diff --git a/Readme.md b/Readme.md
index a9bfc24..fdd9046 100644
--- a/Readme.md
+++ b/Readme.md
@@ -9,16 +9,16 @@ background script from a tab’s content script, or vice versa.
 
 In `background.js`:
 
-    import { makeRemotelyCallable } from 'webextension-rpc'
+    import { RpcServer } from 'webextension-rpc'
     async function myFunc(arg) {
       return arg * 2
     }
-    makeRemotelyCallable({ myFunc })
+    new RpcServer({ myFunc })
 
 In `content_script.js`:
 
-    import { remoteFunction } from 'webextension-rpc'
-    const myRemoteFunc = remoteFunction('myFunc')
+    import { RpcClient } from 'webextension-rpc'
+    const myRemoteFunc = new RpcClient().func('myFunc')
     await myRemoteFunc(21) // 42!
 
 Note that the remote function always returns a `Promise`, which resolves with the remote function’s
@@ -39,30 +39,42 @@ This module is published [on npm](https://www.npmjs.com/package/webextension-rpc
 
 Run `npm install webextension-rpc` or equivalent, and in your code import what you need, e.g.:
 
-    import { makeRemotelyCallable } from 'webextension-rpc'
-
-Or copy its `lib/index.js` and import from that if you prefer (this module has no dependencies).
+    import { RpcClient, RpcServer } from 'webextension-rpc'
 
 
 ## API
 
-### `remoteFunction(functionName, { tabId })`
+### RpcClient
+
+#### `new RpcClient(options?)` (constructor)
+
+Instantiate the RpcClient.
+
+Arguments:
+- `options` (object, optional):
+  - `options.tabId` (number): The id of the tab whose content script is the remote side. Leave undefined or
+    null to invoke functions in the background script (from a content script).
+
+#### `func(functionName, options?)`
 
 Create a proxy function that invokes the specified remote function.
 
+Arguments:
 - `functionName` (string, required): name of the function as registered on the remote side.
-- `options` (object, optional):
-  - `tabId` (number): The id of the tab whose content script is the remote side. Leave undefined
-    to call the background script (from a content script).
+- `options` (object, optional): override any options passed to the constructor.
+
+
+### RpcServer
 
-### `makeRemotelyCallable(functions, { insertExtraArg })`
+#### `new RpcServer(functions)` (constructor)
 
-Register one or more functions to enable remote scripts to call them. Arguments:
+Register one or more functions to enable remote scripts to call them.
+
+Arguments:
 
 - `functions` (object, required): An object with a `{ functionName: function }` mapping. Each
   function will be remotely callable using the given name.
 
-
 ### `injectRpcInfo`
 
 If the special symbol `injectRpcInfo` is passed as the first argument to a proxy function, this
diff --git a/example/background-script.ts b/example/background-script.ts
index 1e3faa4..037a564 100644
--- a/example/background-script.ts
+++ b/example/background-script.ts
@@ -1,19 +1,19 @@
-import { remoteFunction, makeRemotelyCallable } from 'webextension-rpc';
+import { RpcClient, RpcServer } from 'webextension-rpc';
 import type { RpcInfo } from 'webextension-rpc';
 // Only import the *types* of the remote script’s functions.
-import type { contentScriptRemoteFunctions } from './content-script';
+import type { contentScriptRpcServer } from './content-script';
 
 // From background to content script.
-const setColour = remoteFunction<typeof contentScriptRemoteFunctions.setColour>(
-  'setColour',
-  { tabId: 123 },
-);
+const contentScriptRpc = new RpcClient<typeof contentScriptRpcServer>({ tabId: 123 });
+const setColour = contentScriptRpc.func('setColour');
 await setColour('blue');
 
