diff --git a/.github/workflows/nss_install_smoke.yml b/.github/workflows/nss_install_smoke.yml
new file mode 100644
index 000000000..25653c869
--- /dev/null
+++ b/.github/workflows/nss_install_smoke.yml
@@ -0,0 +1,62 @@
+name: NSS install smoke
+
+# Phase 4 of nss_first_class_sampler — verifies that `pip install autofit[nss]`
+# remains a single safe command. Runs on every PR (so we catch local regressions
+# in pyproject.toml before merging) and on a weekly cron (so we catch upstream
+# regressions in handley-lab/blackjax or yallup/nss that drift past the pinned
+# SHAs).
+
+on:
+  pull_request:
+  schedule:
+    # Sundays at 03:00 UTC — quiet hours, catches upstream drift weekly.
+    - cron: '0 3 * * 0'
+
+jobs:
+  fresh_venv_install:
+    name: Fresh-venv install of autofit[nss]
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - name: Checkout PyAutoConf
+        uses: actions/checkout@v4
+        with:
+          repository: PyAutoLabs/PyAutoConf
+          path: PyAutoConf
+      - name: Checkout PyAutoFit
+        uses: actions/checkout@v4
+        with:
+          path: PyAutoFit
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - name: Upgrade pip
+        run: python -m pip install --upgrade pip
+      - name: Install PyAutoConf
+        run: python -m pip install -e PyAutoConf
+      - name: Install PyAutoFit with [nss] extra
+        run: python -m pip install -e PyAutoFit[nss]
+      - name: Import smoke
+        run: |
+          python -c "
+          import autofit as af
+          import nss.ns
+          import blackjax
+
+          # Confirm the right blackjax fork landed (mainline does not have
+          # blackjax.ns.adaptive — this attribute access is the canonical
+          # check for the handley-lab fork).
+          import blackjax.ns.adaptive
+          assert hasattr(blackjax.ns.adaptive, 'init'), (
+              'blackjax.ns.adaptive.init missing — wrong blackjax installed?'
+          )
+
+          # Confirm af.NSS instantiates without ImportError.
+          search = af.NSS(n_live=10, num_mcmc_steps=2, num_delete=5, termination=-1.0)
+          assert search.checkpoint_interval == 100
+
+          print('NSS install smoke: OK')
+          print(f'  blackjax version: {blackjax.__version__}')
+          print(f'  nss installed at: {nss.__file__}')
+          "
diff --git a/autofit/non_linear/search/nest/nss/search.py b/autofit/non_linear/search/nest/nss/search.py
index 21a668439..c91919ef5 100644
--- a/autofit/non_linear/search/nest/nss/search.py
+++ b/autofit/non_linear/search/nest/nss/search.py
@@ -240,11 +240,11 @@ def __init__(
 
         if not _HAS_NSS:
             raise ImportError(
-                "af.NSS requires the optional `nss` package. Install via\n"
-                "    pip install git+https://github.com/yallup/nss.git\n"
-                "(Phase 4 of the nss_first_class_sampler roadmap will ship a\n"
-                "`pip install autofit[nss]` extra — track the progress in\n"
-                "PyAutoPrompt/autofit/nss_install_simplification.md.)"
+                "af.NSS requires the optional `nss` package and the matching "
+                "`handley-lab/blackjax` fork. Install via:\n"
+                "    pip install autofit[nss]\n"
+                "The extra pins specific upstream commits — see PyAutoFit's "
+                "pyproject.toml `[project.optional-dependencies] nss` entry."
             )
 
         super().__init__(
diff --git a/pyproject.toml b/pyproject.toml
index 01a60e96b..94e70813d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -76,6 +76,24 @@ optional = [
     "nautilus-sampler==1.0.5",
     "zeus-mcmc==2.5.4",
 ]
+# Phase 4 of nss_first_class_sampler — single safe install for `af.NSS`.
+# Pins the `handley-lab/blackjax` fork (BSD-3-Clause) which carries the
+# `blackjax.ns.adaptive.init` entrypoint that mainline blackjax lacks,
+# plus `yallup/nss` (BSD-3-Clause) on top. Both pinned to specific SHAs
+# so installs are reproducible. `fastprogress<1.1` keeps `fastprogress`
+# from pulling `python-fasthtml` (1.1.5 transitively requires it).
+# Re-pin SHAs periodically — see `autofit/install_nss.py` for the manual
+# fallback when pip's resolver cannot sequence the URL deps cleanly.
+nss = [
+    "fastprogress<1.1",
+    # blackjax pinned to ef45acd2 (May 2026 "Merge pull request #60 — double_compile").
+    # Locally-validated against the entire Phase 1-3 nss_first_class_sampler work.
+    # HEAD as of pin time (5dbf89a9) introduced `numpy>=1.25` which conflicts with
+    # autofit's `anesthetic==2.8.14` (numpy<2.0). Bump only after both pins move
+    # together — see PyAutoPrompt/issued/nss_install_simplification.md notes #3-4.
+    "blackjax @ git+https://github.com/handley-lab/blackjax.git@ef45acd2f2fa0cca15adbdcd3ff7cb3a98987cb5",
+    "nss @ git+https://github.com/yallup/nss.git@69159b0f4a3a53123b9eec7df91e4ed3885e4dc4",
+]
 docs=[
     "sphinx",
     "furo",