Routing

Use this skill when API path design, handler registration, route composition, and parameter behavior are core to the task.

Execution Workflow

1. Define endpoint contracts with semantic handler decorators and strict type annotations.
2. Choose composition model:
3. Register simple handlers directly on app.
4. Use routers for path grouping and nested route trees.
5. Use controllers for OOP-style endpoint grouping under a shared controller path.
6. Choose parameter sources and constraints (path/query/header/cookie/layered parameters).
7. Apply route metadata and indexing (name, opt) for discoverability and policy hooks.
8. Validate reverse-routing, unique path+method combinations, and layered precedence behavior.

Implementation Rules

• Keep URL design stable, resource-oriented, and version-aware.
• Keep handlers thin; delegate business logic to services.
• Prefer semantic decorators (@get, @post, etc.) over @route() for clearer OpenAPI operations.
• Use explicit path converters and narrow parameter types.
• Treat dynamic registration as an exception path; avoid it for routine topology.
• Apply metadata and guards at the narrowest effective scope.
• Keep type annotations complete for all handler params and return values.

Routing Topology: App, Router, Controller

App-level registration

• Litestar(route_handlers=[...]) is the root registration point.
• Registered components are appended to root path /.
• A handler can be attached to multiple paths by passing a list of paths to the decorator.

Dynamic registration

• Use app.register(handler) when runtime registration is genuinely required.
• App instance is available from connection objects (Request, WebSocket, ASGIConnection), so dynamic registration can be called from handlers/middleware/dependencies.
• Dynamic registration should be used sparingly because it increases operational complexity.

Routers

• Router can register controllers, handlers, and nested routers.
• Nested routers compose their paths; registering router A on router B appends A’s path under B’s path.

Controllers

• Controllers are Controller subclasses with a class-level path.
• Controller path prefixes each route handler method path.
• If path is omitted, it defaults to /.
• Path + HTTP method combinations must remain unique per effective routing tree.

Registering Components Multiple Times

• Controllers can be registered multiple times across different routers; each router creates its own controller instance.
• Standalone handlers can also be registered multiple times; Litestar copies handlers per registration context.
• Routers can be nested, but once a router is registered it cannot be re-registered.

Route Handlers

Core behaviors

• Handlers are built with Litestar decorators on functions or controller methods.
• Both sync and async callables are supported.
• For sync handlers:
• sync_to_thread=True runs in threadpool (safe for blocking code).
• sync_to_thread=False signals non-blocking sync behavior.
• Leaving sync behavior implicit can emit warnings.

Declaring paths

• Path can be positional (@get("/x")) or keyword (@get(path="/x")).
• Path can be a list for multi-path registration and optional path-parameter patterns.

Type annotation requirements

• All handler arguments and return values must be typed.
• Missing annotations raise ImproperlyConfiguredException during app startup.

Reserved kwargs injection

Reserved names include:

• cookies, headers, query
• request (HTTP handlers)
• socket (WebSocket handlers)
• scope, state, body (body for HTTP handlers)

If collisions occur, use alternative parameter naming patterns.

HTTP handlers

• @route() maps to HTTPRouteHandler but is generally discouraged for normal CRUD-style APIs.
• Prefer semantic decorators:
• @get, @post, @put, @patch, @delete, @head

WebSocket handlers

• @websocket() supports low-level socket handling directly.
• For higher-level real-time patterns, use dedicated WebSocket architecture patterns (see cross-skill handoff).

ASGI handlers

• @asgi() / ASGIRouteHandler supports custom ASGI apps.
• ASGI handler signature is constrained to scope, receive, send.
• ASGI handlers must be async.

Mounting ASGI Apps

• Use @asgi(..., is_mount=True) to mount sub-app behavior on a path prefix.
• Mounted handlers receive all traffic under that prefix.
• With copy_scope=True, forwarded scope["path"] is rewritten relative to mount root.
• This is useful for integrating third-party ASGI apps.

