
3. PDS XRPC `com.atproto.identity.resolveHandle`
4. Public API fallback `https://public.api.bsky.app` (if enabled)
5. Slingshot mini-doc (if configured)

**DID -> Document**:
1. `did:web`: HTTPS `.well-known/did.json`
2. `did:plc`: PLC directory or Slingshot
3. PDS XRPC `com.atproto.identity.resolveDid`

**Methods are generic over backing type**:
```rust
// resolve_handle accepts any Handle<S> backing type.
let did: Did = resolver.resolve_handle(&handle).await?;
// Returns Did (= Did<SmolStr>) for owned results.

// resolve_did_doc same pattern.
let response = resolver.resolve_did_doc(&did).await?;
let doc = response.parse_validated()?;  // Validates doc.id matches requested DID.

// Combined: get PDS endpoint.
let pds_url = resolver.pds_for_did(&did).await?;
```

**DidDocResponse pattern** (same as XRPC responses):

```rust
let response = resolver.resolve_did_doc(&did).await?;

// Borrow from buffer.
let doc: DidDocument<CowStr<'_>> = response.parse()?;

// Validate doc ID.
let doc = response.parse_validated()?;  // Error if doc.id != requested DID.

// Convert to owned.
let doc: DidDocument = response.into_owned()?;
```

**Mini-doc fallback**: If full DID document parsing fails, automatically tries parsing as `MiniDoc` (Slingshot's minimal format) and synthesizes a minimal `DidDocument`. This is transparent to caller.

**OAuthResolver trait**: Auto-implemented for `JacquardResolver`. Adds OAuth metadata resolution:

```rust
// High-level: accepts handle, DID, or HTTPS URL.
let (server_metadata, doc_opt) = resolver.resolve_oauth("alice.bsky.social").await?;

// From identity (handle or DID).
let (server_metadata, doc) = resolver.resolve_from_identity("alice.bsky.social").await?;

// From service URL (PDS or entryway).
let server_metadata = resolver.resolve_from_service(&pds_url).await?;

// Verify issuer authority over DID.
let pds = resolver.verify_issuer(&server_metadata, &sub_did).await?;
```

### jacquard-axum (server-side)

**Note**: jacquard-axum is temporarily out of the workspace while the extractor is redesigned for the BosStr type system.

**ExtractXrpc**: Type-safe XRPC request extraction, parameterised on `S: BosStr`.

```rust
use jacquard_axum::{ExtractXrpc, IntoRouter};
use jacquard::api::com_atproto::identity::resolve_handle::ResolveHandleRequest;
use jacquard::common::DefaultStr;

async fn handle_resolve(
    ExtractXrpc(req): ExtractXrpc<ResolveHandleRequest, DefaultStr>
) -> Json<ResolveHandleOutput> {
    let did = resolve_handle_logic(&req.handle).await;
    Json(ResolveHandleOutput { did, extra_data: Default::default() })
}

// Automatic routing.
let app = Router::new()
    .merge(ResolveHandleRequest::into_router(handle_resolve));
```

**Query vs Procedure**:
- Query (GET): Extracts from query string via `serde_html_form`
- Procedure (POST): Calls `Request::decode_body()` (default: JSON, override for CBOR)

**Custom encodings**:

```rust
impl XrpcRequest for MyRequest {
    fn decode_body(body: &[u8]) -> Result<Box<Self>> {
        let req = serde_ipld_dagcbor::from_slice(body)?;
        Ok(Box::new(req))
    }
}
```

**Service auth** (feature: `service-auth`):

```rust
use jacquard_axum::service_auth::{ServiceAuthConfig, ExtractServiceAuth};

let config = ServiceAuthConfig::new(
    Did::new_static("did:web:feedgen.example.com")?,
    resolver,
);

async fn handler(
    ExtractServiceAuth(auth): ExtractServiceAuth,
) -> String {
    format!("Authenticated as {}", auth.did())
}

let app = Router::new()
    .route("/xrpc/app.bsky.feed.getFeedSkeleton", get(handler))
    .with_state(config);
```

**Method binding (`lxm` claim)**: Default enabled. Binds JWTs to specific XRPC methods to prevent token reuse across endpoints.

