diff --git a/.github/actions/python-prepare/action.yaml b/.github/actions/python-prepare/action.yaml
index 00224f21..6dd49f9f 100644
--- a/.github/actions/python-prepare/action.yaml
+++ b/.github/actions/python-prepare/action.yaml
@@ -22,20 +22,42 @@ inputs:
       Path to the pyproject.toml file, relative to the repository root.
       Defaults to `pyproject.toml`.
 
+outputs:
+  pip-extra-index-url:
+    description: |
+      The PIP_EXTRA_INDEX_URL value used to reach AWS CodeArtifact for this run
+      (empty string if no code-artefact-role was supplied). Requirements files
+      generated or installed by this action never have the index URL embedded
+      in them, so later steps needing CodeArtifact access (e.g. `python -m build`)
+      must set this as their own PIP_EXTRA_INDEX_URL env var.
+    value: ${{ steps.code-artifact-login.outputs.token != '' && format('https://aws:{0}@telicent-{1}.d.codeartifact.eu-west-2.amazonaws.com/pypi/telicent-code-artifacts/simple/', steps.code-artifact-login.outputs.token, steps.creds.outputs.aws-account-id) || '' }}
+
 runs:
   using: composite
   steps:
     - name: Checkout
       uses: actions/checkout@v6
+
     - name: Set up Python ${{ inputs.version }}
       uses: actions/setup-python@v5.6.0
       with:
         python-version: ${{ inputs.version }}
-    - name: Prepare pip and install pip-tools
-      shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
+
+    - name: Resolve requirements path
+      shell: bash
       run: |
-        python -m pip install --upgrade "pip<25.3"
-        python -m pip install pip-tools
+        req_dir=$(dirname "${{ inputs.pyproject-toml-path }}")
+        echo "REQUIREMENTS_PATH=$req_dir/requirements.txt" >> "$GITHUB_ENV"
+
+    - name: Prepare pip
+      shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
+      run: python -m pip install --upgrade "pip<25.3"
+
+    - name: Install pip-tools
+      if: hashFiles(env.REQUIREMENTS_PATH) == ''
+      shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
+      run: python -m pip install pip-tools
+
     - name: Configure CodeArtifact AWS credentials
       if: inputs.code-artefact-role != ''
       id: creds
@@ -44,6 +66,7 @@ runs:
         role-to-assume: ${{ inputs.code-artefact-role }}
         aws-region: eu-west-2
         mask-aws-account-id: true
+
     - name: CodeArtifact login
       id: code-artifact-login
       if: inputs.code-artefact-role != ''
@@ -51,12 +74,31 @@ runs:
       with:
         domain: telicent
         owner: "${{ steps.creds.outputs.aws-account-id }}"
+
+    - name: Warn if no committed requirements.txt was found
+      if: hashFiles(env.REQUIREMENTS_PATH) == ''
+      shell: bash
+      run: echo "::warning::No committed $REQUIREMENTS_PATH found - generating one from ${{ inputs.pyproject-toml-path }} on the fly. This project is out of date; commit a hash-pinned requirements.txt (pip-compile --generate-hashes) instead."
+
     - name: Generate requirements
+      if: hashFiles(env.REQUIREMENTS_PATH) == ''
       shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
-      run: python -m piptools compile ${{ inputs.extra-args }} -o requirements.txt ${{ inputs.pyproject-toml-path }}
+      run: python -m piptools compile --generate-hashes --no-emit-index-url ${{ inputs.extra-args }} -o ${{ env.REQUIREMENTS_PATH }} ${{ inputs.pyproject-toml-path }}
       env:
         PIP_EXTRA_INDEX_URL: ${{ steps.code-artifact-login.outputs.token != '' && format('https://aws:{0}@telicent-{1}.d.codeartifact.eu-west-2.amazonaws.com/pypi/telicent-code-artifacts/simple/', steps.code-artifact-login.outputs.token, steps.creds.outputs.aws-account-id) || '' }}
+
+    - name: Verify requirements are hash-pinned
+      if: hashFiles(env.REQUIREMENTS_PATH) != ''
+      shell: bash
+      run: |
+        if ! grep -q -- '--hash=' "$REQUIREMENTS_PATH"; then
+          echo "::error::$REQUIREMENTS_PATH exists but contains no --hash= entries. Committed requirements files must be generated with 'pip-compile --generate-hashes' for supply-chain integrity."
