emacs/doc/misc/eglot.texi

\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename ../../eglot.info
@settitle Eglot: The Emacs Client for the Language Server Protocol
@documentlanguage en
@include docstyle.texi
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@copying
This manual is for Eglot, the Emacs LSP client.

Copyright @copyright{} 2022--2026 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual'',
and with the Back-Cover Texts as in (a) below.  A copy of the license is
included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Emacs misc features
@direntry
* Eglot: (eglot).             Language Server Protocol client for Emacs.
@end direntry

@titlepage
@sp 4
@c The title is printed in a large font.
@center @titlefont{User's Guide}
@sp 1
@center @titlefont{to}
@sp 1
@center @titlefont{Eglot: The Emacs LSP Client}
@ignore
@sp 2
@center release 1.8
@c -release-
@end ignore
@sp 3
@center Jo@~ao T@'avora & Eli Zaretskii
@c -date-

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top Eglot

@cindex LSP
@cindex language server protocol
Eglot is the Emacs client for the @dfn{Language Server Protocol}
(@acronym{LSP}).  The name ``Eglot'' is an acronym that stands for
@ifhtml
``@emph{E}macs Poly@emph{glot}''.
@end ifhtml
@ifnothtml
``Emacs polyGLOT''.
@end ifnothtml
@footnote{ A @dfn{polyglot} is a person who is able to use several
languages.  } Eglot provides infrastructure and a set of commands for
enriching the source code editing capabilities of Emacs via LSP@.  LSP
is a standardized communications protocol between source code editors
(such as Emacs) and language servers---programs external to Emacs which
analyze the source code on behalf of Emacs.  The protocol allows Emacs
to receive various source code services from the server, such as
description and location of function calls, types of variables, class
definitions, syntactic errors, etc.  This way, Emacs doesn't need to
implement the language-specific parsing and analysis capabilities in its
own code, but is still capable of providing sophisticated editing
features that rely on such capabilities, such as automatic code
completion, go-to definition of function/class, documentation of symbol
at-point, refactoring, on-the-fly diagnostics, and more.

Eglot itself is completely language-agnostic, but it can support any
programming language for which there is a language server and an Emacs
major mode.

This manual documents how to configure, use, and customize Eglot.  To
read this manual from within Emacs, type @kbd{M-x eglot-manual
@key{RET}}.

@insertcopying

@menu
* Quick Start::                 For the impatient.
* Eglot and LSP Servers::       How to work with language servers.
* Using Eglot::                 Important Eglot commands and variables.
* Customizing Eglot::           Eglot customization and advanced features.
* Advanced server configuration::  Fine-tune a specific language server
* Multi-server support:: Use more than one server in a buffer
* Extending Eglot::             Writing Eglot extensions in Elisp
* Troubleshooting Eglot::       Troubleshooting and reporting bugs.
* GNU Free Documentation License::  The license for this manual.
* Index::
@end menu
@end ifnottex

@node Quick Start
@chapter Quick Start
@cindex quick start

This chapter provides concise instructions for setting up and using
Eglot with your programming project in common usage scenarios.  For more
detailed instructions regarding Eglot setup, @pxref{Eglot and LSP
Servers}.  @xref{Using Eglot}, for detailed description of using Eglot,
and see @ref{Customizing Eglot}, for adapting Eglot to less common use
patterns.

Here's how to start using Eglot with your programming project:

@enumerate
@item
Select and install a language server.

Eglot comes pre-configured for many popular language servers, see the
value of @code{eglot-server-programs}.  If the server(s) mentioned there
satisfy your needs for the programming language(s) with which you want
to use Eglot, you just need to make sure those servers are installed on
your system and Eglot can find them.

Alternatively, install one or more servers of your choice and tell Eglot
where to find them.  To do that, you may add to the value of
@code{eglot-server-programs}, as described in @ref{Setting Up LSP
Servers} or have Eglot prompt you about it (@pxref{Eglot Commands}).

@item
Turn on Eglot for your project.

To start using Eglot for a project, type @kbd{M-x eglot @key{RET}} in a
buffer visiting any file that belongs to the project.  This starts the
language server configured for the programming language of that buffer,
and causes Eglot to start @dfn{managing} file-visiting buffers related
to that programming language.  This includes files that are already
visited at the time the @code{eglot} command is invoked, as well as any
files visited after this invocation.

The notion of a ``project'' used by Eglot is the same Emacs uses
(@pxref{Projects,,, emacs, GNU Emacs Manual}): in the simplest case, the
``project'' is the single file you are editing, but it can also be all
the files in a single directory or a directory tree under some version
control system, such as Git.

There are alternate ways of starting Eglot; see @ref{Starting Eglot}
for details.

@item
Use Eglot.

Most Eglot facilities are integrated into Emacs features, such as ElDoc,
Flymake, Xref, and Imenu.  However, Eglot also provides commands of its
own, mainly to perform tasks by the language server, such as @kbd{M-x
eglot-rename} (to rename an identifier across the entire project),
@kbd{M-x eglot-format} (to reformat and reindent code), and some others.
@xref{Eglot Commands}, for the detailed list of Eglot commands.

@item
That's it!
@end enumerate

@node Eglot and LSP Servers
@chapter Eglot and LSP Servers

This chapter describes how to set up Eglot for your needs, and how to
start it.

@menu
* Setting Up LSP Servers::   How to configure LSP servers for your needs.
* Starting Eglot::              Ways of starting Eglot for your project.
* Shutting Down LSP Servers::
@end menu

@node Setting Up LSP Servers
@section Setting Up LSP Servers
@cindex setting up LSP server for Eglot
@cindex LSP server for Eglot, setting up
@cindex language server for Eglot

For Eglot to be useful, it must first be combined with a suitable
language server.  Usually, that means running the server program locally
as a child process of Emacs (@pxref{Processes,,, elisp, GNU Emacs Lisp
Reference Manual}) and communicating with it via the standard input and
output streams.

The language server program must be installed separately, and is not
further discussed in this manual; refer to the documentation of the
particular server(s) you want to install.

To use a language server, Eglot must know how to start it and which
programming languages each server supports.  This information is
commonly provided by the variable @code{eglot-server-programs}.

@defvar eglot-server-programs
This variable associates major modes with names and command-line
arguments of the language server programs corresponding to the
programming language of each major mode.  It provides all the
information that Eglot needs to know about the programming language of
the source you are editing.

The value of the variable is an alist, whose elements are of the form
@w{@code{(@var{major-mode} . @var{server})}}.

The @var{major-mode} of the alist elements can be either a symbol of an
Emacs major mode or a list of the form @w{@code{(@var{mode} :language-id
@var{id})}}, with @var{mode} being a major-mode symbol and @var{id} a
string that identifies the language to the server (if Eglot cannot by
itself convert the major-mode to the language identifier string required
by the server).  In addition, @var{major-mode} can be a list of several
major modes specified in one of the above forms -- this means a running
instance of the associated server is responsible for files of multiple
major modes or languages in the project.

The @var{server} part of the alist elements can be one of the following:

@table @code
@item (@var{program} @var{args}@dots{})
This says to invoke @var{program} with zero or more arguments
@var{args}; the program is expected to communicate with Emacs via the
standard input and standard output streams.

@item (@var{program} @var{args}@dots{} :initializationOptions @var{options}@dots{})
@var{program} is invoked with @var{args} but @var{options} specifies how
to construct the @samp{:initializationOptions} JSON object to pass the
server on during the LSP handshake (@pxref{Advanced server
configuration}).

@item (@var{host} @var{port} @var{args}@dots{})
Here @var{host} is a string and @var{port} is a positive integer
specifying a TCP connection to a remote server.  The @var{args} are
passed to @code{open-network-stream}, e.g.@: if the connection needs to
use encryption or other non-default parameters (@pxref{Network,,, elisp,
GNU Emacs Lisp Reference Manual}).

@item (@var{program} @var{args}@dots{} :autoport @var{moreargs}@dots{})
@var{program} is started with a command line constructed from @var{args}
followed by an available server port and the rest of arguments in
@var{moreargs}; Eglot then establishes a TCP connection with the server
via that port on the local host.

@item @var{function}
This should be a function of a single argument: non-@code{nil} if the
connection was requested interactively (e.g., by the @code{eglot}
command), otherwise @code{nil}.  The function should return a value of
any of the forms described above.  This allows interaction with the user
for determining the program to start and its command-line arguments.
@end table

@end defvar

Eglot comes with a fairly complete set of associations of major-modes to
popular language servers predefined.  If you need to add server
associations to the default list, use @code{add-to-list}.  For example,
if there is a hypothetical language server program @command{fools} for
the language @code{Foo} which is supported by an Emacs major-mode
@code{foo-mode}, you can add it to the alist like this:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               '(foo-mode . ("fools" "--stdio"))))
@end lisp

This will invoke the program @command{fools} with the command-line
argument @option{--stdio} in support of editing source files for which
Emacs turns on @code{foo-mode}, and will communicate with the program
via the standard streams.  As usual with invoking programs, the
executable file @file{fools} should be in one of the directories
mentioned by the @code{exec-path} variable (@pxref{Subprocess
Creation,,, elisp, GNU Emacs Lisp Reference Manual}), for Eglot to be
able to find it.

