diff --git a/src/cloudcode/index.js b/src/cloudcode/index.js
index 806f402..b1d43f5 100644
--- a/src/cloudcode/index.js
+++ b/src/cloudcode/index.js
@@ -12,12 +12,12 @@
 // Re-export public API
 export { sendMessage } from './message-handler.js';
 export { sendMessageStream } from './streaming-handler.js';
-export { listModels, fetchAvailableModels, getModelQuotas, getSubscriptionTier } from './model-api.js';
+export { listModels, fetchAvailableModels, getModelQuotas, getSubscriptionTier, isValidModel } from './model-api.js';
 
 // Default export for backwards compatibility
 import { sendMessage } from './message-handler.js';
 import { sendMessageStream } from './streaming-handler.js';
-import { listModels, fetchAvailableModels, getModelQuotas, getSubscriptionTier } from './model-api.js';
+import { listModels, fetchAvailableModels, getModelQuotas, getSubscriptionTier, isValidModel } from './model-api.js';
 
 export default {
     sendMessage,
@@ -25,5 +25,6 @@ export default {
     listModels,
     fetchAvailableModels,
     getModelQuotas,
-    getSubscriptionTier
+    getSubscriptionTier,
+    isValidModel
 };
diff --git a/src/cloudcode/model-api.js b/src/cloudcode/model-api.js
index 0dc6cf4..b567235 100644
--- a/src/cloudcode/model-api.js
+++ b/src/cloudcode/model-api.js
@@ -9,10 +9,18 @@ import {
     ANTIGRAVITY_HEADERS,
     LOAD_CODE_ASSIST_ENDPOINTS,
     LOAD_CODE_ASSIST_HEADERS,
-    getModelFamily
+    getModelFamily,
+    MODEL_VALIDATION_CACHE_TTL_MS
 } from '../constants.js';
 import { logger } from '../utils/logger.js';
 
+// Model validation cache
+const modelCache = {
+    validModels: new Set(),
+    lastFetched: 0,
+    fetchPromise: null  // Prevents concurrent fetches
+};
+
 /**
  * Check if a model is supported (Claude or Gemini)
  * @param {string} modelId - Model ID to check
@@ -46,6 +54,10 @@ export async function listModels(token) {
         description: modelData.displayName || modelId
     }));
 
+    // Warm the model validation cache
+    modelCache.validModels = new Set(modelList.map(m => m.id));
+    modelCache.lastFetched = Date.now();
+
     return {
         object: 'list',
         data: modelList
@@ -246,3 +258,71 @@ export async function getSubscriptionTier(token) {
     logger.warn('[CloudCode] Failed to detect subscription tier from all endpoints. Defaulting to free.');
     return { tier: 'free', projectId: null };
 }
+
+/**
+ * Populate the model validation cache
+ * @param {string} token - OAuth access token
+ * @param {string} [projectId] - Optional project ID
+ * @returns {Promise<void>}
+ */
+async function populateModelCache(token, projectId = null) {
+    const now = Date.now();
+
+    // Check if cache is fresh
+    if (modelCache.validModels.size > 0 && (now - modelCache.lastFetched) < MODEL_VALIDATION_CACHE_TTL_MS) {
+        return;
+    }
+
+    // If already fetching, wait for it
+    if (modelCache.fetchPromise) {
+        await modelCache.fetchPromise;
+        return;
+    }
+
+    // Start fetch
+    modelCache.fetchPromise = (async () => {
+        try {
+            const data = await fetchAvailableModels(token, projectId);
+            if (data && data.models) {
+                const validIds = Object.keys(data.models).filter(modelId => isSupportedModel(modelId));
+                modelCache.validModels = new Set(validIds);
+                modelCache.lastFetched = Date.now();
+                logger.debug(`[CloudCode] Model cache populated with ${validIds.length} models`);
+            }
+        } catch (error) {
+            logger.warn(`[CloudCode] Failed to populate model cache: ${error.message}`);
+            // Don't throw - validation should degrade gracefully
+        } finally {
+            modelCache.fetchPromise = null;
+        }
+    })();
+
+    await modelCache.fetchPromise;
+}
+
+/**
+ * Check if a model ID is valid (exists in the available models list)
+ * Uses a cached model list with TTL-based refresh
+ * @param {string} modelId - Model ID to validate
+ * @param {string} token - OAuth access token for cache population
+ * @param {string} [projectId] - Optional project ID
+ * @returns {Promise<boolean>} True if model is valid
+ */
+export async function isValidModel(modelId, token, projectId = null) {
+    try {
+        // Populate cache if needed
+        await populateModelCache(token, projectId);
+
+        // If cache is populated, validate against it
+        if (modelCache.validModels.size > 0) {
+            return modelCache.validModels.has(modelId);
+        }
+
+        // Cache empty (fetch failed) - fail open, let API validate
+        return true;
+    } catch (error) {
+        logger.debug(`[CloudCode] Model validation error: ${error.message}`);
+        // Fail open - let the API validate
+        return true;
+    }
+}
diff --git a/src/constants.js b/src/constants.js
index 082315b..3a93493 100644
--- a/src/constants.js
+++ b/src/constants.js
@@ -156,6 +156,9 @@ export const GEMINI_SKIP_SIGNATURE = 'skip_thought_signature_validator';
 // Cache TTL for Gemini thoughtSignatures (2 hours)
 export const GEMINI_SIGNATURE_CACHE_TTL_MS = 2 * 60 * 60 * 1000;
 
+// Cache TTL for model validation (5 minutes)
+export const MODEL_VALIDATION_CACHE_TTL_MS = 5 * 60 * 1000;
+
 /**
  * Get the model family from model name (dynamic detection, no hardcoded list).
  * @param {string} modelName - The model name from the request
@@ -295,6 +298,7 @@ export default {
     GEMINI_MAX_OUTPUT_TOKENS,
     GEMINI_SKIP_SIGNATURE,
     GEMINI_SIGNATURE_CACHE_TTL_MS,
+    MODEL_VALIDATION_CACHE_TTL_MS,
     getModelFamily,
     isThinkingModel,
     OAUTH_CONFIG,
diff --git a/src/server.js b/src/server.js
index 5a06cc2..7c9c1b6 100644
--- a/src/server.js
+++ b/src/server.js
@@ -8,7 +8,7 @@ import express from 'express';
 import cors from 'cors';
 import path from 'path';
 import { fileURLToPath } from 'url';
-import { sendMessage, sendMessageStream, listModels, getModelQuotas, getSubscriptionTier } from './cloudcode/index.js';
+import { sendMessage, sendMessageStream, listModels, getModelQuotas, getSubscriptionTier, isValidModel } from './cloudcode/index.js';
 import { mountWebUI } from './webui/index.js';
 import { config } from './config.js';
 
@@ -720,6 +720,18 @@ app.post('/v1/messages', async (req, res) => {
 
         const modelId = requestedModel;
 
+        // Validate model ID before processing
+        const { account: validationAccount } = accountManager.selectAccount();
+        if (validationAccount) {
+            const token = await accountManager.getTokenForAccount(validationAccount);
+            const projectId = validationAccount.subscription?.projectId || null;
+            const valid = await isValidModel(modelId, token, projectId);
+
+            if (!valid) {
+                throw new Error(`invalid_request_error: Invalid model: ${modelId}. Use /v1/models to see available models.`);
+            }
+        }
+
         // Optimistic Retry: If ALL accounts are rate-limited for this model, reset them to force a fresh check.
         // If we have some available accounts, we try them first.
         if (accountManager.isAllRateLimited(modelId)) {