MPP-4539: feat(relay): add optional parameter

CHANGELOG.md

@@ -20,6 +20,10 @@
 * Added `eval-jexl` command to nimbus-cli for evaluating JEXL targeting expressions against the app context. Useful for testing and debugging targeting expressions on iOS and Android.
 ([#7160](https://github.com/mozilla/application-services/pull/7160))
 
+### Relay
+
+* Added `X-Relay-Client` header to all Relay API requests with automatic platform detection (`appservices-ios`, `appservices-android`, etc.) to help the backend distinguish mobile vs desktop requests for telemetry.
+
 [Full Changelog](In progress)
 
 # v148.0 (_2026-01-12_)

components/relay/src/lib.rs

@@ -124,6 +124,7 @@ impl RelayClient {
         if let Some(ref token) = self.auth_token {
             request = request.header(header_names::AUTHORIZATION, format!("Bearer {}", token))?;
         }
+        request = request.header("X-Relay-Client", format!("appservices-{}", std::env::consts::OS))?;
         Ok(request)
     }
 }
@@ -132,6 +133,10 @@ impl RelayClient {
 impl RelayClient {
     /// Creates a new `RelayClient` instance.
     ///
+    /// The client automatically includes an `X-Relay-Client` header with the platform
+    /// identifier based on the target OS (e.g., "appservices-ios", "appservices-android",
+    /// "appservices-macos", etc.) to help the Relay backend distinguish mobile vs desktop requests.
+    ///
     /// # Parameters
     /// - `server_url`: Base URL for the Relay API.
     /// - `auth_token`: Optional relay-scoped access token (see struct docs).
@@ -621,6 +626,50 @@ mod tests {
         assert!(address.enabled);
     }
 
+    #[test]
+    fn test_create_address_with_platform_header() {
+        viaduct_dev::init_backend_dev();
+
+        let expected_platform = format!("appservices-{}", std::env::consts::OS);
+
+        let address_json = r#"{
+            "mask_type": "alias",
+            "enabled": true,
+            "description": "Test",
+            "generated_for": "example.com",
+            "block_list_emails": false,
+            "used_on": "example.com",
+            "id": 1,
+            "address": "test123",
+            "domain": 2,
+            "full_address": "test123@mozmail.com",
+            "created_at": "2021-01-01T00:00:00Z",
+            "last_modified_at": "2021-01-01T00:00:00Z",
+            "last_used_at": null,
+            "num_forwarded": 0,
+            "num_blocked": 0,
+            "num_level_one_trackers_blocked": 0,
+            "num_replied": 0,
+            "num_spam": 0
+        }"#;
+
+        let _mock = mock("POST", "/api/v1/relayaddresses/")
+            .match_header("authorization", "Bearer mock_token")
+            .match_header("x-relay-client", expected_platform.as_str())
+            .with_status(201)
+            .with_body(address_json)
+            .create();
+
+        let client = RelayClient::new(mockito::server_url(), Some("mock_token".to_string()));
+
+        let address = client
+            .expect("success")
+            .create_address("Test", "example.com", "example.com")
+            .expect("should create address with platform header");
+
+        assert_eq!(address.full_address, "test123@mozmail.com");
+    }
+
     fn mock_profile_json(
         id: i64,
         has_premium: bool,