Feature: Expose per-flight CO2 emissions from API response

[Paullyoung]

Feature: Expose per-flight CO2 emissions from API response

Summary

Google Flights returns estimated CO2 emissions for every flight in the API response, but fli currently discards this data during parsing. Adding emissions to FlightResult would make fli the only programmatic tool that surfaces Google's per-flight carbon estimates.

Where the data lives

The emissions data is in leg_block[22] of the raw API response, alongside the flight details already being parsed in _parse_flights_data. Specifically:

Index	Field	Example (JFK-LAX nonstop)	Unit
[22][7]	Flight CO2 emissions	240000	grams CO2
[22][8]	Route typical (median) emissions	364000	grams CO2
[22][10]	Route high emissions benchmark	411000	grams CO2
[22][3]	% difference from typical	-34	percentage points

Verified across multiple flights, routes, and cabin classes. The values match what Google Flights displays in its UI (e.g., "240 kg CO2, 34% less than typical").

Proposed changes

1. Add emissions fields to FlightResult

class FlightResult(BaseModel):
    legs: list[FlightLeg]
    price: NonNegativeFloat
    currency: str | None = None
    duration: PositiveInt
    stops: NonNegativeInt
    # New fields (optional to avoid breaking existing consumers)
    emissions_grams: int | None = None        # per-flight CO2 in grams
    typical_emissions_grams: int | None = None # route median CO2 in grams
    emissions_diff_pct: int | None = None      # % vs typical (-34 = 34% less)

2. Parse emissions in SearchFlights._parse_flights_data

# In _parse_flights_data, after existing parsing:
emissions_grams = None
typical_emissions_grams = None
emissions_diff_pct = None
try:
    emissions_block = data[0][22]
    if emissions_block and len(emissions_block) > 10:
        emissions_grams = emissions_block[7]
        typical_emissions_grams = emissions_block[8]
        emissions_diff_pct = emissions_block[3]
except (IndexError, TypeError):
    pass

3. Surface in CLI and MCP output

Add an --emissions flag or include by default in --format json. For the MCP server, include in the tool response so AI assistants can factor emissions into recommendations.

Why this matters

• Google is the only major flight search that provides per-flight emissions estimates, and right now there's no programmatic way to access them
• The EmissionsFilter and SortBy.EMISSIONS options already exist in fli, but users can't see the actual numbers they're filtering/sorting by
• Growing demand from developers building sustainability-aware travel tools

Testing notes

I verified the data across:

• Multiple routes (JFK-LAX, LAX-SYD, LAX-MEL)
• Nonstop and connecting flights
• Economy and business class

The [22] block is consistently present. When emissions data isn't available, the block is either None or the relevant indices are None, so the try/except fallback handles it cleanly.