diff --git a/ui/actions/mute-rules/mute-rules.test.ts b/ui/actions/mute-rules/mute-rules.test.ts
new file mode 100644
index 0000000000..cfb7f5daf9
--- /dev/null
+++ b/ui/actions/mute-rules/mute-rules.test.ts
@@ -0,0 +1,87 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { fetchMock, getAuthHeadersMock, revalidatePathMock } = vi.hoisted(
+  () => ({
+    fetchMock: vi.fn(),
+    getAuthHeadersMock: vi.fn(),
+    revalidatePathMock: vi.fn(),
+  }),
+);
+
+vi.mock("@/lib/helper", () => ({
+  apiBaseUrl: "https://api.test/api/v1",
+  getAuthHeaders: getAuthHeadersMock,
+}));
+
+vi.mock("next/cache", () => ({
+  revalidatePath: revalidatePathMock,
+}));
+
+import { createMuteRule } from "./mute-rules";
+
+const NAME_CONFLICT_DETAIL = "A mute rule with this name already exists.";
+
+const errorResponse = (contentType: string, body: string, status = 400) =>
+  new Response(body, {
+    status,
+    headers: { "Content-Type": contentType },
+  });
+
+const nameConflictBody = JSON.stringify({
+  errors: [
+    {
+      detail: NAME_CONFLICT_DETAIL,
+      status: "400",
+      source: { pointer: "/data/attributes/name" },
+      code: "invalid",
+    },
+  ],
+});
+
+const muteRuleFormData = () => {
+  const formData = new FormData();
+  formData.set("name", "Root account has a hardware MFA device enabled");
+  formData.set("reason", "Not our approach here with SSO");
+  formData.set("finding_ids", JSON.stringify(["finding-1"]));
+  return formData;
+};
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  vi.stubGlobal("fetch", fetchMock);
+  vi.spyOn(console, "error").mockImplementation(() => {});
+  getAuthHeadersMock.mockResolvedValue({ Authorization: "Bearer token" });
+});
+
+describe("createMuteRule", () => {
+  it("should return only the error detail for a JSON:API error response", async () => {
+    fetchMock.mockResolvedValue(
+      errorResponse("application/vnd.api+json", nameConflictBody),
+    );
+
+    const result = await createMuteRule(null, muteRuleFormData());
+
+    expect(result?.errors?.general).toBe(NAME_CONFLICT_DETAIL);
+    expect(revalidatePathMock).not.toHaveBeenCalled();
+  });
+
+  it("should return only the error detail for a plain JSON error response", async () => {
+    fetchMock.mockResolvedValue(
+      errorResponse("application/json", nameConflictBody),
+    );
+
+    const result = await createMuteRule(null, muteRuleFormData());
+
+    expect(result?.errors?.general).toBe(NAME_CONFLICT_DETAIL);
+  });
+
+  it("should return the response text for a non-JSON error response", async () => {
+    fetchMock.mockResolvedValue(
+      errorResponse("text/plain", "Bad gateway", 502),
+    );
+
+    const result = await createMuteRule(null, muteRuleFormData());
+
+    expect(result?.errors?.general).toBe("Bad gateway");
+  });
+});
diff --git a/ui/actions/mute-rules/mute-rules.ts b/ui/actions/mute-rules/mute-rules.ts
index c74c4a7e2a..1ce9a8b3bc 100644
--- a/ui/actions/mute-rules/mute-rules.ts
+++ b/ui/actions/mute-rules/mute-rules.ts
@@ -156,7 +156,8 @@ export const createMuteRule = async (
       let errorMessage = `Failed to create mute rule: ${response.statusText}`;
       const responseContentType = response.headers.get("content-type");
       try {
-        if (responseContentType?.includes("application/json")) {
+        // The API answers with application/vnd.api+json
+        if (responseContentType?.includes("json")) {
           const errorData = await response.json();
           const jsonApiError = (
             errorData as {
diff --git a/ui/changelog.d/mute-rule-error-toast-raw-json.fixed.md b/ui/changelog.d/mute-rule-error-toast-raw-json.fixed.md
new file mode 100644
index 0000000000..d59b218727
--- /dev/null
+++ b/ui/changelog.d/mute-rule-error-toast-raw-json.fixed.md
@@ -0,0 +1 @@
+Mute rule creation errors show the API error message instead of the raw JSON:API response body