 // From content to background script.
-export const backgroundScriptRemoteFunctions = makeRemotelyCallable({
+const backgroundScriptRpcServer = new RpcServer({
   async duplicateTab(rpcInfo: RpcInfo, active: boolean) {
     const newTab = await browser.tabs.duplicate(rpcInfo.tab.id, { active });
     return newTab.id;
   },
+  async timesTwo(x: number) { return 2 * x },
 });
+export type { backgroundScriptRpcServer }
diff --git a/example/content-script.ts b/example/content-script.ts
index e381709..d9ff878 100644
--- a/example/content-script.ts
+++ b/example/content-script.ts
@@ -1,22 +1,20 @@
 import {
-  remoteFunction,
-  makeRemotelyCallable,
+  RpcClient,
+  RpcServer,
   injectRpcInfo,
 } from 'webextension-rpc';
 // Only import the *types* of the remote script’s functions.
-import type { backgroundScriptRemoteFunctions } from './background-script';
+import type { backgroundScriptRpcServer } from './background-script';
 
 // From background to content script.
-export const contentScriptRemoteFunctions = makeRemotelyCallable({
+const contentScriptRpcServer = new RpcServer({
   async setColour(colour: string) {
     document.body.style.backgroundColor = colour;
   },
 });
+export type { contentScriptRpcServer }
 
