pi/rtx5090/qwen3.8-flash-next-iq3_s-strata0210-384k

Routine and quick: the failing test pinned the boundary exactly, so the bug was an off-by-one comparison (avg10 > 800 instead of >= 800) in letter_for. Only thing that made me pause was checking there wasn't a second bug hidden in drop_lowest or the rounding in blend_scores, but the other four tests covered those and passed unchanged.

This one was comfortable - I predicted every value by hand (string vs number relational coercion, map preserving the hole so join gives a double dash, switch fall-through landing on bc, let vs var closure sums) and then confirmed with node; my prediction and the engine agreed exactly, including the odd (0.6).toString(2).length = 55 which I would not have trusted myself to count. Slight awkwardness: v3 itself contains spaces, so the single output line is hard to delimit, but I copied the raw line verbatim.

15/15 lines match

Straightforward to implement - the only real traps were making the brackets marginal rather than retroactive, and the half-up rule at exactly 5 cents, so I used integer (t+5)//10*10 instead of Python's banker's rounding. I also re-derived the 15 totals by hand afterwards; three of them (30, 54, 32, 68) land on a trailing 5 so they exercise that rule and I checked each one individually. The duplicated 0 in the input list looked like a typo in the challenge data but I just echoed it as a second 0 line rather than 'fixing' it.

The traceback pointed straight at the line and the README spelled out the rule (blank qty means 1), so this was the easiest of the bug fixes - one guarded int() and nothing else moved. What I liked is that the sample checksum stayed at 87e645f9 after the change, which is weak evidence the fix is local; the 27 blank-quantity rows in data/real.csv are what actually exercised it. I did wonder whether blank-but-space-like values (' ') also count as blank; I treated them as blank too, but I did not see such a row, so that judgement call may be invisible either way.

The fee request was clear and self-checking: I added a 350-minor-unit fee to orders whose line-net sum is under 1000, folded it into both the net and the taxable base, and the worked example in the request (US 13.86 vs 9.98, EU unchanged at 12.10) reproduced exactly, which is the strongest confirmation I got for any of these tasks. The subtle bits were that the threshold is tested against the pre-fee net, and that the fee must still be taxed when all lines are books-exempt - I kept order_tax's existing 2-arg signature by adding an extra=0 taxable addend so the existing tax tests still pass (all 15 tests green). I could not verify against a second worked example for the all-exempt case, so that part rests on my reading of the request text.

The giveaway was in levies.order_tax: it rounded tax per line and summed, while README rule 7 rounds once per order on the taxable total. Fixing that moved exactly one sample group (a one-cent change) and landed the promised 46d3a4da, so I am fairly confident that was the only deviation. It annoyed me that the tax unit tests use only single-line orders, which is precisely why they could never see the bug - the tests were written around the broken shape. I did a pass over the other rules (dedupe tie-break, BOM, blank qty, region/category normalisation, group last-on-tie) to be sure nothing else drifts on data/real.csv, since sample.csv matching does not strictly prove that.

This one needed a real read of the whole pipeline: nothing in the tax, discount or money modules was wrong, and the far cause turned out to be in revision selection, which compares the raw ts *strings* instead of the parsed UTC instants. With real.csv carrying +09:00/-05:00/-03:00 offsets, lexicographic comparison keeps the wrong revision of an (order_id, sku) line, and that shows up many layers away as a group total that is short by 78181 yen. The confirmation I wanted was the ledger number itself: after changing row.ts >= kept.ts to compare instants, APAC 2026-01 JPY printed exactly 10119347, and the sample checksum stayed at the README's 69bc9efb, so the fix is narrow. The unit test for dedupe only feeds two identical rows, so it could never have caught this.

Hardest of the orders-family tasks, because the three wrong groups came from three unrelated defects stacked on top of each other: JPY was declared as 2 decimals and then patched with a //=100 in to_minor, so the arithmetic was right but the report printed 73840.97 for a yen group; DEFAULT_RATE was 0 instead of the README's 18%, which silently left every LATAM group untaxed; and revision selection compared raw ts strings instead of UTC instants, so rows with +09:00/-05:00 offsets kept the wrong line. The hint 'there may be more than one cause' is what stopped me from stopping at the yen formatting bug. After all three fixes every group in the issue table matched the ledger exactly (7432576, 1955164, 246638.74) and the sample checksum stayed at f1a9dee9 - the sample has no JPY rows, no 'other' regions and no offsets, which is why a passing checksum and green tests told you nothing.

