diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..074d23c
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,31 @@
+name: ci
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  check:
+    name: format / lint / spec
+    runs-on: ubuntu-24.04-arm
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+
+      - name: install crystal
+        uses: crystal-lang/install-crystal@v1
+        with:
+          crystal: latest
+
+      - name: install shards
+        run: shards install
+
+      - name: check format
+        run: crystal tool format --check
+
+      - name: lint with ameba
+        run: ./bin/ameba
+
+      - name: run specs
+        run: crystal spec
diff --git a/.gitignore b/.gitignore
index ed141fe..0248cdc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,7 +6,8 @@ jspm_packages
 .serverless
 
 bootstrap
+bin
 lib
 env.yml
-
-.idea/
+
+.idea/
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 765f0e9..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,6 +0,0 @@
-language: crystal
-
-# Uncomment the following if you'd like Travis to run specs and check code formatting
-# script:
-#   - crystal spec
-#   - crystal tool format --check
diff --git a/shard.lock b/shard.lock
index 4feacb6..f612c4d 100644
--- a/shard.lock
+++ b/shard.lock
@@ -1,6 +1,6 @@
-version: 1.0
+version: 2.0
 shards:
-  clim:
-    github: at-grandpa/clim
-    version: 0.4.1
+  ameba:
+    git: https://github.com/crystal-ameba/ameba.git
+    version: 1.6.4
 
diff --git a/shard.yml b/shard.yml
index 8bb7853..dec4eb0 100644
--- a/shard.yml
+++ b/shard.yml
@@ -8,6 +8,11 @@ targets:
   bootstrap:
     main: src/main.cr
 
-crystal: 1.11.2
+crystal: ">= 1.20.0"
+
+development_dependencies:
+  ameba:
+    github: crystal-ameba/ameba
+    version: ~> 1.6
 
 license: MIT
diff --git a/spec/github/models_spec.cr b/spec/github/models_spec.cr
new file mode 100644
index 0000000..d6759cb
--- /dev/null
+++ b/spec/github/models_spec.cr
@@ -0,0 +1,113 @@
+require "../spec_helper"
+
+private def subject_from(type : String, url = "", latest_comment_url = "")
+  Github::Subject.from_json({
+    type:               type,
+    title:              "title",
+    url:                url,
+    latest_comment_url: latest_comment_url,
+  }.to_json)
+end
+
+describe Github::Subject do
+  describe "#update?" do
+    it "is true for tracked subject types" do
+      [
+        Github::Subject::Type::PULL_REQUEST,
+        Github::Subject::Type::ISSUE,
+        Github::Subject::Type::COMMIT,
+        Github::Subject::Type::DISCUSSION,
+      ].each do |type|
+        subject_from(type).update?.should be_true
+      end
+    end
+
+    it "is false for unknown subject types" do
+      subject_from("Release").update?.should be_false
+    end
+  end
+
+  describe "#color" do
+    it "returns a distinct color per known type" do
+      subject_from(Github::Subject::Type::PULL_REQUEST).color.should eq "#F6CEE3"
+      subject_from(Github::Subject::Type::ISSUE).color.should eq "#A9D0F5"
+      subject_from(Github::Subject::Type::COMMIT).color.should eq "#f5d7a9"
+      subject_from(Github::Subject::Type::DISCUSSION).color.should eq "#7fffd4"
+    end
+
+    it "falls back to a default color for unknown types" do
+      subject_from("Release").color.should eq "#D8D8D8"
+    end
+  end
+
+  describe "#comment_url" do
+    it "prefers latest_comment_url when present" do
+      subject = subject_from("Issue", url: "u", latest_comment_url: "c")
+      subject.comment_url.should eq "c"
+    end
+
+    it "falls back to url when latest_comment_url is blank" do
+      subject = subject_from("Issue", url: "u", latest_comment_url: "")
+      subject.comment_url.should eq "u"
+    end
+  end
+end
+
+describe Github::Notification do
+  describe "#mention?" do
+    it "is true for reasons that mention the user" do
+      Github::Notification::MENTION_REASONS.each do |reason|
+        notification_from(reason).mention?.should be_true
+      end
+    end
+
+    it "is false for non-mention reasons" do
+      notification_from("subscribed").mention?.should be_false
+      notification_from("ci_activity").mention?.should be_false
+    end
+  end
+
+  it "parses a GitHub notifications API payload" do
+    notifications = Array(Github::Notification).from_json(NOTIFICATIONS_FIXTURE)
+    notifications.size.should eq 1
+
+    notification = notifications.first
+    notification.reason.should eq "mention"
+    notification.subject.type.should eq "Issue"
+    notification.subject.title.should eq "Spurious failure"
+    notification.repository.full_name.should eq "octocat/Hello-World"
+    notification.mention?.should be_true
+  end
+end
+
+private def notification_from(reason : String)
+  Github::Notification.from_json({
+    reason:     reason,
+    subject:    {type: "Issue", title: "title"},
+    repository: {owner: {login: "octocat"}},
+  }.to_json)
+end
+
+NOTIFICATIONS_FIXTURE = <<-JSON
+[
+  {
+    "reason": "mention",
+    "subject": {
+      "title": "Spurious failure",
+      "url": "https://api.github.com/repos/octocat/Hello-World/issues/1",
+      "latest_comment_url": "https://api.github.com/repos/octocat/Hello-World/issues/comments/1",
+      "type": "Issue"
+    },
+    "repository": {
+      "full_name": "octocat/Hello-World",
+      "html_url": "https://github.com/octocat/Hello-World",
+      "owner": {
+        "login": "octocat",
+        "avatar_url": "https://github.com/images/error/octocat.gif",
+        "html_url": "https://github.com/octocat"
+      }
+    },
+    "subscription_url": "https://api.github.com/notifications/threads/1/subscription"
+  }
+]
+JSON
diff --git a/spec/github_notifications_slack_spec.cr b/spec/github_notifications_slack_spec.cr
deleted file mode 100644
index 1a4c2f2..0000000
--- a/spec/github_notifications_slack_spec.cr
+++ /dev/null
@@ -1,9 +0,0 @@
-require "./spec_helper"
-
-describe GithubNotificationsSlack do
-  # TODO: Write tests
-
-  it "works" do
-    false.should eq(true)
-  end
-end
diff --git a/spec/slack/models_spec.cr b/spec/slack/models_spec.cr
new file mode 100644
index 0000000..26da433
--- /dev/null
+++ b/spec/slack/models_spec.cr
@@ -0,0 +1,22 @@
+require "../spec_helper"
+
+describe Slack::Attachment do
+  it "omits unset fields when serialized" do
+    json = Slack::Attachment.new(text: "hello", color: "#000000").to_json
+    parsed = JSON.parse(json)
+
+    parsed["text"].should eq "hello"
+    parsed["color"].should eq "#000000"
+    parsed.as_h.has_key?("title").should be_false
+  end
+end
+
+describe Slack::Post do
+  it "wraps attachments under an attachments key" do
+    post = Slack::Post.new([Slack::Attachment.new(text: "a")])
+    parsed = JSON.parse(post.to_json)
+
+    parsed["attachments"].as_a.size.should eq 1
+    parsed["attachments"][0]["text"].should eq "a"
+  end
+end
diff --git a/spec/spec_helper.cr b/spec/spec_helper.cr
index 6ca3d64..eb8812d 100644
--- a/spec/spec_helper.cr
+++ b/spec/spec_helper.cr
@@ -1,2 +1,3 @@
 require "spec"
-require "../src/github_notifications_slack"
+require "../src/github/models"
+require "../src/slack/models"
diff --git a/src/error/usecase.cr b/src/error/usecase.cr
index 178b6c8..6fe7697 100644
--- a/src/error/usecase.cr
+++ b/src/error/usecase.cr
@@ -3,20 +3,21 @@ require "../slack/repository"
 
 module Error
   class Usecase
-    def alert(err)
-      slack = Slack::PostRepository.new ENV["ALERT_WEBHOOK_URL"]
+    def initialize(@slack_repo : Slack::PostRepository, @slack_id : String, @env : String)
+    end
 
