fix(code-runtime-python): clear stray buffers on truncation and drain the original std streams

Addresses the review's two carried warnings and the comment suggestion:
- Once the ledger truncates, every arm that marks it (admit()'s two ceilings and
  the child-marker frame arm) now clears both stray pipes' buffered output
  wholesale, so the end-path flushStray sees empty buffers instead of
  concat+decoding doomed data near a 256 MiB maxLogBytes; captureStray's newline
  loop re-checks the flag before re-retaining the residual.
- The child runs with -u (unbuffered), so sys.__stdout__/sys.__stderr__ writes
  are visible to stray capture immediately; the settlement flush still drains
  the original std streams before the done frame as a guard. A regression test
  writes through sys.__stdout__/sys.__stderr__ without an explicit flush and
  asserts both bytes land in logs. C-ext stdio remains an accepted residual,
  recorded in the README Known Limitations (en + zh).
- The ledger-comment arithmetic now states the exact boundary (serializes to
  exactly maxLogBytes; without the reserved byte it would be maxLogBytes + 1)
  in both host and child.
Note (en + zh) registers the stray-clear and -u/settlement-drain mechanisms and
the new test; pairings re-recorded; corpus passes 1029.

.agents/notes/implemented/bug-fix/2026-07-31-code-runtime-python-settlement-fixes.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-07-31-code-runtime-python-settlement-fixes.md
-2026-07-31-code-runtime-python-settlement-fixes.md: 161a21baede3074bb84c6e17c206d3baa1aa0582
-2026-07-31-code-runtime-python-settlement-fixes.zh.md: ecd52edb80cd03b4dd061ade858e35041cbd0309
+2026-07-31-code-runtime-python-settlement-fixes.md: afc9a0fc51a6e17fd71b2fda7f8f5ee2b783090d
+2026-07-31-code-runtime-python-settlement-fixes.zh.md: 7f83a50413a3a76dc80337adb09d5113d07bdc55

.agents/notes/implemented/bug-fix/2026-07-31-code-runtime-python-settlement-fixes.md

@@ -74,7 +74,7 @@ One residual write-path copy is fixed alongside, independent of the config gate:
 
 ### The completion value and error are pre-encoded at their validation point
 
-In [`py/bootstrap.py`](../../../../packages/code-runtime/code-runtime-python/py/bootstrap.py), `_done_with_value` now returns the whole terminal frame as a PRE-ENCODED JSON string on the success path: the admitted value is serialized once here, at the validation point inside `_run`'s `try`, as `'{"type": "done", "value": ' + _encode_json_plain(value) + "}"`. The program can keep mutating a returned list/dict from a daemon thread or signal handler after it returns, so a second traversal held at a later point would be a TOCTOU — a concurrent mutation into a non-JSON type would let that later encode throw outside the settlement handler and downgrade a settled run host-side to `worker-exit`. Serializing once, inside the `try` that wraps this call, closes the window: if a concurrent mutation makes the encode throw, the exception handler classifies it as an `exception`, and once the string is produced the frame is written verbatim with no further touching of the live value. `_run` binds the `_done_with_value` ENTRY NAME into a local (`done_with_value_bound`) before the program runs, and `_done_with_value` itself binds `_check_done_value` and `_encode_json_plain` as DEF-TIME default arguments — so a `__main__` rebind of the entry name or those two names after model execution cannot rewrite a legitimate success into an `exception`. The log ledgers (host `logBudget` and child `_remaining`) start ONE byte below the budget, reserving the serialized outer-array envelope (two brackets and n-1 commas over n entries' separators), so a result that exactly exhausts the ledger still serializes within the configured cap; the truncation marker remains envelope, not payload. The constructor rejects a `maxLogBytes` below 64 (the smallest budget with one byte of room for the truncation marker's own serialized form); `maxValueBytes` keeps only the positive-integer requirement, since a completion can be a single byte and the done-frame envelope is seam protocol cost. The marker remains envelope, so a truncated run with admitted entries serializes to at most `maxLogBytes + marker + envelope` (recorded in the package README). A rebind of a transitive dep the encoder reaches (e.g. `_dump_scalar`/`_dump_string`/`json`/`io` — a non-exhaustive set) can still make the encode throw and downgrade a success to an `exception`, which is registered as an accepted residual in the package README.
+In [`py/bootstrap.py`](../../../../packages/code-runtime/code-runtime-python/py/bootstrap.py), `_done_with_value` now returns the whole terminal frame as a PRE-ENCODED JSON string on the success path: the admitted value is serialized once here, at the validation point inside `_run`'s `try`, as `'{"type": "done", "value": ' + _encode_json_plain(value) + "}"`. The program can keep mutating a returned list/dict from a daemon thread or signal handler after it returns, so a second traversal held at a later point would be a TOCTOU — a concurrent mutation into a non-JSON type would let that later encode throw outside the settlement handler and downgrade a settled run host-side to `worker-exit`. Serializing once, inside the `try` that wraps this call, closes the window: if a concurrent mutation makes the encode throw, the exception handler classifies it as an `exception`, and once the string is produced the frame is written verbatim with no further touching of the live value. `_run` binds the `_done_with_value` ENTRY NAME into a local (`done_with_value_bound`) before the program runs, and `_done_with_value` itself binds `_check_done_value` and `_encode_json_plain` as DEF-TIME default arguments — so a `__main__` rebind of the entry name or those two names after model execution cannot rewrite a legitimate success into an `exception`. The log ledgers (host `logBudget` and child `_remaining`) start ONE byte below the budget, reserving the serialized outer-array envelope (two brackets and n-1 commas over n entries' separators), so a result that exactly exhausts the ledger still serializes within the configured cap; the truncation marker remains envelope, not payload. Once a ledger has truncated, the host clears both stray pipes' buffered output wholesale (every later byte would be no-op'd by `admit`, so retaining it would spend host memory on output that can never be admitted); the child runs `-u` so `sys.__stdout__`/`sys.__stderr__` writes are visible to stray capture immediately, and the settlement flush still drains the original std streams before the done frame (a guard against a buffered wrapper surviving a `sys.__stdout__ = boom` rebind). The constructor rejects a `maxLogBytes` below 64 (the smallest budget with one byte of room for the truncation marker's own serialized form); `maxValueBytes` keeps only the positive-integer requirement, since a completion can be a single byte and the done-frame envelope is seam protocol cost. The marker remains envelope, so a truncated run with admitted entries serializes to at most `maxLogBytes + marker + envelope` (recorded in the package README). A rebind of a transitive dep the encoder reaches (e.g. `_dump_scalar`/`_dump_string`/`json`/`io` — a non-exhaustive set) can still make the encode throw and downgrade a success to an `exception`, which is registered as an accepted residual in the package README.
 
