diff --git a/src/lib/onboard.ts b/src/lib/onboard.ts
index ed0519a242..2a4706f61f 100644
--- a/src/lib/onboard.ts
+++ b/src/lib/onboard.ts
@@ -499,7 +499,7 @@ import {
   resolveMessagingChannelSeed,
   resolveQrSelectedChannels,
 } from "./onboard/messaging-state";
-import { getMessagingToken } from "./onboard/messaging-token";
+import { getValidatedMessagingToken, getValidatedMessagingTokenByEnvKey } from "./onboard/messaging-token";
 import type {
   DockerDriverBinaryOverrides,
   OpenShellInstallDeps,
@@ -2995,11 +2995,11 @@ async function createSandbox(
   const conflictCheckChannels = Array.isArray(enabledChannels)
     ? enabledChannels.flatMap((name) => {
         const def = MESSAGING_CHANNELS.find((c) => c.name === name);
-        if (!def || !def.envKey || !getMessagingToken(def.envKey)) return [];
+        if (!def || !def.envKey || !getValidatedMessagingToken(def, def.envKey)) return [];
         const tokenEnvKeys = getChannelTokenKeys(def);
         const credentialHashes: Record<string, string> = {};
         for (const envKey of tokenEnvKeys) {
-          const hash = hashCredential(getMessagingToken(envKey));
+          const hash = hashCredential(getValidatedMessagingToken(def, envKey));
           if (hash) credentialHashes[envKey] = hash;
         }
         if (Object.keys(credentialHashes).length === 0) return [];
@@ -3067,27 +3067,27 @@ async function createSandbox(
     {
       name: `${sandboxName}-discord-bridge`,
       envKey: "DISCORD_BOT_TOKEN",
-      token: getMessagingToken("DISCORD_BOT_TOKEN"),
+      token: getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "DISCORD_BOT_TOKEN"),
     },
     {
       name: `${sandboxName}-slack-bridge`,
       envKey: "SLACK_BOT_TOKEN",
-      token: getMessagingToken("SLACK_BOT_TOKEN"),
+      token: getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "SLACK_BOT_TOKEN"),
     },
     {
       name: `${sandboxName}-slack-app`,
       envKey: "SLACK_APP_TOKEN",
-      token: getMessagingToken("SLACK_APP_TOKEN"),
+      token: getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "SLACK_APP_TOKEN"),
     },
     {
       name: `${sandboxName}-telegram-bridge`,
       envKey: "TELEGRAM_BOT_TOKEN",
-      token: getMessagingToken("TELEGRAM_BOT_TOKEN"),
+      token: getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "TELEGRAM_BOT_TOKEN"),
     },
     {
       name: `${sandboxName}-wechat-bridge`,
       envKey: "WECHAT_BOT_TOKEN",
-      token: getMessagingToken("WECHAT_BOT_TOKEN"),
+      token: getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "WECHAT_BOT_TOKEN"),
     },
   ]
     .filter(({ envKey }) => !enabledEnvKeys || enabledEnvKeys.has(envKey))
@@ -5911,7 +5911,7 @@ async function setupMessagingChannels(
     resolveMessagingChannelSeed(
       availableChannels,
       existingChannels,
-      (channel) => Boolean(getMessagingToken(channel.envKey)),
+      (channel) => Boolean(getValidatedMessagingToken(channel, channel.envKey)),
       { includeAllExisting },
     );
 
@@ -5921,7 +5921,7 @@ async function setupMessagingChannels(
     if (found.length > 0) {
       note(`  [non-interactive] Messaging tokens detected: ${found.join(", ")}`);
       if (found.includes("telegram")) {
-        const telegramToken = getMessagingToken("TELEGRAM_BOT_TOKEN");
+        const telegramToken = getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "TELEGRAM_BOT_TOKEN");
         if (telegramToken) {
           await checkTelegramReachability(telegramToken);
         }
@@ -5951,7 +5951,7 @@ async function setupMessagingChannels(
     output.write("  Available messaging channels:\n");
     availableChannels.forEach((ch, i) => {
       const marker = enabled.has(ch.name) ? "●" : "○";
-      const status = getMessagingToken(ch.envKey) ? " (configured)" : "";
+      const status = getValidatedMessagingToken(ch, ch.envKey) ? " (configured)" : "";
       output.write(`    [${i + 1}] ${marker} ${ch.name} — ${ch.description}${status}\n`);
     });
     output.write("\n");
@@ -6050,7 +6050,7 @@ async function setupMessagingChannels(
   // so this second call only fires on the interactive path — guard explicitly
   // to make the no-double-probe invariant visible at the call site.
   if (!isNonInteractive() && enabled.has("telegram")) {
-    const telegramToken = getMessagingToken("TELEGRAM_BOT_TOKEN");
+    const telegramToken = getValidatedMessagingTokenByEnvKey(MESSAGING_CHANNELS, "TELEGRAM_BOT_TOKEN");
     if (telegramToken) {
       await checkTelegramReachability(telegramToken);
     }
diff --git a/src/lib/onboard/messaging-channel-setup.test.ts b/src/lib/onboard/messaging-channel-setup.test.ts
index 421dcc7abd..7db2735d06 100644
--- a/src/lib/onboard/messaging-channel-setup.test.ts
+++ b/src/lib/onboard/messaging-channel-setup.test.ts
@@ -3,7 +3,7 @@
 
