diff --git a/src/cliproxy/composite-validator.ts b/src/cliproxy/composite-validator.ts
new file mode 100644
index 00000000..981b58e0
--- /dev/null
+++ b/src/cliproxy/composite-validator.ts
@@ -0,0 +1,118 @@
+/**
+ * Shared validation helpers for composite CLIProxy variants.
+ * Used by API routes, service layer, and config loader to avoid contract drift.
+ */
+
+import { CLIPROXY_SUPPORTED_PROVIDERS, CompositeTierConfig } from '../config/unified-config-types';
+import type { CLIProxyProvider } from './types';
+
+export const VALID_COMPOSITE_TIERS = ['opus', 'sonnet', 'haiku'] as const;
+export type CompositeTierName = (typeof VALID_COMPOSITE_TIERS)[number];
+
+interface CompositeValidationOptions {
+  defaultTier?: unknown;
+  requireAllTiers?: boolean;
+}
+
+type CompositeTierInput = Partial<Record<CompositeTierName, CompositeTierConfig>>;
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value);
+}
+
+function isValidProvider(provider: unknown): provider is CLIProxyProvider {
+  return (
+    typeof provider === 'string' &&
+    CLIPROXY_SUPPORTED_PROVIDERS.includes(provider as CLIProxyProvider)
+  );
+}
+
+export function validateCompositeDefaultTier(defaultTier: unknown): string | null {
+  if (
+    defaultTier !== undefined &&
+    !VALID_COMPOSITE_TIERS.includes(defaultTier as CompositeTierName)
+  ) {
+    return `Invalid default_tier '${String(defaultTier)}': must be one of ${VALID_COMPOSITE_TIERS.join(', ')}`;
+  }
+  return null;
+}
+
+/**
+ * Validate composite tier payload.
+ *
+ * Create mode (`requireAllTiers=true`): all tiers required.
+ * Update mode (`requireAllTiers=false`): partial tiers allowed.
+ */
+export function validateCompositeTiers(
+  tiers: unknown,
+  options: CompositeValidationOptions = {}
+): string | null {
+  const { defaultTier, requireAllTiers = false } = options;
+
+  const defaultTierError = validateCompositeDefaultTier(defaultTier);
+  if (defaultTierError) {
+    return defaultTierError;
+  }
+
+  if (!isRecord(tiers)) {
+    return "Invalid tiers payload: expected object with tier keys ('opus', 'sonnet', 'haiku')";
+  }
+
+  const tierMap = tiers as CompositeTierInput;
+
+  for (const tier of VALID_COMPOSITE_TIERS) {
+    const tierValue = tierMap[tier];
+
+    if (requireAllTiers && tierValue === undefined) {
+      return `Missing required tier '${tier}': all tiers (opus, sonnet, haiku) required for create`;
+    }
+
+    if (tierValue === undefined) {
+      continue;
+    }
+
+    if (!isRecord(tierValue)) {
+      return `Invalid tier config for '${tier}': expected object with provider and model`;
+    }
+
+    const provider = tierValue.provider;
+    const model = tierValue.model;
+
+    if (typeof provider !== 'string' || typeof model !== 'string') {
+      return `Invalid tier config for '${tier}': requires 'provider' and 'model' strings`;
+    }
+
+    if (!model.trim()) {
+      return `Invalid model for tier '${tier}': model cannot be empty or whitespace`;
+    }
+
+    if (!isValidProvider(provider)) {
+      return `Invalid provider '${provider}' for tier '${tier}': must be one of ${CLIPROXY_SUPPORTED_PROVIDERS.join(', ')}`;
+    }
+
+    if (tierValue.fallback !== undefined) {
+      const fallback = tierValue.fallback;
+      if (!isRecord(fallback)) {
+        return `Invalid fallback config for tier '${tier}': expected object with provider and model`;
+      }
+
+      if (typeof fallback.provider !== 'string' || typeof fallback.model !== 'string') {
+        return `Invalid fallback config for tier '${tier}': requires 'provider' and 'model' strings`;
+      }
+
+      if (!fallback.model.trim()) {
+        return `Invalid fallback model for tier '${tier}': model cannot be empty or whitespace`;
+      }
+
+      if (!isValidProvider(fallback.provider)) {
+        return `Invalid fallback provider '${fallback.provider}' for tier '${tier}': must be one of ${CLIPROXY_SUPPORTED_PROVIDERS.join(', ')}`;
+      }
+
+      if (fallback.provider === provider && fallback.model === model) {
+        return `Circular fallback in tier '${tier}': fallback cannot point to same provider and model`;
+      }
+    }
+  }
+
+  return null;
+}
diff --git a/src/cliproxy/config/thinking-config.ts b/src/cliproxy/config/thinking-config.ts
index cda3a7ee..69ec1762 100644
--- a/src/cliproxy/config/thinking-config.ts
+++ b/src/cliproxy/config/thinking-config.ts
@@ -108,6 +108,16 @@ export function applyThinkingConfig(
     return result;
   }
 
