diff --git a/src/Aspire.Hosting/ResourceBuilderExtensions.cs b/src/Aspire.Hosting/ResourceBuilderExtensions.cs
index 364d0c793c9..13c148cc7e5 100644
--- a/src/Aspire.Hosting/ResourceBuilderExtensions.cs
+++ b/src/Aspire.Hosting/ResourceBuilderExtensions.cs
@@ -179,12 +179,14 @@ public static IResourceBuilder<T> WithEnvironment<T>(this IResourceBuilder<T> bu
         {
             builder.WithEnvironment(async context =>
             {
-                var url = await externalService.Resource.UrlParameter.GetValueAsync(context.CancellationToken).ConfigureAwait(false);
-
                 // In publish mode we can't validate the parameter value so we'll just use it without validating.
-                if (!context.ExecutionContext.IsPublishMode && !ExternalServiceResource.UrlIsValidForExternalService(url, out var _, out var message))
+                if (!context.ExecutionContext.IsPublishMode)
                 {
-                    throw new DistributedApplicationException($"The URL parameter '{externalService.Resource.UrlParameter.Name}' for the external service '{externalService.Resource.Name}' is invalid: {message}");
+                    var url = await externalService.Resource.UrlParameter.GetValueAsync(context.CancellationToken).ConfigureAwait(false);
+                    if (!ExternalServiceResource.UrlIsValidForExternalService(url, out var _, out var message))
+                    {
+                        throw new DistributedApplicationException($"The URL parameter '{externalService.Resource.UrlParameter.Name}' for the external service '{externalService.Resource.Name}' is invalid: {message}");
+                    }
                 }
 
                 context.EnvironmentVariables[name] = externalService.Resource.UrlParameter;
diff --git a/tests/Aspire.Hosting.Tests/ExternalServiceTests.cs b/tests/Aspire.Hosting.Tests/ExternalServiceTests.cs
index 51933630ebf..a4c1acb6956 100644
--- a/tests/Aspire.Hosting.Tests/ExternalServiceTests.cs
+++ b/tests/Aspire.Hosting.Tests/ExternalServiceTests.cs
@@ -444,6 +444,36 @@ public async Task ExternalServiceWithParameterHttpHealthCheckResolvesUrlAsync()
         Assert.Contains(healthCheckKey, result.Entries.Keys);
     }
 
+    [Fact]
+    public async Task ExternalServiceWithParameterPublishManifest()
+    {
+        using var builder = TestDistributedApplicationBuilder.Create(DistributedApplicationOperation.Publish);
+
+        var urlParam = builder.AddParameter("external-url");
+        var externalService = builder.AddExternalService("external", urlParam);
+
+        var project = builder.AddProject<TestProject>("project")
+                     .WithReference(externalService)
+                     .WithEnvironment("EXTERNAL_SERVICE", externalService);
+
+        var manifest = await ManifestUtils.GetManifest(project.Resource);
+
+        Assert.Equal(
+            """
+            {
+              "type": "project.v0",
+              "path": "testproject",
+              "env": {
+                "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EXCEPTION_LOG_ATTRIBUTES": "true",
+                "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EVENT_LOG_ATTRIBUTES": "true",
+                "OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY": "in_memory",
+                "services__external__default__0": "{external-url.value}",
+                "EXTERNAL_SERVICE": "{external-url.value}"
+              }
+            }
+            """, manifest.ToString());
+    }
+
     private sealed class TestProject : IProjectMetadata
     {
         public string ProjectPath => "testproject";