+    def alert(err)
       message = "エラーみたい…確認してみよっか"
       attachment = Slack::Attachment.new(
         fallback: message,
-        pretext: "<@#{ENV["SLACK_ID"]}> #{message}",
+        pretext: "<@#{@slack_id}> #{message}",
         color: "#EB4646",
         title: err.message,
-        text: err.backtrace.join('\n'),
-        footer: "github_notifications_slack (#{ENV["ENV"]})",
+        text: err.backtrace?.try(&.join('\n')),
+        footer: "github_notifications_slack (#{@env})",
         footer_icon: "",
       )
-      slack.send_attachment attachment
+      @slack_repo.send_attachment attachment
 
       {msg: "ng"}
     end
diff --git a/src/github/models.cr b/src/github/models.cr
index 685f1fb..67e97db 100644
--- a/src/github/models.cr
+++ b/src/github/models.cr
@@ -1,39 +1,37 @@
 require "json"
 
 module Github
-  class Notifications
+  class Notification
     include JSON::Serializable
 
-    property subject : Subject
-    property reason : String
-    property repository : Repository
-    property subscription_url : String?
+    MENTION_REASONS = [
+      "assign",
+      "author",
+      "comment",
+      "invitation",
+      "mention",
+      "team_mention",
+      "review_requested",
+      # "ci_activity",
+    ]
+
+    getter subject : Subject
+    getter reason : String
+    getter repository : Repository
+    getter subscription_url : String?
 
     def mention? : Bool
-      [
-        "assign",
-        "author",
-        "comment",
-        "invitation",
-        "mention",
-        "team_mention",
-        "review_requested",
-        # "ci_activity",
-      ].includes?(reason)
+      reason.in?(MENTION_REASONS)
     end
   end
 
   class Subject
     include JSON::Serializable
 
-    property type : String
-    property title : String?
-
-    @[JSON::Field(emit_null: false)]
-    property url : String = ""
-
-    @[JSON::Field(emit_null: false)]
-    property latest_comment_url : String = ""
+    getter type : String
+    getter title : String?
+    getter url : String = ""
+    getter latest_comment_url : String = ""
 
     module Type
       PULL_REQUEST = "PullRequest"
@@ -42,13 +40,15 @@ module Github
       DISCUSSION   = "Discussion"
     end
 
+    UPDATE_TYPES = [
+      Type::PULL_REQUEST,
+      Type::ISSUE,
+      Type::COMMIT,
+      Type::DISCUSSION,
+    ]
+
     def update? : Bool
-      [
-        Type::PULL_REQUEST,
-        Type::ISSUE,
-        Type::COMMIT,
-        Type::DISCUSSION,
-      ].includes?(type)
+      type.in?(UPDATE_TYPES)
     end
 
     def color : String
@@ -67,28 +67,24 @@ module Github
     end
 
     def comment_url : String
-      if !latest_comment_url.blank?
-        latest_comment_url
-      else
-        url
-      end
+      latest_comment_url.presence || url
     end
   end
 
   class Repository
     include JSON::Serializable
 
-    property full_name : String?
-    property html_url : String?
-    property owner : User
+    getter full_name : String?
+    getter html_url : String?
+    getter owner : User
   end
 
   class Comment
     include JSON::Serializable
 
-    property user : User
-    property html_url : String?
-    property body : String?
+    getter user : User
+    getter html_url : String?
+    getter body : String?
 
     def initialize(@body)
       @user = User.new
@@ -98,9 +94,9 @@ module Github
   class User
     include JSON::Serializable
 
