[ranxianglei]

(ai: zai/glm-5.3)

Fixes #4185. Branch: 2026-10-22_clamp-forward-escape (base fe5ba591).

Root cause

The #3146 fresh-session escape hatch (src/server.ts, if (unknownBaseline && !growthArmed)) keyed on unknownBaseline = anonymous && session.stats.lastInputTokens <= 0. The anonymous conjunct is a #553 trigger-side concern; dsh/codex/claude-code lanes stamp their own session ids, so their FIRST over-headroom request skipped the gate, hit the final fail-fast (413), and noteLocalRefusal (#3846) pinned every retry — the session was dead from message one (2026-10-21 raccoon incident: ~227K calibrated estimate vs the 192001 headroom target while payload + clamped output budget fit the 256000 native window).

Fix — three-arm arbitrate gate

const deterministicExhaustion = f?.kind === "exhausted" && f.deterministic;
const noUpstreamEvidence = session.stats.lastInputTokens <= 0 && statedWall <= 0 && deterministicExhaustion;
// outbound budget read from the prepared (already-clamped) body
const nativeFitAfterClamp = statedWall <= 0 && outboundBudgetTokens !== undefined
    && resolvedNativeWindow > 0 && outboundCalibrated + outboundBudgetTokens <= resolvedNativeWindow;
if ((unknownBaseline || noUpstreamEvidence || nativeFitAfterClamp) && !growthArmed) { /* #2490 doom → #3846 → forward once */ }

• unknownBaseline — original [Bug] 新会话（无 usage 基线）preflight 把字符数当 token——折叠其实成功（2263796→1111212）却 fail-fast 502，上游实际只计费 540828 #3146 anonymous arm, byte-for-byte unchanged.
• noUpstreamEvidence — extends "zero evidence → arbitrate" to IDENTIFIED fresh sessions, gated to DETERMINISTIC exhaustion (zero compressible ranges after maximum folding, 413 Deadlock: When an authenticated session exceeds the reservation window with zero collapsibles, the #3146 forwarding | 413 死锁:带身份会话超预留窗口且零可折叠时,#3146 转发门因 anonymous 条件不触发,会话从首条消息起永久不可用 #4185's shape). Relay-dependent failures (transient summary 503/429/524, empty/truncated summaries) keep the classic 502 fail-fast — the walk may recover, and forwarding then would hammer the upstream (fix: honor anthropic-beta context-1m to size the context window #301/[Bug] Codex 额度中断后恢复长会话，preflight SSE 摘要为空并持续 502 #726).
• nativeFitAfterClamp — regardless of baseline: the outbound body is already plugin mode: no hard backstop when the agent ignores the compress nudge — context grows past the window and upstream 400s #453/clampOutgoingOutput sizes max_tokens against the raw native window while preflight enforces the reserve-adjusted one — up to 25% of slack the clamp never sees #2096-clamped in prepare, so when outbound view + declared output budget fit the NATIVE window, forwarding is not a guaranteed-400; the headroom reservation is policy, not physics. Closed when statedWall > 0 (upstream already stated its wall) — no loop.

All arms keep the #2490 doomed-bytes 413, the #3846 local-refusal pin, and the growthArmed breaker.

Evidence

• Red on master: test 1 of tests/issue4185-identified-fresh-forward.test.ts reproduces the incident 413 ("exceeds the model window … NOT forwarded") with the exact raccoon shape (identified session, giant protected tool result, fresh baseline); green with this patch (forwarded once, usage baseline settles).
• 4 new tests: fresh-identified forward (arm2), armed-baseline forward with clamped max_tokens === 4096 (arm3), doomed-bytes still 413, genuinely-over-native still 413.
• No regression: preflight-empty-summary 9/9, preflight-stream-summary-config 6/6, wire-exit-gap-cells 5/5, preflight-fork-estimate ([Bug] 新会话（无 usage 基线）preflight 把字符数当 token——折叠其实成功（2263796→1111212）却 fail-fast 502，上游实际只计费 540828 #3146) 2/2, codex e2e green (ACP_TEST_E2E=1).
• Earlier revision (arm2 ungated) broke 8 relay-failure fail-fast tests; caught by re-running the full local matrix on both this tree and pristine master — fixed by the deterministicExhaustion gate above.

Non-goals

• 413 message wording (窗口溢出报错 #736/fix: name the operator-shrunk window in preflight fail-fast errors #737), thinking-model budget burn (【bug】空轮重试门闸把 reasoning 记成可见正文 —— Responses 协议下 item.content[] 带 reasoning 的退化轮永不重试（客户端拿到空轮） #2756/[Bug][DSH] 长会话切换自定义模型后输出预算恢复到 249 token，连续 thinking-only / max-tokens（#1840 零上限来源场景） #2904), cold-load re-fold cost (v0.1.198 升级后老会话丢失压缩状态:197 折叠过的会话以 117 万 token 原文重放,预压缩超过客户端耐心,老会话看起来「炸了」 #3573).
• Master test lanes are currently red from [PR] Reserve output headroom on every wire (drop the Anthropic exemption) #4184 ([BUG] acp-proxy upstream HTTP 400: Request with 1,198,269 tokens exceeds the model's 1,048,576 token limit, consistently | [BUG] acp-proxy upstream HTTP 400：请求 1,198,269 tokens 超过模型上限 1,048,576，一直复现——是插件压缩未生效，还是 DSH 的问题？ #3900) — tracked separately in master test lanes red after #4184 (#3900 always-reserve-headroom): stale expectations across codex-window-align + side-request-isolation (+3 more) #4194; the remaining test-lane failures on this PR are that inherited set, not from this change.

[github-actions]

📦 Built Package Artifact

Branch: 2026-10-22_clamp-forward-escape (4f65ae3)

Option A — Install this exact build (recommended)

npm install -g [email protected]

This exact version is immutable and reproducible. The rolling pr tag tracks the newest test build across all PRs.

Option B — Download artifact

1. Download the artifact from the Actions run
2. Extract the tarball and install:

tar xzf billion-context-pr4188.tgz
npm install -g package

---

Install instructions are refreshed on each push.

[github-actions]

📦 Merged Build Published

This merge to master was published as 0.0.736-master.399c8bc (rolling master tag).

npm install -g [email protected]  # this exact build
npm install -g billion-context@master  # always the newest merged build

npm view billion-context@master prints the current version at any time.