lsp-extract-function

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• SKILL.md
• patterns.md · references/

SKILL.md (first lines)

> 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:

From blackwell-systems/agent-lsp, MIT.

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