diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 587fffa058..9705c44642 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,8 +33,12 @@ jobs:
           PIP_NO_CACHE_DIR: 1
         run: |
           sudo apt-get update && sudo apt-get install -y build-essential cmake
-          python -m pip install -U pip pytest
-          pip install -e . --no-cache-dir
+          python -m pip install -U pip "setuptools<81" wheel numpy pytest
+          # CPU-only torch: the CUDA stack no longer fits the runner's disk
+          # quota. --no-build-isolation then reuses it for the build instead
+          # of resolving a second copy. See tests/smoke_tests/Dockerfile.
+          pip install --index-url https://download.pytorch.org/whl/cpu torch
+          pip install -e . --no-build-isolation --no-cache-dir
           python setup.py build_ext --inplace --force
 
       - name: Unit tests
@@ -70,8 +74,12 @@ jobs:
           PIP_NO_CACHE_DIR: 1
         run: |
           sudo apt-get update && sudo apt-get install -y build-essential cmake xvfb libgl1-mesa-dev libegl1-mesa-dev libx11-dev libxcursor-dev libxrandr-dev libxinerama-dev libxi-dev libxxf86vm-dev
-          python -m pip install -U pip
-          pip install -e . --no-cache-dir
+          python -m pip install -U pip "setuptools<81" wheel numpy
+          # CPU-only torch: the CUDA stack no longer fits the runner's disk
+          # quota. --no-build-isolation then reuses it for the build instead
+          # of resolving a second copy. See tests/smoke_tests/Dockerfile.
+          pip install --index-url https://download.pytorch.org/whl/cpu torch
+          pip install -e . --no-build-isolation --no-cache-dir
           python setup.py build_ext --inplace --force
 
       - name: C drive tests
@@ -103,8 +111,12 @@ jobs:
           PIP_NO_CACHE_DIR: 1
         run: |
           sudo apt-get update && sudo apt-get install -y build-essential cmake
-          python -m pip install -U pip pytest jupytext nbclient ipykernel
-          pip install -e . --no-cache-dir
+          python -m pip install -U pip "setuptools<81" wheel numpy pytest jupytext nbclient ipykernel
+          # CPU-only torch: the CUDA stack no longer fits the runner's disk
+          # quota. --no-build-isolation then reuses it for the build instead
+          # of resolving a second copy. See tests/smoke_tests/Dockerfile.
+          pip install --index-url https://download.pytorch.org/whl/cpu torch
+          pip install -e . --no-build-isolation --no-cache-dir
           python setup.py build_ext --inplace --force
 
       - name: Run notebooks
@@ -151,8 +163,12 @@ jobs:
           PIP_NO_CACHE_DIR: 1
         run: |
           sudo apt-get update && sudo apt-get install -y build-essential cmake libgl1-mesa-dev libegl1-mesa-dev libx11-dev libxcursor-dev libxrandr-dev libxinerama-dev libxi-dev libxxf86vm-dev
-          python -m pip install -U pip
-          pip install -e . --no-cache-dir
+          python -m pip install -U pip "setuptools<81" wheel numpy
+          # CPU-only torch: the CUDA stack no longer fits the runner's disk
+          # quota. --no-build-isolation then reuses it for the build instead
+          # of resolving a second copy. See tests/smoke_tests/Dockerfile.
+          pip install --index-url https://download.pytorch.org/whl/cpu torch
+          pip install -e . --no-build-isolation --no-cache-dir
           python setup.py build_ext --inplace --force
 
       - name: Test simulator performance
diff --git a/.github/workflows/install.yml b/.github/workflows/install.yml
index 7b5a5c4703..25bde9f831 100644
--- a/.github/workflows/install.yml
+++ b/.github/workflows/install.yml
@@ -38,7 +38,19 @@ jobs:
           sudo apt-get clean
           sudo rm -rf ~/.cache/pip /tmp/* /var/tmp/*
 
-      - name: Install pufferlib
+      - name: Install pufferlib (Linux)
+        if: runner.os == 'Linux'
+        run: |
+          python -m pip install -U pip "setuptools<81" wheel numpy
+          # CPU-only torch. The CUDA stack is ~3 GB of wheels and no longer
+          # fits the runner's disk quota; see the Install steps in ci.yml.
+          pip install --index-url https://download.pytorch.org/whl/cpu torch
+          pip install -e . --no-build-isolation
+
+      - name: Install pufferlib (macOS)
+        if: runner.os != 'Linux'
         run: |
           python -m pip install -U pip
+          # Deliberately a plain install: macOS wheels are already CPU-only, so
+          # this keeps the default PEP 517 build-isolation path under test.
           pip install -e .