diff --git a/packages/mcp-core/src/api-client/client.test.ts b/packages/mcp-core/src/api-client/client.test.ts
index 874b9f40d..dfdc4a550 100644
--- a/packages/mcp-core/src/api-client/client.test.ts
+++ b/packages/mcp-core/src/api-client/client.test.ts
@@ -10,6 +10,7 @@ import { ConfigurationError } from "../errors";
 import { parseSentryUrl } from "../internal/url-helpers";
 import { SentryApiService } from "./client";
 import {
+  ApiClientError,
   ApiNotFoundError,
   ApiPermissionError,
   ApiRateLimitError,
@@ -108,6 +109,71 @@ describe("validateEvents error bodies", () => {
   );
 });
 
+describe("validateEvents non-JSON responses", () => {
+  afterEach(() => mswServer.resetHandlers());
+
+  it("classifies the endpoint's empty feature-gate 400 as a client error", async () => {
+    mswServer.use(
+      http.get(
+        "https://sentry.io/api/0/organizations/test-org/events/validate/",
+        () => new HttpResponse(null, { status: 400 }),
+      ),
+    );
+    const api = new SentryApiService({
+      host: "sentry.io",
+      accessToken: "test-token",
+    });
+    await expect(
+      api.validateEvents({ organizationSlug: "test-org" }),
+    ).rejects.toBeInstanceOf(ApiValidationError);
+  });
+
+  it.each(["text/plain", "text/html"])(
+    "classifies a 400 %s response as a client error",
+    async (contentType) => {
+      mswServer.use(
+        http.get(
+          "https://sentry.io/api/0/organizations/test-org/events/validate/",
+          () =>
+            new HttpResponse("Invalid query", {
+              status: 400,
+              headers: { "Content-Type": contentType },
+            }),
+        ),
+      );
+      const api = new SentryApiService({
+        host: "sentry.io",
+        accessToken: "test-token",
+      });
+      await expect(
+        api.validateEvents({ organizationSlug: "test-org" }),
+      ).rejects.toBeInstanceOf(ApiValidationError);
+    },
+  );
+
+  it("does not classify a successful HTML response as user input", async () => {
+    mswServer.use(
+      http.get(
+        "https://sentry.io/api/0/organizations/test-org/events/validate/",
+        () =>
+          new HttpResponse("<html>Unexpected response</html>", {
+            status: 200,
+            headers: { "Content-Type": "text/html" },
+          }),
+      ),
+    );
+    const api = new SentryApiService({
+      host: "sentry.io",
+      accessToken: "test-token",
+    });
+    const result = api.validateEvents({ organizationSlug: "test-org" });
+    await expect(result).rejects.toThrow(
+      "Expected JSON response but received HTML",
+    );
+    await expect(result).rejects.not.toBeInstanceOf(ApiClientError);
+  });
+});
+
 describe("API bearer token validation", () => {
   it("removes edge padding before sending a request", async () => {
     let authorization: string | null = null;
diff --git a/packages/mcp-core/src/api-client/client.ts b/packages/mcp-core/src/api-client/client.ts
index 7b6ae29c1..21f3e9368 100644
--- a/packages/mcp-core/src/api-client/client.ts
+++ b/packages/mcp-core/src/api-client/client.ts
@@ -3634,6 +3634,19 @@ export class SentryApiService {
       undefined,
       { ...opts, allowStatuses: [400] },
     );
+    // The endpoint can reject with an empty or non-JSON 400, such as when its
+    // feature gate is unavailable. That is still a client error, not a server bug.
+    const contentType = response.headers.get("content-type");
+    if (response.status === 400 && !contentType?.includes("application/json")) {
+      const text = await response.text();
+      throw createApiError(
+        `Expected JSON response but received ${contentType || "unknown content type"} ` +
+          `(${response.status} ${response.statusText})`,
+        response.status,
+        undefined,
+        text,
+      );
+    }
     const body = await readJsonResponse(response);
     // ParseError responses use a detail envelope instead of validation results.
     // Keep malformed validation results reportable, even when their status is 400.