diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 474d69c..0948be7 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -102,15 +102,44 @@ jobs:
     continue-on-error: true
     steps:
       - uses: actions/checkout@v7
+      # PyBaMM main is a uv workspace that also holds the solvers (packages/pybammsolvers).
+      # Their version is only bumped at a release, so the PyPI wheel cannot be told apart
+      # from main's source, which may need it (main called a solver function that no
+      # release had). So both are built from main's source, as PyBaMM's own tests do.
+      - uses: actions/checkout@v7
+        with:
+          repository: pybamm-team/PyBaMM
+          ref: main
+          path: pybamm-main
+          submodules: recursive
+          # pybamm's version comes from its git tags: the history without file contents
+          fetch-depth: 0
+          filter: blob:none
+          persist-credentials: false
       - uses: astral-sh/setup-uv@v10.3.2
+      # Building SUNDIALS and SuiteSparse takes most of the time. They are built into
+      # packages/pybammsolvers/.idaklu, which the solver module finds through an absolute
+      # RPATH, so that folder is cached, keyed on what it is built from: the submodule pins
+      # and the build script. On a cache hit only the solver module itself is compiled.
+      - name: Solver libraries cache key
+        id: solver-libraries
+        working-directory: pybamm-main/packages/pybammsolvers
+        run: >-
+          echo "key=idaklu-${{ runner.os }}-$(git rev-parse HEAD:./sundials
+          HEAD:./SuiteSparse HEAD:./install_KLU_Sundials.py | sha256sum | cut -c1-16)"
+          >> "$GITHUB_OUTPUT"
+      - uses: actions/cache@v6
+        with:
+          path: pybamm-main/packages/pybammsolvers/.idaklu
+          key: ${{ steps.solver-libraries.outputs.key }}
+      # The compilers and BLAS that PyBaMM's tests install
+      - run: sudo apt-get update && sudo apt-get install -y gfortran libopenblas-dev
       - run: uv sync
-      # PyBaMM main is a uv workspace (packages/pybamm, packages/pybammsolvers, ...); install
-      # the pybamm package only. Its workspace source for pybammsolvers would compile the
-      # solvers (SuiteSparse, SUNDIALS, which need BLAS), so take pybammsolvers from PyPI
-      # instead. If main ever requires an unreleased pybammsolvers, build it with BLAS.
+      # Without the workspace sources: through them pybamm pulls in pybammsolvers as an
+      # editable install, which rebuilds itself on import, without its build tools
       - run: >-
-          uv pip install --no-sources-package pybammsolvers
-          "pybamm @ git+https://github.com/pybamm-team/PyBaMM.git@main#subdirectory=packages/pybamm"
+          uv pip install --no-sources ./pybamm-main/packages/pybammsolvers
+          ./pybamm-main/packages/pybamm
       - run: uv run --no-sync pytest
 
   changelog: