The api_messages build used a shallow msg.copy(), decoupling only
top-level fields. Every nested container (tool_calls entries and their
function dicts, multimodal content-part lists, reasoning_details) stayed
aliased to the persisted history, so ANY in-place transform on the send
copy silently rewrote the stored transcript.
Probed every send-path transform against that aliasing shape on main:
content strip loop safe (top-level reassign)
_canonicalize_api_tool_calls (repair) LEAKED <- #80616's fix
_sanitize_messages_surrogates LEAKED (multimodal parts,
tc ids/args, reasoning)
_sanitize_messages_non_ascii LEAKED (multimodal parts)
_sanitize_api_messages safe
_drop_thinking_only_and_merge_users safe
The retry loop already believed the copies were independent - it
sanitizes messages AND api_messages separately (~L3555) - so the
aliasing was accidental everywhere.
Fix at the chokepoint: _clone_message_for_send clones every container
(dict/list) recursively while sharing immutable leaves, so every
downstream in-place transform - current and future - is safe by
construction. Cost is container-count, not string-bytes: 100KB argument
strings and base64 payloads are shared (measured ~0.5ms vs ~0.1ms per
1500-message build; noise next to one json round-trip). Same clone
applied to the prefill-message insert (same class, same pipeline).
The class-wide invariant test runs the full send-path transform
pipeline over an adversarial fixture (malformed args, surrogates,
non-ASCII, multimodal parts, reasoning fields) and asserts the history
stays byte-identical; an AST contract pins the build-site wiring so the
shallow copy can't quietly return. Both mutation-verified: reverting
the clone to shallow fails 4 isolation tests, unwiring the build site
fails the AST contract.
0xGr1mm's branch fix (previous commit) remains as defense in depth at
the exact site the #80498 incident hit; his regression tests and the
class-wide invariant give layered coverage.
kshitij