+  // Explicit "off" (CLI override or manual config override) must disable ALL tier thinking.
+  const explicitOffOverride =
+    thinkingOverride === 'off' ||
+    (thinkingOverride === undefined &&
+      thinkingConfig.mode === 'manual' &&
+      thinkingConfig.override === 'off');
+  if (explicitOffOverride) {
+    return result;
+  }
+
   // Get base model to check thinking support
   const baseModel = result.ANTHROPIC_MODEL || '';
   if (!supportsThinking(provider, baseModel)) {
@@ -152,10 +162,8 @@ export function applyThinkingConfig(
   }
   thinkingValue = validation.value;
 
-  // P1 FIX: If validation says 'off' AND no per-tier thinking config, skip ALL processing
-  // This distinguishes between:
-  // 1. "off" with no per-tier config → no thinking anywhere
-  // 2. "off" with per-tier config → skip main model, process tiers with their own values
+  // If auto-detection resolves default tier to "off", skip the main model but still allow
+  // explicit per-tier thinking values for other tiers.
   if (thinkingValue === 'off') {
     const hasPerTierThinking =
       compositeTierThinking &&
diff --git a/src/cliproxy/executor/index.ts b/src/cliproxy/executor/index.ts
index 26abbf81..050a8d6b 100644
--- a/src/cliproxy/executor/index.ts
+++ b/src/cliproxy/executor/index.ts
@@ -840,6 +840,18 @@ export async function execClaudeWithCLIProxy(
     compositeDefaultTier: cfg.compositeDefaultTier,
   });
 
+  if (cfg.isComposite && cfg.compositeTiers && cfg.compositeDefaultTier) {
+    const mode = useRemoteProxy
+      ? proxyConfig.protocol === 'https'
+        ? 'remote-https'
+        : 'remote-http'
+      : 'local';
+    const defaultTierProvider = cfg.compositeTiers[cfg.compositeDefaultTier]?.provider ?? provider;
+    log(
+      `Composite self-check: mode=${mode}, baseUrl=${env.ANTHROPIC_BASE_URL || 'unset'}, defaultTier=${cfg.compositeDefaultTier}, defaultProvider=${defaultTierProvider}`
+    );
+  }
+
   const webSearchEnv = getWebSearchHookEnv();
   logEnvironment(env, webSearchEnv, verbose);
 
diff --git a/src/cliproxy/services/variant-service.ts b/src/cliproxy/services/variant-service.ts
index 76ce8814..b9048d86 100644
--- a/src/cliproxy/services/variant-service.ts
+++ b/src/cliproxy/services/variant-service.ts
@@ -17,6 +17,7 @@ import { isUnifiedMode } from '../../config/unified-config-loader';
 import { deleteConfigForPort } from '../config-generator';
 import { hasActiveSessions, deleteSessionLockForPort } from '../session-tracker';
 import { warn } from '../../utils/ui';
+import { validateCompositeTiers } from '../composite-validator';
 import {
   createSettingsFile,
   createSettingsFileUnified,
@@ -309,6 +310,14 @@ export function createCompositeVariant(
   try {
     const { name, defaultTier, tiers } = options;
 
