fix(count_tokens): count structured Anthropic blocks (#2419)

Estimate tokens for tool_use, tool_result, thinking, system, and tools
blocks instead of text only, so count_tokens no longer returns 0 for
structured content and breaks Claude Code auto-compaction (#2337).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/api/v1/messages/count_tokens/route.js

@@ -11,6 +11,64 @@ export async function OPTIONS() {
   return new Response(null, { headers: CORS_HEADERS });
 }
 
+function countValueChars(value) {
+  if (value == null) return 0;
+  if (typeof value === "string") return value.length;
+  if (typeof value === "number" || typeof value === "boolean") {
+    return String(value).length;
+  }
+  if (Array.isArray(value)) {
+    return value.reduce((total, item) => total + countValueChars(item), 0);
+  }
+  if (typeof value === "object") {
+    return Object.entries(value).reduce((total, [key, item]) => {
+      return total + key.length + countValueChars(item);
+    }, 0);
+  }
+  return 0;
+}
+
+function countContentBlockChars(block) {
+  if (block == null) return 0;
+  if (typeof block === "string") return block.length;
+  if (typeof block !== "object") return countValueChars(block);
+
+  switch (block.type) {
+    case "text":
+      return countValueChars(block.text);
+    case "tool_use":
+      return countValueChars(block.name) + countValueChars(block.input);
+    case "tool_result":
+      return countValueChars(block.content);
+    case "thinking":
+      return countValueChars(block.thinking);
+    default:
+      return countValueChars(block);
+  }
+}
+
+function countMessageChars(message) {
+  if (!message || typeof message !== "object") return 0;
+  const content = message.content;
+
+  if (typeof content === "string") return content.length;
+  if (Array.isArray(content)) {
+    return content.reduce((total, block) => total + countContentBlockChars(block), 0);
+  }
+  return countValueChars(content);
+}
+
+export function estimateAnthropicInputTokens(body = {}) {
+  const messages = Array.isArray(body.messages) ? body.messages : [];
+  let totalChars = countValueChars(body.system) + countValueChars(body.tools);
+
+  for (const msg of messages) {
+    totalChars += countMessageChars(msg);
+  }
+
+  return Math.ceil(totalChars / 4);
+}
+
 /**
  * POST /v1/messages/count_tokens - Mock token count response
  */
@@ -25,23 +83,7 @@ export async function POST(request) {
     });
   }
 
-  // Estimate token count based on content length
-  const messages = body.messages || [];
-  let totalChars = 0;
-  for (const msg of messages) {
-    if (typeof msg.content === "string") {
-      totalChars += msg.content.length;
-    } else if (Array.isArray(msg.content)) {
-      for (const part of msg.content) {
-        if (part.type === "text" && part.text) {
-          totalChars += part.text.length;
-        }
-      }
-    }
-  }
-
-  // Rough estimate: ~4 chars per token
-  const inputTokens = Math.ceil(totalChars / 4);
+  const inputTokens = estimateAnthropicInputTokens(body);
 
   return new Response(JSON.stringify({
     input_tokens: inputTokens

tests/unit/count-tokens.test.js

@@ -0,0 +1,84 @@
+import { describe, expect, it } from "vitest";
+
+import { POST } from "../../src/app/api/v1/messages/count_tokens/route.js";
+
+async function countTokens(body) {
+  const response = await POST(new Request("https://9router.local/v1/messages/count_tokens", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  }));
+
+  expect(response.status).toBe(200);
+  return response.json();
+}
+
+describe("Anthropic count_tokens estimator", () => {
+  it("preserves the existing plain text estimate", async () => {
+    const result = await countTokens({
+      messages: [
+        {
+          role: "user",
+          content: "hello world",
+        },
+      ],
+    });
+
+    expect(result.input_tokens).toBe(3);
+  });
+
+  it("counts tool and thinking content blocks that carry context", async () => {
+    const result = await countTokens({
+      messages: [
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "tool_use",
+              id: "toolu_01",
+              name: "Read",
+              input: { file_path: "/tmp/example.txt" },
+            },
+            {
+              type: "thinking",
+              thinking: "Need to inspect the file before answering.",
+            },
+          ],
+        },
+        {
+          role: "user",
+          content: [
+            {
+              type: "tool_result",
+              tool_use_id: "toolu_01",
+              content: "line1 line2 line3 some file content here",
+            },
+          ],
+        },
+      ],
+    });
+
+    expect(result.input_tokens).toBeGreaterThan(0);
+  });
+
+  it("counts system prompts and tool definitions", async () => {
+    const result = await countTokens({
+      system: "You are a coding assistant.",
+      tools: [
+        {
+          name: "Read",
+          description: "Read a file",
+          input_schema: {
+            type: "object",
+            properties: {
+              file_path: { type: "string" },
+            },
+          },
+        },
+      ],
+      messages: [],
+    });
+
+    expect(result.input_tokens).toBeGreaterThan(0);
+  });
+});