diff --git a/.github/workflows/inspect-r-api-update.yml b/.github/workflows/inspect-r-api-update.yml
index dc0db3d4..03f43de9 100644
--- a/.github/workflows/inspect-r-api-update.yml
+++ b/.github/workflows/inspect-r-api-update.yml
@@ -72,7 +72,11 @@ jobs:
       - name: Install Python build and test tools
         run: |
           python -m pip install -U pip
-          python -m pip install build scikit-build-core nanobind pytest ruff mypy numpy scipy
+          # numpy pinned <2.5: the R parity cache keys hash test inputs generated via
+          # multivariate_normal (LAPACK SVD). numpy 2.5.x bundles an OpenBLAS whose SVD
+          # kernels differ on some runner CPUs, changing inputs bit-for-bit and causing
+          # cache misses. Re-evaluate at the next full live-R cache regeneration.
+          python -m pip install build scikit-build-core nanobind pytest ruff mypy "numpy<2.5" scipy
           python -m pip install hypothesis pytest-benchmark pytest-xdist
 
       - name: Plan R API parity review
diff --git a/.github/workflows/native-backend-ci.yml b/.github/workflows/native-backend-ci.yml
index 67f09d6f..8ed7976c 100644
--- a/.github/workflows/native-backend-ci.yml
+++ b/.github/workflows/native-backend-ci.yml
@@ -25,7 +25,11 @@ jobs:
       - name: Install build tools
         run: |
           python -m pip install -U pip
-          python -m pip install build scikit-build-core nanobind pytest ruff mypy numpy scipy
+          # numpy pinned <2.5: the R parity cache keys hash test inputs generated via
+          # multivariate_normal (LAPACK SVD). numpy 2.5.x bundles an OpenBLAS whose SVD
+          # kernels differ on some runner CPUs, changing inputs bit-for-bit and causing
+          # cache misses. Re-evaluate at the next full live-R cache regeneration.
+          python -m pip install build scikit-build-core nanobind pytest ruff mypy "numpy<2.5" scipy
           python -m pip install hypothesis pytest-benchmark pytest-xdist
 
       - name: Install package editable
diff --git a/.github/workflows/parity-autofix.yml b/.github/workflows/parity-autofix.yml
index 004ad94f..861211ca 100644
--- a/.github/workflows/parity-autofix.yml
+++ b/.github/workflows/parity-autofix.yml
@@ -86,7 +86,11 @@ jobs:
       - name: Install Python build and test tools
         run: |
           python -m pip install -U pip
-          python -m pip install build scikit-build-core nanobind pytest ruff mypy numpy scipy
+          # numpy pinned <2.5: the R parity cache keys hash test inputs generated via
+          # multivariate_normal (LAPACK SVD). numpy 2.5.x bundles an OpenBLAS whose SVD
+          # kernels differ on some runner CPUs, changing inputs bit-for-bit and causing
+          # cache misses. Re-evaluate at the next full live-R cache regeneration.
+          python -m pip install build scikit-build-core nanobind pytest ruff mypy "numpy<2.5" scipy
           python -m pip install hypothesis pytest-benchmark pytest-xdist
 
       - name: Install package editable
diff --git a/.github/workflows/sync-from-nns-core.yml b/.github/workflows/sync-from-nns-core.yml
index d0daa2ee..a0927c5b 100644
--- a/.github/workflows/sync-from-nns-core.yml
+++ b/.github/workflows/sync-from-nns-core.yml
@@ -69,7 +69,11 @@ jobs:
       - name: Install build tools
         run: |
           python -m pip install -U pip
-          python -m pip install build scikit-build-core nanobind pytest ruff mypy numpy scipy
+          # numpy pinned <2.5: the R parity cache keys hash test inputs generated via
+          # multivariate_normal (LAPACK SVD). numpy 2.5.x bundles an OpenBLAS whose SVD
+          # kernels differ on some runner CPUs, changing inputs bit-for-bit and causing
+          # cache misses. Re-evaluate at the next full live-R cache regeneration.
+          python -m pip install build scikit-build-core nanobind pytest ruff mypy "numpy<2.5" scipy
           python -m pip install hypothesis pytest-benchmark pytest-xdist
 
       - name: Vendor NNS-core snapshot