diff --git a/package-lock.json b/package-lock.json
index 79b762c..e5e8a64 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,7 +12,8 @@
         "async-mutex": "^0.5.0",
         "better-sqlite3": "^12.5.0",
         "cors": "^2.8.5",
-        "express": "^4.18.2"
+        "express": "^4.18.2",
+        "gpt-tokenizer": "^2.5.0"
       },
       "bin": {
         "antigravity-claude-proxy": "bin/cli.js"
@@ -395,7 +396,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "baseline-browser-mapping": "^2.9.0",
         "caniuse-lite": "^1.0.30001759",
@@ -1231,6 +1231,12 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/gpt-tokenizer": {
+      "version": "2.9.0",
+      "resolved": "https://registry.npmjs.org/gpt-tokenizer/-/gpt-tokenizer-2.9.0.tgz",
+      "integrity": "sha512-YSpexBL/k4bfliAzMrRqn3M6+it02LutVyhVpDeMKrC/O9+pCe/5s8U2hYKa2vFLD5/vHhsKc8sOn/qGqII8Kg==",
+      "license": "MIT"
+    },
     "node_modules/has-flag": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/has-flag/-/has-flag-4.0.0.tgz",
@@ -1416,7 +1422,6 @@
       "integrity": "sha512-/imKNG4EbWNrVjoNC/1H5/9GFy+tqjGBHCaSsN+P2RnPqjsLmv6UD3Ej+Kj8nBWaRAwyk7kK5ZUc+OEatnTR3A==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "jiti": "bin/jiti.js"
       }
@@ -1793,7 +1798,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "nanoid": "^3.3.11",
         "picocolors": "^1.1.1",
@@ -2611,7 +2615,6 @@
       "integrity": "sha512-3ofp+LL8E+pK/JuPLPggVAIaEuhvIz4qNcf3nA1Xn2o/7fb7s/TYpHhwGDv1ZU3PkBluUVaF8PyCHcm48cKLWQ==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@alloc/quick-lru": "^5.2.0",
         "arg": "^5.0.2",
@@ -2736,7 +2739,6 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
diff --git a/package.json b/package.json
index dec5b4f..93f51dc 100644
--- a/package.json
+++ b/package.json
@@ -60,7 +60,8 @@
     "async-mutex": "^0.5.0",
     "better-sqlite3": "^12.5.0",
     "cors": "^2.8.5",
-    "express": "^4.18.2"
+    "express": "^4.18.2",
+    "gpt-tokenizer": "^2.5.0"
   },
   "devDependencies": {
     "@tailwindcss/forms": "^0.5.7",
diff --git a/src/cloudcode/count-tokens.js b/src/cloudcode/count-tokens.js
new file mode 100644
index 0000000..32188de
--- /dev/null
+++ b/src/cloudcode/count-tokens.js
@@ -0,0 +1,297 @@
+/**
+ * Token Counter Implementation for antigravity-claude-proxy
+ *
+ * Implements Anthropic's /v1/messages/count_tokens endpoint
+ * Uses hybrid approach: local estimation for text, API call for complex content
+ *
+ * @see https://platform.claude.com/docs/en/api/messages-count-tokens
+ */
+
+import { encode } from 'gpt-tokenizer';
+import { logger } from '../utils/logger.js';
+import { buildCloudCodeRequest, buildHeaders } from './request-builder.js';
+import { ANTIGRAVITY_ENDPOINT_FALLBACKS } from '../constants.js';
+
+/**
+ * Estimate tokens for text content using GPT tokenizer
+ * Claude uses a similar tokenizer to GPT-4 (cl100k_base)
+ *
+ * @param {string} text - Text to tokenize
+ * @returns {number} Estimated token count
+ */
+function estimateTextTokens(text) {
+    if (!text) return 0;
+    try {
+        return encode(text).length;
+    } catch (error) {
+        // Fallback: rough estimate of 4 chars per token
+        return Math.ceil(text.length / 4);
+    }
+}
+
+/**
+ * Check if content contains complex blocks (images, documents)
+ * These require API call for accurate counting
+ *
+ * @param {Object} request - Anthropic request
+ * @returns {boolean} True if complex content detected
+ */
+function hasComplexContent(request) {
+    const { messages = [], system } = request;
+
+    for (const message of messages) {
+        const content = message.content;
+        if (Array.isArray(content)) {
+            for (const block of content) {
+                if (block.type === 'image' || block.type === 'document') {
+                    return true;
+                }
+            }
+        }
+    }
+
+    // Check system prompt for complex content
+    if (Array.isArray(system)) {
+        for (const block of system) {
+            if (block.type !== 'text') {
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
+
+/**
+ * Extract text from message content
+ *
+ * @param {string|Array} content - Message content
+ * @returns {string} Concatenated text
+ */
+function extractText(content) {
+    if (typeof content === 'string') {
+        return content;
+    }
+
+    if (Array.isArray(content)) {
+        return content
+            .filter(block => block.type === 'text')
+            .map(block => block.text)
+            .join('\n');
+    }
+
+    return '';
+}
+
+/**
+ * Count tokens locally using tokenizer
+ *
+ * @param {Object} request - Anthropic format request
+ * @returns {number} Estimated token count
+ */
+function countTokensLocally(request) {
+    const { messages = [], system, tools } = request;
+    let totalTokens = 0;
+
+    // Count system prompt tokens
+    if (system) {
+        if (typeof system === 'string') {
+            totalTokens += estimateTextTokens(system);
+        } else if (Array.isArray(system)) {
+            for (const block of system) {
+                if (block.type === 'text') {
+                    totalTokens += estimateTextTokens(block.text);
+                }
+            }
+        }
+    }
+
+    // Count message tokens
+    for (const message of messages) {
+        // Add overhead for role and structure (~4 tokens per message)
+        totalTokens += 4;
+        totalTokens += estimateTextTokens(extractText(message.content));
+
+        // Handle tool_use and tool_result blocks
+        if (Array.isArray(message.content)) {
+            for (const block of message.content) {
+                if (block.type === 'tool_use') {
+                    totalTokens += estimateTextTokens(block.name);
+                    totalTokens += estimateTextTokens(JSON.stringify(block.input));
+                } else if (block.type === 'tool_result') {
+                    if (typeof block.content === 'string') {
+                        totalTokens += estimateTextTokens(block.content);
+                    } else if (Array.isArray(block.content)) {
+                        totalTokens += estimateTextTokens(extractText(block.content));
+                    }
+                } else if (block.type === 'thinking') {
+                    totalTokens += estimateTextTokens(block.thinking);
+                }
+            }
+        }
+    }
+
+    // Count tool definitions
+    if (tools && tools.length > 0) {
+        for (const tool of tools) {
+            totalTokens += estimateTextTokens(tool.name);
+            totalTokens += estimateTextTokens(tool.description || '');
+            totalTokens += estimateTextTokens(JSON.stringify(tool.input_schema || {}));
+        }
+    }
+
+    return totalTokens;
+}
+
+/**
+ * Count tokens via Google Cloud Code API
+ * Makes a dry-run request to get accurate token count
+ *
+ * @param {Object} anthropicRequest - Anthropic format request
+ * @param {Object} accountManager - Account manager instance
+ * @returns {Promise<number>} Accurate token count from API
+ */
+async function countTokensViaAPI(anthropicRequest, accountManager) {
+    const account = accountManager.pickNext(anthropicRequest.model);
+    if (!account) {
+        throw new Error('No accounts available for token counting');
+    }
+
+    const token = await accountManager.getTokenForAccount(account);
+    const project = await accountManager.getProjectForAccount(account, token);
+
+    // Build request with minimal max_tokens to avoid generating content
+    const countRequest = {
+        ...anthropicRequest,
+        max_tokens: 1,
+        stream: false
+    };
+
+    const payload = buildCloudCodeRequest(countRequest, project);
+
+    // Try endpoints until one works
+    for (const endpoint of ANTIGRAVITY_ENDPOINT_FALLBACKS) {
+        try {
+            const url = `${endpoint}/v1internal:generateContent`;
+
+            const response = await fetch(url, {
+                method: 'POST',
+                headers: buildHeaders(token, anthropicRequest.model, 'application/json'),
+                body: JSON.stringify(payload)
+            });
+
+            if (!response.ok) {
+                logger.debug(`[TokenCounter] Error at ${endpoint}: ${response.status}`);
+                continue;
+            }
+
+            const data = await response.json();
+            const usageMetadata = data.usageMetadata || data.response?.usageMetadata || {};
+
+            return usageMetadata.promptTokenCount || 0;
+
+        } catch (error) {
+            logger.debug(`[TokenCounter] Error at ${endpoint}: ${error.message}`);
+            continue;
+        }
+    }
+
+    throw new Error('Failed to count tokens via API');
+}
+
+/**
+ * Count tokens in a message request
+ * Implements Anthropic's /v1/messages/count_tokens endpoint
+ *
+ * @param {Object} anthropicRequest - Anthropic format request with messages, model, system, tools
