diagnose-nex-file

Metadata

Description

┌─ write_file ────
  ↳ Successfully wrote 1376 bytes to /home/erik/household/src/main.rs
└─

$ cat copenhagen_weather_forecast.txt
  ↳ Copenhagen Weather Forecast (June 28 - July 3, 2026) (2 lines)  ← truncated

## Description
The file_write tool inside nex (the in-process LLM agent loop) is reporting successful writes but the resulting file on disk is truncated. Observed transcript from .chat-0 in ~/household 2026-05-04:

```
┌─ write_file ────
  ↳ Successfully wrote 1376 bytes to /home/erik/household/src/main.rs
└─

$ cat copenhagen_weather_forecast.txt
  ↳ Copenhagen Weather Forecast (June 28 - July 3, 2026) (2 lines)  ← truncated
```

Multiple write attempts with similar pattern: 'Successfully wrote 1289 bytes / 1376 bytes / 1405 bytes' but the resulting file shows only the first line or two.

User report 2026-05-04: 'there are deep problems with the file writing tool.'

## Likely cause hypotheses

1. **Buffer not flushed**: nex's file_write tool may write to a buffered FileWriter that isn't flushed before the subsequent cargo run / cat reads. The 'wrote N bytes' is the bytes-queued, not bytes-on-disk.

2. **Path resolution mismatch**: tool's idea of cwd differs from the bash subprocess's cwd. Write goes to /tmp/Z; bash cat reads /home/erik/household/Z. Net: file 'exists' but not where bash looks. (Less likely given that cargo run + cat both work consistently against the same files.)

3. **Race with cargo run**: tool writes to main.rs; cargo run rebuilds the binary; main.rs then truncated as part of build (overwritten by cargo's own write or similar). Unlikely but possible if cargo and the file_write tool both touch main.rs.

4. **Encoding or newline normalization**: tool writes 1376 bytes as UTF-8 / CRLF / etc., but the file system stores fewer bytes after normalization. Doesn't match 'truncated to first 2 lines' shape though.

5. **Tool concurrency bug**: multiple write_file calls in flight, some race. Unlikely if each tool call is awaited.

Most likely #1 (buffer) or a path-resolution / cwd issue (#2 in different shape).

## Investigation
1. Read nex's file_write tool source — likely in src/nex/tools/ or similar
2. Check whether it explicitly flushes / closes the file handle before returning success
3. Reproduce: a deterministic minimal case (write 1500 bytes, immediately cat, observe truncation)
4. Compare with claude/codex CLIs' file write behavior — do they have the same issue? (Unlikely since user has been using claude/codex chats successfully)
5. Bonus: any path-construction logic that might produce the wrong file path

## Deliverable
`wg log` entry with:
- Reproduction of the truncation deterministically
- Root cause with file:line citation
- Concrete fix proposal
- Whether other nex tools have similar buffer / path / flush bugs

## Validation
- [ ] Truncation reproduced in a controlled test
- [ ] Root cause identified with file:line
- [ ] Fix proposal concrete
- [ ] No source modifications — diagnose only

