diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3045d8939..76ecfc4a2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -194,6 +194,9 @@ jobs:
       - name: Build
         run: npm run build
 
+      - name: Bundle budget (warn-only until a baseline is enforced)
+        run: npm run check:bundle-budget
+
       - name: Deployment boot smoke
         if: github.event_name == 'workflow_dispatch' || github.event_name == 'schedule' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/heads/release/')
         env:
diff --git a/bundle-budget.json b/bundle-budget.json
new file mode 100644
index 000000000..84feae937
--- /dev/null
+++ b/bundle-budget.json
@@ -0,0 +1,6 @@
+{
+  "$comment": "Client JS bundle-size budget. Warn-only until a baseline is captured: after a known-good `npm run build`, run `npm run check:bundle-budget -- --update` to set totalGzipBytes, then flip enforce to true so CI fails on >tolerancePct growth. See scripts/check-bundle-budget.mjs.",
+  "enforce": false,
+  "tolerancePct": 10,
+  "totalGzipBytes": null
+}
diff --git a/package.json b/package.json
index 683023ce3..d7a9ecfb8 100644
--- a/package.json
+++ b/package.json
@@ -18,6 +18,7 @@
     "ensure": "node scripts/ensure-local-server.mjs",
     "build": "node scripts/guard-next-build.mjs && node --max-old-space-size=8192 ./node_modules/next/dist/bin/next build --webpack && node scripts/check-client-bundle-secrets.mjs",
     "build:analyze": "node scripts/build-analyze.mjs",
+    "check:bundle-budget": "node scripts/check-bundle-budget.mjs",
     "start": "node scripts/dev-free-port.mjs start",
     "lint": "node --max-old-space-size=8192 ./node_modules/eslint/bin/eslint.js src tests scripts worker supabase playwright eslint.config.mjs next.config.ts playwright.config.ts playwright.visual.config.ts vitest.config.mts --max-warnings 0 --no-error-on-unmatched-pattern",
     "typecheck": "node ./node_modules/typescript/bin/tsc --noEmit",
diff --git a/scripts/check-bundle-budget.mjs b/scripts/check-bundle-budget.mjs
new file mode 100644
index 000000000..c1a17349c
--- /dev/null
+++ b/scripts/check-bundle-budget.mjs
@@ -0,0 +1,144 @@
+#!/usr/bin/env node
+/**
+ * check-bundle-budget — guard client JS bundle size against regression.
+ *
+ * The repo did perf/bundle-hygiene work, but nothing stops a regression: the next
+ * heavy dependency or an accidental client-side import of a big module (e.g. pulling
+ * a snapshot into a client component) silently undoes it. This measures the built
+ * client chunks and compares total gzip size against a committed baseline
+ * (bundle-budget.json), failing when growth exceeds the tolerance.
+ *
+ * ROLLOUT: warn-only until a baseline is captured and enforcement is turned on.
+ *   - `bundle-budget.json` ships with `enforce: false` and `totalGzipBytes: null`.
+ *   - After a known-good production build, run `--update` to record the baseline.
+ *   - Flip `enforce` to true to make CI fail on >tolerancePct growth.
+ * Reads .next/static/chunks/**.js. If no build output exists it prints a note and
+ * exits 0 (so it never breaks a run that didn't build).
+ *
+ * Flags: --update (write current measurement as the baseline), --json.
+ */
+import { gzipSync } from "node:zlib";
+import { existsSync, readdirSync, readFileSync, writeFileSync } from "node:fs";
+import path from "node:path";
+import { fileURLToPath, pathToFileURL } from "node:url";
+
+const root = path.join(path.dirname(fileURLToPath(import.meta.url)), "..");
+const CHUNKS_DIR = path.join(root, ".next", "static", "chunks");
+const BUDGET_PATH = path.join(root, "bundle-budget.json");
+
+function walkJsFiles(dir) {
+  const out = [];
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) out.push(...walkJsFiles(full));
+    else if (entry.isFile() && entry.name.endsWith(".js")) out.push(full);
+  }
+  return out;
+}
+
+/** Measure client chunks → { files, totalRawBytes, totalGzipBytes, largest[] }. */
+export function measureChunks(files) {
+  const measured = files.map(({ name, buffer }) => ({
+    name,
+    rawBytes: buffer.length,
+    gzipBytes: gzipSync(buffer).length,
+  }));
+  const totalRawBytes = measured.reduce((sum, f) => sum + f.rawBytes, 0);
+  const totalGzipBytes = measured.reduce((sum, f) => sum + f.gzipBytes, 0);
+  const largest = [...measured].sort((a, b) => b.gzipBytes - a.gzipBytes).slice(0, 5);
+  return { files: measured.length, totalRawBytes, totalGzipBytes, largest };
+}
+
+/**
+ * Pure comparison. Returns { status: "ok"|"warn"|"fail", overPct, ... }.
+ * - no baseline → "warn" (nothing to compare yet).
+ * - within tolerance → "ok".
+ * - over tolerance → "fail" if enforcing, else "warn".
+ */
+export function compareToBudget(current, budget) {
+  const baseline = budget?.totalGzipBytes ?? null;
+  const tolerancePct = budget?.tolerancePct ?? 10;
+  const enforce = Boolean(budget?.enforce);
+  if (baseline == null) {
+    return { status: "warn", reason: "no baseline recorded", overPct: null, baseline, tolerancePct, enforce };
+  }
+  const overPct = ((current.totalGzipBytes - baseline) / baseline) * 100;
+  const withinTolerance = overPct <= tolerancePct;
+  return {
+    status: withinTolerance ? "ok" : enforce ? "fail" : "warn",
+    reason: withinTolerance ? "within tolerance" : `+${overPct.toFixed(1)}% vs baseline (tolerance ${tolerancePct}%)`,
+    overPct,
+    baseline,
+    tolerancePct,
+    enforce,
+  };
+}
+
+function kb(bytes) {
+  return `${(bytes / 1024).toFixed(1)} KiB`;
+}
+
+function loadBudget() {
+  try {
+    return JSON.parse(readFileSync(BUDGET_PATH, "utf8"));
+  } catch {
+    return { enforce: false, tolerancePct: 10, totalGzipBytes: null };
+  }
+}
+
+function main() {
+  const asJson = process.argv.includes("--json");
+  const update = process.argv.includes("--update");
+
+  if (!existsSync(CHUNKS_DIR)) {
+    console.log(
+      `[bundle-budget] no build output at ${path.relative(root, CHUNKS_DIR)} — run \`npm run build\` first. Skipping.`,
+    );
+    process.exit(0);
+  }
+
+  const files = walkJsFiles(CHUNKS_DIR).map((full) => ({
+    name: path.relative(CHUNKS_DIR, full),
+    buffer: readFileSync(full),
+  }));
+  const current = measureChunks(files);
+  const budget = loadBudget();
+
+  if (update) {
+    const next = { ...budget, totalGzipBytes: current.totalGzipBytes, updatedAt: new Date().toISOString() };
+    writeFileSync(BUDGET_PATH, JSON.stringify(next, null, 2) + "\n");
+    console.log(`[bundle-budget] baseline updated to ${kb(current.totalGzipBytes)} gzip (${current.files} chunks).`);
+    process.exit(0);
+  }
+
+  const verdict = compareToBudget(current, budget);
+  if (asJson) {
+    console.log(JSON.stringify({ current, verdict }, null, 2));
+  } else {
+    console.log(
+      `[bundle-budget] client chunks: ${current.files} files, ${kb(current.totalGzipBytes)} gzip (${kb(current.totalRawBytes)} raw).`,
+    );
+    if (verdict.baseline != null)
+      console.log(`[bundle-budget] baseline ${kb(verdict.baseline)} gzip; ${verdict.reason}.`);
+    console.log("[bundle-budget] largest chunks (gzip):");
+    for (const c of current.largest) console.log(`  ${kb(c.gzipBytes).padStart(12)}  ${c.name}`);
+  }
+
+  if (verdict.status === "fail") {
+    console.error(
+      `[bundle-budget] FAIL — ${verdict.reason}. Refresh intentionally with \`npm run check:bundle-budget -- --update\`.`,
+    );
+    process.exit(1);
+  }
+  if (verdict.status === "warn" && verdict.baseline == null) {
+    console.log(
+      "[bundle-budget] warn-only: capture a baseline with --update after a known-good build, then set enforce:true.",
+    );
+  } else if (verdict.status === "warn") {
+    console.warn(`[bundle-budget] WARN (not enforced) — ${verdict.reason}.`);
+  }
+  process.exit(0);
+}
+
+const invokedDirectly = process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href;
