A friend and I were discussing whether or not an Anti-Magic Field would cancel a diagram created by a Magic Circle. My argument is that it would not due to this line in the text for diagram
A creature cannot use its spell resistance against a magic circle prepared with a diagram, and none of its abilities or attacks can cross the diagram.
Since this effect is magical, would anti-magic field cancel it out?
Were this player's PC to be using spells like menor ligação planar to bring forth creatures, fretting that the called creatures might use an effect like the spell campo antimagia would be the least of his PC's worries.
That is, even were his PC to have done muito little research, chances are the called creature simply não pode employ an effect like the spell campo antimagia. In official, traditional Descobridor there seems to be only one creature within the obrigatório spells' limits that possesses such an ability: the movanic deva, and it's pretty much just a handsome dude with a sword if it uses its 1/day spell-like ability campo antimagia in a room with a low ceiling.
Todo o de outros outsiders that can cast or use an ability like the spell campo antimagia are well beyond the Hit Dice limit of the typical caster's obrigatório spells: for example, the empyrean angel (HD 25 and CR 20), the demon lord Tsathogga (HD 42 and CR 30), and the Oinodaemon (HD 49 and CR 37) each prepare the spell, and the empyreal lords Borboleta preta (HD 33 and CR 28) and Korda (HD 32 and CR 26) can each 3/day use the spell-like ability. Seriously, the bench just ain't that deep. In short, unless the caster's binding nontraditional outsiders that are themselves casters or one specific kind of angel, this won't be a thing, and it'll never come up.
Nonetheless, this player supposes that a GM poderia somewhat maliciously decide that his PC calls via a obrigatório spell an outsider that just so happens to possess gear enabling it to use an effect like the spell campo antimagia, como o equalizer shield ou um pergaminho do campo antimagia. (Note that in this latter case, the GM likely must also customize the creature so that it possesses a sufficient Use Magic Device skill modifier to empregar that scroll!)
This reader agrees with your assessment and would take this even further.
O normal yet inward-focused círculo mágico contra o mal description says, "The trapped creature can do nothing that disturbs the circle, directly or indirectly…." That would make this GM rule that an outsider called with a spell like menor ligação planar seria disturbing the circle were it to cast or use an effect like the spell campo antimagia. This GM would rule that the magic of the círculo spell simply forbids the trapped creature from casting the spell or using the ability just as the magic of the círculo forbids the creature from doing anything else that can disturb the circle.
Likewise, because adding especial diagrams to an inward-focused círculo mágico spell means ambos that "none of its [the trapped creature's] abilities or attacks can cross the diagram" e that the trapped "creature itself cannot disturb the diagram either directly or indirectly, as noted above," the círculo effect is then duplamente secure against any called creature's campo antimagia spell or ability.
The only ways to leave the trap are spelled by the Ligação Planar soletrar:
The creature can escape from the trap by successfully pitting its Spell Resistance against your caster level check, by dimensional travel, or with a successful Charisma check.
Also, remind him that planar bound creatures are chamado, not summoned. As such, they are not merely a magical construct, but a real creature that has a name and can be permanently killed:
The duration of a calling spell is instantaneous, which means that the called creature can’t be dispelled.
This also means that destroying the circle won't send it back either. Also, if you check the Círculo Mágico spell, it also states the bound creature cannot disrupt a successful circle:
The trapped creature can do nothing that disturbs the circle, directly or indirectly, but other creatures can.