+    const validationError = validateCompositeTiers(tiers, {
+      defaultTier,
+      requireAllTiers: true,
+    });
+    if (validationError) {
+      return { success: false, error: validationError };
+    }
+
     // Validate all tier providers against backend compatibility
     const tierNames: Array<'opus' | 'sonnet' | 'haiku'> = ['opus', 'sonnet', 'haiku'];
     for (const tier of tierNames) {
@@ -387,6 +396,15 @@ export function updateCompositeVariant(
       haiku: { ...existing.tiers.haiku, ...updates.tiers?.haiku },
     };
 
+    const newDefaultTier = updates.defaultTier ?? existing.default_tier ?? 'sonnet';
+    const validationError = validateCompositeTiers(mergedTiers, {
+      defaultTier: newDefaultTier,
+      requireAllTiers: true,
+    });
+    if (validationError) {
+      return { success: false, error: validationError };
+    }
+
     // Validate all tier providers against backend compatibility
     const tierNames: Array<'opus' | 'sonnet' | 'haiku'> = ['opus', 'sonnet', 'haiku'];
     for (const tier of tierNames) {
@@ -396,13 +414,6 @@ export function updateCompositeVariant(
       }
     }
 
-    const newDefaultTier = updates.defaultTier ?? existing.default_tier ?? 'sonnet';
-
-    // Delete old settings file
-    if (existing.settings) {
-      deleteSettingsFile(existing.settings);
-    }
-
     // Create new settings file with updated config
     const settingsPath = createCompositeSettingsFile(
       name,
diff --git a/src/config/unified-config-loader.ts b/src/config/unified-config-loader.ts
index 0824597d..773259c4 100644
--- a/src/config/unified-config-loader.ts
+++ b/src/config/unified-config-loader.ts
@@ -26,9 +26,9 @@ import {
   ThinkingConfig,
   DashboardAuthConfig,
   ImageAnalysisConfig,
-  CLIPROXY_SUPPORTED_PROVIDERS,
   CursorConfig,
 } from './unified-config-types';
+import { validateCompositeTiers } from '../cliproxy/composite-validator';
 import { isUnifiedConfigEnabled } from './feature-flags';
 
 const CONFIG_YAML = 'config.yaml';
@@ -212,32 +212,17 @@ export function loadUnifiedConfig(): UnifiedConfig | null {
  * Warns about invalid providers in composite variant configurations.
  */
 function validateCompositeVariants(config: UnifiedConfig): void {
-  const validProviders = new Set<string>(CLIPROXY_SUPPORTED_PROVIDERS);
   const variants = config.cliproxy?.variants;
   if (!variants) return;
 
