merl

The one editor you need when agents write the code.

An agent works in one terminal split. merl sits in the other. You read what the agent wrote, walk its branch hunk by hunk, jump from a changed line into the code around it, fix the one line that is wrong, and go back to reading.

That is the whole tool: find your way around a project, review a diff with the code around it, and now and then type a secret into a .env. No setup, no config, no modes. It runs wherever a terminal does, SSH and tmux included.

_{A checkout of gitea, 5,500 files. Nothing was indexed or configured first.}

Install

brew install maksim-burtsev/tap/merl

Or take a prebuilt binary for macOS (arm64) or Linux (x86_64, arm64) from the releases:

curl -fsSL https://github.com/Maksim-Burtsev/merl/releases/latest/download/merl-aarch64-apple-darwin.tar.gz | tar xz
sudo mv merl /usr/local/bin/

Or build it (Rust 1.85 or newer):

cargo install --git https://github.com/Maksim-Burtsev/merl

Then:

merl                 # the current directory
merl FILE:LINE[:COL] # what compilers, linters and grep print, pasted straight in
merl --review        # the branch you are on, as a diff over the real files
merl --tutor         # every key, hands on, in about ten minutes
merl --drill [N]     # N tasks (20) that name what to do, not the key; the keys you skip come most
merl --keys          # how often you press each key and miss it, the unused last

Why

Agents write most of the code now. Your part is to understand it and to review it, and merl does those two things out of the box, with nothing to configure and nothing to switch off.

• Understand the code. Open a file by a few letters of its name, search the project as you type, go to a definition or list its usages, and come back with [. It follows you into the standard library and the dependencies.

• Review a branch. merl --review draws the branch's diff over the real files, so from any changed line you can look up what it calls and who else uses it. It stays current while the agent keeps working, and a file you have walked through gets a tick that goes when the file changes. A review, step by step.

  

• Touch up a line. Enter, type, Esc. It saves itself. Enough for a typo, a constant, or a secret you would rather not hand to an agent. More on editing.

I spent years switching things off in VS Code to get down to a tree, a highlighted file, go to definition and search. Vim and Helix want weeks of learning and a config first. merl starts there. It has no autocomplete, and it took me a while to notice: most of an editor is for typing code, and I had stopped typing it.

What merl is not

• No plugins.
• No language server and no index. Nothing to install per language, nothing to wait for.
• No splits and no tabs. Your terminal has those.
• No mouse.
• No keymap to write. The keys are below and they stay.
• No autocomplete, no snippets, no refactoring. The agent types.

None of these is on a roadmap. If you live in Vim or Helix and like it there, you do not need merl.

Keys

The dozen you will use every day. ? inside merl shows the rest.

Every key

Key	Action
o / Ctrl+E	Open a file (fuzzy)
Ctrl+N	New file: type its path, Enter creates and edits it
/ / Ctrl+F	Find in the open file
n / N	Next / previous match
s	Search the project
d / F12	Go to definition of the word under the cursor, or its implementations
D	Project symbols (fuzzy)
u / Shift+F12	Usages of the word under the cursor
[ / ]	Back / forward in the jump history
c / C	Review: next / previous hunk, on to the next file
m	Review: mark the file as viewed, or take the mark off
: / Ctrl+G	Go to line
t	Show or hide the file tree
T	Pick a theme (live preview)
w	Wrap long lines, or cut them at the edge and scroll sideways
Tab	Switch focus between tree and code
Enter	Edit at the cursor (Esc returns to navigation)
Ctrl+S	Save now (edits are saved on their own after a pause)
Ctrl+R	Reload from disk, dropping unsaved edits
Ctrl+Z / Ctrl+Y	Undo / redo
Ctrl+C	Copy the selection, or the line, to the clipboard
Edit: Ctrl+X	Cut the selection, or the line
Edit: Alt+Backspace / Alt+Delete	Delete the word before / after the cursor
Arrows	Move the cursor; Up / Down go by screen row
Shift+Up / Shift+Down	Extend the selection by a screen row
Shift+Left / Shift+Right	Extend the selection by a char
Alt+Left / Alt+Right	Move one word
Alt+Shift+Left / Right	Extend the selection by a word
Ctrl+Shift+Left / Right	Extend the selection to the start / end of the screen row, then of the line
v	Select the word, then the line, then the paragraph
Ctrl+D / Ctrl+U	Move half a screen down / up
{ / }	Previous / next paragraph (blank line)
PgUp / PgDn	Move one screen
Home / End	Start / end of the screen row, then of the line
Ctrl+Home / Ctrl+End	Start / end of the file
Esc	Close an overlay, leave edit mode, or clear selection and find
?	This help
q	Quit
Tree: Up / Down	Move
Tree: Enter	Open the file, or expand the directory
Tree: Left / Right	Collapse / expand
Picker: Up / Down	Move
Picker: Enter	Accept
Picker: Esc	Cancel
Picker: PgUp / PgDn	Move one page
Help: Up / Down	Scroll

Languages

d, u and D work in all of these, with no language server and no index: a project works the moment you open it. In Python, TypeScript and Go, d also follows the type of the receiver (self.repo.save), and it always says how it found its target.

What each rule reads and what it refuses to guess: docs/navigation.md.

Themes

T lists the themes and repaints merl in the one under the cursor as it moves. Enter keeps it, Esc puts the old one back. The default is tokyonight-moon. They were picked to sit in for hours: no neon, and light themes that look like paper.

docs/themes.md has a screenshot of each and how to add your own.

Config

There is none to write. T remembers your theme in ~/.config/merl/config.toml, and autosave_delay_ms (1000) lives there too.

Terminals

Any terminal, and the same keys over SSH and in tmux. Terminals keep Cmd for themselves, so copy is Ctrl+C and quit is q. Terminal.app is the weak one: no Shift+arrows, no Ctrl+Home, no copy.

In Ghostty, keybind = performable:cmd+c=copy_to_clipboard:mixed makes Cmd+C and Cmd+X work too.

Status

I use merl every working day. 1.0 is next, and what is left for it is in the milestone. Issues are welcome.

License

MIT. Syntax definitions come from bat via two-face; the themes keep their authors' licences.