diff --git a/lib/share/rate-limit-core.test.ts b/lib/share/rate-limit-core.test.ts
new file mode 100644
index 0000000..0d5d8e8
--- /dev/null
+++ b/lib/share/rate-limit-core.test.ts
@@ -0,0 +1,53 @@
+import { describe, expect, test } from "bun:test";
+import { enforceFixedWindowRateLimit } from "@/lib/share/rate-limit-core";
+
+function memoryIncrement() {
+  const counts = new Map<string, number>();
+  return async (key: string) => {
+    const count = (counts.get(key) ?? 0) + 1;
+    counts.set(key, count);
+    return count;
+  };
+}
+
+describe("share fixed-window rate limits", () => {
+  for (const [name, limit] of [
+    ["sender shares", 10],
+    ["bot requests", 60],
+    ["comments", 30],
+  ] as const) {
+    test(`allows ${limit} ${name} and rejects request ${limit + 1}`, async () => {
+      const increment = memoryIncrement();
+      const options = {
+        key: name,
+        limit,
+        windowSeconds: 3_600,
+        increment,
+        now: 3_601_000,
+      };
+
+      for (let request = 1; request <= limit; request += 1) {
+        await expect(enforceFixedWindowRateLimit(options)).resolves.toBeUndefined();
+      }
+      await expect(enforceFixedWindowRateLimit(options)).rejects.toMatchObject({
+        status: 429,
+        retryAfter: 3_599,
+      });
+    });
+  }
+
+  test("fails closed when Redis is unavailable", async () => {
+    await expect(
+      enforceFixedWindowRateLimit({
+        key: "sender:1",
+        limit: 10,
+        windowSeconds: 3_600,
+        increment: async () => {
+          throw new Error("offline");
+        },
+      }),
+    ).rejects.toEqual(
+      expect.objectContaining({ status: 503 }),
+    );
+  });
+});
diff --git a/lib/share/rate-limit-core.ts b/lib/share/rate-limit-core.ts
new file mode 100644
index 0000000..c3b4e01
--- /dev/null
+++ b/lib/share/rate-limit-core.ts
@@ -0,0 +1,41 @@
+import { ShareHttpError } from "@/lib/share/http-error";
+
+export type FixedWindowIncrement = (
+  key: string,
+  expiresInSeconds: number,
+) => Promise<number>;
+
+export async function enforceFixedWindowRateLimit({
+  key,
+  limit,
+  windowSeconds,
+  increment,
+  now = Date.now(),
+}: {
+  key: string;
+  limit: number;
+  windowSeconds: number;
+  increment: FixedWindowIncrement;
+  now?: number;
+}) {
+  const nowSeconds = Math.floor(now / 1000);
+  const window = Math.floor(nowSeconds / windowSeconds);
+  const retryAfter = windowSeconds - (nowSeconds % windowSeconds);
+
+  let count: number;
+  try {
+    count = await increment(
+      `erika:share:${key}:${window}`,
+      windowSeconds + 1,
+    );
+  } catch {
+    throw new ShareHttpError("Rate limiting is temporarily unavailable", 503);
+  }
+
+  if (!Number.isFinite(count)) {
+    throw new ShareHttpError("Rate limiting is temporarily unavailable", 503);
+  }
+  if (count > limit) {
+    throw new ShareHttpError("Rate limit exceeded", 429, retryAfter);
+  }
+}
diff --git a/lib/share/rate-limit.ts b/lib/share/rate-limit.ts
index 643f3de..2d57cdb 100644
--- a/lib/share/rate-limit.ts
+++ b/lib/share/rate-limit.ts
@@ -1,7 +1,7 @@
 import "server-only";
 
 import { getRedisClient } from "@/lib/redis";
-import { ShareHttpError } from "@/lib/share/http-error";
+import { enforceFixedWindowRateLimit } from "@/lib/share/rate-limit-core";
 
 const FIXED_WINDOW_SCRIPT = `
 local count = redis.call("INCR", KEYS[1])
@@ -20,25 +20,20 @@ export async function enforceShareRateLimit({
   limit: number;
   windowSeconds: number;
 }) {
-  const nowSeconds = Math.floor(Date.now() / 1000);
-  const window = Math.floor(nowSeconds / windowSeconds);
-  const retryAfter = windowSeconds - (nowSeconds % windowSeconds);
-
-  try {
-    const redis = await getRedisClient();
-    const count = Number(
-      await redis.send("EVAL", [
-        FIXED_WINDOW_SCRIPT,
-        "1",
-        `erika:share:${key}:${window}`,
-        String(windowSeconds + 1),
-      ])
-    );
-    if (count > limit) {
-      throw new ShareHttpError("Rate limit exceeded", 429, retryAfter);
-    }
-  } catch (error) {
-    if (error instanceof ShareHttpError) throw error;
-    throw new ShareHttpError("Rate limiting is temporarily unavailable", 503);
-  }
+  return enforceFixedWindowRateLimit({
+    key,
+    limit,
+    windowSeconds,
+    increment: async (redisKey, expiresInSeconds) => {
+      const redis = await getRedisClient();
+      return Number(
+        await redis.send("EVAL", [
+          FIXED_WINDOW_SCRIPT,
+          "1",
+          redisKey,
+          String(expiresInSeconds),
+        ]),
+      );
+    },
+  });
 }