diff --git a/packages/bots/slack/src/index.test.ts b/packages/bots/slack/src/index.test.ts
index 44bdfaf8..8b17a427 100644
--- a/packages/bots/slack/src/index.test.ts
+++ b/packages/bots/slack/src/index.test.ts
@@ -33,6 +33,10 @@ function captureFetch(responses: Array<{ ok: boolean; ts?: string; error?: strin
 
 function signHeaders(body: string, signingKey = 'synthetic-signing-key'): Record<string, string> {
   const timestamp = String(Math.floor(Date.now() / 1000));
+  return signHeadersAt(body, timestamp, signingKey);
+}
+
+function signHeadersAt(body: string, timestamp: string, signingKey = 'synthetic-signing-key'): Record<string, string> {
   return {
     'x-slack-request-timestamp': timestamp,
     'x-slack-signature': slackSignature(body, timestamp, signingKey),
@@ -199,6 +203,24 @@ describe('Slack bot adapter', () => {
     await closeable.close();
   });
 
+  it('uses the default timestamp tolerance when config contains an invalid value', async () => {
+    const { fetcher } = captureFetch();
+    let server: Server | undefined;
+    const closeable = await bot.register(
+      ctx(),
+      [],
+      { port: 0, fetch: fetcher, timestampToleranceSeconds: Number.NaN, onServerReady: (value) => { server = value; } },
+    );
+    const body = JSON.stringify({ type: 'event_callback', event: { type: 'message', channel: 'C123' } });
+    const staleTimestamp = String(Math.floor(Date.now() / 1000) - 600);
+
+    const response = await post(serverPort(server!), '/slack/events', body, signHeadersAt(body, staleTimestamp));
+
+    expect(response.status).toBe(401);
+    expect(JSON.parse(response.body)).toEqual({ ok: false, error: 'invalid_signature' });
+    await closeable.close();
+  });
+
   it('dispatches signed block action payloads as interactions', async () => {
     const seen: BotEvent[] = [];
     const { calls, fetcher } = captureFetch([{ ok: true, ts: '1700000002.000100' }]);
diff --git a/packages/bots/slack/src/index.ts b/packages/bots/slack/src/index.ts
index 650fe337..babed8e3 100644
--- a/packages/bots/slack/src/index.ts
+++ b/packages/bots/slack/src/index.ts
@@ -416,12 +416,16 @@ function verifySlackSignature(
   signingKey: string,
   toleranceSeconds = DEFAULT_TIMESTAMP_TOLERANCE_SECONDS,
 ): boolean {
+  const effectiveToleranceSeconds = normalizeTimestampToleranceSeconds(toleranceSeconds);
   const timestamp = firstHeader(headers["x-slack-request-timestamp"]);
   const signature = firstHeader(headers["x-slack-signature"]);
   if (!timestamp || !signature) return false;
   const timestampNumber = Number(timestamp);
   if (!Number.isFinite(timestampNumber)) return false;
-  if (toleranceSeconds > 0 && Math.abs(Math.floor(Date.now() / 1000) - timestampNumber) > toleranceSeconds) {
+  if (
+    effectiveToleranceSeconds > 0
+    && Math.abs(Math.floor(Date.now() / 1000) - timestampNumber) > effectiveToleranceSeconds
+  ) {
     return false;
   }
   const expected = slackSignature(rawBody, timestamp, signingKey);
@@ -430,6 +434,12 @@ function verifySlackSignature(
   return actual.length === expectedBuffer.length && timingSafeEqual(actual, expectedBuffer);
 }
 
+function normalizeTimestampToleranceSeconds(value: number): number {
+  if (value === 0) return 0;
+  if (Number.isFinite(value) && value > 0) return value;
+  return DEFAULT_TIMESTAMP_TOLERANCE_SECONDS;
+}
+
 export function slackSignature(rawBody: string, timestamp: string, signingKey: string): string {
   const digest = createHmac("sha256", signingKey)
     .update(`v0:${timestamp}:${rawBody}`, "utf8")