diff --git a/src/cliproxy/executor/env-resolver.ts b/src/cliproxy/executor/env-resolver.ts
index 3d0a449f..40daf8f5 100644
--- a/src/cliproxy/executor/env-resolver.ts
+++ b/src/cliproxy/executor/env-resolver.ts
@@ -81,55 +81,74 @@ export function buildClaudeEnvironment(config: ProxyChainConfig): Record<string,
     compositeDefaultTier,
   } = config;
 
-  // Build base env vars - composite, remote, or local
+  // Build base env vars - check remote mode first
   let envVars: NodeJS.ProcessEnv;
 
-  if (isComposite && compositeTiers && compositeDefaultTier) {
-    // Composite variant: root URL, per-tier models from different providers
-    envVars = getCompositeEnvVars(
-      compositeTiers,
-      compositeDefaultTier,
-      localPort,
-      customSettingsPath
-    );
-  } else if (useRemoteProxy && remoteConfig) {
+  if (useRemoteProxy && remoteConfig) {
     if (httpsTunnel && tunnelPort) {
-      // HTTPS remote via local tunnel - use HTTP to tunnel
-      envVars = getRemoteEnvVars(
-        provider,
-        {
-          host: '127.0.0.1',
-          port: tunnelPort,
-          protocol: 'http', // Tunnel speaks HTTP locally
-          authToken: remoteConfig.authToken,
-        },
-        customSettingsPath
-      );
+      // HTTPS remote via local tunnel
+      if (isComposite && compositeTiers && compositeDefaultTier) {
+        envVars = getCompositeEnvVars(
+          compositeTiers,
+          compositeDefaultTier,
+          tunnelPort,
+          customSettingsPath
+        );
+      } else {
+        envVars = getRemoteEnvVars(
+          provider,
+          {
+            host: '127.0.0.1',
+            port: tunnelPort,
+            protocol: 'http', // Tunnel speaks HTTP locally
+            authToken: remoteConfig.authToken,
+          },
+          customSettingsPath
+        );
+      }
     } else {
       // HTTP remote - direct connection
-      envVars = getRemoteEnvVars(
-        provider,
-        {
-          host: remoteConfig.host,
-          port: remoteConfig.port,
-          protocol: remoteConfig.protocol,
-          authToken: remoteConfig.authToken,
-        },
-        customSettingsPath
-      );
+      if (isComposite && compositeTiers && compositeDefaultTier) {
+        envVars = getCompositeEnvVars(
+          compositeTiers,
+          compositeDefaultTier,
+          remoteConfig.port,
+          customSettingsPath
+        );
+      } else {
+        envVars = getRemoteEnvVars(
+          provider,
+          {
+            host: remoteConfig.host,
+            port: remoteConfig.port,
+            protocol: remoteConfig.protocol,
+            authToken: remoteConfig.authToken,
+          },
+          customSettingsPath
+        );
+      }
     }
   } else {
     // Local proxy mode
-    const remoteRewriteConfig = remoteConfig
-      ? {
-          host: remoteConfig.host,
-          port: remoteConfig.port,
-          protocol: remoteConfig.protocol,
-          authToken: remoteConfig.authToken,
-        }
-      : undefined;
+    if (isComposite && compositeTiers && compositeDefaultTier) {
+      envVars = getCompositeEnvVars(
+        compositeTiers,
+        compositeDefaultTier,
+        localPort,
+        customSettingsPath
+      );
+    } else {
+      const remoteRewriteConfig = remoteConfig
+        ? {
+            host: remoteConfig.host,
+            port: remoteConfig.port,
+            protocol: remoteConfig.protocol,
+            authToken: remoteConfig.authToken,
+          }
+        : undefined;
 
-    envVars = getEffectiveEnvVars(provider, localPort, customSettingsPath, remoteRewriteConfig);
+      envVars = getEffectiveEnvVars(provider, localPort, customSettingsPath, remoteRewriteConfig);
+    }
   }
 
   // Extract per-tier thinking from composite config
diff --git a/src/cliproxy/services/variant-service.ts b/src/cliproxy/services/variant-service.ts
index 0ab7c1c8..76ce8814 100644
--- a/src/cliproxy/services/variant-service.ts
+++ b/src/cliproxy/services/variant-service.ts
@@ -380,11 +380,11 @@ export function updateCompositeVariant(
       return { success: false, error: `Variant '${name}' is not a composite variant` };
     }
 
-    // Merge tiers (keep unchanged tiers from existing)
+    // Deep merge tiers to preserve optional fields (fallback, thinking, account)
     const mergedTiers = {
-      opus: updates.tiers?.opus ?? existing.tiers.opus,
-      sonnet: updates.tiers?.sonnet ?? existing.tiers.sonnet,
-      haiku: updates.tiers?.haiku ?? existing.tiers.haiku,
+      opus: { ...existing.tiers.opus, ...updates.tiers?.opus },
+      sonnet: { ...existing.tiers.sonnet, ...updates.tiers?.sonnet },
+      haiku: { ...existing.tiers.haiku, ...updates.tiers?.haiku },
     };
 
     // Validate all tier providers against backend compatibility
diff --git a/src/web-server/routes/variant-routes.ts b/src/web-server/routes/variant-routes.ts
index 4faad646..df55bfd6 100644
--- a/src/web-server/routes/variant-routes.ts
+++ b/src/web-server/routes/variant-routes.ts
@@ -31,13 +31,12 @@ function validateCompositeTiers(
   if (defaultTier && !VALID_TIERS.includes(defaultTier as (typeof VALID_TIERS)[number])) {
     return `Invalid default_tier '${defaultTier}': must be one of ${VALID_TIERS.join(', ')}`;
   }
-  // Validate each tier
+  // Validate each tier (partial updates allowed - only validate tiers present in request)
   for (const tier of VALID_TIERS) {
-    if (
-      !tiers[tier] ||
-      typeof tiers[tier].provider !== 'string' ||
-      typeof tiers[tier].model !== 'string'
-    ) {
+    // Skip validation for tiers not present in the request (partial updates allowed)
+    if (tiers[tier] === undefined) continue;
+
+    if (typeof tiers[tier].provider !== 'string' || typeof tiers[tier].model !== 'string') {
       return `Invalid tier config for '${tier}': requires 'provider' and 'model' strings`;
     }
     // Validate non-empty model string (whitespace-only is also invalid)