   for (const [name, variant] of Object.entries(variants)) {
     if ('type' in variant && variant.type === 'composite') {
-      // Guard against malformed composite variants
-      if (!variant.tiers || typeof variant.tiers !== 'object') {
-        console.warn(`[!] Composite variant '${name}' missing tiers object, skipping validation`);
-        continue;
-      }
-
-      for (const [tier, tierConfig] of Object.entries(variant.tiers as Record<string, unknown>)) {
-        if (!tierConfig || typeof tierConfig !== 'object') {
-          console.warn(
-            `[!] Variant '${name}': invalid config in ${tier} tier (expected object, got ${tierConfig === null ? 'null' : typeof tierConfig})`
-          );
-          continue;
-        }
-
-        const provider = (tierConfig as { provider?: unknown }).provider;
-        if (typeof provider !== 'string' || !validProviders.has(provider)) {
-          console.warn(
-            `[!] Variant '${name}': invalid provider '${String(provider)}' in ${tier} tier`
-          );
-        }
+      const error = validateCompositeTiers(variant.tiers, {
+        defaultTier: variant.default_tier,
+        requireAllTiers: true,
+      });
+      if (error) {
+        console.warn(`[!] Variant '${name}': invalid composite config (${error})`);
       }
     }
   }
diff --git a/src/web-server/routes/variant-routes.ts b/src/web-server/routes/variant-routes.ts
index 209e0dac..14aa4039 100644
--- a/src/web-server/routes/variant-routes.ts
+++ b/src/web-server/routes/variant-routes.ts
@@ -7,7 +7,6 @@
 import { Router, Request, Response } from 'express';
 import { isReservedName, RESERVED_PROFILE_NAMES } from '../../config/reserved-names';
 import type { CLIProxyProvider } from '../../cliproxy/types';
-import { CLIPROXY_SUPPORTED_PROVIDERS } from '../../config/unified-config-types';
 import {
   createVariant,
   removeVariant,
@@ -17,64 +16,13 @@ import {
   createCompositeVariant,
   updateCompositeVariant,
 } from '../../cliproxy/services/variant-service';
+import {
+  validateCompositeDefaultTier,
+  validateCompositeTiers,
+} from '../../cliproxy/composite-validator';
 
 const router = Router();
 
-const VALID_TIERS = ['opus', 'sonnet', 'haiku'] as const;
-
-/** Validate composite tiers shape and provider/default_tier values. Returns error string or null. */
-function validateCompositeTiers(
-  tiers: Record<string, { provider?: string; model?: string }>,
-  defaultTier?: string,
-  requireAllTiers = false
-): string | null {
-  // Validate default_tier
-  if (defaultTier && !VALID_TIERS.includes(defaultTier as (typeof VALID_TIERS)[number])) {
-    return `Invalid default_tier '${defaultTier}': must be one of ${VALID_TIERS.join(', ')}`;
-  }
-  // Validate each tier
-  for (const tier of VALID_TIERS) {
-    const tierValue = tiers[tier];
-
-    // For POST/create: all tiers required
-    if (requireAllTiers && tierValue === undefined) {
-      return `Missing required tier '${tier}': all tiers (opus, sonnet, haiku) required for create`;
-    }
-
-    // Skip validation for tiers not present in the request (PUT partial updates)
-    if (tierValue === undefined) continue;
-
-    // Guard against null tier values
-    if (tierValue === null || typeof tierValue !== 'object') {
-      return `Invalid tier config for '${tier}': expected object with provider and model`;
-    }
-
-    if (typeof tierValue.provider !== 'string' || typeof tierValue.model !== 'string') {
-      return `Invalid tier config for '${tier}': requires 'provider' and 'model' strings`;
-    }
-    // Validate non-empty model string (whitespace-only is also invalid)
-    if (!tiers[tier].model?.trim()) {
-      return `Invalid model for tier '${tier}': model cannot be empty or whitespace`;
-    }
-    if (!CLIPROXY_SUPPORTED_PROVIDERS.includes(tiers[tier].provider as CLIProxyProvider)) {
-      return `Invalid provider '${tiers[tier].provider}' for tier '${tier}': must be one of ${CLIPROXY_SUPPORTED_PROVIDERS.join(', ')}`;
-    }
-    // Check for circular fallback (fallback points to same provider+model)
-    const tierConfig = tiers[tier] as {
-      provider: string;
-      model: string;
-      fallback?: { provider?: string; model?: string };
-    };
-    if (tierConfig.fallback) {
-      const fb = tierConfig.fallback;
-      if (fb.provider === tierConfig.provider && fb.model === tierConfig.model) {
-        return `Circular fallback in tier '${tier}': fallback cannot point to same provider and model`;
-      }
-    }
-  }
-  return null;
-}
-
 /**
  * GET /api/cliproxy - List cliproxy variants
  * Uses variant-service for consistent behavior with CLI
@@ -132,7 +80,10 @@ router.post('/', (req: Request, res: Response): void => {
     }
 
