diff --git a/src/auth/profile-detector.ts b/src/auth/profile-detector.ts
index d2c63087..2c402d18 100644
--- a/src/auth/profile-detector.ts
+++ b/src/auth/profile-detector.ts
@@ -130,10 +130,27 @@ class ProfileDetector {
       // Handle composite variants
       if ('type' in variant && variant.type === 'composite') {
         const composite = variant as CompositeVariantConfig;
+
+        // Defensive: check for missing tiers or default_tier
+        if (!composite.tiers || !composite.default_tier) {
+          console.warn(
+            `[!] Warning: Composite variant '${profileName}' has missing tiers or default_tier`
+          );
+          return null;
+        }
+
+        const defaultTierConfig = composite.tiers[composite.default_tier];
+        if (!defaultTierConfig) {
+          console.warn(
+            `[!] Warning: Composite variant '${profileName}' missing config for default tier '${composite.default_tier}'`
+          );
+          return null;
+        }
+
         return {
           type: 'cliproxy',
           name: profileName,
-          provider: composite.tiers[composite.default_tier].provider as CLIProxyProfileName,
+          provider: defaultTierConfig.provider as CLIProxyProfileName,
           settingsPath: composite.settings,
           port: composite.port,
           isComposite: true,
diff --git a/src/cliproxy/config/env-builder.ts b/src/cliproxy/config/env-builder.ts
index 38b7cc28..f3d5dfae 100644
--- a/src/cliproxy/config/env-builder.ts
+++ b/src/cliproxy/config/env-builder.ts
@@ -461,17 +461,31 @@ export function getCompositeEnvVars(
   }
 
   const validPort = validatePort(port);
-  const defaultModel = tiers[defaultTier].model;
 
-  return {
+  // Defensive: handle missing tiers gracefully
+  const opusModel = tiers.opus?.model;
+  const sonnetModel = tiers.sonnet?.model;
+  const haikuModel = tiers.haiku?.model;
+  const defaultModel = tiers[defaultTier]?.model;
+
+  // If default tier is missing, we cannot proceed meaningfully
+  if (!defaultModel) {
+    throw new Error(`Missing model for default tier '${defaultTier}'`);
+  }
+
+  const env: Record<string, string> = {
     ...globalEnv,
     ...additionalEnvVars,
     // Root URL — CLIProxyAPI routes based on model name in request body
     ANTHROPIC_BASE_URL: `http://127.0.0.1:${validPort}`,
     ANTHROPIC_AUTH_TOKEN: getEffectiveApiKey(),
     ANTHROPIC_MODEL: defaultModel,
-    ANTHROPIC_DEFAULT_OPUS_MODEL: tiers.opus.model,
-    ANTHROPIC_DEFAULT_SONNET_MODEL: tiers.sonnet.model,
-    ANTHROPIC_DEFAULT_HAIKU_MODEL: tiers.haiku.model,
   };
+
+  // Only set tier env vars if the tier exists
+  if (opusModel) env.ANTHROPIC_DEFAULT_OPUS_MODEL = opusModel;
+  if (sonnetModel) env.ANTHROPIC_DEFAULT_SONNET_MODEL = sonnetModel;
+  if (haikuModel) env.ANTHROPIC_DEFAULT_HAIKU_MODEL = haikuModel;
+
+  return env;
 }
diff --git a/src/cliproxy/executor/env-resolver.ts b/src/cliproxy/executor/env-resolver.ts
index 1af44fa7..3d0a449f 100644
--- a/src/cliproxy/executor/env-resolver.ts
+++ b/src/cliproxy/executor/env-resolver.ts
@@ -136,9 +136,9 @@ export function buildClaudeEnvironment(config: ProxyChainConfig): Record<string,
   let compositeTierThinking: { opus?: string; sonnet?: string; haiku?: string } | undefined;
   if (isComposite && compositeTiers) {
     const tierThinking: { opus?: string; sonnet?: string; haiku?: string } = {};
-    if (compositeTiers.opus.thinking) tierThinking.opus = compositeTiers.opus.thinking;
-    if (compositeTiers.sonnet.thinking) tierThinking.sonnet = compositeTiers.sonnet.thinking;
-    if (compositeTiers.haiku.thinking) tierThinking.haiku = compositeTiers.haiku.thinking;
+    if (compositeTiers.opus?.thinking) tierThinking.opus = compositeTiers.opus.thinking;
+    if (compositeTiers.sonnet?.thinking) tierThinking.sonnet = compositeTiers.sonnet.thinking;
+    if (compositeTiers.haiku?.thinking) tierThinking.haiku = compositeTiers.haiku.thinking;
     if (Object.keys(tierThinking).length > 0) {
       compositeTierThinking = tierThinking;
     }