diff --git a/packages/mcp-cloudflare/src/server/index.ts b/packages/mcp-cloudflare/src/server/index.ts
index 0c7e91c3..84034ef0 100644
--- a/packages/mcp-cloudflare/src/server/index.ts
+++ b/packages/mcp-cloudflare/src/server/index.ts
@@ -3,6 +3,10 @@ import * as Sentry from "@sentry/cloudflare";
 import { logWarn } from "@sentry/mcp-core/telem/logging";
 import { SCOPES } from "../constants";
 import app from "./app";
+import {
+  UTM_SOURCE_ATTRIBUTE,
+  resolveUtmSourceFromUrl,
+} from "./lib/attribution";
 import { resolveClientFamily } from "./lib/client-family";
 import { redirectUriHasUserInfo } from "./lib/html-utils";
 import sentryMcpHandler from "./lib/mcp-handler";
@@ -189,7 +193,16 @@ const wrappedOAuthProvider = {
     }
 
     const clientFamily = resolveClientFamily(request.headers.get("user-agent"));
-    Sentry.getActiveSpan()?.setAttribute("app.client.family", clientFamily);
+    const activeSpan = Sentry.getActiveSpan();
+    activeSpan?.setAttribute("app.client.family", clientFamily);
+    // Set utm_source early on /mcp requests so the attribute is present even
+    // if the request is rejected before reaching mcp-handler.ts.
+    if (url.pathname.startsWith("/mcp")) {
+      const utmSource = resolveUtmSourceFromUrl(url);
+      if (utmSource) {
+        activeSpan?.setAttribute(UTM_SOURCE_ATTRIBUTE, utmSource);
+      }
+    }
 
     // Reject registrations with userinfo-spoofed redirect URIs before the
     // library stores the client (e.g. host@example.io).
diff --git a/packages/mcp-cloudflare/src/server/lib/attribution.test.ts b/packages/mcp-cloudflare/src/server/lib/attribution.test.ts
new file mode 100644
index 00000000..7eb62ace
--- /dev/null
+++ b/packages/mcp-cloudflare/src/server/lib/attribution.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import {
+  UTM_SOURCE_ATTRIBUTE,
+  resolveUtmSource,
+  resolveUtmSourceFromUrl,
+} from "./attribution";
+
+describe("UTM_SOURCE_ATTRIBUTE", () => {
+  it("is app.utm_source", () => {
+    expect(UTM_SOURCE_ATTRIBUTE).toBe("app.utm_source");
+  });
+});
+
+describe("resolveUtmSource", () => {
+  it.each([
+    // Known server-side values
+    ["plugin", "plugin"],
+    // Any other non-empty value buckets to "other"
+    ["unknown-source", "other"],
+    // Client-side values are intentionally not known server-side
+    ["sentry-mcp-settings-docs-btn", "other"],
+    // Absent / empty → null (do not set the attribute)
+    ["", null],
+    [null, null],
+    [undefined, null],
+  ])("maps %s → %s", (input, expected) => {
+    expect(resolveUtmSource(input)).toBe(expected);
+  });
+});
+
+describe("resolveUtmSourceFromUrl", () => {
+  it("reads utm_source from URL search params", () => {
+    const url = new URL("https://mcp.sentry.dev/mcp?utm_source=plugin");
+    expect(resolveUtmSourceFromUrl(url)).toBe("plugin");
+  });
+
+  it("returns null when utm_source is absent", () => {
+    const url = new URL("https://mcp.sentry.dev/mcp");
+    expect(resolveUtmSourceFromUrl(url)).toBeNull();
+  });
+
+  it("buckets unknown values to other", () => {
+    const url = new URL("https://mcp.sentry.dev/mcp?utm_source=something-new");
+    expect(resolveUtmSourceFromUrl(url)).toBe("other");
+  });
+});
diff --git a/packages/mcp-cloudflare/src/server/lib/attribution.ts b/packages/mcp-cloudflare/src/server/lib/attribution.ts
new file mode 100644
index 00000000..fbb01a28
--- /dev/null
+++ b/packages/mcp-cloudflare/src/server/lib/attribution.ts
@@ -0,0 +1,39 @@
+// Server-side attribution helpers. These live separately from client-side
+// attribution (client/utils/attribution.ts) because the two contexts have
+// different known utm_source values and different APIs (no window/document
+// on the server).
+
+/**
+ * Span/metric attribute name for the bucketed utm_source value.
+ * Use this constant in all call sites to avoid drift.
+ */
+export const UTM_SOURCE_ATTRIBUTE = "app.utm_source";
+
+/**
+ * Buckets a raw utm_source query param value into a fixed allow-list so it
+ * is safe to use as a metric/span attribute (raw values are unbounded
+ * cardinality). Returns null when the param is absent so callers can skip
+ * setting the attribute entirely — absence means "no UTM source", which is
+ * different from "unknown UTM source".
+ *
+ * Known server-side values:
+ *   "plugin" — MCP server URL tagged by the sentry-for-ai AI plugin
+ */
+export function resolveUtmSource(
+  raw: string | null | undefined,
+): string | null {
+  if (!raw) return null;
+  switch (raw) {
+    case "plugin":
+      return raw;
+    default:
+      return "other";
+  }
+}
+
+/**
+ * Convenience wrapper that reads utm_source directly from a URL object.
+ */
+export function resolveUtmSourceFromUrl(url: URL): string | null {
+  return resolveUtmSource(url.searchParams.get("utm_source"));
+}
diff --git a/packages/mcp-cloudflare/src/server/lib/mcp-handler.ts b/packages/mcp-cloudflare/src/server/lib/mcp-handler.ts
index 1810ab1c..ed979315 100644
--- a/packages/mcp-cloudflare/src/server/lib/mcp-handler.ts
+++ b/packages/mcp-cloudflare/src/server/lib/mcp-handler.ts
@@ -28,6 +28,7 @@ import {
   checkRateLimit,
 } from "../utils/rate-limiter";
 import { setSentryUserFromRequest } from "../utils/sentry-user";
+import { UTM_SOURCE_ATTRIBUTE, resolveUtmSourceFromUrl } from "./attribution";
 import { resolveClientFamily } from "./client-family";
 import { verifyConstraintsAccess } from "./constraint-utils";
 
