diff --git a/.github/workflows/pplns-parse-seedpin.yml b/.github/workflows/pplns-parse-seedpin.yml
new file mode 100644
index 000000000..5030319a5
--- /dev/null
+++ b/.github/workflows/pplns-parse-seedpin.yml
@@ -0,0 +1,46 @@
+name: PPLNS parse seed-pin guard
+
+# Permanent regression guard for the PPLNS parse property suite.
+#
+# The random-seeded web-static job (build.yml) can pass even when a specific
+# fast-check sequence still reproduces a bug, because it never replays that
+# sequence. This job pins seed -1679627146 — the counterexample that found
+# the non-finite legacy bare-number regression (input [Infinity] ->
+# totalPrimary Infinity, fixed in PR #51) — so the exact failing path is
+# re-exercised deterministically on every change to the parser or its tests.
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - "web-static/sharechain-explorer/**"
+      - ".github/workflows/pplns-parse-seedpin.yml"
+  pull_request:
+    branches: [master]
+    paths:
+      - "web-static/sharechain-explorer/**"
+      - ".github/workflows/pplns-parse-seedpin.yml"
+
+jobs:
+  seedpin:
+    name: PPLNS parse seed-pin replay
+    runs-on: ubuntu-24.04
+    defaults:
+      run:
+        working-directory: web-static/sharechain-explorer
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v5
+        with:
+          node-version: "22"
+          cache: npm
+          cache-dependency-path: web-static/sharechain-explorer/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Replay pinned seed -1679627146 against PPLNS parse property suite
+        run: |
+          node --import tsx --import ./.ci-seed-pin.mjs \
+            --test "tests/unit/pplns-parse-properties.test.ts"
diff --git a/web-static/sharechain-explorer/.ci-seed-pin.mjs b/web-static/sharechain-explorer/.ci-seed-pin.mjs
new file mode 100644
index 000000000..81352119d
--- /dev/null
+++ b/web-static/sharechain-explorer/.ci-seed-pin.mjs
@@ -0,0 +1,12 @@
+// CI-only fast-check seed pin for the PPLNS parse property suite.
+//
+// Replays seed -1679627146 — the counterexample that surfaced the
+// non-finite legacy bare-number regression (shrunk input [Infinity],
+// totalPrimary -> Infinity). Fixed in PR #51 (parseLegacyShape now routes
+// the bare-number branch through num()); this preload makes that exact
+// path part of permanent CI, so a random-seeded run can never again pass
+// while the pinned sequence still fails.
+//
+// Loaded via: node --import ./.ci-seed-pin.mjs  (see pplns-parse-seedpin.yml)
+import fc from "fast-check";
+fc.configureGlobal({ ...fc.readConfigureGlobal(), seed: -1679627146 });