Sometimes, multiple servers are acceptable alternatives for handling a
given major-mode.  In those cases, you may combine the helper function
@code{eglot-alternatives} with the functional form of
@code{eglot-server-programs}.

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(foo-mode . ,(eglot-alternatives
                               '(("fools" "--stdio")
                                 ("phewls" "--fast"))))))
@end lisp

If you have @command{fools} and @command{phewls} installed, the function
produced by @code{eglot-alternatives} will prompt for the server to use
in @code{foo-mode} buffers.  Else it will use whichever is available.

@node Starting Eglot
@section Starting Eglot
@cindex starting Eglot
@cindex activating Eglot for a project

@findex eglot
The most common way to start Eglot is to simply visit a source file of a
given language and use the command @kbd{M-x eglot}.  This starts the
language server suitable for the visited file's major-mode, and attempts
to connect to it.  If the connection to the language server is
successful, you will see the @code{[eglot:@var{project}]} indicator on
the mode line which reflects the server that was started.  If the server
program couldn't be started or connection to it failed, you will see an
error message; in that case, try to troubleshoot the problem as
described in @ref{Troubleshooting Eglot}.  Once a language server was
successfully started and Eglot connected to it, you can immediately
start using the Emacs features supported by Eglot, as described in
@ref{Eglot Features}.

A single Eglot session for a certain major-mode usually serves all the
buffers under that mode which visit files from the same project, so you
don't need to invoke @kbd{M-x eglot} again when you visit another file
from the same project which is edited using the same major-mode.  This
is because Eglot uses the Emacs project infrastructure, as described in
@ref{Eglot and Buffers}, and this knows about files that belong to the
same project.  Thus, after starting an Eglot session for some buffer,
that session is automatically reused when visiting files in the same
project with the same major-mode.

@findex eglot-ensure
Alternatively, you could configure Eglot to start automatically for one
or more major-modes from the respective mode hooks.  Here's an example
for a hypothetical @code{foo-mode}:

@lisp
 (add-hook 'foo-mode-hook 'eglot-ensure)
@end lisp

@noindent
The function @code{eglot-ensure} will start an Eglot session for each
buffer in which @code{foo-mode} is turned on, if there isn't already an
Eglot session that handles the buffer.  Note that this variant of
starting an Eglot session is non-interactive, so it should be used only
when you are confident that Eglot can be started reliably for any file
which may be visited with the major-mode in question.

Note that it's often difficult to establish this confidence fully, so it
may be wise to use the interactive command @code{eglot} instead.  You
only need to invoke it once per project, as all other files visited
within the same project will automatically be managed with no further
user intervention needed.

When Eglot connects to a language server for the first time in an Emacs
session, it runs the hook @code{eglot-connect-hook} (@pxref{Other
Variables}).

@node Shutting Down LSP Servers
@section Shutting Down LSP Servers
@cindex shutting down LSP server

When Eglot is turned on, it arranges for turning itself off
automatically if the language server process terminates.  Turning off
Eglot means that it shuts down the server connection, ceases its
management of all the buffers that use the server connection which was
terminated, deactivates its minor mode, and restores the original values
of the Emacs variables that Eglot changed when it was turned on.
@xref{Eglot and Buffers}, for more details of what Eglot management of a
buffer entails.

@findex eglot-shutdown
You can also shut down a language server manually, by using the command
@kbd{M-x eglot-shutdown}.  This prompts for the server (unless there's
only one connection and it's used in the current buffer), and then shuts
it down.  By default, it also kills the server's events buffer
(@pxref{Troubleshooting Eglot}), but a prefix argument prevents that.

Alternatively, you can customize the variable @code{eglot-autoshutdown}
to a non-@code{nil} value, in which case Eglot will automatically shut
down the language server process when the last buffer served by that
language server is killed.  The default of this variable is @code{nil},
so that visiting another file would automatically activate Eglot even
when the project which started Eglot with the server no longer has any
buffer associated with it.  This default allows you to start a server
only once in each Emacs session.

@node Using Eglot
@chapter Using Eglot

This chapter describes in detail the features that Eglot provides and
how it does that.  It also provides reference sections for Eglot
commands and variables.

@menu
* Eglot Features::
* Eglot and Buffers::
* Eglot Commands::
@end menu

@node Eglot Features
@section Eglot Features
@cindex features in buffers supported by Eglot

While Eglot is enabled in a buffer, it is said to be @dfn{managing} it,
using LSP and the specific capabilities of the language server to
activate and enhance modern IDE features in Emacs.  Some of these
features are provided via other Emacs packages, and some via Eglot
directly (@pxref{Eglot Commands}).

Here's an overview of the main features that Eglot provides:

@itemize @bullet
@item
At-point documentation: when point is near a symbol or an identifier,
information about the identifier's significance in the program (its
function or method signature, its type, its custom documentation string,
etc.) is made available via the ElDoc package (@pxref{Programming
Language Doc,,, emacs, GNU Emacs Manual}).

@item
On-the-fly diagnostic annotations, via the Flymake package
(@pxref{Top,,, Flymake, GNU Flymake manual}).  Eglot's Flymake backend
replaces other Flymake backends while it is managing a buffer, and
enhances diagnostics with interactive server-suggested fixes (so-called
@dfn{code actions}, @pxref{Eglot Commands}).

@item
Finding definitions and uses of identifiers, via Xref (@pxref{Xref,,,
emacs, GNU Emacs Manual}).  Eglot provides a backend for the Xref
capabilities which uses the language server's understanding of the
program source.  In particular, it eliminates the need to generate tags
tables (@pxref{Tags tables,,, emacs, GNU Emacs Manual}) for languages
which are only supported by the @code{etags} backend.

@item
Buffer navigation by name of function, class, method, etc., via Imenu
(@pxref{Imenu,,, emacs, GNU Emacs Manual}).  Eglot provides its own
variant of @code{imenu-create-index-function}, which generates the index
for the buffer based on the language server's program source analysis.

@item
Enhanced completion of symbol at point by the @code{completion-at-point}
command (@pxref{Symbol Completion,,, emacs, GNU Emacs Manual}).  This
uses the language server's parser data for the completion candidates.

@item
Server-suggested code refactorings.  The ElDoc package is also leveraged
to retrieve so-called @dfn{code actions} nearby point.  When such
suggestions are available they are annotated with a special indication
and can be easily invoked by the user with the @code{eglot-code-action}
command (@pxref{Eglot Commands}).

@item
On-the-fly succinct informative annotations, so-called @dfn{inlay
hints}.  Eglot adds special intangible text nearby certain identifiers,
be it the type of a variable, or the name of a formal parameter in a
function call.  @xref{Eglot Commands}, and the
@code{eglot-inlay-hints-mode} minor mode.

@item
Enhanced source code fontification via LSP @dfn{semantic tokens}.  Eglot
can leverage the language server's understanding of the role and meaning
of identifiers in the code to provide more accurate syntax highlighting.
@xref{Eglot Commands}, and the @code{eglot-semantic-tokens-mode} minor
mode.

@item
Display of function call and type hierarchies via the
@code{eglot-show-call-hierarchy} and @code{eglot-show-type-hierarchy}
commands (@pxref{Eglot Commands}).

@item
Code reformatting via the @code{eglot-format} and related commands
(@pxref{Eglot Commands}).  Automatic reformatting of source code is also
supported and is activated automatically as you type.

@item
If a completion package such as the Company package (a popular
third-party completion package providing @code{company-mode}), is
installed, Eglot enhances it by providing completion candidates based on
the language server's analysis of the source code.  (Company can be
installed from GNU ELPA.)

@item
If YASnippet, a popular third-party package for automatic insertion of
code templates (snippets), is installed, and the language server
supports snippet completion candidates, Eglot arranges for the
completion package to instantiate these snippets using YASnippet.
(YASnippet can be installed from GNU ELPA.)

@item
If the popular third-party package @code{markdown-mode} is installed,
and the server provides at-point documentation formatted as Markdown in
addition to plain text, Eglot arranges for the ElDoc package to enrich
this text with fontifications and other nice formatting before
displaying it to the user.  This makes the documentation shown by ElDoc
look nicer on display.

@item
In addition to enabling and enhancing other features and packages, Eglot
also provides a number of user commands based on the capabilities of
language servers.  Examples include renaming symbols with
@kbd{eglot-rename} and asking to automatically correct problems with
@kbd{eglot-code-actions}.  @xref{Eglot Commands}.
@end itemize

Not all servers support the full set of LSP capabilities, but most of
them support enough to enable the basic set of features mentioned above.

Conversely, some servers offer capabilities for which no equivalent
Emacs package exists yet, and so Eglot cannot (yet) expose these
capabilities to Emacs users.  However, @xref{Extending Eglot}.

