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Unofficial Relace MCP Server

Unofficial — Personal project, not affiliated with Relace.

Built with AI — Developed entirely with AI assistance (Antigravity, Codex, Cursor, Github Copilot, Windsurf).

MCP server providing AI-powered code editing and intelligent codebase exploration tools.

Without	With agentic_search + fast_apply
Manual grep, misses related files	Ask naturally, get precise locations
Edits break imports elsewhere	Traces imports and call chains
Full rewrites burn tokens	Describe changes, no line numbers
Line number errors corrupt code	10,000+ tokens/sec merging

Features

• Fast Apply — Apply code edits at 10,000+ tokens/sec via Relace API
• Agentic Search — Agentic codebase exploration with natural language queries
• Agentic Retrieval — Hybrid semantic hints + live code exploration, with stale-hint support and explicit cloud maintenance (enable with MCP_SEARCH_RETRIEVAL=1; choose the backend with MCP_RETRIEVAL_BACKEND)
• Cloud Search — Semantic code search over cloud-synced repositories

Quick Start

Prerequisites: uv, git, ripgrep (recommended)

Using the relace backend (default): get your API key from Relace Dashboard, then add to your MCP client:

Cursor

~/.cursor/mcp.json

{
  "mcpServers": {
    "relace": {
      "command": "uv",
      "args": ["tool", "run", "relace-mcp"],
      "env": {
        "RELACE_API_KEY": "rlc-your-api-key",
        "MCP_BASE_DIR": "/absolute/path/to/your/project"
      }
    }
  }
}

Claude Code

claude mcp add relace \
  --env RELACE_API_KEY=rlc-your-api-key \
  --env MCP_BASE_DIR=/absolute/path/to/your/project \
  -- uv tool run relace-mcp

Windsurf

~/.codeium/windsurf/mcp_config.json

{
  "mcpServers": {
    "relace": {
      "command": "uv",
      "args": ["tool", "run", "relace-mcp"],
      "env": {
        "RELACE_API_KEY": "rlc-your-api-key",
        "MCP_BASE_DIR": "/absolute/path/to/your/project"
      }
    }
  }
}

VS Code

.vscode/mcp.json

{
  "mcp": {
    "servers": {
      "relace": {
        "type": "stdio",
        "command": "uv",
        "args": ["tool", "run", "relace-mcp"],
        "env": {
          "RELACE_API_KEY": "rlc-your-api-key",
          "MCP_BASE_DIR": "${workspaceFolder}"
        }
      }
    }
  }
}

Codex CLI

~/.codex/config.toml

[mcp_servers.relace]
command = "uv"
args = ["tool", "run", "relace-mcp"]

[mcp_servers.relace.env]
RELACE_API_KEY = "rlc-your-api-key"
MCP_BASE_DIR = "/absolute/path/to/your/project"

Configuration

Variable	Required	Description
RELACE_API_KEY	✅*	API key from Relace Dashboard; required for Relace providers and cloud tools
MCP_SEARCH_RETRIEVAL	❌	Set to 1 to register the agentic_retrieval tool
MCP_RETRIEVAL_BACKEND	❌	Semantic retrieval backend: relace (default), codanna, chunkhound, or none
MCP_RETRIEVAL_HINT_POLICY	❌	Retrieval hint policy: prefer-stale (default) or strict
MCP_BACKGROUND_INDEX_MONITOR	❌	Opt-in periodic refresh monitor for local indexes; requires a pinned MCP_BASE_DIR and a local backend
MCP_BACKGROUND_INDEX_INTERVAL_SECONDS	❌	Periodic local index monitor interval in seconds (default: 300)
MCP_BACKGROUND_INDEX_INITIAL_DELAY_SECONDS	❌	Initial delay before the first periodic local index check in seconds (default: 30)
MCP_SEARCH_TURN_STATUS_MODE	❌	Turn-status user message policy: always (default), final-only, or off
SEARCH_BASH_TOOLS	❌	Enable bash during search runs: 1 (on, default), 0 (off)
SEARCH_LSP_TOOLS	❌	Enable LSP-assisted search: 1 (on), 0 (off, default)
MCP_BASE_DIR	❌	Project root override (auto-detected via MCP Roots → Git → workspace storage → CWD)
MCP_LOGGING	❌	File logging: off (default), safe, full
MCP_DOTENV_PATH	❌	Path to .env file for centralized config

