diff --git a/runtime/doc/lsp.txt b/runtime/doc/lsp.txt
index 8a0b144e83..3151a17417 100644
--- a/runtime/doc/lsp.txt
+++ b/runtime/doc/lsp.txt
@@ -1719,7 +1719,7 @@ extract_completion_items({result})
       • {result}  (table) The result of a `textDocument/completion` request
 
     Return: ~
-        (table) List of completion items
+        lsp.CompletionItem[] List of completion items
 
     See also: ~
       • https://microsoft.github.io/language-server-protocol/specification#textDocument_completion
@@ -2014,7 +2014,7 @@ text_document_completion_list_to_complete_items({result}, {prefix})
       • {prefix}  (string) the prefix to filter the completion items
 
     Return: ~
-        (table) { matches = complete-items table, incomplete = bool }
+        table[] items
 
     See also: ~
       • complete-items
diff --git a/runtime/lua/vim/lsp.lua b/runtime/lua/vim/lsp.lua
index 4b1da94d77..82a88772bd 100644
--- a/runtime/lua/vim/lsp.lua
+++ b/runtime/lua/vim/lsp.lua
@@ -2273,24 +2273,6 @@ function lsp.buf_notify(bufnr, method, params)
   return resp
 end
 
----@private
-local function adjust_start_col(lnum, line, items, encoding)
-  local min_start_char = nil
-  for _, item in pairs(items) do
-    if item.textEdit and item.textEdit.range.start.line == lnum - 1 then
-      if min_start_char and min_start_char ~= item.textEdit.range.start.character then
-        return nil
-      end
-      min_start_char = item.textEdit.range.start.character
-    end
-  end
-  if min_start_char then
-    return util._str_byteindex_enc(line, min_start_char, encoding)
-  else
-    return nil
-  end
-end
-
 --- Implements 'omnifunc' compatible LSP completion.
 ---
 ---@see |complete-functions|
@@ -2307,82 +2289,7 @@ function lsp.omnifunc(findstart, base)
   if log.debug() then
     log.debug('omnifunc.findstart', { findstart = findstart, base = base })
   end
-
-  local bufnr = resolve_bufnr()
-  local clients = lsp.get_clients({ bufnr = bufnr, method = ms.textDocument_completion })
-  local remaining = #clients
-  if remaining == 0 then
-    return findstart == 1 and -1 or {}
-  end
-
-  -- Then, perform standard completion request
-  if log.info() then
-    log.info('base ', base)
-  end
-
-  local win = api.nvim_get_current_win()
-  local pos = api.nvim_win_get_cursor(win)
-  local line = api.nvim_get_current_line()
-  local line_to_cursor = line:sub(1, pos[2])
-  local _ = log.trace() and log.trace('omnifunc.line', pos, line)
-
-  -- Get the start position of the current keyword
-  local match_pos = vim.fn.match(line_to_cursor, '\\k*$') + 1
-  local items = {}
-
-  local startbyte
-
-  local function on_done()
-    local mode = api.nvim_get_mode()['mode']
-    if mode == 'i' or mode == 'ic' then
-      vim.fn.complete(startbyte or match_pos, items)
-    end
-  end
-
-  for _, client in ipairs(clients) do
-    local params = util.make_position_params(win, client.offset_encoding)
-    client.request(ms.textDocument_completion, params, function(err, result)
-      if err then
-        log.warn(err.message)
-      end
-      if result and vim.fn.mode() == 'i' then
-        -- Completion response items may be relative to a position different than `textMatch`.
-        -- Concrete example, with sumneko/lua-language-server:
-        --
-        -- require('plenary.asy|
-        --         ▲       ▲   ▲
-        --         │       │   └── cursor_pos: 20
-        --         │       └────── textMatch: 17
-        --         └────────────── textEdit.range.start.character: 9
-        --                                 .newText = 'plenary.async'
-        --                  ^^^
-        --                  prefix (We'd remove everything not starting with `asy`,
-        --                  so we'd eliminate the `plenary.async` result
-        --
-        -- `adjust_start_col` is used to prefer the language server boundary.
-        --
-        local encoding = client.offset_encoding
-        local candidates = util.extract_completion_items(result)
-        local curstartbyte = adjust_start_col(pos[1], line, candidates, encoding)
-        if startbyte == nil then
-          startbyte = curstartbyte
-        elseif curstartbyte ~= nil and curstartbyte ~= startbyte then
-          startbyte = match_pos
-        end
-        local prefix = startbyte and line:sub(startbyte + 1) or line_to_cursor:sub(match_pos)
-        local matches = util.text_document_completion_list_to_complete_items(result, prefix)
-        vim.list_extend(items, matches)
-      end
-      remaining = remaining - 1
-      if remaining == 0 then
-        vim.schedule(on_done)
-      end
-    end, bufnr)
-  end
-
-  -- Return -2 to signal that we should continue completion so that we can
-  -- async complete.
-  return -2
+  return require('vim.lsp._completion').omnifunc(findstart, base)
 end
 
