diff --git a/src/index.ts b/src/index.ts
index 4c6e2254..0b7b4b43 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,4 +1,4 @@
-import { readFileSync, writeFileSync, existsSync } from 'node:fs';
+import { readFileSync } from 'node:fs';
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
 import { Command } from 'commander';
@@ -10,6 +10,7 @@ import {
   saveConfig,
   isSetupComplete,
   getMercuryHome,
+  appendToMercuryEnv,
   ensureCreatorField,
   clearTelegramAccess,
   isProviderConfigured,
@@ -539,18 +540,6 @@ async function promptValidatedValue(
   }
 }
 
-function appendToEnv(key: string, value: string): void {
-  const envPath = join(getMercuryHome(), '.env');
-  let envContent = '';
-  if (existsSync(envPath)) {
-    envContent = readFileSync(envPath, 'utf-8');
-  }
-  const lines = envContent.split('\n').filter((l: string) => !l.startsWith(`${key}=`) && l.trim() !== '');
-  lines.push(`${key}=${value}`);
-  writeFileSync(envPath, lines.join('\n') + '\n', 'utf-8');
-  process.env[key] = value;
-}
-
 function parseGithubRepo(input: string): { owner: string; repo: string } | null {
   const trimmed = input.trim().replace(/\/+$/, '');
   const urlMatch = trimmed.match(/github\.com\/([^/]+)\/([^/]+)/);
@@ -1078,7 +1067,7 @@ async function configure(existingConfig?: MercuryConfig): Promise<void> {
     const ghTokenCurrent = process.env.GITHUB_TOKEN ? ` [${maskKey(process.env.GITHUB_TOKEN)}]` : '';
     const ghToken = await ask(chalk.white(`  2. GitHub PAT${ghTokenCurrent}: `));
     if (ghToken) {
-      appendToEnv('GITHUB_TOKEN', ghToken);
+      appendToMercuryEnv('GITHUB_TOKEN', ghToken);
     }
 
     if (config.github.username || process.env.GITHUB_TOKEN) {
@@ -1122,14 +1111,14 @@ async function configure(existingConfig?: MercuryConfig): Promise<void> {
     const spotifyClientId = await ask(chalk.white(`  1. Spotify Client ID${spotifyIdCurrent}: `));
     if (spotifyClientId) {
       config.spotify.clientId = spotifyClientId;
-      appendToEnv('SPOTIFY_CLIENT_ID', spotifyClientId);
+      appendToMercuryEnv('SPOTIFY_CLIENT_ID', spotifyClientId);
     }
 
     const spotifySecretCurrent = isReconfig && config.spotify.clientSecret ? ` [${maskKey(config.spotify.clientSecret)}]` : '';
     const spotifyClientSecret = await ask(chalk.white(`  2. Spotify Client Secret${spotifySecretCurrent}: `));
     if (spotifyClientSecret) {
       config.spotify.clientSecret = spotifyClientSecret;
-      appendToEnv('SPOTIFY_CLIENT_SECRET', spotifyClientSecret);
+      appendToMercuryEnv('SPOTIFY_CLIENT_SECRET', spotifyClientSecret);
     }
 
     if (spotifyClientId || spotifyClientSecret) {
diff --git a/src/utils/config.test.ts b/src/utils/config.test.ts
new file mode 100644
index 00000000..b0a6e19e
--- /dev/null
+++ b/src/utils/config.test.ts
@@ -0,0 +1,47 @@
+import { existsSync, mkdtempSync, readFileSync, rmSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+let mercuryHome = '';
+
+async function loadConfigModule() {
+  vi.resetModules();
+  vi.stubEnv('MERCURY_HOME', mercuryHome);
+  return import('./config.js');
+}
+
+describe('appendToMercuryEnv', () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    if (mercuryHome) {
+      rmSync(mercuryHome, { recursive: true, force: true });
+      mercuryHome = '';
+    }
+  });
+
+  it('creates the Mercury home directory and env file on first write', async () => {
+    mercuryHome = join(mkdtempSync(join(tmpdir(), 'mercury-config-test-')), 'nested-home');
+    const { appendToMercuryEnv } = await loadConfigModule();
+
+    appendToMercuryEnv('GITHUB_TOKEN', 'ghp_test_token');
+
+    const envPath = join(mercuryHome, '.env');
+    expect(existsSync(envPath)).toBe(true);
+    expect(readFileSync(envPath, 'utf-8')).toBe('GITHUB_TOKEN=ghp_test_token\n');
+    expect(process.env.GITHUB_TOKEN).toBe('ghp_test_token');
+  });
+
+  it('replaces an existing key without duplicating unrelated env entries', async () => {
+    mercuryHome = mkdtempSync(join(tmpdir(), 'mercury-config-test-'));
+    const { appendToMercuryEnv } = await loadConfigModule();
+
+    appendToMercuryEnv('GITHUB_TOKEN', 'ghp_old_token');
+    appendToMercuryEnv('SPOTIFY_CLIENT_ID', 'spotify-client');
+    appendToMercuryEnv('GITHUB_TOKEN', 'ghp_new_token');
+
+    expect(readFileSync(join(mercuryHome, '.env'), 'utf-8')).toBe(
+      'SPOTIFY_CLIENT_ID=spotify-client\nGITHUB_TOKEN=ghp_new_token\n',
+    );
+  });
+});
diff --git a/src/utils/config.ts b/src/utils/config.ts
index e78b8e55..e71bad41 100644
--- a/src/utils/config.ts
+++ b/src/utils/config.ts
@@ -16,6 +16,27 @@ export function getMercuryHome(): string {
   return process.env.MERCURY_HOME || MERCURY_HOME;
 }
 
+export function appendToMercuryEnv(key: string, value: string): void {
+  const dir = getMercuryHome();
+  if (!existsSync(dir)) {
+    mkdirSync(dir, { recursive: true });
+  }
+
+  const envPath = join(dir, '.env');
+  let envContent = '';
+  if (existsSync(envPath)) {
+    envContent = readFileSync(envPath, 'utf-8');
+  }
+
+  const lines = envContent
+    .split('\n')
+    .filter((line) => !line.startsWith(`${key}=`) && line.trim() !== '');
+
+  lines.push(`${key}=${value}`);
+  process.env[key] = value;
+  writeFileSync(envPath, lines.join('\n') + '\n', 'utf-8');
+}
+
 export function getMemoryDir(): string {
   return join(getMercuryHome(), 'memory');
 }