emend

A Python refactoring CLI built on tree-sitter with a Rust backend. The name means "to make corrections to a text" — which is what it does, but with AST-aware precision instead of find-and-replace.

Two complementary systems: structured edits use selectors like file.py::func[params][0] for precise changes to symbol metadata, and pattern transforms use capture variables like print($X) → logger.info($X) for code-pattern search and replace.

Installation

Using uv with free-threaded Python (recommended for best performance)

uv tool install --python 3.14t emend

Python 3.13+ ships a free-threaded variant (3.13t, 3.14t) that removes the GIL. emend's Rust core (emend_core) is already GIL-free (built with #[pymodule(gil_used = false)]), so on free-threaded Python it can run parallel file scans with no lock contention — meaning find, lint, analyze refs, and edit rename across large codebases are significantly faster.

We recommend 3.14t for the latest free-threaded runtime. Python 3.13t remains supported, and both variants can run the optional MCP server:

uv tool install --python 3.14t emend   # latest free-threaded runtime
uv tool install --python 3.13t emend   # free-threaded 3.13

Using uv (standard Python)

uv tool install emend

Using pip

pip install emend

MCP server

To use emend as an MCP server for LLM-based clients, install the optional mcp extra:

pip install emend[mcp]

Then start the server:

emend mcp                              # stdio transport (default)
emend mcp --transport sse --port 8080  # SSE transport

The optional extra installs MCP SDK v2 (mcp>=2.0).

Adding emend to Claude Code

The quickest way is the claude mcp add CLI:

# Add for the current project (default scope: local)
claude mcp add --transport stdio emend -- emend mcp

# Or share with your team via .mcp.json (project scope)
claude mcp add --transport stdio --scope project emend -- emend mcp

If you installed with uv tool install, make sure the emend binary is on your PATH, or use the full path:

claude mcp add --transport stdio emend -- uvx emend mcp

You can also add it by editing configuration JSON directly. For a personal setup, run:

claude mcp add-json emend '{"type":"stdio","command":"emend","args":["mcp"]}'

For a team-shared setup, add a .mcp.json file to your project root (and commit it to version control):

{
  "mcpServers": {
    "emend": {
      "type": "stdio",
      "command": "emend",
      "args": ["mcp"]
    }
  }
}

Verify it's connected:

claude mcp list          # from the terminal

Or inside Claude Code, type /mcp to see all connected servers and their status.

See the Claude Code MCP documentation for details on scopes, environment variables, and managed configurations.

Run emend --help to verify. Full documentation at lucaswiman.github.io/emend.

Vim / Neovim plugin

emend ships with a Vim/Neovim plugin for interactive code search. Install with vim-plug:

Plug 'lucaswiman/emend', { 'rtp': 'vim' }

Then use :Emend to open the search prompt, or :Emend parse to search directly. The plugin communicates with emend tool editor-server via JSON-RPC over stdio pipes — the server stays warm for sub-5ms lookups.

For local development, point to your checkout:

Plug '~/src/emend', { 'rtp': 'vim' }
let g:emend_command = '~/src/emend/.venv/bin/emend'

See vim/README.md for full documentation.

Indexing (recommended for large codebases)

After installing, run emend tool index in your project root to pre-build caches:

emend tool index                  # index current directory
emend tool index src/ --jobs 8    # index specific directory with 8 workers

This parses every Python file and builds a qualified-name index, so subsequent cross-project operations (analyze refs, edit rename, analyze graph, analyze deadcode) are significantly faster. The cache is stored in .emend/cache/parse.db (automatically gitignored and dockerignored) and is keyed by file content hash, so it self-invalidates when files change. Each Git worktree keeps its own mutable parse.db and facts.db snapshots so switching branches cannot contaminate another worktree's analysis.

The MCP server (emend mcp) starts without eagerly indexing the whole project; analysis tools build or refresh the facts they need on demand.

MCP Server

The MCP server now uses a smaller discriminated tool surface:

1. search — Code search, symbol lookup, and summary listing
2. transform — Pattern replace plus selector-based edits, adds, removes, copies, moves, and renames
3. references — References, callers, and callees
4. analyze — Graph, deadcode, impact, semantic context, and flow analysis
5. check — Unified project rules from .emend/rules.yaml
6. mappings — Cross-repo identifier and module mappings
7. grammar_and_cookbook — Syntax reference and cookbook text

See the grammar_and_cookbook.rst reference for full command documentation.

