Constructive review — this closes the billing half of #24029, but I think it leaves a gap (and, for a common config, can relocate the silent-substitution problem rather than fix it).

What the PR does well

_resolve_openrouter_aux_model() makes auxiliary tasks defer to the user's fallback_providers OpenRouter entry instead of the hardcoded _OPENROUTER_MODEL. That correctly addresses the "don't silently use a hardcoded paid model" complaint.

Gap 1 — meta-routers aren't concrete models

The method returns the first fallback_providers entry where provider == "openrouter" and model is non-empty, verbatim. That assumes the configured model is a concrete slug (e.g. …:free). But openrouter/auto and openrouter/free are meta-routers, not models — openrouter/auto selects a model per-request via NotDiamond, non-deterministically by design.

A very common reliability config is:

fallback_providers:
  - provider: openrouter
    model: openrouter/auto
  - provider: anthropic
    model: anthropic/claude-sonnet-4-20250514

With this PR, _resolve_openrouter_aux_model() returns openrouter/auto, so auxiliary tasks (title generation, compression, vision, web summarization) now get routed through the auto-router. The aux model is no longer hardcoded — but it's now whatever the router picks, with no record of what that was.

Gap 2 — the disclosure half of #24029 is still open

The issue's expected behavior included: "at minimum, log a clear warning that a paid/substitute model is being used." This PR adds no such surfacing — the only log on this path is a generic logger.debug. So a substitution still happens invisibly; it's just sourced from config now instead of a constant.

Evidence that the substitution is invisible

I hit the same fallback_providers → openrouter/auto path on a (non-auxiliary) cron run, instrumented via a telemetry plugin that records each call's response model into SQLite. The run was launched on one model and executed entirely on another, with status: ok and no warning.

Run record (declared at session start):

session_id  cron_7fc40f2b1cef_20260606_154226
model       nous/hermes-4-70b-thinking
provider    openrouter
status      ok
api_calls   6
cost_usd    0.00445815

Per-call records (response_model) for that same session:

All 6 calls ran on google/gemini-2.5-flash-lite, estimated=0 (real usage, not estimated); api_calls in the run record matches the 6 rows and the per-call costs sum exactly to the run's cost_usd; and nothing anywhere records the declared nous/hermes-4-70b-thinking as executed. The router's selection is returned in each response's model field — the data exists, it's just never surfaced. This PR would route aux tasks into exactly that opaque path for any config with openrouter/auto in fallback_providers.

Suggested adjustments

1. In _resolve_openrouter_aux_model(), detect meta-router slugs (openrouter/auto, openrouter/free). Either skip them when picking the aux model, or accept them but capture and surface the resolved model from the response rather than logging the router slug.
2. Add the disclosure Auxiliary tasks silently fall back to paid OpenRouter models, bypassing user's free-only configuration #24029 asked for — a single notice along the lines of aux task: requested X, ran Y — and make it cover this fallback path, not only the hardcoded one.