Add agent information to UserAgent

src/Stripe.net/Infrastructure/Public/SystemNetHttpClient.cs

@@ -62,6 +62,17 @@ private static readonly Lazy<System.Net.Http.HttpClient> LazyDefaultHttpClient
 
         private string stripeClientUserAgentString;
 
+        internal static readonly (string EnvVar, string Slug)[] AIAgents = new (string EnvVar, string Slug)[]
+        {
+            ("ANTIGRAVITY_CLI_ALIAS", "antigravity"),
+            ("CLAUDECODE", "claude_code"),
+            ("CLINE_ACTIVE", "cline"),
+            ("CODEX_SANDBOX", "codex_cli"),
+            ("CURSOR_AGENT", "cursor"),
+            ("GEMINI_CLI", "gemini_cli"),
+            ("OPENCODE", "open_code"),
+        };
+
         // Deprecated in .NET 9; but tls 1.2 became a default after .NET Framework 4.7
 #if !NET9_0_OR_GREATER
         static SystemNetHttpClient()
@@ -189,6 +200,25 @@ await response.Content.ReadAsStreamAsync().ConfigureAwait(false))
             };
         }
 
+        private static string DetectAIAgent()
+        {
+            return DetectAIAgent(System.Environment.GetEnvironmentVariable);
+        }
+
+        internal static string DetectAIAgent(Func<string, string> getEnv)
+        {
+            foreach (var (envVar, slug) in AIAgents)
+            {
+                var val = getEnv(envVar);
+                if (!string.IsNullOrEmpty(val))
+                {
+                    return slug;
+                }
+            }
+
+            return string.Empty;
+        }
+
         private async Task<(HttpResponseMessage responseMessage, int retries)> SendHttpRequest(
             StripeRequest request,
             CancellationToken cancellationToken)
@@ -302,6 +332,12 @@ private string BuildStripeClientUserAgentString()
                 values.Add("application", this.appInfo);
             }
 
+            var aiAgent = DetectAIAgent();
+            if (!string.IsNullOrEmpty(aiAgent))
+            {
+                values.Add("ai_agent", aiAgent);
+            }
+
             return JsonUtils.SerializeObject(values, Formatting.None);
         }
 
@@ -314,6 +350,12 @@ private string BuildUserAgentString(ApiMode apiMode)
                 userAgent += " " + this.appInfo.FormatUserAgent();
             }
 
+            var aiAgent = DetectAIAgent();
+            if (!string.IsNullOrEmpty(aiAgent))
+            {
+                userAgent += $" AIAgent/{aiAgent}";
+            }
+
             return userAgent;
         }
 

src/StripeTests/Infrastructure/Public/SystemNetHttpClientTest.cs

@@ -1,5 +1,6 @@
 namespace StripeTests
 {
+    using System;
     using System.Linq;
     using System.Net;
     using System.Net.Http;
@@ -132,6 +133,67 @@ public void CanInspectEnableTelemetry()
             Assert.True(client.EnableTelemetry);
         }
 
+        [Fact]
+        public void TestDetectAIAgent()
+        {
+            var result = SystemNetHttpClient.DetectAIAgent(
+                key => key == "CLAUDECODE" ? "1" : null);
+            Assert.Equal("claude_code", result);
+        }
+
+        [Fact]
+        public void TestDetectAIAgentNoEnvVars()
+        {
+            var result = SystemNetHttpClient.DetectAIAgent(key => null);
+            Assert.Equal(string.Empty, result);
+        }
+
+        [Fact]
+        public void TestDetectAIAgentFirstMatchWins()
+        {
+            var result = SystemNetHttpClient.DetectAIAgent(
+                key => key == "CURSOR_AGENT" || key == "OPENCODE" ? "1" : null);
+            Assert.Equal("cursor", result);
+        }
+
+        [Fact]
+        public async Task AIAgentIncludedInHeaders()
+        {
+            var responseMessage = new HttpResponseMessage(HttpStatusCode.OK);
+            responseMessage.Content = new StringContent("Hello world!");
+            this.MockHttpClientFixture.MockHandler.Protected()
+                .Setup<Task<HttpResponseMessage>>(
+                    "SendAsync",
+                    ItExpr.IsAny<HttpRequestMessage>(),
+                    ItExpr.IsAny<CancellationToken>())
+                .Returns(Task.FromResult(responseMessage));
+
+            Environment.SetEnvironmentVariable("CLAUDECODE", "1");
+            try
+            {
+                var client = new SystemNetHttpClient(
+                    httpClient: new HttpClient(this.MockHttpClientFixture.MockHandler.Object));
+                var request = new StripeRequest(
+                    this.StripeClient,
+                    HttpMethod.Post,
+                    "/foo",
+                    null,
+                    null);
+                await client.MakeRequestAsync(request);
+
+                this.MockHttpClientFixture.MockHandler.Protected()
+                    .Verify(
+                        "SendAsync",
+                        Times.Once(),
+                        ItExpr.Is<HttpRequestMessage>(m => this.VerifyAIAgentHeaders(m.Headers)),
+                        ItExpr.IsAny<CancellationToken>());
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("CLAUDECODE", null);
+            }
+        }
+
         private bool VerifyHeaders(HttpRequestHeaders headers)
         {
             var userAgent = headers.UserAgent.ToString();
@@ -156,5 +218,16 @@ private bool VerifyHeaders(HttpRequestHeaders headers)
 
             return true;
         }
+
+        private bool VerifyAIAgentHeaders(HttpRequestHeaders headers)
+        {
+            var userAgent = headers.UserAgent.ToString();
+            var userAgentJson = JObject.Parse(headers.GetValues("X-Stripe-Client-User-Agent").First());
+
+            Assert.Contains("AIAgent/claude_code", userAgent);
+            Assert.Equal("claude_code", userAgentJson.Value<string>("ai_agent"));
+
+            return true;
+        }
     }
 }