diff --git a/action.yaml b/action.yaml
index 7db4b056..7ba11cd4 100644
--- a/action.yaml
+++ b/action.yaml
@@ -12,6 +12,7 @@ inputs:
   github-repository:
     description: "Repository to update. The current repository will be used by default"
     required: false
+    default: ${{ github.repository }}
   github-token:
     description: "Github Personal Access Token with permission to create branches on repo"
     required: false
diff --git a/package-lock.json b/package-lock.json
index 1aac7820..b55d8ea0 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -25,6 +25,7 @@
         "all-contributors-cli": "^6.24.0",
         "ava": "^5.1.0",
         "ts-node": "^10.9.1",
+        "ts-pattern": "^4.0.6",
         "typescript": "^4.9.3",
         "xo": "^0.53.1"
       }
@@ -6933,6 +6934,12 @@
         }
       }
     },
+    "node_modules/ts-pattern": {
+      "version": "4.0.6",
+      "resolved": "https://registry.npmjs.org/ts-pattern/-/ts-pattern-4.0.6.tgz",
+      "integrity": "sha512-sFHQYD4KoysBi7e7a2mzDPvRBeqA4w+vEyRE+P5MU9VLq8eEYxgKCgD9RNEAT+itGRWUTYN+hry94GDPLb1/Yw==",
+      "dev": true
+    },
     "node_modules/tsconfig-paths": {
       "version": "3.14.1",
       "resolved": "https://registry.npmjs.org/tsconfig-paths/-/tsconfig-paths-3.14.1.tgz",
@@ -13389,6 +13396,12 @@
         "yn": "3.1.1"
       }
     },
+    "ts-pattern": {
+      "version": "4.0.6",
+      "resolved": "https://registry.npmjs.org/ts-pattern/-/ts-pattern-4.0.6.tgz",
+      "integrity": "sha512-sFHQYD4KoysBi7e7a2mzDPvRBeqA4w+vEyRE+P5MU9VLq8eEYxgKCgD9RNEAT+itGRWUTYN+hry94GDPLb1/Yw==",
+      "dev": true
+    },
     "tsconfig-paths": {
       "version": "3.14.1",
       "resolved": "https://registry.npmjs.org/tsconfig-paths/-/tsconfig-paths-3.14.1.tgz",
diff --git a/package.json b/package.json
index 954987ed..f993eda4 100644
--- a/package.json
+++ b/package.json
@@ -43,6 +43,7 @@
     "all-contributors-cli": "^6.24.0",
     "ava": "^5.1.0",
     "ts-node": "^10.9.1",
+    "ts-pattern": "^4.0.6",
     "typescript": "^4.9.3",
     "xo": "^0.53.1"
   },
@@ -54,7 +55,6 @@
       "ava/no-ignored-test-files": 0,
       "n/file-extension-in-import": 0,
       "node/prefer-global/process": 0,
-      "node/prefer-global/buffer": 0,
       "import/extensions": 0,
       "unicorn/prefer-node-protocol": 0
     }
diff --git a/src/check.ts b/src/check.ts
deleted file mode 100644
index c6e6133b..00000000
--- a/src/check.ts
+++ /dev/null
@@ -1,167 +0,0 @@
-import type Buffer from 'buffer'
-import fs from 'fs'
-import process from 'process'
-import fetch from 'node-fetch'
-import * as core from '@actions/core'
-
-/**
- * Checks connection with Maven Central, throws error if unable to connect.
- */
-export async function mavenCentral(): Promise<void> {
-  const response = await fetch('https://repo1.maven.org/maven2/')
-
-  if (!response.ok) {
-    throw new Error('Unable to connect to Maven Central')
-  }
-
-  core.info('✓ Connected to Maven Central')
-}
-
-/**
- * Reads the Github Token from the `github-token` input. Throws error if the
- * input is empty or returns the token in case it is not.
- *
- * @returns {string} The Github Token read from the `github-token` input.
- */
-export function githubToken(): string {
-  const token: string = core.getInput('github-token')
-
-  if (token === '') {
-    throw new Error('You need to provide a Github token in the `github-token` input')
-  }
-
-  core.info('✓ Github Token provided as input')
-
-  return token
-}
-
-const defaultRepoConfLocation = '.github/.scala-steward.conf'
-
-/**
- * Reads the path of the file containing the default Scala Steward configuration.
- *
- * If the provided file does not exist and is not the default one it will throw an error.
- * On the other hand, if it exists it will be returned, otherwise; it will return `undefined`.
- *
- * @returns {string | undefined} The path indicated in the `repo-config` input, if it
- *                               exists; otherwise, `undefined`.
- */
-export function defaultRepoConf(): string | undefined {
-  const path = core.getInput('repo-config')
-
-  const fileExists = fs.existsSync(path)
-
-  if (!fileExists && path !== defaultRepoConfLocation) {
-    throw new Error(`Provided default repo conf file (${path}) does not exist`)
-  }
-
-  if (fileExists) {
-    core.info(`✓ Default Scala Steward configuration set to: ${path}`)
-
-    return path
-  }
-
-  return undefined
-}
-
-/**
- * Reads a Github repository from the `github-repository` input. Fallback to the
- * `GITHUB_REPOSITORY` environment variable.
- *
- * Throws error if the fallback fails or returns the repository in case it doesn't.
- *
- * If the `branch` input is set, the selected branch will be added for update instead
- * of the default one.
- *
- * @returns {string} The Github repository read from the `github-repository` input
- *                   or the `GITHUB_REPOSITORY` environment variable.
- */
-export function githubRepository(): string {
-  const repo: string | undefined
-    = core.getInput('github-repository') || process.env.GITHUB_REPOSITORY
-
-  if (repo === undefined) {
-    throw new Error(
-      'Unable to read Github repository from `github-repository` '
-        + 'input or `GITHUB_REPOSITORY` environment variable',
-    )
-  }
-
-  const branches = core.getInput('branches').split(',').filter(Boolean)
-
-  if (branches.length === 1) {
-    const branch = branches[0]
-
-    core.info(`✓ Github Repository set to: ${repo}. Will update ${branch} branch.`)
-
-    return `- ${repo}:${branch}`
-  }
-
-  if (branches.length > 1) {
-    core.info(`✓ Github Repository set to: ${repo}. Will update ${branches.join(', ')} branches.`)
-
-    return branches.map((branch: string) => `- ${repo}:${branch}`).join('\n')
-  }
-
-  core.info(`✓ Github Repository set to: ${repo}.`)
-
-  return `- ${repo}`
-}
-
-/**
- * Reads the path of the file containing the list of repositories to update  from the `repos-file`
- * input.
- *
- * If the input isn't provided this function will return `undefined`.
- * On the other hand, if it is provided, it will check if the path exists:
- * - If the file exists, its contents will be returned.
- * - If it doesn't exists, an error will be thrown.
- *
- * @returns {string | undefined} The contents of the file indicated in `repos-file` input, if is
- *                               defined; otherwise, `undefined`.
- */
-export function reposFile(): Buffer | undefined {
-  const file: string = core.getInput('repos-file')
-
-  if (!file) {
-    return undefined
-  }
-
-  if (fs.existsSync(file)) {
-    core.info(`✓ Using multiple repos file: ${file}`)
-
-    return fs.readFileSync(file)
-  }
-
-  throw new Error(`The path indicated in \`repos-file\` (${file}) does not exist`)
-}
-
-/**
- * Checks that Github App ID and private key are set together, writes the key to a temporary file.
- *
- * Throws error if only one of the two inputs is set.
- *
- * @returns {{id: string, keyFile: string} | undefined} App ID and path to the private key file or
- * undefined if both inputs are empty.
- */
-export function githubAppInfo(): {id: string; keyFile: string} | undefined {
-  const id: string = core.getInput('github-app-id')
-  const key: string = core.getInput('github-app-key')
-
-  if (!id && !key) {
-    return undefined
-  }
-
-  if (id && key) {
-    const keyFile = `${fs.mkdtempSync('tmp-')}/github-app-private-key.pem`
-    fs.writeFileSync(keyFile, key)
-
-    core.info(`✓ Github App ID: ${id}`)
-    core.info(`✓ Github App private key is written to: ${keyFile}`)
-    return {id, keyFile}
-  }
-
-  throw new Error(
-    '`github-app-id` and `github-app-key` inputs have to be set together. One of them is missing',
-  )
-}
diff --git a/src/coursier.ts b/src/coursier.ts
index f211feaf..6015f535 100644
--- a/src/coursier.ts
+++ b/src/coursier.ts
@@ -5,6 +5,7 @@ import * as core from '@actions/core'
 import * as tc from '@actions/tool-cache'
 import * as io from '@actions/io'
 import * as exec from '@actions/exec'
+import {type NonEmptyString} from './types'
 