+if (invokedDirectly) main();
diff --git a/scripts/ci-change-scope.mjs b/scripts/ci-change-scope.mjs
index 317714125..a975b6363 100644
--- a/scripts/ci-change-scope.mjs
+++ b/scripts/ci-change-scope.mjs
@@ -140,6 +140,7 @@ const sourcePatterns = ["data", "src", "tests", "scripts", "worker", "playwright
 const coveragePatterns = ["data", "src", "tests", "vitest.config.mts"];
 
 const buildPatterns = [
+  "bundle-budget.json",
   "data",
   "src",
   "worker",
@@ -149,6 +150,7 @@ const buildPatterns = [
   "postcss.config.mjs",
   "package.json",
   "package-lock.json",
+  "scripts/check-bundle-budget.mjs",
   /^scripts\/(check-node-engine|guard-next-build|dev-free-port|ensure-local-server)\.(?:cjs|mjs)$/,
 ];
 
@@ -428,6 +430,13 @@ function selfTest() {
     workflow_changed: false,
     build_changed: true,
   });
+  assertScope("bundle-budget-config", ["bundle-budget.json"], {
+    build_changed: true,
+  });
+  assertScope("bundle-budget-checker", ["scripts/check-bundle-budget.mjs"], {
+    source_changed: true,
+    build_changed: true,
+  });
   assertScope("container", ["Dockerfile.worker", "worker/python/requirements.txt"], {
     container_changed: true,
     build_changed: true,
diff --git a/tests/bundle-budget.test.ts b/tests/bundle-budget.test.ts
new file mode 100644
index 000000000..e8831f48f
--- /dev/null
+++ b/tests/bundle-budget.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from "vitest";
+import { compareToBudget, measureChunks } from "../scripts/check-bundle-budget.mjs";
+
+const buf = (n: number) => Buffer.alloc(n, "a"); // highly compressible; gzip < raw
+
+describe("measureChunks", () => {
+  it("sums raw and gzip bytes and ranks the largest", () => {
+    const m = measureChunks([
+      { name: "a.js", buffer: buf(1000) },
+      { name: "b.js", buffer: buf(4000) },
+    ]);
+    expect(m.files).toBe(2);
+    expect(m.totalRawBytes).toBe(5000);
+    expect(m.totalGzipBytes).toBeGreaterThan(0);
+    expect(m.totalGzipBytes).toBeLessThan(m.totalRawBytes);
+    expect(m.largest[0].name).toBe("b.js");
+  });
+});
+
+describe("compareToBudget", () => {
+  it("warns when there is no baseline", () => {
+    const v = compareToBudget({ totalGzipBytes: 1000 }, { enforce: true, tolerancePct: 10, totalGzipBytes: null });
+    expect(v.status).toBe("warn");
+    expect(v.reason).toMatch(/no baseline/);
+  });
+
+  it("passes within tolerance", () => {
+    const v = compareToBudget({ totalGzipBytes: 1050 }, { enforce: true, tolerancePct: 10, totalGzipBytes: 1000 });
+    expect(v.status).toBe("ok");
+    expect(v.overPct).toBeCloseTo(5, 5);
+  });
+
+  it("fails over tolerance when enforcing", () => {
+    const v = compareToBudget({ totalGzipBytes: 1200 }, { enforce: true, tolerancePct: 10, totalGzipBytes: 1000 });
+    expect(v.status).toBe("fail");
+    expect(v.overPct).toBeCloseTo(20, 5);
+  });
+
+  it("only warns over tolerance when not enforcing", () => {
+    const v = compareToBudget({ totalGzipBytes: 1200 }, { enforce: false, tolerancePct: 10, totalGzipBytes: 1000 });
+    expect(v.status).toBe("warn");
+  });
+
+  it("treats exactly-at-tolerance as ok", () => {
+    const v = compareToBudget({ totalGzipBytes: 1100 }, { enforce: true, tolerancePct: 10, totalGzipBytes: 1000 });
+    expect(v.status).toBe("ok");
+  });
+});