 // From content to background script.
-const duplicateMe =
-  remoteFunction<typeof backgroundScriptRemoteFunctions.duplicateTab>(
-    'duplicateTab',
-  );
+const duplicateMe = new RpcClient<typeof backgroundScriptRpcServer>().func('duplicateTab');
 // The injectRpcInfo placeholder will be replaced by actual info.
 const newTabId = await duplicateMe(injectRpcInfo, true);
diff --git a/package.json b/package.json
index 93edcda..8f8d1da 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,21 @@
   "description": "Remote Procedure Call implementation for WebExtensions, to easily call functions across content scripts and background script.",
   "main": "./lib/index.js",
   "types": "./lib/index.d.ts",
+  "exports": {
+    ".": {
+      "import": "./lib/index.js",
+      "types": "./lib/index.d.ts"
+    },
+    "./RpcClient": {
+      "import": "./lib/RpcClient.js",
+      "types": "./lib/RpcClient.d.ts"
+    },
+    "./RpcServer": {
+      "import": "./lib/RpcServer.js",
+      "types": "./lib/RpcServer.d.ts"
+    }
+  },
+  "files": ["lib"],
   "scripts": {
     "prepare": "tsc",
     "test": "ava"
diff --git a/src/RpcClient.ts b/src/RpcClient.ts
new file mode 100644
index 0000000..04c21f8
--- /dev/null
+++ b/src/RpcClient.ts
@@ -0,0 +1,138 @@
+import type { RpcServer } from "./RpcServer"
+import { isRpcResponseMessage, RPC_CALL } from "./common"
+import type { AsyncFunction, RpcCallMessage, RpcResponseMessage, ReplaceRpcInfo } from "./common"
+
+export type { AsyncFunction }
+
+/**
+ * Error thrown when the remote function could not be found/executed.
+ */
+export class RpcError extends Error {
+	constructor(message: string) {
+		super(message)
+		this.name = this.constructor.name
+	}
+}
+
+/**
+ * Error thrown when the remote function threw an error.
+ */
+export class RemoteError extends Error {
+	constructor(message: string) {
+		super(message)
+		this.name = this.constructor.name
+	}
+}
+
+/**
+ * If the special symbol `injectRpcInfo` is passed as the first argument to a proxy function, this
+ * argument will be replaced on the executing side by an `RpcInfo` object.
+ */
+export const injectRpcInfo = Symbol('RpcInfo')
+
+export interface RpcOptions {
+	/**
+	 * The id of the tab whose content script is the remote side. Leave undefined
+	 * to call the background script (from a content script).
+	 */
+	tabId?: number,
+}
+
+/**
+ * RpcFunction<F> equals the function F, except for two tweaks:
+ * - In the parameters, any RpcInfo is swapped for rpcInfoSymbol.
+ * - In the return type, nested promises become one promise.
+ */
+export type RpcFunction<F extends AsyncFunction> = (
+	...args: ReplaceRpcInfo<Parameters<F>, typeof injectRpcInfo>
+) => Promise<Awaited<ReturnType<F>>>
+
+type FunctionsOf<R extends RpcServer> = R['functions']
+
+/**
+ * Define a ‘connection’ for remote procedure calls.
+ * @param options.tabId - The id of the tab whose content script is the remote side. Leave undefined
+ * to call functions of the background script (from a content script).
+ */
+export class RpcClient<TheRpcServer extends RpcServer> {
+	tabId: number | undefined
+
+	constructor(options: RpcOptions = {}) {
+		this.tabId = options.tabId;
+
+	}
+
+	/**
+	 * Create a proxy function that invokes the specified remote function.
+	 * @param funcName - Name of the function as registered on the remote side.
+	 * @param options.tabId - Overrides the `tabId` passed to `RpcClient`.
+	 * @returns The proxy function.
+	 */
+	func<FunctionName extends string & keyof FunctionsOf<TheRpcServer>>(
+		funcName: FunctionName,
+		{
+			tabId = this.tabId,
+		}: RpcOptions = {},
+	): RpcFunction<FunctionsOf<TheRpcServer>[FunctionName]> {
+		type RemoteFunction = FunctionsOf<TheRpcServer>[FunctionName]
+
+		const otherSide = (tabId !== undefined)
+			? "the tab's content script"
+			: 'the background script'
+
+		const f: RpcFunction<RemoteFunction> = async function (...args): ReturnType<RpcFunction<RemoteFunction>> {
+			const message: RpcCallMessage = {
+				__WEBEXTENSION_RPC_MESSAGE__: RPC_CALL,
+				funcName,
+				args,
+				addRpcInfoAsArgument: false,
+			}
+
+			if (args.includes(injectRpcInfo)) {
+				const argIndex = args.indexOf(injectRpcInfo)
+				message.addRpcInfoAsArgument = argIndex