     // Validate tiers shape, providers, and default_tier (all tiers required for create)
-    const tierError = validateCompositeTiers(tiers, default_tier, true);
+    const tierError = validateCompositeTiers(tiers, {
+      defaultTier: default_tier,
+      requireAllTiers: true,
+    });
     if (tierError) {
       res.status(400).json({ error: tierError });
       return;
@@ -221,26 +172,30 @@ router.put('/:name', (req: Request, res: Response): void => {
 
       // Validate tiers shape, providers, and default_tier if provided
       if (tiers) {
-        const tierError = validateCompositeTiers(tiers, default_tier);
+        const tierError = validateCompositeTiers(tiers, {
+          defaultTier: default_tier,
+        });
         if (tierError) {
           res.status(400).json({ error: tierError });
           return;
         }
-      } else if (
-        default_tier &&
-        !VALID_TIERS.includes(default_tier as (typeof VALID_TIERS)[number])
-      ) {
-        res.status(400).json({
-          error: `Invalid default_tier '${default_tier}': must be one of ${VALID_TIERS.join(', ')}`,
-        });
-        return;
+      } else {
+        const defaultTierError = validateCompositeDefaultTier(default_tier);
+        if (defaultTierError) {
+          res.status(400).json({
+            error: defaultTierError,
+          });
+          return;
+        }
       }
 
       const result = updateCompositeVariant(name, { defaultTier: default_tier, tiers });
 
       if (!result.success) {
         const status = result.error?.includes('not found') ? 404 : 400;
-        res.status(status).json({ error: result.error });
+        res.status(status).json({
+          error: result.error,
+        });
         return;
       }
 
diff --git a/tests/unit/cliproxy/composite-env-routing.test.ts b/tests/unit/cliproxy/composite-env-routing.test.ts
new file mode 100644
index 00000000..64076d48
--- /dev/null
+++ b/tests/unit/cliproxy/composite-env-routing.test.ts
@@ -0,0 +1,60 @@
+/**
+ * Composite environment routing tests.
+ */
+
+import { describe, it, expect } from 'bun:test';
+import { buildClaudeEnvironment } from '../../../src/cliproxy/executor/env-resolver';
+
+const tiers = {
+  opus: { provider: 'agy' as const, model: 'claude-opus-4-6-thinking' },
+  sonnet: { provider: 'gemini' as const, model: 'gemini-2.5-pro' },
+  haiku: { provider: 'codex' as const, model: 'gpt-5.1-codex-mini' },
+};
+
+describe('buildClaudeEnvironment - composite remote routing', () => {
+  it('uses remote base URL and auth token for direct remote composite mode', () => {
+    const env = buildClaudeEnvironment({
+      provider: 'agy',
+      useRemoteProxy: true,
+      remoteConfig: {
+        host: 'remote.example.com',
+        port: 9443,
+        protocol: 'https',
+        authToken: 'remote-auth-token',
+      },
+      localPort: 8318,
+      verbose: false,
+      isComposite: true,
+      compositeTiers: tiers,
+      compositeDefaultTier: 'sonnet',
+    });
+
+    expect(env.ANTHROPIC_BASE_URL).toBe('https://remote.example.com:9443');
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('remote-auth-token');
+    expect(env.ANTHROPIC_BASE_URL).not.toContain('/api/provider/');
+  });
+
+  it('uses local tunnel endpoint for HTTPS remote composite mode', () => {
+    const env = buildClaudeEnvironment({
+      provider: 'agy',
+      useRemoteProxy: true,
+      remoteConfig: {
+        host: 'remote.example.com',
+        port: 9443,
+        protocol: 'https',
+        authToken: 'remote-auth-token',
+      },
+      httpsTunnel: {} as never,
+      tunnelPort: 9911,
+      localPort: 8318,
+      verbose: false,
+      isComposite: true,
+      compositeTiers: tiers,
+      compositeDefaultTier: 'sonnet',
+    });
+
+    expect(env.ANTHROPIC_BASE_URL).toBe('http://127.0.0.1:9911');
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('remote-auth-token');
+    expect(env.ANTHROPIC_BASE_URL).not.toContain('/api/provider/');
+  });
+});
diff --git a/tests/unit/cliproxy/composite-thinking.test.ts b/tests/unit/cliproxy/composite-thinking.test.ts
index 30d8cf5e..0112d6b9 100644
--- a/tests/unit/cliproxy/composite-thinking.test.ts
+++ b/tests/unit/cliproxy/composite-thinking.test.ts
@@ -284,6 +284,56 @@ describe('applyThinkingConfig - composite variant integration', () => {
     expect(result.ANTHROPIC_DEFAULT_HAIKU_MODEL).toBe('claude-haiku-4-5-20251001');
   });
 