Route Indexing and Reverse Routing

• Set explicit unique name on handlers for stable lookup.
• Name uniqueness is required; duplicates raise ImproperlyConfiguredException.
• route_reverse(name, **params) can build a path.
• request.url_for(name, **params) builds absolute URLs.
• Avoid reverse lookups on handlers with multiple ambiguous matching paths; result selection can be unpredictable.

Arbitrary Route Metadata with opt

• All route decorators accept opt={...} metadata.
• Arbitrary kwargs on route decorators are merged into opt.
• opt is available to guards, request route handler context, and ASGI scope.
• opt is layered (app/router/controller/handler) and merged by precedence.
• Closest layer to the handler wins on key conflicts.

Signature Namespace Resolution

• Litestar builds runtime signature models for handlers/dependencies.
• If types are only imported under TYPE_CHECKING, resolve them via:
• signature_types=[...] on app/router/controller/handler layer as needed.
• signature_namespace={...} when alias names differ from runtime type __name__.

Default signature namespace includes:

• Headers, ImmutableState, Receive, Request, Scope, Send, State, WebSocket, WebSocketScope

Parameters: Path, Query, Header, Cookie, Layered

Path parameters

• Declared in route path as {name:type}, for example {user_id:int}.
• Supported path converter types include:
• date, datetime, decimal, float, int, path, str, time.
• Path-converter type and function annotation do not have to match 1:1 if coercion is valid.
• Path params can exist in path without being declared in the function signature; they still validate and document.

Parameter() helper

• Use Parameter(...) with Annotated[...] for extra validation and OpenAPI metadata.
• Supports constraints and docs attributes (for example gt, lt, titles/descriptions/examples).

Query parameters

• Any non-path function kwarg is treated as query param by default.
• Query parameters are required by default.
• Defaults produce optional-like behavior at runtime for omitted values.
• Optional annotations (str | None / Optional[str]) express nullable query params.
• Query values are coercible into richer types (for example datetime, numbers, lists).

Alternative names and constraints

• Remap URL query names via Parameter(query="externalName").
• Apply validation constraints via Parameter(...), for example numeric bounds.

Enum query parameter docs

• Enum docstrings feed schema descriptions by default.
• Use schema_component_key when same enum needs different parameter descriptions across endpoints.

Header and cookie parameters

• Must be declared with Parameter(header="...") or Parameter(cookie="...").
• Behavior otherwise follows query-parameter parsing/validation model.

Layered parameters

• Parameters can be declared at app/router/controller/handler levels.
• Layered declarations participate in validation and OpenAPI generation.
• Handler-local declarations can further narrow constraints from outer layers.
• Path parameters cannot be declared in non-handler layers.

Validation Checklist

• Confirm route_handlers topology resolves to intended final paths.
• Confirm unique path+method operations and unique handler names.
• Confirm controller and router path composition behaves as intended.
• Confirm sync handlers have explicit sync_to_thread decisions.
• Confirm type annotations exist for all handler args and return values.
• Confirm reserved kwargs and injected dependencies do not collide unexpectedly.
• Confirm query/header/cookie/path parameters validate and coerce as expected.
• Confirm layered parameters enforce outer constraints and handler overrides correctly.
• Confirm reverse routing (route_reverse / url_for) is deterministic for indexed handlers.
• Confirm opt metadata merge behavior is correct across layers.
• Confirm mounted ASGI apps receive expected rewritten path semantics.
• Confirm OpenAPI output matches effective paths, params, and operation decomposition.

Cross-Skill Handoffs

• Use litestar-requests and litestar-responses for transport contract depth.
• Use litestar-authentication and litestar-dependency-injection for route-scoped security/services.
• Use litestar-websockets for higher-level websocket session architecture.
• Use litestar-openapi to verify operation IDs, parameter schemas, and route docs fidelity.

Litestar References

• https://docs.litestar.dev/latest/usage/routing/index.html
• https://docs.litestar.dev/latest/usage/routing/overview.html
• https://docs.litestar.dev/latest/usage/routing/handlers.html
• https://docs.litestar.dev/latest/usage/routing/parameters.html