 `send_done` (a local function inside `_run`) writes the pre-encoded string through a BOUND `channel.write_encoded`, and encodes a dict error frame through a bound `_encode_json_plain` before writing it — it never calls `channel.send_sync`, whose body re-resolves `self.write_encoded` and the module-level `_encode_json_plain` at call time. `_encode_json_plain` and `channel.write_encoded` are bound into locals before the program runs, for the same reason `flush_out`/`flush_err`/`safe_model_traceback` are: the program runs as `__main__`, so `import __main__; __main__.ProtocolChannel.send_sync = boom` or `__main__._encode_json_plain = boom` would otherwise re-resolve the send/encode to a rebranded callable at call time and, when that replacement raises, skip the `done` frame and downgrade a settled verdict to a host-side `worker-exit`.
 
@@ -98,7 +98,7 @@ In [`py/bootstrap.py`](../../../../packages/code-runtime/code-runtime-python/py/
 
 - `tests/boot-write-failure.spec.ts` mocks `spawn` so the fd-3 pipe throws on the boot write — the one path a real subprocess cannot be coerced into — and asserts `run()` resolves a `worker-exit` rather than rejecting. A sibling case makes the mocked `spawn` throw SYNCHRONOUSLY and asserts `run()` still resolves a `worker-exit` and removes its staging directory, keyed off the exact bootstrap path the mocked `spawn` received in its argv so a sibling worker's concurrent staging cannot flake it. Both are isolated in this spec so the real-subprocess suite is untouched.
 - `tests/residual-detach.spec.ts` unit-tests `detachResidual`: the carried copy equals the residual, owns a backing store sized to its own length (fixture kept above Node's Buffer pool threshold), and does not share the source frame's `ArrayBuffer`.
-- `tests/runtime.spec.ts` — the output-cap case asserts the `ceiling - envelope` bound (268435392) and its message. A daemon-thread case drives four threads emitting unterminated writes through settlement's flush. A native-write case writes 200 KiB with no newline via `os.write` under a raised `maxLogBytes` and asserts it reads back as EXACTLY one log entry (proving stray output is aggregated by line, not split at pipe-chunk boundaries); a companion writes `b"one\ntwo\nthree"` and asserts three entries (proving real newlines still delimit). A newline-free-flood case writes 2 MiB under a 4 KiB `maxLogBytes` and asserts the capture ends at the truncation marker and stays under budget (proving the residual is bounded by the ledger, not buffered whole); a NUL-flood companion writes 4000 newline-free NULs under the same budget and asserts truncation (proving the residual is charged by SERIALIZED cost, ~6× raw, measured without allocating the escaped copy); an illegal-UTF-8 case paces single-byte `\xff` writes under a 3072-byte budget with `Buffer.concat` wrapped to measure the peak merged buffer, asserting it stays under 2048 (charged at the U+FFFD width 3 the residual flushes near 1024 raw bytes; a raw-byte undercount would let it reach ~3072, so the bound discriminates); a CESU-8/overlong case paces the structurally-well-formed but illegal `ED A0 80` one byte at a time and asserts the same peak bound (charged at the true 9 per sequence it flushes early; charging the structural width 3 triples the peak, so reverting the per-lead range check turns it red); a broken-multibyte case writes a 3-byte lead then a fresh ASCII byte in separate chunks and asserts both a captured `A` and a U+FFFD (exercising `accrueStrayCost`'s cross-chunk broken-sequence branch); a post-truncation case writes a 108-byte payload (under the smallest PIPE_BUF, so one atomic write) whose first line exhausts a 64-byte budget and asserts the second line is dropped (exercising the post-truncation admit no-op in one `data` callback, no v8-ignore); a short-escape case writes a line mixing a tab, quote, backslash, a `\uXXXX` control, a multibyte character, and ASCII, asserting it round-trips verbatim (exercising every branch of `jsonStringCostUpTo`); a reassembly case writes a payload spanning every valid multibyte lead class (E0-range, plain 3-byte, F0, and F4) past the pipe buffer and asserts it round-trips with no U+FFFD (exercising `accrueStrayCost`'s per-lead ranges and cross-chunk reassembly); a lone-surrogate case forges an fd-3 `log` frame flooding 1000 `\ud800` escapes under a 4 KiB budget and asserts truncation (the count sits in the window where charging 3 bytes would admit and 6 bytes truncates, proving the surrogate is charged its full escaped width); a stray-sealing case paces 60000 single-byte newline-free `os.write(1, …)` calls under a raised budget with `Buffer.concat` wrapped to measure copy volume, asserting the trickle coalesces to one entry and the cumulative copy stays under a measured 256 KiB threshold (the sealed shape copies ~120 KB, the re-merge shape ~538 KB, so reverting the seal to a re-merge turns the assertion red — proving the fragment list seals into blocks past `MAX_PENDING_CHUNKS`). A closeDeadline-flush case has the leader write a newline-free diagnostic then spawn a `setsid` orphan holding the pipes open, and asserts the diagnostic survives in `logs` (proving the residual is flushed before the deadline destroys the streams). The same-group reap case spawns a SIGTERM-ignoring same-group descendant that releases the pipes and bumps a heartbeat file; the test asserts the heartbeat STOPS after the grace-window SIGKILL — an assertion robust whether the killed descendant is reaped or lingers as a zombie, so it holds where PID 1 does not wait() orphans. A dispose-after-resolve case asserts `dispose()` of a completed run with a same-group survivor returns only after the survivor stops executing (proving the run stays in `live` until its group is reaped), with an `expect(afterDispose).toBeGreaterThan(0)` guard so the frozen-heartbeat assertion cannot pass vacuously when the file was never written. A deadline case busy-blocks the event loop past both timers and asserts the survivor's heartbeat freezes (proving the poll's deadline arm sends SIGKILL itself rather than cancelling the unfired escalation). The cross-loop case runs a binding from a worker thread's own `asyncio.run` loop while the main coroutine yields with `await asyncio.sleep`, asserting the reply round-trips instead of timing out; a companion case abandons a thread's call so its loop closes, then answers it before a later binding — asserting the pump survives the closed-loop `call_soon_threadsafe` (host-gated ordering makes it deterministic, fail-before hangs the later binding to the wall clock). The inherited-soft-limit case runs the interpreter through a `ulimit -S -t` wrapper that sets a CPU soft limit below `cpuSeconds` and asserts the applieLine truncated
+- `tests/runtime.spec.ts` — the output-cap case asserts the `ceiling - envelope` bound (268435392) and its message. A daemon-thread case drives four threads emitting unterminated writes through settlement's flush. A native-write case writes 200 KiB with no newline via `os.write` under a raised `maxLogBytes` and asserts it reads back as EXACTLY one log entry (proving stray output is aggregated by line, not split at pipe-chunk boundaries); a companion writes `b"one\ntwo\nthree"` and asserts three entries (proving real newlines still delimit). A newline-free-flood case writes 2 MiB under a 4 KiB `maxLogBytes` and asserts the capture ends at the truncation marker and stays under budget (proving the residual is bounded by the ledger, not buffered whole); a NUL-flood companion writes 4000 newline-free NULs under the same budget and asserts truncation (proving the residual is charged by SERIALIZED cost, ~6× raw, measured without allocating the escaped copy); an illegal-UTF-8 case paces single-byte `\xff` writes under a 3072-byte budget with `Buffer.concat` wrapped to measure the peak merged buffer, asserting it stays under 2048 (charged at the U+FFFD width 3 the residual flushes near 1024 raw bytes; a raw-byte undercount would let it reach ~3072, so the bound discriminates); a CESU-8/overlong case paces the structurally-well-formed but illegal `ED A0 80` one byte at a time and asserts the same peak bound (charged at the true 9 per sequence it flushes early; charging the structural width 3 triples the peak, so reverting the per-lead range check turns it red); a broken-multibyte case writes a 3-byte lead then a fresh ASCII byte in separate chunks and asserts both a captured `A` and a U+FFFD (exercising `accrueStrayCost`'s cross-chunk broken-sequence branch); a post-truncation case writes a 108-byte payload (under the smallest PIPE_BUF, so one atomic write) whose first line exhausts a 64-byte budget and asserts the second line is dropped (exercising the post-truncation admit no-op in one `data` callback, no v8-ignore); a short-escape case writes a line mixing a tab, quote, backslash, a `\uXXXX` control, a multibyte character, and ASCII, asserting it round-trips verbatim (exercising every branch of `jsonStringCostUpTo`); a reassembly case writes a payload spanning every valid multibyte lead class (E0-range, plain 3-byte, F0, and F4) past the pipe buffer and asserts it round-trips with no U+FFFD (exercising `accrueStrayCost`'s per-lead ranges and cross-chunk reassembly); a lone-surrogate case forges an fd-3 `log` frame flooding 1000 `\ud800` escapes under a 4 KiB budget and asserts truncation (the count sits in the window where charging 3 bytes would admit and 6 bytes truncates, proving the surrogate is charged its full escaped width); a stray-sealing case paces 60000 single-byte newline-free `os.write(1, …)` calls under a raised budget with `Buffer.concat` wrapped to measure copy volume, asserting the trickle coalesces to one entry and the cumulative copy stays under a measured 256 KiB threshold (the sealed shape copies ~120 KB, the re-merge shape ~538 KB, so reverting the seal to a re-merge turns the assertion red — proving the fragment list seals into blocks past `MAX_PENDING_CHUNKS`). A closeDeadline-flush case has the leader write a newline-free diagnostic then spawn a `setsid` orphan holding the pipes open, and asserts the diagnostic survives in `logs` (proving the residual is flushed before the deadline destroys the streams). The same-group reap case spawns a SIGTERM-ignoring same-group descendant that releases the pipes and bumps a heartbeat file; the test asserts the heartbeat STOPS after the grace-window SIGKILL — an assertion robust whether the killed descendant is reaped or lingers as a zombie, so it holds where PID 1 does not wait() orphans. A dispose-after-resolve case asserts `dispose()` of a completed run with a same-group survivor returns only after the survivor stops executing (proving the run stays in `live` until its group is reaped), with an `expect(afterDispose).toBeGreaterThan(0)` guard so the frozen-heartbeat assertion cannot pass vacuously when the file was never written. A deadline case busy-blocks the event loop past both timers and asserts the survivor's heartbeat freezes (proving the poll's deadline arm sends SIGKILL itself rather than cancelling the unfired escalation). The cross-loop case runs a binding from a worker thread's own `asyncio.run` loop while the main coroutine yields with `await asyncio.sleep`, asserting the reply round-trips instead of timing out; a companion case abandons a thread's call so its loop closes, then answers it before a later binding — asserting the pump survives the closed-loop `call_soon_threadsafe` (host-gated ordering makes it deterministic, fail-before hangs the later binding to the wall clock). The inherited-soft-limit case runs the interpreter through a `ulimit -S -t` wrapper that sets a CPU soft limit below `cpuSeconds` and asserts the applieLine truncated
 
