ycmd: a code-completion & comprehension server

ycmd is a server that provides APIs for code-completion and other code-comprehension use-cases like semantic GoTo commands (and others). For certain filetypes, ycmd can also provide diagnostic errors and warnings.

ycmd was originally part of [YouCompleteMe][ycm]'s codebase, but has been split out into a separate project so that it can be used in editors other than Vim.

Check [the API documentation][api-docs] if you want to implement a client. A good way to learn how to interact with ycmd is by reading through (and running) the [example_client.py][example-client] file. See the [README for the examples][example-readme] folder for details on how to run the example client.

Known ycmd clients:

• [YouCompleteMe][ycm]: Vim client, stable and exposes all ycmd features.
• [emacs-ycmd][]: Emacs client.
• [you-complete-me][atom-you-complete-me]: Atom client.
• [YcmdCompletion][sublime-ycmd-completion]: Sublime client
• [sublime-ycmd][sublime-ycmd]: Sublime Text 3 client.
• [kak-ycmd][]: Kakoune client.
• [you-complete-me][vscode-you-complete-me]: VSCode client.
• [gycm][]: Geany client.
• [nano-ycmd][]: GNU nano client.

Feel free to send a pull request adding a link to your client here if you've built one.

Building

If you're looking to develop ycmd, see the [instructions for running the tests][test-setup].

This is all for Ubuntu Linux. Details on getting ycmd running on other OS's can be found in [YCM's instructions][ycm-install] (ignore the Vim-specific parts). Note that ycmd runs on Python 3.12.0+.

First, install the minimal dependencies:

sudo apt install build-essential cmake python3-dev

Next, install the language specific dependencies you need:

• sudo apt install golang-go for Go.
• sudo apt install npm for JavaScript and TypeScript.
• sudo apt install mono-devel for C#.
• sudo apt install openjdk-8-jre for Java.

When you first clone the repository you'll need to update the submodules:

git submodule update --init --recursive

Then run python3 build.py --all or any of the specific completers listed by python3 build.py --help. This should get you going.

For more detailed instructions on building ycmd, see [YCM's instructions][ycm-install] (ignore the Vim-specific parts).

Supported compilers

• GCC 8 and later
• Clang 7 and later
• Microsoft Visual Studio 2017 v 15.7 and later

API notes

• All strings going into and out of the server are UTF-8 encoded.
• All lines end with \n.
• All line and column numbers are 1-based, not 0-based. They are also byte offsets, not Unicode codepoint offsets.
• All file paths are full, absolute paths.
• All requests to the server must include an [HMAC][] in the x-ycm-hmac HTTP header. The HMAC is computed from the shared secret passed to the server on startup and the request/response body. The digest algorithm is SHA-256. The server will also include the HMAC in its responses; you must verify it before using the response. See [example_client.py][example-client] to see how it's done.
• API is documented in swagger and published on [the website][api-docs].

How ycmd works

There are several completion engines in ycmd. The most basic one is an identifier-based completer that collects all of the identifiers in the file provided in the completion request, other files of the same filetype that were provided previously and any tags files produced by ctags. This engine is non-semantic.

There are also several semantic engines in YCM. There's [clangd][clangd]-based completer that both provide semantic completion for C-family languages. There's also a Jedi-based completer for semantic completion for Python, an OmniSharp-based completer for C#, a [gopls][gopls]-based completer for Go (using [gopls][gopls] for jumping to definitions), a TSServer-based completer for JavaScript and TypeScript, a [jdt.ls][jdtls]-based server for Java, and a [RLS][]-based completer for Rust. More will be added with time.

There are also other completion engines, like the filepath completer (part of the identifier completer).

The server will automatically detect which completion engine would be the best in any situation. On occasion, it queries several of them at once, merges the outputs and presents the results.

Semantic engines are triggered only after semantic "triggers" are inserted in the code. If the request received shows that the user's cursor is after the last character in string foo; foo. in a C# file, this would trigger the semantic engine to examine members of foo because . is a [default semantic trigger][trigger-defaults] for C# (triggers can be changed dynamically). If the text were string foo; foo.zoo, semantic completion would still be triggered (the trigger is behind the zoo word the user is typing) and the results would be filtered with the zoo query.

Semantic completion can also be forced by setting force_semantic: true in the JSON data for the completion request, but you should only do this if the user explicitly requested semantic completion with a keyboard shortcut; otherwise, leave it up to ycmd to decide when to use which engine.

The reason why semantic completion isn't always used even when available is because the semantic engines can be slow and because most of the time, the user doesn't actually need semantic completion.

There are two main use-cases for code-completion:

1. The user knows which name they're looking for, they just don't want to type the whole name.
2. The user either doesn't know the name they need or isn't sure what the name is. This is also known as the "API exploration" use-case.

The first use case is the most common one and is trivially addressed with the identifier completion engine (which BTW is blazing fast). The second one needs semantic completion.

Completion string filtering

A critical thing to note is that the completion filtering is NOT based on the input being a string prefix of the completion (but that works too). The input needs to be a [subsequence][] match of a completion. This is a fancy way of saying that any input characters need to be present in a completion string in the order in which they appear in the input. So abc is a subsequence of xaybgc, but not of xbyxaxxc.

Completion string ranking

The subsequence filter removes any completions that do not match the input, but then the sorting system kicks in. It's a bit involved, but roughly speaking "word boundary" (WB) subsequence character matches are "worth" more than non-WB matches. In effect, this means given an input of "gua", the completion "getUserAccount" would be ranked higher in the list than the "Fooguxa" completion (both of which are subsequence matches). A word-boundary character are all capital characters, characters preceded by an underscore and the first letter character in the completion string.

Auto-shutdown if no requests for a while

If the server hasn't received any requests for a while (controlled by the --idle_suicide_seconds ycmd flag), it will shut itself down. This is useful for cases where the process that started ycmd dies without telling ycmd to die too or if ycmd hangs (this should be extremely rare).

If you're implementing a client for ycmd, ensure that you have some sort of keep-alive background thread that periodically pings ycmd (just call the /healthy handler, although any handler will do).

You can also turn this off by passing --idle_suicide_seconds=0, although that isn't recommended.

Exit codes

During startup, ycmd attempts to load the ycm_core library and exits with one of the following return codes if unsuccessful:

• 3: unexpected error while loading the library;
• 4: the ycm_core library is missing;
• 7: the version of the ycm_core library is outdated.
• 8: server is started with python; recompile with python3.

User-level customization

You can provide settings to ycmd on server startup. There's a [default_settings.json][def-settings] file that you can tweak. See the [Options section in YCM's User Guide][options] for a description on what each option does. Pass the path to the modified settings file to ycmd as an --options_file=/path/to/file flag. Note that you must set the hmac_secret setting (encode the value with [base64][]). Because the file you are passing contains a secret token, ensure that you are creating the temporary file in a secure way (the [mkstemp()][mkstemp] Linux system call is a good idea; use something similar for other OS's).

After it starts up, ycmd will delete the settings file you provided after it reads it.

The settings file is something your editor should produce based on values your user has configured. There's also an extra file (.ycm_extra_conf.py) your user is supposed to provide to configure certain semantic completers. More information on it can also be found in the [corresponding section of YCM's User Guide][extra-conf-doc].

.ycm_extra_conf.py specification

The .ycm_extra_conf.py module may define the following functions:

Settings( **kwargs )

This function allows users to configure the language completers on a per project basis or globally. Currently, it is required by the libclang-based completer and optional for other completers. The following arguments can be retrieved from the kwargs dictionary and are common to all completers:

• language: an identifier of the completer that called the function. Its value is python for the Python