Usage

emend <command> [options]

Workflow

All mutating commands default to dry-run, showing a diff of proposed changes. Re-run with --apply to write them. You'll probably want to run a formatter (black/ruff/isort) afterward, since emend doesn't try to preserve exact formatting.

Selector Syntax

Three types of selectors:

Symbol Selectors

file.py::Class.method.nested   # Nested symbol path
file.py::func                  # Module-level symbol

Extended Selectors (with components)

file.py::func[params]           # Function parameters
file.py::func[params][ctx]      # Specific parameter (by name)
file.py::func[params][0]        # Specific parameter (by index)
file.py::func[returns]          # Return annotation
file.py::func[decorators]       # Decorator list
file.py::MyClass[bases]         # Base classes
file.py::func[body]             # Function body

Pseudo-class Selectors

file.py::func[params]:KEYWORD_ONLY       # Keyword-only parameter slot
file.py::func[params]:POSITIONAL_ONLY    # Positional-only parameter slot

Line Selectors

file.py:42                      # Single line
file.py:42-100                  # Line range

Wildcard Selectors

file.py::*[params]              # All function parameters
file.py::Test*[decorators]      # Test class parameters
file.py::*.*[returns]           # All method return types
file.py::Class.*[body]          # All method bodies in Class

Wildcards support glob patterns:

• * - Match any symbol at this level
• Test* - Match symbols starting with Test
• *.* - Match any method in any class
• Class.* - Match any method in Class

Selector Grammar (Lark)

start: selector

selector: file_path DOUBLE_COLON symbol_path? component*

file_path: PATH
symbol_path: symbol_segment ("." symbol_segment)*
symbol_segment: WILDCARD | IDENTIFIER
component: "[" COMPONENT_NAME "]" accessor? pseudo_class?
accessor: "[" (IDENTIFIER | INT) "]"
pseudo_class: PSEUDO_CLASS

COMPONENT_NAME: "params" | "returns" | "decorators" | "bases" | "body" | "imports"
DOUBLE_COLON: "::"
PATH: /[^:]+/
WILDCARD: "*" | /[a-zA-Z_*][a-zA-Z0-9_*]*/
IDENTIFIER: /[a-zA-Z_][a-zA-Z0-9_]*/
INT: /-?\d+/
PSEUDO_CLASS: /:KEYWORD_ONLY|:POSITIONAL_ONLY|:POSITIONAL_OR_KEYWORD/

Commands

The public CLI is organized around a small set of top-level commands:

• find — Search for patterns, selectors, symbols, or summaries.
• edit — Code changes and refactors. Subcommands: set, rm, delete, add, replace, cp, rename, mv, batch, saturate.
• analyze — Read-only analysis. Subcommands: refs, graph, deadcode, impact, types, trace, facts, cfg, dsl.
• tool — Operational and debugging commands. Subcommands: index, editor-search, editor-server.
• check — Unified project rules from .emend/rules.yaml.
• lint / policy — Focused rule runners kept for compatibility and targeted workflows.
• map — Identifier and module mappings.
• mcp — Start the MCP server.

Hidden compatibility aliases still work. For example, emend rm ... maps to emend edit rm ..., and old read commands like search, grep, show, get, and lookup route to emend find.

Examples

Search & Read Examples

# Search by pattern (pattern mode)
emend find 'print($X)' src/
emend find 'assertEqual($A, $B)' tests/ --output count

# Search by symbol (lookup mode)
emend find file.py::func
emend find src/ --kind function --name test_*
emend find file.py --output json

# Extract function parameters
emend find api.py::handler[params]
emend find 'api.py::*[params]'  # Wildcard: all function params in file

# Get return types
emend find 'src/**/*.py::*[returns]' --output metadata

# List symbols in a module
emend find file.py                          # Tree view
emend find file.py --output summary::flat   # Flat list
emend find file.py --depth 2                # Limit nesting depth