 --- Provides an interface between the built-in client and a `formatexpr` function.
diff --git a/runtime/lua/vim/lsp/_completion.lua b/runtime/lua/vim/lsp/_completion.lua
new file mode 100644
index 0000000000..efd1aaacf7
--- /dev/null
+++ b/runtime/lua/vim/lsp/_completion.lua
@@ -0,0 +1,210 @@
+local M = {}
+local api = vim.api
+local lsp = vim.lsp
+local protocol = lsp.protocol
+local ms = protocol.Methods
+
+---@param input string unparsed snippet
+---@return string parsed snippet
+local function parse_snippet(input)
+  local ok, parsed = pcall(function()
+    return require('vim.lsp._snippet_grammar').parse(input)
+  end)
+  return ok and tostring(parsed) or input
+end
+
+--- Returns text that should be inserted when selecting completion item. The
+--- precedence is as follows: textEdit.newText > insertText > label
+---
+--- See https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
+---
+---@param item lsp.CompletionItem
+---@return string
+local function get_completion_word(item)
+  if item.textEdit ~= nil and item.textEdit.newText ~= nil and item.textEdit.newText ~= '' then
+    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
+      return item.textEdit.newText
+    else
+      return parse_snippet(item.textEdit.newText)
+    end
+  elseif item.insertText ~= nil and item.insertText ~= '' then
+    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
+      return item.insertText
+    else
+      return parse_snippet(item.insertText)
+    end
+  end
+  return item.label
+end
+
+---@param result lsp.CompletionList|lsp.CompletionItem[]
+---@return lsp.CompletionItem[]
+local function get_items(result)
+  if result.items then
+    return result.items
+  end
+  return result
+end
+
+--- Turns the result of a `textDocument/completion` request into vim-compatible
+--- |complete-items|.
+---
+---@param result lsp.CompletionList|lsp.CompletionItem[] Result of `textDocument/completion`
+---@param prefix string prefix to filter the completion items
+---@return table[]
+---@see complete-items
+function M._lsp_to_complete_items(result, prefix)
+  local items = get_items(result)
+  if vim.tbl_isempty(items) then
+    return {}
+  end
+
+  local function matches_prefix(item)
+    return vim.startswith(get_completion_word(item), prefix)
+  end
+
+  items = vim.tbl_filter(matches_prefix, items) --[[@as lsp.CompletionItem[]|]]
+  table.sort(items, function(a, b)
+    return (a.sortText or a.label) < (b.sortText or b.label)
+  end)
+
+  local matches = {}
+  for _, item in ipairs(items) do
+    local info = ''
+    local documentation = item.documentation
+    if documentation then
+      if type(documentation) == 'string' and documentation ~= '' then
+        info = documentation
+      elseif type(documentation) == 'table' and type(documentation.value) == 'string' then
+        info = documentation.value
+      else
+        vim.notify(
+          ('invalid documentation value %s'):format(vim.inspect(documentation)),
+          vim.log.levels.WARN
+        )
+      end
+    end
+    local word = get_completion_word(item)
+    table.insert(matches, {
+      word = word,
+      abbr = item.label,
+      kind = protocol.CompletionItemKind[item.kind] or 'Unknown',
+      menu = item.detail or '',
+      info = #info > 0 and info or nil,
+      icase = 1,
+      dup = 1,
+      empty = 1,
+      user_data = {
+        nvim = {
+          lsp = {
+            completion_item = item,
+          },
+        },
+      },
+    })
+  end
+  return matches
+end
+
+---@param items lsp.CompletionItem[]
+local function adjust_start_col(lnum, line, items, encoding)
+  local min_start_char = nil
+  for _, item in pairs(items) do
+    if item.textEdit and item.textEdit.range.start.line == lnum - 1 then