-    property login : String?
-    property avatar_url : String?
-    property html_url : String?
+    getter login : String?
+    getter avatar_url : String?
+    getter html_url : String?
 
     def initialize
     end
@@ -109,7 +105,7 @@ module Github
   class Error
     include JSON::Serializable
 
-    property message : String
-    property documentation_url : String?
+    getter message : String
+    getter documentation_url : String?
   end
 end
diff --git a/src/github/repository.cr b/src/github/repository.cr
index 14012eb..1dcf3ea 100644
--- a/src/github/repository.cr
+++ b/src/github/repository.cr
@@ -14,26 +14,26 @@ module Github
       end
     end
 
-    def find_notifications_unread : Array(Notifications)
+    def find_notifications_unread : Array(Notification)
       res = @github.get "/notifications"
-      if res.status_code >= 500
+      if res.status.server_error?
         Serverless::Lambda.print_log "return 5xx error from notifications api"
-        return Array(Notifications).new
-      elsif res.status_code == 401
+        return [] of Notification
+      elsif res.status.unauthorized?
         # GitHub が断続的に 401 を返すことがあるため、毎分の次回実行に任せてスキップする。
         # トークン失効などの恒久的な 401 までサイレントに握りつぶす点は本来リトライや
         # 連続失敗の監視で区別すべきだが、個人用途の通知ツールであり実装コストに
         # 見合わないため割り切る。
         Serverless::Lambda.print_log "return 401 error from notifications api, skip"
-        return Array(Notifications).new
-      elsif res.status_code >= 400
+        return [] of Notification
+      elsif res.status.client_error?
         Serverless::Lambda.print_log "return 4xx error from notifications api"
         err = Error.from_json res.body
-        raise "notifications api retrun client error: #{err.message}"
+        raise "notifications api return client error: #{err.message}"
       end
 
       Serverless::Lambda.print_log "notifications body: #{res.body}"
-      Array(Notifications).from_json(res.body)
+      Array(Notification).from_json(res.body)
     end
 
     def find_comment_by_url(url : String) : Comment
@@ -43,21 +43,21 @@ module Github
       end
 
       res = @github.get url
-      if res.status_code >= 500
+      if res.status.server_error?
         Serverless::Lambda.print_log "return 5xx error from comments api"
-        return Comment.new "comments api retrun server error"
-      elsif res.status_code >= 400
+        return Comment.new "comments api return server error"
+      elsif res.status.client_error?
         Serverless::Lambda.print_log "return 4xx error from comments api"
         err = Error.from_json res.body
-        return Comment.new "comments api retrun client error: #{err.message}"
+        return Comment.new "comments api return client error: #{err.message}"
       end
 
       begin
         Serverless::Lambda.print_log "comment body: #{res.body}"
         Comment.from_json res.body
       rescue
-        Serverless::Lambda.print_log "faild parse comment data"
-        Comment.new "faild parse comment data"
+        Serverless::Lambda.print_log "failed parse comment data"
+        Comment.new "failed parse comment data"
       end
     end
 
diff --git a/src/github/usecase.cr b/src/github/usecase.cr
index fd749a5..4857ba0 100644
--- a/src/github/usecase.cr
+++ b/src/github/usecase.cr
@@ -4,21 +4,22 @@ require "../slack/models"
 
 module Github
   class Usecase
-    def to_slack_attachment(notify : Notifications, pretext : String, message : String) : Slack::Attachment
-      repo = NotificationRepository.new ENV["GITHUB_TOKEN"]
+    def initialize(@repo : NotificationRepository, @slack_id : String)
+    end
 
