sglang/.claude/rules/schedule-batch-out-of-place-mutation.md

paths

paths
python/sglang/srt/**/*.py

Mutate ScheduleBatch fields out of place

Never mutate a ScheduleBatch direct field in place — no .extend()/.append(), +=/-=/|=, tensor .add_()/.fill_(), or slice/index assignment. Build the new value and rebind the field:

# Bad
self.reqs.extend(other.reqs)
self.seq_lens.add_(1)
self.extend_lens[i] -= encoder_len

# Good
self.reqs = self.reqs + other.reqs
self.seq_lens = self.seq_lens + 1
lens = self.extend_lens[:]; lens[i] -= encoder_len; self.extend_lens = lens  # loop a copy, rebind once

Why: copy() snapshots and the overlap scheduler's queued references rely on old objects staying frozen (the prerequisite for the per-step immutable ScheduleBatch refactor); derived values like seq_lens_sum only recompute safely when fields are rebound.

Out of scope (mutation intended): penalizer buffers, CUDA graph static buffers, ForwardBatch fields, attention-backend metadata.