Avoid mutating ScheduleBatch fields in place (#30672)

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

@@ -0,0 +1,30 @@
+---
+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:
+
+```python
+# 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.