diff --git a/src/cliproxy/executor/env-resolver.ts b/src/cliproxy/executor/env-resolver.ts
index ace7a18d..dbe49ad6 100644
--- a/src/cliproxy/executor/env-resolver.ts
+++ b/src/cliproxy/executor/env-resolver.ts
@@ -24,7 +24,7 @@ import { stripClaudeCodeEnv } from '../../utils/shell-executor';
 import { CodexReasoningProxy } from '../codex-reasoning-proxy';
 import { ToolSanitizationProxy } from '../tool-sanitization-proxy';
 import { HttpsTunnelProxy } from '../https-tunnel-proxy';
-import { normalizeModelIdForProvider } from '../model-id-normalizer';
+import { MODEL_ENV_VAR_KEYS, normalizeModelIdForProvider } from '../model-id-normalizer';
 
 export interface RemoteProxyConfig {
   host: string;
@@ -61,6 +61,38 @@ export interface ProxyChainConfig {
   compositeDefaultTier?: 'opus' | 'sonnet' | 'haiku';
 }
 
+const CODEX_EFFORT_SUFFIX_REGEX = /^(.*)-(xhigh|high|medium)$/i;
+const EXTENDED_CONTEXT_SUFFIX_REGEX = /\[1m\]$/i;
+
+function normalizeCodexModelForDirectUpstream(model: string): string {
+  const withoutExtendedContext = model.trim().replace(EXTENDED_CONTEXT_SUFFIX_REGEX, '').trim();
+  if (!withoutExtendedContext) return withoutExtendedContext;
+
+  const effortMatch = withoutExtendedContext.match(CODEX_EFFORT_SUFFIX_REGEX);
+  if (!effortMatch?.[1] || !effortMatch[2]) {
+    return withoutExtendedContext;
+  }
+
+  return `${effortMatch[1].trim()}(${effortMatch[2].toLowerCase()})`;
+}
+
+function normalizeCodexEnvForDirectUpstream(envVars: NodeJS.ProcessEnv): NodeJS.ProcessEnv {
+  let nextEnv: NodeJS.ProcessEnv | null = null;
+
+  for (const key of MODEL_ENV_VAR_KEYS) {
+    const value = envVars[key];
+    if (typeof value !== 'string' || value.trim().length === 0) continue;
+
+    const normalizedValue = normalizeCodexModelForDirectUpstream(value);
+    if (normalizedValue === value) continue;
+
+    if (!nextEnv) nextEnv = { ...envVars };
+    nextEnv[key] = normalizedValue;
+  }
+
+  return nextEnv ?? envVars;
+}
+
 /**
  * Build final environment variables for Claude CLI execution
  * Handles proxy chain ordering and integration with hooks
@@ -190,6 +222,14 @@ export function buildClaudeEnvironment(config: ProxyChainConfig): Record<string,
   // Auto-enabled for Gemini, opt-in for Claude (--1m flag)
   applyExtendedContextConfig(envVars, provider, extendedContextOverride);
 