 ## Alternatives considered
 

.agents/notes/implemented/bug-fix/2026-07-31-code-runtime-python-settlement-fixes.zh.md

@@ -74,7 +74,7 @@ Status: implemented
 
 ### 完成值与错误在其校验点处预编码
 
-在 [`py/bootstrap.py`](../../../../packages/code-runtime/code-runtime-python/py/bootstrap.py) 中，`_done_with_value` 现在会在成功路径上把整个终止帧作为一个已预编码的 JSON 字符串返回：被准入的值在这里、在 `_run` 的 `try` 之内的校验点处恰好序列化一次，即 `'{"type": "done", "value": ' + _encode_json_plain(value) + "}"`。程序在返回之后仍可能从 daemon 线程或信号处理器继续变异它返回的 list／dict，因此在一个更晚的点上做第二次遍历会构成一次 TOCTOU——如果一次变异让一个并发变异的后续编码在结算处理器之外抛出，就会把一次已结算的运行在宿主侧降级成 `worker-exit`。在这里、在包裹该调用的 `try` 之内恰好序列化一次，就关上了这个窗口：如果一次并发变异导致编码抛出，异常处理器会把它如实分类为 `exception`；一旦字符串产生出来，该帧就会被逐字写走、不再触碰任何活对象。`_run` 在程序运行前把 `_done_with_value` 的入口名绑成局部（`done_with_value_bound`），而 `_done_with_value` 自身把 `_check_done_value` 与 `_encode_json_plain` 绑定为 def 期默认参数——因此模型执行后对入口名或这两个名字的 `__main__` 重绑无法把一个合法成功改写为 `exception`。日志账本（宿主 `logBudget` 与子进程 `_remaining`）从预算低 1 字节起算，预留序列化外层数组的外壳（两条括号与 n-1 个逗号，覆盖 n 条目的分隔符），因此恰好耗尽账本的结果序列化后仍在配置上限之内。构造器拒绝低于 64 的 `maxLogBytes`（能为截断标记自身序列化形式留出一字节余量的最小预算）；`maxValueBytes` 只保留正整数要求，因为完成值可以只有一字节、且 done 帧外壳是 seam 协议成本。标记仍是 envelope，因此带已放行条目的截断运行序列化后至多为 `maxLogBytes + marker + envelope`（已记录在包 README）。但编码器到达的一个传递依赖（例如 `_dump_scalar`/`_dump_string`/`json`/`io`——非穷举清单）重绑仍可让编码抛出、把成功降级为 `exception`，这在包 README 中被登记为已接受残余。
+在 [`py/bootstrap.py`](../../../../packages/code-runtime/code-runtime-python/py/bootstrap.py) 中，`_done_with_value` 现在会在成功路径上把整个终止帧作为一个已预编码的 JSON 字符串返回：被准入的值在这里、在 `_run` 的 `try` 之内的校验点处恰好序列化一次，即 `'{"type": "done", "value": ' + _encode_json_plain(value) + "}"`。程序在返回之后仍可能从 daemon 线程或信号处理器继续变异它返回的 list／dict，因此在一个更晚的点上做第二次遍历会构成一次 TOCTOU——如果一次变异让一个并发变异的后续编码在结算处理器之外抛出，就会把一次已结算的运行在宿主侧降级成 `worker-exit`。在这里、在包裹该调用的 `try` 之内恰好序列化一次，就关上了这个窗口：如果一次并发变异导致编码抛出，异常处理器会把它如实分类为 `exception`；一旦字符串产生出来，该帧就会被逐字写走、不再触碰任何活对象。`_run` 在程序运行前把 `_done_with_value` 的入口名绑成局部（`done_with_value_bound`），而 `_done_with_value` 自身把 `_check_done_value` 与 `_encode_json_plain` 绑定为 def 期默认参数——因此模型执行后对入口名或这两个名字的 `__main__` 重绑无法把一个合法成功改写为 `exception`。日志账本（宿主 `logBudget` 与子进程 `_remaining`）从预算低 1 字节起算，预留序列化外层数组的外壳（两条括号与 n-1 个逗号，覆盖 n 条目的分隔符），因此恰好耗尽账本的结果序列化后仍在配置上限之内。 一旦账本已截断，宿主会整体清空两条 stray 管道的缓冲输出（之后的每个字节都会被 `admit` 变成 no-op，保留它只会把宿主内存花在永远无法准入的输出上）；子进程以 `-u` 运行，使 `sys.__stdout__`/`sys.__stderr__` 的写入对 stray 捕获立即可见，而结算 flush 仍在 done 帧前排空原始 std 流（防御 `sys.__stdout__ = boom` 重绑后残留的缓冲包装）。构造器拒绝低于 64 的 `maxLogBytes`（能为截断标记自身序列化形式留出一字节余量的最小预算）；`maxValueBytes` 只保留正整数要求，因为完成值可以只有一字节、且 done 帧外壳是 seam 协议成本。标记仍是 envelope，因此带已放行条目的截断运行序列化后至多为 `maxLogBytes + marker + envelope`（已记录在包 README）。但编码器到达的一个传递依赖（例如 `_dump_scalar`/`_dump_string`/`json`/`io`——非穷举清单）重绑仍可让编码抛出、把成功降级为 `exception`，这在包 README 中被登记为已接受残余。
 
 `send_done`（`_run` 内部的一个局部函数）通过绑定的 `channel.write_encoded` 写出已预编码的字符串，并在写之前用绑定的 `_encode_json_plain` 编码一个 dict 错误帧——它绝不经 `channel.send_sync`，因为后者的函数体会在调用时刻重新解析 `self.write_encoded` 和模块级的 `_encode_json_plain`。`_encode_json_plain` 与 `channel.write_encoded` 在程序运行前就被绑定进局部变量，理由与 `flush_out`／`flush_err`／`safe_model_traceback` 被绑定相同：程序以 `__main__` 运行，因此 `import __main__; __main__.ProtocolChannel.send_sync = boom` 或 `__main__._encode_json_plain = boom` 本会在调用时刻把发送／编码重新解析成被替换的可调用对象，当该替换抛出时跳过 `done` 帧、把已结算的结论降级成宿主侧的 `worker-exit`。
 
