REPL Configuration

Behavior on connect

Normally, when you first establish a REPL connection, the REPL buffer is auto-displayed in a separate window. You can suppress this behavior like this:

(setq cider-repl-pop-to-buffer-on-connect nil)

If you want the REPL buffer to be auto-displayed, but don’t want it to be focused, use this:

(setq cider-repl-pop-to-buffer-on-connect 'display-only)

Behavior on switch

By default C-c C-z will display the REPL buffer in a different window. You can make C-c C-z switch to the CIDER REPL buffer in the current window:

(setq cider-repl-display-in-current-window t)

Customizing the REPL prompt

You can customize the REPL buffer prompt by setting cider-repl-prompt-function to a function that takes one argument, a namespace name. For convenience, CIDER provides three functions that implement common formats:

  • cider-repl-prompt-lastname:

  • cider-repl-prompt-abbreviated:

  • cider-repl-prompt-default:


By default, CIDER uses cider-repl-prompt-default.

You may, of course, write your own function. For example, in leiningen there are two namespaces with similar names - leiningen.classpath and leiningen.core.classpath. To make them easily recognizable you can either use the default value or you can opt to show only two segments of the namespace and still be able to know which is the REPL’s current namespace. Here is an example function that will do exactly that:

(defun cider-repl-prompt-show-two (namespace)
  "Return a prompt string with the last 2 segments of NAMESPACE."
  (let ((names (reverse (subseq (reverse (split-string namespace "\\.")) 0 2))))
    (concat (car names) "." (cadr names) "> ")))

TAB Completion

You can control the TAB key behavior in the REPL using the cider-repl-tab-command variable. While the default command cider-repl-indent-and-complete-symbol should be an adequate choice for most users, it’s very easy to switch to another command if you wish to. For instance if you’d like TAB to only indent (maybe because you’re used to completing with M-TAB) use the following:

(setq cider-repl-tab-command #'indent-for-tab-command)

Customizing the Return key’s behavior

Ordinarily, Return immediate sends a form for evaluation. If you want to insert a newline into the REPL buffer as you’re editing, you can do so using C-j. If you are entering a lot of longer forms that span multiple lines, it may be more convenient to change the keybindings:

(define-key cider-repl-mode-map (kbd "RET") #'cider-repl-newline-and-indent)
(define-key cider-repl-mode-map (kbd "C-<return>") #'cider-repl-return)

This will make Return insert a newline into the REPL buffer and C-Return send the form off for evaluation.

Auto-scrolling the REPL on Output

Prior to version 0.21.0, the REPL buffer would be automatically re-centered whenever any output was printed, so that the prompt was on the bottom line of the window, displaying the maximum possible amount of output above it. This is no longer the default behavior — you can now replicate it by setting the built-in option scroll-conservatively, for example:

(add-hook 'cider-repl-mode-hook '(lambda () (setq scroll-conservatively 101)))

Display Output Preceding the REPL Window Boundaries

If your REPL prompt is at the beginning of the REPL window (e.g. you pressed C-l a couple of times to recenter your REPL window) and some output is displayed there from an interactive evaluation in a Clojure buffer, this output won’t automatically be scrolled into view. If you want to force such output to be displayed you’ll need to add the following to your config:

(setq cider-repl-display-output-before-window-boundaries t)
This behaviour was changed in CIDER 1.7, as the auto-scrolling of output before the REPL prompt is rarely needed, but it’s current implementation is very slow.

Auto-trimming the REPL buffer

This functionality is disabled by default.

As noted previously, the REPL buffer’s performance will degrade if its size is allowed to grow infinitely. You can obviously clear the REPL manually from time to time, but CIDER also has some auto-trimming functionality that can simplify the process for you.

Auto-trimming can be enabled by setting cider-repl-buffer-size-limit to an integer. By setting a limit to the number of characters in the buffer, the buffer can be trimmed (from its beginning) after each evaluation if the set limit has been exceeded. Here’s how you can set the size limit in your Emacs config:

(setq cider-repl-buffer-size-limit 100000)
You can also trigger auto-trimming manually by invoking the command cider-repl-trim-buffer or the REPL shortcut trim.

Result Prefix

You can change the string used to prefix REPL results:

(setq cider-repl-result-prefix ";; => ")

Which then results in the following REPL output:

user> (+ 1 2)
;; => 3

By default, REPL results have no prefix.

