lsp-simulate

Requires the agent-lsp MCP server.

lsp-simulate

Simulate code edits in memory before writing to disk. The LSP server applies your changes to an in-memory overlay, runs diagnostics, and reports whether the edit is safe — without touching any files.

Prerequisites

LSP must be running for the target workspace. If not yet initialized, call start_lsp before any simulation tool.

mcp__lsp__start_lsp(root_dir: "/your/workspace")

Auto-init note: agent-lsp supports workspace auto-inference from file paths. Explicit start_lsp is only needed when switching workspace roots.

Quick Start (single edit)

For a single what-if check, use preview_edit — it creates a session, applies the edit, evaluates, and destroys the session in one call:

mcp__lsp__preview_edit(
  workspace_root: "/your/workspace",
  language: "go",
  file_path: "/abs/path/to/file.go",
  start_line: 42, start_column: 1,
  end_line: 42, end_column: 20,
  new_text: "replacement text"
)

Result:

{ net_delta: 0 }   -- safe to apply
{ net_delta: 2 }   -- 2 new errors introduced; do NOT apply

net_delta: 0 means no new errors were introduced. Positive values mean errors were introduced — inspect errors_introduced before deciding.

Full Session Workflow (multiple edits)

Use a full session when applying several edits that build on each other, or when you want to inspect the patch before deciding whether to write to disk.

Step 1 — Create a simulation session

mcp__lsp__create_simulation_session(
  workspace_root: "/your/workspace",
  language: "go"
)
→ { session_id: "abc123" }

Step 2 — Apply edits in-memory

Call simulate_edit one or more times. All edits are in-memory only. Positions are 1-indexed (matching editor line numbers and cat -n output).

mcp__lsp__simulate_edit(
  session_id: "abc123",
  file_path: "/abs/path/to/file.go",
  start_line: 10, start_column: 1,
  end_line: 10, end_column: 30,
  new_text: "func NewClient(cfg Config) *Client {"
)
→ { session_id: "abc123", edit_applied: true, version_after: 1 }