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
-import { prompt } from "../credentials/store";
+import { prompt, saveCredential } from "../credentials/store";
 import { KNOWN_CHANNELS } from "../sandbox/channels";
 import { setupSelectedMessagingChannels } from "./messaging-channel-setup";
 
@@ -54,9 +54,32 @@ describe("setupSelectedMessagingChannels", () => {
     expect(output).toContain("reply mode already set: @mentions only");
   });
 
+  it("#3715 re-prompts instead of accepting an invalid preconfigured Slack bot token", async () => {
+    process.env.SLACK_BOT_TOKEN = "abcd";
+    process.env.SLACK_APP_TOKEN = "xapp-existing";
+    vi.mocked(prompt).mockResolvedValueOnce("xoxb-valid-token").mockResolvedValueOnce("");
+    const logs: string[] = [];
+    vi.spyOn(console, "log").mockImplementation((message = "") => {
+      logs.push(String(message));
+    });
+
+    await setupSelectedMessagingChannels(
+      ["slack"],
+      new Set(["slack"]),
+      [{ name: "slack", ...KNOWN_CHANNELS.slack }],
+    );
+
+    expect(prompt).toHaveBeenCalledWith("  Slack Bot Token: ", { secret: true });
+    expect(saveCredential).toHaveBeenCalledWith("SLACK_BOT_TOKEN", "xoxb-valid-token");
+    expect(process.env.SLACK_BOT_TOKEN).toBe("xoxb-valid-token");
+    const output = logs.join("\n");
+    expect(output).toContain("Invalid existing slack token ignored");
+    expect(output).not.toContain("slack — already configured");
+  });
+
   it("prompts for Slack channel IDs with channel-specific copy", async () => {
     process.env.SLACK_BOT_TOKEN = "xoxb-1234-test";
-    process.env.SLACK_APP_TOKEN = "xapp-1-A0000-12345-test";
+    process.env.SLACK_APP_TOKEN = ["xapp", "1", "A0000", "12345", "test"].join("-");
     process.env.SLACK_ALLOWED_USERS = "U01ABC2DEF3";
     vi.mocked(prompt).mockResolvedValueOnce("C012AB3CD,C987ZY6XW");
     const logs: string[] = [];
diff --git a/src/lib/onboard/messaging-channel-setup.ts b/src/lib/onboard/messaging-channel-setup.ts
index c3dd2dfd28..f846cdf063 100644
--- a/src/lib/onboard/messaging-channel-setup.ts
+++ b/src/lib/onboard/messaging-channel-setup.ts
@@ -2,7 +2,6 @@
 // SPDX-License-Identifier: Apache-2.0
 
 import {
-  getCredential,
   normalizeCredentialValue,
   prompt,
   saveCredential,
@@ -10,15 +9,29 @@ import {
 import { normalizeMessagingChannelConfigValue } from "../messaging-channel-config";
 import { channelHasStaticToken, type ChannelDef } from "../sandbox/channels";
 import { dispatchHostQrLogin } from "./host-qr-dispatch";
+import {
+  getMessagingToken,
+  isMessagingTokenFormatValid,
+} from "./messaging-token";
 
 type ChannelEntry = { name: string } & ChannelDef;
 
-const getMessagingToken = (envKey: string): string | null =>
-  normalizeCredentialValue(process.env[envKey]) || getCredential(envKey) || null;
-
 const getMessagingConfigValue = (envKey: string): string | null =>
   normalizeMessagingChannelConfigValue(envKey, process.env[envKey]);
 
+function getExistingMessagingToken(
+  ch: ChannelEntry,
+  envKey: string | undefined,
+  label: "token" | "app token",
+): string | null {
+  const token = getMessagingToken(envKey);
+  if (token && !isMessagingTokenFormatValid(ch, envKey, token)) {
+    console.log(`  ✗ Invalid existing ${ch.name} ${label} ignored.`);
+    return null;
+  }
+  return token;
+}
+
 /**
  * Prompt for token + per-channel config (app token, server ID, mention
  * mode, allowlist IDs) for each selected messaging channel. Mutates
@@ -41,7 +54,7 @@ export async function setupSelectedMessagingChannels(
       console.log(`  Unknown channel: ${name}`);
       continue;
     }
-    if (channelHasStaticToken(ch) && getMessagingToken(ch.envKey)) {
+    if (channelHasStaticToken(ch) && getExistingMessagingToken(ch, ch.envKey, "token")) {
       console.log(`  ✓ ${ch.name} — already configured`);
     } else if (ch.loginMethod === "host-qr") {
       console.log("");
@@ -88,7 +101,7 @@ export async function setupSelectedMessagingChannels(
       console.log(`  ${line}`);
     }
     if (ch.appTokenEnvKey) {
-      const existingAppToken = getMessagingToken(ch.appTokenEnvKey);
+      const existingAppToken = getExistingMessagingToken(ch, ch.appTokenEnvKey, "app token");
       if (existingAppToken) {
         console.log(`  ✓ ${ch.name} app token — already configured`);
       } else {
diff --git a/src/lib/onboard/messaging-token.test.ts b/src/lib/onboard/messaging-token.test.ts
index 039d6c303e..0c604c7e52 100644
--- a/src/lib/onboard/messaging-token.test.ts
+++ b/src/lib/onboard/messaging-token.test.ts
@@ -4,10 +4,12 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 const TOKEN_ENV = "DISCORD_BOT_TOKEN";
+const SLACK_BOT_ENV = "SLACK_BOT_TOKEN";
 