 /**
  * Install `coursier` and add its executable to the `PATH`.
@@ -94,17 +95,16 @@ export async function install(app: string): Promise<void> {
  *
  * Refer to [coursier](https://get-coursier.io/docs/cli-launch) for more information.
  *
- * @param {string} org - The application's organization.
- * @param {string} app - The application's artifact name.
- * @param {string} version - The application's version.
- * @param {(string | string[])[]} args - The args to pass to the application launcher.
+ * @param app - The application's artifact name.
+ * @param version - The application's version.
+ * @param args - The args to pass to the application launcher.
  */
 export async function launch(
   app: string,
-  version: string,
+  version: NonEmptyString | undefined,
   args: Array<string | string[]> = [],
 ): Promise<void> {
-  const name = version ? `${app}:${version}` : app
+  const name = version ? `${app}:${version.value}` : app
 
   core.startGroup(`Launching ${name}`)
 
diff --git a/src/files.ts b/src/files.ts
new file mode 100644
index 00000000..c997d8eb
--- /dev/null
+++ b/src/files.ts
@@ -0,0 +1,14 @@
+/**
+ * Represent file operations performed by the action
+ */
+export type Files = {
+  /**
+   * Read file contents from the filesystem.
+   */
+  readFileSync: (path: string, encoding: 'utf8') => string;
+
+  /**
+   * Returns `true` if the provided path exists.
+   */
+  existsSync: (path: string) => boolean;
+}
diff --git a/src/github.ts b/src/github.ts
index 4a72974b..9dde80bf 100644
--- a/src/github.ts
+++ b/src/github.ts
@@ -1,5 +1,6 @@
 import {getOctokit} from '@actions/github'
 import * as core from '@actions/core'
+import {nonEmpty, type NonEmptyString} from './types'
 
 const emailErrorMessage
   = 'Unable to find author\'s email. Either ensure that the token\'s Github Account has the email '
@@ -13,11 +14,11 @@ const nameErrorMessage
  * Returns the login, email and name of the authenticated user using
  * the provided Github token.
  *
- * @param {string} token - The token whose user data will be extracted.
- * @returns {Promise<AuthUser>} The login, email and name of token's user.
+ * @param token - The token whose user data will be extracted.
+ * @returns The login, email and name of token's user.
  */
-export async function getAuthUser(token: string): Promise<AuthUser> {
-  const github = getOctokit(token)
+export async function getAuthUser(token: NonEmptyString): Promise<AuthUser> {
+  const github = getOctokit(token.value)
 
   try {
     const auth = await github.rest.users.getAuthenticated()
@@ -35,21 +36,21 @@ export async function getAuthUser(token: string): Promise<AuthUser> {
           throw new Error('Unable to retrieve user information from Github')
         }
 
-        return login
+        return nonEmpty(login)
       },
       email() {
         if (!email) {
           throw new Error(emailErrorMessage)
         }
 
-        return email
+        return nonEmpty(email)
       },
       name() {
         if (!name) {
           throw new Error(nameErrorMessage)
         }
 
-        return name
+        return nonEmpty(name)
       },
     }
   } catch (error: unknown) {
@@ -58,15 +59,15 @@ export async function getAuthUser(token: string): Promise<AuthUser> {
     // https://github.community/t/github-actions-bot-email-address/17204/6
     // https://api.github.com/users/github-actions%5Bbot%5D
     return {
-      login: () => 'github-actions[bot]',
-      email: () => '41898282+github-actions[bot]@users.noreply.github.com',
-      name: () => 'github-actions[bot]',
+      login: () => nonEmpty('github-actions[bot]'),
+      email: () => nonEmpty('41898282+github-actions[bot]@users.noreply.github.com'),
+      name: () => nonEmpty('github-actions[bot]'),
     }
   }
 }
 
 type AuthUser = {
-  email: () => string;
-  login: () => string;
-  name: () => string;
+  email: () => NonEmptyString | undefined;
+  login: () => NonEmptyString | undefined;
+  name: () => NonEmptyString | undefined;
 }
diff --git a/src/healthcheck.ts b/src/healthcheck.ts
new file mode 100644
index 00000000..48e90e93
--- /dev/null
+++ b/src/healthcheck.ts
@@ -0,0 +1,23 @@
+import {type Logger} from './logger'
+import {type HttpClient} from './http'
+
+export class HealthCheck {
+  static from(logger: Logger, httpClient: HttpClient) {
+    return new HealthCheck(logger, httpClient)
+  }
+
+  constructor(private readonly logger: Logger, private readonly httpClient: HttpClient) {}
+
+  /**
+   * Checks connection with Maven Central, throws error if unable to connect.
+   */
+  async mavenCentral(): Promise<void> {
+    const success = await this.httpClient.run('https://repo1.maven.org/maven2/').then(response => response.ok)
+
+    if (!success) {
+      throw new Error('Unable to connect to Maven Central')
+    }
+
+    this.logger.info('✓ Connected to Maven Central')
+  }
+}
diff --git a/src/http.ts b/src/http.ts
new file mode 100644
index 00000000..c6057163
--- /dev/null
+++ b/src/http.ts
@@ -0,0 +1,11 @@
+/**
+ * Represents an HTTP client
+ */
+export type HttpClient = {
+  run: (url: string) => Promise<Response>;
+}
+
+export type Response = {
+  ok: boolean;
+  status: number;
+}
diff --git a/src/input.ts b/src/input.ts
new file mode 100644
index 00000000..4414962e
--- /dev/null
+++ b/src/input.ts
@@ -0,0 +1,196 @@
+import {type Files} from './files'
+import {type Logger} from './logger'
+import {nonEmpty, type NonEmptyString} from './types'
+
+/**
+ * Retrieves (and sanitize) inputs.
+ */
+export class Input {
+  static from(inputs: {getInput: (name: string) => string}, files: Files, logger: Logger) {
+    return new Input(inputs, files, logger)
+  }
+
+  constructor(
+    private readonly inputs: {getInput: (name: string) => string},
+    private readonly files: Files,
+    private readonly logger: Logger,
+  ) {}
+
+  /**
+   * Returns every input for this action.
+   */
+  all() {
+    return {
+      github: {
+        token: this.githubToken(),
+        app: this.githubAppInfo(),
+        apiUrl: nonEmpty(this.inputs.getInput('github-api-url')),
+      },
+      steward: {
+        defaultConfiguration: this.defaultRepoConf(),
+        repos: this.reposFile() ?? this.githubRepository(),
+        cacheTtl: nonEmpty(this.inputs.getInput('cache-ttl')),
+        version: nonEmpty(this.inputs.getInput('scala-steward-version')),
+        timeout: nonEmpty(this.inputs.getInput('timeout')),
+        ignoreOptsFiles: /true/i.test(this.inputs.getInput('ignore-opts-files')),
+        extraArgs: nonEmpty(this.inputs.getInput('other-args')),
+      },
+      migrations: {
+        scalafix: nonEmpty(this.inputs.getInput('scalafix-migrations')),
+        artifacts: nonEmpty(this.inputs.getInput('artifact-migrations')),
+      },
+      commits: {
+        sign: {
+          enabled: /true/i.test(this.inputs.getInput('sign-commits')),
+          key: nonEmpty(this.inputs.getInput('signing-key')),
+        },
+        author: {
+          email: nonEmpty(this.inputs.getInput('author-email')),
+          name: nonEmpty(this.inputs.getInput('author-name')),
+        },
+      },
+    }
+  }
+
+  /**
+   * Reads the Github Token from the `github-token` input. Throws error if the
+   * input is empty or returns the token in case it is not.
+   *
+   * @returns {string} The Github Token read from the `github-token` input.
+   */
+  githubToken(): NonEmptyString {
+    const token = nonEmpty(this.inputs.getInput('github-token'))
+
+    if (!token) {
+      throw new Error('You need to provide a Github token in the `github-token` input')
+    }
+
+    this.logger.info('✓ Github Token provided as input')
+
+    return token
+  }
+
+  /**
+   * Reads the path of the file containing the default Scala Steward configuration.
+   *
+   * If the provided file does not exist and is not the default one it will throw an error.
+   * On the other hand, if it exists it will be returned, otherwise; it will return `undefined`.
+   *
+   * @returns {string | undefined} The path indicated in the `repo-config` input, if it
+   *                               exists; otherwise, `undefined`.
+   */
+  defaultRepoConf(): NonEmptyString | undefined {
+    const path = nonEmpty(this.inputs.getInput('repo-config'))
+
+    if (!path) {
+      return undefined
+    }
+
+    const fileExists = this.files.existsSync(path.value)
+
+    if (!fileExists && path.value !== '.github/.scala-steward.conf') {
+      throw new Error(`Provided default repo conf file (${path.value}) does not exist`)
+    }
+
+    if (fileExists) {
+      this.logger.info(`✓ Default Scala Steward configuration set to: ${path.value}`)
+
+      return path
+    }
+
+    return undefined
+  }
+
+  /**
+   * Returns the GitHub repository set to update.
+   *
+   * It reads it from the `github-repository` input.
+   *
+   * Throws error if input is empty or missing.
+   *
+   * If the `branches` input is set, the selected branches will be added.
+   *
+   * @returns {string} The Github repository read from the `github-repository` input.
+   */
+  githubRepository(): string {
+    const repo = nonEmpty(this.inputs.getInput('github-repository'))
+
+    if (!repo) {
+      throw new Error('Unable to read Github repository from `github-repository` input')
+    }
+
+    const branches = this.inputs.getInput('branches').split(',').filter(Boolean)
+
+    if (branches.length === 1) {
+      const branch = branches[0]
+
+      this.logger.info(`✓ Github Repository set to: ${repo.value}. Will update ${branch} branch.`)
+
+      return `- ${repo.value}:${branch}`
+    }
+
+    if (branches.length > 1) {
+      this.logger.info(`✓ Github Repository set to: ${repo.value}. Will update ${branches.join(', ')} branches.`)
+
+      return branches.map((branch: string) => `- ${repo.value}:${branch}`).join('\n')
+    }
+
+    this.logger.info(`✓ Github Repository set to: ${repo.value}.`)
+
+    return `- ${repo.value}`
+  }
+
+  /**
+   * Reads the path of the file containing the list of repositories to update  from the `repos-file`
+   * input.
+   *
+   * If the input isn't provided this function will return `undefined`.
+   * On the other hand, if it is provided, it will check if the path exists:
+   * - If the file exists, its contents will be returned.
+   * - If it doesn't exists, an error will be thrown.
+   *
+   * @returns {string | undefined} The contents of the file indicated in `repos-file` input, if is
+   *                               defined; otherwise, `undefined`.
+   */
+  reposFile(): string | undefined {
+    const file = nonEmpty(this.inputs.getInput('repos-file'))
+
+    if (!file) {
+      return undefined
+    }
+
+    if (this.files.existsSync(file.value)) {
+      this.logger.info(`✓ Using multiple repos file: ${file.value}`)
+
+      return this.files.readFileSync(file.value, 'utf8')
+    }
+
+    throw new Error(`The path indicated in \`repos-file\` (${file.value}) does not exist`)
+  }
+
+  /**
+   * Checks that Github App ID and private key are set together.
+   *
+   * Throws error if only one of the two inputs is set.
+   *
+   * @returns {{id: string, key: string} | undefined} App ID and key or undefined if both inputs are empty.
+   */
+  githubAppInfo(): {id: NonEmptyString; key: NonEmptyString} | undefined {
+    const id = nonEmpty(this.inputs.getInput('github-app-id'))
+    const key = nonEmpty(this.inputs.getInput('github-app-key'))
+
+    if (!id && !key) {
+      return undefined
+    }
+
+    if (id && key) {
+      this.logger.info(`✓ Github App ID: ${id.value}`)
+      this.logger.info('✓ Github App private key will be written to the Scala Steward workspace')
+      return {id, key}
+    }
+
+    throw new Error(
+      '`github-app-id` and `github-app-key` inputs have to be set together. One of them is missing',
+    )
+  }
+}
diff --git a/src/logger.ts b/src/logger.ts
new file mode 100644
index 00000000..611d33fd
--- /dev/null
+++ b/src/logger.ts
@@ -0,0 +1,18 @@
+/**
+ * Represents the logger used across the action.
+ */
+export type Logger = {
+  info(message: string): void;
+
+  debug(message: string): void;
+
+  error(message: string): void;
+
+  warning(message: string): void;
+}
+
+// eslint-disable-next-line @typescript-eslint/no-redeclare, @typescript-eslint/naming-convention
+export const Logger = {
+  noOp: {info() {}, debug() {}, error() {}, warning() {}}, // eslint-disable-line @typescript-eslint/no-empty-function
+
+}
diff --git a/src/main.ts b/src/main.ts
index 9c876190..a7ce933c 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,11 +1,17 @@
-import {Buffer} from 'buffer'
 import process from 'process'
+import fs from 'fs'
 import * as core from '@actions/core'
+import fetch from 'node-fetch'
 import * as github from './github'
-import * as check from './check'
+import {HealthCheck} from './healthcheck'
 import * as workspace from './workspace'
 import * as coursier from './coursier'
+import {type Logger} from './logger'
+import {Input} from './input'
+import {type HttpClient} from './http'
 import * as mill from './mill'
+import {type Files} from './files'
+import {nonEmpty, NonEmptyString} from './types'
 