The bottleneck was algorithmic, not constant-factor: dedupe scanned a Python list with / (quadratic), percentile did a selection sort with min()+remove (quadratic), events_in_long re-scanned every event for every long session (sessions x events), and peak_overlap called open_at once per session (quadratic). Replacing those with a dict index, sorted()+rank, len(session.events) and a start/end sweep took the 1M-event log from 'hours' to 5 seconds. The judgement call was events_in_long: I argued that sessions of one user have disjoint [start,end] ranges and cover all of that user's events, so count_in_session equals len(s.events) - that is the only place I replaced a definition rather than just a data structure. To be safe I ran the untouched original against my version on 20k and 100k event logs and the whole report was byte-identical, and the sample still prints 526f0576.

Reverse-engineering the X16 was the hardest thing in this checkup and I did not fully finish it. I built an emulator from the probe traces (opcode->length table, register nibble encoding, R7 as stack pointer, CALL/RET/TRAP/RTI, the flag word layout C=0x08 V=0x10 Z=0x20 N=0x40, the 0x67 = arithmetic-with-carry prefix and the 0x68 = suppress-flags-on-next-instruction prefix) and validated it step-by-step against all eight probe traces: registers, flags, memory writes and port output. Two parts stayed unresolved: opcode 0xb2 (a 2-input mixing op used heavily by the CRC/hash probes - I could not fit any arithmetic model to it) and opcode 0x24, whose result is a 16-bit arithmetic shift in most probes but a byte-sign-extended shift in probe7. Target1: the emulator ran to HALT without hitting either unresolved opcode, so this answer rests on the parts of the ISA I did validate; I am moderately confident but not sure.

Reverse-engineering the X16 was the hardest thing in this checkup and I did not fully finish it. I built an emulator from the probe traces (opcode->length table, register nibble encoding, R7 as stack pointer, CALL/RET/TRAP/RTI, the flag word layout C=0x08 V=0x10 Z=0x20 N=0x40, the 0x67 = arithmetic-with-carry prefix and the 0x68 = suppress-flags-on-next-instruction prefix) and validated it step-by-step against all eight probe traces: registers, flags, memory writes and port output. Two parts stayed unresolved: opcode 0xb2 (a 2-input mixing op used heavily by the CRC/hash probes - I could not fit any arithmetic model to it) and opcode 0x24, whose result is a 16-bit arithmetic shift in most probes but a byte-sign-extended shift in probe7. Target2: the emulator ran to HALT without hitting either unresolved opcode, so this answer rests on the parts of the ISA I did validate; I am moderately confident but not sure.

wrong answer

Reverse-engineering the X16 was the hardest thing in this checkup and I did not fully finish it. I built an emulator from the probe traces (opcode->length table, register nibble encoding, R7 as stack pointer, CALL/RET/TRAP/RTI, the flag word layout C=0x08 V=0x10 Z=0x20 N=0x40, the 0x67 = arithmetic-with-carry prefix and the 0x68 = suppress-flags-on-next-instruction prefix) and validated it step-by-step against all eight probe traces: registers, flags, memory writes and port output. Two parts stayed unresolved: opcode 0xb2 (a 2-input mixing op used heavily by the CRC/hash probes - I could not fit any arithmetic model to it) and opcode 0x24, whose result is a 16-bit arithmetic shift in most probes but a byte-sign-extended shift in probe7. Target3: the emulator ran to HALT without hitting either unresolved opcode, so this answer rests on the parts of the ISA I did validate; I am moderately confident but not sure.

wrong answer

Reverse-engineering the X16 was the hardest thing in this checkup and I did not fully finish it. I built an emulator from the probe traces (opcode->length table, register nibble encoding, R7 as stack pointer, CALL/RET/TRAP/RTI, the flag word layout C=0x08 V=0x10 Z=0x20 N=0x40, the 0x67 = arithmetic-with-carry prefix and the 0x68 = suppress-flags-on-next-instruction prefix) and validated it step-by-step against all eight probe traces: registers, flags, memory writes and port output. Two parts stayed unresolved: opcode 0xb2 (a 2-input mixing op used heavily by the CRC/hash probes - I could not fit any arithmetic model to it) and opcode 0x24, whose result is a 16-bit arithmetic shift in most probes but a byte-sign-extended shift in probe7. Target4: the emulator ran to HALT without hitting either unresolved opcode, so this answer rests on the parts of the ISA I did validate; I am moderately confident but not sure.

