diff --git a/src/ClientBuilder.js b/src/ClientBuilder.js
index d4ae9d5..8fa8b37 100644
--- a/src/ClientBuilder.js
+++ b/src/ClientBuilder.js
@@ -50,6 +50,7 @@ class ClientBuilder {
 		this.baseUrl = undefined;
 		this.proxy = undefined;
 		this.customHeaders = {};
+		this.appendHeaders = {};
 		this.debug = undefined;
 		this.licenses = [];
 		this.customQueries = new Map();
@@ -133,6 +134,29 @@ class ClientBuilder {
 		return this;
 	}
 
+	/**
+	 * Appends the provided value to the existing header value using the specified separator,
+	 * rather than replacing it. This is useful for single-value headers like User-Agent.
+	 * @param key The header name.
+	 * @param value The value to append.
+	 * @param separator The separator to use when joining values.
+	 * @return ClientBuilder <b>this</b> to accommodate method chaining.
+	 */
+	withAppendedHeader(key, value, separator) {
+		if (this.appendHeaders[key]) {
+			if (this.appendHeaders[key].separator !== separator) {
+				throw new Error(
+					`Conflicting separators for appended header "${key}": ` +
+					`existing "${this.appendHeaders[key].separator}" vs new "${separator}"`,
+				);
+			}
+			this.appendHeaders[key].values.push(value);
+		} else {
+			this.appendHeaders[key] = { values: [value], separator };
+		}
+		return this;
+	}
+
 	/**
 	 * Enables debug mode, which will print information about the HTTP request and response to console.log
 	 * @return ClientBuilder <b>this</b> to accommodate method chaining.
@@ -216,7 +240,7 @@ class ClientBuilder {
 			const retrySender = new RetrySender(this.maxRetries, signingSender, new Sleeper());
 			agentSender = new AgentSender(retrySender);
 		}
-		const customHeaderSender = new CustomHeaderSender(agentSender, this.customHeaders);
+		const customHeaderSender = new CustomHeaderSender(agentSender, this.customHeaders, this.appendHeaders);
 		const baseUrlSender = new BaseUrlSender(customHeaderSender, this.baseUrl);
 		const licenseSender = new LicenseSender(baseUrlSender, this.licenses);
 		const customQuerySender = new CustomQuerySender(licenseSender, this.customQueries);
diff --git a/src/CustomHeaderSender.ts b/src/CustomHeaderSender.ts
index 33b7110..5609740 100644
--- a/src/CustomHeaderSender.ts
+++ b/src/CustomHeaderSender.ts
@@ -1,17 +1,40 @@
 import { Request, Response, Sender } from "./types";
 
+export interface AppendHeader {
+	values: string[];
+	separator: string;
+}
+
 export default class CustomHeaderSender {
 	private sender: Sender;
 	private customHeaders: Record<string, string>;
+	private appendHeaders: Record<string, AppendHeader>;
 
-	constructor(innerSender: Sender, customHeaders: Record<string, string>) {
+	constructor(
+		innerSender: Sender,
+		customHeaders: Record<string, string>,
+		appendHeaders: Record<string, AppendHeader> = {},
+	) {
 		this.sender = innerSender;
 		this.customHeaders = customHeaders;
+		this.appendHeaders = appendHeaders;
 	}
 
 	send(request: Request): Promise<Response> {
-		for (let key in this.customHeaders) {
-			request.headers[key] = this.customHeaders[key];
+		const headers = request.headers as Record<string, string>;
+
+		for (const [key, value] of Object.entries(this.customHeaders)) {
+			headers[key] = value;
+		}
+
+		for (const [key, { values, separator }] of Object.entries(this.appendHeaders)) {
+			const appendValue = values.join(separator);
+			const existing = headers[key];
+			if (existing) {
+				headers[key] = existing + separator + appendValue;
+			} else {
+				headers[key] = appendValue;
+			}
 		}
 
 		return new Promise((resolve, reject) => {
diff --git a/tests/test_CustomHeaderSender.ts b/tests/test_CustomHeaderSender.ts
index 70cbc13..01c7598 100644
--- a/tests/test_CustomHeaderSender.ts
+++ b/tests/test_CustomHeaderSender.ts
@@ -1,20 +1,20 @@
 import { expect } from "chai";
-import CustomHeaderSender from "../src/CustomHeaderSender.js";
+import CustomHeaderSender, { AppendHeader } from "../src/CustomHeaderSender.js";
 import Request from "../src/Request.js";
 import Response from "../src/Response.js";
 import { Sender } from "../src/types";
 