+  it('should disable all tier thinking when CLI override is explicitly off', () => {
+    const envVars: NodeJS.ProcessEnv = {
+      ANTHROPIC_MODEL: 'claude-sonnet-4-5-thinking',
+      ANTHROPIC_DEFAULT_OPUS_MODEL: 'claude-opus-4-6-thinking',
+      ANTHROPIC_DEFAULT_SONNET_MODEL: 'claude-sonnet-4-5-thinking',
+      ANTHROPIC_DEFAULT_HAIKU_MODEL: 'claude-haiku-4-5-20251001',
+    };
+
+    const compositeTierThinking = {
+      opus: 'xhigh',
+      sonnet: 'medium',
+      haiku: 'low',
+    };
+
+    const result = applyThinkingConfig(
+      envVars,
+      'agy' as CLIProxyProvider,
+      'off',
+      compositeTierThinking
+    );
+
+    expect(result.ANTHROPIC_MODEL).toBe('claude-sonnet-4-5-thinking');
+    expect(result.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe('claude-opus-4-6-thinking');
+    expect(result.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe('claude-sonnet-4-5-thinking');
+    expect(result.ANTHROPIC_DEFAULT_HAIKU_MODEL).toBe('claude-haiku-4-5-20251001');
+  });
+
+  it('uses per-tier provider capability checks for mixed-provider composites', () => {
+    const envVars: NodeJS.ProcessEnv = {
+      ANTHROPIC_MODEL: 'gemini-2.5-pro',
+      ANTHROPIC_DEFAULT_OPUS_MODEL: 'gemini-2.5-pro',
+      ANTHROPIC_DEFAULT_SONNET_MODEL: 'claude-sonnet-4-5-thinking',
+      ANTHROPIC_DEFAULT_HAIKU_MODEL: 'claude-haiku-4-5-20251001',
+    };
+
+    const result = applyThinkingConfig(
+      envVars,
+      'gemini' as CLIProxyProvider,
+      undefined,
+      {
+        sonnet: 'high',
+      },
+      {
+        sonnet: { provider: 'agy' as CLIProxyProvider },
+      }
+    );
+
+    expect(result.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe('claude-sonnet-4-5-thinking(high)');
+  });
+
   it('should handle numeric budgets in per-tier thinking', () => {
     const envVars: NodeJS.ProcessEnv = {
       ANTHROPIC_MODEL: 'claude-sonnet-4-5-thinking',
diff --git a/tests/unit/cliproxy/composite-validator.test.ts b/tests/unit/cliproxy/composite-validator.test.ts
new file mode 100644
index 00000000..3701b089
--- /dev/null
+++ b/tests/unit/cliproxy/composite-validator.test.ts
@@ -0,0 +1,80 @@
+/**
+ * Unit tests for shared composite variant validation.
+ */
+
+import { describe, it, expect } from 'bun:test';
+import {
+  validateCompositeDefaultTier,
+  validateCompositeTiers,
+} from '../../../src/cliproxy/composite-validator';
+
+const validTier = {
+  provider: 'agy',
+  model: 'claude-sonnet-4-5-thinking',
+};
+
+describe('validateCompositeDefaultTier', () => {
+  it('accepts valid tier names', () => {
+    expect(validateCompositeDefaultTier('opus')).toBeNull();
+    expect(validateCompositeDefaultTier('sonnet')).toBeNull();
+    expect(validateCompositeDefaultTier('haiku')).toBeNull();
+  });
+
+  it('rejects invalid tier names', () => {
+    const error = validateCompositeDefaultTier('invalid-tier');
+    expect(error).toContain("Invalid default_tier 'invalid-tier'");
+  });
+});
+
+describe('validateCompositeTiers', () => {
+  it('rejects missing required tiers in create mode', () => {
+    const error = validateCompositeTiers(
+      {
+        opus: validTier,
+        sonnet: validTier,
+      },
+      { defaultTier: 'sonnet', requireAllTiers: true }
+    );
+
+    expect(error).toContain("Missing required tier 'haiku'");
+  });
+
+  it('rejects null tier objects', () => {
+    const error = validateCompositeTiers(
+      {
+        opus: null,
+        sonnet: validTier,
+        haiku: validTier,
+      },
+      { defaultTier: 'sonnet', requireAllTiers: true }
+    );
+
+    expect(error).toContain("Invalid tier config for 'opus'");
+  });
+
+  it('accepts partial updates in update mode', () => {
+    const error = validateCompositeTiers(
+      {
+        opus: { provider: 'gemini', model: 'gemini-2.5-pro' },
+      },
+      { defaultTier: 'sonnet' }
+    );
+
+    expect(error).toBeNull();
+  });
+
+  it('rejects circular fallback definitions', () => {
+    const error = validateCompositeTiers(
+      {
+        opus: {
+          provider: 'gemini',
+          model: 'gemini-2.5-pro',
+          fallback: { provider: 'gemini', model: 'gemini-2.5-pro' },
+        },
+      },
+      { defaultTier: 'opus' }
+    );
+
+    expect(error).toContain("Circular fallback in tier 'opus'");
+  });
+});
diff --git a/tests/unit/cliproxy/composite-variant-service.test.ts b/tests/unit/cliproxy/composite-variant-service.test.ts
index 93745c25..5f291315 100644
--- a/tests/unit/cliproxy/composite-variant-service.test.ts
+++ b/tests/unit/cliproxy/composite-variant-service.test.ts
@@ -6,7 +6,10 @@ import * as fs from 'fs';
 import * as os from 'os';
 import * as path from 'path';
 import { describe, it, expect, beforeEach, afterEach } from 'bun:test';
-import { updateCompositeVariant } from '../../../src/cliproxy/services/variant-service';
+import {
+  createCompositeVariant,
+  updateCompositeVariant,
+} from '../../../src/cliproxy/services/variant-service';
 import {
   saveCompositeVariantUnified,
   listVariantsFromConfig,
@@ -173,6 +176,51 @@ cliproxy:
     expect(result.variant?.tiers?.haiku.provider).toBe('gemini');
   });
 
