See errors in buffers

When a syntax check in the current buffer has finished Flycheck reports the results of the check in the current buffer in two ways:

Additionally Flycheck indicates its current state and the number of errors and warnings in the mode line.

The following screenshot illustrates how this looks like in the default Emacs color theme. It shows an info, a warning and an error annotation, from top to bottom. Please also note the fringe indicators on the left side and the emphasized mode line indicator in the bottom right corner:

Flycheck showing info, warning and error annotations


The colours of fringe icons and the whole appearance of the error highlights depend on the active color theme. Although red, orange and green or blue seem to be somewhat standard colours for Flycheck’s annotations across many popular themes, please take a closer look at your color theme if you’re in doubt about the meaning of a Flycheck highlight.

Error levels

All errors that syntax checkers report have a level which tells you the severity of the error. Flycheck has three built-in levels:

Severe errors like syntax or type errors.
Potential but not fatal mistakes which you should likely fix nonetheless.
Purely informational messages which inform about notable things in the current buffer, or provide additional help to fix errors or warnings.

Each error level has a distinct highlighting and colour which helps you to identify the severity of each error right in the buffer.

Error highlights

Flycheck highlights errors directly in the buffer according to flycheck-highlighting-mode and flycheck-highlighting-style.

Most checkers report a single error position, not a range, so Flycheck typically needs to guess how far to extend the highlighting: by default, it highlights the whole symbol at the location reported by the checker, as in the screenshot above, but you can change that range (or even disable highlighting completely) using flycheck-highlighting-mode.

defcustom flycheck-highlighting-mode

How Flycheck chooses which buffer region to highlight:

Do not highlight anything at all.
Highlight the whole line and discard any information about the column.
Highlight the column of the error if any, otherwise like lines.
Highlight the entire symbol around the error column if any, otherwise like columns. This is this default.
Highlight the entire expression around the error column if any, otherwise like columns.


In some major modes sexps is very slow, because discovering expression boundaries is costly.

The built-in python-mode is known to suffer from this issue.

Be careful when enabling this mode.

Conversely, when a checker reports a range, Flycheck uses that.

The style of the highlighting is determined by the value of flycheck-highlighting-style. By default, Flycheck highlights error text with a face indicating the severity of the error (typically, this face applies a coloured wavy underline). Instead of faces, however, Flycheck can also indicate erroneous text by inserting delimiters around it (checkers sometimes report errors that span a large region of the buffer, making underlines distracting, so in fact Flycheck only applies a face if the error spans less than 5 lines; this is achieved using the conditional style described below).

defcustom flycheck-highlighting-style

How Flycheck highlights error regions.

Do not indicate error regions.
Apply a face to erroneous text.
(delimiters BEFORE AFTER)
Bracket the error text between BEFORE and AFTER, which can be strings, images, etc. Chars are handled specially: they are repeated twice to form double brackets.
(conditional NLINES S1 S2)
Chose between styles S1 and S2: S1 if the error covers up to NLINES, and S2 otherwise.

To change the style of the underline or use different colours in the level-face style, customize the following faces, which are used depending on the error level:

defface flycheck-error
defface flycheck-warning
defface flycheck-info

The highlighting face for error, warning and info levels respectively.

Delimiters use the same faces as the fringe icons described below, in addition to the flycheck-error-delimiter face; delimited text has the flycheck-delimited-error face, which is empty by default.

defface flycheck-error-delimiter

The face applied to BEFORE and AFTER delimiters.

defface flycheck-delimited-error

The face applied to error text in delimiters style.

Fringe and margin icons

In GUI frames, Flycheck also adds indicators to the fringe—the left or right border of an Emacs window—to help you identify erroneous lines quickly. These indicators consist of a rightward-pointing double arrow shape coloured in the colour of the corresponding error level. By default the arrow is 8 pixels wide, but a 16 pixels version is used if the fringe is wide enough.


Flycheck extensions can define custom error levels with different fringe indicators. Furthermore some Emacs distributions like Spacemacs redefine Flycheck’s error levels to use different indicators. If you’re using such a distribution please take a look at its documentation if you’re unsure about the appearance of Flycheck’s indicators.