-      comment = repo.find_comment_by_url notify.subject.comment_url
+    def to_slack_attachment(notify : Notification, pretext : String, message : String) : Slack::Attachment
+      comment = @repo.find_comment_by_url notify.subject.comment_url
       Slack::Attachment.new(
         fallback: pretext,
         author_name: comment.user.login,
         author_icon: comment.user.avatar_url,
         author_link: comment.user.html_url,
-        pretext: "#{notify.mention? ? "<@#{ENV["SLACK_ID"]}> " : ""}#{pretext}",
+        pretext: "#{notify.mention? ? "<@#{@slack_id}> " : ""}#{pretext}",
         color: notify.subject.color,
         title: notify.subject.title,
         title_link: comment.html_url,
         text: comment.body,
-        footer: !notify.repository.full_name.nil? ? notify.repository.full_name : "github",
+        footer: notify.repository.full_name || "github",
         footer_icon: notify.repository.owner.avatar_url,
       )
     end
diff --git a/src/main.cr b/src/main.cr
index 99505ad..497e72d 100644
--- a/src/main.cr
+++ b/src/main.cr
@@ -1,14 +1,27 @@
 require "./runtime/lambda"
+require "./github/repository"
+require "./github/usecase"
 require "./notify/usecase"
+require "./slack/repository"
 require "./error/usecase"
 
+# 必須の環境変数は起動時に解決し、欠如していればこの時点で失敗させる。
+GITHUB_TOKEN      = ENV["GITHUB_TOKEN"]
+WEBHOOK_URL       = ENV["WEBHOOK_URL"]
+ALERT_WEBHOOK_URL = ENV["ALERT_WEBHOOK_URL"]
+SLACK_ID          = ENV["SLACK_ID"]
+APP_ENV           = ENV["ENV"]
+
 Serverless::Lambda.handler "github_notifications_slack" do |_|
   begin
-    notify_uc = Notify::Usecase.new
-    notify_uc.check_notifications
-  rescue err
-    err_uc = Error::Usecase.new
-    err_uc.alert err
-    raise err
+    github_repo = Github::NotificationRepository.new GITHUB_TOKEN
+    github_uc = Github::Usecase.new github_repo, SLACK_ID
+    slack_repo = Slack::PostRepository.new WEBHOOK_URL
+
+    Notify::Usecase.new(github_repo, github_uc, slack_repo).check_notifications
+  rescue error
+    alert_repo = Slack::PostRepository.new ALERT_WEBHOOK_URL
+    Error::Usecase.new(alert_repo, SLACK_ID, APP_ENV).alert error
+    raise error
   end
 end
diff --git a/src/notify/usecase.cr b/src/notify/usecase.cr
index 84c059f..d7955b8 100644
--- a/src/notify/usecase.cr
+++ b/src/notify/usecase.cr
@@ -1,16 +1,18 @@
-require "../github/models"
 require "../github/repository"
 require "../github/usecase"
-require "../slack/models"
 require "../slack/repository"
 
 module Notify
   class Usecase
-    def check_notifications
-      github_repo = Github::NotificationRepository.new ENV["GITHUB_TOKEN"]
-      github_uc = Github::Usecase.new
+    def initialize(
+      @github_repo : Github::NotificationRepository,
+      @github_uc : Github::Usecase,
+      @slack_repo : Slack::PostRepository,
+    )
+    end
 
-      notices = github_repo
+    def check_notifications
+      notices = @github_repo
         .find_notifications_unread
         .map do |item|
           message =
@@ -21,14 +23,12 @@ module Notify
             end
           pretext = "[#{item.subject.type}] #{message}"
 
-          github_uc.to_slack_attachment item, pretext, message
+          @github_uc.to_slack_attachment item, pretext, message
         end
 
-      if notices.size != 0
-        slack_repo = Slack::PostRepository.new ENV["WEBHOOK_URL"]
-        slack_repo.send_attachments notices
-
-        github_repo.notification_to_read
+      unless notices.empty?
+        @slack_repo.send_attachments notices
+        @github_repo.notification_to_read
       end
 
       {msg: "ok"}
diff --git a/src/runtime/lambda.cr b/src/runtime/lambda.cr
index 932fdae..e7a3c0e 100644
--- a/src/runtime/lambda.cr
+++ b/src/runtime/lambda.cr
@@ -1,11 +1,14 @@
 require "json"
+require "log"
 require "http/client"
 
 module Serverless
   module Lambda
     extend self
 
-    def handler(name : String)
+    Log = ::Log.for("lambda")
+
+    def handler(name : String, &)
       return if name != ENV["_HANDLER"]
 
