diff --git a/.husky/pre-commit b/.husky/pre-commit
index dfbf41f..92c418a 100755
--- a/.husky/pre-commit
+++ b/.husky/pre-commit
@@ -16,3 +16,6 @@ fi
 # staged but doesn't match what rebuild-state would produce. Catches the case
 # where someone hand-edits _state.json without going through rebuild-state.
 node scripts/work/state-sync-guard.mjs || exit 1
+
+# 4. Check library decision traces for new runtime deps in feature/core packages.
+node scripts/library-decisions/check.mjs || exit 1
diff --git a/scripts/library-decisions/check.mjs b/scripts/library-decisions/check.mjs
new file mode 100644
index 0000000..1bdfae8
--- /dev/null
+++ b/scripts/library-decisions/check.mjs
@@ -0,0 +1,155 @@
+#!/usr/bin/env node
+/**
+ * Pre-commit guard: refuses the commit if a new runtime dependency in a
+ * feature- or core-tier package lacks a staged approved library trace.
+ *
+ * Exit codes:
+ *   0 — all staged deps have approved traces (or are app-tier / devDeps / peerDeps)
+ *   1 — one or more feature/core deps are missing an approved trace
+ */
+import { execSync } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { parseFrontmatter } from "./schema.mjs";
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const DEFAULT_REPO_ROOT = path.resolve(__dirname, "..", "..");
+
+/** Derive package tier from its repo-relative path. */
+function deriveTier(relPath) {
+  if (relPath.startsWith("apps/")) return "app";
+  if (relPath.startsWith("packages/core-")) return "core";
+  if (relPath.startsWith("packages/")) return "feature";
+  return "skip"; // root package.json or unknown path
+}
+
+function stagedFilesList(repoRoot) {
+  return execSync("git diff --cached --name-only", {
+    cwd: repoRoot,
+    encoding: "utf8",
+  })
+    .split("\n")
+    .filter(Boolean);
+}
+
+/**
+ * Return the names of runtime deps that are new in the staged version of
+ * relPath compared to HEAD. Returns [] when the file can't be read.
+ */
+function getNewRuntimeDeps(relPath, repoRoot) {
+  let staged;
+  try {
+    staged = JSON.parse(
+      execSync(`git show ":${relPath}"`, { cwd: repoRoot, encoding: "utf8" }),
+    );
+  } catch {
+    return [];
+  }
+  let base = {};
+  try {
+    base = JSON.parse(
+      execSync(`git show "HEAD:${relPath}"`, {
+        cwd: repoRoot,
+        encoding: "utf8",
+      }),
+    );
+  } catch {
+    // New file or initial commit — treat all staged deps as new
+  }
+  const baseDeps = new Set(Object.keys(base.dependencies ?? {}));
+  return Object.keys(staged.dependencies ?? {}).filter((d) => !baseDeps.has(d));
+}
+
+/**
+ * Search the staged-files list for a trace file whose name ends with
+ * `-<depName>.md` inside docs/library-decisions/.
+ */
+function findStagedTrace(depName, staged) {
+  const safe = depName.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+  const re = new RegExp(`^docs/library-decisions/[^/]+-${safe}\\.md$`);
+  return staged.find((f) => re.test(f)) ?? null;
+}
+
+/**
+ * Run the library-decisions check against repoRoot.
+ *
+ * Returns an array of error objects, each with:
+ *   { pkgJson, dep, reason: "no-trace" | "not-approved" | "parse-error", ... }
+ *
+ * An empty array means the commit is clean.
+ */
+export function checkLibraryDecisions(repoRoot = DEFAULT_REPO_ROOT) {
+  const staged = stagedFilesList(repoRoot);
+  const pkgJsons = staged.filter(
+    (f) => f === "package.json" || f.endsWith("/package.json"),
+  );
+  const errors = [];
+
+  for (const relPath of pkgJsons) {
+    const tier = deriveTier(relPath);
+    if (tier === "app" || tier === "skip") continue;
+
+    for (const dep of getNewRuntimeDeps(relPath, repoRoot)) {
+      const traceFile = findStagedTrace(dep, staged);
+      if (!traceFile) {
+        errors.push({ pkgJson: relPath, dep, reason: "no-trace" });
+        continue;
+      }
+      try {
+        const content = execSync(`git show ":${traceFile}"`, {
+          cwd: repoRoot,
+          encoding: "utf8",
+        });
+        const fm = parseFrontmatter(content);
+        if (fm.decision !== "approved") {
+          errors.push({
+            pkgJson: relPath,
+            dep,
+            reason: "not-approved",
+            decision: fm.decision,
+          });
+        }
+      } catch (e) {
+        errors.push({
+          pkgJson: relPath,
+          dep,
+          reason: "parse-error",
+          detail: String(e.message),
+        });
+      }
+    }
+  }
+
+  return errors;
+}
+
+// CLI entry point — only runs when executed directly, not when imported.
+if (process.argv[1] === fileURLToPath(import.meta.url)) {
+  const errors = checkLibraryDecisions();
+  if (!errors.length) process.exit(0);
+
+  console.error(
+    "✗ library-decisions/check: new runtime deps require an approved trace.\n",
+  );
+
+  const groups = {};
+  for (const e of errors) {
+    (groups[e.pkgJson] ??= []).push(e);
+  }
+  for (const [pkg, errs] of Object.entries(groups)) {
+    console.error(`  ${pkg}:`);
+    for (const e of errs) {
+      const msg =
+        e.reason === "no-trace"
+          ? "no staged trace in docs/library-decisions/"
+          : e.reason === "not-approved"
+            ? `trace decision is "${e.decision}" (expected "approved")`
+            : `trace parse error — ${e.detail}`;
+      console.error(`    ✗ ${e.dep}: ${msg}`);
+    }
+  }
+  console.error(
+    "\n  Evaluate the library first: .claude/skills/evaluate-library/SKILL.md",
+  );
+  process.exit(1);
+}
diff --git a/scripts/library-decisions/check.test.mjs b/scripts/library-decisions/check.test.mjs
new file mode 100644
index 0000000..31b0708
--- /dev/null
+++ b/scripts/library-decisions/check.test.mjs
@@ -0,0 +1,166 @@
+import { test, describe } from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { execSync } from "node:child_process";
+import { checkLibraryDecisions } from "./check.mjs";
+
+/** Create a temp git repo with one initial commit so HEAD exists. */
+function makeRepo() {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "libcheck-"));
+  const g = (cmd) => execSync(cmd, { cwd: dir, stdio: "pipe" });
+  g("git init");
+  g("git config user.email test@test.com");
+  g("git config user.name Test");
+  g("git config commit.gpgsign false");
+  fs.writeFileSync(path.join(dir, ".gitkeep"), "");
+  g("git add .gitkeep");
+  g("git commit -m init");
+  return { dir, g };
+}
+
+/** Write a package.json under relDir and commit it as the baseline. */
+function commitPkg(dir, g, relDir, pkg) {
+  const pkgDir = path.join(dir, relDir);
+  fs.mkdirSync(pkgDir, { recursive: true });
+  fs.writeFileSync(
+    path.join(pkgDir, "package.json"),
+    JSON.stringify(pkg, null, 2),
+  );
+  g(`git add ${relDir}/package.json`);
+  g("git commit -m add-pkg");
+}
+
+/** Overwrite a package.json and stage the result (no new commit). */
+function stagePkg(dir, g, relDir, pkg) {
+  fs.writeFileSync(
+    path.join(dir, relDir, "package.json"),
+    JSON.stringify(pkg, null, 2),
+  );
+  g(`git add ${relDir}/package.json`);
+}
+
+function traceFm(depName, decision) {
+  return `package: ${depName}
+version: "^1.0.0"
+tier: feature
+decision: ${decision}
+date: 2026-05-14
+deciders: [alice]
+adr: null
+filter-results:
+  license: MIT
+  types: native
+  maintenance: active
+  boundary-fit: pass
+  shadow-check: pass
+  eu-residency: ok
+  cve-scan: clean
+  named-consumer: pass
+verification-commands:
+  - pnpm audit --audit-level=moderate`;
+}
+
+/** Write a trace file and stage it. */
+function stageTrace(dir, g, depName, decision = "approved") {
+  const traceDir = path.join(dir, "docs", "library-decisions");
+  fs.mkdirSync(traceDir, { recursive: true });
+  const file = `2026-05-14-${depName}.md`;
+  fs.writeFileSync(
+    path.join(traceDir, file),
+    `---\n${traceFm(depName, decision)}\n---\n\n`,
+  );
+  g(`git add docs/library-decisions/${file}`);
+}
+
+describe("checkLibraryDecisions", () => {
+  test("new feature-tier dep without trace → exit 1", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", { dependencies: {} });
+    stagePkg(dir, g, "packages/feat-a", {
+      dependencies: { "new-lib": "^1.0.0" },
+    });
+
+    const errs = checkLibraryDecisions(dir);
+
+    assert.equal(errs.length, 1);
+    assert.equal(errs[0].dep, "new-lib");
+    assert.equal(errs[0].reason, "no-trace");
+  });
+
+  test("new feature-tier dep with approved trace staged → exit 0", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", { dependencies: {} });
+    stagePkg(dir, g, "packages/feat-a", {
+      dependencies: { "new-lib": "^1.0.0" },
+    });
+    stageTrace(dir, g, "new-lib", "approved");
+
+    assert.deepEqual(checkLibraryDecisions(dir), []);
+  });
+
+  test("new feature-tier dep with rejected-decision trace staged → exit 1", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", { dependencies: {} });
+    stagePkg(dir, g, "packages/feat-a", {
+      dependencies: { "new-lib": "^1.0.0" },
+    });
+    stageTrace(dir, g, "new-lib", "rejected");
+
+    const errs = checkLibraryDecisions(dir);
+
+    assert.equal(errs.length, 1);
+    assert.equal(errs[0].dep, "new-lib");
+    assert.equal(errs[0].reason, "not-approved");
+    assert.equal(errs[0].decision, "rejected");
+  });
+
+  test("new app-tier dep → exit 0", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "apps/web", { dependencies: {} });
+    stagePkg(dir, g, "apps/web", { dependencies: { "new-lib": "^1.0.0" } });
+
+    assert.deepEqual(checkLibraryDecisions(dir), []);
+  });
+
+  test("new devDependency → exit 0", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", {});
+    stagePkg(dir, g, "packages/feat-a", {
+      devDependencies: { "test-lib": "^1.0.0" },
+    });
+
+    assert.deepEqual(checkLibraryDecisions(dir), []);
+  });
+
+  test("multi-file diff with mixed pass/fail → exit 1 with per-package report", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", { dependencies: {} });
+    commitPkg(dir, g, "packages/feat-b", { dependencies: {} });
+    stagePkg(dir, g, "packages/feat-a", {
+      dependencies: { "lib-a": "^1.0.0" },
+    });
+    stagePkg(dir, g, "packages/feat-b", {
+      dependencies: { "lib-b": "^1.0.0" },
+    });
+    stageTrace(dir, g, "lib-a", "approved"); // feat-a passes; no trace for lib-b
+
+    const errs = checkLibraryDecisions(dir);
+
+    assert.equal(errs.length, 1);
+    assert.equal(errs[0].pkgJson, "packages/feat-b/package.json");
+    assert.equal(errs[0].dep, "lib-b");
+    assert.equal(errs[0].reason, "no-trace");
+  });
+
+  test("peerDependencies-only change → exit 0", () => {
+    const { dir, g } = makeRepo();
+    commitPkg(dir, g, "packages/feat-a", {});
+    stagePkg(dir, g, "packages/feat-a", {
+      peerDependencies: { react: "^18.0.0" },
+    });
+
+    assert.deepEqual(checkLibraryDecisions(dir), []);
+  });
+});