diff --git a/scripts/postinstall.js b/scripts/postinstall.js
index a2466ff9..c761c920 100755
--- a/scripts/postinstall.js
+++ b/scripts/postinstall.js
@@ -125,12 +125,14 @@ function createConfigFiles() {
     // runs `ccs gemini` or `ccs codex` for first time (requires OAuth auth first)
     const configPath = path.join(ccsDir, 'config.json');
     if (!fs.existsSync(configPath)) {
+      // NOTE: No 'default' entry - when no profile specified, CCS passes through
+      // to Claude's native auth without --settings flag. This prevents env var
+      // pollution from affecting the default profile.
       const config = {
         profiles: {
           glm: '~/.ccs/glm.settings.json',
           glmt: '~/.ccs/glmt.settings.json',
-          kimi: '~/.ccs/kimi.settings.json',
-          default: '~/.claude/settings.json'
+          kimi: '~/.ccs/kimi.settings.json'
         }
       };
 
@@ -141,23 +143,38 @@ function createConfigFiles() {
 
       console.log('[OK] Created config: ~/.ccs/config.json');
     } else {
-      // Update existing config with glmt if missing (migration for v3.x users)
+      // Update existing config (migration for older versions)
       const config = JSON.parse(fs.readFileSync(configPath, 'utf8'));
       // Ensure profiles object exists
       if (!config.profiles) {
         config.profiles = {};
       }
       let configUpdated = false;
+
+      // Migration: Add glmt if missing (v3.x)
       if (!config.profiles.glmt) {
         config.profiles.glmt = '~/.ccs/glmt.settings.json';
         configUpdated = true;
       }
+
+      // Migration: Remove 'default' entry pointing to ~/.claude/settings.json (v5.4.0)
+      // This entry caused the default profile to pass --settings flag, which could
+      // pick up stale env vars (ANTHROPIC_BASE_URL) from previous profile sessions.
+      // Fix: Let CCS pass through to Claude's native auth without --settings flag.
+      if (config.profiles.default === '~/.claude/settings.json') {
+        delete config.profiles.default;
+        configUpdated = true;
+        console.log('[OK] Removed legacy default profile (now uses native Claude auth)');
+      }
+
       // NOTE: gemini/codex profiles added on-demand, not during migration
       if (configUpdated) {
         const tmpPath = `${configPath}.tmp`;
         fs.writeFileSync(tmpPath, JSON.stringify(config, null, 2) + '\n', 'utf8');
         fs.renameSync(tmpPath, configPath);
-        console.log('[OK] Updated config with glmt profile');
+        if (!config.profiles.glmt) {
+          console.log('[OK] Updated config with glmt profile');
+        }
       } else {
         console.log('[OK] Config exists: ~/.ccs/config.json (preserved)');
       }
diff --git a/src/auth/profile-detector.ts b/src/auth/profile-detector.ts
index 2886090d..eab38008 100644
--- a/src/auth/profile-detector.ts
+++ b/src/auth/profile-detector.ts
@@ -178,10 +178,21 @@ class ProfileDetector {
     const config = this.readConfig();
 
     if (config.profiles && config.profiles['default']) {
+      const settingsPath = config.profiles['default'];
+      // Safety net: If default points to ~/.claude/settings.json, treat as pass-through
+      // to avoid loading stale env vars from previous profile sessions (issue #37).
+      // The ~/.claude/settings.json is Claude's native config - let Claude handle it.
+      if (settingsPath.includes('.claude') && settingsPath.endsWith('settings.json')) {
+        return {
+          type: 'default',
+          name: 'default',
+          message: 'Using native Claude auth (no custom env vars)',
+        };
+      }
       return {
         type: 'settings',
         name: 'default',
-        settingsPath: config.profiles['default'],
+        settingsPath,
       };
     }