WebUI keeps two parallel arrays per session: messages (display transcript) and
context_messages (what's sent to the model). In large/compacted sessions they
diverge. Forking / in-place truncation copies a prefix of both and relies on
truncate_context_for_display_keep() to translate a display keep-index into the
matching context index — an aligner that prefers a stable per-row `id`. But the
model-context rows carried neither id nor timestamp (a 989-session live scan
found 0 with any id), so alignment fell back to fragile content-signature
matching that goes ambiguous on repeated tool calls / empty assistant turns.
This closes the DATA gap (the alignment logic itself already prefers `id` and
handles the compacted case via the matcher, shipped in #5563): mint a monotonic,
session-unique integer `id` on the per-turn result rows AFTER the context restore
and BEFORE both arrays are built, so the display and model-context copies of a
logical row share the same id.
- api/streaming.py: new _assign_stable_message_ids(); _restore_reasoning_metadata
carries `id` forward across turns (as it already does timestamp); wired into all
three streaming commit sites (main turn, retry, self-heal).
- api/routes.py: same mint on the runs/MoA _handle_chat_sync commit path.
- api/gateway_chat.py: mints ids on the two new rows of a gateway turn.
- The `id` is stripped before the provider API call (not in _API_SAFE_MSG_KEYS),
same treatment as timestamp — nothing new reaches the provider.
session_ops.py alignment was already delivered by #5563 (id-preferring matcher +
compacted-case fall-through), so no change is needed there now; tests updated to
assert the post-#5563 behavior (id-bearing = exact cut; id-less = errs toward
under-keeping, never the old raw-index mis-cut).
Co-authored-by: b3nw <b3nw@users.noreply.github.com>