Finally, it's worth noting that, by default, Eglot generally turns on
all features that it @emph{can} turn on.  It's possible to opt out of
features via the @code{eglot-ignored-server-capabilities} user option
(@pxref{Customizing Eglot}) and the @code{eglot-managed-mode-hook} hook
that runs after Eglot starts managing a buffer (@pxref{Eglot and
Buffers}).

@node Eglot and Buffers
@section Buffers, Projects, and Eglot
@cindex buffers managed by Eglot
@cindex projects and Eglot

@cindex workspace
One of the main strong points of using a language server is that a
language server has a broad view of the program: it considers more than
just the single source file you are editing.  Ideally, the language
server should know about all the source files of your program which are
written in the language supported by the server.  In LSP parlance, the
set of the source files of a program is known as a @dfn{workspace}.  The
Emacs equivalent of a workspace is a @dfn{project} (@pxref{Projects,,,
emacs, GNU Emacs Manual}).  Eglot fully supports Emacs projects, and
considers the file in whose buffer Eglot is turned on as belonging to a
project.  In the simplest case, that file is the entire project, i.e.@:
your project consists of a single file.  But there are other more
complex projects:

@itemize @bullet
@item
A single-directory project: several source files in a single common
directory.

@item
A VC project: source files in a directory hierarchy under some VCS,
e.g.@: a VCS repository (@pxref{Version Control,,, emacs, GNU Emacs
Manual}).

@item
An EDE project: source files in a directory hierarchy managed via the
Emacs Development Environment (@pxref{EDE,,, emacs, GNU Emacs
Manual}).
@end itemize

Eglot uses Emacs's project management infrastructure to figure out which
files and buffers belong to what project, so any kind of project
supported by that infrastructure is automatically supported by Eglot.

When Eglot starts a server program, it does so in the project's root
directory, which is usually the top-level directory of the project's
directory hierarchy.  This ensures the language server has the same
comprehensive view of the project's files as you do.

For example, if you visit the file @file{~/projects/fooey/lib/x.foo} and
@file{x.foo} belongs to a project rooted at @file{~/projects/fooey}
(perhaps because a @file{.git} directory exists there), then @kbd{M-x
eglot} causes the server program to start with that root as the current
working directory.  The server then will analyze not only the file
@file{lib/x.foo} you visited, but likely also all the other @file{*.foo}
files under the @file{~/projects/fooey} directory.

In some cases, additional information specific to a given project will
need to be provided to the language server when starting it.  The
variable @code{eglot-workspace-configuration} (@pxref{Customizing
Eglot}) exists for that purpose.  It specifies the parameters and their
values to communicate to each language server which needs that.

When Eglot is active for a project, the project's buffers are treated
specially in several ways:

@itemize @bullet
@cindex mode-line indication of language server
@cindex mouse clicks on mode-line, and Eglot
@vindex eglot-menu
@item
All of the project's file-visiting buffers under the same major-mode are
served by a single language server connection.  (If the project uses
several programming languages, there will usually be a separate server
connection for each group of files written in the same language and
using the same Emacs major-mode.)  Eglot adds the
@samp{[eglot:@var{project}]} indication to the mode line of each such
buffer, where @var{server} is the name of the server and @var{project}
identifies the project by its root directory.  Clicking the mouse on the
Eglot mode-line indication activates a menu with server-specific items.
The layout of the Eglot mode-line can be changed by customizing
@code{eglot-mode-line-format} (@pxref{Customization Variables}).

@item
For each buffer in which Eglot is active, it notifies the language
server that Eglot is @dfn{managing} the file visited by that buffer.
This tells the language server that the file's contents on disk may no
longer be up-to-date due to unsaved edits.  Eglot reports to the server
any changes in the text of each managed buffer, to make the server aware
of unsaved changes.  This includes your editing of the buffer and also
changes done automatically by other Emacs features and commands.
Killing a buffer relinquishes its management by Eglot and notifies the
server that the file on disk is up-to-date.

@item
When you visit a file under the same project, whether an existing or a
new file, its buffer is automatically added to the set of buffers
managed by Eglot, and the server which supports the buffer's major-mode
is notified about that.  Thus, visiting a non-existent file
@file{/home/joe/projects/fooey/lib/y.foo} in the above example will
notify the server of the @file{*.foo} files' language that a new file
was added to the project, even before the file appears on disk.  The
special Eglot minor mode is also turned on automatically in the buffer
visiting the file.

@item
Eglot turns on a special minor mode in each buffer it manages.  This
minor mode ensures the server is notified about files Eglot manages.
Unlike other minor-modes, this special minor mode is @emph{not}
activated manually by the user, but automatically, as the result of
starting an Eglot session for the buffer.  When Eglot stops managing the
buffer, the minor mode is turned off, and all the settings that Eglot
changed are restored to their original values.

@vindex eglot-managed-mode-hook
@vindex eglot-managed-p
@item
The minor mode's main job is to ``hook up'' Eglot to other Emacs
functionality, i.e.  arrange for these features to receive information
from the language server, by changing special variables controlling
these features.  @xref{Eglot Features}.

@item
To customize this, Eglot's special minor mode provides a hook variable
@code{eglot-managed-mode-hook} specifically to alter Eglot's interaction
with these variables in each buffer.  This hook is run both when the
minor mode is turned on and when it's turned off; use the function
@code{eglot-managed-p} to tell if current buffer is still being managed
or not.
@end itemize

For example, to turn off @dfn{inlay hints} and @dfn{semantic tokens} by
default but still retain the ability to turn them on again
interactively use this:

@lisp
  (add-hook 'eglot-managed-mode-hook
    (lambda ()
      (eglot-inlay-hints-mode -1)
      (eglot-semantic-tokens-mode -1)))
@end lisp

@node Eglot Commands
@section Eglot Commands
@cindex commands, Eglot

This section provides a reference for the most commonly used Eglot
commands:

@ftable @code
@item M-x eglot
This command adds the current buffer and the file it visits to the group
of buffers and files managed by Eglot on behalf of a suitable language
server.  If a language server for the buffer's @code{major-mode}
(@pxref{Major Modes,,, emacs, GNU Emacs Manual}) is not yet running, it
will be started; otherwise the buffer and its file will be added to
those managed by an existing server session.

The command attempts to figure out the buffer's major mode and the
suitable language server; in case it fails, it might prompt for the
major mode to use and for the server program to start.  If invoked with
@kbd{C-u}, it always prompts for the server program, and if invoked with
@kbd{C-u C-u}, it also prompts for the major mode.

If the language server is successfully started and contacted, this
command arranges for any other buffers belonging to the same project and
using the same major mode to use the same language server session.  That
includes any buffers created by visiting files after this command
succeeds to connect to a language server.

All the Emacs features that are capable of using Eglot services
(@pxref{Eglot Features}) are automatically configured by this command to
start using the language server via Eglot.  To customize which Emacs
features will be configured to use Eglot, use the
@code{eglot-stay-out-of} option (@pxref{Customizing Eglot}).

@item M-x eglot-reconnect
This command shuts down the current connection to the language server
and immediately restarts it using the same options used originally.
This can sometimes be useful to unclog a partially malfunctioning server
connection.

@item M-x eglot-shutdown
This command shuts down a language server.  It prompts for a language
server to shut down (unless there's only one server session, and it
manages the current buffer).  Then the command shuts down the server and
stops managing the buffers the server was used for.  Emacs features
(@pxref{Eglot Features}) that Eglot configured to work with the language
server are restored back to their original configuration.

Normally, this command kills the buffers used for communicating with the
language server, but if invoked with a prefix argument @kbd{C-u}, the
command doesn't kill those buffers, allowing them to be used for
diagnostics and problem reporting (@pxref{Troubleshooting Eglot}).

@item M-x eglot-shutdown-all
This command shuts down all the language servers active in the current
Emacs session.  As with @code{eglot-shutdown}, invoking this command
with a prefix argument avoids killing the buffers used for
communications with the language servers.

@item M-x eglot-rename
This command renames the program symbol (a.k.a.@: @dfn{identifier}) at
point to another name.  It prompts for the new name of the symbol, and
then modifies all the files in the project which are managed by the
language server of the current buffer to implement the renaming.

@item M-x eglot-format
This command reformats the active region according to the
language server rules.  If no region is active, it reformats the entire
current buffer.

@item M-x eglot-format-buffer
This command reformats the current buffer, in the same manner as
@code{eglot-format} does.

@cindex code actions
@item M-x eglot-code-actions
@itemx M-x eglot-code-action-organize-imports
@itemx M-x eglot-code-action-quickfix
@itemx M-x eglot-code-action-extract
@itemx M-x eglot-code-action-inline
@itemx M-x eglot-code-action-rewrite
These commands allow you to invoke the so-called @dfn{code actions}:
requests for the language server to provide editing commands for
correcting, refactoring or beautifying your code.  These commands may
affect more than one visited file belonging to the project.