**JTI replay protection**: NOT built-in. You must implement:

```rust
if let Some(jti) = auth.jti() {
    if state.seen_jtis.contains(jti) {
        return Err(StatusCode::UNAUTHORIZED);
    }
    state.seen_jtis.insert(jti.to_string(), auth.exp);
}
```

**Common mistakes**:
- Forgetting the second type parameter on `ExtractXrpc<Request, S>` (defaults to `DefaultStr`)
- Using wrong trait (use `XrpcEndpoint` marker, not `XrpcRequest`)
- Not implementing JTI tracking (allows replay attacks)
- Disabling method binding without understanding security implications

### jacquard-repo (repository primitives)

**MST (Merkle Search Tree)**: Immutable, persistent data structure.

```rust
use jacquard_repo::mst::Mst;
use jacquard_repo::storage::MemoryBlockStore;

let storage = MemoryBlockStore::new();
let mst = Mst::new(storage.clone());

// IMMUTABLE: Always reassign.
let mst = mst.add("app.bsky.feed.post/abc", record_cid).await?;
let mst = mst.add("app.bsky.feed.post/xyz", record_cid2).await?;

// Persist and get root CID.
let root_cid = mst.persist().await?;
```

**Key validation**: `[a-zA-Z0-9._:~-]+` with exactly one `/` separator. Max 256 bytes. Format: `collection/rkey`.

**Diff operations**:

```rust
let diff = old_mst.diff(&new_mst).await?;
diff.validate_limits()?;  // Enforce 200 op limit (protocol).

// Convert to different formats.
let verified_ops = diff.to_verified_ops();  // For batch().
let repo_ops = diff.to_repo_ops();  // For firehose.
```

**Commits**:

```rust
use jacquard_repo::commit::Commit;

// Create and sign.
let commit = Commit::new_unsigned(did, data_cid, rev, prev_cid)
    .sign(&signing_key)?;

// Verify.
commit.verify(&public_key)?;
```

**Supported signature algorithms**: Ed25519, ECDSA P-256, ECDSA secp256k1.

**Firehose validation**:

```rust
// Sync v1.0 (requires prev MST state).
let new_root = commit.validate_v1_0(prev_mst_root, prev_storage, pubkey).await?;

// Sync v1.1 (inductive, requires prev_data field + op prev CIDs).
let new_root = commit.validate_v1_1(pubkey).await?;
```

**v1.1 inductive validation**: Inverts operations on claimed result, verifies inverted result matches `prev_data`. Requires:
- `prev_data` field in commit
- All operations have `prev` CIDs for updates/deletes
- All required MST blocks in CAR

**CAR I/O**:

```rust
// Read.
let parsed = parse_car_bytes(&bytes)?;
let root_cid = parsed.root;
let blocks = parsed.blocks;  // BTreeMap<CID, Bytes>

// Write.
let bytes = write_car_bytes(root_cid, blocks)?;
```

**BlockStore trait**: Pluggable storage backend.

```rust
#[trait_variant::make(Send)]  // Conditionally Send on non-WASM.
pub trait BlockStore: Clone {
    async fn get(&self, cid: &CID) -> Result<Option<Bytes>>;
    async fn put(&self, data: &[u8]) -> Result<CID>;
    async fn apply_commit(&self, commit: CommitData) -> Result<()>;  // Atomic.
}
```

**Implementations**:
- `MemoryBlockStore`: In-memory (testing)
- `FileBlockStore`: File-based (persistent)
- `LayeredBlockStore`: Read-through cache (e.g., temp over persistent for firehose)

**Repository API** (high-level):

```rust
use jacquard_repo::repo::Repository;

let repo = Repository::create(storage, did, signing_key, None).await?;

// Single operations (don't auto-commit).
repo.create_record(collection, rkey, cid).await?;
let old_cid = repo.update_record(collection, rkey, new_cid).await?;
let deleted_cid = repo.delete_record(collection, rkey).await?;

// Batch commit.
let ops = vec![
    RecordWriteOp::Create { collection, rkey, record },
    RecordWriteOp::Update { collection, rkey, record, prev },
];
let (repo_ops, commit_data) = repo.create_commit(&ops, &did, prev, &key).await?;
repo.apply_commit(commit_data).await?;
```

**Common mistakes**:
- Forgetting immutability (not reassigning MST operations)
- Not calling `validate_limits()` before creating commits (protocol violation)
- Using v1.1 validation without `prev_data` field (will fail)
- Missing `prev` CIDs on update/delete operations for v1.1
- Not implementing `Clone` cheaply on custom `BlockStore` (use `Arc` internally)
- Ignoring CID mismatch errors (indicates data corruption)