+          exit 1
+        fi
+
     - name: Install requirements
       if: inputs.install-requirements == 'true'
       shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
-      run: pip install -r requirements.txt
+      run: pip install --require-hashes -r ${{ env.REQUIREMENTS_PATH }}
+      env:
+        PIP_EXTRA_INDEX_URL: ${{ steps.code-artifact-login.outputs.token != '' && format('https://aws:{0}@telicent-{1}.d.codeartifact.eu-west-2.amazonaws.com/pypi/telicent-code-artifacts/simple/', steps.code-artifact-login.outputs.token, steps.creds.outputs.aws-account-id) || '' }}
diff --git a/.github/actions/python-prepare/test-fixtures/generate-only/pyproject.toml b/.github/actions/python-prepare/test-fixtures/generate-only/pyproject.toml
new file mode 100644
index 00000000..b536a7a0
--- /dev/null
+++ b/.github/actions/python-prepare/test-fixtures/generate-only/pyproject.toml
@@ -0,0 +1,8 @@
+[project]
+name = "test-fixture-generate-only"
+version = "0.0.1"
+dependencies = ["certifi==2024.8.30"]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
diff --git a/.github/actions/python-prepare/test-fixtures/missing-hashes/pyproject.toml b/.github/actions/python-prepare/test-fixtures/missing-hashes/pyproject.toml
new file mode 100644
index 00000000..48dd02fe
--- /dev/null
+++ b/.github/actions/python-prepare/test-fixtures/missing-hashes/pyproject.toml
@@ -0,0 +1,8 @@
+[project]
+name = "test-fixture-missing-hashes"
+version = "0.0.1"
+dependencies = ["certifi==2024.8.30"]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
diff --git a/.github/actions/python-prepare/test-fixtures/missing-hashes/requirements.txt b/.github/actions/python-prepare/test-fixtures/missing-hashes/requirements.txt
new file mode 100644
index 00000000..8cbfdc93
--- /dev/null
+++ b/.github/actions/python-prepare/test-fixtures/missing-hashes/requirements.txt
@@ -0,0 +1,6 @@
+#
+# This requirements.txt was committed without hashes (e.g. hand-written or
+# generated with plain `pip-compile`, not `pip-compile --generate-hashes`).
+# It is used as a fixture to verify python-prepare rejects it.
+#
+certifi==2024.8.30
diff --git a/.github/actions/python-prepare/test-fixtures/valid-hashes/pyproject.toml b/.github/actions/python-prepare/test-fixtures/valid-hashes/pyproject.toml
new file mode 100644
index 00000000..9f500dca
--- /dev/null
+++ b/.github/actions/python-prepare/test-fixtures/valid-hashes/pyproject.toml
@@ -0,0 +1,8 @@
+[project]
+name = "test-fixture-valid-hashes"
+version = "0.0.1"
+dependencies = ["certifi==2024.8.30"]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
diff --git a/.github/actions/python-prepare/test-fixtures/valid-hashes/requirements.txt b/.github/actions/python-prepare/test-fixtures/valid-hashes/requirements.txt
new file mode 100644
index 00000000..5aaef15e
--- /dev/null
+++ b/.github/actions/python-prepare/test-fixtures/valid-hashes/requirements.txt
@@ -0,0 +1,10 @@
+#
+# This file is autogenerated by pip-compile with Python 3.12
+# by the following command:
+#
+#    pip-compile --generate-hashes --no-emit-index-url --output-file=requirements.txt pyproject.toml
+#
+certifi==2024.8.30 \
+    --hash=sha256:922820b53db7a7257ffbda3f597266d435245903d80737e34f8a45ff3e3230d8 \
+    --hash=sha256:bec941d2aa8195e248a60b31ff9f0558284cf01a52591ceda73ea9afffd69fd9
+    # via test-fixture-valid-hashes (pyproject.toml)
diff --git a/.github/actions/python-test/action.yaml b/.github/actions/python-test/action.yaml
index e79cf93b..dc3e973d 100644
--- a/.github/actions/python-test/action.yaml
+++ b/.github/actions/python-test/action.yaml
@@ -35,6 +35,7 @@ runs:
   using: composite
   steps:
     - name: Prepare Python
+      id: prepare