The command @code{eglot-code-actions} asks the server if there are any
code actions for any point in the buffer or contained in the active
region.  If there are, you have the choice to execute one of them via
the minibuffer.

A common use of code actions is fixing the Flymake error diagnostics
issued by Eglot (@pxref{Top,,, Flymake, GNU Flymake manual}).  Clicking
on a diagnostic with @kbd{mouse-2} invokes
@code{eglot-code-actions-at-mouse} which pops up a menu of available
code actions.  The variable @code{eglot-diagnostics-map} can be used to
control the mouse binding.

Other commands execute a specific code action.  For example,
@code{eglot-code-action-organize-imports} rearranges the program's
@dfn{imports}---declarations of modules whose capabilities the program
uses.

@cindex inlay hints
@item M-x eglot-inlay-hints-mode
This command toggles LSP @dfn{inlay hints} on and off for the current
buffer.  Inlay hints are small text annotations to specific parts of the
whole buffer, not unlike diagnostics, but designed to help readability
instead of indicating problems.  For example, a C++ language server can
serve hints about positional parameter names in function calls and a
variable's automatically deduced type.  Inlay hints help the user not
have to remember these things by heart.

@cindex momentary inlay hints
@item eglot-momentary-inlay-hints
When bound to a single key in @code{eglot-mode-map}
(@pxref{Customization Variables}), this will arrange for inlay hints to
be displayed as long as the key is held down, and then hidden shortly
after it is released.  The best way to set it up is something like this:

@lisp
(define-key eglot-mode-map [f7] 'eglot-momentary-inlay-hints)
@end lisp

@noindent
Note that Emacs doesn't support binding to \"key up\" events, so this
command offers an approximation by estimating your system keyboard delay
and repeat rate.

@cindex semantic tokens
@item M-x eglot-semantic-tokens-mode
This command toggles LSP @dfn{semantic tokens} fontification on and off
for the current buffer.  Semantic tokens provide enhanced syntax
highlighting based on the language server's semantic analysis of the
code, going beyond traditional regular-expression-based fontification.
For example, the server can distinguish between different kinds of
variables (parameters, local variables, fields) or indicate whether a
function is being declared or called.

@vindex eglot-semantic-token-types
@vindex eglot-semantic-token-modifiers
The @code{eglot-semantic-faces} customization group contains the user
options @code{eglot-semantic-token-types} and
@code{eglot-semantic-token-modifiers}, which control which semantic
token types and modifiers reported by the language server Eglot should
consider when fontifying.  For performance reasons, the server
connection has to be restarted before changes take effect.

@cindex semantic token faces
The @code{eglot-semantic-faces} customization group also contains the
face definitions for controlling the appearance of each token.

@cindex type hierarchy
@item M-x eglot-show-type-hierarchy
Pop up a special buffer showing a interactive tree which represents a
hierarchy of subtypes and supertypes for the symbol at point.

@cindex call hierarchy
@item M-x eglot-call-type-hierarchy
Pop up a special buffer showing a interactive tree which represents a
hierarchy of callers and callee for the symbol at point.
@end ftable

The following Eglot commands are used less commonly, mostly for
diagnostic and troubleshooting purposes:

@ftable @code
@item M-x eglot-events-buffer
This command pops up the events buffer used for communication with the
language server of the current buffer.

@item M-x eglot-stderr-buffer
This command pops up the buffer with the debug info printed by the
language server to its standard error stream.

@item M-x eglot-forget-pending-continuations
Forget pending requests for the server of the current buffer.
@c FIXME: Better description of the need.

@item M-x eglot-signal-didChangeConfiguration
This command updates the language server configuration according to the
current value of the variable @code{eglot-workspace-configuration}
(@pxref{Customizing Eglot}).

@item M-x eglot-clear-status
Clear the last JSONRPC error for the server of the current buffer.
Eglot keeps track of erroneous situations encountered by the server in
its mode-line indication so that the user may inspect the communication
leading up to it (@pxref{Troubleshooting Eglot}).  If the situation is
deemed uninteresting or temporary, this command can be used to
``forget'' the error.  Note that the command @code{M-x eglot-reconnect}
can sometimes be used to unclog a temporarily malfunctioning server.
@end ftable

As described in @ref{Eglot Features} most features associated with Eglot
are actually provided by other Emacs packages and features, and Eglot
only enhances them by allowing them to use the information coming from
the language servers.  For completeness, here's the list of commands of
those other packages that are very commonly used in Eglot-managed
buffers:

@c Not @ftable, because the index entries should mention Eglot
@table @code
@cindex ElDoc, and Eglot
@cindex documentation using Eglot
@item M-x eldoc
Ask the ElDoc system for help at point.

@cindex Flymake, and Eglot
@cindex on-the-fly diagnostics using Eglot
@item M-x flymake-show-buffer-diagnostics
Ask Flymake system to display diagnostics for the current buffer.

@item M-x flymake-show-project-diagnostics
Ask Flymake to list diagnostics for all the files in the current
project.

@cindex xref, and Eglot
@cindex finding definitions of identifiers using Eglot
@item M-x xref-find-definitions
Ask Xref to go the definition of the identifier at point.

@cindex imenu navigation using Eglot
@item M-x imenu
Let the user navigate the program source code using buffer index,
categorizing program elements by syntactic class (class, method,
variable, etc.) and offering completion.

@cindex symbol completion using Eglot
@item M-x completion-at-point
Request completion of the symbol at point.
@end table

@node Customizing Eglot
@chapter Customizing Eglot
@cindex customizing Eglot

To have Eglot behave like you want it, sometimes it suffices to
customize Emacs packages and features that Eglot uses but to do so for
all of Emacs (regardless of Eglot) .  For example, to configure the
amount of space taken up by documentation in the echo area, customize
the ElDoc variable @code{eldoc-echo-area-use-multiline-p}.  To configure
Flymake faces used for errors and warnings, customize the
@code{flymake-error} and @code{flymake-warning} faces.

Compared to the breadth of its functionality, Eglot itself has a
relatively small number of customization variables.  There is
additionally a set of variables that are usually set from Elisp snippets
(@pxref{Init File,,, emacs, The GNU Emacs Manual}).

Other times, it's necessary to have finer control of Eglot's interaction
with these features.  @xref{More Customization}.

@menu
* Customization Variables::
* Other Variables::
* More Customization::
@end menu

@node Customization Variables
@section Customization Variables
@cindex variables, Eglot

This section provides a reference for the Eglot user options.

@vtable @code
@item eglot-autoreconnect
This option controls the ability to reconnect automatically to the
language server when Eglot detects that the server process terminated
unexpectedly.  The default value @code{3} means to attempt reconnection
only if the previous successful connection lasted for more than that
number of seconds; a different positive value changes the minimal length
of the connection to trigger reconnection.  A value of @code{t} means
always reconnect automatically, and @code{nil} means never reconnect (in
which case you will need to reconnect manually using @kbd{M-x eglot}).

@item eglot-connect-timeout
This specifies the number of seconds before connection attempt to a
language server times out.  The value of @code{nil} means never time
out.  The default is 30 seconds.

@item eglot-sync-connect
This setting is mainly important for connections which are slow to
establish.  Whereas the variable @code{eglot-connect-timeout} controls
how long to wait for, this variable controls whether to block Emacs's
user interface while waiting.  The default value is @code{3}; a positive
value means block for that many seconds, then wait for the connection in
the background.  The value of @code{t} means block during the whole
waiting period.  The value of @code{nil} or @code{0} means don't block
at all during the waiting period.

@item eglot-events-buffer-config
This configures the size and format of the Eglot events buffer.
@xref{Eglot Commands, eglot-events-buffer}, for how to access that
buffer.  If the value is changed, the connection should be restarted
using @kbd{M-x eglot-reconnect} for the new value to take effect.
@c FIXME: Shouldn't the defcustom do this by itself using the :set
@c attribute?  Maybe not because reconnecting is a complex task.
@xref{Troubleshooting Eglot}, for when this could be useful.

@item eglot-autoshutdown
If this is non-@code{nil}, Eglot shuts down a language server when the
last buffer managed by it is killed.  @xref{Shutting Down LSP Servers}.
The default is @code{nil}; if you want to shut down a server, use
@kbd{M-x eglot-shutdown} (@pxref{Eglot Commands}).

@item eglot-confirm-server-edits
Various Eglot commands and code actions result in the language server
sending editing commands to Emacs.  If this option's value is
non-@code{nil}, Eglot will ask for confirmation before performing edits
proposed by the language server.  This option's value can be crafted to
require this confirmation for specific commands or only when the edit
affects files not yet visited by the user.  Consult this option's
docstring for more information.

@item eglot-ignored-server-capabilities
This variable's value is a list of language server capabilities that
Eglot should not use.  The default is @code{nil}: Eglot uses all of the
capabilities supported by each server.

@item eglot-extend-to-xref
If this is non-@code{nil}, and @kbd{M-.} (@code{xref-find-definitions})
lands you in a file outside of your project, such as a system-installed
library or header file, transiently consider that file as managed by the
same language server.  That file is still outside your project
(i.e. @code{project-find-file} won't find it), but Eglot and the server
will consider it to be part of the workspace.  The default is
@code{nil}.