+  // Fallback compatibility for Codex:
+  // CLIProxyAPI provider lookup recognizes model(level) suffix form, while CCS stores codex
+  // effort aliases as model-level (e.g., gpt-5.3-codex-high). If codex reasoning proxy is
+  // unavailable, normalize to model(level) to avoid "unknown provider for model ..." failures.
+  if (provider === 'codex' && !isComposite && !codexReasoningPort) {
+    envVars = normalizeCodexEnvForDirectUpstream(envVars);
+  }
+
   // Determine the final ANTHROPIC_BASE_URL based on active proxies
   // Chain order: Claude CLI → [CodexReasoningProxy] → [ToolSanitizationProxy] → CLIProxy
   let finalBaseUrl = envVars.ANTHROPIC_BASE_URL;
diff --git a/tests/unit/cliproxy/env-resolver-codex-fallback.test.ts b/tests/unit/cliproxy/env-resolver-codex-fallback.test.ts
new file mode 100644
index 00000000..0162ed26
--- /dev/null
+++ b/tests/unit/cliproxy/env-resolver-codex-fallback.test.ts
@@ -0,0 +1,95 @@
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { afterEach, describe, expect, it } from 'bun:test';
+import { buildClaudeEnvironment } from '../../../src/cliproxy/executor/env-resolver';
+
+const tempDirs: string[] = [];
+
+function createCodexSettingsFile(models: {
+  defaultModel: string;
+  opusModel: string;
+  sonnetModel: string;
+  haikuModel: string;
+}): string {
+  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ccs-codex-fallback-'));
+  tempDirs.push(tempDir);
+
+  const settingsPath = path.join(tempDir, 'codex-test.settings.json');
+  fs.writeFileSync(
+    settingsPath,
+    JSON.stringify(
+      {
+        env: {
+          ANTHROPIC_BASE_URL: 'http://127.0.0.1:8317/api/provider/codex',
+          ANTHROPIC_AUTH_TOKEN: 'test-token',
+          ANTHROPIC_MODEL: models.defaultModel,
+          ANTHROPIC_DEFAULT_OPUS_MODEL: models.opusModel,
+          ANTHROPIC_DEFAULT_SONNET_MODEL: models.sonnetModel,
+          ANTHROPIC_DEFAULT_HAIKU_MODEL: models.haikuModel,
+        },
+      },
+      null,
+      2
+    ) + '\n'
+  );
+
+  return settingsPath;
+}
+
+describe('buildClaudeEnvironment codex fallback normalization', () => {
+  afterEach(() => {
+    while (tempDirs.length > 0) {
+      const tempDir = tempDirs.pop();
+      if (tempDir) {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    }
+  });
+
+  it('normalizes codex effort aliases when reasoning proxy is unavailable', () => {
+    const settingsPath = createCodexSettingsFile({
+      defaultModel: 'gpt-5.3-codex-high',
+      opusModel: 'gpt-5.3-codex-xhigh',
+      sonnetModel: 'gpt-5.3-codex-high',
+      haikuModel: 'gpt-5-mini-medium',
+    });
+
+    const env = buildClaudeEnvironment({
+      provider: 'codex',
+      useRemoteProxy: false,
+      localPort: 8317,
+      customSettingsPath: settingsPath,
+      verbose: false,
+    });
+
+    expect(env.ANTHROPIC_MODEL).toBe('gpt-5.3-codex(high)');
+    expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe('gpt-5.3-codex(xhigh)');
+    expect(env.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe('gpt-5.3-codex(high)');
+    expect(env.ANTHROPIC_DEFAULT_HAIKU_MODEL).toBe('gpt-5-mini(medium)');
+  });
+
+  it('keeps codex effort aliases when reasoning proxy is active', () => {
+    const settingsPath = createCodexSettingsFile({
+      defaultModel: 'gpt-5.3-codex-high',
+      opusModel: 'gpt-5.3-codex-xhigh',
+      sonnetModel: 'gpt-5.3-codex-high',
+      haikuModel: 'gpt-5-mini-medium',
+    });
+
+    const env = buildClaudeEnvironment({
+      provider: 'codex',
+      useRemoteProxy: false,
+      localPort: 8317,
+      customSettingsPath: settingsPath,
+      codexReasoningPort: 9444,
+      verbose: false,
+    });
+
+    expect(env.ANTHROPIC_MODEL).toBe('gpt-5.3-codex-high');
+    expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe('gpt-5.3-codex-xhigh');
+    expect(env.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe('gpt-5.3-codex-high');
+    expect(env.ANTHROPIC_DEFAULT_HAIKU_MODEL).toBe('gpt-5-mini-medium');
+    expect(env.ANTHROPIC_BASE_URL).toBe('http://127.0.0.1:9444/api/provider/codex');
+  });
+});