@@ -202,6 +203,9 @@ const mcpHandler: ExportedHandler<Env> = {
     // Check for experimental mode query parameter
     const isExperimentalMode = url.searchParams.get("experimental") === "1";
 
+    // Read utm_source for attribution tracking
+    const utmSource = resolveUtmSourceFromUrl(url);
+
     // Extract OAuth props from ExecutionContext (set by OAuth provider)
     const oauthCtx = ctx as OAuthExecutionContext;
 
@@ -232,6 +236,9 @@ const mcpHandler: ExportedHandler<Env> = {
       "app.server.mode.experimental",
       isExperimentalMode,
     );
+    if (utmSource) {
+      activeSpan?.setAttribute(UTM_SOURCE_ATTRIBUTE, utmSource);
+    }
 
     // Parse and validate granted skills (primary authorization method)
     // Legacy tokens without grantedSkills are no longer supported
diff --git a/packages/mcp-cloudflare/src/server/metrics.ts b/packages/mcp-cloudflare/src/server/metrics.ts
index 55f9b21a..34b07fd6 100644
--- a/packages/mcp-cloudflare/src/server/metrics.ts
+++ b/packages/mcp-cloudflare/src/server/metrics.ts
@@ -1,4 +1,8 @@
 import * as Sentry from "@sentry/cloudflare";
+import {
+  UTM_SOURCE_ATTRIBUTE,
+  resolveUtmSourceFromUrl,
+} from "./lib/attribution";
 import { resolveClientFamily } from "./lib/client-family";
 import type { OAuthErrorTelemetry } from "./oauth/telemetry";
 
@@ -87,6 +91,7 @@ function getMcpRequestAttributes(request: Request, url: URL) {
     clientFamily: resolveClientFamily(request.headers.get("user-agent")),
     agentMode: url.searchParams.get("agent") === "1",
     experimentalMode: url.searchParams.get("experimental") === "1",
+    utmSource: resolveUtmSourceFromUrl(url),
   };
 }
 
@@ -132,6 +137,9 @@ function getMetricAttributes(
     attributes["app.server.mode.experimental"] = getBooleanAttribute(
       mcpAttributes.experimentalMode,
     );
+    if (mcpAttributes.utmSource) {
+      attributes[UTM_SOURCE_ATTRIBUTE] = mcpAttributes.utmSource;
+    }
   }
 
   return attributes;
@@ -184,6 +192,9 @@ export function annotateTrackedRequestSpan(
       "app.server.mode.experimental",
       mcpAttributes.experimentalMode,
     );
+    if (mcpAttributes.utmSource) {
+      activeSpan.setAttribute(UTM_SOURCE_ATTRIBUTE, mcpAttributes.utmSource);
+    }
   }
 
   if (options?.responseReason) {