Edit Examples

# Update return type
emend edit set api.py::handler[returns] "Response" --apply

# Add parameter with default value
emend edit add api.py::handler[params] "timeout: int = 30" --apply

# Add keyword-only parameter
emend edit add "api.py::handler[params]:KEYWORD_ONLY" "debug: bool" --apply

# Insert parameter before specific param
emend edit add api.py::handler[params] "ctx: Context" --before user_id --apply

# Remove a specific parameter
emend edit rm api.py::handler[params][deprecated_arg] --apply

# Edit multiple symbols at once (wildcards)
emend edit set 'file.py::*[decorators]' "@dataclass" --apply

Pattern Transform Examples

# Simple find and replace (dry-run by default)
emend edit replace 'print($X)' 'logger.info($X)' file.py

# Replace within a specific scope
emend edit replace 'old_var' 'new_var' api.py --where process --apply

# Replace with pattern capture
emend edit replace 'get_field($N)' 'field$N' api.py --where process --apply

# String content interpolation: ${X.content} strips quotes from a captured string literal
emend edit replace 'Union["$X", $Y]' '$X | $Y' src/ --apply

# Find all pattern matches
emend find 'print($X)' src/ --output location

# Multi-rule batch operations
emend edit batch rules.json --apply

Symbol Management Examples

# Find all references to a symbol
emend analyze refs models.py::User --json
emend analyze refs models.py::User --writes-only    # Only write references
emend analyze refs models.py::User --calls-only     # Only function calls

# Rename a symbol project-wide
emend edit rename models.py::User --to Account --apply

# Move a symbol to another file (updates imports)
emend edit mv utils.py::parse_date helpers/dates.py --apply

# Copy a symbol to another file
emend edit cp workflow.py::Builder._build.helper tasks.py --dedent --apply

# Remove a symbol or component
emend edit rm file.py::deprecated_func --apply

# List symbols using find
emend find workflow.py --depth 3

Pattern Syntax

Patterns support metavariables for capturing:

# Single expression
emend find 'print($MSG)' src/

# Multiple arguments with capture
emend find 'func($A, $B)' src/

# Variable arguments
emend find 'func($...ARGS)' src/

# Type constraints
emend find 'range($N:int)' src/

# Anonymous metavariables
emend find 'func($_, $ARG)' src/

# Structural constraints (via --where)
emend find 'print($X)' src/ --within 'async def'
emend find 'await $X' src/ --where 'not if __debug__'

# Supported pattern types:
#   Literals: $X, $MSG:str, $N:int, 3.14
#   Calls: func($X), obj.method($A, $B)
#   Operations: $A + $B, $A and $B, not $X, $X[$Y]
#   Collections: ($A, $B), [$X, $Y], {$K: $V}
#   Control: return $X, assert $A == $B, raise $EXC

Pattern Grammar (Lark)

start: pattern

pattern: (code_chunk | metavar)+

metavar: DOLLAR (ELLIPSIS)? METAVAR_NAME TYPE_CONSTRAINT?
       | DOLLAR UNDERSCORE

DOLLAR: "$"
ELLIPSIS: "..."
UNDERSCORE: "_"
METAVAR_NAME: /[A-Z][A-Z0-9_]*/
TYPE_CONSTRAINT: /:!?(?:expr|stmt|identifier|int|str|float|call|attr|any)/
code_chunk: /[^$:]+/ | ":"

The code_chunk rule excludes colons (/[^$:]+/) to prevent consuming colons that are part of type constraints (e.g., $MSG:str). A standalone colon is matched by the alternative | ":" for patterns containing colons outside of type constraints.

Diff Patch Format

- pattern_to_find
+ replacement_pattern

- another_pattern
+ another_replacement

Lines prefixed with - are matched; corresponding + lines are the replacement. Blank lines separate rules.

Linting

emend's canonical rules file is .emend/rules.yaml. emend check is the unified entry point, while emend lint and emend policy remain available for focused workflows and compatibility.

# .emend/rules.yaml
macros:
  print_call: "print($...ARGS)"

rules:
  no-print:
    match: "{print_call}"
    not-within: "def test_*"
    message: "Use logger instead of print"
    fix: "logger.info($...ARGS)"

  no-open-without-encoding:
    match: "open($PATH)"
    message: "Specify encoding when calling open()"
    fix: "open($PATH, encoding='utf-8')"

# Run all configured checks
emend check src/

# Auto-fix match rules that have a fix
emend check src/ --fix

# Focus on match-style lint rules only
emend lint src/ --rule no-print

Suppress violations inline with # noqa comments:

print("keep this")  # noqa
print("keep this")  # noqa: emend:no-print
print("keep this")  # noqa: E501, emend:no-print  # mixed with other linters

