protolint

protolint is the pluggable linting/fixing utility for Protocol Buffer files (proto2+proto3):

• Runs fast because this works without compiler.
• Easy to follow the official style guide. The rules and the style guide correspond to each other exactly.
  • Fixer automatically fixes all the possible official style guide violations.
• Allows to disable rules with a comment in a Protocol Buffer file.
  • It is useful for projects which must keep API compatibility while enforce the style guide as much as possible.
  • Some rules can be automatically disabled by inserting comments to the spotted violations.
• Loads plugins to contain your custom lint rules.
• Undergone testing for all rules.
• Many integration supports.
  • protoc plugin
  • Editor integration
  • GitHub Action
  • CI Integration

Demo

Once MCP server configured, you can ask any MCP clients like Claude Desktop to lint and fix your Protocol Buffer files like this:

Also, vim-protolint works like the following.

MCP Server

protolint now includes support for the Model Context Protocol (MCP), which allows AI models to interact with protolint directly.

Usage

protolint --mcp

For detailed documentation on how to use and integrate protolint's MCP server functionality, see the MCP documentation.

Installation

Via Homebrew

protolint can be installed for Mac or Linux using Homebrew via the yoheimuta/protolint tap.

brew tap yoheimuta/protolint
brew install protolint

Since homebrew-core includes protolint, you can also install it by just brew install protolint. This is the default tap that is installed by default. It's easier, but not maintained by the same author. To keep it updated, I recommend you run brew tap yoheimuta/protolint first.

Via GitHub Releases

You can also download a pre-built binary from this release page:

• https://github.com/yoheimuta/protolint/releases

In the downloads section of each release, you can find pre-built binaries in .tar.gz packages.

Use the maintained Docker image

protolint ships a Docker image yoheimuta/protolint that allows you to use protolint as part of your Docker workflow.

❯❯❯ docker run --volume "$(pwd):/workspace" --workdir /workspace yoheimuta/protolint lint _example/proto
[_example/proto/invalidFileName.proto:1:1] File name should be lower_snake_case.proto.
[_example/proto/issue_88/oneof_options.proto:11:5] Found an incorrect indentation style "    ". "  " is correct.
[_example/proto/issue_88/oneof_options.proto:12:5] Found an incorrect indentation style "    ". "  " is correct.

From Source

The binary can be installed from source if Go is available. However, I recommend using one of the pre-built binaries instead because it doesn't include the version info.

go install github.com/yoheimuta/protolint/cmd/protolint@latest

Within JavaScript / TypeScript

You can use protolint using your nodejs package manager like npm or yarn.

$ npm install protolint --save-dev

This will add a reference to a development dependency to your local package.json.

During install, the postinstall.js script will be called. It will download the matching protolint from github. Just like @electron/get, you can bypass the download using the following environment variables:

Environment Variable	Default value	Description
PROTOLINT_MIRROR_HOST	https://github.com	HTTP/Web server base url hosting the binaries
PROTOLINT_MIRROR_REMOTE_PATH	yoheimuta/protolint/releases/download	Path to the archives on the remote host
PROTOLINT_MIRROR_USERNAME		HTTP Basic auth user name
PROTOLINT_MIRROR_PASSWORD		HTTP Basic auth password
PROTOLINT_PROXY		HTTP(S) Proxy with optional auth data

Within the remote path, the archives from the releases page must be mirrored.

After that, you can use npx protolint (with all supplied protolint arguments) within your dev-scripts.

{
  ...
  "scripts": {
    "protoc": "....",
    "preprotoc": "npx protolint"
  },
  ...
}

You can add a protolint node to your package.json which may contain the content of protolint.yml below the lint node, i.e. the root element of the configuration will be protolint.

If you want to get an output that matches the TSC compiler, use reporter tsc.

Within Python projects

You can use protolint as a linter within your python projects, the wheel protolint-bin on pypi contains the pre-compiled binaries for various platforms. Just add the desired version to your pyproject.toml or requirements.txt.

The wheels downloaded will contain the compiled go binaries for protolint and protoc-gen-protolint. Your platform must be compatible with the supported binary platforms.

You can add the linter configuration to the tools.protolint package in pyproject.toml.