@item eglot-mode-map
This variable is the keymap for binding Eglot-related command.  It is in
effect only as long as the buffer is managed by Eglot.  By default, it
is empty, with the single exception: @kbd{C-h .} is remapped to invoke
@code{eldoc-doc-buffer}.  You can bind additional commands in this map.
For example:

@lisp
  (define-key eglot-mode-map (kbd "C-c r") 'eglot-rename)
  (define-key eglot-mode-map (kbd "C-c o") 'eglot-code-action-organize-imports)
  (define-key eglot-mode-map (kbd "C-c h") 'eldoc)
  (define-key eglot-mode-map (kbd "<f6>") 'xref-find-definitions)
  (define-key eglot-mode-map (kbd "<f7>") 'eglot-momentary-inlay-hints)
@end lisp

@cindex progress
@item eglot-report-progress
Set this variable to non-nil if you'd like progress notifications coming
from the language server to be handled by Emacs's progress reporting
facilities.  If the value is the symbol @code{messages} the message
buffer is used, else the progress is reported in the mode line.

@cindex request cancellation
@item eglot-advertise-cancellation
Setting this variable to true causes Eglot to send special cancellation
notification for certain stale client request.  This may help some LSP
servers avoid doing costly but ultimately useless work on behalf of the
client, improving overall performance.

@item eglot-code-action-indications
This variable controls the indication of code actions available at
point.  Value is a list of symbols, more than one can be specified:

@itemize @minus
@item
@code{eldoc-hint}: ElDoc is used to hint about at-point actions.
@item
@code{margin}: A special indicator appears in the margin of the line
that point is currently on.
@item
@code{nearby}: An interactive special indicator appears near point.
@item
@code{mode-line}: An interactive special indicator appears in the mode
line.
@end itemize

@code{margin} and @code{nearby} are incompatible.  If the list is empty,
ElDoc will not hint about at-point actions.

@item eglot-code-action-indicator
This variable is a string determining what the special indicator looks
like.

@item eglot-mode-line-format
This variables controls the shape and look of Eglot's mode line.  Its
value is a mode line ``construct'', generally a list of symbols and
strings.  The following symbols are meaningful and useful for Eglot's
mode line:
@itemize @minus
@item
@code{eglot-mode-line-menu}: access Eglot's main menu.  See also
@code{eglot-menu-string};
@item
@code{eglot-mode-line-session}: indicate current project and access
current session's menu;
@item
@code{eglot-mode-line-error}: an indication of recent LSP errors;
@item
@code{eglot-mode-line-pending-requests}: number of pending LSP requests;
@item
@code{eglot-mode-line-progress}: progress reporter widgets;
@item
@code{eglot-mode-line-action-suggestion}: LSP code actions at point.
@end itemize
@end vtable

@node Other Variables
@section Other Variables

This section provides a reference for Eglot variables that are meant to
be changed from Elisp snippets.

@c @table, not @vtable, because some of the variables are indexed
@c elsewhere
@table @code
@item eglot-server-programs
This variable determines which language server to start for each
supported major mode, and how to invoke that server's program.
@xref{Setting Up LSP Servers}, for the details.

@vindex eglot-strict-mode
@item eglot-strict-mode
This is @code{nil} by default, meaning that Eglot is generally lenient
about non-conforming servers.  If you need to debug a server, set this
to @w{@code{(disallow-non-standard-keys enforce-required-keys)}}.

@vindex eglot-server-initialized-hook
@item eglot-server-initialized-hook
A hook run after the server object is successfully initialized (which
includes launching the process) but before any LSP communication is
attempted.  Each function receives a @code{eglot-lsp-server} instance as
argument.

@vindex eglot-connect-hook
@item eglot-connect-hook
A hook run after the LSP connection to the server is successfully
established.  Each function receives a @code{eglot-lsp-server} instance
as argument.  @xref{Starting Eglot}.

@item eglot-managed-mode-hook
A hook run after Eglot started or stopped managing a buffer.
@xref{Eglot and Buffers}, for details of its usage.

@vindex eglot-stay-out-of
@item eglot-stay-out-of
This variable's value lists Emacs features that Eglot shouldn't
automatically try to manage on the user's behalf.  It is useful, for
example, when you need to use non-LSP Flymake or Company back-ends.  To
have Eglot stay away from some Emacs feature, add that feature's symbol
or a regexp that will match a symbol's name to the list: for example,
the symbol @code{xref} to leave Xref alone, or the string @samp{company}
to stay away from your Company customizations.  Here's an example:

@lisp
(add-to-list 'eglot-stay-out-of 'flymake)
@end lisp

Note that you can still configure the excluded Emacs features manually
to use Eglot in your @code{eglot-managed-mode-hook} or via some other
mechanism.
@end table

@node More Customization
@section More Customization

As explained earlier (@pxref{Eglot and Buffers}), Eglot hooks onto
existing Emacs packages and features by setting special variables in its
special minor mode.  To gain finer control of Eglot's operation, the
@code{eglot-managed-mode-hook} should be used to tweak, subdue or
completely override Eglot's setting of these variables.

@itemize @bullet
@item
Eglot hooks onto ElDoc's @code{eldoc-documentation-functions} variable,
adding a number of functions to it:
@itemize @minus
@item @code{eglot-hover-eldoc-function}, to display general information
about an identifier.
@item @code{eglot-signature-eldoc-function}, to display function
signature information.
@item @code{eglot-highlight-eldoc-function}, to highlight nearby
manifestations of an identifier.
@item @code{eglot-code-action-suggestion}, to retrieve relevant code
actions at point.
@end itemize

A simple tweak to remove at-point identifier information for
hypothetical language @code{Foo} may look like:

@lisp
(add-hook 'foo-mode-hook
  (lambda ()
    (add-hook 'eglot-managed-mode-hook
      (lambda ()
        (remove-hook 'eldoc-documentation-functions
                     'eglot-hover-eldoc-function
                     nil t))
      nil t)))
@end lisp

For more sophisticated changes, we can take advantage of ElDoc's
well-defined protocols.  @xref{Programming Language Doc,,, emacs, GNU
Emacs Manual}.  For instance, the functions in
@code{eldoc-display-functions} control displays of the at-point
documentation produced by @code{eldoc-documentation-functions}.  As an
example, to ensure that hover documentation specifically is never output
into the Echo area (in all programming modes):

@lisp
(require 'cl-lib)

(defun my/special-eglot-hover-function (cb &rest _ignored)
  "Same as `eglot-hover-eldoc-function`, but skip the echo area."
  (eglot-hover-eldoc-function (lambda (info &rest _ignore)
                                (funcall cb info :echo 'skip))))

;; substitute 'eldoc-hover-eldoc-function with our "fixed" variant.
(add-hook
  'eglot-managed-mode-hook
  (lambda ()
    (setq-local eldoc-documentation-functions
                (cl-substitute #'my/special-eglot-hover-function
                               'eglot-hover-eldoc-function
                               eldoc-documentation-functions))))
@end lisp

@item
Eglot hooks onto Flymake's @code{flymake-diagnostic-functions} variable,
completely replacing its value with Eglot's own
@code{eglot-flymake-backend} function.  Additional backends may be added
to work alongside Eglot's.

@lisp
(add-hook 'foo-mode-hook
  (lambda ()
    (add-hook 'eglot-managed-mode-hook
      (lambda ()
        (add-hook 'flymake-diagnostic-functions
                  #'my/special-foo-mode-backend
                  nil t))
      nil t)))
@end lisp

@item
Eglot hooks onto Xref's @code{xref-backend-functions} variable, adding
its own @code{eglot-xref-backend} function to it.

@item
Eglot hooks onto Imenu's @code{imenu-create-index-function}.

@item
Eglot hooks onto Emacs's @code{completion-at-point-functions}.

@item
Eglot hooks onto Company's @code{company-backends} variable, ensuring
only the @code{company-capf} backend lives there.  This ensures the same
set of completions as regular @code{completion-at-point}.

@item
Eglot hooks onto YASnippet by automatically activating
@code{yas-minor-mode} when needed to expand server-provided snippets.
@end itemize

@node Advanced server configuration
@chapter Advanced server configuration

Though many language servers work well out-of-the-box, most allow
fine-grained control of their operation via specific configuration
options that are transmitted over the LSP protocol and vary from server
to server.  A small number of servers require such special configuration
to work acceptably, or even to work at all.

After having setup a server executable program in
@code{eglot-server-programs} (@pxref{Setting Up LSP Servers}) and
ensuring Eglot can invoke it, you may want to take advantage of some of
these options.  You should first distinguish two main kinds of server
configuration:

@itemize @bullet
@item
Project-specific, applying to a specific project;

@item
User-specific, applying to all projects the server is used for.
@end itemize

When you have decided which kind you need, the following sections
teach how Eglot's user variables can be used to achieve it:

@menu
* Project-specific configuration::
* User-specific configuration::
* JSONRPC objects in Elisp::
@end menu

It's important to note that not all servers allow both kinds of
configuration, nor is it guaranteed that user options can be copied over
to project options, and vice-versa.  When in doubt, consult your
language server's documentation.

