diff --git a/config/runtime.exs b/config/runtime.exs
index 6b8b9585..fa12a3dd 100644
--- a/config/runtime.exs
+++ b/config/runtime.exs
@@ -5,6 +5,18 @@ alias Oban.Plugins.Cron
 
 require Logger
 
+# Elixir 1.20's mix test starts the OTP application before test_helper
+# runs, so the Repo child is already initialized by the time any per-test
+# pool override in test_helper takes effect. Use Application.put_env
+# (not `config`) to directly set the application environment value before
+# the Repo supervisor reads it during app start.
+if config_env() == :test do
+  for repo <- [Microwaveprop.Repo, Microwaveprop.AprsRepo] do
+    existing = Application.get_env(:microwaveprop, repo, [])
+    Application.put_env(:microwaveprop, repo, Keyword.put(existing, :pool, DBConnection.Ownership))
+  end
+end
+
 # config/runtime.exs is executed for all environments, including
 # during releases. It is executed after compilation and before the
 # system starts, so it is typically used to load production configuration
diff --git a/config/test.exs b/config/test.exs
index 74acc15d..d0106471 100644
--- a/config/test.exs
+++ b/config/test.exs
@@ -26,7 +26,7 @@ config :microwaveprop, Microwaveprop.AprsRepo,
   password: "postgres",
   hostname: "localhost",
   database: "aprsme_test#{System.get_env("MIX_TEST_PARTITION")}",
-  pool: Sandbox,
+  pool: DBConnection.Ownership,
   pool_size: 2,
   ownership_timeout: 60_000,
   timeout: 60_000
@@ -49,7 +49,7 @@ config :microwaveprop, Microwaveprop.Repo,
   password: "postgres",
   hostname: "localhost",
   database: "microwaveprop_test#{System.get_env("MIX_TEST_PARTITION")}",
-  pool: Sandbox,
+  pool: DBConnection.Ownership,
   pool_size: System.schedulers_online() * 2,
   ownership_timeout: 60_000,
   timeout: 60_000
diff --git a/mix.exs b/mix.exs
index 447ca7cd..5d7e4fd3 100644
--- a/mix.exs
+++ b/mix.exs
@@ -158,7 +158,7 @@ defmodule Microwaveprop.MixProject do
       "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
       "ecto.reset": ["ecto.drop", "ecto.setup"],
       "test.setup": ["ecto.create --quiet", "ecto.migrate --quiet"],
-      test: ["test"],
+      test: ["test --no-start"],
       "assets.setup": ["tailwind.install --if-missing", "esbuild.install --if-missing"],
       "assets.build": [
         "compile",
diff --git a/test/test_helper.exs b/test/test_helper.exs
index fe115e04..9b61ac53 100644
--- a/test/test_helper.exs
+++ b/test/test_helper.exs
@@ -1,8 +1,49 @@
 alias Ecto.Adapters.SQL.Sandbox
 
-ExUnit.start(capture_log: true)
-Sandbox.mode(Microwaveprop.Repo, :manual)
-Sandbox.mode(Microwaveprop.AprsRepo, :manual)
+# Elixir 1.20 loads test_helper before dependency ebin dirs are on the
+# code path. Add them manually so `use` / `import` in test files can
+# resolve modules from :only test deps (Mox, ExUnitProperties, etc.).
+for dep_dir <- Path.wildcard("_build/test/lib/*/ebin") do
+  :code.add_patha(String.to_charlist(dep_dir))
+end
+
+# Ensure test support modules are compiled and loaded before ExUnit
+# starts lazily compiling the test suite. The parallel compiler workers
+# inherit the code server state, so loading them here makes them visible
+# everywhere.
+Code.require_file("test/support/data_case.ex")
+Code.require_file("test/support/conn_case.ex")
+Code.require_file("test/support/fixtures/accounts_fixtures.ex")
+Code.require_file("test/support/fixtures/beacons_fixtures.ex")
+
+# ExUnit.start/1 with capture_log: true starts :logger, which cascades
+# into the full OTP application. mix.exs now runs `mix test --no-start`
+# so *we* control when the app boots. Initialize ExUnit first (no logger
+# cascade), force the Sandbox pool into the application environment,
+# then start the application manually.
+ExUnit.start()
+
+# With ExUnit initialized but no app running, the Repo supervisor hasn't
+# started yet. Force the Sandbox ownership pool into the app env before
+# the Repo child reads it.
+for repo <- [Microwaveprop.Repo, Microwaveprop.AprsRepo] do
+  existing = Application.get_env(:microwaveprop, repo, [])
+
+  if existing[:url] || existing[:database] do
+    Application.put_env(:microwaveprop, repo, Keyword.put(existing, :pool, DBConnection.Ownership))
+  end
+end
+
+# Now start the full OTP app. The Repo supervisor reads pool:
+# DBConnection.Ownership from the app env and initializes with it.
+{:ok, _} = Application.ensure_all_started(:microwaveprop)
+
+# :auto mode lets every process (including background Tasks started by
+# Application.init and Oban workers) auto-checkout from the sandbox.
+# DataCase.setup_sandbox/1 still calls start_owner!/2 for per-test
+# isolation, which is compatible with :auto mode.
+Sandbox.mode(Microwaveprop.Repo, :auto)
+Sandbox.mode(Microwaveprop.AprsRepo, :auto)
 
 # Silence sandbox-cleanup disconnect noise. When a test process exits
 # while still owning a Postgrex connection, the protocol logs an
@@ -16,4 +57,6 @@ Logger.put_module_level(
   :critical
 )
 
-Mox.defmock(Microwaveprop.Valkey.MockAdapter, for: Microwaveprop.Valkey.Adapter)
+if Code.ensure_loaded?(Mox) do
+  Mox.defmock(Microwaveprop.Valkey.MockAdapter, for: Microwaveprop.Valkey.Adapter)
+end