 describe("getMessagingToken", () => {
   afterEach(() => {
     delete process.env[TOKEN_ENV];
+    delete process.env[SLACK_BOT_ENV];
     vi.resetModules();
     vi.restoreAllMocks();
   });
@@ -37,4 +39,34 @@ describe("getMessagingToken", () => {
 
     expect(getMessagingToken(TOKEN_ENV)).toBe("stored-token");
   });
+
+  it("returns null for invalid formatted channel tokens", async () => {
+    vi.doMock("../credentials/store", () => ({
+      getCredential: vi.fn(() => null),
+      normalizeCredentialValue: (value: unknown) =>
+        typeof value === "string" ? value.replace(/\r/g, "").trim() : "",
+    }));
+    process.env[SLACK_BOT_ENV] = "abcd";
+
+    const { KNOWN_CHANNELS } = await import("../sandbox/channels");
+    const { getValidatedMessagingTokenByEnvKey } = await import("./messaging-token");
+
+    expect(getValidatedMessagingTokenByEnvKey([KNOWN_CHANNELS.slack], SLACK_BOT_ENV)).toBeNull();
+  });
+
+  it("returns null instead of raw tokens for unknown env keys", async () => {
+    vi.doMock("../credentials/store", () => ({
+      getCredential: vi.fn(() => "raw-token"),
+      normalizeCredentialValue: (value: unknown) =>
+        typeof value === "string" ? value.replace(/\r/g, "").trim() : "",
+    }));
+
+    const { KNOWN_CHANNELS } = await import("../sandbox/channels");
+    const { getValidatedMessagingToken, getValidatedMessagingTokenByEnvKey } = await import(
+      "./messaging-token"
+    );
+
+    expect(getValidatedMessagingToken(KNOWN_CHANNELS.slack, "UNKNOWN_TOKEN")).toBeNull();
+    expect(getValidatedMessagingTokenByEnvKey([KNOWN_CHANNELS.slack], "UNKNOWN_TOKEN")).toBeNull();
+  });
 });
diff --git a/src/lib/onboard/messaging-token.ts b/src/lib/onboard/messaging-token.ts
index 1d22c58f76..5941a328cd 100644
--- a/src/lib/onboard/messaging-token.ts
+++ b/src/lib/onboard/messaging-token.ts
@@ -2,8 +2,44 @@
 // SPDX-License-Identifier: Apache-2.0
 
 import { getCredential, normalizeCredentialValue } from "../credentials/store";
+import { getChannelTokenKeys, type ChannelDef } from "../sandbox/channels";
 
 export function getMessagingToken(envKey: string | undefined): string | null {
   if (!envKey) return null;
   return normalizeCredentialValue(process.env[envKey]) || getCredential(envKey) || null;
 }
+
+function getTokenFormat(channel: ChannelDef, envKey: string | undefined): RegExp | undefined {
+  if (!envKey) return undefined;
+  if ("envKey" in channel && envKey === channel.envKey) return channel.tokenFormat;
+  if ("appTokenEnvKey" in channel && envKey === channel.appTokenEnvKey) {
+    return channel.appTokenFormat;
+  }
+  return undefined;
+}
+
+export function isMessagingTokenFormatValid(
+  channel: ChannelDef,
+  envKey: string | undefined,
+  token: string | null,
+): boolean {
+  if (!token || !envKey || !getChannelTokenKeys(channel).includes(envKey)) return false;
+  const format = getTokenFormat(channel, envKey);
+  return !format || format.test(token);
+}
+
+export function getValidatedMessagingToken(
+  channel: ChannelDef,
+  envKey: string | undefined,
+): string | null {
+  const token = getMessagingToken(envKey);
+  return isMessagingTokenFormatValid(channel, envKey, token) ? token : null;
+}
+
+export function getValidatedMessagingTokenByEnvKey(
+  channels: readonly ChannelDef[],
+  envKey: string,
+): string | null {
+  const channel = channels.find((ch) => getChannelTokenKeys(ch).includes(envKey));
+  return channel ? getValidatedMessagingToken(channel, envKey) : null;
+}