Usage

protolint lint example.proto example2.proto # file mode, specify multiple specific files
protolint lint .                            # directory mode, search for all .proto files recursively
protolint .                                 # same as "protolint lint ."
protolint lint -config_path=path/to/your_protolint.yaml . # use path/to/your_protolint.yaml
protolint lint -config_dir_path=path/to .   # search path/to for .protolint.yaml
protolint lint -fix .                       # automatically fix some of the problems reported by some rules
protolint lint -fix -auto_disable=next .    # this is preferable when you want to fix problems while maintaining the compatibility. Automatically fix some problems and insert disable comments to the other problems. The available values are next and this.
protolint lint -auto_disable=next .         # automatically insert disable comments to the other problems. 
protolint lint -v .                         # with verbose output to investigate the parsing error
protolint lint -no-error-on-unmatched-pattern . # exits with success code even if no file is found (file & directory mode)
protolint lint -reporter junit .            # output results in JUnit XML format
protolint lint -output_file=path/to/out.txt # output results to path/to/out.txt
protolint lint -plugin ./my_custom_rule1 -plugin ./my_custom_rule2 .   # run custom lint rules.
protolint list                              # list all current lint rules being used
protolint version                           # print protolint version
protolint --version                         # print protolint version (global flag)
protolint -v                                # print protolint version (when used as the only argument)

protolint does not require configuration by default, for the majority of projects it should work out of the box.

Version Control Integration

protolint is available as a pre-commit hook. Add this to your .pre-commit-config.yaml in your repository to run protolint with Go:

repos:
  - repo: https://github.com/yoheimuta/protolint
    rev: v0.56.3 # Select a release here
    hooks:
      - id: protolint

For Docker-based execution, use the protolint-docker hook:

repos:
  - repo: https://github.com/yoheimuta/protolint
    hooks:
      - id: protolint-docker # Uses yoheimuta/protolint:latest by default

Important: For Docker hooks, the rev parameter is meaningless for versioning per pre-commit's design. To pin to a specific protolint version, specify the Docker image tag in the entry:

repos:
  - repo: https://github.com/yoheimuta/protolint
    hooks:
      - id: protolint-docker
        entry: yoheimuta/protolint:v0.55.6 lint # Pin to specific version

Editor Integration

Visual Studio Code

• vscode-protolint
• protolint-vscode

JetBrains IntelliJ IDEA, GoLand, WebStorm, PHPStorm, PyCharm...

• intellij-protolint

Vim(ALE engine)

• ale's built-in support

Vim(Syntastic)

• vim-protolint

GitHub Action

A GitHub Action to run protolint in your workflows

• github/super-linter
• plexsystems/protolint-action
• yoheimuta/action-protolint - Integrated with reviewdog

CI Integration

Jenkins Plugins

• warnings-ng and any that use violatons-lib

Environment specific output

It is possible to format your linting according to the formatting of the CI/CD environment. The environment must be set using the output format. Currently, the following output is realized:

Environment	Command Line Value	Description	Example
Github Actions	ci-gh	Github Help	::warning file=example.proto,line=10,col=20,title=ENUM_NAMES_UPPER_CAMEL_CASE::EnumField name \"SECOND.VALUE\" must be CAPITALS_WITH_UNDERSCORES
Azure DevOps	ci-az	Azure DevOps Help	##vso[task.logissue type=warning;sourcepath=example.proto;linenumber=10;columnnumber=20;code=ENUM_NAMES_UPPER_CAMEL_CASE;]EnumField name \"SECOND.VALUE\" must be CAPITALS_WITH_UNDERSCORES
Gitlab CI/CD	ci-glab	Reverse Engineered from Examples	WARNING: ENUM_NAMES_UPPER_CAMEL_CASE example.proto(10,20) : EnumField name \"SECOND.VALUE\" must be CAPITALS_WITH_UNDERSCORES

You can also use the generic ci formatter, which will create a generic problem matcher.

With the ci-env value, you can specify the template from the following environment variables:

Environment Variable	Priority	Meaning
PROTOLINT_CIREPORTER_TEMPLATE_STRING	1	String containing a Go-template
PROTOLINT_CIREPORTER_TEMPLATE_FILE	2	Path to a file containing a Go-template

The resulting line-feed must not be added, as it will be added automatically.

