What is the color identity of a card with a hybrid mana symbol in its mana cost for Commander deckbuilding?

In Commander, a card's color identity is determined by every mana symbol that appears on the card — in its mana cost, rules text, and characteristic-defining abilities. Hybrid mana symbols count as both colors they represent, not just one or the other (CR 903.4, CR 202.2c).

This means a card like Beseech the Mirror, which has {2}{B/R} in its mana cost, has a color identity of black and red. You cannot include it in a deck whose commander is only black — you would need a commander that includes red in its color identity as well.

The reasoning is straightforward: hybrid symbols are defined as belonging to both colors simultaneously (CR 107.4e). Since color identity captures every color of mana symbol on the card, a hybrid symbol contributes both colors to the identity.

Concrete example: Flame-Kin Zealot has the hybrid symbol {R/W} in its cost. Its color identity is red AND white. You may only include it in a Commander deck whose commander has both red and white in its color identity (e.g., Aurelia, the Warleader). A mono-red commander like Krenko, Mob Boss cannot lead a deck containing it.

There is no exception or workaround: even if you could pay the hybrid cost using only one color of mana, the symbol still locks both colors into the card's identity for deckbuilding purposes (CR 903.5a).