### jacquard-lexicon (code generation)

**ONLY use `just` commands** for code generation:

```bash
just lex-gen      # Fetch + generate
just lex-fetch    # Fetch only
just generate-api # Generate from existing lexicons
```

**Union collision detection**: When multiple namespaces have similar type names in a union, foreign refs get prefixed with second NSID segment:

```
app.bsky.embed.images -> BskyImages
sh.custom.embed.images -> CustomImages
```

**Builder heuristics**:
- Has builder: 1+ required fields, not all bare `S`
- Has `Default`: 0 required fields OR all required are bare `S`

**Empty objects**: Generate as empty structs with `#[lexicon]` attribute (adds `extra_data`), not as `Data<S>`.

**Local refs**: `#fragment` normalized to `{current_nsid}#fragment` during generation.

**Feature generation**: Tracks cross-namespace dependencies:

```toml
net_anisota = ["app_bsky"]  # Uses Bluesky embeds
```

**Token types**: Unit structs with `Display` impl:

```rust
pub struct ClickthroughAuthor;
impl Display for ClickthroughAuthor {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "clickthroughAuthor")
    }
}
```

**Common mistakes**: Running codegen commands manually without `just` (wrong flags, paths).

---

## Anti-patterns to avoid

### Roundtripping through String

```rust
// BAD.
let did_str = did.as_str().to_string();
let did2 = Did::new_owned(&did_str)?;

// GOOD.
let did2 = did.clone();
// Or for type conversion:
let did2: Did<String> = did.convert();
```

### Using serde_json::Value

```rust
// NEVER.
let value: serde_json::Value = serde_json::from_slice(bytes)?;
let post: Post = serde_json::from_value(value)?;

// ALWAYS.
let data: Data = serde_json::from_slice(bytes)?;
let post: Post = from_data(&data)?;
```

### Using FromStr when new_static() or new_owned() suffices

```rust
// SLOWER (FromStr always allocates into SmolStr).
let did: Did = "did:plc:abc".parse()?;

// BETTER (validates and wraps into SmolStr).
let did = Did::new_owned("did:plc:abc")?;

// BEST for static strings (zero allocation).
let did = Did::new_static("did:plc:abc")?;
```

### Not handling Unknown variant in unions

```rust
// WILL NOT COMPILE (missing Unknown variant).
match embed {
    PostEmbed::Images(img) => { /* ... */ }
    PostEmbed::Video(vid) => { /* ... */ }
}

// HANDLE ALL VARIANTS.
match embed {
    PostEmbed::Images(img) => { /* ... */ }
    PostEmbed::Video(vid) => { /* ... */ }
    _ => { /* Unknown or other variants */ }
}
```

### Forgetting MST immutability

```rust
// WRONG (loses result).
mst.add(key, cid).await?;

// CORRECT (reassign).
let mst = mst.add(key, cid).await?;
```


### Using DeserializeOwned with CowStr or &str backing types

`DeserializeOwned` (i.e. `for<'de> Deserialize<'de>`) works perfectly with `SmolStr`-backed types. This is fine:

```rust
// FINE -- SmolStr-backed types implement DeserializeOwned.
let post: Post = serde_json::from_slice(&bytes)?;
let post: Post<SmolStr> = serde_json::from_reader(reader)?;
response.into_output()?;  // Uses DeserializeOwned internally.
```

However, it does NOT work with `CowStr<'_>` or `&str` backing types, because those need to borrow from the input buffer:

```rust
// WRONG -- CowStr needs to borrow from input.
fn bad(data: &[u8]) -> Post<CowStr<'static>> {
    serde_json::from_slice(data).unwrap()  // Can't borrow from data!
}

// CORRECT -- use method-level lifetime for zero-copy.
fn good(data: &[u8]) -> Post<CowStr<'_>> {
    serde_json::from_slice(data).unwrap()  // Borrows from data.
}

// ALSO CORRECT -- use SmolStr when you don't need zero-copy.
fn also_good(data: &[u8]) -> Post {
    serde_json::from_slice(data).unwrap()  // DeserializeOwned, no borrowing needed.
}
```

**Rule**: `DeserializeOwned` is fine for `SmolStr`-backed types (the default). Only avoid it when you specifically want zero-copy with `CowStr<'_>` or `&str`. Use `.into_output()` for owned results and `.parse::<CowStr<'_>>()` for zero-copy.

