diff --git a/Makefile b/Makefile
index 207384a..702467c 100644
--- a/Makefile
+++ b/Makefile
@@ -66,7 +66,7 @@ bench-check: ## Type-check benchmark code without running it
 	fi
 
 .PHONY: check
-check: format-check lint test bench-check ## Run all checks (format-check, lint, test, bench-check)
+check: format-check lint test bench-check viewer-test ## Run all checks (format-check, lint, test, bench-check, viewer-test)
 
 .PHONY: clean
 clean: ## Remove build output
@@ -99,6 +99,14 @@ export-fixtures: ## Regenerate plan JSON for every tools/exporter/examples/*.sce
 examples: export-fixtures ## Regenerate fixtures from scenarios and run them through plan-runner against their oracles.
 	@cargo test -p plan-runner --test examples
 
+.PHONY: viewer-test
+viewer-test: ## Check the plan viewer's JS engine against every fixture oracle (needs Node)
+	@if ! command -v node >/dev/null 2>&1; then \
+		echo "node not found. Skipping plan viewer engine check."; \
+	else \
+		node tools/plan-viewer/test.js; \
+	fi
+
 VIEWER_PORT ?= 8000
 
 .PHONY: viewer
diff --git a/tools/plan-viewer/index.html b/tools/plan-viewer/index.html
index 3965e01..6ccbab0 100644
Binary files a/tools/plan-viewer/index.html and b/tools/plan-viewer/index.html differ
diff --git a/tools/plan-viewer/test.js b/tools/plan-viewer/test.js
new file mode 100644
index 0000000..4dc3dd1
--- /dev/null
+++ b/tools/plan-viewer/test.js
@@ -0,0 +1,87 @@
+#!/usr/bin/env node
+// Parity check for the plan viewer's JavaScript engine.
+//
+// Extracts the engine script block from index.html and runs every fixture
+// in crates/plan-runner/fixtures/ through it, requiring the result to match
+// the fixture's expected_bindings oracle, exactly as the Rust runner does.
+// Also checks the provenance helpers: every output row of every node must
+// have at least one contributing row in each of its inputs.
+//
+// The Rust crates remain the correctness oracle; this test only catches the
+// viewer drifting away from them.
+
+"use strict";
+
+const fs = require("fs");
+const path = require("path");
+const vm = require("vm");
+
+const html = fs.readFileSync(path.join(__dirname, "index.html"), "utf8");
+const match = html.match(/<script id="engine">([\s\S]*?)<\/script>/);
+if (!match) {
+  console.error("engine script block not found in index.html");
+  process.exit(1);
+}
+const sandbox = { module: { exports: {} }, console };
+vm.runInNewContext(match[1], sandbox, { filename: "index.html#engine" });
+const engine = sandbox.module.exports;
+
+const fixturesDir = path.join(__dirname, "..", "..", "crates", "plan-runner", "fixtures");
+const files = fs.readdirSync(fixturesDir).filter((f) => f.endsWith(".json")).sort();
+if (files.length === 0) {
+  console.error("no fixtures found in " + fixturesDir);
+  process.exit(1);
+}
+
+// Every output row must have at least one contributing row in each input.
+function checkProvenance(plan, tables, results) {
+  const byId = new Map(plan.query.nodes.map((n) => [n.id, n]));
+  for (const node of plan.query.nodes) {
+    const rel = results.get(node.id);
+    for (const row of rel.rows) {
+      const bindings = {};
+      rel.columns.forEach((c, i) => { bindings[c] = row[i]; });
+      if (node.action.scan) {
+        const scan = node.action.scan;
+        const matched = engine.scanMatches(tables.get(scan.table), scan.columns, bindings);
+        if (matched.length === 0) {
+          throw new Error("node " + node.id + ": output row has no matching fact row");
+        }
+      } else {
+        for (const id of [node.action.join.left, node.action.join.right]) {
+          const matched = engine.filterByBindings(results.get(id), bindings);
+          if (matched.length === 0) {
+            throw new Error(
+              "node " + node.id + ": output row has no contributing row in node " + id);
+          }
+        }
+      }
+    }
+  }
+}
+
+let failures = 0;
+for (const file of files) {
+  const plan = JSON.parse(fs.readFileSync(path.join(fixturesDir, file), "utf8"));
+  try {
+    const tables = engine.buildTables(plan);
+    const results = engine.executePlan(plan, tables);
+    const verdict = engine.verifyBindings(plan, results.get(plan.query.root));
+    if (verdict.status !== "match") {
+      throw new Error("bindings " + verdict.status +
+        (verdict.message ? ": " + verdict.message : ""));
+    }
+    checkProvenance(plan, tables, results);
+    engine.layoutQuery(plan.query);
+    console.log("ok   " + file);
+  } catch (err) {
+    failures += 1;
+    console.log("FAIL " + file + " (" + err.message + ")");
+  }
+}
+
+if (failures > 0) {
+  console.error(failures + " of " + files.length + " fixtures failed");
+  process.exit(1);
+}
+console.log("all " + files.length + " fixtures match their oracles");