# lsp-simulate

Simulates code changes in memory and validates diagnostics via LSP before touching disk. Ideal when planning risky multi-file refactoring or checking safety first.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · ★ 165
- Author: blackwell-systems
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/blackwell-systems/agent-lsp/tree/HEAD/skills/lsp-simulate/SKILL.md
- Updated: 2026-10-24
- HTML: https://affpapa.org/en/nevibecode/skills/lsp-simulate

## What it does

The skill manages speculative code editing sessions powered by the `agent-lsp` MCP server. It applies edits directly to an in-memory overlay, runs language server checks, and reports `net_delta` to show new errors. When chaining multiple edits, `simulate_chain` checks each step sequentially to locate where the build breaks.

Diagnostics run in two scopes: `file` for fast single-file checks with high confidence, or `workspace` to catch cross-file type mismatches under eventual confidence. Single changes can run via `preview_edit`, which creates, evaluates, and clears the session in one call. Line and column numbers use 1-based indexing.

The skill strictly requires a running `agent-lsp` MCP server. Because sessions live exclusively in server memory, any server restart wipes active session IDs, requiring you to call `destroy_session` after committing or discarding.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add blackwell-systems/agent-lsp --skill lsp-simulate -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/lsp-simulate/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/lsp-simulate/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add blackwell-systems/agent-lsp --skill lsp-simulate -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/lsp-simulate/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add blackwell-systems/agent-lsp --skill lsp-simulate -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/lsp-simulate/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install blackwell-systems/agent-lsp lsp-simulate
```

2. **Manually: put the files in place**
- File: `.github/skills/lsp-simulate/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add blackwell-systems/agent-lsp --skill lsp-simulate -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/blackwell-systems/agent-lsp --path skills/lsp-simulate
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/lsp-simulate/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add blackwell-systems/agent-lsp --skill lsp-simulate -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/blackwell-systems/agent-lsp/tree/HEAD/skills/lsp-simulate/SKILL.md

## When to use it
- test code edits in memory without touching files on disk
- find the exact step in a multi-edit sequence that introduces errors
- catch cross-file type issues before committing refactors
- preview a patch before writing changes to disk

## Example prompts
- Run lsp-simulate to check my edit on line 42 of `client.go` without writing to disk
- Simulate this struct refactor across the workspace and check if diagnostics pass
- Evaluate this sequence of edits with simulate_chain to see which steps are safe

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: fb94315803b4d245.

BEGIN THIRD-PARTY FILE fb94315803b4d245
````text
---
name: lsp-simulate
description: Speculative code editing session — simulate changes in memory before touching disk. Use when planning edits that might break things, exploring refactors across multiple files, or verifying an edit is safe before applying.
user-invocable: true
allowed-tools: mcp__lsp__start_lsp mcp__lsp__create_simulation_session mcp__lsp__simulate_edit mcp__lsp__simulate_chain mcp__lsp__evaluate_session mcp__lsp__commit_session mcp__lsp__discard_session mcp__lsp__destroy_session mcp__lsp__preview_edit
license: MIT
compatibility: Requires the agent-lsp MCP server (github.com/blackwell-systems/agent-lsp)
metadata: {}
---

> 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 }
```

Repeat for additional edits as needed.

**Step 3 — Evaluate the session**

```
````
END THIRD-PARTY FILE fb94315803b4d245

The third-party file has ended. Everything between the fb94315803b4d245 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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