diff --git a/src/cliproxy/proxy/__tests__/tool-sanitization-proxy-integration.test.ts b/src/cliproxy/proxy/__tests__/tool-sanitization-proxy-integration.test.ts
index 335019cf..3bb65862 100644
--- a/src/cliproxy/proxy/__tests__/tool-sanitization-proxy-integration.test.ts
+++ b/src/cliproxy/proxy/__tests__/tool-sanitization-proxy-integration.test.ts
@@ -436,6 +436,155 @@ describe('ToolSanitizationProxy Integration', () => {
       }
     });
 
+    it('translates Codex fast aliases before forwarding to provider routes', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      try {
+        await fetch(`http://127.0.0.1:${port}/api/provider/codex/v1/messages?beta=true`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            model: 'gpt-5.5-fast',
+            messages: [],
+          }),
+        });
+
+        const sentBody = lastRequest!.body as Record<string, unknown>;
+        expect(sentBody.model).toBe('gpt-5.5');
+        expect(sentBody.service_tier).toBe('priority');
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('translates Codex effort and fast aliases before forwarding to provider routes', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      try {
+        await fetch(`http://127.0.0.1:${port}/api/provider/codex/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            model: 'gpt-5.5-fast-high',
+            messages: [],
+            reasoning: { summary: 'auto' },
+          }),
+        });
+
+        const sentBody = lastRequest!.body as Record<string, unknown>;
+        expect(sentBody.model).toBe('gpt-5.5');
+        expect((sentBody.reasoning as Record<string, unknown>).summary).toBe('auto');
+        expect((sentBody.reasoning as Record<string, unknown>).effort).toBe('high');
+        expect(sentBody.service_tier).toBe('priority');
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('folds Codex system messages into the first user message before forwarding', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      try {
+        await fetch(`http://127.0.0.1:${port}/api/provider/codex/v1/messages?beta=true`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            model: 'gpt-5.4',
+            system: [{ type: 'text', text: 'Always be concise.' }],
+            messages: [
+              { role: 'system', content: 'Use JSON.' },
+              { role: 'user', content: [{ type: 'text', text: 'hello' }] },
+            ],
+          }),
+        });
+
+        const sentBody = lastRequest!.body as Record<string, unknown>;
+        const sentMessages = sentBody.messages as Array<Record<string, unknown>>;
+
+        expect(sentBody.system).toBeUndefined();
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].role).toBe('user');
+        expect(sentMessages[0].content).toEqual([
+          { type: 'text', text: 'Always be concise.\n\nUse JSON.' },
+          { type: 'text', text: 'hello' },
+        ]);
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('strips blank Codex system messages before forwarding', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      try {
+        await fetch(`http://127.0.0.1:${port}/api/provider/codex/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            model: 'gpt-5.4',
+            system: '   ',
+            messages: [
+              { role: 'system', content: [{ type: 'text', text: '  ' }] },
+              { role: 'user', content: 'hello' },
+            ],
+          }),
+        });
+
+        const sentBody = lastRequest!.body as Record<string, unknown>;
+        const sentMessages = sentBody.messages as Array<Record<string, unknown>>;
+
+        expect(sentBody.system).toBeUndefined();
+        expect(sentMessages).toEqual([{ role: 'user', content: 'hello' }]);
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('does not apply Codex alias or system rewrites on explicit non-Codex provider routes', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      try {
+        await fetch(`http://127.0.0.1:${port}/api/provider/gemini/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            model: 'gpt-5.5-fast',
+            system: 'Keep this as a provider-level system field.',
+            messages: [
+              { role: 'system', content: 'Keep this as a message.' },
+              { role: 'user', content: 'hello' },
+            ],
+          }),
+        });
+
+        const sentBody = lastRequest!.body as Record<string, unknown>;
+        expect(sentBody.model).toBe('gpt-5.5-fast');
+        expect(sentBody.service_tier).toBeUndefined();
+        expect(sentBody.system).toBe('Keep this as a provider-level system field.');
+        expect(sentBody.messages).toEqual([
+          { role: 'system', content: 'Keep this as a message.' },
+          { role: 'user', content: 'hello' },
+        ]);
+      } finally {
+        proxy.stop();
+      }
+    });
+
     for (const model of [
       'gpt-5.3-codex-xhigh',
       'gpt-5.1-codex-mini',
diff --git a/src/cliproxy/proxy/tool-sanitization-proxy.ts b/src/cliproxy/proxy/tool-sanitization-proxy.ts
index a49ca71a..a86cc7ad 100644
--- a/src/cliproxy/proxy/tool-sanitization-proxy.ts
+++ b/src/cliproxy/proxy/tool-sanitization-proxy.ts
@@ -22,6 +22,7 @@ import {
   extractProviderFromPathname,
   getDeniedModelIdReasonForProvider,
   normalizeModelIdForRouting,
+  parseCodexModelTuningAlias,
   stripCodexEffortSuffix,
 } from '../ai-providers/model-id-normalizer';
 import { getModelMaxLevel } from '../model-catalog';
@@ -63,6 +64,7 @@ const GEMINI_UNSUPPORTED_TOOL_FIELDS = new Set([
 ]);
 
 const CODEX_UNSUPPORTED_TOOL_FIELDS = new Set(['cache_control']);
+const CODEX_FAST_SERVICE_TIER = 'priority';
 const EXTENDED_CONTEXT_SUFFIX_REGEX = /\[1m\]$/i;
 const LEGACY_CODEX_MODEL_ID_REGEX = /^gpt-5(?:\.\d+)?-codex(?:-(?:mini|max))?$/i;
 
