diff --git a/src/channels/official-channels-store.ts b/src/channels/official-channels-store.ts
index 00624d28..0c8cbc98 100644
--- a/src/channels/official-channels-store.ts
+++ b/src/channels/official-channels-store.ts
@@ -22,6 +22,8 @@ export interface OfficialChannelTokenStatus {
   processEnvAvailable: boolean;
 }
 
+const MAX_OFFICIAL_CHANNEL_TOKEN_LENGTH = 4096;
+
 function getResolvedStateDirOverride(
   channelId: OfficialChannelId,
   envOverrides?: NodeJS.ProcessEnv | null
@@ -293,6 +295,11 @@ export function setConfiguredOfficialChannelToken(
   if (!normalized) {
     throw new Error(`${envKey} cannot be empty or multiline.`);
   }
+  if (normalized.length > MAX_OFFICIAL_CHANNEL_TOKEN_LENGTH) {
+    throw new Error(
+      `${envKey} cannot exceed ${MAX_OFFICIAL_CHANNEL_TOKEN_LENGTH} characters.`
+    );
+  }
 
   const envPath = getOfficialChannelEnvPath(channelId);
   const currentContent = readFileIfExists(envPath) ?? '';
diff --git a/src/web-server/routes/channels-routes.ts b/src/web-server/routes/channels-routes.ts
index 044dd5ca..89a67cef 100644
--- a/src/web-server/routes/channels-routes.ts
+++ b/src/web-server/routes/channels-routes.ts
@@ -178,7 +178,8 @@ router.put('/:channelId/token', (req: Request, res: Response): void => {
     res.json({ success: true, tokenConfigured: true, tokenPath });
   } catch (error) {
     const message = (error as Error).message;
-    const statusCode = message.includes('cannot be empty') ? 400 : 500;
+    const statusCode =
+      message.includes('cannot be empty') || message.includes('cannot exceed') ? 400 : 500;
     res.status(statusCode).json({ error: message });
   }
 });
diff --git a/tests/unit/channels/official-channels-store.test.ts b/tests/unit/channels/official-channels-store.test.ts
index e114c000..7731ad92 100644
--- a/tests/unit/channels/official-channels-store.test.ts
+++ b/tests/unit/channels/official-channels-store.test.ts
@@ -56,6 +56,16 @@ describe('official channels token store', () => {
     expect(readConfiguredOfficialChannelToken('telegram')).toBe('telegram-secret');
   });
 
+  it('rejects oversized official channel tokens before writing channel state', () => {
+    const envPath = getOfficialChannelEnvPath('discord');
+
+    expect(() => setConfiguredOfficialChannelToken('discord', 'x'.repeat(4097))).toThrow(
+      'DISCORD_BOT_TOKEN cannot exceed 4096 characters.'
+    );
+    expect(fs.existsSync(envPath)).toBe(false);
+    expect(hasConfiguredOfficialChannelToken('discord')).toBe(false);
+  });
+
   it('uses the official state-dir override when one is configured', () => {
     const originalDiscordStateDir = process.env.DISCORD_STATE_DIR;
     process.env.DISCORD_STATE_DIR = path.join(tempHome, 'discord-state');
diff --git a/tests/unit/web-server/channels-routes.test.ts b/tests/unit/web-server/channels-routes.test.ts
index f414f742..5afc77ad 100644
--- a/tests/unit/web-server/channels-routes.test.ts
+++ b/tests/unit/web-server/channels-routes.test.ts
@@ -194,4 +194,16 @@ describe('web-server channels-routes', () => {
       else delete process.env.DISCORD_BOT_TOKEN;
     }
   });
+
+  it('rejects oversized token writes with a 400 response', async () => {
+    const response = await putJson(baseUrl, '/api/channels/discord/token', {
+      token: 'x'.repeat(4097),
+    });
+
+    expect(response.status).toBe(400);
+    expect(await response.json()).toEqual({
+      error: 'DISCORD_BOT_TOKEN cannot exceed 4096 characters.',
+    });
+    expect(fs.existsSync(path.join(tempHome, '.claude', 'channels', 'discord', '.env'))).toBe(false);
+  });
 });