Can you permanency shrink item on a magic weapon in pathfinder? Can I have a magic Longspear that can shrink on command?
You are able to shrink one nonmagical item (if it is within the size limit) to 1/16 of its normal size in each dimension (to about 1/4,000 the original volume and mass). This change effectively reduces the object's size by four categories. Optionally, you can also change its now shrunken composition to a clothlike one. Objects changed by a shrink item spell can be returned to normal composition and size merely by tossing them onto any solid surface or by a word of command from the original caster. Even a burning fire and its fuel can be shrunk by this spell. Restoring the shrunken object to its normal size and composition ends the spell.Shrink item can be made permanent with a permanency spell, in which case the affected object can be shrunk and expanded an indefinite number of times, but only by the original caster.
Bolding mine.
Not with the Shrink Item spell. You could have a masterwork one shrunk, and keep it and an oil of greater magic weapon in your pocket for emergencies though.
In 3.5 there were a few different ways to re-size spears in the magic item compendium (collapsable, changeling) but I'm unsure if any found their way to official Pathfinder products.
No... if the spell is not on the list for permanency, it can't happen.
Which Shrink Item is...
And then of course there is the following:
The GM may allow other spells to be made permanent.
followed by the fact there are spells that came out after the core rulebook that have spells that explicitly state they can be made permanent with the permanency spell.
These spells of course are not on the list.
The only problem with the OP's plan is the fact the weapon is magical -- which makes it something shrink item can't affect.
If you have a masterwork shrinking item can it be enchanted at that point? Would you be adding to an already magic weapon at that point?
Once the weapon becomes magic then it is no longer a valid target for the shrink item spell.
Shrink item says it's non-magical only.
You want to read all the magic creation rules on p.549-551 and work with your GM. You can use the spell effect if you use it that way. Probably something like adding to the spear a 'command word spell effect'
spell level x caster level x 1800gp
Shrink Item's 3rd level, so minimum caster level is 5th, so it would add 27,000gp to the cost of the item. It's a 'different attribute" so your GM might let you cut that in half. Crafting actually costs half that do it yourself, and you can add limits to reduce it further, like times per day. All depands on how much your GM wants to deal with and is willing to let you do, but it's all in there.