diff --git a/src/build.js b/src/build.js
index cc5ae5c..cb4c558 100644
--- a/src/build.js
+++ b/src/build.js
@@ -33,9 +33,18 @@ function runTrace(config, flowName, args, opts) {
       LOOPFINDER_ALLOW_NETWORK: opts.allowNetwork ? '1' : '',
       LOOPFINDER_ALLOW_SUBPROCESS: opts.allowSubprocess ? '1' : '',
     };
+    // A record left by an earlier build must not pass for this run's: if the script fails to start,
+    // the old record would be used as if it were new, and declared loops would look broken.
+    fs.rmSync(cache, { force: true });
     const r = spawnSync(py ? config.python : process.execPath,
       py ? [TRACE_PY, script, ...args.slice(1)] : ['-r', TRACE_JS, script, ...args.slice(1)],
       { cwd: config.root, env, encoding: 'utf8', timeout: opts.timeout || 600000 });
+    // Windows ships a `python` that only points to the Microsoft Store (exit 9009), so a missing
+    // interpreter shows up either as a spawn error or as that message.
+    if (py && !fs.existsSync(cache) && (r.error?.code === 'ENOENT' || r.status === 9009 || /Python was not found/.test(r.stderr || ''))) {
+      throw new Error(`could not start Python to trace ${args[0]} ("python": ${JSON.stringify(config.python)} in the config).\n`
+        + 'Install Python 3, or set "python" in loopfinder/config.json to the interpreter (for example "py" or a full path).');
+    }
     if (!fs.existsSync(cache)) {
       throw new Error(`trace did not finish: ${args.join(' ')} (exit ${r.status})\n${(r.stderr || '').slice(-2000)}`);
     }
diff --git a/test/core.test.js b/test/core.test.js
index af35f32..f8b9c6c 100644
--- a/test/core.test.js
+++ b/test/core.test.js
@@ -274,3 +274,23 @@ test('demo: builds the expected loops and leaves the workspace untouched', () =>
   const feedNode = graph.nodes.find(n => n.id === 'web:Garden feed');
   assert.ok(feedNode, 'the blocked fetch is still recorded and labelled');
 });
+
+test('build: a Python that cannot start is named, and an old record is not used in its place', () => {
+  const src = path.join(__dirname, '..', 'examples', 'demo');
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'loopfinder-nopy-'));
+  fs.cpSync(src, dir, { recursive: true });
+  const cfgFile = path.join(dir, 'loopfinder', 'config.json');
+  const cfg = JSON.parse(fs.readFileSync(cfgFile, 'utf8'));
+  const out = fs.mkdtempSync(path.join(os.tmpdir(), 'lf-nopy-out-'));
+  cfg.output = path.join(out, 'flow.json');
+  cfg.cache = path.join(out, 'traces');
+  cfg.python = 'loopfinder-no-such-python';
+  fs.writeFileSync(cfgFile, JSON.stringify(cfg));
+  // A record from an earlier build, as if Python had been there last time.
+  fs.mkdirSync(cfg.cache, { recursive: true });
+  const stale = path.join(cfg.cache, 'reading__weekly_summary.py.json');
+  fs.writeFileSync(stale, JSON.stringify({ reads: [], writes: [], network: [], subprocess: [] }));
+  assert.throws(() => build(loadConfig(cfgFile), { warn: () => {} }),
+    e => /could not start Python/.test(e.message) && e.message.includes('loopfinder-no-such-python'));
+  assert.ok(!fs.existsSync(stale), 'the old record was left to pass for this run');
+});