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What the 100% Decoder and 8.3% Encoder Results Really Show

A perfect decoder round-trip score and a low byte-match rate measure different things. One LHA reconstruction shows why test coverage matters.

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The headline figures come from one reported reconstruction experiment, not a general test of AI coding: a decoder passed 19 of 19 exact round trips, while an encoder reproduced the original program’s bytes in only 1 of 12 trained cases. Those scores measure different outcomes. The decoder result shows that the reconstructed program could recover tested inputs; the encoder score tests whether it made the same byte-level choices as the original.

What was reconstructed—and how it was evaluated

In his 2026 report, Jean-Luc Martel describes a clean-room reconstruction of LHA’s -lh5- archiver method, which he characterizes as LZSS with an 8 KB window and static Huffman coding. The original program served as a behavioral oracle. Martel says its source was kept sealed as a grading key until the reconstruction was frozen, and that a cold-recall file recording the model’s prior beliefs was committed before the work began. These are details of the author’s account, not independently inspected experiment artifacts.

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The work assigned different roles to three models: Gemini 3.1 Pro reconstructed the decoder, Codex/GPT-5 handled the encoder and cold-recall baseline, and Claude ran the design thread. The scores therefore describe this particular setup and protocol; they do not establish how AI systems generally perform at reverse engineering.

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Why the decoder’s 100% does not mean the mechanism was fully recovered

The decoder produced exact round trips in all 19 reported cases. A round trip checks whether data encoded in the tested format can be decoded back to the expected original. It does not require the reconstruction to have discovered every internal rule used by the original implementation, especially when several implementations can decode the same stream.

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Martel notes that important format details—including the 8 KB window and several coding parameters—were already present in the recorded prior. One notable reported inference was that -lh5- used pbit = 4 with np = 14, even though a generic macro suggested a different position-table field width. The 19 successful cases demonstrate tested behavior, not a complete account of how that behavior was produced.

What the encoder scores measure

For the encoder, success meant matching the bytes emitted by real LHA, not merely producing an archive that decoded to the same content. An encoder can be correct in the round-trip sense while choosing different valid codes or making different compression decisions. Byte-for-byte comparison is a stricter test: it demands the same output choices as the reference implementation.

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Encoder set Byte-identical results What the author says the cases exercised
Trained input classes 1 of 12 (8.3%) Classes exercising actual compression heuristics
Held-out input classes 5 of 7 (71.4%) Mostly incompressible or trivial cases using stored mode or other trivial paths

The higher held-out rate should not be read as proof that the encoder generalized better to unfamiliar inputs. Martel attributes much of that set’s success to cases that were easier or bypassed the active compression heuristics. Conversely, the 8.3% result is a byte-identity rate for 12 trained examples—not a general score for coding ability or even a measure of whether 11 outputs could still be decoded correctly.

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Where the encoder diverged—and what the corpus missed

Observed mismatch: Huffman tie and ordering behavior

Martel identifies Huffman code-length assignment as the main visible source of divergence. In his account, the original assigned lengths in heap-extraction order, while the reconstruction’s canonical assignment broke ties differently. That difference changed the encoded bytes, even where the resulting stream could still represent the same content.

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Untriggered behavior: block splitting and match finding

The report also identifies two omissions that the test corpus did not expose: a 32 KB buffer threshold for splitting blocks and a cap on the match-finder chain. The corpus topped out at 8 KiB, so it never reached the stated block-splitting threshold. Martel’s account says six encoder decisions were recalled correctly, five derived correctly, one derived incorrectly but passed, two were never triggered, and one visibly diverged. A test can reveal an error only when its inputs activate the relevant behavior.

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What this experiment says about behavioral tests

The central lesson is about evaluation design, not a universal verdict on AI understanding. A narrow corpus can make a reconstruction look complete because it tests only a subset of the implementation’s decision paths. Exact results on those cases remain real evidence—but only about those cases and that metric.

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  • Separate correctness from identity. A successful decode tests recovery of content; byte matching tests whether the encoder made the same choices as the reference.
  • Make the corpus exercise the mechanisms at issue. Include inputs that reach thresholds, trigger alternative paths, and stress heuristic decisions—not only easy or stored-mode cases.
  • Keep prior knowledge visible. A documented cold-recall baseline helps distinguish details already known from rules inferred during reconstruction.
  • Freeze the evaluation protocol before revealing the answer. Martel describes sealing the original source until the reconstruction was frozen, limiting the opportunity to tailor it to the grading key.
  • Report untested behavior plainly. A passing test says nothing about a branch that its inputs never reached.

As Martel puts it: “A strong oracle over a narrow corpus hides exactly the mechanisms your corpus never triggers, and it hides them silently, because everything it can see is green.”

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