+				message.args[argIndex] = null
+			}
+
+			// Try send the message and await the response.
+			let response: RpcResponseMessage<RemoteFunction>
+			try {
+				response = (tabId !== undefined && tabId !== null)
+					? await browser.tabs.sendMessage(tabId, message)
+					: await browser.runtime.sendMessage(message)
+			} catch (err) {}
+
+			// Check if we got an error or no response.
+			if (response === undefined) {
+				throw new RpcError(
+					`Got no response when trying to call '${funcName}'. `
+					+ `Did you enable RPC in ${otherSide}?`
+				)
+			}
+
+			// Check if it was *our* listener that responded.
+			if (!isRpcResponseMessage(response)) {
+				throw new RpcError(
+					`RPC got a response from an interfering listener while calling '${funcName}' in `
+					+ `${otherSide}`
+				)
+			}
+
+			// If we could not invoke the function on the other side, throw an error.
+			if ('rpcError' in response) {
+				throw new RpcError(response.rpcError)
+			}
+
+			// Return the value or throw the error we received from the other side.
+			if ('errorMessage' in response) {
+				throw new RemoteError(response.errorMessage)
+			} else {
+				return response.returnValue
+			}
+		}
+
+		// Give it a name, could be helpful in debugging
+		Object.defineProperty(f, 'name', { value: `${funcName}_RPC` })
+		return f
+	}
+}
diff --git a/src/RpcServer.ts b/src/RpcServer.ts
new file mode 100644
index 0000000..021277f
--- /dev/null
+++ b/src/RpcServer.ts
@@ -0,0 +1,75 @@
+import { isRpcCallMessage, RPC_RESPONSE } from './common'
+import type { AsyncFunction, RpcCallMessage, RpcResponseMessage } from './common'
+
+export type { AsyncFunction }
+
+/**
+ * If the special symbol `injectRpcInfo` is passed as the first argument to a proxy function, this
+ * argument will be replaced on the executing side by an `RpcInfo` object.
+ */
+ export interface RpcInfo {
+	tab: browser.tabs.Tab,
+}
+
+/**
+ * Register one or more functions to enable remote scripts to call them.
+ *
+ * @param functions - A `{ functionName: function }` mapping. Each function will be remotely
+ * callable using the given name.
+ */
+export class RpcServer<Fs extends Record<string, AsyncFunction> = Record<string, AsyncFunction>> {
+	constructor(
+		public readonly functions: Fs,
+	) {
+		browser.runtime.onMessage.addListener(this.incomingRPCListener.bind(this))
+	}
+
+	// TODO Avoid conflict if there are multiple listeners.
+	private incomingRPCListener(
+		message: any,
+		sender: browser.runtime.MessageSender,
+	): undefined | Promise<RpcResponseMessage> {
+		if (!isRpcCallMessage(message)) return
+
+		// TODO Support extension popups and other pages, not just background script & tabs.
+		// Each page gets the message, so we may need to name each endpoint.
+		// Then here we should return if the message was not for us.
+
+		return this.executeRpc(message, sender)
+	}
+
+	private async executeRpc(message: RpcCallMessage, sender: browser.runtime.MessageSender): Promise<RpcResponseMessage> {
+		const funcName = message.funcName
+		const func = this.functions[funcName]
+		if (func === undefined) {
+			console.error(`Received RPC for unknown function: ${funcName}`)
+			return {
+				rpcError: `No such function registered for RPC: ${funcName}`,
+				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
+			}
+		}
+
+		const args = message.args
+
+		if (message.addRpcInfoAsArgument !== false) {
+			const rpcInfo: RpcInfo = {
+				tab: sender.tab,
+			}
+			args[message.addRpcInfoAsArgument] = rpcInfo
+		}
+
+		// Run the function, return the result.
+		try {
+			const returnValue = await func(...args)
+			return {
+				returnValue,
+				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
+			}
+		} catch (error) {
+			return {
+				errorMessage: error.message,
+				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
+			}
+		}
+	}
+}
diff --git a/src/common.ts b/src/common.ts
new file mode 100644
index 0000000..bbfb65e
--- /dev/null
+++ b/src/common.ts
@@ -0,0 +1,55 @@
+import type { RpcInfo } from './RpcServer'
+
+export type AsyncFunction = Function & ((...args: any[]) => Promise<any>)
+
+// Our secret tokens to recognise our messages
+export const RPC_CALL = '__RPC_CALL__'
+export const RPC_RESPONSE = '__RPC_RESPONSE__'