+  it('should preserve optional tier fields when updating provider/model only', () => {
+    const initialConfig: CompositeVariantConfig = {
+      type: 'composite',
+      default_tier: 'sonnet',
+      tiers: {
+        opus: {
+          provider: 'agy',
+          model: 'claude-opus-4-6-thinking',
+          fallback: { provider: 'gemini', model: 'gemini-2.5-flash' },
+          thinking: 'xhigh',
+          account: 'team-a',
+        },
+        sonnet: { provider: 'agy', model: 'claude-sonnet-4-5-thinking' },
+        haiku: { provider: 'agy', model: 'claude-haiku-4-5-20251001' },
+      },
+      settings: 'cliproxy/composite-test.settings.json',
+      port: 8318,
+    };
+    saveCompositeVariantUnified('test', initialConfig);
+
+    const settingsDir = path.join(tmpDir, 'cliproxy');
+    fs.mkdirSync(settingsDir, { recursive: true });
+    fs.writeFileSync(
+      path.join(settingsDir, 'composite-test.settings.json'),
+      JSON.stringify({ env: {} }),
+      'utf-8'
+    );
+
+    const result = updateCompositeVariant('test', {
+      tiers: {
+        opus: { provider: 'gemini', model: 'gemini-2.5-pro' },
+      },
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.variant?.tiers?.opus.provider).toBe('gemini');
+    expect(result.variant?.tiers?.opus.model).toBe('gemini-2.5-pro');
+    expect(result.variant?.tiers?.opus.fallback).toEqual({
+      provider: 'gemini',
+      model: 'gemini-2.5-flash',
+    });
+    expect(result.variant?.tiers?.opus.thinking).toBe('xhigh');
+    expect(result.variant?.tiers?.opus.account).toBe('team-a');
+  });
+
   it('should return error when variant does not exist', () => {
     const result = updateCompositeVariant('nonexistent', {
       tiers: {
@@ -219,6 +267,87 @@ cliproxy:
   });
 });
 
