diff --git a/src/CCAI.NET/SMS/SMSService.cs b/src/CCAI.NET/SMS/SMSService.cs
index 63e7976..93c4ba0 100644
--- a/src/CCAI.NET/SMS/SMSService.cs
+++ b/src/CCAI.NET/SMS/SMSService.cs
@@ -123,6 +123,17 @@ public async Task<SMSResponse> SendAsync(SMSRequest request, CancellationToken c
         // Prepare the endpoint and data
         var endpoint = $"/clients/{_client.GetClientId()}/campaigns/direct";
         
+        // The API carries custom data per recipient (as "messageData"), so a campaign-level
+        // CustomData is applied to every account that does not already define its own
+        if (!string.IsNullOrEmpty(request.CustomData))
+        {
+            accountsList = accountsList
+                .Select(account => account.CustomData is null
+                    ? account with { CustomData = request.CustomData }
+                    : account)
+                .ToList();
+        }
+        
         var campaignData = new SMSCampaign
         {
             Accounts = accountsList,
diff --git a/tests/CCAI.NET.Tests/SMS/SMSCustomDataWebhookTests.cs b/tests/CCAI.NET.Tests/SMS/SMSCustomDataWebhookTests.cs
index 8534c2b..4d8918d 100644
--- a/tests/CCAI.NET.Tests/SMS/SMSCustomDataWebhookTests.cs
+++ b/tests/CCAI.NET.Tests/SMS/SMSCustomDataWebhookTests.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
 using System.Net;
+using System.Runtime.CompilerServices;
 using System.Text.Json;
 using CCAI.NET.SMS;
 using Moq;
@@ -194,4 +195,137 @@ public void SMSRequest_CreateSingle_WithCustomData_SetsAllFields()
         Assert.Equal(customAccountId, account.CustomAccountId);
         Assert.Equal(customData, account.CustomData);
     }
+
+    [Fact]
+    public async Task SendAsync_WithCampaignLevelCustomData_AppliesItToEveryAccount()
+    {
+        // Arrange
+        var customData = "OrderBatch-42";
+
+        var accounts = new[]
+        {
+            new Account { FirstName = "John", LastName = "Test", Phone = "+15551234567" },
+            new Account { FirstName = "Jane", LastName = "Test", Phone = "+15557654321" }
+        };
+
+        var capturedRequestBody = SetupCapture();
+
+        // Act
+        await _smsService.SendAsync(
+            accounts: accounts,
+            message: "Hello ${FirstName}!",
+            title: "Custom Data Campaign",
+            customData: customData);
+
+        // Assert
+        var sentAccounts = GetSentAccounts(capturedRequestBody);
+        Assert.Equal(2, sentAccounts.GetArrayLength());
+
+        foreach (var sentAccount in sentAccounts.EnumerateArray())
+        {
+            Assert.Equal(customData, sentAccount.GetProperty("messageData").GetString());
+        }
+    }
+
+    [Fact]
+    public async Task SendAsync_WithCampaignLevelCustomData_DoesNotOverrideAccountCustomData()
+    {
+        // Arrange
+        var accounts = new[]
+        {
+            new Account { FirstName = "John", LastName = "Test", Phone = "+15551234567", CustomData = "PerAccount-1" },
+            new Account { FirstName = "Jane", LastName = "Test", Phone = "+15557654321" }
+        };
+
+        var capturedRequestBody = SetupCapture();
+
+        // Act
+        await _smsService.SendAsync(
+            accounts: accounts,
+            message: "Hello ${FirstName}!",
+            title: "Custom Data Campaign",
+            customData: "CampaignLevel");
+
+        // Assert
+        var sentAccounts = GetSentAccounts(capturedRequestBody);
+        Assert.Equal("PerAccount-1", sentAccounts[0].GetProperty("messageData").GetString());
+        Assert.Equal("CampaignLevel", sentAccounts[1].GetProperty("messageData").GetString());
+    }
+
+    [Fact]
+    public async Task SendAsync_WithoutCustomData_LeavesMessageDataUnset()
+    {
+        // Arrange
+        var accounts = new[]
+        {
+            new Account { FirstName = "John", LastName = "Test", Phone = "+15551234567" }
+        };
+
+        var capturedRequestBody = SetupCapture();
+
+        // Act
+        await _smsService.SendAsync(
+            accounts: accounts,
+            message: "Hello ${FirstName}!",
+            title: "No Custom Data Campaign");
+
+        // Assert
+        var sentAccounts = GetSentAccounts(capturedRequestBody);
+        Assert.Equal(JsonValueKind.Null, sentAccounts[0].GetProperty("messageData").ValueKind);
+    }
+
+    [Fact]
+    public async Task SendAsync_WithCampaignLevelCustomData_DoesNotMutateCallerAccounts()
+    {
+        // Arrange
+        var account = new Account { FirstName = "John", LastName = "Test", Phone = "+15551234567" };
+        var accounts = new[] { account };
+
+        SetupCapture();
+
+        // Act
+        await _smsService.SendAsync(
+            accounts: accounts,
+            message: "Hello ${FirstName}!",
+            title: "Custom Data Campaign",
+            customData: "OrderBatch-42");
+
+        // Assert
+        Assert.Null(account.CustomData);
+        Assert.Null(accounts[0].CustomData);
+    }
+
+    /// <summary>
+    /// Capture the campaign body handed to the client and return a holder for it
+    /// </summary>
+    private StrongBox<object?> SetupCapture()
+    {
+        var capturedRequestBody = new StrongBox<object?>(null);
+
+        _mockClient
+            .Setup(c => c.RequestAsync<SMSResponse>(
+                It.IsAny<HttpMethod>(),
+                It.IsAny<string>(),
+                It.IsAny<object>(),
+                It.IsAny<CancellationToken>(),
+                It.IsAny<Dictionary<string, string>>()))
+            .Callback<HttpMethod, string, object, CancellationToken, Dictionary<string, string>>(
+                (method, url, body, token, headers) => capturedRequestBody.Value = body)
+            .ReturnsAsync(new SMSResponse { Id = "msg-123", Status = "sent" });
+
+        return capturedRequestBody;
+    }
+
+    /// <summary>
+    /// Serialize the captured campaign body and return its "accounts" array as sent on the wire
+    /// </summary>
+    private static JsonElement GetSentAccounts(StrongBox<object?> capturedRequestBody)
+    {
+        Assert.NotNull(capturedRequestBody.Value);
+
+        var requestJson = JsonSerializer.Serialize(capturedRequestBody.Value);
+        using var document = JsonDocument.Parse(requestJson);
+
+        return document.RootElement.GetProperty("accounts").Clone();
+    }
 }
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