-describe("A custom header sender", function () {
-	it("adds custom headers to the request.", function () {
-		class MockSender implements Sender {
-			request?: Request;
+class MockSender implements Sender {
+	request?: Request;
 
-			send = (request: Request): Promise<Response> => {
-				this.request = request;
-				return Promise.resolve(new Response(200, {}));
-			};
-		}
+	send = (request: Request): Promise<Response> => {
+		this.request = request;
+		return Promise.resolve(new Response(200, {}));
+	};
+}
 
+describe("A custom header sender", function () {
+	it("adds custom headers to the request.", function () {
 		const mockSender = new MockSender();
 		const customHeaders = {
 			a: "1",
@@ -30,4 +30,97 @@ describe("A custom header sender", function () {
 		expect("b" in mockSender.request!.headers).to.equal(true);
 		expect((mockSender.request!.headers as Record<string, string>)["b"]).to.equal("2");
 	});
+
+	it("appended headers are joined with separator.", function () {
+		const mockSender = new MockSender();
+		const appendHeaders: Record<string, AppendHeader> = {
+			"User-Agent": { values: ["custom-value"], separator: " " },
+		};
+		const customHeaderSender = new CustomHeaderSender(mockSender, {}, appendHeaders);
+		const request = new Request(undefined, {
+			"Content-Type": "application/json; charset=utf-8",
+			"User-Agent": "base-value",
+		});
+
+		customHeaderSender.send(request);
+
+		expect((mockSender.request!.headers as Record<string, string>)["User-Agent"]).to.equal(
+			"base-value custom-value",
+		);
+	});
+
+	it("appended headers set the value when no existing header is present.", function () {
+		const mockSender = new MockSender();
+		const appendHeaders: Record<string, AppendHeader> = {
+			"User-Agent": { values: ["custom-value"], separator: " " },
+		};
+		const customHeaderSender = new CustomHeaderSender(mockSender, {}, appendHeaders);
+		const request = new Request();
+
+		customHeaderSender.send(request);
+
+		expect((mockSender.request!.headers as Record<string, string>)["User-Agent"]).to.equal(
+			"custom-value",
+		);
+	});
+
+	it("customHeaders value is used as base when appendHeaders targets the same key.", function () {
+		const mockSender = new MockSender();
+		const customHeaders = { "User-Agent": "custom-base" };
+		const appendHeaders: Record<string, AppendHeader> = {
+			"User-Agent": { values: ["extra"], separator: " " },
+		};
+		const customHeaderSender = new CustomHeaderSender(mockSender, customHeaders, appendHeaders);
+		const request = new Request(undefined, {
+			"User-Agent": "original",
+		});
+
+		customHeaderSender.send(request);
+
+		expect((mockSender.request!.headers as Record<string, string>)["User-Agent"]).to.equal(
+			"custom-base extra",
+		);
+	});
+
+	it("multiple appended header values are accumulated.", function () {
+		const mockSender = new MockSender();
+		const appendHeaders: Record<string, AppendHeader> = {
+			"User-Agent": { values: ["foo", "bar"], separator: " " },
+		};
+		const customHeaderSender = new CustomHeaderSender(mockSender, {}, appendHeaders);
+		const request = new Request(undefined, {
+			"User-Agent": "base-value",
+		});
+
+		customHeaderSender.send(request);
+
+		expect((mockSender.request!.headers as Record<string, string>)["User-Agent"]).to.equal(
+			"base-value foo bar",
+		);
+	});
+
+	it("withAppendedHeader throws on conflicting separators for the same key.", function () {
+		// Test the withAppendedHeader logic directly (ClientBuilder can't be easily
+		// constructed in tsx/cjs tests due to instanceof interop issues).
+		const appendHeaders: Record<string, AppendHeader> = {};
+		const withAppendedHeader = (key: string, value: string, separator: string) => {
+			if (appendHeaders[key]) {
+				if (appendHeaders[key].separator !== separator) {
+					throw new Error(
+						`Conflicting separators for appended header "${key}": ` +
+							`existing "${appendHeaders[key].separator}" vs new "${separator}"`,
+					);
+				}
+				appendHeaders[key].values.push(value);
+			} else {
+				appendHeaders[key] = { values: [value], separator };
+			}
+		};
+
+		withAppendedHeader("User-Agent", "a", " ");
+
+		expect(() => withAppendedHeader("User-Agent", "b", "/")).to.throw(
+			'Conflicting separators for appended header "User-Agent"',
+		);
+	});
 });