+describe('createCompositeVariant', () => {
+  let tmpDir: string;
+  let originalCcsDir: string | undefined;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ccs-composite-test-'));
+    originalCcsDir = process.env.CCS_DIR;
+    process.env.CCS_DIR = tmpDir;
+
+    const configPath = path.join(tmpDir, 'config.yaml');
+    fs.writeFileSync(
+      configPath,
+      `version: 2
+accounts: {}
+profiles: {}
+preferences:
+  theme: system
+  telemetry: false
+  auto_update: true
+cliproxy:
+  oauth_accounts: {}
+  providers:
+    - gemini
+    - codex
+    - agy
+  variants: {}
+`,
+      'utf-8'
+    );
+  });
+
+  afterEach(() => {
+    if (originalCcsDir !== undefined) {
+      process.env.CCS_DIR = originalCcsDir;
+    } else {
+      delete process.env.CCS_DIR;
+    }
+
+    if (tmpDir && fs.existsSync(tmpDir)) {
+      fs.rmSync(tmpDir, { recursive: true, force: true });
+    }
+  });
+
+  it('returns validation error for missing required tier in create flow', () => {
+    const result = createCompositeVariant({
+      name: 'broken',
+      defaultTier: 'sonnet',
+      tiers: {
+        opus: { provider: 'gemini', model: 'gemini-2.5-pro' },
+        sonnet: { provider: 'agy', model: 'claude-sonnet-4-5-thinking' },
+      } as unknown as {
+        opus: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+        sonnet: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+        haiku: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+      },
+    });
+
+    expect(result.success).toBe(false);
+    expect(result.error).toContain("Missing required tier 'haiku'");
+  });
+
+  it('returns validation error for null tier payload in create flow', () => {
+    const result = createCompositeVariant({
+      name: 'broken-null',
+      defaultTier: 'sonnet',
+      tiers: {
+        opus: null,
+        sonnet: { provider: 'agy', model: 'claude-sonnet-4-5-thinking' },
+        haiku: { provider: 'agy', model: 'claude-haiku-4-5-20251001' },
+      } as unknown as {
+        opus: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+        sonnet: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+        haiku: { provider: 'gemini' | 'codex' | 'agy'; model: string };
+      },
+    });
+
+    expect(result.success).toBe(false);
+    expect(result.error).toContain("Invalid tier config for 'opus'");
+  });
+});
+
 describe('saveCompositeVariantUnified', () => {
   let tmpDir: string;
   let originalCcsDir: string | undefined;