The following fields are available:

Severity : The severity as string (either note, warning or error)

File : Path to the file containing the error

Line : Line within the file containing the error (starting position)

Column : Column within the file containing the error (starting position)

Rule : The name of the rule that is faulting

Message : The error message that descibes the error

Producing an output file and an CI/CD Error stream

You can create a specific output matching your CI/CD environment and also create an output file, e.g. for your static code analysis tools like github CodeQL or SonarQube.

This can be done by adding the --add-reporter flag. Please note, that the value must be formatted <reporter-name>:<output-file-path> (omitting < and >).

$ protolint --reporter ci-gh --add-reporter sarif:/path/to/my/output.sarif.json proto/*.proto

Use as a protoc plugin

protolint also maintains a binary protoc-gen-protolint that performs the lint functionality as a protoc plugin. See cmd/protoc-gen-protolint/README.md in detail.

This is useful in situations where you already have a protoc plugin workflow.

Call from Go code

You can also use protolint from Go code. See Go Documentation and lib/lint_test.go in detail.

import (
    "bytes"

    "github.com/yoheimuta/protolint/lib"
)

func main() {
    args := []string{"-config_path", "path/to/your_protolint.yaml", "."}
    var stdout bytes.Buffer
    var stderr bytes.Buffer

    err := lib.Lint(args, &stdout, &stderr)
    if err != nil {
        // Handle error
    }

    // Process output in stdout and stderr
}

Rules

See internal/addon/rules in detail.

The rule set follows:

• Official Style Guide. This is enabled by default. Basically, these rules can fix the violations by appending -fix option.
• Unofficial Style Guide. This is disabled by default. You can enable each rule with .protolint.yaml.

The -fix option on the command line can automatically fix all the problems reported by fixable rules. See Fixable columns below.

The -auto_disable option on the command line can automatically disable all the problems reported by auto-disable rules. This feature is helpful when fixing the existing violations breaks the compatibility. See AutoDisable columns below.

• *1: These rules are not supposed to support AutoDisable because the fixes don't break their compatibilities. You should run the protolint with -fix.

Official	Fixable	AutoDisable	ID	Purpose
Yes	✅	✅	ENUM_FIELD_NAMES_PREFIX	Verifies that enum field names are prefixed with its ENUM_NAME_UPPER_SNAKE_CASE.
Yes	✅	✅	ENUM_FIELD_NAMES_UPPER_SNAKE_CASE	Verifies that all enum field names are CAPITALS_WITH_UNDERSCORES.
Yes	✅	✅	ENUM_FIELD_NAMES_ZERO_VALUE_END_WITH	Verifies that the zero value enum should have the suffix (e.g. "UNSPECIFIED", "INVALID"). The default is "UNSPECIFIED". You can configure the specific suffix with .protolint.yaml.
Yes	✅	✅	ENUM_NAMES_UPPER_CAMEL_CASE	Verifies that all enum names are CamelCase (with an initial capital).
Yes	✅	*1	FILE_NAMES_LOWER_SNAKE_CASE	Verifies that all file names are lower_snake_case.proto. You can configure the excluded files with .protolint.yaml.
Yes	✅	✅	FIELD_NAMES_LOWER_SNAKE_CASE	Verifies that all field names are underscore_separated_names.
Yes	✅	*1	IMPORTS_SORTED	Verifies that all imports are sorted.
Yes	✅	✅	MESSAGE_NAMES_UPPER_CAMEL_CASE	Verifies that all message names are CamelCase (with an initial capital).
Yes	✅	*1	ORDER	Verifies that all files should be ordered in the specific manner.
Yes	✅	*1	PACKAGE_NAME_LOWER_CASE	Verifies that the package name should only contain lowercase letters.
Yes	✅	✅	RPC_NAMES_UPPER_CAMEL_CASE	Verifies that all rpc names are CamelCase (with an initial capital).
Yes	✅	✅	SERVICE_NAMES_UPPER_CAMEL_CASE	Verifies that all service names are CamelCase (with an initial capital).
Yes	✅	✅	REPEATED_FIELD_NAMES_PLURALIZED	Verifies that repeated field names are pluralized names.
Yes	✅	*1	QUOTE_CONSISTENT	Verifies that the use of quote for strings is consistent. The default is double quoted. You can configure the specific quote with .protolint.yaml.
Yes	✅	*1	INDENT	Enforces a consistent indentation style. The default style is 2 spaces. Inserting appropriate new lines is also forced by default. You can configure the detail with .protolint.yaml.
Yes	✅	*1	PROTO3_FIELDS_AVOID_REQUIRED	Verifies that all fields should avoid required for proto3.
Yes	_	✅	PROTO3_GROUPS_AVOID	Verifies that all groups should be avoided for proto3.
Yes	_	*1	MAX_LINE_LENGTH	Enforces a maximum line length. The length of a line is defined as the number of Unicode characters in the line. The default is 80 characters. You can configure the detail with .protolint.yaml.
No	_	-	SERVICE_NAMES_END_WITH	Enforces a consistent suffix for service names. You can configure the specific suffix with .protolint.yaml.
No	_	-	FIELD_NAMES_EXCLUDE_PREPOSITIONS	Verifies that all field names don't include prepositions (e.g. "for", "during", "at"). You can configure the specific prepositions and excluded keywords with .protolint.yaml.
No	_	-	MESSAGE_NAMES_EXCLUDE_PREPOSITIONS	Verifies that all message names don't include prepositions (e.g. "With", "For"). You can configure the specific prepositions and excluded keywords with .protolint.yaml.
No	_	-	RPC_NAMES_CASE	Verifies that all rpc names conform to the specified convention. You need to configure the specific convention with .protolint.yaml.
No	_	-	MESSAGES_HAVE_COMMENT	Verifies that all messages have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	SERVICES_HAVE_COMMENT	Verifies that all services have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	RPCS_HAVE_COMMENT	Verifies that all rps have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	FIELDS_HAVE_COMMENT	Verifies that all fields have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	ENUMS_HAVE_COMMENT	Verifies that all enums have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	ENUM_FIELDS_HAVE_COMMENT	Verifies that all enum fields have a comment. You can configure to enforce Golang Style comments with .protolint.yaml.
No	_	-	FILE_HAS_COMMENT	Verifies that a file starts with a doc comment.
No	_	-	SYNTAX_CONSISTENT	Verifies that syntax is a specified version. The default is proto3. You can configure the version with .protolint.yaml.
No	_	-	FIELD_NUMBERS_ORDER_ASCENDING	Verifies the order of fields is ascending. For enums, honors option allow_alias = true; by allowing adjacent equal numbers (non-decreasing).