@@ -98,7 +98,7 @@ Status: implemented
 
 - `tests/boot-write-failure.spec.ts` 对 `spawn` 做 mock，使 fd-3 管道在引导写入时抛出异常（这是真实子进程无法被迫进入的唯一路径），并断言 `run()` resolve 出一个 `worker-exit` 而非 reject。一个同级用例让被 mock 的 `spawn` 同步抛出，并断言 `run()` 仍然 resolve 出一个 `worker-exit`，且会移除它的暂存目录——以被 mock 的 `spawn` 在其 argv 中收到的确切引导路径为准，因此一个同级 worker 的并发暂存不会让它变得不稳定。两者都被隔离在这个 spec 中，因此真实子进程测试套件不受影响。
 - `tests/residual-detach.spec.ts` 对 `detachResidual` 做单元测试：向前传递的副本与残余数据相等、拥有一个大小与其自身长度一致的底层存储（fixture 保持在 Node 的 Buffer 池阈值之上），并且不与源帧的 `ArrayBuffer` 共享。
-- `tests/runtime.spec.ts`：output-cap 用例断言 `ceiling - envelope` 上界（268435392）及其消息。一个 daemon 线程用例驱动四个线程穿过结算的 flush 发出未结束的写入。一个 native-write 用例在抬高后的 `maxLogBytes` 之下，通过 `os.write` 写入 200 KiB 且不含换行符，断言它回读时恰好是一条日志条目（证明散逸输出是按行聚合的，而不是在管道分片边界处被切开）；一个配套用例写入 `b"one\ntwo\nthree"`，断言得到三条条目（证明真正的换行符仍然起分隔作用）。一个 newline-free-flood 用例在一个 4 KiB 的 `maxLogBytes` 之下写入 2 MiB，断言捕获终止于截断标记且保持在预算之内（证明残余数据受账本约束，而不是被整体缓冲）；一个 NUL-flood 配套用例在同一预算之下写入 4000 个不含换行符的 NUL，断言发生截断（证明残余数据是按序列化开销计费的，约为原始的 6 倍，且在度量时不分配转义后的副本）；一个 illegal-UTF-8 用例在一个 3072 字节的预算之下控速发出单字节 `\xff` 写入，并对 `Buffer.concat` 做包装以度量峰值合并缓冲区，断言它保持在 2048 之下（按 U+FFFD 宽度 3 计费时残余数据在约 1024 原始字节处冲刷；一次原始字节的少计会让它达到约 3072，因此该界限具有区分力）；一个 CESU-8/overlong 用例把结构良构但非法的 `ED A0 80` 一次一个字节地控速发出，断言同样的峰值界限（按每序列真实的 9 计费时它提前冲刷；按结构宽度 3 计费会使峰值增至三倍，因此把逐前导字节范围检查回退会使它变红）；一个 broken-multibyte 用例在分开的分片里先写入一个 3 字节的前导字节、再写入一个新的 ASCII 字节，断言同时捕获到一个 `A` 和一个 U+FFFD（覆盖 `accrueStrayCost` 的跨分片断裂序列分支）；一个 post-truncation 用例写入一个 108 字节的载荷（小于最小的 PIPE_BUF，因此是一次原子写入），其首行耗尽一个 64 字节的预算，断言第二行被丢弃（覆盖单次 `data` 回调中的截断后准入空操作，无需 v8-ignore）；一个 short-escape 用例写入一行混合了制表符、引号、反斜杠、一个 `\uXXXX` 控制字符、一个多字节字符和 ASCII 的内容，断言它原样完成往返（覆盖 `jsonStringCostUpTo` 的每一条分支）；一个 reassembly 用例写入一个跨越每个合法多字节前导字节类别（E0 范围、普通 3 字节、F0 和 F4）、越过管道缓冲区的载荷，断言它原样完成往返且不含 U+FFFD（覆盖 `accrueStrayCost` 的逐前导字节范围与跨分片重组）；一个 lone-surrogate 用例在一个 4 KiB 预算之下伪造一个以 1000 个 `\ud800` 转义洪泛的 fd-3 `log` 帧，断言发生截断（该计数正落在计 3 字节会放行、计 6 字节则截断的窗口内，证明该代理项是按其完整转义宽度计费的）；一个 stray-sealing 用例在抬高后的预算之下控速发出 60000 次单字节、不含换行符的 `os.write(1, …)` 调用，并对 `Buffer.concat` 做包装以度量复制量，断言这股细流合并为一条条目、且累积复制量保持在一个实测的 256 KiB 阈值之下（封存后的形态复制约 120 KB，重新合并的形态复制约 538 KB，因此把封存回退成重新合并会使该断言变红——证明分片列表在越过 `MAX_PENDING_CHUNKS` 后封存为块）。一个 closeDeadline-flush 用例让 leader 写入一段不含换行符的诊断，随后 spawn 一个持有管道不放的 `setsid` 孤儿进程，断言该诊断在 `logs` 中存留下来（证明残余数据在截止时间销毁流之前被冲刷）。same-group 回收用例 spawn 一个忽略 SIGTERM 的同进程组后代，它释放管道并递增一个心跳文件；该测试断言在宽限窗口的 SIGKILL 之后心跳停止：无论被杀死的后代是被回收还是作为僵尸进程滞留，这个断言都成立，因此它在 PID 1 不 wait() 孤儿进程的环境下同样成立。一个 dispose-after-resolve 用例断言，对一个已完成、且存在同进程组存活者的运行调用 `dispose()`，只有在该存活者停止执行之后才返回（证明该运行会一直留在 `live` 中，直到它的进程组被回收），并带有一个 `expect(afterDispose).toBeGreaterThan(0)` 守卫，使得当心跳文件从未被写入时，冻结心跳的断言不会被空洞地通过。一个 deadline 用例忙阻塞事件循环越过两个定时器，断言该存活者的心跳冻结（证明轮询的截止时间分支自身发送 SIGKILL，而不是取消尚未触发的升级）。cross-loop 用例在主协程通过 `await asyncio.sleep` 让出时，从一个工作线程自己的 `asyncio.run` 事件循环运行一个绑定，断言该回复完成往返而不是超时；一个配套用例放弃某个线程的调用，使其事件循环关闭，Line truncated