Dead code detection

Configure dead-code rules for emend check/emend lint in .emend/rules.yaml. The standalone emend analyze deadcode command uses the equivalent CLI flags shown below:

# .emend/rules.yaml
deadcode: true

# Or with options:
deadcode:
  enabled: true
  kind: function                          # Only functions (or "class")
  include-private: true                   # Default: include private symbols/methods
  exclude-test-references: true           # Default: ignore refs from tests
  unused-modules: true                    # Default: report unimported modules
  strings-count-as-references: true       # String literals count as refs
  message: "Symbol appears to be unused"

# Run as part of unified checks
emend check src/

# Or use the standalone command
emend analyze deadcode src/
emend analyze deadcode src/ --include-test-references --json
emend analyze deadcode src/ --exclude-private --no-unused-modules

Suppress false positives inline:

def my_entry_point():  # noqa: emend:deadcode
    ...

Mapping Store

emend includes a built-in mapping store for cross-service identifier mappings and module-to-repo mappings. Stored in .emend/mappings.yaml.

# Identifier mappings — cross-service relationships
emend map add backend "UserService.create" gateway "POST /api/v1/users" \
    --source-kind function --target-kind endpoint
emend map search "UserService"

# Module mappings — map module prefixes to repos or local dirs
emend map add-module payments --repo org/payments-service
emend map add-module shared.utils --path /home/user/shared-utils
emend map resolve payments.models.Order

Module mappings that reference GitHub repos are automatically cloned (via gh) and checked out as git worktrees under ~/.cache/emend/repo-checkouts/. Set EMEND_CACHE_DIR to relocate this cache.

pre-commit integration

emend can run as a pre-commit hook. Add to your .pre-commit-config.yaml:

repos:
  - repo: https://github.com/lucaswiman/emend
    rev: v0.5.0  # replace with desired version tag
    hooks:
      - id: emend-lint

This runs emend lint on staged Python files using your .emend/rules.yaml config.

To auto-fix violations, add args: ["--fix"] to the hook configuration.

Development

Installing from Source

Clone the repository and install for development:

git clone https://github.com/lucaswiman/emend
cd emend

# Using make (creates a free-threaded venv, compiles the Rust extension, installs dev deps)
make venv

# Or manually (requires maturin and a Rust toolchain):
uv venv .venv --python 3.13t
uv pip install maturin
.venv/bin/maturin develop -E dev

Running Tests

# Run all tests
make test

# Run specific test file
make test TESTS=tests/test_emend/test_add_parameter.py

# Run specific test
make test TESTS="tests/test_emend/test_add_parameter.py::test_add_parameter_with_default"

Project Structure

emend/
├── src/emend/
│   ├── cli.py                # CLI assembly and compatibility layer
│   ├── cli_find.py           # `find`
│   ├── cli_edit.py           # `edit` subcommands
│   ├── cli_analysis.py       # `analyze` subcommands
│   ├── cli_tooling.py        # `tool` and `mcp`
│   ├── cli_checks.py         # `check`, `lint`, `policy`
│   ├── cli_map.py            # `map`
│   ├── transform.py          # Core search/edit/analysis engine
│   ├── pattern.py            # Pattern parsing and compilation
│   ├── query.py              # Symbol querying with filters
│   ├── ast_commands.py       # AST-based command implementations
│   ├── ast_utils.py          # AST traversal utilities
│   ├── component_selector.py # Extended selector parsing
│   ├── lint.py               # Pattern-based linter engine
│   ├── mcp_server.py         # MCP server (optional, requires emend[mcp])
│   └── grammars/
│       ├── selector.lark     # Extended selector grammar
│       └── pattern.lark      # Pattern grammar
├── rust/                     # emend_core Rust extension (bundled in wheel)
│   ├── src/lib.rs            # PyO3 bindings, GIL-free module definition
│   └── Cargo.toml
├── vim/                      # Vim/Neovim plugin (JSON-RPC over stdio)
│   ├── plugin/emend.vim      # Commands (:Emend, :EmendSearch, etc.)
│   ├── autoload/emend.vim    # RPC client, server lifecycle
│   ├── autoload/emend/ui.vim # Split-pane search UI
│   └── doc/emend.txt         # Vim help (:help emend)
├── tests/test_emend/         # Test suite
├── Makefile
└── pyproject.toml            # maturin build (bundles Rust + Python in one wheel)

License

MPL 2.0