I recommend that you add all_default: true in .protolint.yaml, because all linters above are automatically enabled so that you can always enjoy maximum benefits whenever protolint is updated.

Here are some examples that show good style enabled by default. - is a bad style, + is a good style:

ENUM_FIELD_NAMES_PREFIX

enum FooBar {
-  UNSPECIFIED = 0;
+  FOO_BAR_UNSPECIFIED = 0;
}

ENUM_FIELD_NAMES_UPPER_SNAKE_CASE

enum Foo {
-  firstValue = 0;
+  FIRST_VALUE = 0;
-  second_value = 1;
+  SECOND_VALUE = 1;
}

ENUM_FIELD_NAMES_ZERO_VALUE_END_WITH

enum Foo {
-  FOO_FIRST = 0;
+  FOO_UNSPECIFIED = 0;
}

ENUM_NAMES_UPPER_CAMEL_CASE

- enum foobar {
+ enum FooBar {
  FIRST_VALUE = 0;
  SECOND_VALUE = 1;
}

FIELD_NAMES_LOWER_SNAKE_CASE

message SongServerRequest {
-  required string SongName = 1;
+  required string song_name = 1;
}

IMPORTS_SORTED

- import public "new.proto";
+ import "myproject/other_protos.proto";
- import "myproject/other_protos.proto";
+ import public "new.proto";

import "google/protobuf/empty.proto";
import "google/protobuf/timestamp.proto";

MESSAGE_NAMES_UPPER_CAMEL_CASE

- message song_server_request {
+ message SongServerRequest {
  required string SongName = 1;
  required string song_name = 1;
}

ORDER

- option java_package = "com.example.foo";
- syntax = "proto3";
- package examplePb;
- message song_server_request { }
- import "other.proto";
+ syntax = "proto3";
+ package examplePb;
+ import "other.proto";
+ option java_package = "com.example.foo";
+ message song_server_request { }

PACKAGE_NAME_LOWER_CASE

- package myPackage
+ package my.package

RPC_NAMES_UPPER_CAMEL_CASE

service FooService {
-  rpc get_something(FooRequest) returns (FooResponse);
+  rpc GetSomething(FooRequest) returns (FooResponse);
}

