diff --git a/lib/stacks/forecast.rb b/lib/stacks/forecast.rb
index 3bb2b0a8..54c82c1e 100644
--- a/lib/stacks/forecast.rb
+++ b/lib/stacks/forecast.rb
@@ -1,6 +1,6 @@
 class Stacks::Forecast
   include HTTParty
-  base_uri 'api.forecastapp.com'
+  base_uri 'https://api.forecastapp.com'
 
   def initialize()
     @headers = {
diff --git a/lib/stacks/twist.rb b/lib/stacks/twist.rb
index 2a961129..4abe1598 100644
--- a/lib/stacks/twist.rb
+++ b/lib/stacks/twist.rb
@@ -1,6 +1,6 @@
 class Stacks::Twist
   include HTTParty
-  base_uri 'api.twist.com/api/v3'
+  base_uri 'https://api.twist.com/api/v3'
 
   def initialize()
     @headers = {
diff --git a/test/lib/stacks/https_base_uri_test.rb b/test/lib/stacks/https_base_uri_test.rb
new file mode 100644
index 00000000..5898c03f
--- /dev/null
+++ b/test/lib/stacks/https_base_uri_test.rb
@@ -0,0 +1,30 @@
+require "test_helper"
+
+# Regression guard for a silent, production-only class of bug:
+#
+# A scheme-less HTTParty `base_uri` (e.g. "api.forecastapp.com") defaults to
+# http://. Forecast (and Twist) 301-redirect http -> https. GETs survive the
+# redirect, but a redirected POST/PUT/DELETE arrives malformed and the API
+# returns 400 — so READS work while WRITES silently fail. Stubbed unit tests
+# never catch it because they don't hit the network; it only surfaces against
+# the live API. `Stacks::Forecast#create_assignment` hitting this is exactly
+# what stopped recurring-assignment materialization from reaching Forecast.
+#
+# Pin every HTTParty API client to an explicit https base_uri.
+class Stacks::HttpsBaseUriTest < ActiveSupport::TestCase
+  CLIENTS = [
+    Stacks::Forecast,
+    Stacks::Twist,
+    Stacks::Runn,
+    Stacks::Deel,
+    Stacks::Notion,
+    Stacks::Apollo,
+  ].freeze
+
+  test "every HTTParty API client uses an https base_uri (writes must not ride an http->https redirect)" do
+    CLIENTS.each do |klass|
+      assert klass.base_uri.to_s.start_with?("https://"),
+        "#{klass}.base_uri must be https to avoid write-mangling redirects; got #{klass.base_uri.inspect}"
+    end
+  end
+end