fix: don't reject same-day flights on UTC servers, and surface real validation errors.

[scottjg]

I was trying this MCP and it didn't really work out of the box for me:

so here's two fixes:

1. Don't return a generic "Invalid parameter value". Neither an agent nor a person can tell what to fix from that. Pydantic already produces a specific message, so surface it: travel_date: Travel date cannot be in the past.

2. Don't reject same-day flights for travelers west of UTC. The server validates that the travel date isn't in the past, but flight times are always local to the origin airport, and it compares against a naive datetime.now() (i.e. whatever timezone the server happens to run in. Mine runs on a platform with a UTC clock, so from ~5pm PST onward, today's date looks like yesterday and every same-day search is rejected. So, we fix validation to allow utc_today - 1. This should cover any possible timezone discrepancy. We also fix the validation to not allow putting a return date before the departure date on round trips.

Greptile Summary

This PR makes flight-date validation timezone-safe and improves validation feedback. The main changes are:

• Allows dates that may still be today west of UTC.
• Rejects round-trip and multi-city segments in reverse date order.
• Returns detailed Pydantic errors from MCP operations.
• Adds tests for UTC boundaries and segment ordering.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
fli/mcp/server.py	Adds actionable formatting and explicit handling for Pydantic validation errors.
fli/models/google_flights/base.py	Adds a UTC-based lower date bound that accounts for global timezone differences.
fli/models/google_flights/dates.py	Applies the shared date bound and uses UTC today when adjusting old start dates.
fli/models/google_flights/flights.py	Rejects reverse-ordered round-trip and multi-city segments.
tests/models/test_date_search_filters_validation.py	Updates date-range validation tests to use the UTC boundary.
tests/models/test_flight_search_filters_validation.py	Adds coverage for round-trip and multi-city segment ordering.
tests/models/test_flight_segment_validation.py	Covers acceptance of the previous UTC day for same-day travel west of UTC.

_{Reviews (1): Last reviewed commit: "fix(models): stop rejecting same-day fli..." | Re-trigger Greptile}

Context used:

• Context used - CLAUDE.md (source)

[alexechoi]

The utc_today − 1 grace window looks like the right call given there's no per-airport timezone data in the repo — it never rejects a date that's still today-or-future for the actual traveler, at the acceptable cost of letting a genuinely-past date through to Google. One edge case I think the loosened rule opens, though:

Inverted date ranges become constructible in DateSearchFilters. With to_date now allowed to be utc_today − 1, validate_and_adjust_from_date can clamp from_date past to_date: DateSearchFilters(from_date=utc_today−10, to_date=utc_today−1) constructs successfully with from_date > to_date and gets sent to Google as-is. The swap logic in validate_date_order is a field validator that has already run by then, so nothing re-checks the ordering. Clamping to min(current_date, to_date) (or re-asserting order at the end of the model validator) would close it — and notably, the reversed-dates assertion removed from test_date_search_today_to_date was covering exactly this path.

Two smaller notes: the "To date must be in the future" message no longer matches the new rule (it now accepts yesterday-UTC), and the multi-city branch reuses the "Return date … cannot be before departure date" wording when comparing leg 3 to leg 2, which will read oddly for someone debugging a 3-leg itinerary.

Coordination point: #226 (just opened) adds another parsed < datetime.now().date() validator in fli/models/google_flights/explore.py — the same naive-now pattern this PR removes. Happy to switch it to earliest_searchable_date() (and add the except ValidationError branch to the new explore executor) once this lands, in whatever shape it lands.

[scottjg]

i updated the pr, as per your review.