diff --git a/app/api/comments/route.test.ts b/app/api/comments/route.test.ts
new file mode 100644
index 0000000..99d5c6f
--- /dev/null
+++ b/app/api/comments/route.test.ts
@@ -0,0 +1,114 @@
+import { NextRequest } from "next/server";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const from = vi.fn();
+const consumeSharedRateLimit = vi.fn();
+const hashIp = vi.fn(() => "hashed-ip");
+const getClientIp = vi.fn(() => "127.0.0.1");
+
+vi.mock("@/lib/supabase/server", () => ({
+  createSupabaseServerClient: () => ({ from }),
+}));
+
+vi.mock("@/lib/supabase/admin", () => ({
+  createSupabaseAdminClient: () => ({ from }),
+}));
+
+vi.mock("@/lib/rate-limit/shared", () => ({
+  consumeSharedRateLimit,
+}));
+
+vi.mock("@/lib/utils/hash", () => ({
+  hashIp,
+  getClientIp,
+}));
+
+function createRequest(body: unknown) {
+  return new NextRequest("http://localhost/api/comments", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+describe("POST /api/comments", () => {
+  beforeEach(() => {
+    vi.resetModules();
+    vi.clearAllMocks();
+    consumeSharedRateLimit.mockResolvedValue({
+      allowed: true,
+      remaining: 4,
+      resetAt: new Date().toISOString(),
+      currentCount: 1,
+    });
+    from.mockReturnValue({
+      insert: vi.fn().mockReturnValue({
+        select: () => ({
+          single: async () => ({
+            data: {
+              id: "comment-1",
+              body: "This comment has enough content.",
+              is_anonymous: false,
+              author_handle: "saved-handle",
+              created_at: "2026-09-13T00:00:00.000Z",
+            },
+            error: null,
+          }),
+        }),
+      }),
+    });
+  });
+
+  it("rejects an author handle longer than 64 characters", async () => {
+    const { POST } = await import("./route");
+    const response = await POST(
+      createRequest({
+        post_id: "post-1",
+        body: "This comment has enough content.",
+        is_anonymous: false,
+        author_handle: "a".repeat(65),
+      }),
+    );
+
+    expect(response.status).toBe(400);
+    await expect(response.json()).resolves.toEqual({
+      error: "Handle must be 64 characters or fewer.",
+    });
+    expect(consumeSharedRateLimit).not.toHaveBeenCalled();
+    expect(from).not.toHaveBeenCalled();
+  });
+
+  it("accepts a 64-character author handle", async () => {
+    const handle = "a".repeat(64);
+    const insertSpy = vi.fn().mockReturnValue({
+      select: () => ({
+        single: async () => ({
+          data: {
+            id: "comment-1",
+            body: "This comment has enough content.",
+            is_anonymous: false,
+            author_handle: handle,
+            created_at: "2026-09-13T00:00:00.000Z",
+          },
+          error: null,
+        }),
+      }),
+    });
+    from.mockReturnValue({ insert: insertSpy });
+
+    const { POST } = await import("./route");
+    const response = await POST(
+      createRequest({
+        post_id: "post-1",
+        body: "This comment has enough content.",
+        is_anonymous: false,
+        author_handle: handle,
+      }),
+    );
+
+    expect(response.status).toBe(201);
+    expect(insertSpy).toHaveBeenCalledWith(
+      expect.objectContaining({ author_handle: handle }),
+    );
+  });
+});
diff --git a/app/api/comments/route.ts b/app/api/comments/route.ts
index 6cc9a14..256277c 100644
--- a/app/api/comments/route.ts
+++ b/app/api/comments/route.ts
@@ -3,6 +3,7 @@ import { createSupabaseServerClient } from "@/lib/supabase/server";
 import { createSupabaseAdminClient } from "@/lib/supabase/admin";
 import { consumeSharedRateLimit } from "@/lib/rate-limit/shared";
 import { hashIp, getClientIp } from "@/lib/utils/hash";
+import { MAX_AUTHOR_HANDLE_LENGTH } from "@/lib/schemas/submit";
 
 interface CommentBody {
   post_id?: unknown;
@@ -47,6 +48,14 @@ export async function POST(req: NextRequest) {
     const isAnon = typeof is_anonymous === "boolean" ? is_anonymous : true;
     const handle =
       typeof author_handle === "string" ? author_handle.trim() : null;
+    if (handle && handle.length > MAX_AUTHOR_HANDLE_LENGTH) {
+      return NextResponse.json(
+        {
+          error: `Handle must be ${MAX_AUTHOR_HANDLE_LENGTH} characters or fewer.`,
+        },
+        { status: 400 },
+      );
+    }
     if (!isAnon && !handle) {
       return NextResponse.json(
         { error: "A handle is required when posting non-anonymously." },
diff --git a/lib/schemas/submit.ts b/lib/schemas/submit.ts
index 371b130..09b38ba 100644
--- a/lib/schemas/submit.ts
+++ b/lib/schemas/submit.ts
@@ -1,6 +1,8 @@
 import { z } from "zod";
 import { isOwnedScreenshotUrl } from "@/lib/utils/urls";
 
+export const MAX_AUTHOR_HANDLE_LENGTH = 64;
+
 export const submitSchema = z.object({
   /** Slug of the agent involved (from AGENTS constant) */
   agentSlug: z.string().min(1, "Please select the AI agent involved.").max(64),
@@ -39,7 +41,7 @@ export const submitSchema = z.object({
   isAnonymous: z.boolean().default(true),
 
   /** Optional display handle or company name (when not anonymous) */
-  authorHandle: z.string().max(64).optional(),
+  authorHandle: z.string().max(MAX_AUTHOR_HANDLE_LENGTH).optional(),
 
   /** Optional email to receive the edit token — never stored long-term */
   email: z