+
+export interface RpcMessage {
+	__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_CALL | typeof RPC_RESPONSE,
+	funcName: string,
+}
+
+export interface RpcCallMessage<F extends AsyncFunction = AsyncFunction> extends RpcMessage {
+	__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_CALL,
+	funcName: F['name'],
+	args: ReplaceRpcInfo<Parameters<F>, null>,
+	addRpcInfoAsArgument: number | false,
+}
+
+export type RpcResponseMessage<F extends AsyncFunction = AsyncFunction> =
+	| RpcResponseResolve<F>
+	| RpcResponseReject
+	| RpcResponseRpcError
+
+interface RpcResponseMessage_base {
+__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_RESPONSE,
+}
+
+interface RpcResponseResolve<F extends AsyncFunction> extends RpcResponseMessage_base {
+	returnValue: ReturnType<F>,
+}
+
+interface RpcResponseReject extends RpcResponseMessage_base {
+	errorMessage: string,
+}
+
+interface RpcResponseRpcError extends RpcResponseMessage_base {
+	rpcError: string,
+}
+
+export function isRpcCallMessage(message: any): message is RpcCallMessage {
+	return !!(message && message['__WEBEXTENSION_RPC_MESSAGE__'] === RPC_CALL)
+}
+
+export function isRpcResponseMessage(message: any): message is RpcResponseMessage {
+	return !!(message && message['__WEBEXTENSION_RPC_MESSAGE__'] === RPC_RESPONSE)
+}
+
+type Tail<T extends any[]> = T extends [infer _Head, ...infer Tail] ? Tail : [];
+
+export type ReplaceRpcInfo<Params extends Parameters<AsyncFunction>, Replacement extends any> =
+	Params extends [RpcInfo, ...any]
+		? [Replacement, ...Tail<Params>]
+		: Params
diff --git a/src/index.ts b/src/index.ts
index 867e57a..9522103 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,215 +1,2 @@
-// Our secret tokens to recognise our messages
-const RPC_CALL = '__RPC_CALL__'
-const RPC_RESPONSE = '__RPC_RESPONSE__'
-
-export class RpcError extends Error {
-	constructor(message) {
-		super(message)
-		this.name = this.constructor.name
-	}
-}
-
-export class RemoteError extends Error {
-	constructor(message) {
-		super(message)
-		this.name = this.constructor.name
-	}
-}
-
-export const injectRpcInfo = Symbol('RpcInfo')
-
-export interface RpcInfo {
-	tab: browser.tabs.Tab,
-}
-
-type AsyncFunction = Function & ((...args: any[]) => Promise<any>)
-
-interface RpcMessage {
-	__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_CALL | typeof RPC_RESPONSE,
-	funcName: string,
-}
-
-interface RpcCallMessage<F extends AsyncFunction = AsyncFunction> extends RpcMessage {
-	__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_CALL,
-	funcName: F['name'],
-	args: ParametersWithRpcInfo<F>,
-	addRpcInfoAsArgument: number | false,
-}
-
-type RpcResponseMessage<F extends AsyncFunction = AsyncFunction> =
-	| RpcResponseResolve<F>
-	| RpcResponseReject
-	| RpcResponseRpcError
-
-interface RpcResponseMessage_base {
-__WEBEXTENSION_RPC_MESSAGE__: typeof RPC_RESPONSE,
-}
-
-interface RpcResponseResolve<F extends AsyncFunction> extends RpcResponseMessage_base {
-	returnValue: ReturnType<F>,
-}
-
-interface RpcResponseReject extends RpcResponseMessage_base {
-	errorMessage: string,
-}
-
-interface RpcResponseRpcError extends RpcResponseMessage_base {
-	rpcError: string,
-}
-
-function isRpcCallMessage(message: any): message is RpcCallMessage {
-	return !!(message && message['__WEBEXTENSION_RPC_MESSAGE__'] === RPC_CALL)
-}
-
-function isRpcResponseMessage(message: any): message is RpcResponseMessage {
-	return !!(message && message['__WEBEXTENSION_RPC_MESSAGE__'] === RPC_RESPONSE)
-}
-
-type Tail<T extends any[]> = T extends [infer _Head, ...infer Tail] ? Tail : [];
-
-type ParametersWithRpcInfo<F extends AsyncFunction> = Parameters<F> extends [RpcInfo, ...any]
-	? [typeof injectRpcInfo, ...Tail<Parameters<F>>]
-	: Parameters<F>
-
-// I thought the type below should allow putting the RpcInfo argument at any position, but it does not work.
-// type ParametersWithRpcInfo<F extends AsyncFunction> = Array<any> & {
-// 	[N in keyof Parameters<F>]: Parameters<F>[N] extends RpcInfo
-// 		? typeof injectRpcInfo
-// 		: Parameters<F>[N]
-// }
-
-/**
- * RpcFunction<F> equals the function F, except for two tweaks:
- * - In the parameters, any RpcInfo is swapped for rpcInfoSymbol.
- * - In the return type, nested promises become one promise.