+ * @param {Object} accountManager - Account manager instance (optional, for API-based counting)
+ * @param {Object} options - Options
+ * @param {boolean} options.useAPI - Force API-based counting (default: false)
+ * @returns {Promise<Object>} Response with input_tokens count
+ */
+export async function countTokens(anthropicRequest, accountManager = null, options = {}) {
+    const { useAPI = false } = options;
+
+    try {
+        let inputTokens;
+
+        // Use API for complex content or when forced
+        if (useAPI || (hasComplexContent(anthropicRequest) && accountManager)) {
+            if (!accountManager) {
+                throw new Error('Account manager required for API-based token counting');
+            }
+            inputTokens = await countTokensViaAPI(anthropicRequest, accountManager);
+            logger.debug(`[TokenCounter] API count: ${inputTokens} tokens`);
+        } else {
+            // Use local estimation for text-only content
+            inputTokens = countTokensLocally(anthropicRequest);
+            logger.debug(`[TokenCounter] Local estimate: ${inputTokens} tokens`);
+        }
+
+        return {
+            input_tokens: inputTokens
+        };
+
+    } catch (error) {
+        logger.warn(`[TokenCounter] Error: ${error.message}, falling back to local estimation`);
+
+        // Fallback to local estimation
+        const inputTokens = countTokensLocally(anthropicRequest);
+        return {
+            input_tokens: inputTokens
+        };
+    }
+}
+
+/**
+ * Express route handler for /v1/messages/count_tokens
+ *
+ * @param {Object} accountManager - Account manager instance
+ * @returns {Function} Express middleware
+ */
+export function createCountTokensHandler(accountManager) {
+    return async (req, res) => {
+        try {
+            const { messages, model, system, tools, tool_choice, thinking } = req.body;
+
+            // Validate required fields
+            if (!messages || !Array.isArray(messages)) {
+                return res.status(400).json({
+                    type: 'error',
+                    error: {
+                        type: 'invalid_request_error',
+                        message: 'messages is required and must be an array'
+                    }
+                });
+            }
+
+            if (!model) {
+                return res.status(400).json({
+                    type: 'error',
+                    error: {
+                        type: 'invalid_request_error',
+                        message: 'model is required'
+                    }
+                });
+            }
+
+            const result = await countTokens(
+                { messages, model, system, tools, tool_choice, thinking },
+                accountManager,
+                { useAPI: false } // Default to local estimation for speed
+            );
+
+            res.json(result);
+
+        } catch (error) {
+            logger.error(`[TokenCounter] Handler error: ${error.message}`);
+            res.status(500).json({
+                type: 'error',
+                error: {
+                    type: 'api_error',
+                    message: error.message
+                }
+            });
+        }
+    };
+}
diff --git a/src/server.js b/src/server.js
index be2c3af..809467e 100644
--- a/src/server.js
+++ b/src/server.js
@@ -9,6 +9,7 @@ import cors from 'cors';
 import path from 'path';
 import { fileURLToPath } from 'url';
 import { sendMessage, sendMessageStream, listModels, getModelQuotas, getSubscriptionTier } from './cloudcode/index.js';
+import { createCountTokensHandler } from './cloudcode/count-tokens.js';
 import { mountWebUI } from './webui/index.js';
 import { config } from './config.js';
 
@@ -597,17 +598,10 @@ app.get('/v1/models', async (req, res) => {
 });
 
 /**
- * Count tokens endpoint (not supported)
+ * Count tokens endpoint - Anthropic Messages API compatible
+ * Uses hybrid approach: local tokenizer for text, API for complex content (images, documents)
  */
-app.post('/v1/messages/count_tokens', (req, res) => {
-    res.status(501).json({
-        type: 'error',
-        error: {
-            type: 'not_implemented',
-            message: 'Token counting is not implemented. Use /v1/messages with max_tokens or configure your client to skip token counting.'
-        }
-    });
-});
+app.post('/v1/messages/count_tokens', createCountTokensHandler(accountManager));
 
 /**
  * Main messages endpoint - Anthropic Messages API compatible