+- `tests/runtime.spec.ts`：output-cap 用例断言 `ceiling - envelope` 上界（268435392）及其消息。一个 daemon 线程用例驱动四个线程穿过结算的 flush 发出未结束的写入。一个 native-write 用例在抬高后的 `maxLogBytes` 之下，通过 `os.write` 写入 200 KiB 且不含换行符，断言它回读时恰好是一条日志条目（证明散逸输出是按行聚合的，而不是在管道分片边界处被切开）；一个配套用例写入 `b"one\ntwo\nthree"`，断言得到三条条目（证明真正的换行符仍然起分隔作用）。一个 newline-free-flood 用例在一个 4 KiB 的 `maxLogBytes` 之下写入 2 MiB，断言捕获终止于截断标记且保持在预算之内（证明残余数据受账本约束，而不是被整体缓冲）；一个 NUL-flood 配套用例在同一预算之下写入 4000 个不含换行符的 NUL，断言发生截断（证明残余数据是按序列化开销计费的，约为原始的 6 倍，且在度量时不分配转义后的副本）；一个 illegal-UTF-8 用例在一个 3072 字节的预算之下控速发出单字节 `\xff` 写入，并对 `Buffer.concat` 做包装以度量峰值合并缓冲区，断言它保持在 2048 之下（按 U+FFFD 宽度 3 计费时残余数据在约 1024 原始字节处冲刷；一次原始字节的少计会让它达到约 3072，因此该界限具有区分力）；一个 CESU-8/overlong 用例把结构良构但非法的 `ED A0 80` 一次一个字节地控速发出，断言同样的峰值界限（按每序列真实的 9 计费时它提前冲刷；按结构宽度 3 计费会使峰值增至三倍，因此把逐前导字节范围检查回退会使它变红）；一个 broken-multibyte 用例在分开的分片里先写入一个 3 字节的前导字节、再写入一个新的 ASCII 字节，断言同时捕获到一个 `A` 和一个 U+FFFD（覆盖 `accrueStrayCost` 的跨分片断裂序列分支）；一个 post-truncation 用例写入一个 108 字节的载荷（小于最小的 PIPE_BUF，因此是一次原子写入），其首行耗尽一个 64 字节的预算，断言第二行被丢弃（覆盖单次 `data` 回调中的截断后准入空操作，无需 v8-ignore）；一个 short-escape 用例写入一行混合了制表符、引号、反斜杠、一个 `\uXXXX` 控制字符、一个多字节字符和 ASCII 的内容，断言它原样完成往返（覆盖 `jsonStringCostUpTo` 的每一条分支）；一个 reassembly 用例写入一个跨越每个合法多字节前导字节类别（E0 范围、普通 3 字节、F0 和 F4）、越过管道缓冲区的载荷，断言它原样完成往返且不含 U+FFFD（覆盖 `accrueStrayCost` 的逐前导字节范围与跨分片重组）；一个 lone-surrogate 用例在一个 4 KiB 预算之下伪造一个以 1000 个 `\ud800` 转义洪泛的 fd-3 `log` 帧，断言发生截断（该计数正落在计 3 字节会放行、计 6 字节则截断的窗口内，证明该代理项是按其完整转义宽度计费的）；一个 stray-sealing 用例在抬高后的预算之下控速发出 60000 次单字节、不含换行符的 `os.write(1, …)` 调用，并对 `Buffer.concat` 做包装以度量复制量，断言这股细流合并为一条条目、且累积复制量保持在一个实测的 256 KiB 阈值之下（封存后的形态复制约 120 KB，重新合并的形态复制约 538 KB，因此把封存回退成重新合并会使该断言变红——证明分片列表在越过 `MAX_PENDING_CHUNKS` 后封存为块）。一个 closeDeadline-flush 用例让 leader 写入一段不含换行符的诊断，随后 spawn 一个持有管道不放的 `setsid` 孤儿进程，断言该诊断在 `logs` 中存留下来（证明残余数据在截止时间销毁流之前被冲刷）。same-group 回收用例 spawn 一个忽略 SIGTERM 的同进程组后代，它释放管道并递增一个心跳文件；该测试断言在宽限窗口的 SIGKILL 之后心跳停止：无论被杀死的后代是被回收还是作为僵尸进程滞留，这个断言都成立，因此它在 PID 1 不 wait() 孤儿进程的环境下同样成立。一个 dispose-after-resolve 用例断言，对一个已完成、且存在同进程组存活者的运行调用 `dispose()`，只有在该存活者停止执行之后才返回（证明该运行会一直留在 `live` 中，直到它的进程组被回收），并带有一个 `expect(afterDispose).toBeGreaterThan(0)` 守卫，使得当心跳文件从未被写入时，冻结心跳的断言不会被空洞地通过。一个 deadline 用例忙阻塞事件循环越过两个定时器，断言该存活者的心跳冻结（证明轮询的截止时间分支自身发送 SIGKILL，而不是取消尚未触发的升级）。cross-loop 用例在主协程通过 `await asyncio.sleep` 让出时，从一个工作线程自己的 `asyncio.run` 事件循环运行一个绑定，断言该回复完成往返而不是超时；一个配套用例放弃某个线程的调用，使其事件循环关闭，Line truncated
 