- */
-export type RpcFunction<F extends AsyncFunction> = (...args: ParametersWithRpcInfo<F>) => Promise<Awaited<ReturnType<F>>>
-
-
-// === Initiating side ===
-
-/**
- * Create a proxy function that invokes the specified remote function.
- * @param funcName - Name of the function as registered on the remote side.
- * @param options.tabId - The id of the tab whose content script is the remote side. Leave undefined
-   to call the background script (from a content script).
- * @returns The proxy function.
- */
-export function remoteFunction<F extends AsyncFunction>(
-	funcName: string,
-	{ tabId }: { tabId?: number } = {},
-): RpcFunction<F> {
-	const otherSide = (tabId !== undefined)
-		? "the tab's content script"
-		: 'the background script'
-
-	const f: RpcFunction<F> = async function (...args): Promise<Awaited<ReturnType<F>>> {
-		const message: RpcCallMessage = {
-			__WEBEXTENSION_RPC_MESSAGE__: RPC_CALL,
-			funcName,
-			args,
-			addRpcInfoAsArgument: false,
-		}
-
-		if (args.includes(injectRpcInfo)) {
-			const argIndex = args.indexOf(injectRpcInfo)
-			message.addRpcInfoAsArgument = argIndex
-			message.args[argIndex] = null
-		}
-
-		// Try send the message and await the response.
-		let response: RpcResponseMessage<F>
-		try {
-			response = (tabId !== undefined)
-				? await browser.tabs.sendMessage(tabId, message)
-				: await browser.runtime.sendMessage(message)
-		} catch (err) {}
-
-		// Check if we got an error or no response.
-		if (response === undefined) {
-			throw new RpcError(
-				`Got no response when trying to call '${funcName}'. `
-				+ `Did you enable RPC in ${otherSide}?`
-			)
-		}
-
-		// Check if it was *our* listener that responded.
-		if (!isRpcResponseMessage(response)) {
-			throw new RpcError(
-				`RPC got a response from an interfering listener while calling '${funcName}' in `
-				+ `${otherSide}`
-			)
-		}
-
-		// If we could not invoke the function on the other side, throw an error.
-		if ('rpcError' in response) {
-			throw new RpcError(response.rpcError)
-		}
-
-		// Return the value or throw the error we received from the other side.
-		if ('errorMessage' in response) {
-			throw new RemoteError(response.errorMessage)
-		} else {
-			return response.returnValue
-		}
-	}
-
-	// Give it a name, could be helpful in debugging
-	Object.defineProperty(f, 'name', { value: `${funcName}_RPC` })
-	return f
-}
-
-
-// === Executing side ===
-
-
-/**
- * Register one or more functions to enable remote scripts to call them.
- *
- * @param functions - A `{ functionName: function }` mapping. Each function will be remotely
- * callable using the given name.
- * @returns The passed `functions` object.
- */
-export function makeRemotelyCallable<Fs extends Record<string, AsyncFunction>>(
-	functions: Fs,
-): typeof functions {
-	browser.runtime.onMessage.addListener(incomingRPCListener)
-	return functions
-
-	async function incomingRPCListener(message: any, sender: browser.runtime.MessageSender): Promise<RpcResponseMessage> {
-		if (!isRpcCallMessage(message)) return
-
-		const funcName = message.funcName
-		const func = functions[funcName]
-		if (func === undefined) {
-			console.error(`Received RPC for unknown function: ${funcName}`)
-			return {
-				rpcError: `No such function registered for RPC: ${funcName}`,
-				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
-			}
-		}
-
-		const args = message.args
-
-		if (message.addRpcInfoAsArgument !== false) {
-			const rpcInfo: RpcInfo = {
-				tab: sender.tab,
-			}
-			args[message.addRpcInfoAsArgument] = rpcInfo
-		}
-
-		// Run the function, return the result.
-		try {
-			const returnValue = await func(...args)
-			return {
-				returnValue,
-				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
-			}
-		} catch (error) {
-			return {
-				errorMessage: error.message,
-				__WEBEXTENSION_RPC_MESSAGE__: RPC_RESPONSE,
-			}
-		}
-	}
-}
+export * from './RpcClient'
+export * from './RpcServer'
diff --git a/test/index.ts b/test/RpcClient.ts
similarity index 71%
rename from test/index.ts
rename to test/RpcClient.ts
index 7434260..c7bce12 100644
--- a/test/index.ts
+++ b/test/RpcClient.ts
@@ -1,7 +1,7 @@
 import test from 'ava'
 import sinon from 'ts-sinon'
 
