Lsp Extract Function

Requires the agent-lsp MCP server.

lsp-extract-function: Extract Code Block into a Named Function

This skill RESTRUCTURES existing code — it takes code that already exists and moves it into a new function. This is distinct from /lsp-generate, which creates NEW code that does not yet exist (stubs, mocks, interface implementations). Use this skill when the code is already written; use /lsp-generate when you need to generate code from scratch.

Invocation: User provides file_path (absolute path), start_line and end_line (1-indexed range), and new_function_name (desired name for the extracted function).

Prerequisites

If LSP is not yet initialized, call mcp__lsp__start_lsp with the workspace root first. Auto-inference applies when file paths are provided, but an explicit start is required when switching workspaces.

Step 1 — Get context (document symbols)

Call mcp__lsp__open_document to open the file, then call mcp__lsp__list_symbols to understand the containing function and scope:

mcp__lsp__open_document({ "file_path": "<file_path>" })
mcp__lsp__list_symbols({ "file_path": "<file_path>" })

This establishes:

• Which function contains the selection
• Whether new_function_name already exists in the file (name collision check)

Mandatory name collision check: If new_function_name already exists as a symbol in the document symbols list, report the conflict and stop immediately:

Cannot extract: function new_function_name already exists in this file. Choose a different name and retry.

Step 2 — Check server capabilities

Call mcp__lsp__get_server_capabilities to understand what the language server supports:

mcp__lsp__get_server_capabilities({})

Check for codeActionProvider in the response. Note whether execute_command is listed in executeCommandProvider.commands. This determines whether the primary path (Step 3) is available.

Step 3 — Primary path: LSP code action

Call mcp__lsp__suggest_fixes with the selection range:

mcp__lsp__suggest_fixes({
  "file_path": "<file_path>",
  "start_line": N,
  "start_column": 1,
  "end_line": M,
  "end_column": 999
})

Filter the returned actions for extract-function actions: include any action whose kind contains "refactor.extract" OR whose title contains both "Extract" and "function" (case-insensitive).

If an extract-function action is found:

• Display the action title to the user
• If the action proposes a different name than new_function_name, ask for confirmation before proceeding
• Execute via mcp__lsp__execute_command if the action has a command field:

  mcp__lsp__execute_command({
    "command": "<action.command.command>",
    "arguments": <action.command.arguments>
  })
  

• OR apply directly via mcp__lsp__apply_edit if the action has an edit field:

  mcp__lsp__apply_edit({ "workspace_edit": <action.edit> })
  

• Skip to Step 5 after applying.

If no extract-function action is found: fall through to Step 4 (manual fallback).

Step 4 — Manual fallback

When no code action is available, perform manual extraction:

a) Analyze the selection

Read the selected lines (start_line through end_line) and identify:

• Parameters: Variables used inside the selection that are declared outside (captured from outer scope — must become function parameters)
• Return values: Variables declared inside the selection that are used outside (must be returned from the extracted function)
• Early returns: Return statements inside the selection (the extracted function must wrap these)

b) Construct and confirm the proposed signature

Build the extracted function signature based on the captured variables analysis. Display the proposed signature to the user before writing:

Proposed extraction:

func new_function_name(param1 Type1, param2 Type2) (ReturnType, error) {
    // selected lines
}

Proceed with this signature? [y/n]

Wait for user confirmation before applying any edit.

c) Apply the extraction (order matters)

Apply edits sequentially — do NOT batch edits from different line regions into a single apply_edit call:

1. First: Replace the selected lines with a call to the new function:

   mcp__lsp__apply_edit({
     "workspace_edit": {
       "changes": {
         "<file_path>": [{
           "range": { "start": { "line": start_line-1, "character": 0 },
                      "end":   { "line": end_line,     "character": 0 } },
           "newText": "    result := new_function_name(args...)\n"
         }]
       }
     }
   })
   

2. Second: Insert the new function definition after the containing function's closing brace:

   mcp__lsp__apply_edit({
     "workspace_edit": {
       "changes": {
         "<file_path>": [{
           "range": { "start": { "line": insert_line, "character": 0 },
                      "end":   { "line": insert_line, "character": 0 } },
           "newText": "\nfunc new_function_name(params) ReturnType {\n    ...\n}\n"
         }]
       }
     }
   })
   

Apply call-site replacement first, then insert the new function. This order preserves line numbers during editing: replacing call site does not shift the insertion point for the new function definition.

Step 5 — Validate

After extraction via either path:

1. Check diagnostics

mcp__lsp__get_diagnostics({ "file_path": "<file_path>" })

If errors are reported, display them with the table of common causes below.

2. Common post-extraction errors

3. Format the document

mcp__lsp__format_document({ "file_path": "<file_path>" })

This cleans up indentation introduced by the extraction.

Output Format

After completing extraction, display:

## Extraction Summary
- File:           path/to/file.go
- Extracted:      lines N–M
- New function:   new_function_name
- Path used:      LSP code action / Manual fallback
- Post-extraction errors: 0

Follow with the Diagnostic Summary if any errors changed (format in references/patterns.md).

Language-Specific Notes

• Go: gopls may offer "Extract function" in code actions for selection ranges. Check code actions first; gopls support varies by version.
• TypeScript/JavaScript: typescript-language-server may offer "Extract to function in global scope" or "Extract to inner function" — filter for these titles in Step 3.
• Python: pylsp and pyright-langserver typically do NOT offer extract-function code actions. Manual fallback (Step 4) is required for Python files.