 /**
  * Runs the action main code. In order it will do the following:
@@ -18,85 +24,52 @@ import * as mill from './mill'
  */
 async function run(): Promise<void> {
   try {
-    await check.mavenCentral()
-    await coursier.selfInstall()
-    const token = check.githubToken()
-    const user = await github.getAuthUser(token)
-
-    const authorEmail = core.getInput('author-email') || user.email()
-    const authorName = core.getInput('author-name') || user.name()
-
-    const githubAppInfo = check.githubAppInfo()
-
-    const defaultRepoConfPath = check.defaultRepoConf()
+    const logger: Logger = core
+    const httpClient: HttpClient = {run: async url => fetch(url)}
+    const files: Files = fs
+    const inputs = Input.from(core, files, logger).all()
+    const healthCheck: HealthCheck = HealthCheck.from(logger, httpClient)
 
-    // Content of the repos.md file either comes from the input file
-    // or is empty (replaced by the Github App info) or is a single repo
-    const reposList
-      = check.reposFile()
-      ?? (githubAppInfo ? Buffer.from('') : Buffer.from(check.githubRepository()))
+    await healthCheck.mavenCentral()
 
-    const workspaceDir = await workspace.prepare(reposList, token)
+    await coursier.selfInstall()
+    await coursier.install('scalafmt')
+    await coursier.install('scalafix')
+    await mill.install()
 
-    const cacheTtl = core.getInput('cache-ttl')
+    const user = await github.getAuthUser(inputs.github.token)
 
+    const workspaceDir = await workspace.prepare(inputs.steward.repos, inputs.github.token, inputs.github.app?.key)
     await workspace.restoreWorkspaceCache(workspaceDir)
 
-    const timeout = core.getInput('timeout')
-
-    const version = core.getInput('scala-steward-version')
-
-    const signCommits = /true/i.test(core.getInput('sign-commits'))
-    const signingKey = core.getInput('signing-key')
-    const ignoreOptionsFiles = /true/i.test(core.getInput('ignore-opts-files'))
-    const githubApiUrl = core.getInput('github-api-url')
-    const scalafixMigrations = core.getInput('scalafix-migrations')
-      ? ['--scalafix-migrations', core.getInput('scalafix-migrations')]
-      : []
-    const artifactMigrations = core.getInput('artifact-migrations')
-      ? ['--artifact-migrations', core.getInput('artifact-migrations')]
-      : []
-    const defaultRepoConf = defaultRepoConfPath ? ['--repo-config', defaultRepoConfPath] : []
-
-    const githubAppArgs = githubAppInfo
-      ? ['--github-app-id', githubAppInfo.id, '--github-app-key-file', githubAppInfo.keyFile]
-      : []
-
     if (process.env.RUNNER_DEBUG) {
       core.debug('Debug mode activated for Scala Steward')
       core.exportVariable('LOG_LEVEL', 'TRACE')
       core.exportVariable('ROOT_LOG_LEVEL', 'TRACE')
     }
 
-    const otherArgs = core.getInput('other-args')
-      ? core.getInput('other-args').split(' ')
-      : []
-
-    await coursier.install('scalafmt')
-    await coursier.install('scalafix')
-    await mill.install()
-
-    await coursier.launch('scala-steward', version, [
-      ['--workspace', `${workspaceDir}/workspace`],
-      ['--repos-file', `${workspaceDir}/repos.md`],
-      ['--git-ask-pass', `${workspaceDir}/askpass.sh`],
-      ['--git-author-email', `${authorEmail}"`],
-      ['--git-author-name', `${authorName}"`],
-      ['--vcs-login', `${user.login()}"`],
-      ['--env-var', '"SBT_OPTS=-Xmx2048m -Xss8m -XX:MaxMetaspaceSize=512m"'],
-      ['--process-timeout', timeout],
-      ['--vcs-api-host', githubApiUrl],
-      ignoreOptionsFiles ? '--ignore-opts-files' : [],
-      signCommits ? '--sign-commits' : [],
-      signingKey ? ['--git-author-signing-key', signingKey] : [],
-      ['--cache-ttl', cacheTtl],
-      scalafixMigrations,
-      artifactMigrations,
-      defaultRepoConf,
+    await coursier.launch('scala-steward', inputs.steward.version, [
+      arg('--workspace', nonEmpty(`${workspaceDir}/workspace`)),
+      arg('--repos-file', nonEmpty(`${workspaceDir}/repos.md`)),
+      arg('--git-ask-pass', nonEmpty(`${workspaceDir}/askpass.sh`)),
+      arg('--git-author-email', inputs.commits.author.email ?? user.email()),
+      arg('--git-author-name', inputs.commits.author.name ?? user.name()),
+      arg('--vcs-login', user.login()),
+      arg('--env-var', nonEmpty('"SBT_OPTS=-Xmx2048m -Xss8m -XX:MaxMetaspaceSize=512m"')),
+      arg('--process-timeout', inputs.steward.timeout),
+      arg('--vcs-api-host', inputs.github.apiUrl),
+      arg('--ignore-opts-files', inputs.steward.ignoreOptsFiles),
+      arg('--sign-commits', inputs.commits.sign.enabled),
+      arg('--git-author-signing-key', inputs.commits.sign.key),
+      arg('--cache-ttl', inputs.steward.cacheTtl),
+      arg('--scalafix-migrations', inputs.migrations.scalafix),
+      arg('--artifact-migrations', inputs.migrations.artifacts),
+      arg('--repo-config', inputs.steward.defaultConfiguration),
+      arg('--github-app-id', inputs.github.app?.id),
+      arg('--github-app-key-file', inputs.github.app ? nonEmpty(`${workspaceDir}/app.pem`) : undefined),
       '--do-not-fork',
       '--disable-sandbox',
-      githubAppArgs,
-      otherArgs,
+      inputs.steward.extraArgs ? inputs.steward.extraArgs.value.split(' ') : [],
     ]).finally(() => {
       workspace.saveWorkspaceCache(workspaceDir).catch((error: unknown) => {
         core.setFailed(` ✕ ${(error as Error).message}`)
@@ -107,5 +80,24 @@ async function run(): Promise<void> {
   }
 }
 
