diff --git a/package.json b/package.json
index 5536a24b6..8b92b89a9 100644
--- a/package.json
+++ b/package.json
@@ -32,12 +32,13 @@
     "build:all": "bun build --compile --target=bun-darwin-arm64 --outfile bin/gbrain-darwin-arm64 src/cli.ts && bun build --compile --target=bun-linux-x64 --outfile bin/gbrain-linux-x64 src/cli.ts",
     "build:schema": "bash scripts/build-schema.sh",
     "build:llms": "bun run scripts/build-llms.ts",
-    "test": "scripts/check-jsonb-pattern.sh && scripts/check-progress-to-stdout.sh && scripts/check-wasm-embedded.sh && bun run typecheck && bun test --timeout=60000",
+    "test": "scripts/check-jsonb-pattern.sh && scripts/check-progress-to-stdout.sh && scripts/check-wasm-embedded.sh && scripts/check-exports-count.sh && bun run typecheck && bun test --timeout=60000",
     "check:wasm": "scripts/check-wasm-embedded.sh",
     "test:e2e": "bash scripts/run-e2e.sh",
     "typecheck": "tsc --noEmit",
     "check:jsonb": "scripts/check-jsonb-pattern.sh",
     "check:progress": "scripts/check-progress-to-stdout.sh",
+    "check:exports-count": "scripts/check-exports-count.sh",
     "postinstall": "command -v gbrain >/dev/null 2>&1 && gbrain apply-migrations --yes --non-interactive || echo '[gbrain] postinstall skipped. If installed via bun install -g github:...: run `gbrain doctor` and `gbrain apply-migrations --yes` manually. See https://github.com/garrytan/gbrain/issues/218' 1>&2",
     "prepublish:clawhub": "bun run build:all",
     "publish:clawhub": "clawhub package publish . --family bundle-plugin"
diff --git a/scripts/check-exports-count.sh b/scripts/check-exports-count.sh
new file mode 100755
index 000000000..160c4bf14
--- /dev/null
+++ b/scripts/check-exports-count.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# CI guard: the public exports surface never shrinks silently (v0.21.0).
+#
+# Precedent: scripts/check-jsonb-pattern.sh + check-progress-to-stdout.sh
+# are grep-based structural guards wired into `bun run test`. This one
+# counts the entries in package.json "exports" and fails when the count
+# drops below the v0.21.0 baseline (17 entries).
+#
+# Policy (from CLAUDE.md):
+#   "Removing any of these is a breaking change going forward."
+#
+# If you're legitimately removing a public export: bump gbrain's minor
+# version, note the removal in CHANGELOG.md under a "Breaking changes"
+# bullet, then bump EXPECTED_COUNT below. Anything else is a regression.
+#
+# Adding a new export: update EXPECTED_COUNT to match AND extend the
+# EXPECTED_EXPORTS list in test/public-exports.test.ts so the runtime
+# contract test pins the canary symbol.
+
+set -euo pipefail
+
+EXPECTED_COUNT=17
+
+# Count top-level keys in the exports object. `node -e` parses JSON
+# reliably without needing jq (which isn't in every CI environment).
+ACTUAL=$(node -e "
+  const pkg = require('./package.json');
+  console.log(Object.keys(pkg.exports || {}).length);
+")
+
+if [ "$ACTUAL" -lt "$EXPECTED_COUNT" ]; then
+  echo "❌ public-exports guard: package.json exports shrank from $EXPECTED_COUNT to $ACTUAL"
+  echo "   Removing a public export is a breaking change (see CLAUDE.md)."
+  echo "   If intentional: bump gbrain minor version + update EXPECTED_COUNT in"
+  echo "   scripts/check-exports-count.sh and EXPECTED_EXPORTS in"
+  echo "   test/public-exports.test.ts, AND add a CHANGELOG 'Breaking changes' bullet."
+  exit 1
+fi
+
+if [ "$ACTUAL" -gt "$EXPECTED_COUNT" ]; then
+  echo "⚠️  public-exports guard: package.json exports grew from $EXPECTED_COUNT to $ACTUAL"
+  echo "   Additive public API change. Update EXPECTED_COUNT in this script + the"
+  echo "   EXPECTED_EXPORTS list in test/public-exports.test.ts to lock the new"
+  echo "   canary symbols."
+  exit 1
+fi
+
+echo "✓ public-exports guard: $ACTUAL entries (matches baseline $EXPECTED_COUNT)"
diff --git a/test/public-exports.test.ts b/test/public-exports.test.ts
new file mode 100644
index 000000000..954f13073
--- /dev/null
+++ b/test/public-exports.test.ts
@@ -0,0 +1,102 @@
+/**
+ * Public exports contract test (v0.21.0 — Lane 2 / R2).
+ *
+ * Reads package.json "exports" at runtime, imports each subpath via the
+ * package name ("gbrain/<subpath>") so it actually exercises the
+ * resolver — then asserts each resolves AND has at least one canary
+ * symbol. Importing from relative paths (e.g. "../src/core/engine.ts")
+ * would bypass the exports map and miss resolver/wiring breakage.
+ *
+ * The canary symbols are concrete values each module re-exports today.
+ * If a refactor renames or removes one, this test fails in CI so the
+ * downstream consumer (gbrain-evals) doesn't silently break first.
+ *
+ * To add a new public export: extend `EXPECTED_EXPORTS` below. To
+ * REMOVE one: bump gbrain's minor (breaking-interface per CLAUDE.md
+ * "Removing any of these is a breaking change going forward").
+ */
+
+import { readFileSync } from 'fs';
+import { resolve } from 'path';
+import { describe, expect, test } from 'bun:test';
+
+interface ExpectedExport {
+  /** Subpath key as it appears in package.json exports. */
+  subpath: string;
+  /** At least one named export that MUST exist at runtime. Chosen from the
+   *  module's current surface; if it goes away, that's a breaking change. */
+  canary: string[];
+}
+
+/**
+ * Canary symbols pinned to the v0.21.0 contract. Changes to this list
+ * are intentional breaking changes to the public exports surface.
+ */
+const EXPECTED_EXPORTS: ExpectedExport[] = [
+  { subpath: 'gbrain', canary: [] }, // root "." export; no single canary — just require import success
+  { subpath: 'gbrain/engine', canary: ['clampSearchLimit', 'MAX_SEARCH_LIMIT'] },
+  { subpath: 'gbrain/types', canary: ['GBrainError'] },
+  { subpath: 'gbrain/operations', canary: ['operations', 'OperationError'] },
+  { subpath: 'gbrain/minions', canary: [] }, // barrel module; re-exports many names
+  { subpath: 'gbrain/engine-factory', canary: [] }, // factory exports a default creator
+  { subpath: 'gbrain/pglite-engine', canary: ['PGLiteEngine'] },
+  { subpath: 'gbrain/link-extraction', canary: ['extractEntityRefs', 'extractPageLinks'] },
+  { subpath: 'gbrain/import-file', canary: ['importFromContent'] },
+  { subpath: 'gbrain/transcription', canary: [] },
+  { subpath: 'gbrain/embedding', canary: ['embed'] },
+  { subpath: 'gbrain/config', canary: ['loadConfig'] },
+  { subpath: 'gbrain/markdown', canary: ['splitBody', 'parseMarkdown', 'serializeMarkdown'] },
+  { subpath: 'gbrain/backoff', canary: [] },
+  { subpath: 'gbrain/search/hybrid', canary: ['hybridSearch', 'rrfFusion'] },
+  { subpath: 'gbrain/search/expansion', canary: ['expandQuery'] },
+  { subpath: 'gbrain/extract', canary: [] },
+];
+
+function readPackageExports(): Record<string, string> {
+  const pkgPath = resolve(import.meta.dir, '..', 'package.json');
+  const pkg = JSON.parse(readFileSync(pkgPath, 'utf8'));
+  return pkg.exports as Record<string, string>;
+}
+
+describe('public exports — package.json exports map', () => {
+  test('has the expected number of subpaths (v0.21.0 locks the surface)', () => {
+    const exports = readPackageExports();
+    const count = Object.keys(exports).length;
+    // Adding new exports: increment this + add to EXPECTED_EXPORTS below.
+    // Removing exports: see CLAUDE.md "Removing any of these is a
+    // breaking change going forward" — bump minor and update this count.
+    expect(count).toBe(17);
+  });
+
+  test('EXPECTED_EXPORTS list matches the exports map exactly (no drift)', () => {
+    const exports = readPackageExports();
+    const exportedSubpaths = Object.keys(exports).map(k => (k === '.' ? 'gbrain' : `gbrain${k.slice(1)}`)).sort();
+    const expectedSubpaths = EXPECTED_EXPORTS.map(e => e.subpath).sort();
+    expect(expectedSubpaths).toEqual(exportedSubpaths);
+  });
+});
+
+describe('public exports — every subpath resolves via package name', () => {
+  for (const entry of EXPECTED_EXPORTS) {
+    test(`${entry.subpath} imports without throwing`, async () => {
+      // Package-path import goes through the exports map — bypassing a
+      // broken/removed subpath surfaces here. Importing "../src/..."
+      // would resolve via filesystem and miss the contract.
+      const mod = await import(entry.subpath);
+      expect(mod).toBeDefined();
+      expect(typeof mod).toBe('object');
+    });
+
+    if (entry.canary.length > 0) {
+      test(`${entry.subpath} exports canary symbols: ${entry.canary.join(', ')}`, async () => {
+        const mod = await import(entry.subpath);
+        for (const name of entry.canary) {
+          expect(mod).toHaveProperty(name);
+          // Must be something truthy — value, class, or function. Not
+          // just a TypeScript type (those don't exist at runtime).
+          expect(mod[name]).toBeDefined();
+        }
+      });
+    }
+  }
+});