fix(runtime): tokenize context compaction budget

docs/proposals/C3-real-tokenizer-followup.md

@@ -1,7 +1,7 @@
 # Proposal C3-followup: 用真实 tokenizer 替换字符估算
 
 > 来源：Roundtable 2026-04-20 surgical compactor 修复；C3 首轮决定先用保守常量 (2 chars/token) 落地
-> 状态：proposed（中间态已落地，真正解决方案待依赖就绪）
+> 状态：implemented（2026-04-22：`js-tiktoken` / `cl100k_base` 已接入 `context-compactor.ts`）
 > 提出：Karpathy (T2)
 > Owner: TBD
 > 估计工时：1 天（换依赖）+ 半天（灰度验证）
@@ -9,7 +9,7 @@
 
 ## 问题陈述
 
-`src/main/agent-runtime/context-compactor.ts` 的 `estimateTokens` 现在用 `Math.ceil(chars / 2)` 做 token 估算，作为 120k 阈值的守门人。
+`src/main/agent-runtime/context-compactor.ts` 曾用 `Math.ceil(chars / 2)` 做 token 估算，作为 120k 阈值的守门人。2026-04-22 已改成 `js-tiktoken` 的 `cl100k_base` 本地编码器，并保留每条 chat message 的固定结构开销。
 
 问题：
 1. **中文 BPE 实际约 1.5-2 tokens/char**：2 chars/token 的常量对 CJK 大致可用，但 mixed 中英 / emoji / code block 的实际比例会漂移。
@@ -29,9 +29,9 @@ pi-ai 0.67.68 没 export tokenizer。Compactor 作为 `transformContext` hook
 
 ## 建议路径
 
-1. 先等 pi-ai 是否在后续 patch 里暴露 `countTokens(messages, model)`——如果暴露，零成本切过去。
-2. 若 6 周内上游没动作，引入 `tiktoken` + 模型 → encoding 映射表（Anthropic 用 `cl100k_base` 近似；Gemini / GPT 各自对应）。
-3. 保留 `estimateTokens(text: string): number` 的对外签名不变，只换内部实现——B3 配对修复和 hook 调用都无需动。
+1. 已接入 `js-tiktoken` + `cl100k_base`，保持 compactor 同步纯函数。
+2. pi-ai 后续若暴露 provider-native tokenizer，可在 `context-compactor.ts` 内替换编码器实现。
+3. hook 调用保持不变，`compactMessagesIfNeeded(messages)` 继续返回原有 `CompactionOutcome`。
 
 ## 不做的事
 

package.json

@@ -43,6 +43,7 @@
     "electron-log": "5.4.3",
     "electron-updater": "6.8.3",
     "framer-motion": "12.36.0",
+    "js-tiktoken": "1.0.21",
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "react-markdown": "^10.1.0",

pnpm-lock.yaml

@@ -32,6 +32,9 @@ importers:
       framer-motion:
         specifier: 12.36.0
         version: 12.36.0(react-dom@19.2.4(react@19.2.4))(react@19.2.4)
+      js-tiktoken:
+        specifier: 1.0.21
+        version: 1.0.21
       react:
         specifier: 19.2.4
         version: 19.2.4
@@ -2944,6 +2947,9 @@ packages:
     resolution: {integrity: sha512-ekilCSN1jwRvIbgeg/57YFh8qQDNbwDb9xT/qu2DAHbFFZUicIl4ygVaAvzveMhMVr3LnpSKTNnwt8PoOfmKhQ==}
     hasBin: true
 
+  js-tiktoken@1.0.21:
+    resolution: {integrity: sha512-biOj/6M5qdgx5TKjDnFT1ymSpM5tbd3ylwDtrQvFQSu0Z7bBYko2dF+W/aUkXUPuk6IVpRxk/3Q2sHOzGlS36g==}
+
   js-tokens@4.0.0:
     resolution: {integrity: sha512-RdJUflcE3cUzKiMqQgsCu06FPu9UdIJO0beYbPhHN4k6apgJtifcoCtT9bcxOpYBtpD2kCM6Sbzg4CausW/PKQ==}
 