Depends on

• done .assign-diagnose-nex-file

Required by

• done .flip-diagnose-nex-file

Log

• 2026-05-04T21:54:24.533244649+00:00 Task paused
• 2026-05-04T21:55:37.282914307+00:00 Task published
• 2026-05-04T21:56:14.836937365+00:00 Lightweight assignment: agent=Careful Programmer (02e87968), exec_mode=light, context_scope=task, reason=Careful Programmer's verification-focused tradeoff is ideal for root-cause diagnosis; code investigation and reproducible bug testing fit the core task.
• 2026-05-04T21:56:15.030298744+00:00 Spawned by coordinator --executor claude --model opus
• 2026-05-04T21:56:28.302189499+00:00 Starting diagnostic — will locate nex file_write tool and check flush/path/cwd hypotheses
• 2026-05-04T22:02:36.652273279+00:00 === ROOT CAUSE === nex's write_file tool itself is correct. The bug lives one layer up, in the OpenAI tool-call argument parser. When a streamed (or non-streamed) tool-call JSON gets cut off mid-string, the recovery path silently closes the string and returns a "successfully recovered" JSON object whose `content` is truncated to whatever bytes happened to arrive. write_file then writes that truncated content and reports `Successfully wrote N bytes` — the value of `N` is the TRUNCATED content length, not the model's intended length. Hypothesis #1 (buffer not flushed) is FALSE. write_file uses std::fs::write (open + write_all + close), no buffering — see src/executor/native/tools/file.rs:285. The reported byte count is content.len() of the &str that arrived in the JSON. Hypothesis #2 (cwd mismatch) is FALSE. resolve_inside_cwd uses env::current_dir() and bash uses the same value captured at nex startup (src/commands/nex.rs:88). They cannot drift in a single process. Hypothesis #3, #4, #5 also rejected after reading the code path. === CALL CHAIN (file:line citations) === 1. Provider returns tool_call.function.arguments as a string. For OpenAI/OpenRouter it can arrive truncated when finish_reason="length", or when the stream is interrupted, or when the model emits an early EOS in the middle of the string. 2. Streaming path: src/executor/native/openai_client.rs:1190-1192 accumulates `arguments` chunks via push_str. End-of-stream finalization parses the final string at openai_client.rs:2700-2704: let input = match serde_json::from_str(&arguments) { Ok(v) => v, Err(e) => make_parse_error_input(&arguments, &e.to_string()), }; 3. Non-streaming path: openai_client.rs:706-708 runs the same pattern. 4. make_parse_error_input (openai_client.rs:2066) calls try_recover_json FIRST, before falling back to the __parse_error marker the agent loop knows how to surface (agent.rs:2407-2428). 5. try_recover_json (openai_client.rs:1905) tries three strategies. Strategy 3 = complete_truncated_json (openai_client.rs:1999-2059). When the open tool-call arguments end mid-string, this scans the chars, finds in_string=true plus depth_brace=1, **closes the unterminated string with `"`** and **closes the open brace**, then returns the result. 6. The "recovered" JSON parses successfully as `{"path":"...","content":"<bytes that happened to arrive>"}`. There is NO marker indicating the value was truncated — input flows on to write_file as if it were a normal call. 7. write_file (file.rs:285-296) writes content via fs::write, returns `Successfully wrote {} bytes` where the byte count == content.len() of the silently truncated string. Disk reflects exactly that — a truncated file. === DETERMINISTIC REPRODUCTION === Existing test test_json_recovery_truncated_mid_string (openai_client.rs:4708-4716) ALREADY proves the recovery happily produces a truncated string from `{"path": "/home/user/fi` → `{"path": "/home/user/fi"}`. Test passes today. End-to-end repro at /tmp/diag-nex-write.py: simulates a 1461-byte intended content, stream cut at 1376 bytes, complete_truncated_json recovers it, the final JSON parses successfully with content len = 1319 bytes. The "wrote 1319" matches the user-reported "wrote 1376 / 1289 / 1405 bytes" pattern (varies with exactly where the cut lands relative to the JSON envelope's framing bytes). === CONCRETE FIX PROPOSAL === Two-layer fix; both should land: (A) In src/executor/native/openai_client.rs:1905 (try_recover_json), Strategy 3 is a DESTRUCTIVE recovery — it fabricates string and bracket-closing bytes that the model never emitted. Make it return a typed result distinguishing SafeRecovery (markdown-strip / balanced-extract) from LossyRecovery (truncation completion). Callers can choose to accept or reject lossy recoveries. (B) In src/executor/native/openai_client.rs:2066 (make_parse_error_input), refuse to use LossyRecovery for tool-call arguments — if the underlying recovery is the truncation-completion path, return a __parse_error payload instead of the recovered value. The agent loop (agent.rs:2407) already knows how to surface that as a tool error, which gives the model a clean signal to retry. Keep the eprintln "Recovered malformed JSON" log but mark it WARN-level when LossyRecovery is used. (C) Bonus (cheap): when finish_reason=="length" / StopReason::MaxTokens AND a tool_use block is present, the agent loop should treat the tool call as poisoned and reject it before dispatch. Today MaxTokens is detected (openai_client.rs:2724) but the tool call still runs. === OTHER NEX TOOLS WITH SIMILAR BUG SHAPE === - edit_file (file.rs:341) — same arguments-parsing path. If `new_string` is cut off mid-content, the edit silently truncates the replacement. `old_string` cuts are self-protective (the match will simply fail). - write_file confirmed. - bash, grep, glob, read_file, etc. — short string/integer arguments rarely approach the truncation boundary, so the bug is much less likely to manifest, but the same recovery path applies. A bash command cut mid-quote could in principle execute a partial command. - Other tools (delegate, summarize, research, reader, map, etc.) — same path. - The Anthropic-format streaming code (src/executor/native/client.rs:752-765) does NOT use try_recover_json. It defaults to serde_json::Value::Null on parse failure, which makes the tool fail loudly with "Missing required parameter". So claude-direct does NOT exhibit this bug — only the OpenAI-compatible path (openrouter:, local:, nex:) does. This matches the user's observation that claude/codex CLI work fine. No buffer/flush bug, no cwd bug, no encoding bug. The truncation is upstream of the file I/O.
• 2026-05-04T22:03:12.004965445+00:00 Validated: end-to-end reproducer at /tmp/diag-nex-write.py confirms truncation; existing cargo test test_json_recovery_truncated_mid_string passes (proves recovery silently truncates). No source modifications made — diagnose only as required.
• 2026-05-04T22:03:52.656967666+00:00 Task pending eval (agent reported done; awaiting `.evaluate-*` to score)
• 2026-05-04T22:05:00.950976259+00:00 PendingEval → Done (evaluator passed; downstream unblocks)