If you are fighting defensively as a standard or full round action, does the fighting modify your action or does it determine your action?

Would a magus be able to use a spell (with a melee attack component), or would he be able to dual wield (weapon plus magic)?

Looking at the ruling, it's not clear if fighting defensively is like a spring attack that replaces your action, or like a power attack that modifiers your action (with the restriction that you have to attack and can't just do other stuff).

From what I can tell it is simply something that changes how the attack functions but not its action economy. Like power attack and the like.

You made me think about my Magus but it is not possible:

Spell Combat (Ex):..... As a full-round action, he can make all of his attacks with his melee weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action (any attack roll made as part of this spell also takes this penalty).

Fighting Defensively as a Full-Round Action: You can choose to fight defensively when taking a full-attack action. If you do so, you take a –4 penalty on all attacks in a round to gain a +2 dodge bonus to AC for until the start your next turn.

Spell combat is a full-round action, not a full attack action.

Diego is right, but I would allow it in my games.

Well don;t I have egg on my face. I had crossed wires in my head so to speak. I was thinking of Combat Expertise.

Combat Expertise works the same way.

Humm no I have to disagree as it states

"You can only choose to use this feat when you declare that you are making an attack or a full-attack action with a melee weapon."

Spell combat and spell strike still make "attacks". It is not stated as an "Attack action" and the full around attack part is "or" not must or anything like this.

RAW you can declare it before any attack no matter where it comes from or how it is triggered. Likely on AOO's if you wished.

At 1st level, a magus learns to cast spells and wield his weapons at the same time. This functions much like two-weapon fighting, but the off-hand weapon is a spell that is being cast.

This is what makes me ask in the first place.

Full Attack

If you get more than one attack per round because your base attack bonus is high enough (see Base Attack Bonus in Classes), because you fight with two weapons or a double weapon, or for some special reason, you must use a full-round action to get your additional attacks

Wording wise, it treats spellcasting as an offhand weapon, so according to these rules, the full round action at least would qualify.

Fighting Defensively as a Standard Action

You can choose to fight defensively when attacking. If you do so, you take a –4 penalty on all attacks in a round to gain a +2 to AC until the start of your next turn.

At 2nd level, whenever a magus casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any weapon he is wielding as part of a melee attack.

Here too it's open enough that because you are attacking as part of your touch spell, you seem to qualify for fighting defensively. If it was worded differently, such as "Fighting defensively is a standard action that...." and would exclude things like vital strike, then it would definitely exclude the spellstrike.

Humm no I have to disagree as it states

"You can only choose to use this feat when you declare that you are making an attack or a full-attack action with a melee weapon."

Spell combat and spell strike still make "attacks". It is not stated as an "Attack action" and the full around attack part is "or" not must or anything like this.

RAW you can declare it before any attack no matter where it comes from or how it is triggered. Likely on AOO's if you wished.

The wording is ambiguous because it could read two ways:

1) ... making (an attack) or (a full-attack action)...
2) ... making (an attack or a full-attack) action...

It'd be the same as telling someone they can have custard or fruit pie for dessert; do you mean custard pie or fruit pie or do you mean just a bowl of custard or a fruit pie. Sounds like a faq topic to me.