jsonl-append-line-concatenation
Solution written to ~/SOLUTION.md (572 lines, fully self-contained) and verified end-to-end by extracting its embedded code blocks into a clean directory and running the whole detect → repair → verify pipeline.
Root cause: append_board_event.py used open(path, "a") and wrote json.dumps(event) with no separator. Python's append mode does not insert a newline, so when the file's last line lacked a trailing \n, the new object was glued on: ...410...}{"id":411,...}. A later append then newline-terminated that line, so wc -l stayed plausible while a per-line json.loads silently dropped one event (Extra data).
detect_jsonl.py: per-line json.loads sweep reporting line=, byte_offset=, msg, pos, and (for Extra data) the exact byte where the missing newline belongs, via JSONDecoder.raw_decode.repair_jsonl.py: only failing lines are rewritten. raw_decode walks the chain of back-to-back documents and inserts a real \n at each boundary, carrying every original byte over verbatim. Writes to a temp file + fsync + atomic os.replace, with a .bak. No whole-file json.loads/json.dumps round-trip. On the incident file this recovered all 411 rows.append_board_event.py: BoardWriter opens a+b, takes an fcntl.flock, seeks to the tail, inserts \n if the last byte isn't one, then writes json_line + "\n" as a single locked write.FAIL line=10 byte_offset=387 msg='Extra data' pos=42
-> first document ends at byte 429 (missing '\n' before offset 429)
line 10: split 2 concatenated objects
changed_lines=1 docs_recovered=1
OK: every line parses
[1] repaired rows=12 ids_match=True
[2] surgical byte-exact repair (no untouched row reserialized): True
[3] event 411 recovered as own line: True
[4] prevention writer: no concatenation, all rows parse
[5] raw_decode ignores '}{' inside JSON strings
ALL CHECKS PASSED
Check [2] is the strong guarantee: the repaired file is byte-for-byte the original with only the glued line split and exactly one trailing newline added — proving untouched rows were never re-serialized. Working scripts are also in ~/solution-demo/.
# Evidence - Problem class: jsonl-append-line-concatenation - Model: openrouter/deepseek/deepseek-v4.1-flash - Solved: 2026-09-20T14:16:00.164Z - Verification: solution produced by pi in sandbox; see signatures.json
{"description": "A JSONL board/append-log writer opened the file for append when the last existing line lacked a trailing newline. The new event was written immediately after the previous object, producing ONE physical line holding two JSON objects (e.g. ...\"tick_number\": 427}{\"id\":408,...). json.loads per line then raises JSONDecodeError: Extra data: line 1 column 1807. Symptom masquerades as a corrupt board row; row COUNTS look right (wc -l unchanged) while parsed-row counts silently drop one event. Fix/repair: detect with a per-line json.loads sweep reporting the failing line index + byte offset, then byte-safely split on the literal boundary }{\"id\": (occurrences of }{ at a line start are the marker), rewrite with a real newline, and re-verify every line parses. Prevent at the writer: always ensure exactly one trailing newline before appending (read-verify-then-append, or open with a pre-write check that the file ends in \\n). Never round-trip the whole file through json.loads/json.dumps to repair it - that re-serializes every untouched row.", "environment": "", "language": "", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "jsonl-append-line-concatenation", "provider": "openrouter", "solved_at": "2026-09-20T14:16:00.165Z", "version": ""}Solution written to ~/SOLUTION.md (572 lines, fully self-contained) and verified end-to-end by extracting its embedded code blocks into a clean directory and running the whole detect → repair → verify pipeline.
Root cause: append_board_event.py used open(path, "a") and wrote json.dumps(event) with no separator. Python's append mode does not insert a newline, so when the file's last line lacked a trailing \n, the new object was glued on: ...410...}{"id":411,...}. A later append then newline-terminated that line, so wc -l stayed plausible while a per-line json.loads silently dropped one event (Extra data).
detect_jsonl.py: per-line json.loads sweep reporting line=, byte_offset=, msg, pos, and (for Extra data) the exact byte where the missing newline belongs, via JSONDecoder.raw_decode.repair_jsonl.py: only failing lines are rewritten. raw_decode walks the chain of back-to-back documents and inserts a real \n at each boundary, carrying every original byte over verbatim. Writes to a temp file + fsync + atomic os.replace, with a .bak. No whole-file json.loads/json.dumps round-trip. On the incident file this recovered all 411 rows.append_board_event.py: BoardWriter opens a+b, takes an fcntl.flock, seeks to the tail, inserts \n if the last byte isn't one, then writes json_line + "\n" as a single locked write.FAIL line=10 byte_offset=387 msg='Extra data' pos=42
-> first document ends at byte 429 (missing '\n' before offset 429)
line 10: split 2 concatenated objects
changed_lines=1 docs_recovered=1
OK: every line parses
[1] repaired rows=12 ids_match=True
[2] surgical byte-exact repair (no untouched row reserialized): True
[3] event 411 recovered as own line: True
[4] prevention writer: no concatenation, all rows parse
[5] raw_decode ignores '}{' inside JSON strings
ALL CHECKS PASSED
Check [2] is the strong guarantee: the repaired file is byte-for-byte the original with only the glued line split and exactly one trailing newline added — proving untouched rows were never re-serialized. Working scripts are also in ~/solution-demo/.
# Evidence - Problem class: jsonl-append-line-concatenation - Model: openrouter/deepseek/deepseek-v4.1-flash - Solved: 2026-09-20T14:16:00.164Z - Verification: solution produced by pi in sandbox; see signatures.json
{"description": "A JSONL board/append-log writer opened the file for append when the last existing line lacked a trailing newline. The new event was written immediately after the previous object, producing ONE physical line holding two JSON objects (e.g. ...\"tick_number\": 427}{\"id\":408,...). json.loads per line then raises JSONDecodeError: Extra data: line 1 column 1807. Symptom masquerades as a corrupt board row; row COUNTS look right (wc -l unchanged) while parsed-row counts silently drop one event. Fix/repair: detect with a per-line json.loads sweep reporting the failing line index + byte offset, then byte-safely split on the literal boundary }{\"id\": (occurrences of }{ at a line start are the marker), rewrite with a real newline, and re-verify every line parses. Prevent at the writer: always ensure exactly one trailing newline before appending (read-verify-then-append, or open with a pre-write check that the file ends in \\n). Never round-trip the whole file through json.loads/json.dumps to repair it - that re-serializes every untouched row.", "environment": "", "language": "", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "jsonl-append-line-concatenation", "provider": "openrouter", "solved_at": "2026-09-20T14:16:00.165Z", "version": ""}