Set ns in REPL

By default cider-repl-set-ns won’t require the target ns, just set it. That’s done with the assumption that you’ve probably evaluated the ns in question already before switching to it (e.g. by doing C-c C-k (cider-load-buffer) in its source buffer). If you want to change this behavior (to avoid calling cider-repl-set-ns and then (require 'my-ns) manually), you can set:

(setq cider-repl-require-ns-on-set t)

Customizing the initial REPL namespace

Normally, the CIDER REPL will start in the user namespace. You can supply an initial namespace for REPL sessions in the repl-options section of your Leiningen project configuration:

:repl-options {:init-ns 'my-ns}

Customize the REPL Buffer’s Name

You can customize the buffer name using the variable cider-session-name-template. See the documentation for this variable for details.


Normally, code in the REPL is font-locked the same way as in clojure-mode. Before CIDER 0.10, by default, REPL input was font-locked with cider-repl-input-face (after pressing Return) and results were font-locked with cider-repl-result-face. If you want to restore the old behavior use:

(setq cider-repl-use-clojure-font-lock nil)

You can temporarily disable the Clojure font-locking by using M-x cider-repl-toggle-clojure-font-lock or the REPL shortcut toggle-font-lock.

Keep in mind that by default cider-repl-input-face simply makes the input bold and cider-repl-result-face is blank (meaning it doesn’t really apply any font-locking to results), so you might want to adjust those faces to your preferences. Some Emacs color themes might be providing different defaults for them.

Using Clojure font-locking in the REPL can impact performance negatively, especially when it comes to font-locking huge results. That, however, is mitigated to a big extent by result streaming.

Font-locking of Results

There are a few things you need to keep in mind about Clojure font-locking of results:

  • When streaming is enabled only single-chunk results will be font-locked as Clojure, as each chunk is font-locked by itself and the results can’t really be combined

  • The font-locking of results is an expensive operation which involves copying the value to a temporary buffer, where we check its integrity and do the actual font-locking.

By default CIDER instructs nREPL to stream data in 4K chunks, but you can easily modify this:

;; let's stream data in 8K chunks
(setq cider-print-buffer-size (8 * 1024))

Setting this to nil will result in using nREPL’s default buffer-size of 1024 bytes. The smaller the print buffer size the faster you’ll get feedback/updates in the REPL, so generally it’s a good idea to stick to some relatively small size.

If you’d like to learn more about the font-locking of results you can check out the definition of clojure-font-lock-as and clojure-font-lock-as-clojure in cider-util.el.

Pretty printing in the REPL

By default the REPL always prints the results of your evaluations using the printing function specified by cider-print-fn.

This behavior was changed in CIDER 0.20. In prior CIDER releases pretty-printing was disabled by default.

You can temporarily disable this behavior and revert to the default behavior (equivalent to clojure.core/pr) using M-x cider-repl-toggle-pretty-printing or the REPL shortcut toggle-pprint.

If you want to disable using cider-print-fn entirely, use:

(setq cider-repl-use-pretty-printing nil)

Note that disabling pretty-printing is not advised. Emacs does not handle well very long lines, so using a printing function that wraps lines beyond a certain width (i.e. any of them except for pr) will keep your REPL running smoothly.

See this section of the documentation for more information on configuring printing.

Displaying images in the REPL

Starting with CIDER 0.17 (AndalucĂ­a) expressions that evaluate to images can be rendered as images in the REPL. You can enable this behavior like this:

(setq cider-repl-use-content-types t)
This setting used to be enabled by default until CIDER 0.25 when it was disabled due to some rough edges with the feature that were never properly addressed. See this bug report for details.

Alternatively, you can toggle this behavior on and off using M-x cider-repl-toggle-content-types or the REPL shortcut toggle-content-types.

REPL type detection

Normally CIDER would detect automatically the type of a REPL (Clojure or ClojureScript), based on information it receives from the track-state middleware, that’s part of cider-nrepl.

In some rare cases (e.g. a bug in cider-nrepl or shadow-cljs) this auto-detection might fail and return the wrong type (e.g. Clojure instead of ClojureScript). You can disable the auto-detection logic like this:

(setq cider-repl-auto-detect-type nil)

Afterwards you can use cider-repl-set-type to set the right type manually.

Using cider-repl-set-type without disabling cider-repl-auto-detect-type won’t do much, as the REPL type will constantly be reset automatically by the track-state middleware.

REPL history

  • To make the REPL history wrap around when CIDER reaches the end:

(setq cider-repl-wrap-history t)
  • To adjust the maximum number of items kept in the REPL history:

(setq cider-repl-history-size 1000) ; the default is 500
  • To store the REPL history in a file:

(setq cider-repl-history-file "path/to/file")

Note that CIDER writes the history to the file when you kill the REPL buffer, which includes invoking cider-quit, or when you quit Emacs.