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).

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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.

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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.

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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.

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Step 3 — Primary path: LSP code action

Call mcp__lsp__suggest_fixes with the selection range: