diff --git a/web/src/filter_builder.test.ts b/web/src/filter_builder.test.ts
index 0c6ed3d1..b46747bb 100644
--- a/web/src/filter_builder.test.ts
+++ b/web/src/filter_builder.test.ts
@@ -242,7 +242,10 @@ const builderFields = [
     valueKind: "id" as const,
     defaultOp: "in",
     suggest: true,
-    ops: [{ key: "in", label: "in", arity: "many" }],
+    ops: [
+      { key: "in", label: "in", arity: "many" },
+      { key: "is", label: "is", arity: "one" },
+    ],
   },
   {
     key: "author",
@@ -436,6 +439,50 @@ describe("id-kind value cells", () => {
     vi.unstubAllGlobals();
   });
 
+  it("opening the menu after a pick browses the WHOLE list (regression)", async () => {
+    const c = filterBuilderData();
+    c.filterTree = {
+      mode: "all",
+      children: [{ field: "library", op: "in", values: ["lib-1"] }],
+    };
+    c.idLabels["lib-1"] = "Books";
+    let url = "";
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async (u: string | URL) => {
+        url = String(u);
+        return { ok: true, json: async () => ({ results: [] }) };
+      }),
+    );
+    // the user opens the cell to ADD another — the stored ids must not
+    // filter the menu down to the picked row (2026-11-01 report: "can't
+    // add more than one").
+    await c.fetchSuggestions("0", [0], c.filterTree.children[0] as FilterRuleState);
+    expect(url).not.toContain("lib-1");
+    expect(url).toContain("library=+");
+    vi.unstubAllGlobals();
+  });
+
+  it("a one-arity op (is) picks ONE value — replace, not toggle", () => {
+    const c = filterBuilderData();
+    c.filterTree = {
+      mode: "all",
+      children: [{ field: "library", op: "is", values: ["lib-1"] }],
+    };
+    c.pickSuggestion("0", [0], { value: "lib-2", display: "Comics", count: 2 });
+    expect((c.filterTree.children[0] as FilterRuleState).values).toEqual(["lib-2"]);
+    expect(c.valueCellText([0])).toBe("Comics");
+  });
+
+  it("the placeholder follows the op's arity", () => {
+    const c = filterBuilderData();
+    expect(c.valuePlaceholderFor({ field: "library", op: "in", values: [] })).toBe(
+      "pick one or more",
+    );
+    expect(c.valuePlaceholderFor({ field: "library", op: "is", values: [] })).toBe("pick one");
+    expect(c.valuePlaceholderFor({ field: "author", op: "contains", values: [] })).toBe("value");
+  });
+
   it("text cells keep writing straight through", () => {
     const c = filterBuilderData();
     c.filterTree = {
diff --git a/web/src/filter_builder.ts b/web/src/filter_builder.ts
index 26fb0376..8fa0ab30 100644
--- a/web/src/filter_builder.ts
+++ b/web/src/filter_builder.ts
@@ -727,12 +727,14 @@ export function filterBuilderData() {
       this.filterTree.children.forEach((child, idx) => visit(child, [idx]));
     },
 
-    // valuePlaceholderFor — the value cell's hint. Id-kind cells are
-    // the few MULTI-VALUE boxes (pick several; the menu keeps toggling
-    // them in) — say so, because nothing else on the page does.
+    // valuePlaceholderFor — the value cell's hint, ARITY-AWARE: the op
+    // decides how many values the cell takes ("in" = several, "is" =
+    // one), and the hint follows the op as it changes.
     valuePlaceholderFor(rule: FilterRuleState): string {
       if (this.fieldIsDays(rule.field)) return "days";
-      if (FIELD_BY_KEY.get(rule.field)?.valueKind === "id") return "pick one or more";
+      if (FIELD_BY_KEY.get(rule.field)?.valueKind === "id") {
+        return this.valueArity(rule) === "many" ? "pick one or more" : "pick one";
+      }
       return "value";
     },
 
@@ -795,8 +797,8 @@ export function filterBuilderData() {
       const meta = FIELD_BY_KEY.get(node.field);
       this.idLabels[s.value] = s.display ?? s.value;
       delete this.idDrafts[path.join("-")]; // the cell shows labels again
-      if (meta?.valueKind === "id") {
-        // id-valued: toggle the id in the comma list.
+      if (meta?.valueKind === "id" && this.valueArity(node) === "many") {
+        // id-valued MULTI: toggle the id in the comma list.
         const ids = node.values.join(",").split(",").filter(Boolean);
         const at = ids.indexOf(s.value);
         if (at >= 0) {
@@ -878,7 +880,7 @@ export function filterBuilderData() {
       if (meta.valueKind === "fixed") return;
       const typed = (
         this.isIdValueRow(path)
-          ? (this.idDrafts[path.join("-")] ?? rule.values.join(","))
+          ? (this.idDrafts[path.join("-")] ?? (quiet ? rule.values.join(",") : ""))
           : (rule.values[0] ?? "")
       ).trim();
       // 1 char is too little to filter on; EMPTY means BROWSE — the