diff --git a/package.json b/package.json
index e09f31f118..a7cae9f43f 100644
--- a/package.json
+++ b/package.json
@@ -14,8 +14,8 @@
   "type": "module",
   "packageManager": "pnpm@10.33.0",
   "scripts": {
-    "pretest": "node scripts/check-no-nohup.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-test-timeout-appeasement.mjs",
-    "pretest:full": "node scripts/check-no-nohup.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-test-timeout-appeasement.mjs",
+    "pretest": "node scripts/check-no-nohup.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-test-timeout-appeasement.mjs && node scripts/check-file-line-count.mjs",
+    "pretest:full": "node scripts/check-no-nohup.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-test-timeout-appeasement.mjs && node scripts/check-file-line-count.mjs",
     "test:gate": "node scripts/check-no-nohup.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-test-timeout-appeasement.mjs && pnpm --filter @fusion/engine test:core && pnpm --filter @runfusion/fusion test:ci-shape",
     "smoke:boot": "node scripts/boot-smoke.mjs",
     "local": "node scripts/start-local.mjs",
diff --git a/scripts/__tests__/check-file-line-count.test.mjs b/scripts/__tests__/check-file-line-count.test.mjs
new file mode 100644
index 0000000000..a611e9f349
--- /dev/null
+++ b/scripts/__tests__/check-file-line-count.test.mjs
@@ -0,0 +1,76 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import {
+  MAX_LINES,
+  countLines,
+  evaluate,
+  formatFailureMessage,
+} from "../check-file-line-count.mjs";
+
+test("countLines counts a trailing-newline file without an extra empty line", () => {
+  assert.equal(countLines("a\nb\nc\n"), 3);
+});
+
+test("countLines counts a file with no trailing newline", () => {
+  assert.equal(countLines("a\nb\nc"), 3);
+});
+
+test("countLines treats an empty file as zero lines", () => {
+  assert.equal(countLines(""), 0);
+});
+
+test("evaluate flags a new file over the cap", () => {
+  const { violations } = evaluate({ "packages/x/src/new.ts": MAX_LINES + 1 }, {});
+  assert.equal(violations.length, 1);
+  assert.equal(violations[0].grandfathered, false);
+  assert.equal(violations[0].ceiling, MAX_LINES);
+});
+
+test("evaluate passes a new file at exactly the cap", () => {
+  const { violations } = evaluate({ "packages/x/src/new.ts": MAX_LINES }, {});
+  assert.equal(violations.length, 0);
+});
+
+test("evaluate allows a grandfathered file at or below its recorded ceiling", () => {
+  const baseline = { "packages/x/src/big.ts": 5000 };
+  const { violations } = evaluate({ "packages/x/src/big.ts": 4800 }, baseline);
+  assert.equal(violations.length, 0);
+});
+
+test("evaluate flags a grandfathered file that grew past its ceiling", () => {
+  const baseline = { "packages/x/src/big.ts": 5000 };
+  const { violations } = evaluate({ "packages/x/src/big.ts": 5001 }, baseline);
+  assert.equal(violations.length, 1);
+  assert.equal(violations[0].grandfathered, true);
+  assert.equal(violations[0].ceiling, 5000);
+});
+
+test("evaluate reports a grandfathered file that shrank as tightenable", () => {
+  const baseline = { "packages/x/src/big.ts": 5000 };
+  const { staleBaseline } = evaluate({ "packages/x/src/big.ts": 4500 }, baseline);
+  assert.equal(staleBaseline.some((s) => s.reason === "shrank"), true);
+});
+
+test("evaluate reports a grandfathered file that dropped under the cap as tightenable", () => {
+  const baseline = { "packages/x/src/big.ts": 5000 };
+  const { violations, staleBaseline } = evaluate(
+    { "packages/x/src/big.ts": MAX_LINES - 1 },
+    baseline,
+  );
+  assert.equal(violations.length, 0);
+  assert.equal(staleBaseline.some((s) => s.reason === "under-cap"), true);
+});
+
+test("evaluate reports a deleted baseline file as tightenable", () => {
+  const baseline = { "packages/x/src/gone.ts": 5000 };
+  const { staleBaseline } = evaluate({}, baseline);
+  assert.equal(staleBaseline.some((s) => s.reason === "deleted"), true);
+});
+
+test("formatFailureMessage cites the file, count, and remediation", () => {
+  const msg = formatFailureMessage([
+    { filePath: "packages/x/src/new.ts", lines: 2500, ceiling: MAX_LINES, grandfathered: false },
+  ]);
+  assert.match(msg, /packages\/x\/src\/new\.ts: 2500 lines/);
+  assert.match(msg, /focused modules/);
+});
diff --git a/scripts/check-file-line-count.mjs b/scripts/check-file-line-count.mjs
new file mode 100644
index 0000000000..faf451939f
--- /dev/null
+++ b/scripts/check-file-line-count.mjs
@@ -0,0 +1,163 @@
+#!/usr/bin/env node
+// Repo-wide guard: hand-written source files may not exceed a hard line-count
+// cap (MAX_LINES). This stops the next god-file from being born while leaving
+// today's known offenders to be refactored down over time.
