How Do Filetype Plugins Interact With Vim Auto-Indent?

I get picky about editor behavior, so I like to think in terms of responsibilities: ftplugins are for configuration and user conveniences, indent scripts are for the algorithm. The key detail is that both operate per-buffer, so they communicate by reading and writing buffer-local options. Order matters because buffer-local options don't merge — they get replaced. That’s why you’ll often see an ftplugin set shiftwidth and an indent/.vim set indentexpr; if indentexpr is present, Vim uses that function to compute indentation regardless of simpler flags like 'smartindent'.

A few nuanced rules I rely on: FileType autocommands trigger ftplugin loading (which commonly runs first), but distributions and plugins can change load order; indent scripts often guard themselves with b:did_indent to avoid double-loading, so you can set that flag manually in an ftplugin to prevent an indent script from running. For persistent tweaks I use after/ftplugin/.vim or after/indent/.vim so my preferences load after built-in scripts. And when debugging, :setlocal, :verbose setlocal, and :scriptnames are my friends — they tell me who stomped on what. In short, treat ftplugin for the style and indent/ for the brain, and use 'after' or b:did_indent when you want to pick the winner.

If you've ever opened a file in Vim and wondered why indentation behaves one way in one project and differently in another, the way filetype plugins and indent scripts interact is the usual culprit. In my messy but beloved setup I keep separate snippets in ~/.vim/ftplugin/ and ~/.vim/indent/ and they each have a job: ftplugin files generally set buffer-local editing options (things like shiftwidth, tabstop, expandtab, mappings) while indent scripts (under indent/) provide indentation logic by setting 'indentexpr', 'cindent', 'indentkeys', or related buffer-local options. Because these are buffer-local, whichever script writes a particular option last wins for that buffer.

Practically that means you can get conflicts. An ftplugin might set 'shiftwidth' to 4 for 'python' and an indent script might expect 2; or an indent script will set 'indentexpr' to a custom function that overrides simpler behaviors such as 'autoindent'. The usual fixes I use are: enable both with :filetype plugin indent on, then put overrides in after/ftplugin/ or after/indent/ so they load later; or explicitly set local options with setlocal in a ftplugin; or prevent an indent script with let b:did_indent = 1 if you deliberately want to skip it. For debugging, :scriptnames shows what got sourced, and :verbose setlocal shiftwidth? / :verbose setlocal indentexpr? tell you who last changed a setting. I like keeping ftplugin for styling and small mappings, and leaving indentation math to indent scripts — but I always keep an 'after' copy for those moments when I need the last word.

I usually keep things simple: filetype plugins and indent scripts both run when filetype detection is enabled, but they have different jobs. The ftplugin configures buffer-local settings and convenience stuff, and the indent script provides the indentation logic (often by setting 'indentexpr' or enabling 'cindent'). Because these scripts set buffer-local options, whichever one runs later or explicitly sets an option last determines the final behavior in that buffer.

If you need to override built-in behavior, put changes in after/ftplugin/ or after/indent/ so they load after the defaults, or disable indent scripts with :filetype indent off. For quick fixes, :verbose setlocal shiftwidth? and :scriptnames show recent changes and loaded scripts. It’s worked for me whenever a project insists on tabs while my brain prefers spaces.

I tend to tinker a lot, so here's the practical short guide I use every time Vim mis-indents: Vim treats filetype plugins (ftplugin/.vim) and indent scripts (indent/.vim) as separate things. The ftplugin usually configures buffer-specific options like tabstop or shiftwidth and adds convenient mappings, while the indent script defines the actual indentation logic by setting 'indentexpr', 'cindent', or 'indentkeys'. If both try to control the same option, the last file that sets it wins for that buffer.

To control the order, you can place custom changes in after/ftplugin/ or after/indent/ so they load later, or set let b:did_indent = 1 inside an ftplugin to skip the indent script. If you want to turn off indent scripts entirely, use :filetype indent off. When things go weird I use :scriptnames to see what loaded and :verbose setlocal indentexpr? to find who changed it. That little workflow saves me from pointless reformatting fights.