# Link Options Reference ## httpLink Terminating link that sends one tRPC operation per HTTP request. ```ts import { httpLink } from '@trpc/client'; httpLink({ url: 'http://localhost:3000/trpc', fetch: customFetch, transformer: superjson, headers: { Authorization: 'Bearer token' }, methodOverride: 'POST', }); ``` | Option | Type | Default | Description | | ---------------- | --------------------------------------------------------------------------------- | -------------- | --------------------------------------------- | | `url` | `string \| URL` | required | Server endpoint URL | | `fetch` | `typeof fetch` | global `fetch` | Fetch ponyfill | | `transformer` | `DataTransformerOptions` | none | Data transformer (e.g. superjson) | | `headers` | `HTTPHeaders \| (opts: { op: Operation }) => HTTPHeaders \| Promise` | `{}` | Static headers object or per-request callback | | `methodOverride` | `'POST'` | none | Force all requests as POST | ## httpBatchLink Terminating link that batches multiple operations into a single HTTP request. ```ts import { httpBatchLink } from '@trpc/client'; httpBatchLink({ url: 'http://localhost:3000/trpc', maxURLLength: 2083, maxItems: 10, headers({ opList }) { return { Authorization: `Bearer ${opList[0]?.context.token}` }; }, transformer: superjson, }); ``` | Option | Type | Default | Description | | ---------------- | --------------------------------------------------------------------------------------- | -------------- | ---------------------------------------------------- | | `url` | `string \| URL` | required | Server endpoint URL | | `fetch` | `typeof fetch` | global `fetch` | Fetch ponyfill | | `transformer` | `DataTransformerOptions` | none | Data transformer | | `headers` | `HTTPHeaders \| (opts: { opList: Operation[] }) => HTTPHeaders \| Promise` | `{}` | Headers callback receives `opList` (array), not `op` | | `maxURLLength` | `number` | `Infinity` | Split batch if URL exceeds this length | | `maxItems` | `number` | `Infinity` | Maximum operations per batch | | `methodOverride` | `'POST'` | none | Force all requests as POST | ## httpBatchStreamLink Terminating link similar to httpBatchLink but streams responses as they arrive instead of waiting for all to complete. ```ts import { httpBatchStreamLink } from '@trpc/client'; httpBatchStreamLink({ url: 'http://localhost:3000/trpc', maxURLLength: 2083, maxItems: 10, transformer: superjson, streamHeader: 'accept', }); ``` | Option | Type | Default | Description | | --------------------------- | --------------------------- | --------------- | ------------------------------------------------------------------------------------------------- | | All `httpBatchLink` options | | | Inherits all httpBatchLink options | | `streamHeader` | `'trpc-accept' \| 'accept'` | `'trpc-accept'` | Header used to signal streaming. Use `'accept'` to avoid CORS preflight on cross-origin requests. | Sends `trpc-accept: application/jsonl` (or `Accept: application/jsonl`). Response arrives as `transfer-encoding: chunked` with `content-type: application/jsonl`. Cannot set response headers (including cookies) after stream begins. ## splitLink Non-terminating link that branches the link chain based on a condition. ```ts import { httpBatchLink, httpLink, httpSubscriptionLink, splitLink, } from '@trpc/client'; splitLink({ condition: (op) => op.type === 'subscription', true: httpSubscriptionLink({ url }), false: httpBatchLink({ url }), }); ``` | Option | Type | Default | Description | | ----------- | ---------------------------- | -------- | ------------------------------------------------------------- | | `condition` | `(op: Operation) => boolean` | required | Route predicate | | `true` | `TRPCLink \| TRPCLink[]` | required | Link(s) for condition=true. Must include a terminating link. | | `false` | `TRPCLink \| TRPCLink[]` | required | Link(s) for condition=false. Must include a terminating link. | Each branch creates its own sub-chain, so both branches need a terminating link. ## loggerLink Non-terminating link that logs operations to the console. ```ts import { loggerLink } from '@trpc/client'; loggerLink({ enabled: (opts) => (process.env.NODE_ENV === 'development' && typeof window !== 'undefined') || (opts.direction === 'down' && opts.result instanceof Error), colorMode: 'ansi', }); ``` | Option | Type | Default | Description | | ------------- | -------------------------------------------------------------------- | ------------------------------------ | -------------------------------- | | `enabled` | `(opts: { direction: 'up' \| 'down'; result?: unknown }) => boolean` | `() => true` | Control when logging is active | | `logger` | `(opts: LoggerOpts) => void` | built-in pretty logger | Custom log function | | `console` | `{ log: Function; error: Function }` | `globalThis.console` | Console implementation | | `colorMode` | `'ansi' \| 'css' \| 'none'` | `'css'` in browser, `'ansi'` in Node | Color output mode | | `withContext` | `boolean` | `false` (true if css) | Include operation context in log | ## retryLink Non-terminating link that retries failed operations. ```ts import { retryLink } from '@trpc/client'; retryLink({ retry(opts) { if (opts.error.data?.code === 'INTERNAL_SERVER_ERROR') { return opts.attempts <= 3; } return false; }, retryDelayMs: (attempt) => Math.min(1000 * 2 ** attempt, 30000), }); ``` | Option | Type | Default | Description | | -------------- | -------------------------------------------- | --------- | --------------------------- | | `retry` | `(opts: { op, error, attempts }) => boolean` | required | Return true to retry | | `retryDelayMs` | `(attempt: number) => number` | `() => 0` | Delay between retries in ms | When used with subscriptions that use `tracked()`, automatically includes the last known event ID on retry. ## wsLink Terminating link for WebSocket connections. Requires a `TRPCWebSocketClient`. ```ts import { createWSClient, wsLink } from '@trpc/client'; const wsClient = createWSClient({ url: 'ws://localhost:3000', connectionParams: () => ({ token: 'supersecret' }), lazy: { enabled: true, closeMs: 10_000 }, keepAlive: { enabled: true, intervalMs: 5_000, pongTimeoutMs: 1_000 }, }); wsLink({ client: wsClient, transformer: superjson, }); ``` ### wsLink Options | Option | Type | Default | Description | | ------------- | ------------------------ | -------- | ------------------------------------ | | `client` | `TRPCWebSocketClient` | required | WebSocket client from createWSClient | | `transformer` | `DataTransformerOptions` | none | Data transformer | ### createWSClient Options | Option | Type | Default | Description | | ------------------------- | ---------------------------------------------------------------------------------------- | ------------------- | ----------------------------------------------------------------- | | `url` | `string \| (() => MaybePromise)` | required | WebSocket server URL | | `connectionParams` | `Record \| null \| (() => MaybePromise \| null>)` | `null` | Auth params sent as first message, available in `createContext()` | | `WebSocket` | `typeof WebSocket` | global `WebSocket` | WebSocket ponyfill | | `retryDelayMs` | `(attemptIndex: number) => number` | exponential backoff | Reconnection delay | | `onOpen` | `() => void` | none | Connection opened callback | | `onError` | `(evt?: Event) => void` | none | Connection error callback | | `onClose` | `(cause?: { code?: number }) => void` | none | Connection closed callback | | `lazy.enabled` | `boolean` | `false` | Close WS after inactivity | | `lazy.closeMs` | `number` | `0` | Idle timeout before closing | | `keepAlive.enabled` | `boolean` | `false` | Send ping messages | | `keepAlive.intervalMs` | `number` | `5000` | Ping interval | | `keepAlive.pongTimeoutMs` | `number` | `1000` | Close if no pong within this time | ## httpSubscriptionLink Terminating link for Server-Sent Events (SSE) subscriptions. ```ts import { httpSubscriptionLink } from '@trpc/client'; import { EventSourcePolyfill } from 'event-source-polyfill'; httpSubscriptionLink({ url: 'http://localhost:3000/trpc', connectionParams: async () => ({ token: 'supersecret' }), transformer: superjson, EventSource: EventSourcePolyfill, eventSourceOptions: async ({ op }) => ({ headers: { authorization: 'Bearer token', }, }), }); ``` | Option | Type | Default | Description | | -------------------- | ------------------------------------------------------------------------------------ | -------------------- | ----------------------------------------- | | `url` | `string \| (() => string \| Promise)` | required | Server endpoint URL | | `connectionParams` | `Record \| null \| (() => MaybePromise<...>)` | none | Serialized as URL query param | | `transformer` | `DataTransformerOptions` | none | Data transformer | | `EventSource` | EventSource constructor | global `EventSource` | EventSource ponyfill for custom headers | | `eventSourceOptions` | `EventSourceInit \| ((opts: { op }) => EventSourceInit \| Promise)` | none | Options passed to EventSource constructor | For cross-domain cookies, use `eventSourceOptions: () => ({ withCredentials: true })`. ## unstable_localLink Terminating link for direct procedure calls without HTTP. Useful for testing and server-side usage. ```ts import { unstable_localLink } from '@trpc/client'; import { appRouter } from './server'; unstable_localLink({ router: appRouter, createContext: async () => ({ db: prisma }), onError: (opts) => console.error('Error:', opts.error), }); ``` | Option | Type | Default | Description | | --------------- | ------------------------------------- | -------- | ------------------------ | | `router` | `AnyRouter` | required | tRPC router instance | | `createContext` | `() => Promise` | required | Context factory per call | | `onError` | `(opts: ErrorHandlerOptions) => void` | none | Error handler | | `transformer` | `DataTransformerOptions` | none | Data transformer |