It's also worth noting that some language servers can read these
settings from configuration files in the user's @code{HOME} directory or
in a project's directory.  For example, the @command{pylsp} Python
server reads the file @file{~/.config/pycodestyle} for user
configuration.  The @command{clangd} C/C++ server reads both
@file{~/.config/clangd/config.yaml} for user configuration and
@file{.clangd} for project configuration.  It may be advantageous to use
these mechanisms instead of Eglot's, as this will probably work with
other LSP clients and may be easier to debug than options riding on the
LSP wire.

@node Project-specific configuration
@section Project-specific configuration
@vindex eglot-workspace-configuration
@cindex workspace configuration

To set project-specific settings, which the LSP specification calls
@dfn{workspace configuration}, the variable
@code{eglot-workspace-configuration} may be used.

This variable is a directory-local variable (@pxref{Directory Variables,
, Per-directory Local Variables, emacs, The GNU Emacs Manual}).  It's
important to recognize that this variable really only makes sense when
set directory-locally.  It usually does not make sense to set it
file-locally or in a major-mode hook.

The most common way to set @code{eglot-workspace-configuration } is
using a @file{.dir-locals.el} file in the root of your project.  If you
can't do that, you may also set it from Elisp code via the
@code{dir-locals-set-class-variables} function.  (@pxref{Directory Local
Variables,,, elisp, GNU Emacs Lisp Reference Manual}).

However you choose to set it, the variable's value is a plist
(@pxref{Property Lists,,, elisp, GNU Emacs Lisp Reference Manual}) with
the following format:

@lisp
 (@var{:server1} @var{plist1} @var{:server2} @var{plist2} @dots{})
@end lisp

@noindent
Here, @var{:server1} and @var{:server2} are keywords whose names
identify the LSP language servers to target.  Consult server
documentation to find out what name to use.  @var{plist1} and
@var{plist2} are plists of options, possibly nesting other plists.

@findex eglot-show-workspace-configuration
When experimenting with workspace settings, you can use the command
@kbd{M-x eglot-show-workspace-configuration} to inspect and debug the
value of this variable in its final JSON form, ready to be sent to the
server (@pxref{JSONRPC objects in Elisp}).  This helper command works
even before actually connecting to the server.

These variable's value doesn't take effect immediately.  That happens
upon establishing the connection, in response to an explicit query from
the server, or when issuing the command @kbd{M-x
eglot-signal-didChangeConfiguration} which notifies the server during an
ongoing Eglot session.

@subsection Examples

For some users, setting @code{eglot-workspace-configuration} is a
somewhat daunting task.  One of the reasons is having to manage the
general Elisp syntax of per-mode directory-local variables, which uses
alists (@pxref{Association Lists,,, elisp, GNU Emacs Lisp Reference
Manual}), and the specific syntax of Eglot's variable, which uses
plists.  Some examples are useful.

Let's say you want to configure two language servers to be used in a
project written in a combination of the Python and Go languages.  You
want to use the @command{pylsp} and @command{gopls} languages servers.
In the documentation of the servers in question (or in some other
editor's configuration file, or in some blog article), you find the
following configuration options in informal dotted-notation syntax:

@example
pylsp.plugins.jedi_completion.include_params: true
pylsp.plugins.jedi_completion.fuzzy: true
pylsp.pylint.enabled: false
gopls.usePlaceholders: true
@end example

To apply this to Eglot, and assuming you chose the
@file{.dir-locals.el} file method, the contents of that file could be:

@lisp
((nil
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))
         :gopls (:usePlaceholders t)))))
 (python-base-mode . ((indent-tabs-mode . nil)))
 (go-mode          . ((indent-tabs-mode . t))))
@end lisp

@noindent
Note how alists are used for associating Emacs mode names with other
alists, associating variable names with variable values.  Then, pay
special attention to how the value of
@code{eglot-workspace-configuration} is a plist and @emph{not} an alist.

The above form sets the value of @code{eglot-workspace-configuration} in
all the buffers inside the project; each language server will then use
only the section of the parameters intended for it, and ignore the rest.
In our example, @command{pylsp} and @command{gopls} are solely
interested in the @code{:pylsp} and @code{:gopls} parameter section
names, respectively.  However, section names don't have to match the
server name and some servers may be even interested in multiple
sections: the @command{typescript-language-server} server is known to be
interested in the @code{:javascript} and @code{:typescript} section
names.

To avoid sending whole configuration sections to servers who are
decidedly not interested in them, the following form may also be used:

@lisp
((python-base-mode
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))))
     (indent-tabs-mode . nil)))
 (go-mode
  . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders t)))
     (indent-tabs-mode . t))))
@end lisp

@noindent
This sets up the value of @code{eglot-workspace-configuration}
separately depending on the major mode of each of that project's
buffers.  @code{python-base-mode} buffers will have the variable set to
@code{(:pylsp (:plugins ...))}.  @code{go-mode} buffers will have the
variable set to @code{(:gopls (:usePlaceholders t))}.

Some servers will issue workspace configuration for specific source
files inside your project.  For example, if you know @code{gopls} is
asking about specific files in the @code{src/imported} subdirectory and
you want to set a different option for @code{gopls.usePlaceholders} ,
you may use something like:

@lisp
((python-base-mode
  . ((eglot-workspace-configuration
      . (:pylsp (:plugins (:jedi_completion (:include_params t
                                             :fuzzy t)
                           :pylint (:enabled :json-false)))))
     (indent-tabs-mode nil)))
 (go-mode
  . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders t)))
     (indent-tabs-mode t)))
 ("src/imported"
   . ((eglot-workspace-configuration
      . (:gopls (:usePlaceholders nil))))))
@end lisp

Finally, if one needs to determine the workspace configuration based on
some dynamic context, @code{eglot-workspace-configuration} can be set to
a function instead of a plist.  The function is called with the
@code{eglot-lsp-server} instance of the connected server (if any) and
with @code{default-directory} set to the root of the project.  The
function should return a plist suitable for use as the variable's value.

@node User-specific configuration
@section User-specific configuration
@cindex initializationOptions
@cindex command-line arguments

This kind of configuration applies to all projects the server is used
for.  Here, there are a number of ways to do this inside Eglot.

A common way is to pass command-line options to the server invocation
via @code{eglot-server-programs}.  Let's say we want to configure where
the @command{clangd} server reads its @code{compile_commands.json} from.
This can be done like so:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(c++-mode . ("clangd" "--compile-commands-dir=/tmp"))))

@end lisp

@noindent
Another way is to have Eglot pass a JSON object to the server during the
LSP handshake.  This is done using the @code{:initializationOptions}
syntax of @code{eglot-server-programs}:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               `(c++-mode . ("clangd" :initializationOptions
                                      (:compilationDatabasePath "/tmp")))))
@end lisp

@noindent
The argument @code{(:compilationDatabasePath "/tmp")} is Emacs's
representation in plist format of a simple JSON object
@code{@{"compilationDatabasePath": "/tmp"@}}.  To learn how to represent
more deeply nested options in this format, @pxref{JSONRPC objects in
Elisp}.

In this case, the two examples achieve exactly the same, but notice how
the option's name has changed between them.

@vindex eglot-workspace-configuration
Finally there is another way to do user-specific configuration of
language servers, which may be used if the methods above are not
supported.  It consists of @emph{globally} setting
@code{eglot-workspace-configuration}, a variable originally intended for
project-specific configuration.  This has the same effect as giving all
your projects a certain default configuration, as described in
@ref{Project-specific configuration}.  Here is an example:

@lisp
(setq-default eglot-workspace-configuration
              '(:pylsp (:plugins (:jedi_completion (:include_params t
                                                    :fuzzy t)
                                  :pylint (:enabled :json-false)))
                :gopls (:usePlaceholders t)))
@end lisp

Note that the global value of @code{eglot-workspace-configuration} is
always overridden if a directory-local value is detected.

@node JSONRPC objects in Elisp
@section JSONRPC objects in Elisp

Emacs's preferred way of representing JSON is via Lisp lists.  In Eglot,
the syntax of this list is the simplest possible (the one with fewer
parenthesis), a plist (@pxref{Property Lists,,, elisp, GNU Emacs Lisp
Reference Manual}).

The plist may be arbitrarily complex, and generally containing other
keyword-value property sub-plists corresponding to JSON sub-objects.

For representing the JSON leaf values @code{true}, @code{false},
@code{null} and @code{@{@}}, you can use the Lisp values @code{t},
@code{:json-false}, @code{nil}, and @code{eglot-@{@}}, respectively.
JSON arrays are represented as Elisp vectors surrounded by square
brackets (@pxref{Vectors,,,elisp,GNU Emacs Lisp Reference Manual}).

For example, the plist

@lisp
(:pylsp (:plugins (:jedi_completion (:include_params t
                                     :fuzzy t
                                     :cache_for ["pandas" "numpy"])
                   :pylint (:enabled :json-false)))
 :gopls (:usePlaceholders t))
@end lisp

@noindent
is serialized by Eglot to the following JSON text:

@example
@{
  "pylsp": @{
    "plugins": @{
      "jedi_completion": @{
        "include_params": true,
        "fuzzy": true,
        "cache_for": [ "pandas", "numpy" ]
      @},
      "pylint": @{
        "enabled": false
      @}
    @}
  @},
  "gopls": @{
    "usePlaceholders": true
  @}
@}
@end example

@node Multi-server support
@chapter Multi-server support
@cindex multiple servers per buffer
@cindex LSP server multiplexer
@cindex per-buffer multiple servers

One of the most frequently requested features for Eglot in close to a
decade of existence is the ability to use more than one LSP server in a
single buffer.  This is distinct from using multiple servers in a
project, where each server manages a disjoint set of files written in
different languages.

The latter case---multiple servers for different files---is
intrinsically supported by Eglot.  For example, in a web project with
JavaScript, CSS, and Python files, Eglot can seamlessly manage separate
language servers for each file type within the same project
(@pxref{Starting Eglot}).  Each buffer communicates with its appropriate
server, and this works out-of-the-box.

However, there are several scenarios where multiple servers per buffer
are useful:

@itemize @bullet
@item
Combining a spell-checking language server like @command{codebook-lsp}
with language-specific servers for C++, Go, or Python files.  The
spell-checker provides diagnostics for comments and strings, while the
language server handles syntax and semantics.

@item
One might want multiple servers to cover different aspects of the same
language.  For Python, you might combine @command{ty} for type checking
with @command{ruff} for linting and formatting.  For JavaScript, you
might use @command{typescript-language-server} for language features
together with @command{eslint} for linting.

@item
When working on multi-language files like Vue @file{.vue} files, which
contain JavaScript, CSS, and HTML embedded in a single file, multiple
servers can manage the different areas of the buffer.
@end itemize

These use cases are not directly supported by Eglot's architecture,
however, you can use a language-agnostic @dfn{LSP server multiplexer}
that sits between Eglot and the actual language servers.  Eglot still
communicates with a single LSP server process in each buffer, but that
process mediates communication to multiple language-specific servers,
meaning that for practical purposes, it's @emph{as if} Eglot was
connected to them directly.

This approach is more powerful and user-friendly than current
workarounds that combine one LSP server in a buffer with additional
non-LSP mechanisms such as extra Flymake backends (@pxref{Top,,,
Flymake, GNU Flymake manual}) for the same buffer.

@menu
* Using Rassumfrassum::   Setup the @code{rass} LSP multiplexer
* Design rationale::      Benefits and drawbacks of LSP multiplexers
@end menu

@node Using Rassumfrassum
@section Using Rassumfrassum

@uref{https://github.com/joaotavora/rassumfrassum, Rassumfrassum} is an
LSP server multiplexer program that fits the bill.  Like most language
servers, it must be installed separately since it is not bundled with
Emacs (at time of writing).  The installation is similar to installing
any other language server, and usually amounts to making sure the
program executable is somewhere in @code{PATH} or @code{exec-path}.

The Rassumfrassum program, invoked via the @command{rass} command, works
by spawning multiple LSP server subprocesses and aggregating their
capabilities, requests, and responses into a single unified LSP
interface.  From Eglot's perspective, it appears to be communicating with
a single server.

To use Rassumfrassum with Eglot, you can start it interactively with a
prefix argument to @code{eglot} and specify the @command{rass} command
followed by the actual servers you want to use, separated by @code{--}:

@example
C-u M-x eglot RET rass -- clangd -- codebook-lsp serve RET
@end example

@noindent
This starts @command{clangd} for C++ language support and
@command{codebook-lsp} for spell-checking in the same buffer.

For Python, you might use:

@example
C-u M-x eglot RET rass -- ty server -- ruff server RET
@end example

@noindent
or simply @kbd{C-u M-x eglot RET rass python}, using the ``preset''
feature. This combines @command{ty} for type checking with
@command{ruff} for linting and formatting.

These configurations can be integrated into the
@code{eglot-server-programs} variable (@pxref{Setting Up LSP Servers})
for automatic use:

@lisp
(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
               '(c-ts-base-mode . ("rass" "--" "clangd" "--"
                                   "codebook-lsp" "serve")))
  (add-to-list 'eglot-server-programs
               '(python-mode . ("rass" "--" "ty" "server" "--"
                                "ruff" "server"))))
@end lisp

@node Design rationale
@section Design rationale

Using an LSP server multiplexer like @command{rass} relieves Eglot from
knowing about the specific characteristics of individual servers and the
complexity of managing multiple simultaneous server connections per
buffer.  This helps preserve the essential features that distinguish
Eglot's code base from other LSP offers for Emacs: simple, performant
and mindful of the core tenet of LSP, which is for a client to be
language-agnostic.

This approach has an additional benefit: because the multiplexer
mediates all communication between Eglot and the servers, it can take
advantage of different optimization opportunities.  For instance, at the
system level it may be multi-threaded to process different JSONRPC
streams in with true parallelism, something which is currently
impossible to do in plain Elisp.  At the LSP-level it can merge server
responses intelligently, truncate unnecessarily large objects, and cache
significant amounts of information in efficient ways.  In many cases,
this can reduce the amount of JSONRPC traffic exchanged with Emacs to
levels well below what would occur if a client connected to multiple
servers separately.  Some of these optimizations may apply even when a
program like @command{rass} is mediating communication to a single
server.

The multiplexer approach is not without drawbacks.  Since LSP is a
relatively large protocol with a decade of existence and many backward
compatibility concerns, combining the responses of servers using completely
different mechanisms of the protocol to respond to the same request
sometimes leads to complexity in covering the corner cases.  However,
offloading this complexity to a completely separate layer has proven
very effective in practice.

@node Extending Eglot
@chapter Extending Eglot

Sometimes it may be useful to extend existing Eglot functionality using
Elisp its public methods.  A good example of when this need may arise is
adding support for a custom LSP protocol extension only implemented by a
specific server.

The best source of documentation for this is probably Eglot source code
itself, particularly the section marked ``API''.

Most of the functionality is implemented with Common-Lisp style generic
functions (@pxref{Generics,,,eieio,EIEIO}) that can be easily extended
or overridden.  The Eglot code itself is an example on how to do this.

The following is a relatively simple example that adds support for the
@code{inactiveRegions} experimental feature introduced in version 17 of
the @command{clangd} C/C++ language server++.

Summarily, the feature works by first having the server detect the
Eglot's advertisement of the @code{inactiveRegions} client capability
during startup, whereupon the language server will report a list of
regions of inactive code for each buffer.  This is usually code
surrounded by C/C++ @code{#ifdef} macros that the preprocessor removes
based on compile-time information.

The language server reports the regions by periodically sending a
@code{textDocument/inactiveRegions} notification for each managed buffer
(@pxref{Eglot and Buffers}).  Normally, unknown server notifications are
ignored by Eglot, but we're going change that.

Both the announcement of the client capability and the handling of the
new notification is done by adding methods to generic functions.

@itemize @bullet
@item
The first method extends @code{eglot-client-capabilities} using a simple
heuristic to detect if current server is @command{clangd} and enables
the @code{inactiveRegion} capability.

@lisp
(cl-defmethod eglot-client-capabilities :around (server)
  (let ((base (cl-call-next-method)))
    (when (cl-find "clangd" (process-command
                              (jsonrpc--process server))
                   :test #'string-match)
      (setf (cl-getf (cl-getf base :textDocument)
                     :inactiveRegionsCapabilities)
            '(:inactiveRegions t)))
    base))
@end lisp

Notice we use an internal function of the @code{jsonrpc.el} library, and
a regexp search to detect @command{clangd}.  An alternative would be to
define a new EIEIO subclass of @code{eglot-lsp-server}, maybe called
@code{eglot-clangd}, so that the method would be simplified:

@lisp
(cl-defmethod eglot-client-capabilities :around ((_s eglot-clangd))
  (let ((base (cl-call-next-method)))
    (setf (cl-getf (cl-getf base :textDocument)
                     :inactiveRegionsCapabilities)
            '(:inactiveRegions t))))
@end lisp

However, this would require that users tweak @code{eglot-server-program}
to tell Eglot instantiate such sub-classes instead of the generic
@code{eglot-lsp-server} (@pxref{Setting Up LSP Servers}).  For the
purposes of this particular demonstration, we're going to use the more
hacky regexp route which doesn't require that.

Note, however, that detecting server versions before announcing new
capabilities is generally not needed, as both server and client are
required by LSP to ignore unknown capabilities advertised by their
counterparts.