+      if min_start_char and min_start_char ~= item.textEdit.range.start.character then
+        return nil
+      end
+      min_start_char = item.textEdit.range.start.character
+    end
+  end
+  if min_start_char then
+    return vim.lsp.util._str_byteindex_enc(line, min_start_char, encoding)
+  else
+    return nil
+  end
+end
+
+---@param findstart integer 0 or 1, decides behavior
+---@param base integer findstart=0, text to match against
+---@return integer|table Decided by {findstart}:
+--- - findstart=0: column where the completion starts, or -2 or -3
+--- - findstart=1: list of matches (actually just calls |complete()|)
+function M.omnifunc(findstart, base)
+  local bufnr = api.nvim_get_current_buf()
+  local clients = lsp.get_clients({ bufnr = bufnr, method = ms.textDocument_completion })
+  local remaining = #clients
+  if remaining == 0 then
+    return findstart == 1 and -1 or {}
+  end
+
+  local log = require('vim.lsp.log')
+  -- Then, perform standard completion request
+  if log.info() then
+    log.info('base ', base)
+  end
+
+  local win = api.nvim_get_current_win()
+  local pos = api.nvim_win_get_cursor(win)
+  local line = api.nvim_get_current_line()
+  local line_to_cursor = line:sub(1, pos[2])
+  log.trace('omnifunc.line', pos, line)
+
+  local word_boundary = vim.fn.match(line_to_cursor, '\\k*$') + 1 --[[@as integer]]
+  local items = {}
+  local startbyte = nil
+
+  local function on_done()
+    local mode = api.nvim_get_mode()['mode']
+    if mode == 'i' or mode == 'ic' then
+      vim.fn.complete(startbyte or word_boundary, items)
+    end
+  end
+
+  local util = vim.lsp.util
+  for _, client in ipairs(clients) do
+    local params = util.make_position_params(win, client.offset_encoding)
+    client.request(ms.textDocument_completion, params, function(err, result)
+      if err then
+        log.warn(err.message)
+      end
+      if result and vim.fn.mode() == 'i' then
+        -- Completion response items may be relative to a position different than `textMatch`.
+        -- Concrete example, with sumneko/lua-language-server:
+        --
+        -- require('plenary.asy|
+        --         ▲       ▲   ▲
+        --         │       │   └── cursor_pos: 20
+        --         │       └────── textMatch: 17
+        --         └────────────── textEdit.range.start.character: 9
+        --                                 .newText = 'plenary.async'
+        --                  ^^^
+        --                  prefix (We'd remove everything not starting with `asy`,
+        --                  so we'd eliminate the `plenary.async` result
+        --
+        -- `adjust_start_col` is used to prefer the language server boundary.
+        --
+        local encoding = client.offset_encoding
+        local candidates = get_items(result)
+        local curstartbyte = adjust_start_col(pos[1], line, candidates, encoding)
+        if startbyte == nil then
+          startbyte = curstartbyte
+        elseif curstartbyte ~= nil and curstartbyte ~= startbyte then
+          startbyte = word_boundary
+        end
+        local prefix = startbyte and line:sub(startbyte + 1) or line_to_cursor:sub(word_boundary)
+        local matches = M._lsp_to_complete_items(result, prefix)
+        vim.list_extend(items, matches)
+      end
+      remaining = remaining - 1
+      if remaining == 0 then
+        vim.schedule(on_done)
+      end
+    end, bufnr)
+  end
+
+  -- Return -2 to signal that we should continue completion so that we can
+  -- async complete.
+  return -2
+end
+
+return M
diff --git a/runtime/lua/vim/lsp/protocol.lua b/runtime/lua/vim/lsp/protocol.lua
index 3a1b16c450..7a48c800c6 100644
--- a/runtime/lua/vim/lsp/protocol.lua
+++ b/runtime/lua/vim/lsp/protocol.lua
@@ -242,6 +242,7 @@ local constants = {
 
