diff --git a/.changeset/x-client-header.md b/.changeset/x-client-header.md
new file mode 100644
index 0000000..ea44b58
--- /dev/null
+++ b/.changeset/x-client-header.md
@@ -0,0 +1,5 @@
+---
+"tesla-fleet-api": minor
+---
+
+A new `client` option names your application, and is sent as the `X-Client` header on every request. `X-Library` is unchanged.
diff --git a/README.md b/README.md
index d5f3f64..ea29c40 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ await site.live_status();
 
 Replies keep the service's `{ response }` envelope. `api.vehicles.byVin` and `api.energySites.byId` are `Map`s of what each factory created. More: [vehicles](docs/fleet_api_vehicles.md), [energy sites](docs/fleet_api_energy_sites.md).
 
-`accessToken` may be a function returning `Promise<string | null>`, which is called for every request. Other options are `fetch`, `logger`, `debug` and `timeout` (milliseconds).
+`accessToken` may be a function returning `Promise<string | null>`, which is called for every request. Other options are `fetch`, `logger`, `debug`, `timeout` (milliseconds) and `client`, a short name for your application sent as the `X-Client` header on every request. It must be printable ASCII, or the constructor throws.
 
 ## Signed commands
 
diff --git a/src/teslafleetapi.ts b/src/teslafleetapi.ts
index f64900b..1890629 100644
--- a/src/teslafleetapi.ts
+++ b/src/teslafleetapi.ts
@@ -57,6 +57,8 @@ export default class TeslaFleetApi {
   timeout: number | null;
   /** Transport identity for debug logging; Teslemetry and Tessie override this. */
   readonly _transport_name: string = "fleet";
+  /** Sent as `X-Client` on every request when set. */
+  client: string | null;
   #fetch: typeof fetch | null;
   // Native private fields, behind accessors: a key must not be reachable by
   // walking the instance, e.g. in a structured log.
@@ -77,11 +79,17 @@ export default class TeslaFleetApi {
     /** Used for every request in place of the global `fetch`. */
     fetch?: typeof fetch;
     timeout?: number;
+    /** A short name for the application, sent as `X-Client` on every request. */
+    client?: string;
   }) {
     this.debug = !!options.debug;
     this.logger = options.logger ?? null;
     this.#fetch = options.fetch ?? null;
     this.timeout = options.timeout ?? null;
+    if (options.client && !/^[\x20-\x7e]+$/.test(options.client)) {
+      throw new Error("client must contain only printable ASCII characters.");
+    }
+    this.client = options.client || null;
     this.accessToken = options.accessToken ?? null;
 
     if (options.server) {
@@ -221,6 +229,9 @@ export default class TeslaFleetApi {
         Authorization: `Bearer ${accessToken}`,
         "X-Library": `node tesla_fleet_api ${__VERSION__}`,
       };
+      if (this.client) {
+        headers["X-Client"] = this.client;
+      }
 
       // Remove null and undefined values from params and json
       const query = new URLSearchParams();
diff --git a/src/teslemetry.ts b/src/teslemetry.ts
index b389d31..7242bb1 100644
--- a/src/teslemetry.ts
+++ b/src/teslemetry.ts
@@ -20,6 +20,8 @@ export type TeslemetryOptions = {
   logger?: Logger;
   fetch?: typeof fetch;
   timeout?: number;
+  /** A short name for the application, sent as `X-Client` on every request. */
+  client?: string;
 };
 
 /** Teslemetry's OAuth dynamic client registration endpoint. */
diff --git a/src/tessie.ts b/src/tessie.ts
index eff8377..cacdcbf 100644
--- a/src/tessie.ts
+++ b/src/tessie.ts
@@ -16,6 +16,8 @@ export type TessieOptions = {
     logger?: Logger;
     fetch?: typeof fetch;
     timeout?: number;
+    /** A short name for the application, sent as `X-Client` on every request. */
+    client?: string;
     /** Default for each command's `wait_for_completion`. */
     wait_for_completion?: boolean;
     /** Default for each command's `max_attempts`. */
diff --git a/test/request.test.ts b/test/request.test.ts
index 2fbe1eb..7c4b690 100644
--- a/test/request.test.ts
+++ b/test/request.test.ts
@@ -141,6 +141,23 @@ describe("_request", () => {
         expect(headersOf(fetch).get("X-Library")).toBe(`node tesla_fleet_api ${version}`);
     });
 
+    it("sends X-Client only when client is set", async () => {
+        const withClient = fakeFetch();
+        const without = fakeFetch();
+
+        await makeApi(withClient, { client: "node-red-contrib-teslemetry" })._request("GET", "api/1/products");
+        await makeApi(without)._request("GET", "api/1/products");
+
+        expect(headersOf(withClient).get("X-Client")).toBe("node-red-contrib-teslemetry");
+        expect(headersOf(withClient).get("X-Library")).toMatch(/^node tesla_fleet_api /);
+        expect(headersOf(without).has("X-Client")).toBe(false);
+    });
+
+    it("rejects a client that is not printable ASCII", () => {
+        expect(() => makeApi(fakeFetch(), { client: "café" })).toThrow(/printable ASCII/);
+        expect(() => makeApi(fakeFetch(), { client: "app\n" })).toThrow(/printable ASCII/);
+    });
+
     it("raises ResponseError for a non-JSON 2xx", async () => {
         const fetch = fakeFetch(() => new Response("<html>ok</html>", { status: 200, headers: { "Content-Type": "text/html" } }));