+/**
+ * Creates an optional argument depending on an input's value.
+ *
+ * @param name Name of the arg being added.
+ * @param value The argument's value, empty string, false booleans or undefined will be skipped.
+ * @returns the argument to add if it should be added; otherwise returns `[]`.
+ */
+function arg(name: string, value: NonEmptyString | boolean | undefined) {
+  if (value instanceof NonEmptyString) {
+    return [name, value.value]
+  }
+
+  if (value === undefined) {
+    return []
+  }
+
+  return value ? [name] : []
+}
+
 // eslint-disable-next-line unicorn/prefer-top-level-await
 void run()
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 00000000..d3203709
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,11 @@
+export class NonEmptyString {
+  static from(string: string): NonEmptyString | undefined {
+    return (string === '') ? undefined : new NonEmptyString(string)
+  }
+
+  private constructor(readonly value: string) {}
+}
+
+export function nonEmpty(string: string): NonEmptyString | undefined {
+  return NonEmptyString.from(string)
+}
diff --git a/src/workspace.ts b/src/workspace.ts
index 7e94582c..bd4a1970 100644
--- a/src/workspace.ts
+++ b/src/workspace.ts
@@ -1,4 +1,3 @@
-import type Buffer from 'buffer'
 import fs from 'fs'
 import os from 'os'
 import path from 'path'
@@ -7,6 +6,7 @@ import * as core from '@actions/core'
 import * as io from '@actions/io'
 import * as exec from '@actions/exec'
 import jsSHA from 'jssha/dist/sha256'
+import {type NonEmptyString} from './types'
 
 /**
  * Gets the first eight characters of the SHA-256 hash value for the
@@ -91,18 +91,25 @@ export async function saveWorkspaceCache(workspace: string): Promise<void> {
  * - Creating a `askpass.sh` file inside workspace containing the Github token.
  * - Making the previous file executable.
  *
- * @param {Buffer} reposList - The Markdown list of repositories to write to the `repos.md` file.
+ * @param {string} reposList - The Markdown list of repositories to write to the `repos.md` file. It is only used if no
+ *                             GitHub App key is provided on `gitHubAppKey` parameter.
  * @param {string} token - The Github Token used to authenticate into Github.
+ * @param {string | undefined} gitHubAppKey - The Github App private key (optional).
  * @returns {string} The workspace directory path
  */
-export async function prepare(reposList: Buffer, token: string): Promise<string> {
+export async function prepare(reposList: string, token: NonEmptyString, gitHubAppKey: NonEmptyString | undefined): Promise<string> {
   try {
     const stewarddir = `${os.homedir()}/scala-steward`
     await io.mkdirP(stewarddir)
 
-    fs.writeFileSync(`${stewarddir}/repos.md`, reposList)
+    if (gitHubAppKey === undefined) {
+      fs.writeFileSync(`${stewarddir}/repos.md`, reposList)
+    } else {
+      fs.writeFileSync(`${stewarddir}/repos.md`, '')
+      fs.writeFileSync(`${stewarddir}/app.pem`, gitHubAppKey.value)
+    }
 
-    fs.writeFileSync(`${stewarddir}/askpass.sh`, `#!/bin/sh\n\necho '${token}'`)
+    fs.writeFileSync(`${stewarddir}/askpass.sh`, `#!/bin/sh\n\necho '${token.value}'`)
     await exec.exec('chmod', ['+x', `${stewarddir}/askpass.sh`], {silent: true})
 