-import { remoteFunction, RpcError, RemoteError, injectRpcInfo } from '../src/index'
+import { RpcClient, RpcError, RemoteError, injectRpcInfo } from '../src/RpcClient'
 
 function mockBrowser() {
 	return {
@@ -23,13 +23,13 @@ test.beforeEach(() => {
 })
 
 test.serial('should create a function', t => {
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	t.is(remoteFunc.name, 'remoteFunc_RPC')
 	t.is(typeof remoteFunc, 'function')
 })
 
 test.serial('should throw an error when unable to sendMessage', async t => {
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	browser.tabs.sendMessage = async () => { throw new Error() }
 	await t.throwsAsync(remoteFunc, {
 		instanceOf: RpcError,
@@ -38,7 +38,7 @@ test.serial('should throw an error when unable to sendMessage', async t => {
 })
 
 test.serial('should call the browser.tabs function when tabId is given', async t => {
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	try {
 		await remoteFunc()
 	} catch (e) {}
@@ -47,7 +47,25 @@ test.serial('should call the browser.tabs function when tabId is given', async t
 })
 
 test.serial('should call the browser.runtime function when tabId is undefined', async t => {
-	const remoteFunc = remoteFunction('remoteFunc')
+	const remoteFunc = new RpcClient().func('remoteFunc')
+	try {
+		await remoteFunc()
+	} catch (e) {}
+	t.true(browser.tabs.sendMessage.notCalled)
+	t.true(browser.runtime.sendMessage.calledOnce)
+})
+
+test.serial('should call the browser.tabs function when tabId is overridden', async t => {
+	const remoteFunc = new RpcClient().func('remoteFunc', { tabId: 123 })
+	try {
+		await remoteFunc()
+	} catch (e) {}
+	t.true(browser.tabs.sendMessage.calledOnce)
+	t.true(browser.runtime.sendMessage.notCalled)
+})
+
+test.serial('should call the browser.runtime function when tabId is overridden as null', async t => {
+	const remoteFunc = new RpcClient({ tabId: 123 }).func('remoteFunc', { tabId: null })
 	try {
 		await remoteFunc()
 	} catch (e) {}
@@ -56,7 +74,7 @@ test.serial('should call the browser.runtime function when tabId is undefined',
 })
 
 test.serial('should send the call message correctly', async t => {
-	const remoteFunc = remoteFunction('remoteFunc')
+	const remoteFunc = new RpcClient().func('remoteFunc')
 	try {
 		await remoteFunc('a', 'b', 'c', 'd')
 	} catch {}
@@ -70,7 +88,7 @@ test.serial('should send the call message correctly', async t => {
 })
 
 test.serial('should handle the RpcInfoSymbol', async t => {
-	const remoteFunc = remoteFunction('remoteFunc')
+	const remoteFunc = new RpcClient().func('remoteFunc')
 	try {
 		await remoteFunc('a', 'b', injectRpcInfo, 'd')
 	} catch {}
@@ -78,14 +96,14 @@ test.serial('should handle the RpcInfoSymbol', async t => {
 	t.deepEqual(browser.runtime.sendMessage.lastCall.args, [{
 		__WEBEXTENSION_RPC_MESSAGE__: '__RPC_CALL__',
 		funcName: 'remoteFunc',
-		args: ['a', 'b', injectRpcInfo, 'd'],
+		args: ['a', 'b', null, 'd'],
 		addRpcInfoAsArgument: 2,
 	}])
 })
 
 test.serial('should throw an "interfering listener" error if response is unrecognised', async t => {
 	browser.tabs.sendMessage = async () => 'some unexpected return value'
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	await t.throwsAsync(remoteFunc, {
 		instanceOf: RpcError,
 		message: /RPC got a response from an interfering listener/,
@@ -96,7 +114,7 @@ test.serial('should throw a "no response" error if response is undefined', async
 	// It seems we can get back undefined when the tab is closed before the response is sent.
 	// In such cases 'no response' seems a better error message than 'interfering listener'.
 	browser.tabs.sendMessage = async () => undefined
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	await t.throwsAsync(remoteFunc, {
 		instanceOf: RpcError,
 		message: /Got no response/,
@@ -108,7 +126,7 @@ test.serial('should throw RemoteError if the response contains an error message'
 		__WEBEXTENSION_RPC_MESSAGE__: '__RPC_RESPONSE__',
 		errorMessage: 'Remote function error',
 	})
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	await t.throwsAsync(remoteFunc, {
 		instanceOf: RemoteError,
 		message: 'Remote function error',
@@ -120,8 +138,6 @@ test.serial('should return the value contained in the response', async t => {
 		__WEBEXTENSION_RPC_MESSAGE__: '__RPC_RESPONSE__',
 		returnValue: 'Remote function return value',
 	})
-	const remoteFunc = remoteFunction('remoteFunc', { tabId: 1 })
+	const remoteFunc = new RpcClient({ tabId: 1 }).func('remoteFunc')
 	t.is(await remoteFunc(), 'Remote function return value')
 })
-
-// TODO Test behaviour of executing side.
diff --git a/tsconfig.json b/tsconfig.json
index 6f0cc17..0503ca3 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -4,9 +4,7 @@
     "declaration": true,
     "outDir": "lib"
   },
-  "files": [
-    "src/index.ts"
-  ],
+  "include": ["src"],
   "ts-node": {
     "transpileOnly": true
   }