+//
+// Existing oversized files are grandfathered via scripts/line-count-baseline.json,
+// which records each file's current line count as its personal ceiling. The
+// baseline is a RATCHET: a grandfathered file may shrink (or stay put) but may
+// never grow past its recorded count, and once it drops to the cap it is removed
+// from the baseline and can never regress. New files get no grandfathering and
+// must stay at or under MAX_LINES.
+//
+// Generated, vendored, and data files are out of scope: only source extensions
+// under SCAN_ROOTS are scanned, and *.d.ts is excluded. Lockfiles, CHANGELOG,
+// locale JSON, and snapshots never match because of the extension filter.
+//
+// Run `node scripts/check-file-line-count.mjs --update` to rewrite the baseline
+// after an intentional, reviewed change to the set of oversized files.
+import { readFileSync, writeFileSync } from "node:fs";
+import { spawnSync } from "node:child_process";
+import { fileURLToPath } from "node:url";
+
+export const MAX_LINES = 2000;
+
+const SCAN_ROOTS = ["packages", "scripts", "plugins"];
+const SOURCE_EXT = /\.(m?[jt]sx?|cjs)$/;
+const DECLARATION_EXT = /\.d\.ts$/;
+
+const BASELINE_PATH = fileURLToPath(new URL("./line-count-baseline.json", import.meta.url));
+
+export function loadBaseline(path = BASELINE_PATH) {
+  try {
+    return JSON.parse(readFileSync(path, "utf8"));
+  } catch {
+    return {};
+  }
+}
+
+export function listTrackedSources() {
+  const result = spawnSync("git", ["ls-files", "--", ...SCAN_ROOTS], {
+    encoding: "utf8",
+    stdio: ["ignore", "pipe", "pipe"],
+  });
+  if (result.status !== 0) {
+    throw new Error(result.stderr?.trim() || "git ls-files failed");
+  }
+  return result.stdout
+    .split("\n")
+    .map((line) => line.trim())
+    .filter(Boolean)
+    .filter((path) => SOURCE_EXT.test(path) && !DECLARATION_EXT.test(path));
+}
+
+export function countLines(content) {
+  if (content === "") return 0;
+  const withoutTrailingNewline = content.endsWith("\n") ? content.slice(0, -1) : content;
+  return withoutTrailingNewline.split(/\r?\n/).length;
+}
+
+// Returns { violations, staleBaseline } for the given file→lineCount map.
+// `violations` are hard failures; `staleBaseline` lists baseline entries that
+// could be tightened (file shrank to/under the cap, or no longer exists).