     core.info('✓ Scala Steward workspace created')
diff --git a/tests/check.test.ts b/tests/check.test.ts
deleted file mode 100644
index 467023a1..00000000
--- a/tests/check.test.ts
+++ /dev/null
@@ -1,147 +0,0 @@
-import fs from 'fs'
-import process from 'process'
-import test from 'ava'
-import * as check from '../src/check'
-
-test.beforeEach(() => {
-  process.env['INPUT_REPOS-FILE'] = ''
-  process.env.GITHUB_REPOSITORY = ''
-  process.env.INPUT_BRANCHES = ''
-  process.env['INPUT_GITHUB-REPOSITORY'] = ''
-  process.env['INPUT_REPO-CONFIG'] = ''
-})
-
-test.serial('`check.reposFile()` should return undefined on missing input', t => {
-  const file = check.reposFile()
-  t.is(file, undefined)
-})
-
-test.serial('`check.reposFile()` should return undefined on empty input', t => {
-  process.env['INPUT_REPOS-FILE'] = ''
-  const file = check.reposFile()
-  t.is(file, undefined)
-})
-
-test.serial('`check.reposFile()` should return contents if file exists', t => {
-  process.env['INPUT_REPOS-FILE'] = 'tests/resources/repos.test.md'
-  const file = check.reposFile() ?? ''
-
-  const expected = '- owner1/repo1\n- owner1/repo2\n- owner2/repo'
-
-  t.is(file.toString(), expected)
-})
-
-test.serial('`check.reposFile()` should throw error if file doesn\'t exists', t => {
-  process.env['INPUT_REPOS-FILE'] = 'this/does/not/exist.md'
-
-  const expected = 'The path indicated in `repos-file` (this/does/not/exist.md) does not exist'
-
-  const error = t.throws(() => check.reposFile(), {instanceOf: Error})
-
-  t.is(error?.message, expected)
-})
-
-test.serial('`check.githubRepository()` should return current repository if input not present', t => {
-  process.env.GITHUB_REPOSITORY = 'owner/repo'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.githubRepository()` should return current repository if input not present with custom branch', t => {
-  process.env.GITHUB_REPOSITORY = 'owner/repo'
-  process.env.INPUT_BRANCHES = '0.1.x'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo:0.1.x'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.githubRepository()` should return current repository if input not present with multiple custom branches', t => {
-  process.env.GITHUB_REPOSITORY = 'owner/repo'
-  process.env.INPUT_BRANCHES = 'main,0.1.x,0.2.x'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo:main\n- owner/repo:0.1.x\n- owner/repo:0.2.x'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.githubRepository()` should return repository from input', t => {
-  process.env['INPUT_GITHUB-REPOSITORY'] = 'owner/repo'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.githubRepository()` should return repository from input with custom branch', t => {
-  process.env['INPUT_GITHUB-REPOSITORY'] = 'owner/repo'
-  process.env.INPUT_BRANCHES = '0.1.x'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo:0.1.x'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.githubRepository()` should return repository from input with multiple custom branches', t => {
-  process.env['INPUT_GITHUB-REPOSITORY'] = 'owner/repo'
-  process.env.INPUT_BRANCHES = 'main,0.1.x,0.2.x'
-
-  const content = check.githubRepository()
-
-  const expected = '- owner/repo:main\n- owner/repo:0.1.x\n- owner/repo:0.2.x'
-
-  t.is(content, expected)
-})
-
-test.serial('`check.defaultRepoConf()` should return the path if it exists', t => {
-  process.env['INPUT_REPO-CONFIG'] = 'tests/resources/.scala-steward.conf'
-
-  const path = check.defaultRepoConf()
-
-  const expected = 'tests/resources/.scala-steward.conf'
-
-  t.is(path, expected)
-})
-
-test.serial('`check.defaultRepoConf()` should return the default path if it exists', t => {
-  fs.writeFileSync('.github/.scala-steward.conf', '')
-  process.env['INPUT_REPO-CONFIG'] = '.github/.scala-steward.conf'
-
-  const path = check.defaultRepoConf()
-
-  const expected = '.github/.scala-steward.conf'
-
-  t.is(path, expected)
-
-  fs.rmSync('.github/.scala-steward.conf')
-})
-
-test.serial('`check.defaultRepoConf()` should return undefined if the default path do not exist', t => {
-  process.env['INPUT_REPO-CONFIG'] = '.github/.scala-steward.conf'
-
-  const path = check.defaultRepoConf()
-
-  t.is(path, undefined)
-})
-
-test.serial('`check.defaultRepoConf()` throws error if provided non-default file do not exist', t => {
-  process.env['INPUT_REPO-CONFIG'] = 'tests/resources/.scala-steward-new.conf'
-
-  const expected = 'Provided default repo conf file (tests/resources/.scala-steward-new.conf) does not exist'
-
-  const error = t.throws(() => check.defaultRepoConf(), {instanceOf: Error})
-
-  t.is(error?.message, expected)
-})
diff --git a/tests/input.test.ts b/tests/input.test.ts
new file mode 100644
index 00000000..b9c38168
--- /dev/null
+++ b/tests/input.test.ts
@@ -0,0 +1,320 @@
+import {fail} from 'assert'
+import test from 'ava'
+import {match} from 'ts-pattern'
+import {type Files} from '../src/files'
+import {Input} from '../src/input'
+import {Logger} from '../src/logger'
+import {nonEmpty} from '../src/types'
+
+test('`Input.all` should return all inputs', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-token', () => '123')
+    .with('repo-config', () => '.github/defaults/.scala-steward.conf')
+    .with('github-repository', () => 'owner/repo')
+    .with('branches', () => '1.0x,2.0x')
+    .with('author-email', () => 'alex@example.com')
+    .with('author-name', () => 'Alex')
+    .with('github-api-url', () => 'github.my-org.com')
+    .with('cache-ttl', () => '20m')
+    .with('timeout', () => '60s')
+    .with('scala-steward-version', () => '1.0')
+    .with('ignore-opts-files', () => 'true')
+    .with('artifact-migrations', () => '.github/artifact-migrations.conf')
+    .with('scalafix-migrations', () => '.github/scalafix-migrations.conf')
+    .with('other-args', () => '--help')
+    .with('sign-commits', () => 'true')
+    .with('signing-key', () => '42')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: name => name === '.github/defaults/.scala-steward.conf',
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const expected = {
+    github: {
+      token: nonEmpty('123'),
+      app: undefined,
+      apiUrl: nonEmpty('github.my-org.com'),
+    },
+    steward: {
+      defaultConfiguration: nonEmpty('.github/defaults/.scala-steward.conf'),
+      repos: '- owner/repo:1.0x\n- owner/repo:2.0x',
+      cacheTtl: nonEmpty('20m'),
+      version: nonEmpty('1.0'),
+      timeout: nonEmpty('60s'),
+      ignoreOptsFiles: true,
+      extraArgs: nonEmpty('--help'),
+    },
+    migrations: {
+      scalafix: nonEmpty('.github/scalafix-migrations.conf'),
