diff --git a/services/api.ts b/services/api.ts
index bed0e8f..edaaa58 100644
--- a/services/api.ts
+++ b/services/api.ts
@@ -1782,6 +1782,65 @@ class ApiService {
     }
   }
 
+  /**
+   * Attach device to deployment via Legacy API
+   * Uses set_deployment endpoint to link a WP sensor to a beneficiary's deployment
+   *
+   * @param deploymentId - The deployment ID to attach the device to
+   * @param wellId - The device's well_id (from BLE scan, e.g., 497)
+   * @param ssid - WiFi network SSID
+   * @param password - WiFi network password
+   */
+  async attachDeviceToDeployment(
+    deploymentId: number,
+    wellId: number,
+    ssid: string,
+    password: string
+  ): Promise<ApiResponse<{ success: boolean }>> {
+    try {
+      const creds = await this.getLegacyWebViewCredentials();
+      if (!creds) {
+        return { ok: false, error: { message: 'Not authenticated with Legacy API', code: 'UNAUTHORIZED' } };
+      }
+
+      // Call set_deployment to attach device
+      const formData = new URLSearchParams({
+        function: 'set_deployment',
+        user_name: creds.userName,
+        token: creds.token,
+        deployment: deploymentId.toString(),
+        devices: JSON.stringify([wellId]),
+        wifis: JSON.stringify([`${ssid}|${password}`]),
+        reuse_existing_devices: '1',
+      });
+
+      console.log('[API] attachDeviceToDeployment: deployment=', deploymentId, 'wellId=', wellId, 'ssid=', ssid);
+
+      const response = await fetch(API_BASE_URL, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+        body: formData.toString(),
+      });
+
+      if (!response.ok) {
+        return { ok: false, error: { message: 'Failed to attach device to deployment' } };
+      }
+
+      const data = await response.json();
+
+      if (data.status !== '200 OK') {
+        console.error('[API] attachDeviceToDeployment failed:', data);
+        return { ok: false, error: { message: data.message || 'Failed to attach device' } };
+      }
+
+      console.log('[API] attachDeviceToDeployment success');
+      return { ok: true, data: { success: true } };
+    } catch (error: any) {
+      console.error('[API] attachDeviceToDeployment error:', error);
+      return { ok: false, error: { message: error.message || 'Network error', code: 'NETWORK_ERROR' } };
+    }
+  }
+
   /**
    * Detach device from beneficiary
    */