+export function evaluate(counts, baseline = loadBaseline()) {
+  const violations = [];
+  for (const [filePath, lines] of Object.entries(counts)) {
+    const ceiling = filePath in baseline ? baseline[filePath] : MAX_LINES;
+    if (lines > ceiling) {
+      violations.push({
+        filePath,
+        lines,
+        ceiling,
+        grandfathered: filePath in baseline,
+      });
+    }
+  }
+
+  const staleBaseline = [];
+  for (const [filePath, recorded] of Object.entries(baseline)) {
+    if (!(filePath in counts)) {
+      staleBaseline.push({ filePath, reason: "deleted" });
+    } else if (counts[filePath] <= MAX_LINES) {
+      staleBaseline.push({ filePath, reason: "under-cap", lines: counts[filePath] });
+    } else if (counts[filePath] < recorded) {
+      staleBaseline.push({ filePath, reason: "shrank", lines: counts[filePath], recorded });
+    }
+  }
+
+  return { violations, staleBaseline };
+}
+
+export function collectCounts(files = listTrackedSources()) {
+  const counts = {};
+  for (const filePath of files) {
+    let content;
+    try {
+      content = readFileSync(filePath, "utf8");
+    } catch {
+      continue;
+    }
+    counts[filePath] = countLines(content);
+  }
+  return counts;
+}
+
+export function formatFailureMessage(violations) {
+  const lines = violations.map(({ filePath, lines: n, ceiling, grandfathered }) =>
+    grandfathered
+      ? `${filePath}: ${n} lines (grandfathered ceiling ${ceiling} — this file grew and must shrink, not expand)`
+      : `${filePath}: ${n} lines (cap ${ceiling})`,
+  );
+  return [
+    `[check-file-line-count] ${violations.length} file(s) exceed the line-count guardrail:`,
+    "",
+    ...lines,
+    "",
+    `New source files must stay at or under ${MAX_LINES} lines. Split the file into`,
+    "focused modules. Grandfathered files (in scripts/line-count-baseline.json) may",
+    "shrink but never grow; refactor them down rather than raising their ceiling.",
+    "If a larger file is genuinely justified, update the baseline with",
+    "`node scripts/check-file-line-count.mjs --update` in a reviewed change.",
+  ].join("\n");
+}
+
+function buildBaseline(counts) {
+  const baseline = {};
+  for (const filePath of Object.keys(counts).sort()) {
+    if (counts[filePath] > MAX_LINES) baseline[filePath] = counts[filePath];
+  }
+  return baseline;
+}
+
+export function main(argv = process.argv.slice(2)) {
+  const counts = collectCounts();
+
+  if (argv.includes("--update")) {
+    const baseline = buildBaseline(counts);
+    writeFileSync(BASELINE_PATH, `${JSON.stringify(baseline, null, 2)}\n`);
+    console.error(
+      `[check-file-line-count] baseline rewritten: ${Object.keys(baseline).length} file(s) over ${MAX_LINES} lines.`,
+    );
+    return 0;
+  }
+
+  const { violations, staleBaseline } = evaluate(counts);
+
+  if (staleBaseline.length > 0) {
+    const shrunk = staleBaseline.filter((s) => s.reason !== "deleted");
+    if (shrunk.length > 0) {
+      console.error(
+        `[check-file-line-count] note: ${staleBaseline.length} baseline entr(ies) can be tightened ` +
+          "(files shrank or were removed). Run with --update to ratchet the baseline down.",
+      );
+    }
+  }
+
+  if (violations.length === 0) return 0;
+  console.error(formatFailureMessage(violations));
+  return 1;
+}
+
+if (process.argv[1] && fileURLToPath(import.meta.url) === process.argv[1]) {
+  process.exitCode = main();
