In Commander does a commander's color identity include mana symbols in activated ability costs?

A card's color identity in Commander is determined by every mana symbol that appears anywhere on the card — including in the mana cost, rules text, and ability costs. This is defined in the Commander format rules (derived from CR 903.4), which state that a card's color identity is its color plus the color of any mana symbol in its rules text.

CR 903.4 specifically reads: "A card's color identity is its color plus the color of any mana symbol in that card's mana cost or rules text." Activated ability costs are part of a card's rules text, so any colored mana symbol there — such as {R} in an activated ability cost like "{R}: This creature gains first strike" — contributes to the card's color identity.

This means even if a card is colorless or mono-colored in its casting cost, a single off-color mana symbol in an activated ability makes it part of that color's identity. You cannot include it in a Commander deck whose commander's color identity does not include that color.

Example: Imagine a card with a white mana cost but an activated ability that costs {U}. Its color identity is White and Blue. You could not include it in a mono-White Commander deck — you would need a commander whose color identity includes both White and Blue.

Note: Reminder text and flavor text are not rules text, so mana symbols appearing there (which is very rare) do not affect color identity. Also, symbols in the color indicator or characteristic-defining abilities are included per CR 903.4.