PI052TextTokenizerStep masked text_labels over the assistant turn's
*content only* — the trailing newline was excluded and no EOS token was
ever a supervised label. So the LM head was never given a stop signal:
at inference select_message decoded to max_new_tokens, producing the
runaway subtask paragraphs and the "}"}"}-style VQA tails.
_format_messages now appends the tokenizer's EOS to each supervised
target turn and extends that turn's span to cover it, so the EOS lands
in text_labels. _shifted_ce then trains "<last content token> -> EOS"
and the model learns to terminate; select_message stops on it.
Inference callers (the runtime's _build_text_batch_pi052) pass no
target_indices / eos_token, so no EOS is baked into the prompt — the
model generates it. Verified end-to-end with the PaliGemma tokenizer:
the supervised span is `<content><eos>` and the trailing newline stays
unsupervised.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Tokenize batched recipe outputs in PI052 so training batches with nested message lists do not crash before model forward.
Co-authored-by: Cursor <cursoragent@cursor.com>
pi052 had the same text-CE collapse bug smolvla2 had — PaliGemma's
embed_prefix flags the language block att=0, so make_att_2d_masks makes
it fully bidirectional and the text cross-entropy degenerates into a
copy task. Ported the three model-specific fixes:
- _mark_target_span_causal: set att=1 on supervised target language
positions so the text-CE is genuine causal next-token prediction.
Applied in both _compute_all_losses_fused and _compute_text_and_fast_loss.
- flow_loss_weight 10.0 -> 5.0: the paper's a=10 swamps the LM head once
the flow-only low_level recipe fires often (matches SmolVLA2Config).
- _flatten_say_tool_calls in the text tokenizer: serialize `say` tool
calls into a <say>...</say> marker so the spoken reply is tokenized
and supervised (PaliGemma's flat prompt has no structured calls, so
they were dropped entirely).
select_message needed no change: pi052's prefix is [images, language]
with no trailing state token, so it already decodes from the last
language token.
Regression tests mirror the smolvla2 attention-masking + tool-call suite.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Pepijn