+      artifacts: nonEmpty('.github/artifact-migrations.conf'),
+    },
+    commits: {
+      sign: {
+        enabled: true,
+        key: nonEmpty('42'),
+      },
+      author: {
+        email: nonEmpty('alex@example.com'),
+        name: nonEmpty('Alex'),
+      },
+    },
+  }
+
+  t.deepEqual(input.all(), expected)
+})
+
+test('`Input.githubAppInfo()` should return GitHub App info', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-app-id', () => '123')
+    .with('github-app-key', () => '42')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const file = input.githubAppInfo()
+
+  t.deepEqual(file, {id: nonEmpty('123'), key: nonEmpty('42')})
+})
+
+test('`Input.githubAppInfo()` should return undefined on missing inputs', t => {
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: () => ''}, files, Logger.noOp)
+
+  const file = input.githubAppInfo()
+
+  t.is(file, undefined)
+})
+
+test('`Input.githubAppInfo()` should return error if only id input present', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-app-id', () => '123')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const expected = '`github-app-id` and `github-app-key` inputs have to be set together. One of them is missing'
+
+  const error = t.throws(() => input.githubAppInfo(), {instanceOf: Error})
+
+  t.is(error?.message, expected)
+})
+
+test('`Input.githubAppInfo()` should return error if only key input present', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-app-key', () => '42')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const expected = '`github-app-id` and `github-app-key` inputs have to be set together. One of them is missing'
+
+  const error = t.throws(() => input.githubAppInfo(), {instanceOf: Error})
+
+  t.is(error?.message, expected)
+})
+
+test('`Input.reposFile()` should return undefined on missing input', t => {
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: () => ''}, files, Logger.noOp)
+
+  const file = input.reposFile()
+  t.is(file, undefined)
+})
+
+test('`Input.reposFile()` should return contents if file exists', t => {
+  const inputs = (name: string) => match(name)
+    .with('repos-file', () => 'repos.md')
+    .otherwise(() => '')
+
+  const contents = '- owner1/repo1\n- owner1/repo2\n- owner2/repo'
+
+  const files: Files = {
+    existsSync: name => match(name).with('repos.md', () => true).run(),
+    readFileSync: name => match(name).with('repos.md', () => contents).run(),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const file = input.reposFile() ?? ''
+
+  t.is(file.toString(), contents)
+})
+
+test('`Input.reposFile()` should throw error if file doesn\'t exists', t => {
+  const inputs = (name: string) => match(name)
+    .with('repos-file', () => 'this/does/not/exist.md')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const expected = 'The path indicated in `repos-file` (this/does/not/exist.md) does not exist'
+
+  const error = t.throws(() => input.reposFile(), {instanceOf: Error})
+
+  t.is(error?.message, expected)
+})
+
+test('`Input.githubRepository()` should return repository from input', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-repository', () => 'owner/repo')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const content = input.githubRepository()
+
+  const expected = '- owner/repo'
+
+  t.is(content, expected)
+})
+
+test('`Input.githubRepository()` should return repository from input with custom branch', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-repository', () => 'owner/repo')
+    .with('branches', () => '0.1.x')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const content = input.githubRepository()
+
+  const expected = '- owner/repo:0.1.x'
+
+  t.is(content, expected)
+})
+
+test('`Input.githubRepository()` should return repository from input with multiple custom branches', t => {
+  const inputs = (name: string) => match(name)
+    .with('github-repository', () => 'owner/repo')
+    .with('branches', () => 'main,0.1.x,0.2.x')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const content = input.githubRepository()
+
+  const expected = '- owner/repo:main\n- owner/repo:0.1.x\n- owner/repo:0.2.x'
+
+  t.is(content, expected)
+})
+
+test('`Input.defaultRepoConf()` should return the path if it exists', t => {
+  const inputs = (name: string) => match(name)
+    .with('repo-config', () => '.scala-steward.conf')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: name => match(name).with('.scala-steward.conf', () => true).run(),
+    readFileSync: () => fail('This should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const path = input.defaultRepoConf()
+
+  const expected = '.scala-steward.conf'
+
+  t.is(path?.value, expected)
+})
+
+test('`Input.defaultRepoConf()` should return the default path if it exists', t => {
+  const inputs = (name: string) => match(name)
+    .with('repo-config', () => '.github/.scala-steward.conf')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: name => match(name).with('.github/.scala-steward.conf', () => true).run(),
+    readFileSync: () => fail('This should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const path = input.defaultRepoConf()
+
+  const expected = '.github/.scala-steward.conf'
+
+  t.is(path?.value, expected)
+})
+
+test('`Input.defaultRepoConf()` should return undefined if the default path do not exist', t => {
+  const inputs = (name: string) => match(name)
+    .with('repo-config', () => '.github/.scala-steward.conf')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const path = input.defaultRepoConf()
+
+  t.is(path, undefined)
+})
+
+test('`Input.defaultRepoConf()` throws error if provided non-default file do not exist', t => {
+  const inputs = (name: string) => match(name)
+    .with('repo-config', () => 'tests/resources/.scala-steward-new.conf')
+    .otherwise(() => '')
+
+  const files: Files = {
+    existsSync: () => false,
+    readFileSync: () => fail('Should not be called'),
+  }
+
+  const input = Input.from({getInput: inputs}, files, Logger.noOp)
+
+  const expected = 'Provided default repo conf file (tests/resources/.scala-steward-new.conf) does not exist'
+
+  const error = t.throws(() => input.defaultRepoConf(), {instanceOf: Error})
+
+  t.is(error?.message, expected)
+})
diff --git a/tests/resources/.scala-steward.conf b/tests/resources/.scala-steward.conf
deleted file mode 100644
index e69de29b..00000000
diff --git a/tests/resources/repos.test.md b/tests/resources/repos.test.md
deleted file mode 100644
index 930ec395..00000000
--- a/tests/resources/repos.test.md
+++ /dev/null
@@ -1,3 +0,0 @@
-- owner1/repo1
-- owner1/repo2
-- owner2/repo
\ No newline at end of file