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I don't have a clear vision of exactly what's too short here, but it sounds a lot like it could be solvable with a cedar shim shingle or a carefully cut piece of PT wood, jammed into the gap. Not sure what the inspector would think of that, but it'll bear a load okay. The holistic solution is probably to cut the bracket off with a sawzall and cut a new post of the correct length, then install a new bracket.

Just added.

Beam as in handrail?

Edit: nvm when you say the top of the posts I thought you meant for the railing. If I'm visualizing what your talking about correctly, shimming it should be fine. Could use composite shims also to prevent it from rotting or rusting.

I've added a picture for more info. My only concern with using a wood shim is the potential instability. Wouldn't in be likely for the shim to degrade over time and leave the gap in place?

Shims are ok as long as they can support the load.

A cedar shingle will crush under relatively low load.

Steel won't crush if you can source it.

Plywood is also good but would likely need to be treated if it's exterior.

In the states one can buy synthetic shims that would likely suffice for a deck load.

Also cutting your own shim stock from treated lumber is an option.

This is exactly what I was hoping. I'll call the permitting office today to make sure there's no fuckery with the inspection, but this is great info for which material to get, thanks!

1/4” piece of steel plate or 1/4 slate use as shim

That's what I was thinking. Thanks!

You should be fine with the metal shim especially if it's inside the bracket.

I had to do something similar for a deck between the screw pile and the beam resting on top. I found a metal supply store and ordered a 2" steel plate cut to size. Then I used exterior metal paint to help prevent rust.

Thanks for the advice. Last thing I want is to fail inspection after working on this massive thing!

You'll want to use a material which has similar properties to the column or stronger. Nothing wrong with a metal plate, but if the columns are PT lumber, then a PT shim should be OK. Wood does compress more across the grain than along it, but not by enough to be a big deal in a deck. Cedar is also pretty rot-resistant. If you've got an inspector coming around, the metal plate others have suggested may be better, if for nothing else than because it looks like you took it seriously. But the way to actually take it seriously is to replace the column.

4x4, 8x8? Cut half way down about 12-20" so you have 2x4 or 4x8. Match that cut with another post section. Trim standing post down to a manageable gap luke 6". Cut new section to height taking into account the new geometry of the standing post. Put both post together with 1 or 2 screws yo hold the position, use carriage bolts with washers on nut to go through both cuts to marry the joint. Could even glue the joint to mitigate water ingress or standing water.

As mentioned, a shim can be called sufficient but it's a bandaid and not a proper fix

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Don't use plywood for anything exposed to the elements. The glue will separate and de-laminate itself. Solid wood only for outside and element exposed construction. Especially a deck. Steel is not a good option either as it will rust. Galvanized is ok and might act like a post cap preventing the post from sponging water through the end grain.

Best method for repair is still splicing or replacing the post

If you go with steel, try to match the types of steel if there is metal to metal contact. I would guess the bracket would be zinc plated or galvanized. Otherwise you will be setting up a galvanic cell whenever it gets rainy or humid, which will accelerate corrosion.

Put a vapor barrier on top of the post.

I would get a "post saddle" like a Simpson strong tie or similar that you can buy at Lows/Home Depot and just shim it with something that wont rot like a piece of synthetic decking cut down to proper size.

Shim it. It’s a compression load, the brackets take the side load.

If ur so worried about shimming it just cut a new post they are cheap enough

This is what I would do. By the time I found something suitable to shim with I could have bought and cut a new post.

isnt galvanizing just a hot dip into a zinc bath?

The problem is that the old bracket won't come out, so I'd need to buy another $25 bracket and hope I don't fuck up the bolt in the process.

Galvanizing is. But galvanic corrosion occurs when dissimilar metals come into contact. The zinc will corrode first in such circumstances, but it's better to avoid the situation in the first place.

There are 2 method Hot dip galvanized is dipped into a hot bath if zinc making a good rust protection (dull grey)

Zinc plating is a thinner zinc coated applied using an electric current (shiny silver) and is less effective and les durable than hot dip.

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OP - don't create some kind of Frankenstein post like this comment is describing. Either shim or replace, trying to splice the post will result in a failed inspection. Plus, it would look ridiculous.

Calling a shim a band-aid, while suggesting you hack together something that only looks like a 6x6, is hilarious.

Keep in mind if you fail inspection, you just need to fix the issue and then get it re-inspected. Not a big deal.

White oak shim wouldn't compress nearly enough to make a problematic difference.

Easiest would be a metal/steel plate

Cut all of the other posts 1/4" shorter than they are; that is, admit you made a mistake, go back to square one, make all posts match.

Also supply to us the printing shown at the bottom left of the image. You want advice and you're not telling us everything you know!

Splicing is a common trade operation.. op speak to your local concil on local codes. Just Google for visual reference.

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My idea was also shim it with white oak, ipe, osage orange or black locust

go to Home Depot and buy a board stretcher