### Calling .into_static() unnecessarily

```rust
// WASTEFUL -- SmolStr types are already owned.
let output = response.into_output()?;
let owned = output.into_static();  // No-op! Already SmolStr-backed.

// USEFUL -- converting from CowStr to SmolStr.
let borrowed: Post<CowStr<'_>> = response.parse()?;
let owned: Post = borrowed.into_static();  // Converts CowStr -> SmolStr.
```

### Using .as_ref() on SmolStr when .as_str() is clearer

`SmolStr` implements `AsRef<str>` but also `AsRef<[u8]>`, which can cause ambiguity:

```rust
// AMBIGUOUS -- compiler can't infer which AsRef.
let s = my_smol.as_ref();

// CLEAR -- use as_str() for concrete SmolStr.
let s: &str = my_smol.as_str();

// FINE -- as_ref() works in generic context where S: BosStr.
fn generic<S: BosStr>(val: &Did<S>) -> &str {
    val.as_ref()
}
```

---

## WASM compatibility

Core crates support `wasm32-unknown-unknown` target:
- jacquard-common
- jacquard-api
- jacquard-identity (no DNS resolution)
- jacquard-oauth

**Pattern**: `#[cfg_attr(not(target_arch = "wasm32"), trait_variant::make(Send))]`

**What's different on WASM**:
- No `Send` bounds on traits
- DNS resolution skipped in handle->DID chain
- Tokio-specific features disabled

**Test WASM compilation**:

```bash
just check-wasm
# or
cargo build --target wasm32-unknown-unknown -p jacquard-common --no-default-features
```

---

## Quick reference

### String type constructors

| Method | Allocates? | Use when |
|--------|-----------|----------|
| `new(S)` | Depends on S | Wrapping an already-constructed S |
| `new_static(&'static str)` | No | Static string literal |
| `new_owned(impl AsRef<str>)` | SmolStr: inline/Arc | Validating from &str |
| `borrow(&self) -> Type<&str>` | No | Cheap temporary access |
| `convert::<B>(self) -> Type<B>` | Depends on B | Cross-type conversion |
| `FromStr::parse()` | SmolStr: inline/Arc | Don't care about explicitness |

### Response parsing

| Method | Backing type | Allocates? | Use when |
|--------|-------------|-----------|----------|
| `.into_output()` | `SmolStr` | Yes (DeserializeOwned) | Most code -- owned, no lifetime concerns |
| `.parse::<CowStr<'_>>()` | `CowStr<'_>` | No (zero-copy) | Performance-critical, response stays alive |
| `.parse::<&str>()` | `&str` | No (zero-copy) | Cheapest, response stays alive |
| `.parse_data()` | `CowStr<'_>` | No | Untyped Data access |
| `.parse_raw()` | `'_` | No | Untyped RawData access |

### XRPC traits

| Trait | Side | Key types |
|-------|------|-----------|
| `XrpcRequest` | Client + Server | `type Response: XrpcResp`, requires `Serialize` |
| `XrpcResp` | Client + Server | `type Output<S: BosStr>`, `type Err: DeserializeOwned` |
| `XrpcEndpoint` | Server | `type Request<S: BosStr>`, `type Response: XrpcResp` |
| `XrpcClient` | Client | `base_uri()`, `send()` |
| `XrpcExt` | Client | Stateless XRPC builder on any HttpClient |

### Session types

| Type | Auth method | Auto-refresh | Storage |
|------|------------|--------------|---------|
| `CredentialSession` | Bearer (app password) | Via refreshSession | SessionStore |
| `OAuthSession` | DPoP (OAuth) | Via token endpoint | ClientAuthStore |

### BlockStore implementations

| Type | Persistent? | Use case |
|------|------------|----------|
| `MemoryBlockStore` | No | Testing |
| `FileBlockStore` | Yes | Production |
| `LayeredBlockStore` | Depends | Read-through cache |