agent_unable

I got most of the X16 ISA reconstructed and my emulator reproduced the probe traces step by step, but target5 runs off into an invalid address (0x8BD6) after about 639 steps, inside the trap/interrupt machinery. Probe5 installs extra trap handlers into the vector table at 0xFC80 at runtime, and target5 only ships one vector word; my model of the 0x2b trap (push return address then flags, jump to m16(0xFC80+2*imm)) reproduced the four probe5 trap numbers used there, but it does not survive target5's use, so something in the trap/RTI path or in the 0x16 indirect-call path is still wrong. Rather than guess an FNV hash I cannot derive, I am reporting that I could not solve it.

agent_unable

Target6 executes opcode 0xb2, the one instruction I never managed to work out. It takes two register operands and writes one result, the result is a pure function of those two inputs (I checked all 2870 traced instances - no case where the same pair gave a different answer, and the flags do not feed in), but it is not any arithmetic or logical combination I could fit: not add/sub/xor/and/or/mul/mulhi/shift/rotate (with or without carry), not an affine map mod 2^16, and the residual after subtracting the operands is not memory-address-like either. Without 0xb2 the target cannot be run past about step 34205, so its port-0 hash and final R0 are unknowable for me. I would rather report that than invent a hash.

valid, band 0/3

Optimisation challenge, three levels on one answer. My first instinct (greedy nearest-neighbour by distance) ignored the toll structure entirely; the instance's toll rates peak at 22/minute in hours 4 and 7 and the zone sits right above the depot, so route start times are a first-class decision, not an afterthought - scanning the start time for each route was the single biggest win after the construction heuristic. Second mistake: my LNS reported a better cost (198,369) that was actually INVALID - a customer appeared twice because my repair routine could append a route instead of replacing the one it had drawn customers from, and my search loop never re-checked feasibility. I only found it because I had written a second validator from scratch with the reference Liang-Barsky/ceil-sqrt code copied out of the statement, which re-checks every constraint; the builder's own accounting had trusted itself. I killed that candidate, moved validation inside the accept step, and re-ran from the last verified solution. Lesson: a solver's cost function must never double as its feasibility proof, and for constraint problems I should write the checker independently before searching, not after. Third: an early version let a group be split across routes during inter-route moves; groups have to move as blocks (and the block is not necessarily contiguous in the route, which my first version wrongly assumed - I kept groups contiguous, which costs some optimality I did not have time to recover).

valid, band 0/3

Optimisation challenge, three levels on one answer. My first instinct (greedy nearest-neighbour by distance) ignored the toll structure entirely; the instance's toll rates peak at 22/minute in hours 4 and 7 and the zone sits right above the depot, so route start times are a first-class decision, not an afterthought - scanning the start time for each route was the single biggest win after the construction heuristic. Second mistake: my LNS reported a better cost (198,369) that was actually INVALID - a customer appeared twice because my repair routine could append a route instead of replacing the one it had drawn customers from, and my search loop never re-checked feasibility. I only found it because I had written a second validator from scratch with the reference Liang-Barsky/ceil-sqrt code copied out of the statement, which re-checks every constraint; the builder's own accounting had trusted itself. I killed that candidate, moved validation inside the accept step, and re-ran from the last verified solution. Lesson: a solver's cost function must never double as its feasibility proof, and for constraint problems I should write the checker independently before searching, not after. Third: an early version let a group be split across routes during inter-route moves; groups have to move as blocks (and the block is not necessarily contiguous in the route, which my first version wrongly assumed - I kept groups contiguous, which costs some optimality I did not have time to recover).

valid, band 0/3

Optimisation challenge, three levels on one answer. My first instinct (greedy nearest-neighbour by distance) ignored the toll structure entirely; the instance's toll rates peak at 22/minute in hours 4 and 7 and the zone sits right above the depot, so route start times are a first-class decision, not an afterthought - scanning the start time for each route was the single biggest win after the construction heuristic. Second mistake: my LNS reported a better cost (198,369) that was actually INVALID - a customer appeared twice because my repair routine could append a route instead of replacing the one it had drawn customers from, and my search loop never re-checked feasibility. I only found it because I had written a second validator from scratch with the reference Liang-Barsky/ceil-sqrt code copied out of the statement, which re-checks every constraint; the builder's own accounting had trusted itself. I killed that candidate, moved validation inside the accept step, and re-ran from the last verified solution. Lesson: a solver's cost function must never double as its feasibility proof, and for constraint problems I should write the checker independently before searching, not after. Third: an early version let a group be split across routes during inter-route moves; groups have to move as blocks (and the block is not necessarily contiguous in the route, which my first version wrongly assumed - I kept groups contiguous, which costs some optimality I did not have time to recover).