+}
diff --git a/scripts/line-count-baseline.json b/scripts/line-count-baseline.json
new file mode 100644
index 0000000000..318e6f4099
--- /dev/null
+++ b/scripts/line-count-baseline.json
@@ -0,0 +1,108 @@
+{
+  "packages/cli/src/__tests__/extension.test.ts": 4186,
+  "packages/cli/src/bin.ts": 2065,
+  "packages/cli/src/commands/__tests__/dashboard.test.ts": 3346,
+  "packages/cli/src/commands/__tests__/serve.test.ts": 2100,
+  "packages/cli/src/commands/__tests__/task.test.ts": 3424,
+  "packages/cli/src/commands/dashboard-tui/app.tsx": 4665,
+  "packages/cli/src/commands/dashboard.ts": 2951,
+  "packages/cli/src/extension.ts": 4704,
+  "packages/core/src/__tests__/agent-store.test.ts": 2997,
+  "packages/core/src/__tests__/central-core.test.ts": 3263,
+  "packages/core/src/__tests__/db.test.ts": 3601,
+  "packages/core/src/__tests__/mission-store.test.ts": 4287,
+  "packages/core/src/__tests__/plugin-loader.test.ts": 2783,
+  "packages/core/src/__tests__/store-settings.test.ts": 2196,
+  "packages/core/src/agent-store.ts": 2946,
+  "packages/core/src/central-core.ts": 3854,
+  "packages/core/src/db.ts": 5770,
+  "packages/core/src/mission-store.ts": 4293,
+  "packages/core/src/store.ts": 16537,
+  "packages/core/src/types.ts": 7163,
+  "packages/dashboard/app/api/legacy.ts": 10565,
+  "packages/dashboard/app/App.tsx": 2295,
+  "packages/dashboard/app/components/__tests__/AgentsView.test.tsx": 2761,
+  "packages/dashboard/app/components/__tests__/App.test.tsx": 4304,
+  "packages/dashboard/app/components/__tests__/ChatView.test.tsx": 5677,
+  "packages/dashboard/app/components/__tests__/GitManagerModal.test.tsx": 3274,
+  "packages/dashboard/app/components/__tests__/ListView.test.tsx": 4113,
+  "packages/dashboard/app/components/__tests__/MailboxView.test.tsx": 2037,
+  "packages/dashboard/app/components/__tests__/ModelOnboardingModal.test.tsx": 4575,
+  "packages/dashboard/app/components/__tests__/PlanningModeModal.planning-flow.test.tsx": 2746,
+  "packages/dashboard/app/components/__tests__/QuickChatFAB.test.tsx": 2816,
+  "packages/dashboard/app/components/__tests__/QuickEntryBox.test.tsx": 4526,
+  "packages/dashboard/app/components/__tests__/SettingsModal.test.tsx": 5375,
+  "packages/dashboard/app/components/__tests__/TaskCard.test.tsx": 5121,
+  "packages/dashboard/app/components/__tests__/TaskChatTab.test.tsx": 2366,
+  "packages/dashboard/app/components/__tests__/TaskDetailModal.inline-editing-and-integrations.test.tsx": 2917,
+  "packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx": 2297,
+  "packages/dashboard/app/components/__tests__/TerminalModal.test.tsx": 5578,
+  "packages/dashboard/app/components/__tests__/UsageIndicator.test.tsx": 2877,
+  "packages/dashboard/app/components/__tests__/WorkflowNodeEditor.test.tsx": 3138,
+  "packages/dashboard/app/components/AgentDetailView.tsx": 5399,
+  "packages/dashboard/app/components/AgentsView.tsx": 2101,
+  "packages/dashboard/app/components/ChatView.tsx": 3964,
+  "packages/dashboard/app/components/GitManagerModal.tsx": 3186,
+  "packages/dashboard/app/components/ListView.tsx": 2397,
+  "packages/dashboard/app/components/MissionManager.tsx": 4999,
+  "packages/dashboard/app/components/ModelOnboardingModal.tsx": 2932,
+  "packages/dashboard/app/components/PlanningModeModal.tsx": 3319,
+  "packages/dashboard/app/components/QuickChatFAB.tsx": 3559,
+  "packages/dashboard/app/components/QuickEntryBox.tsx": 2206,