       ENV["SSL_CERT_FILE"] = "/etc/pki/tls/cert.pem"
@@ -19,10 +22,10 @@ module Serverless
           body = yield event
           header = nil
           url = "http://#{ENV["AWS_LAMBDA_RUNTIME_API"]}/2018-06-01/runtime/invocation/#{request_id}/response"
-        rescue err
+        rescue error
           body = {
             msg: "Internal Lambda Error",
-            err: err.message,
+            err: error.message,
           }
           header = HTTP::Headers{"Lambda-Runtime-Function-Error-Type" => "Unhandled"}
           url = "http://#{ENV["AWS_LAMBDA_RUNTIME_API"]}/2018-06-01/runtime/invocation/#{request_id}/error"
@@ -32,10 +35,11 @@ module Serverless
       end
     end
 
+    # CloudWatch では改行ごとにログエントリが分割されるため、改行を除去して
+    # 1 エントリにまとめつつ、長い本文は適度なチャンクに分割して出力する。
     def print_log(log : String)
-      log.split(//).each_slice(50000) do |line|
-        puts `echo '#{line.join.gsub(/(\r\n|\r|\n|\f)/, "")}'`
-        STDOUT.flush
+      log.gsub(/(\r\n|\r|\n|\f)/, "").each_char.each_slice(50000) do |chunk|
+        Log.info { chunk.join }
       end
     end
   end
diff --git a/src/slack/models.cr b/src/slack/models.cr
index 4711b2a..107777d 100644
--- a/src/slack/models.cr
+++ b/src/slack/models.cr
@@ -4,17 +4,17 @@ module Slack
   class Attachment
     include JSON::Serializable
 
-    property fallback : String?
-    property author_name : String?
-    property author_icon : String?
-    property author_link : String?
-    property pretext : String?
-    property color : String?
-    property title : String?
-    property title_link : String?
-    property text : String?
-    property footer : String?
-    property footer_icon : String?
+    getter fallback : String?
+    getter author_name : String?
+    getter author_icon : String?
+    getter author_link : String?
+    getter pretext : String?
+    getter color : String?
+    getter title : String?
+    getter title_link : String?
+    getter text : String?
+    getter footer : String?
+    getter footer_icon : String?
 
     def initialize(
       @fallback = nil,
@@ -27,7 +27,7 @@ module Slack
       @title_link = nil,
       @text = nil,
       @footer = nil,
-      @footer_icon = nil
+      @footer_icon = nil,
     )
     end
   end
@@ -35,7 +35,7 @@ module Slack
   class Post
     include JSON::Serializable
 
-    property attachments : Array(Attachment)
+    getter attachments : Array(Attachment)
 
     def initialize(@attachments)
     end
diff --git a/src/slack/repository.cr b/src/slack/repository.cr
index a1d04dc..eea7f1f 100644
--- a/src/slack/repository.cr
+++ b/src/slack/repository.cr
@@ -5,24 +5,20 @@ require "./models"
 
 module Slack
   class PostRepository
-    def initialize(@url : String)
-      @uri = URI.parse @url
-    end
-
-    private def send_post(post : Post)
-      HTTP::Client.post(@uri,
-        body: post.to_json
-      )
+    def initialize(url : String)
+      @uri = URI.parse url
     end
 
     def send_attachment(attachment : Attachment)
-      post = Post.new [attachment]
-      send_post post
+      send_attachments [attachment]
     end
 
     def send_attachments(attachments : Array(Attachment))
-      post = Post.new attachments
-      send_post post
+      send_post Post.new(attachments)
+    end
+
+    private def send_post(post : Post)
+      HTTP::Client.post(@uri, body: post.to_json)
     end
   end
 end