diff --git a/.github/actions/install-nixie/CHANGELOG.md b/.github/actions/install-nixie/CHANGELOG.md
new file mode 100644
index 00000000..be285b94
--- /dev/null
+++ b/.github/actions/install-nixie/CHANGELOG.md
@@ -0,0 +1,7 @@
+# Changelog
+
+## v1.0.0 (Unreleased)
+
+- Add pinned Nixie and Merman CLI installation.
+- Prefer `cargo binstall` for Merman and fall back to a locked source build.
+- Support version overrides for Nixie, Merman CLI, and Python.
diff --git a/.github/actions/install-nixie/README.md b/.github/actions/install-nixie/README.md
new file mode 100644
index 00000000..f4abd41d
--- /dev/null
+++ b/.github/actions/install-nixie/README.md
@@ -0,0 +1,62 @@
+# Install Nixie
+
+Install pinned Nixie and Merman CLI releases for Mermaid validation.
+
+The action installs Nixie through `uv` and its Merman rendering backend through
+Cargo. It uses `cargo binstall` when available and falls back to a locked
+`cargo install` build from crates.io.
+
+## Inputs
+
+| Name             | Description                              | Required | Default |
+| ---------------- | ---------------------------------------- | -------- | ------- |
+| `nixie-version`  | Nixie CLI version to install             | no       | `1.1.0` |
+| `merman-version` | Merman CLI version to install            | no       | `0.7.0` |
+| `python-version` | Python version used to install Nixie     | no       | `3.14`  |
+
+## Outputs
+
+| Name   | Description                                             |
+| ------ | ------------------------------------------------------- |
+| _None_ | The action emits no outputs.                            |
+
+## Usage
+
+```yaml
+- name: Set up Rust
+  uses: leynos/shared-actions/.github/actions/setup-rust@v1
+
+- name: Install Nixie
+  uses: leynos/shared-actions/.github/actions/install-nixie@v1
+
+- name: Validate Mermaid diagrams
+  run: nixie --renderer merman
+```
+
+To override the pinned versions:
+
+```yaml
+- uses: leynos/shared-actions/.github/actions/install-nixie@v1
+  with:
+    nixie-version: "1.1.0"
+    merman-version: "0.7.0"
+    python-version: "3.14"
+```
+
+## Behaviour
+
+- **Prerequisites**: `cargo` and `uv` must already be available on `PATH`.
+  The repository's `setup-rust` action provisions both tools and
+  `cargo-binstall`.
+- **Merman installation**: When `cargo binstall` is available, the action
+  installs the selected Merman release from a binary package with locked
+  metadata. Otherwise it builds the exact selected release from crates.io with
+  `cargo install --locked`.
+- **Nixie installation**: The action uses `uv tool install` with the selected
+  Python and exact Nixie release.
+- **Failure behaviour**: Missing prerequisites and failed installations stop
+  the action immediately with a non-zero exit status.
+
+## Release history
+
+See [CHANGELOG](CHANGELOG.md).
diff --git a/.github/actions/install-nixie/action.yml b/.github/actions/install-nixie/action.yml
new file mode 100644
index 00000000..4744b738
--- /dev/null
+++ b/.github/actions/install-nixie/action.yml
@@ -0,0 +1,44 @@
+name: Install Nixie
+description: Install pinned Nixie and Merman CLI releases for Mermaid validation
+inputs:
+  nixie-version:
+    description: Nixie CLI version to install
+    required: false
+    default: "1.1.0"
+  merman-version:
+    description: Merman CLI version to install
+    required: false
+    default: "0.7.0"
+  python-version:
+    description: Python version used by uv to install Nixie
+    required: false
+    default: "3.14"
+runs:
+  using: composite
+  steps:
+    - name: Install Nixie and Merman CLI
+      shell: bash
+      env:
+        NIXIE_VERSION: ${{ inputs.nixie-version }}
+        MERMAN_VERSION: ${{ inputs.merman-version }}
+        PYTHON_VERSION: ${{ inputs.python-version }}
+      run: |
+        set -euo pipefail
+
+        if ! command -v cargo >/dev/null 2>&1; then
+          echo "::error::cargo is required to install merman-cli" >&2
+          exit 1
+        fi
+        if ! command -v uv >/dev/null 2>&1; then
+          echo "::error::uv is required to install nixie-cli" >&2
+          exit 1
+        fi
+
+        if cargo binstall --version >/dev/null 2>&1; then
+          cargo binstall --no-confirm --locked "merman-cli@${MERMAN_VERSION}"
+        else
+          echo "cargo-binstall unavailable; building merman-cli from crates.io"
+          cargo install --locked merman-cli --version "=${MERMAN_VERSION}"
+        fi
+
+        uv tool install --python "${PYTHON_VERSION}" "nixie-cli==${NIXIE_VERSION}"
diff --git a/.github/actions/install-nixie/tests/test_action.py b/.github/actions/install-nixie/tests/test_action.py
new file mode 100644
index 00000000..7d332ebd
--- /dev/null
+++ b/.github/actions/install-nixie/tests/test_action.py
@@ -0,0 +1,152 @@
+"""Behavioural tests for the install-nixie composite action."""
+
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+import typing as typ
+from pathlib import Path
+
+import pytest
+import yaml
+
+ACTION_PATH = Path(__file__).resolve().parents[1] / "action.yml"
+
+
+def _load_action() -> dict[str, typ.Any]:
+    """Load the install-nixie action manifest."""
+    return yaml.safe_load(ACTION_PATH.read_text(encoding="utf-8"))
+
+
+def _install_script() -> str:
+    """Return the action's installation shell fragment."""
