diff --git a/package.json b/package.json
index 0033c8ad..d2e73fb5 100644
--- a/package.json
+++ b/package.json
@@ -31,6 +31,7 @@
     "@types/markdown-it": "^14.2.0",
     "@types/turndown": "^5.0.6",
     "autoprefixer": "^10.4.27",
+    "playwright": "^1.64.1",
     "tailwindcss": "^3.4.19",
     "typescript": "^5.9.3",
     "vite": "^7.3.1"
diff --git a/scripts/filter_builder_harness.js b/scripts/filter_builder_harness.js
new file mode 100644
index 00000000..d9c174e0
--- /dev/null
+++ b/scripts/filter_builder_harness.js
@@ -0,0 +1,96 @@
+// Filter-builder UI regression harness (faceted plan Phase 9 follow-up).
+//
+// Drives the real bookshelf in headless Chromium: opens the filter
+// drawer, types an author value, and sweeps every author operator,
+// asserting the live count fetch behaves (200s, correct numbers, no
+// console errors). Born from the mis-keyed operator table that made
+// every non-self-named operator (doesn't contain, is empty, at least…)
+// a silent 400 — the class of bug only a real browser catches.
+//
+// Prerequisites: dev stack up (make dev / docker compose), playwright
+// installed (cd web && npm i -D playwright && npx playwright install
+// chromium — the repo root's devDependency already carries it), and a
+// reachable server.
+//
+// Usage:  NODE_PATH=<repo>/node_modules node scripts/filter_builder_harness.js
+// Env:    HARNESS_BASE (default http://localhost:8765), HARNESS_LOGIN,
+//         HARNESS_PASSWORD.
+const { chromium } = require("playwright");
+
+const BASE = process.env.HARNESS_BASE || "http://localhost:8765";
+const LOGIN = process.env.HARNESS_LOGIN || "testuser@tests.bookhoard.internal";
+const PASSWORD = process.env.HARNESS_PASSWORD || "Test@Pass123!";
+const AUTHOR = "Andrzej Sapkowski";
+const OPS_SWEEP = ["doesn't contain", "is", "isn't", "is empty", "contains"];
+
+
+(async () => {
+  const browser = await chromium.launch();
+  const context = await browser.newContext();
+  const page = await context.newPage();
+
+  const events = [];
+  page.on("console", (m) => {
+    if (m.type() === "error" || m.type() === "warning") {
+      events.push(`[console.${m.type()}] ${m.text()}`);
+    }
+  });
+  page.on("pageerror", (e) => events.push(`[pageerror] ${e.message}`));
+  page.on("response", async (r) => {
+    if (r.url().includes("filter-count")) {
+      let body = "";
+      try { body = (await r.text()).slice(0, 120); } catch {}
+      events.push(`[net] filter-count ${r.status()} ${body}`);
+    }
+  });
+
+  const login = await context.request.post(`${BASE}/api/auth/login`, {
+    data: { login: LOGIN, password: PASSWORD },
+  });
+  const loginBody = await login.json();
+  await context.addInitScript((t) => localStorage.setItem("token", t), loginBody.access_token);
+  console.log("login:", login.status());
+
+  await page.goto(`${BASE}/bookshelf`);
+  await page.waitForLoadState("networkidle");
+  await page.evaluate(() => {
+    const el = document.getElementById("library-select");
+    const empty = [...el.options].find((o) => o.value === "");
+    if (empty) {
+      el.value = "";
+      el.dispatchEvent(new Event("change", { bubbles: true }));
+    }
+  });
+  await page.waitForTimeout(800);
+
+  await page.click('button:has-text("Filters")');
+  await page.waitForTimeout(300);
+  const valueInput = page.locator(".fb-value").first();
+  await valueInput.click();
+  await valueInput.fill(AUTHOR);
+  await page.waitForTimeout(1200);
+  const countEl = page.locator('span[x-show*="matchCount"]').first();
+  console.log("contains:", JSON.stringify(await countEl.textContent()));
+
+  for (const label of OPS_SWEEP) {
+    await page.locator(".fb-picker-op").first().click();
+    await page.waitForTimeout(150);
+    await page
+      .locator(".fb-menu-item:visible")
+      .filter({ hasText: new RegExp(`^${label.replace(/[\W_]+/g, "\\$&")}$`) })
+      .first()
+      .click();
+    await page.waitForTimeout(1300);
+    console.log(`after "${label}":`, JSON.stringify(await countEl.textContent()));
+  }
+
+  // contains-with-empty-value is incomplete (counter hidden) — refill
+  // and confirm the count returns.
+  await valueInput.fill(AUTHOR);
+  await page.waitForTimeout(1300);
+  console.log("refilled contains:", JSON.stringify(await countEl.textContent()));
+
+  console.log("--- warnings/errors + count calls ---");
+  for (const l of events) console.log(l);
+  await browser.close();
+})().catch((e) => { console.error("HARNESS ERROR:", e.message); process.exit(1); });
diff --git a/web/src/filter_builder.ts b/web/src/filter_builder.ts
index b12db2e8..ba169a2e 100644
--- a/web/src/filter_builder.ts
+++ b/web/src/filter_builder.ts
@@ -30,26 +30,32 @@ const op = (
   arity: OpMeta["arity"],
 ): OpMeta => ({ key, label, negated, arity });
 
-export const OPS: Record<string, OpMeta> = {
-  contains: op("contains", "contains", false, "many"),
-  not_contains: op("doesn't contain", "doesn't contain", true, "many"),
-  is: op("is", "is", false, "many"),
-  is_not: op("isn't", "isn't", true, "many"),
-  empty: op("is empty", "is empty", false, "none"),
-  is_true: op("is true", "is true", false, "none"),
-  is_false: op("is false", "is false", false, "none"),
-  between: op("between", "between", false, "two"),
-  at_least: op("at least", "at least", false, "one"),
-  at_most: op("at most", "at most", false, "one"),
-  within_days: op("within the last", "within the last", false, "one"),
-  has: op("has", "has", false, "none"),
-  missing: op("missing", "missing", false, "none"),
-  unrated: op("unrated", "unrated", false, "none"),
-  in: op("in", "in", false, "many"),
-  not_in: op("isn't in", "isn't in", true, "many"),
-  in_any: op("in any", "in any", false, "none"),
-  in_none: op("in none", "in none", false, "none"),
-};
+// The map key DERIVES from meta.key — the wire value and the display
+// label cannot drift apart again (a mis-keyed entry threw every
+// non-self-named operator out of the allow-list: "doesn't contain" was
+// being sent literally, and the server rightly refused it).
+export const OPS: Record<string, OpMeta> = Object.fromEntries(
+  [
+    op("contains", "contains", false, "many"),
+    op("not_contains", "doesn't contain", true, "many"),
+    op("is", "is", false, "many"),
+    op("is_not", "isn't", true, "many"),
+    op("empty", "is empty", false, "none"),
+    op("is_true", "is true", false, "none"),
+    op("is_false", "is false", false, "none"),
+    op("between", "between", false, "two"),
+    op("at_least", "at least", false, "one"),
+    op("at_most", "at most", false, "one"),
+    op("within_days", "within the last", false, "one"),
+    op("has", "has", false, "none"),
+    op("missing", "missing", false, "none"),
+    op("unrated", "unrated", false, "none"),
+    op("in", "in", false, "many"),
+    op("not_in", "isn't in", true, "many"),
+    op("in_any", "in any", false, "none"),
+    op("in_none", "in none", false, "none"),
+  ].map((m) => [m.key, m]),
+);
 
 export interface FieldMeta {
   key: string;