diff --git a/shard.yml b/shard.yml
index 8bb7853..015c1cd 100644
--- a/shard.yml
+++ b/shard.yml
@@ -8,6 +8,6 @@ targets:
   bootstrap:
     main: src/main.cr
 
-crystal: 1.11.2
+crystal: ">= 1.20.0"
 
 license: MIT
diff --git a/src/error/usecase.cr b/src/error/usecase.cr
index 178b6c8..b785e8c 100644
--- a/src/error/usecase.cr
+++ b/src/error/usecase.cr
@@ -12,7 +12,7 @@ module Error
         pretext: "<@#{ENV["SLACK_ID"]}> #{message}",
         color: "#EB4646",
         title: err.message,
-        text: err.backtrace.join('\n'),
+        text: err.backtrace?.try(&.join('\n')),
         footer: "github_notifications_slack (#{ENV["ENV"]})",
         footer_icon: "",
       )
diff --git a/src/github/models.cr b/src/github/models.cr
index 685f1fb..d39f033 100644
--- a/src/github/models.cr
+++ b/src/github/models.cr
@@ -4,36 +4,34 @@ module Github
   class Notifications
     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..57abcd0 100644
--- a/src/github/repository.cr
+++ b/src/github/repository.cr
@@ -16,20 +16,20 @@ module Github
 
     def find_notifications_unread : Array(Notifications)
       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 Notifications
+      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 Notifications
+      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}"
@@ -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..084430b 100644
--- a/src/github/usecase.cr
+++ b/src/github/usecase.cr
@@ -18,7 +18,7 @@ module Github
         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/notify/usecase.cr b/src/notify/usecase.cr
index 84c059f..3b40fea 100644
--- a/src/notify/usecase.cr
+++ b/src/notify/usecase.cr
@@ -24,7 +24,7 @@ module Notify
           github_uc.to_slack_attachment item, pretext, message
         end
 
-      if notices.size != 0
+      unless notices.empty?
         slack_repo = Slack::PostRepository.new ENV["WEBHOOK_URL"]
         slack_repo.send_attachments notices
 
diff --git a/src/runtime/lambda.cr b/src/runtime/lambda.cr
index 932fdae..ce70d76 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"
@@ -32,10 +35,15 @@ 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。
+    # 中間の Array(Char) を作らずに済むよう文字列スライスで分割する。
     def print_log(log : String)
-      log.split(//).each_slice(50000) do |line|
-        puts `echo '#{line.join.gsub(/(\r\n|\r|\n|\f)/, "")}'`
-        STDOUT.flush
+      cleaned = log.gsub(/(\r\n|\r|\n|\f)/, "")
+      offset = 0
+      while offset < cleaned.size
+        Log.info { cleaned[offset, 50000] }
+        offset += 50000
       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