   -- Defines whether the insert text in a completion item should be interpreted as
   -- plain text or a snippet.
+  --- @enum lsp.InsertTextFormat
   InsertTextFormat = {
     -- The primary text to be inserted is treated as a plain string.
     PlainText = 1,
diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 42c1508cbf..7ccb8a38b1 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -548,7 +548,7 @@ end
 --- `textDocument/completion` request, which may return one of
 --- `CompletionItem[]`, `CompletionList` or null.
 ---@param result table The result of a `textDocument/completion` request
----@return table List of completion items
+---@return lsp.CompletionItem[] List of completion items
 ---@see https://microsoft.github.io/language-server-protocol/specification#textDocument_completion
 function M.extract_completion_items(result)
   if type(result) == 'table' and result.items then
@@ -619,47 +619,6 @@ function M.parse_snippet(input)
   return tostring(parsed)
 end
 
---- Sorts by CompletionItem.sortText.
----
---see https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
-local function sort_completion_items(items)
-  table.sort(items, function(a, b)
-    return (a.sortText or a.label) < (b.sortText or b.label)
-  end)
-end
-
---- Returns text that should be inserted when selecting completion item. The
---- precedence is as follows: textEdit.newText > insertText > label
---see https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
-local function get_completion_word(item)
-  if item.textEdit ~= nil and item.textEdit.newText ~= nil and item.textEdit.newText ~= '' then
-    local insert_text_format = protocol.InsertTextFormat[item.insertTextFormat]
-    if insert_text_format == 'PlainText' or insert_text_format == nil then
-      return item.textEdit.newText
-    else
-      return M.parse_snippet(item.textEdit.newText)
-    end
-  elseif item.insertText ~= nil and item.insertText ~= '' then
-    local insert_text_format = protocol.InsertTextFormat[item.insertTextFormat]
-    if insert_text_format == 'PlainText' or insert_text_format == nil then
-      return item.insertText
-    else
-      return M.parse_snippet(item.insertText)
-    end
-  end
-  return item.label
-end
-
---- Some language servers return complementary candidates whose prefixes do not
---- match are also returned. So we exclude completion candidates whose prefix
---- does not match.
-local function remove_unmatch_completion_items(items, prefix)
-  return vim.tbl_filter(function(item)
-    local word = get_completion_word(item)
-    return vim.startswith(word, prefix)
-  end, items)
-end
-
 --- According to LSP spec, if the client set `completionItemKind.valueSet`,
 --- the client must handle it properly even if it receives a value outside the
 --- specification.
@@ -678,56 +637,10 @@ end
 --- from |vim.lsp.buf.completion()|, which may be one of `CompletionItem[]`,
 --- `CompletionList` or `null`
 ---@param prefix (string) the prefix to filter the completion items
----@return table { matches = complete-items table, incomplete = bool }
+---@return table[] items
 ---@see complete-items
 function M.text_document_completion_list_to_complete_items(result, prefix)
-  local items = M.extract_completion_items(result)
-  if vim.tbl_isempty(items) then
-    return {}
-  end
-
-  items = remove_unmatch_completion_items(items, prefix)
-  sort_completion_items(items)
-
-  local matches = {}
-
-  for _, completion_item in ipairs(items) do
-    local info = ''
-    local documentation = completion_item.documentation
-    if documentation then
-      if type(documentation) == 'string' and documentation ~= '' then
-        info = documentation
-      elseif type(documentation) == 'table' and type(documentation.value) == 'string' then
-        info = documentation.value
-      else
-        vim.notify(
-          ('invalid documentation value %s'):format(vim.inspect(documentation)),
-          vim.log.levels.WARN
-        )
-      end
-    end
-
-    local word = get_completion_word(completion_item)
-    table.insert(matches, {
-      word = word,
-      abbr = completion_item.label,
-      kind = M._get_completion_item_kind_name(completion_item.kind),
-      menu = completion_item.detail or '',
-      info = #info > 0 and info or nil,
-      icase = 1,
-      dup = 1,
-      empty = 1,
-      user_data = {
-        nvim = {
-          lsp = {
-            completion_item = completion_item,
-          },
-        },
-      },
-    })
-  end
-
-  return matches
+  return require('vim.lsp._completion')._lsp_to_complete_items(result, prefix)
 end
 
 --- Like vim.fn.bufwinid except it works across tabpages.