---

## Common operations

### Making an XRPC call

```rust
use jacquard::api::app_bsky::feed::get_author_feed::GetAuthorFeed;

let request = GetAuthorFeed::new()
    .actor("alice.bsky.social".into())
    .limit(50)
    .build();

let response = agent.send(request).await?;
let output = response.into_output()?;

for post in output.feed {
    println!("{}: {}", post.post.author.handle, post.post.uri);
}
```

### Creating a record

```rust
use jacquard::api::app_bsky::feed::post::Post;
use jacquard::common::types::string::Datetime;

let post = Post::builder()
    .text("Hello ATProto from Jacquard!")
    .created_at(Datetime::now())
    .build();

agent.create_record(post, None).await?;
```

### Resolving identity

```rust
use jacquard::identity::PublicResolver;

let resolver = PublicResolver::default();

// Handle -> DID.
let did = resolver.resolve_handle(&handle).await?;

// DID -> PDS endpoint.
let pds = resolver.pds_for_did(&did).await?;

// Combined.
let (did, pds) = resolver.pds_for_handle(&handle).await?;
```

### OAuth login

```rust
use jacquard::oauth::client::OAuthClient;
use jacquard::client::FileAuthStore;

let oauth = OAuthClient::with_default_config(
    FileAuthStore::new("./auth.json")
);

let session = oauth.login_with_local_server(
    "alice.bsky.social",
    Default::default(),
    Default::default(),
).await?;

let agent = Agent::from(session);
```

### Working with different backing types

```rust
// Default SmolStr-backed (most common).
let did = Did::new_owned("did:plc:abc123")?;

// Cheap borrow for function calls.
let borrowed: Did<&str> = did.borrow();
some_function(&borrowed);

// Zero-copy parsing.
let response = agent.send(request).await?;
let output: GetPostOutput<CowStr<'_>> = response.parse()?;
// Process output while response is alive...

// Convert to owned when needed.
let owned: GetPostOutput = output.into_static();
```

### Server-side XRPC handler

```rust
use jacquard_axum::{ExtractXrpc, IntoRouter};
use jacquard::common::DefaultStr;
use axum::{Router, Json};

async fn handler(
    ExtractXrpc(req): ExtractXrpc<MyRequest, DefaultStr>
) -> Json<MyOutput> {
    // Process request.
    Json(output)
}

let app = Router::new()
    .merge(MyRequest::into_router(handler));
```

### MST operations

```rust
use jacquard_repo::mst::Mst;
use jacquard_repo::storage::MemoryBlockStore;

let storage = MemoryBlockStore::new();
let mst = Mst::new(storage.clone());

let mst = mst.add("app.bsky.feed.post/abc123", record_cid).await?;
let mst = mst.add("app.bsky.feed.post/xyz789", record_cid2).await?;

let root_cid = mst.persist().await?;
```

---

## Documentation links

- [docs.rs/jacquard](https://docs.rs/jacquard/latest/jacquard/)
- [docs.rs/jacquard-common](https://docs.rs/jacquard-common/latest/jacquard_common/)
- [docs.rs/jacquard-api](https://docs.rs/jacquard-api/latest/jacquard_api/)
- [docs.rs/jacquard-oauth](https://docs.rs/jacquard-oauth/latest/jacquard_oauth/)
- [docs.rs/jacquard-identity](https://docs.rs/jacquard-identity/latest/jacquard_identity/)
- [docs.rs/jacquard-repo](https://docs.rs/jacquard-repo/latest/jacquard_repo/)
- [docs.rs/jacquard-axum](https://docs.rs/jacquard-axum/latest/jacquard_axum/)
- [Repository](https://tangled.org/@nonbinary.computer/jacquard)

---

## Philosophy summary

Jacquard is designed for **correctness**, **performance**, and **ergonomics** in that order. It favors:

1. **Validation at construction time** - Invalid inputs fail fast, not deep in your code
2. **Flexible string backing** - `SmolStr` for owned types with `DeserializeOwned` support, `CowStr`/`&str` for zero-copy when needed
3. **Explicit type control** - You choose the backing type via `S: BosStr`, with sensible defaults
4. **Type safety without boilerplate** - Generated bindings just work, with strong typing and builders
5. **Batteries included, but replaceable** - High-level `Agent` for convenience, low-level primitives for control

**When in doubt**: Use the default `SmolStr`-backed types, call `.into_output()` for owned responses, use `.borrow()` for cheap access, and trust the type system. Jacquard is designed to guide you toward correct, performant code.