@@ -90,6 +92,118 @@ function isKnownCodexModelId(model: string | undefined): boolean {
   );
 }
 
+function isCodexRequest(providerFromPath: string | null, model: unknown): boolean {
+  return (
+    providerFromPath === 'codex' ||
+    (providerFromPath === null && typeof model === 'string' && isKnownCodexModelId(model))
+  );
+}
+
+function applyCodexModelTuningAlias(body: Record<string, unknown>): Record<string, unknown> {
+  if (typeof body.model !== 'string') {
+    return body;
+  }
+
+  const parsed = parseCodexModelTuningAlias(body.model);
+  if (!parsed || !isKnownCodexModelId(parsed.baseModel)) {
+    return body;
+  }
+
+  const tunedBody: Record<string, unknown> = { ...body, model: parsed.baseModel };
+
+  if (parsed.effort) {
+    const existingReasoning = isRecord(body.reasoning) ? body.reasoning : {};
+    tunedBody.reasoning = {
+      ...existingReasoning,
+      effort: parsed.effort,
+    };
+  }
+
+  if (parsed.serviceTier) {
+    tunedBody.service_tier = CODEX_FAST_SERVICE_TIER;
+  }
+
+  return tunedBody;
+}
+
+function extractSystemText(content: unknown): string {
+  if (typeof content === 'string') {
+    return content;
+  }
+
+  if (!Array.isArray(content)) {
+    return '';
+  }
+
+  return content
+    .filter((block): block is { type: unknown; text?: unknown } => isRecord(block))
+    .filter((block) => block.type === 'text' && typeof block.text === 'string')
+    .map((block) => block.text as string)
+    .join('\n\n');
+}
+
+function prependSystemTextToContent(content: unknown, systemText: string): unknown {
+  if (Array.isArray(content)) {
+    return [{ type: 'text', text: systemText }, ...content];
+  }
+  if (typeof content === 'string') {
+    return `${systemText}\n\n${content}`;
+  }
+  return systemText;
+}
+
+function foldCodexSystemMessages(body: Record<string, unknown>): Record<string, unknown> {
+  const systemTexts: string[] = [];
+  let removedSystem = false;
+  const nextBody = { ...body };
+
+  if (body.system !== undefined) {
+    removedSystem = true;
+    delete nextBody.system;
+    const systemText = extractSystemText(body.system).trim();
+    if (systemText) {
+      systemTexts.push(systemText);
+    }
+  }
+
+  const rawMessages = Array.isArray(body.messages) ? body.messages : [];
+  const messages = rawMessages.filter((message) => {
+    if (!isRecord(message) || message.role !== 'system') {
+      return true;
+    }
+    removedSystem = true;
+    const systemText = extractSystemText(message.content).trim();
+    if (systemText) {
+      systemTexts.push(systemText);
+    }
+    return false;
+  });
+
+  if (!removedSystem) {
+    return body;
+  }
+
+  if (systemTexts.length > 0) {
+    const systemPrefix = systemTexts.join('\n\n');
+    const firstUserIndex = messages.findIndex(
+      (message) => isRecord(message) && message.role === 'user'
+    );
+
+    if (firstUserIndex >= 0 && isRecord(messages[firstUserIndex])) {
+      const firstUserMessage = messages[firstUserIndex];
+      messages[firstUserIndex] = {
+        ...firstUserMessage,
+        content: prependSystemTextToContent(firstUserMessage.content, systemPrefix),
+      };
+    } else {
+      messages.unshift({ role: 'user', content: systemPrefix });
+    }
+  }
+
+  nextBody.messages = messages;
+  return nextBody;
+}
+
 function getUnsupportedToolFields(
   providerFromPath: string | null,
   model: string | undefined
@@ -352,6 +466,11 @@ export class ToolSanitizationProxy {
         }
       }
 
+      if (isRecord(modifiedBody) && isCodexRequest(providerFromPath, modifiedBody.model)) {
+        const tunedBody = applyCodexModelTuningAlias(modifiedBody);
+        modifiedBody = foldCodexSystemMessages(tunedBody);
+      }
+
       // Sanitize tools if present
       if (isRecord(modifiedBody) && Array.isArray(modifiedBody.tools)) {
         // Step 1: Sanitize input_schema properties (remove non-standard JSON Schema properties)