+  "packages/dashboard/app/components/SettingsModal.tsx": 3239,
+  "packages/dashboard/app/components/TaskCard.tsx": 2528,
+  "packages/dashboard/app/components/TaskDetailModal.tsx": 4568,
+  "packages/dashboard/app/components/WorkflowNodeEditor.tsx": 4381,
+  "packages/dashboard/app/hooks/__tests__/useChat.test.ts": 4097,
+  "packages/dashboard/app/hooks/__tests__/useQuickChat.test.ts": 2706,
+  "packages/dashboard/app/hooks/__tests__/useTasks.test.ts": 2582,
+  "packages/dashboard/src/__tests__/chat-manager.test.ts": 2636,
+  "packages/dashboard/src/__tests__/file-service.test.ts": 2123,
+  "packages/dashboard/src/__tests__/github.test.ts": 2222,
+  "packages/dashboard/src/__tests__/plugin-routes.test.ts": 2069,
+  "packages/dashboard/src/__tests__/routes-agents.test.ts": 4921,
+  "packages/dashboard/src/__tests__/routes-auth.test.ts": 3980,
+  "packages/dashboard/src/__tests__/routes-automation.test.ts": 2393,
+  "packages/dashboard/src/__tests__/routes-github.test.ts": 2855,
+  "packages/dashboard/src/__tests__/routes-nodes-sync.test.ts": 2476,
+  "packages/dashboard/src/__tests__/routes-planning.test.ts": 4346,
+  "packages/dashboard/src/__tests__/routes-settings.test.ts": 3122,
+  "packages/dashboard/src/__tests__/routes-tasks-ops.test.ts": 4158,
+  "packages/dashboard/src/__tests__/routes-tasks.test.ts": 2696,
+  "packages/dashboard/src/__tests__/server.test.ts": 2964,
+  "packages/dashboard/src/__tests__/usage.test.ts": 4327,
+  "packages/dashboard/src/chat.ts": 2193,
+  "packages/dashboard/src/github.ts": 4178,
+  "packages/dashboard/src/mission-routes.ts": 3948,
+  "packages/dashboard/src/planning.ts": 2696,
+  "packages/dashboard/src/routes.ts": 5251,
+  "packages/dashboard/src/routes/register-git-github.ts": 5637,
+  "packages/dashboard/src/routes/register-settings-memory-routes.ts": 2421,
+  "packages/dashboard/src/routes/register-task-workflow-routes.ts": 3738,
+  "packages/dashboard/src/server.ts": 2338,
+  "packages/engine/src/__tests__/executor-pause.test.ts": 2836,
+  "packages/engine/src/__tests__/executor-prompt.test.ts": 2572,
+  "packages/engine/src/__tests__/executor-recovery.test.ts": 3600,
+  "packages/engine/src/__tests__/executor-step-session.test.ts": 3731,
+  "packages/engine/src/__tests__/executor-worktree.test.ts": 2534,
+  "packages/engine/src/__tests__/heartbeat-executor.test.ts": 3944,
+  "packages/engine/src/__tests__/merger-merge-lifecycle.test.ts": 3253,
+  "packages/engine/src/__tests__/merger-verification.test.ts": 3163,
+  "packages/engine/src/__tests__/mission-execution-loop.test.ts": 2455,
+  "packages/engine/src/__tests__/pi-create-fn-agent.test.ts": 2191,
+  "packages/engine/src/__tests__/project-engine.test.ts": 2851,
+  "packages/engine/src/__tests__/scheduler.test.ts": 5395,
+  "packages/engine/src/__tests__/self-healing.test.ts": 9641,
+  "packages/engine/src/__tests__/step-session-executor.test.ts": 2911,
+  "packages/engine/src/__tests__/triage.test.ts": 4514,
+  "packages/engine/src/agent-heartbeat.ts": 4548,
+  "packages/engine/src/agent-tools.ts": 3584,
+  "packages/engine/src/executor.ts": 15657,
+  "packages/engine/src/merger.ts": 12643,
+  "packages/engine/src/pi.ts": 2421,
+  "packages/engine/src/project-engine.ts": 3663,
+  "packages/engine/src/scheduler.ts": 2638,
+  "packages/engine/src/self-healing.ts": 10316,
+  "packages/engine/src/triage.ts": 2793,
+  "plugins/fusion-plugin-roadmap/src/dashboard/RoadmapsView.tsx": 2559
+}