@item
The second method implements @code{eglot-handle-notification} to process
the server notification for the LSP method
@code{textDocument/inactiveRegions}.  For each region received it
creates an overlay applying the @code{shadow} face to the region.
Overlays are recreated every time a new notification of this kind is
received.

To learn about how @command{clangd}'s special JSONRPC notification
message is structured in detail you could consult that server's
documentation.  Another possibility is to evaluate the first
capability-announcing method, reconnect to the server and peek in the
events buffer (@pxref{Eglot Commands, eglot-events-buffer}).  You could
find something like:

@lisp
[server-notification] Mon Sep  4 01:10:04 2023:
(:jsonrpc "2.0" :method "textDocument/inactiveRegions" :params
          (:textDocument
           (:uri "file:///path/to/file.cpp")
           :regions
           [(:start (:character 0 :line 18)
             :end (:character 58 :line 19))
            (:start (:character 0 :line 36)
             :end (:character 1 :line 38))]))
@end lisp

This reveals that the @code{textDocument/inactiveRegions} notification
contains a @code{:textDocument} property to designate the managed buffer
and an array of LSP regions under the @code{:regions} property.  Notice
how the message (originally in JSON format), is represented as Elisp
plists (@pxref{JSONRPC objects in Elisp}).

The Eglot generic function machinery will automatically destructure the
incoming message, so these two properties can simply be added to the new
method's lambda list as @code{&key} arguments.  Also, the
@code{eglot-uri-to-path} and @code{eglot-range-region} may be used to
easily parse the LSP @code{:uri} and @code{:start ... :end ...}  objects
to obtain Emacs objects for file names and positions.

The remainder of the implementation consists of standard Elisp
techniques to loop over arrays, manage buffers and overlays.

@lisp
(cl-defmethod eglot-handle-notification
  (_server (_method (eql textDocument/inactiveRegions))
           &key regions textDocument &allow-other-keys)
  (if-let* ((path (expand-file-name (eglot-uri-to-path
                                     (cl-getf textDocument :uri))))
            (buffer (find-buffer-visiting path)))
      (with-current-buffer buffer
        (remove-overlays nil nil 'inactive-code t)
        (cl-loop
         for r across regions
         for (beg . end) = (eglot-range-region r)
         for ov = (make-overlay beg end)
         do
         (overlay-put ov 'face 'shadow)
         (overlay-put ov 'inactive-code t)))))
@end lisp

@end itemize

After evaluating these two additions and reconnecting to the
@command{clangd} language server (version 17), the result will be that
all the inactive code in the buffer will be nicely grayed out using the
LSP server knowledge about current compile time preprocessor defines.

@node Troubleshooting Eglot
@chapter Troubleshooting Eglot
@cindex troubleshooting Eglot

This chapter documents commands and variables that can be used to
troubleshoot Eglot problems.  It also provides guidelines for
reporting Eglot bugs in a way that facilitates their resolution.

When you encounter problems with Eglot, try first using the commands
@kbd{M-x eglot-events-buffer} and @kbd{M-x eglot-stderr-buffer}.  They
pop up special buffers that can be used to inspect the communications
between the Eglot and language server.  In many cases, this will
indicate the problems or at least provide a hint.

@menu
* Performance::
* Getting the latest version::
* Reporting bugs::
@end menu

@node Performance
@section Performance
@cindex performance problems, with Eglot
A common and easy-to-fix cause of performance problems in Eglot
(especially in older versions) is its events buffer, since it represents
additional work that Eglot must do (@pxref{Eglot Commands,
eglot-events-buffer}).  If you find Eglot is operating correctly but
slowly, try to customize the variable @code{eglot-events-buffer-config}
(@pxref{Customization Variables}) and set its @code{:size} property to
0.  This will disable recording any events and may speed things up.

In other situations, the cause of poor performance lies in the language
server itself.  Some servers use aggressive caching and other techniques
to improve their performance.  Often, this can be tweaked by changing
the server configuration (@pxref{Advanced server configuration}).

Another aspect that may cause performance degradation is the amount of
JSONRPC information exchanged with Emacs.  Using an LSP program like
@ref{Using Rassumfrassum,Rassumfrassum} may alleviate such problems.

@node Getting the latest version
@section Getting the latest version
@cindex upgrading Eglot

To install the latest Eglot in an Emacs version that does not bundle
Eglot, use @kbd{M-x package-install}.

Often, a newer Eglot version exists that has fixed a longstanding bug,
has more LSP features, or just better supports a particular language
server.  Recent Eglot versions can self-update via the command @kbd{M-x
eglot-upgrade-eglot}.  This will replace any currently installed version
with the newest one available from the ELPA archives configured in
@code{package-archives}.

You can also update Eglot through other methods, such as
@code{use-package} (@pxref{Installing packages,,, use-package,
use-package User Manual}), @code{package-install}, @code{list-packages}
or the newer @code{package-upgrade} (@pxref{Packages,,, emacs, GNU Emacs
Manual}).  However, do read the docstrings of the command you intend to
use before you use it, as some of them may not work in exactly the same
way across Emacs versions, meaning your configuration may be not
portable.

@node Reporting bugs
@section Reporting bugs
@cindex bug reports

If you think you have found a bug, we want to hear about it.  Before
reporting a bug, keep in mind that interaction with language servers
represents a large quantity of unknown variables.  Therefore, it is
generally both @emph{difficult} and @emph{absolutely essential} that the
maintainers reproduce bugs exactly as they happened to you, the user.

To report an Eglot bug, send e-mail to @email{bug-gnu-emacs@@gnu.org}.

To understand how to write this email, get acquainted with Emacs's bug
reporting guidelines (@pxref{Bugs,,, emacs, GNU Emacs Manual}).  Then,
follow this Eglot-specific checklist:

@enumerate
@item
Include the transcript of JSONRPC events obtained from the buffer popped
up by @kbd{M-x eglot-events-buffer}.  You may narrow down the transcript
if you are sure of where the problematic exchange is, but it's safer to
include the whole transcript, either attached or inline.

@item
If Emacs signaled an error (an error message was seen or heard), make
sure to repeat the process after turning on @code{debug-on-error} via
@kbd{M-x toggle-debug-on-error}.  This normally produces a backtrace of
the error that should also be attached to the bug report.

@item
Include a description of how the maintainer should obtain, install, and
configure the language server you used.  Maintainers usually have access
to GNU/Linux systems, though not necessarily the distribution that you
may be using.  If possible, try to replicate the problem with the
C/C@t{++} or Python servers, as these are very easy to install.

@item
If using an LSP multiplexer server like @ref{Using Rassumfrassum,
Rassumfrassum}, first verify if the program replicates by using one of
the multiplexed servers directly.  If it doesn't the problem lies in the
LSP multiplexer program and should be reported there.

@item
Include a description of a @emph{minimal} project directory where Eglot
should be started for the problem to happen.  Describe each file's name
and its contents, or---sometimes better--- zip that project directory
completely and attach it.  Alternatively, you can supply the address of
a public Git repository.

@item
Include versions of the software used.  The Emacs version can be
obtained with @kbd{M-x emacs-version}.

We welcome bug reports about all Eglot versions, but it is helpful to
first check if the problem isn't already fixed in the latest version
(@pxref{Getting the latest version}).

It's also essential to include the version of ELPA packages that are
explicitly or implicitly loaded.  The optional but popular Company or
Markdown packages are distributed as GNU ELPA packages, not to mention
Eglot itself in some situations.  Prefer reproducing the problem with
built-in Treesit major modes like @code{go-ts-mode} or
@code{rust-ts-mode} since the non-ts modes for such languages are
usually provided by ELPA packages, and it's often easy to miss them.

If you can't reproduce your bug without ELPA packages, you may find the
ones you're using in @code{~/.emacs.d/elpa} (or what is in
@code{package-user-dir}).  Including a listing of files in that
directory is a way to tell the maintainers about ELPA package versions.

@item
Include a recipe to replicate the problem with @emph{a clean Emacs run}.
The invocation @code{emacs -Q -f package-initialize} starts Emacs with
no configuration and initializes the ELPA packages.  A very minimal
@file{.emacs} initialization file (10 lines or less) is also acceptable
and good means to describe changes to variables.

There is usually no need to include @code{require} statements in the
recipe, as Eglot's functionality uses autoloads.

Likewise, there is rarely the need to use things like @code{use-package}
or @code{eglot-ensure}.  This just makes the recipe harder to follow.
Prefer setting variables with @code{setq} and adding to hooks with
@code{add-hook}.  Prefer starting Eglot with @code{M-x eglot}.

@item
Make sure to double check all the above elements and re-run the recipe
to see that the problem is reproducible.  Following the recipe should
produce event transcript and error backtraces that are very similar to
the ones you included.  If the problem only happens sometimes, mention
this in your report.
@end enumerate

Please keep in mind that some problems reported against Eglot may
actually be bugs in the language server or the Emacs feature/package
that used Eglot to communicate with the language server.  Eglot is, in
many cases, just a frontend to that functionality.

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Index
@unnumbered Index
@printindex cp

@bye