+    steps = _load_action()["runs"]["steps"]
+    assert len(steps) == 1, "install-nixie should have one atomic install step"
+    run_script = steps[0].get("run")
+    assert isinstance(run_script, str), "install step must define a shell script"
+    return run_script
+
+
+def _write_executable(path: Path, content: str) -> None:
+    """Write an executable command stub."""
+    path.write_text(content, encoding="utf-8")
+    path.chmod(0o755)
+
+
+def _run_install_script(
+    tmp_path: Path,
+    *,
+    binstall_available: bool,
+    include_cargo: bool = True,
+    include_uv: bool = True,
+) -> subprocess.CompletedProcess[str]:
+    """Execute the install fragment against deterministic command stubs."""
+    bash = shutil.which("bash")
+    if bash is None:
+        pytest.skip("bash not found on PATH")
+
+    stubs_dir = tmp_path / "stubs"
+    stubs_dir.mkdir()
+    calls_path = tmp_path / "calls"
+    if include_cargo:
+        binstall_status = 0 if binstall_available else 1
+        _write_executable(
+            stubs_dir / "cargo",
+            f"""#!/bin/bash
+set -euo pipefail
+if [ "${{1:-}}" = "binstall" ] && [ "${{2:-}}" = "--version" ]; then
+  exit {binstall_status}
+fi
+printf 'cargo' >> "$CALLS_PATH"
+printf ' <%s>' "$@" >> "$CALLS_PATH"
+printf '\n' >> "$CALLS_PATH"
+""",
+        )
+    if include_uv:
+        _write_executable(
+            stubs_dir / "uv",
+            """#!/bin/bash
+set -euo pipefail
+printf 'uv' >> "$CALLS_PATH"
+printf ' <%s>' "$@" >> "$CALLS_PATH"
+printf '\n' >> "$CALLS_PATH"
+""",
+        )
+
+    env = {
+        **os.environ,
+        "CALLS_PATH": calls_path.as_posix(),
+        "MERMAN_VERSION": "0.7.0",
+        "NIXIE_VERSION": "1.1.0",
+        "PATH": stubs_dir.as_posix(),
+        "PYTHON_VERSION": "3.14",
+    }
+    return subprocess.run(  # noqa: S603,TID251 - exercise the bash fragment.
+        [bash, "-c", _install_script()],
+        cwd=tmp_path,
+        env=env,
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+
+
+def test_manifest_exposes_pinned_version_inputs() -> None:
+    """The action should expose the reviewed Nixie toolchain pins."""
+    manifest = _load_action()
+
+    assert manifest["runs"]["using"] == "composite"
+    assert manifest["inputs"]["nixie-version"]["default"] == "1.1.0"
+    assert manifest["inputs"]["merman-version"]["default"] == "0.7.0"
+    assert manifest["inputs"]["python-version"]["default"] == "3.14"
+
+
+def test_install_script_prefers_cargo_binstall(tmp_path: Path) -> None:
+    """Merman should use a locked binary install when cargo-binstall exists."""
+    result = _run_install_script(tmp_path, binstall_available=True)
+
+    assert result.returncode == 0, result.stderr
+    assert (tmp_path / "calls").read_text(encoding="utf-8").splitlines() == [
+        "cargo <binstall> <--no-confirm> <--locked> <merman-cli@0.7.0>",
+        "uv <tool> <install> <--python> <3.14> <nixie-cli==1.1.0>",
+    ]
+
+
+def test_install_script_falls_back_to_cargo_install(tmp_path: Path) -> None:
+    """Merman should use a locked source build without cargo-binstall."""
+    result = _run_install_script(tmp_path, binstall_available=False)
+
+    assert result.returncode == 0, result.stderr
+    assert "cargo-binstall unavailable" in result.stdout
+    assert (tmp_path / "calls").read_text(encoding="utf-8").splitlines() == [
+        "cargo <install> <--locked> <merman-cli> <--version> <=0.7.0>",
+        "uv <tool> <install> <--python> <3.14> <nixie-cli==1.1.0>",
+    ]
+
+
+@pytest.mark.parametrize(
+    ("include_cargo", "include_uv", "expected_error"),
+    [
+        (False, True, "cargo is required to install merman-cli"),
+        (True, False, "uv is required to install nixie-cli"),
+    ],
+    ids=["missing-cargo", "missing-uv"],
+)
+def test_install_script_reports_missing_prerequisite(
+    tmp_path: Path,
+    *,
+    include_cargo: bool,
+    include_uv: bool,
+    expected_error: str,
+) -> None:
+    """Missing runner prerequisites should produce actionable errors."""
+    result = _run_install_script(
+        tmp_path,
+        binstall_available=False,
+        include_cargo=include_cargo,
+        include_uv=include_uv,
+    )
+
+    assert result.returncode == 1
+    assert expected_error in result.stderr
diff --git a/.gitignore b/.gitignore
index 4bb8ceaa..ae8f7f23 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,7 @@ lib-cov
 
 # Coverage directory used by tools like istanbul
 coverage
+.coverage
 *.lcov
 
 # nyc test coverage
diff --git a/README.md b/README.md
index 6f7f3772..d0b62306 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,7 @@ GitHub Actions
 | Export Cargo metadata     | `.github/actions/export-cargo-metadata`                                            | v1           |
 | Export Postgres URL       | `.github/actions/export-postgres-url`                                              | v1           |
 | Generate coverage         | `.github/actions/generate-coverage`                                                | v1           |
+| Install Nixie             | `.github/actions/install-nixie`                                                    | unreleased   |
 | Linux packages            | `.github/actions/linux-packages`                                                   | v1           |
 | macOS package             | `.github/actions/macos-package`                                                    | v1           |
 | Ratchet coverage          | `.github/actions/ratchet-coverage`                                                 | v1           |