You can customise the location of these indicators (left or right fringe) with flycheck-indication-mode, which also lets you turn off these indicators completely; additionally, you can move these indicators into the margins instead of the fringes:

defcustom flycheck-indication-mode

How Flycheck indicates errors and warnings in the buffer fringes:

left-fringe or right-fringe
Use the left or right fringe respectively. Fringes can only contain monochrome bitmaps, so Flycheck draws small pixel-art arrows.
left-margin or right-margin
Use the left or right margin respectively. Margins can support all of Emacs’ rendering facilities, so Flycheck uses the » character, which scales with the font size.
Do not indicate errors and warnings in the fringe or in the margin.

By default, Emacs displays fringes, but not margins. With left-margin and right-margin indication modes, you will need to enable margins in your .emacs. For example:

(setq-default left-fringe-width 1 right-fringe-width 8
              left-margin-width 1 right-margin-width 0)

If you intend to use margins only with Flycheck, consider using flycheck-set-indication-mode in a hook instead; this function adjusts margins and fringes for the current buffer.

(setq-default flycheck-indication-mode 'left-margin)
(add-hook 'flycheck-mode-hook #'flycheck-set-indication-mode)

That function sets fringes and margins to reasonable (but opinionated) defaults, according to flycheck-indication-mode. To set your own margin and fringe widths, use a hook and call flycheck-refresh-fringes-and-margins, like this:

;; Show indicators in the left margin
(setq flycheck-indication-mode 'left-margin)

;; Adjust margins and fringe widths…
(defun my/set-flycheck-margins ()
  (setq left-fringe-width 8 right-fringe-width 8
        left-margin-width 1 right-margin-width 0)

;; …every time Flycheck is activated in a new buffer
(add-hook 'flycheck-mode-hook #'my/set-flycheck-margins)

The following faces control the colours of fringe and margin indicators.

defface flycheck-fringe-error
defface flycheck-fringe-warning
defface flycheck-fringe-info

The icon faces for error, warning and info levels respectively.

When an error spans multiple lines, Flycheck displays a hatch pattern in the fringes or vertical dots in the margins to indicate the extent of the error.

To change the fringe bitmap or the symbol used in the margins, use the function flycheck-redefine-standard-error-levels.

Mode line

Like all minor modes Flycheck also has a mode line indicator. You can see it in the bottom right corner of the above screenshot. By default the indicator shows Flycheck’s current state via one of the following texts:

FlyC* Flycheck is checking the buffer currently.
FlyC There are no errors or warnings in the current buffer.
FlyC:3/5 There are three errors and five warnings in the current buffer.
FlyC- Flycheck did not find a syntax checker for the current buffer. Take a look at the list of supported languages and type C-c ! v to see what checkers are available for the current buffer.
FlyC! The last syntax check failed. Inspect the *Messages* buffer look for error messages, and consider reporting a bug.
FlyC? The last syntax check had a dubious result. The definition of a syntax checker may have a bug. Inspect the *Messages* buffer and consider reporting a bug.

You can entirely customise the mode line indicator with flycheck-mode-line:

defcustom flycheck-mode-line

A “mode line construct” for Flycheck’s mode line indicator.

See also

Mode Line Data(elisp)
Documentation of mode line constructs.
A Flycheck extension which puts emojis into Flycheck’s mode line indicator.
A Flycheck extension which colours the entire mode line according to Flycheck’s status.

Error thresholds

To avoid flooding a buffers with excessive highlighting, cluttering the appearance and slowing down Emacs, Flycheck takes precautions against syntax checkers that report a large number of errors exceeding flycheck-checker-error-threshold:

defcustom flycheck-checker-error-threshold

The maximum number of errors a syntax checker is allowed to report.

If a syntax checker reports more errors the error information is discarded. To not run into the same issue again on the next syntax check the syntax checker is automatically added to flycheck-disabled-checkers in this case to disable it for the next syntax check.

Clear results

You can explicitly remove all highlighting and indication and all error information from a buffer:

C-c ! C
M-x flycheck-clear

Clear all reported errors, all highlighting and all indication icons from the current buffer.

C-u C-c ! C
C-u M-x flycheck-clear

Like C-c ! C but also interrupt any syntax check currently running. Use this command if you think that Flycheck is stuck.