RPC_NAMES_UPPER_CAMEL_CASE

- service foo_service {
+ service FooService {
  rpc get_something(FooRequest) returns (FooResponse);
  rpc GetSomething(FooRequest) returns (FooResponse);
}

REPEATED_FIELD_NAMES_PLURALIZED

-  repeated string song_name = 1;
+  repeated string song_names = 1;

INDENT

 enum enumAllowingAlias {
   UNKNOWN = 0;
-        option allow_alias = true;
+  option allow_alias = true;
   STARTED = 1;
-     RUNNING = 2 [(custom_option) = "hello world"];
+  RUNNING = 2 [(custom_option) = "hello world"];
- }
+}

-   message TestMessage { string test_field = 1; }
+ message TestMessage {
+  string test_field = 1;
+}

QUOTE_CONSISTENT

 option java_package = "com.example.foo";
- option go_package = 'example';
+ option go_package = "example";

Creating your custom rules

protolint is the pluggable linter so that you can freely create custom lint rules.

A complete sample project (aka plugin) is included in this repo under the _example/plugin directory.

Reporters

protolint comes with several built-in reporters(aka. formatters) to control the appearance of the linting results.

You can specify a reporter using the -reporter flag on the command line. For example, -reporter junit uses the junit reporter.

The built-in reporter options are:

• plain (default)
• junit
• json
• sarif
• sonar (SonarQube generic issue format)
• unix
• tsc (compatible to TypeScript compiler)

Configuring

Disable rules in a Protocol Buffer file

Rules can be disabled with a comment inside a Protocol Buffer file with the following format. The rules will be disabled until the end of the file or until the linter sees a matching enable comment:

// protolint:disable <ruleID1> [<ruleID2> <ruleID3>...]
...
// protolint:enable <ruleID1> [<ruleID2> <ruleID3>...]

It's also possible to modify a disable command by appending :next or :this for only applying the command to this(current) or the next line respectively.

For example:

enum Foo {
  // protolint:disable:next ENUM_FIELD_NAMES_UPPER_SNAKE_CASE
  firstValue = 0;    // no error
  second_value = 1;  // protolint:disable:this ENUM_FIELD_NAMES_UPPER_SNAKE_CASE
  THIRD_VALUE = 2;   // spits out an error
}

Setting the command-line option -auto_disable to next or this inserts disable commands whenever spotting problems.

You can specify -fix option together. The rules supporting auto_disable suppress the violations instead of fixing them that cause a schema incompatibility.

Config file

protolint can operate using a config file named .protolint.yaml.

Refer to _example/config/.protolint.yaml for the config file specification.

protolint will automatically search a current working directory for the config file by default and successive parent directories all the way up to the root directory of the filesystem. And it can search the specified directory with -config_dir_path flag. It can also search the specified file with --config_path flag.

Exit codes

When linting files, protolint will exit with one of the following exit codes:

• 0: Linting was successful and there are no linting errors.
• 1: Linting was successful and there is at least one linting error.
• 2: Linting was unsuccessful due to all other errors, such as parsing, internal, and runtime errors.

Motivation

There exists the similar protobuf linters as of 2018/12/20.

One is a plug-in for Google's Protocol Buffers compiler.

• When you just want to lint the files, it may be tedious to create the compilation environment.
• And it generally takes a lot of time to compile the files than to parse the files.

Other is a command line tool which also lints Protocol Buffer files.

• While it has a lot of features other than lint, it seems cumbersome for users who just want the linter.
• The lint rule slants towards to be opinionated.
• Further more, the rule set and the official style guide don't correspond to each other exactly. It requires to understand both rules and the guide in detail, and then to combine the rules accurately.

Other tools

I wrote an article comparing various Protocol Buffer Linters, including protolint, on 2019/12/17.

• https://qiita.com/yoheimuta/items/da7678fcd046b93a2637
  • NOTE: This one is written in Japanese.

Dependencies

• go-protoparser

Development

Release

To streamline the release process and reduce human error, a release.sh script is included in the repository. This script automates the steps required to create and push a new release tag.

How to Use

Run the following command to create a new release:

bash release.sh <version> [message]

License

The MIT License (MIT)