* Optional if both: (1) APPLY_PROVIDER and SEARCH_PROVIDER are non-Relace providers, and (2) MCP_RETRIEVAL_BACKEND is codanna, chunkhound, or none.

For .env usage, encoding settings, custom LLM providers, and more, see docs/advanced.md.

Tools

Always available: fast_apply, agentic_search. agentic_retrieval requires MCP_SEARCH_RETRIEVAL=1. Cloud tools are available only when MCP_RETRIEVAL_BACKEND=relace. index_status is available for relace, codanna, and chunkhound, and hidden when MCP_RETRIEVAL_BACKEND=none.

Use MCP-native discovery surfaces: list_tools() for tools and list_resources() for resources.

For detailed parameters, see docs/tools.md.

Language Support

LSP tools use external language servers installed on your system.

Language	Language Server	Install Command
Python	basedpyright	(bundled)
TypeScript/JS	typescript-language-server	npm i -g typescript-language-server typescript
Go	gopls	go install golang.org/x/tools/gopls@latest
Rust	rust-analyzer	rustup component add rust-analyzer

Dashboard

Real-time terminal UI for monitoring operations.

pip install relace-mcp[tools]
relogs

For detailed usage, see docs/dashboard.md.

Benchmark

Evaluate agentic_search performance using the Loc-Bench code localization dataset.

git clone https://github.com/possible055/relace-mcp.git
cd relace-mcp
uv sync --extra benchmark

# Build dataset from Hugging Face
uv run --extra benchmark python -m benchmark.cli.build_locbench --output artifacts/data/raw/locbench_v1.jsonl

# Run evaluation
uv run --extra benchmark python -m benchmark.cli.run --dataset artifacts/data/raw/locbench_v1.jsonl --limit 20

All benchmark artifacts are written under benchmark/.data/.

For grid search, analysis tools, and metrics interpretation, see docs/benchmark.md.

Platform Support

Platform	Status	Notes
Linux	✅ Fully supported	Primary development platform
macOS	✅ Fully supported	All features available
Windows	⚠️ Partial	bash tool unavailable; use WSL for full functionality

Troubleshooting

Error or message	Solution
RELACE_API_KEY is required ...	Set RELACE_API_KEY when using Relace providers or cloud tools
NEEDS_MORE_CONTEXT	Include 1-3 unique anchor lines near the target block
INVALID_PATH	Verify the path exists and is inside MCP_BASE_DIR or an allowed extra path
FILE_TOO_LARGE	File exceeds 10MB; split the change into smaller files or edits
ENCODING_ERROR	Set RELACE_DEFAULT_ENCODING explicitly for non-UTF-8 projects
AUTH_ERROR	Verify the API key and provider configuration
RATE_LIMIT	Retry later or reduce request volume
NETWORK_ERROR / TIMEOUT_ERROR	Check network access and retry
APPLY_NOOP	Add more specific anchors or concrete new lines so the merge can produce a diff
MARKER_LEAKAGE	Ensure placeholder markers are used only as placeholders, not expected literal output
TRUNCATION_DETECTED	Split large deletion-heavy edits or use explicit remove directives
BLAST_RADIUS_EXCEEDED	Break the change into smaller, more local edits

Development

git clone https://github.com/possible055/relace-mcp.git
cd relace-mcp
uv sync --extra dev --extra benchmark
uv run pytest
uv run --extra dev --extra benchmark pytest benchmark/tests -q

License

MIT