pi0_fast: fix off-by-one in next-token positions; add regression test #707
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aryanrahar
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Michael-Equi,
jimmyt857,
kvablack and
uzhilinsky
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This PR fixes an off-by-one bug in pi0_fast.py when computing positions for the next decoded token. Prefix token positions are 0-indexed (0..L-1), so the first new token after a prefix of length L must be placed at position L (not L+1).
Fix: drop the + 1 when building positions.
Fixes #705.
Changes-
src/openpi/models/pi0_fast.py: 1-line change to compute contiguous, zero-indexed positions.
src/openpi/models/pi0_fast_positions_test.py: small regression test asserting:
next token after length L is at L
advancing step increments positions by 1
Scope / Compatibility
Affects JAX pi0-FAST decode path only.
No API changes. No impact on the PyTorch path (pi0-FAST not supported there).
No performance impact.
How I tested
uv run pytest -q src/openpi/models/pi0_fast_positions_test.py passes.
Verified the minimal repro before/after values locally.
Ran ruff + ruff-format hooks.