       uses: telicent-oss/shared-workflows/.github/actions/python-prepare@main
       with:
         code-artefact-role: ${{ inputs.code-artefact-role}}
@@ -51,7 +52,13 @@ runs:
     - name: Unit tests
       shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
       run: ${{ inputs.test-command }}
+    - name: Install build
+      if: ${{ inputs.skip-build == 'false' }}
+      shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
+      run: python -m pip install build
     - name: Build
       if: ${{ inputs.skip-build == 'false' }}
       shell: ${{ runner.os == 'Windows' && 'pwsh' || 'bash' }}
-      run: python -m build
\ No newline at end of file
+      run: python -m build
+      env:
+        PIP_EXTRA_INDEX_URL: ${{ steps.prepare.outputs.pip-extra-index-url }}
\ No newline at end of file
diff --git a/.github/workflows/test-changes-to-python-prepare-action.yaml b/.github/workflows/test-changes-to-python-prepare-action.yaml
new file mode 100644
index 00000000..d59ee6e9
--- /dev/null
+++ b/.github/workflows/test-changes-to-python-prepare-action.yaml
@@ -0,0 +1,77 @@
+name: Test Changes to Python Prepare Action
+
+on:
+  push:
+    paths:
+      - .github/actions/python-prepare/**
+      - .github/workflows/test-changes-to-python-prepare-action.yaml
+  workflow_dispatch:
+
+jobs:
+  no-requirements-file-generates-hashed-one:
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
+      fail-fast: false
+    runs-on: ${{ matrix.os }}
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+      - name: Prepare Python
+        uses: ./.github/actions/python-prepare
+        with:
+          version: "3.12"
+          install-requirements: "true"
+          pyproject-toml-path: .github/actions/python-prepare/test-fixtures/generate-only/pyproject.toml
+      - name: "Fail if requirements.txt was not generated"
+        shell: bash
+        run: test -f .github/actions/python-prepare/test-fixtures/generate-only/requirements.txt
+      - name: "Fail if generated requirements.txt has no hashes"
+        shell: bash
+        run: grep -q -- '--hash=' .github/actions/python-prepare/test-fixtures/generate-only/requirements.txt
+
+  committed-hashed-file-is-used-as-is:
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
+      fail-fast: false
+    runs-on: ${{ matrix.os }}
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+      - name: Prepare Python
+        uses: ./.github/actions/python-prepare
+        with:
+          version: "3.12"
+          install-requirements: "true"
+          pyproject-toml-path: .github/actions/python-prepare/test-fixtures/valid-hashes/pyproject.toml
+      - name: "Fail if the committed requirements.txt was regenerated/modified"
+        shell: bash
+        run: git diff --exit-code -- .github/actions/python-prepare/test-fixtures/valid-hashes/requirements.txt
+
+  committed-unhashed-file-is-rejected:
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
+      fail-fast: false
+    runs-on: ${{ matrix.os }}
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+      - name: "Prepare Python (expected to fail)"
+        id: prepare
+        continue-on-error: true
+        uses: ./.github/actions/python-prepare
+        with:
+          version: "3.12"
+          install-requirements: "true"
+          pyproject-toml-path: .github/actions/python-prepare/test-fixtures/missing-hashes/pyproject.toml
+      - name: "Fail the job if python-prepare did not reject the un-hashed file"
+        if: steps.prepare.outcome != 'failure'
+        shell: bash
+        run: |
+          echo "Expected python-prepare to fail on a committed requirements.txt with no --hash= entries, but it succeeded"
+          exit 1