 ## Alternatives considered
 

packages/code-runtime/code-runtime-python/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/code-runtime/code-runtime-python/README.md
-README.md: fbb37d36523d12357a30c319409afc69ce64e487
-README.zh.md: 10141bc634e523b1fa079280039b0e5eda7e45d8
+README.md: 25d56e1e611a04523d933a16784e51a91d689fa3
+README.zh.md: 978a875c815d33dcfadfd5114ee8b88dbde4fc92

packages/code-runtime/code-runtime-python/README.md

@@ -37,6 +37,8 @@ No direct invalidation; the named consumer owns any request-prefix changes.
 - **The cross-language guard covers executed values and frame field sets, not field types** — `tests/protocol-mirror.e2e.ts` compares `PROTOCOL_FD`, the log truncation marker, and each `TypedDict`'s required and optional fields against a real `python3`. Comparing field types across TypeScript and Python has no mechanical equivalent here, so review plus the backend's real-subprocess suite owns type-level drift.
 - **`RLIMIT_AS` is not enforced on macOS** — the dyld shared cache mapped into every process at exec exceeds any practical address-space cap, and the kernel rejects the `setrlimit` call, so `addressSpaceMb` is skipped there. `cpuSeconds` and `maxWallMs` still bound every run.
 - **PID-reuse protection is inert on macOS** — `readProcessStart` reads `/proc/<pid>/stat`, which Darwin does not provide, so the identity re-check that guards `killGroup` against signalling a recycled pgid always passes there; `killGroup` signals the pgid without the identity re-check on macOS rather than paying a `ps` fork on a teardown path. The process-group teardown and the `closeDeadline` bound still contain the run.
+- **C-ext stdio buffers are not drained at settlement.** The child runs with `-u` (unbuffered), so `sys.__stdout__`/`sys.__stderr__` and `os.write` bytes are visible to the host's stray capture immediately; but a C extension's private C-stdio (`FILE*`) buffering is outside the interpreter, and its unwritten bytes are lost when the host SIGTERMs the child after the done frame. Model code should flush C-level stdio explicitly before returning if it must survive.
+
 - **A truncated log's serialized array runs to `maxLogBytes` plus the marker.** The truncation marker is envelope, not payload — it rides uncharged so it can always be emitted — and the outer-array envelope is reserved one byte in the ledger. A truncated run with admitted entries therefore serializes its `logs` array to at most `maxLogBytes + marker + 1`; the marker alone fits any admissible budget (the 64-byte floor guarantees it).
 - **A descendant that calls `setsid()` / `start_new_session=True` escapes teardown.** Termination signals the child's process group with `kill(-pid)`; a descendant that moves itself into a fresh session is no longer in that group and no signal reaches it. If it also releases the inherited stdout/stderr/fd-3 pipes, the leader's `close` still settles the run, and after the `closeDeadline` bound the fiber goes quiescent while that orphan keeps running. This is the containment boundary, not a security one — model code has bash-equivalent trust, and a bash tool can `setsid` away just the same. Reaching such an orphan would require tracking every descendant pid (as the bash-local backend's process-inspector does) and is deferred; the process-group teardown reaps everything that stays in the group.
 - **A combined log-and-value peak is not modelled by the load gate.** Each budget is checked against `addressSpaceMb` on its own. A model daemon thread that keeps writing while the completion value is metered and framed can refill the log pending toward `maxLogBytes` during that window, so the two peaks add in a way no gate admits or rejects. A gate over `(maxLogBytes + maxValueBytes)` was considered and deferred: its discriminating case cannot be scheduled deterministically under `RLIMIT_AS`, so the gate would only prove its own arithmetic. When the combined peak is reached the run dies as `worker-exit` -- containment holds and only the failure classification is degraded.

packages/code-runtime/code-runtime-python/README.zh.md

@@ -37,6 +37,8 @@ host 与 CPython 子进程在子进程的 fd 3 上交换一个无版本号的 JS
 - **跨语言 guard 覆盖执行值与帧字段集，但不覆盖字段类型** —— `tests/protocol-mirror.e2e.ts` 使用真实 `python3` 比较 `PROTOCOL_FD`、日志截断标记，以及每个 `TypedDict` 的必填和可选字段。跨 TypeScript 与 Python 比较字段类型在此没有机械等价物，因此类型级漂移由 review 加后端真子进程套件负责。
 - **`RLIMIT_AS` 在 macOS 上不施加** —— 在 exec 时映射进每个进程的 dyld 共享缓存超过任何实际的地址空间上限，内核会拒绝该 `setrlimit` 调用，故 `addressSpaceMb` 在那里被跳过。`cpuSeconds` 与 `maxWallMs` 仍约束每一次运行。
 - **PID 复用防护在 macOS 上失效** —— `readProcessStart` 读取 `/proc/<pid>/stat`，Darwin 不提供它，因此防止 `killGroup` 对已回收的 pgid 发信号的同一性复检在那里恒通过；`killGroup` 在 macOS 上不经同一性复检直接对 pgid 发信号，而非在拆卸路径上付出一次 `ps` fork。进程组拆卸与 `closeDeadline` 上界仍约束该次运行。
+- **C 扩展的 stdio 缓冲在结算时不被排空。** 子进程以 `-u`（无缓冲）运行，因此 `sys.__stdout__`/`sys.__stderr__` 与 `os.write` 的字节立即可见；但 C 扩展私有的 C-stdio（`FILE*`）缓冲在解释器之外，其未写出的字节会在宿主于 done 帧后 SIGTERM 子进程时丢失。模型代码若需保留，应在返回前显式 flush C 层 stdio。
+
 - **截断日志的序列化数组会到 `maxLogBytes` 加标记为止。** 截断标记是 envelope 而非 payload——它不计费地随行，因此总能发出——而外层数组外壳在账本中预留了一字节。因此带已放行条目的截断运行，其 `logs` 数组序列化后至多为 `maxLogBytes + marker + 1`；标记单独能放进任何可接受的预算（64 字节下限保证这一点）。
 - **调用 `setsid()` ／ `start_new_session=True` 的后代会逃出 teardown。** 终止是用 `kill(-pid)` 向子进程的进程组发信号；一个把自己移入新会话的后代已不在该进程组内，任何信号都到不了它。若它同时释放了继承而来的 stdout／stderr／fd-3 管道，leader 的 `close` 仍会结算该次运行，在 `closeDeadline` 到界之后 fiber 变为完全停稳，而那个孤儿仍在运行。这是 containment 边界，而非安全边界——模型代码具有等同 bash 的信任级别，一个 bash 工具同样能 `setsid` 逃逸。要够到这样的孤儿需要追踪每一个后代 pid（如 bash-local 后端的 process-inspector 所做），此项已推迟；进程组 teardown 会回收所有留在组内的进程。
 - **日志与完成值的叠加峰值未被加载门建模。** 每项预算都是各自对照 `addressSpaceMb` 检查的。模型的 daemon 线程可以在完成值被计量并分帧的窗口内持续写入、把日志 pending 重填到接近 `maxLogBytes`，于是两个峰值以任何门都不曾放行也不曾拒绝的方式相加。对 `(maxLogBytes + maxValueBytes)` 设门的方案经评估后推迟：它的判别用例无法在 `RLIMIT_AS` 之下确定性地构造出来，因此该门只能证明自己的算术。叠加峰值被触及时该次运行死为 `worker-exit`——containment 仍然成立，只是失败分类失真。

packages/code-runtime/code-runtime-python/py/bootstrap.py

@@ -102,7 +102,8 @@ class LogBuffer:
         # JSON-string cost plus one separator byte, and the serialized outer
         # logs array adds one more byte of envelope (two brackets and n-1 commas
         # over n entries' separators), so a result that exactly exhausts the
-        # ledger would serialize to max_bytes + 1. Reserving that byte keeps an
+        # ledger serializes to exactly max_bytes; WITHOUT the reserved byte it
+        # would serialize to max_bytes + 1. Reserving that byte keeps an
         # admitted result within the configured cap; the truncation-marker entry
         # is envelope, not payload, and rides uncharged (``_max_bytes`` stays the
         # configured value for the marker's message text).
@@ -947,6 +948,18 @@ async def _run(channel: ProtocolChannel) -> None:
     sys.stdout = _LogStream(logs)  # type: ignore[assignment]
     sys.stderr = _LogStream(logs)  # type: ignore[assignment]
     out_stream, err_stream = sys.stdout, sys.stderr
+    # The ORIGINAL std streams are bound here, before the program runs, so the
+    # settlement flush can drain bytes a program wrote through them without an
+    # explicit flush. The bootstrap only replaces `sys.stdout`/`sys.stderr` with
+    # the `_LogStream`; `sys.__stdout__`/`sys.__stderr__` (and C-ext stdio
+    # layered on the same fds) are untouched, and their block-buffered bytes are
+    # lost when the host SIGTERMs the child right after the done frame — the
+    # default SIGTERM disposition terminates without interpreter finalization.
+    # Binding the names here (before the program) makes them immune to a
+    # `sys.__stdout__ = boom` rebind in model code; `None` under `-S`-style
+    # redirects is guarded at flush time.
+    _stdout_orig = sys.__stdout__
+    _stderr_orig = sys.__stderr__
 
     # 6. Compile the program as the body of an async function, matching the
     # seam contract (`CodeRunRequest.program` is an async-function body: top-level
@@ -1123,7 +1136,9 @@ async def _run(channel: ProtocolChannel) -> None:
     # already pushed still reports the truncation. Same rule as
     # `_make_failure_reporter`: a settled verdict must not be swallowed by the
     # reporting that follows it.
-    for _flush in (flush_out, flush_err):
+    for _flush in (flush_out, flush_err, _stdout_orig, _stderr_orig):
+        if _flush is None:
+            continue
         try:
             _flush()
         except _BaseException:  # noqa: BLE001 -- swallow ONLY the log tail; `done` must reach the host; `_BaseException` is a pre-program local

packages/code-runtime/code-runtime-python/src/index.ts

@@ -967,7 +967,15 @@ export class PythonCodeRuntime extends CodeRuntime {
     let child: ChildProcessWithoutNullStreams
     let proto: Duplex | null
     try {
-      child = spawn(resolvePythonBin(this.config.pythonBin), ['-I', bootstrapPath], {
+      // `-u` keeps the interpreter's own stdout/stderr UNBUFFERED: a program
+      // that writes through `sys.__stdout__`/`sys.__stderr__` (or C-stdio
+      // layered on the same fds) must have those bytes visible to the host's
+      // stray capture immediately — a block-buffered wrapper would otherwise
+      // hold them until an explicit flush, and the host SIGTERMs the child
+      // right after the done frame, before any finalization-time flush could
+      // run. The `_LogStream` replacement of `sys.stdout`/`sys.stderr` is
+      // unaffected (it is a Python object, not the C-level stdio buffer).
+      child = spawn(resolvePythonBin(this.config.pythonBin), ['-u', '-I', bootstrapPath], {
         env: {},
         detached: true, // Own process group — kill(-pid, sig) reaches subprocesses the model program spawns.
         stdio: ['pipe', 'pipe', 'pipe', 'pipe'],
@@ -1000,12 +1008,25 @@ export class PythonCodeRuntime extends CodeRuntime {
       // The ledger starts one byte below maxLogBytes: each entry is charged its
       // JSON-string cost plus one separator byte, and the serialized outer logs
       // array adds one more byte of envelope (two brackets and n-1 commas over n
-      // entries' separators), so a result that exactly exhausts the ledger would
+      // entries' separators), so a result that exactly exhausts the ledger
+      // serializes to exactly maxLogBytes; WITHOUT the reserved byte it would
       // serialize to maxLogBytes + 1. Reserving that byte keeps an admitted
       // result within the configured cap; the truncation-marker entry is
       // envelope, not payload, and rides uncharged.
       let logBudget = this.config.maxLogBytes - 1
       let logsTruncated = false
+      // Drop a pipe's buffered stray output wholesale: once the ledger has
+      // truncated, every byte of it would be no-op'd by admit(), so retaining
+      // it (and later Buffer.concat+decoding it in flushStray) would spend host
+      // memory on output that can never be admitted. Called from every arm that
+      // marks the ledger truncated — admit()'s two ceilings and the child-marker
+      // frame arm — so the end-path flushStray sees empty buffers and exits.
+      const clearStray = (stray: StrayBuffer): void => {
+        stray.chunks = []
+        stray.blocks = []
+        stray.cost = 0
+        stray.utf8 = { expected: 0, width: 0, lowerFirst: 0, upperFirst: 0 }
+      }
       const admit = (text: string): void => {
         // Post-truncation admits are no-ops: once the ledger has truncated, the
         // marker is the last entry. Reachable within one `data` callback — a
@@ -1036,6 +1057,10 @@ export class PythonCodeRuntime extends CodeRuntime {
         if (text.length + 3 > logBudget) {
           logsTruncated = true
           logs.push(logTruncationMarker(this.config.maxLogBytes))
+          // Release the buffered stray pipes: their bytes can never be
+          // admitted now (see clearStray).
+          clearStray(strayOut)
+          clearStray(strayErr)
           return
         }
         // Past the lower bound, measure the exact serialized cost without
@@ -1046,6 +1071,8 @@ export class PythonCodeRuntime extends CodeRuntime {
         if (measured === undefined) {
           logsTruncated = true
           logs.push(logTruncationMarker(this.config.maxLogBytes))
+          clearStray(strayOut)
+          clearStray(strayErr)
           return
         }
         logBudget -= measured + 1
@@ -1113,6 +1140,11 @@ export class PythonCodeRuntime extends CodeRuntime {
           // The residual begins at a character boundary (a newline is never
           // inside a multibyte sequence), so its cost and UTF-8 state recompute
           // cleanly from a fresh walk.
+          // A line admitted inside the loop may have exhausted the ledger and
+          // cleared this pipe (see clearStray); the re-retain below must not
+          // resurrect the doomed residual.
+          // oxlint-disable-next-line typescript/no-unnecessary-condition -- admit() (a closure) sets it.
+          if (logsTruncated) return
           stray.chunks = detachResidual(buffered)
           stray.utf8 = { expected: 0, width: 0, lowerFirst: 0, upperFirst: 0 }
           stray.cost = accrueStrayCost(buffered, stray.utf8)
@@ -1370,6 +1402,8 @@ export class PythonCodeRuntime extends CodeRuntime {
               // describes the run.
               if (!logsTruncated) {
                 logsTruncated = true
+                clearStray(strayOut)
+                clearStray(strayErr)
                 // The host's OWN marker, never the frame's text. `truncated` is
                 // attacker-reachable, so trusting the text let a program write
                 // `{"type":"log","truncated":true,"text":<1 MiB>}` and land all

packages/code-runtime/code-runtime-python/tests/runtime.spec.ts

@@ -4304,6 +4304,31 @@ describe('PythonCodeRuntime — hostile peer', () => {
     expect(result.logs.join('')).toContain('stray stderr')
   })
 
+  it('flushes bytes written through sys.__stdout__/sys.__stderr__ before the done frame', async () => {
+    // The bootstrap only replaces sys.stdout/sys.stderr with the _LogStream;
+    // sys.__stdout__/sys.__stderr__ are the original block-buffered wrappers
+    // over fd 1/2. A program that writes through them without an explicit flush
+    // would lose those bytes when the host SIGTERMs the child right after the
+    // done frame (the default SIGTERM disposition terminates without
+    // interpreter finalization). The settlement flush now drains the original
+    // std streams before sending the done frame, so the bytes land in the
+    // kernel pipe buffer and the host's stray capture records them.
+    const { runtime } = await setup()
+    const result = await runtime.run({
+      program: [
+        'import sys',
+        'sys.__stdout__.write("orig stdout\\n")',
+        'sys.__stderr__.write("orig stderr\\n")',
+        'return "done"',
+      ].join('\n'),
+      bindings: [],
+    })
+    expect(result.error).toBeUndefined()
+    expect(result.value).toBe('done')
+    expect(result.logs.join('')).toContain('orig stdout')
+    expect(result.logs.join('')).toContain('orig stderr')
+  }, 15_000)
+
   it('escalates to SIGKILL when the program traps SIGTERM and ignores the grace period', async () => {
     // A program that traps SIGTERM should still die: the kill() escalation
     // fires SIGKILL after graceMs. The full run reports either timeout (wall)