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A model can report a finite, convincing training loss while never learning when to stop. In Panagiotis (Panos) Gkilis’s reported audio-model case, the end-of-sequence (EOS) token had the same integer ID as the loss function’s ignore_index, so every EOS target was excluded from training. The run did not crash, and its loss curve did not reveal the missing supervision.

How the integer collision removed EOS from the loss

In the reported autoregressive stage, audio tokens occupied IDs 0 through 1023, and EOS was assigned ID 1024. The output layer therefore had 1025 classes. The problematic configuration was conceptually:

nn.Linear(d_model, NUM_AUDIO_TOKENS + 1)  # 1025 outputs: audio 0–1023, EOS 1024
F.cross_entropy(logits, targets, ignore_index=NUM_AUDIO_TOKENS)

Here NUM_AUDIO_TOKENS is 1024, making the ignore sentinel equal to the valid EOS class. Cross-entropy omits target positions whose value equals ignore_index; it does not train the model to predict those targets. Thus, EOS positions contributed no loss, even though EOS was a valid output class.

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This is a collision in a particular configuration, not evidence that every model using an ignore index is affected. The key is the relationship among target IDs, output width, and ignore sentinel—not the integer in isolation.

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Why a healthy-looking loss is possible

Ignored positions are excluded before the loss is calculated. The resulting value can remain finite and plausible because it measures the targets that were included, not the EOS targets that were omitted. A smooth or improving aggregate loss cannot establish that every intended behavior is being supervised.

Why output width matters at each training stage

The same sentinel can mean different things in different stages. In the reported setup, 1024 was outside the range of a 1024-class non-autoregressive stage, but it was a valid class in the 1025-class autoregressive stage. Inspect each stage’s target vocabulary and output width together; checking whether a number looks like a sentinel in one part of the code is not enough.

For a layer with C outputs indexed from 0 to C - 1, any sentinel used by the loss must be distinct from every valid target ID. If EOS is deliberately included as a class, it must remain eligible as a target rather than being accidentally treated as padding or otherwise ignored.

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What the reported measurements show—and do not show

Gkilis reports two related results from his experiments. They illustrate how the loss can fail to track stopping behavior, but they are measurements from his reported setup, not independently validated benchmarks or a guarantee about other training runs.

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Reported result Measurement Interpretation
Minimal two-arm reproduction, 2026 Broken arm: final loss 0.0035; fixed arm: 0.0034 The final loss values were very close despite the broken arm’s EOS-supervision issue.
Separate 200-epoch run, utterance-held-out evaluation (n=32) At epoch 100: mean P(stop) 0.4655; stop was argmax for 18/32 cases. At epoch 150: mean P(stop) 0.2159; stop was argmax for 8/32. The author reports training loss improved by 20% from epoch 100 to 150 while mean P(stop) fell by 54%.
After correction, as reported by Gkilis Mean P(stop) 0.4655; stop was argmax in 18/32 held-out cases; one end-to-end synthesis stopped at frame 203 with a 350-frame ceiling. Stopping behavior was demonstrated in that synthesis, but the reported results do not establish general performance.

The post-correction account also reports that mean P(stop) plateaued around 0.35–0.47 despite trying two learning rates and increasing the data from 224 to 1313 utterances, a 5.9-fold increase. The cause of that plateau was not confirmed. The source identifies a related Zenodo paper dated August 2026, DOI 10.5281/zenodo.21864658; the detailed experimental claims above are attributed to Gkilis’s article.

How to check a training pipeline for this bug

  1. Write down the class range for each stage. Record the output-layer width and the valid target IDs after tokenization. For the reported autoregressive example, 1025 outputs mean valid IDs 0–1024.
  2. Compare the ignore value against valid targets. Confirm the configured ignore_index cannot equal EOS or any other class that should receive supervision.
  3. Inspect collated batches. Check targets after tokenization and collation, not only the original examples. Verify that EOS IDs are present where expected and that the loss mask does not exclude them.
  4. Track which classes reach the loss. During an initial epoch, count positive target occurrences that actually contribute to loss. A class that never reaches the loss despite being expected is a structural warning; interpret low counts in light of sequence length and dataset coverage.
  5. Evaluate the behavior the model must learn. For stopping, inspect P(stop) at terminal frames, EOS rank or argmax frequency, and whether generation terminates autonomously. Do not use aggregate loss as the only capability check.

Useful linter checks

  • Flag a configuration when the ignore sentinel is within the valid output-class range and can therefore match a real target class.
  • Track which output classes appear as supervised positive targets during an initial epoch. Gate dead-class alerts on broad class coverage so a short or sparse run does not trigger the same warning as a structurally excluded class.
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What to conclude from loss and task metrics

Loss answers how well the model predicts the targets that the objective actually includes. It cannot, by itself, tell you whether a target was accidentally excluded or whether the model has learned a task-level behavior such as ending a sequence. Gkilis’s conclusion from the reported experiments was: “The training loss is not a sufficient statistic for model capability.” For an EOS-based generator, pair loss with explicit checks of EOS supervision and stopping behavior.

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