diff --git a/tools/diagnose-toolcall-truncation.ts b/tools/diagnose-toolcall-truncation.ts
new file mode 100644
index 0000000..67d4826
--- /dev/null
+++ b/tools/diagnose-toolcall-truncation.ts
@@ -0,0 +1,183 @@
+#!/usr/bin/env npx tsx
+/**
+ * copilot-api streaming tool call 截断诊断脚本
+ * 
+ * 测试 copilot-api 的 streaming response 是否会截断 tool_calls 的 arguments。
+ * 已知问题：非 Linux 平台（Windows/macOS）的 copilot-api 在 streaming 模式下
+ * 可能截断长 tool call arguments（>2000 字符）。
+ * 
+ * 用法：
+ *   COPILOT_API_URL=http://localhost:4141 npx tsx diagnose-toolcall-truncation.ts
+ * 
+ * 需要 copilot-api 在本地运行。
+ * 
+ * 🐉 Created by 敖丙 (RAKU) — 2026-03-31
+ */
+
+const COPILOT_API_URL = process.env.COPILOT_API_URL || "http://localhost:4141";
+const MODEL = process.env.MODEL || "claude-sonnet-4.6";
+
+interface StreamDelta {
+  tool_calls?: Array<{
+    index: number;
+    function?: { name?: string; arguments?: string };
+  }>;
+}
+
+async function testStreamingToolCall(charTarget: number): Promise<{
+  passed: boolean;
+  expected: number;
+  received: number;
+  truncated: boolean;
+  error?: string;
+}> {
+  const prompt = `You must call the "echo_back" tool with a JSON argument "data" containing a string of exactly ${charTarget} 'x' characters. Do not explain, just call the tool immediately. The data field should be: "${"x".repeat(charTarget)}"`;
+
+  const body = {
+    model: MODEL,
+    stream: true,
+    messages: [
+      { role: "system", content: "You are a test assistant. Always use tools when instructed. Never refuse." },
+      { role: "user", content: prompt },
+    ],
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "echo_back",
+          description: "Echoes back the data string. Used for testing.",
+          parameters: {
+            type: "object",
+            properties: {
+              data: { type: "string", description: "The string to echo back" },
+            },
+            required: ["data"],
+          },
+        },
+      },
+    ],
+    tool_choice: { type: "function", function: { name: "echo_back" } },
+  };
+
+  try {
+    const resp = await fetch(`${COPILOT_API_URL}/v1/chat/completions`, {
+      method: "POST",
+      headers: { "Content-Type": "application/json", Authorization: "Bearer dummy" },
+      body: JSON.stringify(body),
+    });
+
+    if (!resp.ok) {
+      return { passed: false, expected: charTarget, received: 0, truncated: false, error: `HTTP ${resp.status}: ${await resp.text()}` };
+    }
+
+    // Parse SSE stream
+    const text = await resp.text();
+    const lines = text.split("\n").filter((l) => l.startsWith("data: ") && l !== "data: [DONE]");
+
+    let collectedArgs = "";
+    let toolName = "";
+
+    for (const line of lines) {
+      try {
+        const json = JSON.parse(line.slice(6));
+        const delta: StreamDelta = json.choices?.[0]?.delta || {};
+        if (delta.tool_calls) {
+          for (const tc of delta.tool_calls) {
+            if (tc.function?.name) toolName = tc.function.name;
+            if (tc.function?.arguments) collectedArgs += tc.function.arguments;
+          }
+        }
+      } catch {
+        // skip non-JSON lines
+      }
+    }
+
+    // Try to parse the collected arguments
+    let dataLength = 0;
+    try {
+      const parsed = JSON.parse(collectedArgs);
+      dataLength = (parsed.data || "").length;
+    } catch {
+      // Arguments might be truncated JSON
+      const match = collectedArgs.match(/"data"\s*:\s*"(x+)/);
+      dataLength = match ? match[1].length : 0;
+    }
+
+    const truncated = dataLength < charTarget * 0.9; // Allow 10% tolerance (LLM might not generate exact count)
+    return {
+      passed: !truncated,
+      expected: charTarget,
+      received: dataLength,
+      truncated,
+    };
+  } catch (err: any) {
+    return { passed: false, expected: charTarget, received: 0, truncated: false, error: err.message };
+  }
+}
+
+async function main() {
+  console.log("╔══════════════════════════════════════════════════════════════╗");
+  console.log("║  copilot-api Streaming Tool Call 截断诊断                    ║");
+  console.log("╚══════════════════════════════════════════════════════════════╝");
+  console.log();
+  console.log(`API:   ${COPILOT_API_URL}`);
+  console.log(`Model: ${MODEL}`);
+  console.log();
+
+  // Test connectivity first
+  try {
+    const ping = await fetch(`${COPILOT_API_URL}/v1/models`, {
+      headers: { Authorization: "Bearer dummy" },
+    });
+    if (!ping.ok) {
+      console.error(`❌ API 连接失败: HTTP ${ping.status}`);
+      process.exit(1);
+    }
+    console.log("✅ API 连接正常\n");
+  } catch (err: any) {
+    console.error(`❌ API 连接失败: ${err.message}`);
+    process.exit(1);
+  }
+
+  const testSizes = [500, 1000, 2000, 3000, 5000];
+  let allPassed = true;
+
+  console.log("测试 | 目标字符数 | 实收字符数 | 结果");
+  console.log("-----|----------|----------|------");
+
+  for (const size of testSizes) {
+    process.stdout.write(`  ${size.toString().padStart(4)}  |`);
+    const result = await testStreamingToolCall(size);
+
+    if (result.error) {
+      console.log(` ${size.toString().padStart(8)} |          | ❌ ${result.error}`);
+      allPassed = false;
+      continue;
+    }
+
+    const status = result.passed ? "✅ PASS" : "❌ TRUNCATED";
+    const ratio = ((result.received / result.expected) * 100).toFixed(0);
+    console.log(` ${result.expected.toString().padStart(8)} | ${result.received.toString().padStart(8)} | ${status} (${ratio}%)`);
+
+    if (!allPassed && result.truncated) allPassed = false;
+    if (result.truncated) allPassed = false;
+  }
+
+  console.log();
+  if (allPassed) {
+    console.log("🎉 所有测试通过！Streaming tool call 没有截断问题。");
+  } else {
+    console.log("⚠️  检测到截断！建议：");
+    console.log("  1. 检查 copilot-api 版本和平台");
+    console.log("  2. Linux 平台通常没有此问题");
+    console.log("  3. macOS/Windows 可以考虑：");
+    console.log("     a) 升级 copilot-api 到最新版");
+    console.log("     b) 使用 stream-strip-proxy 中间层");
+    console.log("     c) 关闭 streaming（性能代价大）");
+    console.log("  4. 参考 RAKU 的经验：迁移到 Linux 后问题消失");
+  }
+
+  process.exit(allPassed ? 0 : 1);
+}
+
+main().catch(console.error);