diff --git a/.github/actions/test-ios-rntester/action.yml b/.github/actions/test-ios-rntester/action.yml
index 2b62f554daa1..cdc2f5213a25 100644
--- a/.github/actions/test-ios-rntester/action.yml
+++ b/.github/actions/test-ios-rntester/action.yml
@@ -27,6 +27,13 @@ inputs:
       config of the 2026-07-03 SocketRocket dual-copy regression).
     required: false
     default: auto
+  build-from-source:
+    description: >-
+      Force building ReactCore/ReactNativeDependencies from source, disabling the
+      prebuilt Maven fallback (RCT_USE_PREBUILT_RNCORE / RCT_USE_RN_DEP, which
+      otherwise default to '1'). Used by the from-source dynamic-frameworks lane.
+    required: false
+    default: false
 
 runs:
   using: composite
@@ -100,6 +107,17 @@ runs:
           export RCT_USE_LOCAL_RN_DEP="/tmp/third-party/ReactNativeDependencies${{ inputs.flavor }}.xcframework.tar.gz"
           export RCT_TESTONLY_RNCORE_TARBALL_PATH="/tmp/ReactCore/ReactCore${{ inputs.flavor }}.xcframework.tar.gz"
         fi
+        if [[ "${{ inputs.build-from-source }}" == "true" ]]; then
+          # Force building ReactCore/ReactNativeDependencies from source. Both
+          # RCT_USE_PREBUILT_RNCORE and RCT_USE_RN_DEP default to '1', so on a stable
+          # branch (where released Maven artifacts exist for the current version) the
+          # install would otherwise consume those prebuilts instead of building from
+          # source. That is wrong for the from-source dynamic-frameworks lane and also
+          # fails when a published artifact predates newer podspec layout changes
+          # (e.g. a React core tarball without ReactNativeHeaders.xcframework).
+          export RCT_USE_PREBUILT_RNCORE=0
+          export RCT_USE_RN_DEP=0
+        fi
 
         cd packages/rn-tester
 
diff --git a/.github/workflows/test-all.yml b/.github/workflows/test-all.yml
index a65219271e15..9579037a44ab 100644
--- a/.github/workflows/test-all.yml
+++ b/.github/workflows/test-all.yml
@@ -136,6 +136,7 @@ jobs:
         with:
           flavor: ${{ matrix.flavor }}
           use-frameworks: true
+          build-from-source: true
           run-unit-tests: false # tests for dynamic frameworks are already run in the test_ios_rntester job; this is to just a test build from source (no prebuilds)
 
   test_ios_rntester: