diff --git a/.tool-versions b/.tool-versions
index 58766197c..aac7389b8 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1 +1 @@
-ruby 3.4.8
+ruby 4.0.5
diff --git a/AGENTS.md b/AGENTS.md
index 255e5dbf7..b53622f7a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -34,7 +34,7 @@ docker compose up
 - Full suite: `docker compose run --rm api rspec`
 - Single spec: `docker compose run --rm api rspec spec/path/to/spec.rb`
 - Lint: `docker compose run --rm api bundle exec rubocop`
-- CI: GitHub Actions with Ruby 3.4, Postgres 12, Redis.
+- CI: GitHub Actions with Ruby 4, Postgres 12, Redis.
 
 ## Where to Look First
 - Routes: `config/routes.rb`. Auth: `config/initializers/omniauth.rb`, `app/helpers/authentication_helper.rb`, `app/controllers/concerns/identifiable.rb`.
diff --git a/Dockerfile b/Dockerfile
index 2275fb630..0fc698486 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:3.4.8-slim-trixie AS base
+FROM ruby:4.0.5-slim-trixie AS base
 RUN gem install bundler \
   && apt-get update \
   && apt-get upgrade --yes \
diff --git a/Gemfile.lock b/Gemfile.lock
index 9b465f7a1..4c21393a0 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -183,9 +183,9 @@ GEM
       logger
     faraday-net_http (3.4.2)
       net-http (~> 0.5)
-    ffi (1.17.2-aarch64-linux-gnu)
-    ffi (1.17.2-arm64-darwin)
-    ffi (1.17.2-x86_64-linux-gnu)
+    ffi (1.17.4-aarch64-linux-gnu)
+    ffi (1.17.4-arm64-darwin)
+    ffi (1.17.4-x86_64-linux-gnu)
     fiber-storage (1.0.1)
     flipper (1.4.2)
       concurrent-ruby (< 2)
@@ -644,7 +644,7 @@ DEPENDENCIES
   webmock
 
 RUBY VERSION
-  ruby 3.4.8
+  ruby 4.0.5
 
 BUNDLED WITH
   4.0.10