diff --git a/src/lib/groomer/llm.test.ts b/src/lib/groomer/llm.test.ts
index 7dae9e6a..b4545b03 100644
--- a/src/lib/groomer/llm.test.ts
+++ b/src/lib/groomer/llm.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it, vi, beforeEach } from "vitest";
 import { callGroomerLLM, buildGroomerResponseSchema } from "./llm";
+import { ALLOWED_GROOMER_LABELS } from "./schema";
 import { getLaneIds } from "@/lib/lane-config";
 
 describe("callGroomerLLM", () => {
@@ -198,6 +199,25 @@ describe("buildGroomerResponseSchema", () => {
     expect(laneId.enum).toEqual(getLaneIds());
     expect(laneId.enum.length).toBeGreaterThan(0);
   });
+
+  it("enum-constrains label arrays to the validator's allowlist", () => {
+    const schema = buildGroomerResponseSchema() as any;
+    expect(schema.properties.labelsToAdd.items.enum).toEqual([...ALLOWED_GROOMER_LABELS]);
+    expect(schema.properties.labelsToRemove.items.enum).toEqual([...ALLOWED_GROOMER_LABELS]);
+    // The exact failure seen in prod: a 4B inventing "type/refactor".
+    expect(schema.properties.labelsToAdd.items.enum).not.toContain("type/refactor");
+  });
+
+  it("bounds proposedTitle to the validator's 10-200 chars (or null)", () => {
+    const schema = buildGroomerResponseSchema() as any;
+    const title = schema.properties.proposedTitle;
+    expect(title.anyOf).toEqual([
+      { type: "null" },
+      { type: "string", minLength: 10, maxLength: 200 },
+    ]);
+    const body = schema.properties.proposedBody;
+    expect(body.anyOf).toEqual([{ type: "null" }, { type: "string", maxLength: 9999 }]);
+  });
 });
 
 describe("callGroomerLLM response_format", () => {
diff --git a/src/lib/groomer/llm.ts b/src/lib/groomer/llm.ts
index d0f48e0a..aab596fe 100644
--- a/src/lib/groomer/llm.ts
+++ b/src/lib/groomer/llm.ts
@@ -1,4 +1,4 @@
-import type { GroomerOutput } from "./schema";
+import { ALLOWED_GROOMER_LABELS, type GroomerOutput } from "./schema";
 import { getConfiguredLanes, getClaimableLanes, getBacklogLane, getLaneIds } from "@/lib/lane-config";
 import { STATUS_LABELS, PRIORITY_LABELS } from "@/types";
 
@@ -82,8 +82,10 @@ export function buildGroomerResponseSchema(): Record<string, unknown> {
     properties: {
       actionability: { type: "string", enum: ["ready", "needs_info", "blocked", "backlog", "already_done"] },
       confidence,
-      labelsToAdd: { type: "array", items: { type: "string" } },
-      labelsToRemove: { type: "array", items: { type: "string" } },
+      // Enum-constrained to the validator's allowlist so the model cannot
+      // invent labels (a 4B happily emits "type/refactor" otherwise).
+      labelsToAdd: { type: "array", items: { type: "string", enum: [...ALLOWED_GROOMER_LABELS] } },
+      labelsToRemove: { type: "array", items: { type: "string", enum: [...ALLOWED_GROOMER_LABELS] } },
       lane: {
         type: "object",
         additionalProperties: false,
@@ -102,8 +104,10 @@ export function buildGroomerResponseSchema(): Record<string, unknown> {
         type: "string",
         enum: ["promote_to_ready", "escalate", "mark_not_ready", "mark_needs_info", "mark_blocked"],
       },
-      proposedTitle: { type: "string" },
-      proposedBody: { type: "string" },
+      // The validator requires 10-200 chars (or omitted/null). Without the
+      // bounds in the grammar a small model emits "" instead of omitting.
+      proposedTitle: { anyOf: [{ type: "null" }, { type: "string", minLength: 10, maxLength: 200 }] },
+      proposedBody: { anyOf: [{ type: "null" }, { type: "string", maxLength: 9999 }] },
     },
   };
 }
diff --git a/src/lib/groomer/schema.ts b/src/lib/groomer/schema.ts
index c3e92e6b..013fd0dd 100644
--- a/src/lib/groomer/schema.ts
+++ b/src/lib/groomer/schema.ts
@@ -33,11 +33,20 @@ export interface ValidationResult {
 
 const validTypeLabels = ["type/bug", "type/feature", "type/chore", "type/research", "type/security"];
 
+/**
+ * Every label the groomer may add or remove. Exported so the LLM response
+ * schema (`buildGroomerResponseSchema`) can enum-constrain label output to
+ * exactly this set — a grammar-constrained small model then cannot invent
+ * labels (e.g. "type/refactor") that this validator would reject.
+ */
+export const ALLOWED_GROOMER_LABELS: readonly string[] = [
+  ...STATUS_LABELS,
+  ...PRIORITY_LABELS,
+  ...validTypeLabels,
+];
+
 function isAllowedLabel(label: string): boolean {
-  if (STATUS_LABELS.some((s) => s === label)) return true;
-  if (PRIORITY_LABELS.some((s) => s === label)) return true;
-  if (validTypeLabels.includes(label)) return true;
-  return false;
+  return ALLOWED_GROOMER_LABELS.includes(label);
 }
 
 function validateLabelList(list: unknown[], kind: string): string[] {