The `sessions` table records only the initial (model, billing_provider)
for a session, so when a user switches models mid-session (via `/model`
or programmatically) every token — including the switched model's — is
attributed to the first model. Insights/billing reports then hide the
cost of the new model entirely (e.g. a session that started on deepseek
and switched to opus shows $0 for opus).
Add a `session_model_usage` table keyed (session_id, model,
billing_provider) that accumulates each per-API-call delta under the
model active at the time of the call. `update_token_counts()` is the
single chokepoint every per-call delta flows through (CLI, gateway,
cron, delegated, codex), so recording there captures accurate
attribution on every platform. Only the incremental path records — the
gateway's `absolute=True` summary overwrite is skipped to avoid
double-counting cumulative totals that can't be split per model. When a
call omits the model, it falls back to the session's recorded model,
matching the existing COALESCE-from-session summary behaviour.
Insights `_compute_model_breakdown` now aggregates tokens and cost from
`session_model_usage`, so a switched session splits correctly across
models, with a defensive fallback to the per-session aggregate for any
session lacking usage rows. A v17 migration backfills one usage row per
existing token-bearing session from its aggregate totals (idempotent via
INSERT OR IGNORE), validated lossless against a 1.3 GB production DB.
Tests: per-model recording, mid-session split, model fallback, absolute
no-double-count, v17 backfill, and an insights-level switch breakdown.
Fixes#51607.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Thomas Connally
cb7f6bbb2e