I've added lsp-workspace-symbol command which takes query as the only parameter. Then it opens result as a separate buffer. Rendering buffer is implemented as lsp-show-workspace-symbol which you can override to, for example, pass matches to fuzzy finder instead. lsp-show-workspace-symbol takes two parameters, first one is project root and second is a list of matches in *grep*-buffer-like format (passed as a single string).

Verified that it works!

One thing here is that fuzzy matching is already built into workspace/symbols request. So ideally, the user-visible UI should be an input-field for a query plus a listing of the results, such that results are automatically updated when the input field changes, and "update" is basically a fresh $workspace/symbols request to the language server.

So, internally having an lsp-worksapce-symbol command with one argument which returns a list of strings is right, but the ideal user-visible API probably should not expose that directly, instead wrapping it into some kind of incremental search UI.

Yes, it's a good idea, I just didn't realize that it's possible to do with Kakoune builtin functionality. Now I think that prompt with -on-change callback is enough to implement it.

As uncovered it's non-trivial to use -on-change with arbitrary commands, but I think I got decent solution. I added lsp-workspace-symbol-incr command which does incremental search using prompt for query input.

Feel free to re-open this issue or create a new one if you think command's UX should be improved or you hit any bugs.