@@ -7998,6 +8004,10 @@ snapshots:
 
   jiti@2.6.1: {}
 
+  js-tiktoken@1.0.21:
+    dependencies:
+      base64-js: 1.5.1
+
   js-tokens@4.0.0: {}
 
   js-yaml@4.1.1:

src/main/agent-runtime/context-compactor.ts

@@ -8,8 +8,8 @@
  * throttle when the context approaches their window.
  *
  * This compactor keeps the hook pure and provider-free: no LLM
- * round-trip, no mutation, no IO. It estimates token count using a
- * 4-chars-per-token heuristic, keeps recent traffic verbatim, and
+ * round-trip, no mutation, no IO. It counts transcript text with a
+ * local tiktoken-compatible encoder, keeps recent traffic verbatim, and
  * collapses older messages into a single "context summary" user turn
  * that preserves the assistant's narrative plus tool-call headlines.
  *
@@ -19,6 +19,7 @@
  */
 
 import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import { getEncoding } from "js-tiktoken";
 
 /** Token-budget threshold that triggers compaction. */
 const COMPACTION_TRIGGER_TOKENS = 120_000;
@@ -28,17 +29,8 @@ const COMPACTION_TRIGGER_TOKENS = 120_000;
  * losing mid-stream state (tool-call pairs, user intent).
  */
 const RECENT_MESSAGE_TAIL = 20;
-/**
- * Conservative tokens-per-char estimator. The former 4 chars/token value
- * under-counts CJK traffic by roughly 2x — Chinese BPE tokenizers land
- * closer to 1.5-2 tokens per character, so a 120k-token gate would
- * silently trip only after the real context already blew past the
- * window. 2 chars/token is a deliberate over-estimate for latin text in
- * exchange for not under-triggering on Chinese agent runs. The proper
- * fix is a real tokenizer — pi-ai 0.67.68 does not export one yet, so
- * see docs/proposals/C3-real-tokenizer-followup.md for the upgrade path.
- */
-const CHARS_PER_TOKEN = 2;
+const CONTEXT_ENCODING = getEncoding("cl100k_base");
+const CHAT_MESSAGE_OVERHEAD_TOKENS = 4;
 /** Per-assistant-message summary cap to keep the compacted turn readable. */
 const MAX_SUMMARY_CHARS_PER_MESSAGE = 160;
 const MAX_TOOL_RESULT_LINES = 12;
@@ -150,7 +142,8 @@ function estimateTokens(messages: AgentMessage[]): number {
 
 function estimateMessageTokens(message: AgentMessage): number {
   const raw = extractTextForEstimate(message as MessageLike);
-  return Math.ceil(raw.length / CHARS_PER_TOKEN);
+  if (raw.length === 0) return CHAT_MESSAGE_OVERHEAD_TOKENS;
+  return CONTEXT_ENCODING.encode(raw).length + CHAT_MESSAGE_OVERHEAD_TOKENS;
 }
 
 function extractTextForEstimate(message: MessageLike): string {

tests/unit/context-compactor-tool-results.test.ts

@@ -30,6 +30,38 @@ function makeLongTranscript(olderFacts: AgentMessage[]): AgentMessage[] {
 }
 
 describe("context compactor tool result facts", () => {
+  it("uses tokenizer counts so long CJK transcripts compact before provider overflow", () => {
+    const messages: AgentMessage[] = [
+      {
+        role: "user",
+        content: [{ type: "text", text: "Start the Chinese market launch review." }],
+        timestamp: 1,
+      } as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "I will keep the plan concise." }],
+        timestamp: 2,
+      } as AgentMessage,
+      {
+        role: "user",
+        content: [{ type: "text", text: "请完成市场复盘，保留所有客户反馈与行动项。 ".repeat(8_000) }],
+        timestamp: 3,
+      } as AgentMessage,
+    ];
+    for (let i = 0; i < 42; i += 1) {
+      messages.push({
+        role: i % 2 === 0 ? "user" : "assistant",
+        content: [{ type: "text", text: `recent ${i}` }],
+        timestamp: 100 + i,
+      } as AgentMessage);
+    }
+
+    const outcome = compactMessagesIfNeeded(messages);
+
+    expect(outcome.changed).toBe(true);
+    expect(outcome.telemetry?.estimatedTokens).toBeGreaterThanOrEqual(120_000);
+  });
+
   it("preserves task, approval, message, status, decision, and error facts from tool results", () => {
     const messages = makeLongTranscript([
       {