diff --git a/.changeset/bold-ravens-wake.md b/.changeset/bold-ravens-wake.md
new file mode 100644
index 000000000..0d1b55798
--- /dev/null
+++ b/.changeset/bold-ravens-wake.md
@@ -0,0 +1,7 @@
+---
+type: Changed
+pr: 1808
+---
+**Internal: third-party descriptor loader enforces `configHome` write-confinement at load time** — `loadRegistry({includeInstalled:true, configHome})` now rejects (skip + warn, fail-closed) any installed third-party host-plugin descriptor whose declared `destSubpath` resolves outside the supplied `configHome`, before it is composed into the registry (ADR-1239 Phase C-2 / #1681 slice 2). The `configHome` option is optional and backward-compatible (omitted → no load-time check; install-time gate still bounds writes). No user-facing change for existing flows.
+
+<!-- docs-exempt: internal loader hardening; no user-facing doc surface until slice 3's MCP server -->
diff --git a/src/capability-loader.cts b/src/capability-loader.cts
index 1a1049dc3..571c2001a 100644
--- a/src/capability-loader.cts
+++ b/src/capability-loader.cts
@@ -101,6 +101,14 @@ export interface LoadRegistryOptions {
   gsdHome?: string;
   /** Override the running GSD version used for engines.gsd satisfaction. */
   hostVersion?: string;
+  /**
+   * Optional configHome root for load-time write-confinement of installed
+   * third-party descriptors (ADR-1239 Phase C-2 / #1681). When set, each
+   * installed overlay's declared destSubpaths must resolve within this root or
+   * the descriptor is rejected fail-closed (skip + warn). Omit to rely on the
+   * install-time gate only (backward-compatible).
+   */
+  configHome?: string;
 }
 
 export interface OverlaySkip {
@@ -473,6 +481,10 @@ export function loadRegistry(options: LoadRegistryOptions = {}): Registry {
   const ledgerMod: LedgerModule = require('./capability-ledger.cjs');
   // eslint-disable-next-line @typescript-eslint/no-require-imports, @typescript-eslint/no-unsafe-assignment
   const consentMod: ConsentModule = require('./capability-consent.cjs');
+  // ADR-1239 Phase C-2 (#1681): load-time configHome confinement for installed
+  // third-party descriptors. Accessed via module ref for stub compatibility.
+  // eslint-disable-next-line @typescript-eslint/no-require-imports, @typescript-eslint/no-unsafe-assignment
+  const externalDescriptorTrust: { assertDescriptorConfined(descriptor: unknown, configHome: string): void; isPathConfined(target: string, root: string): boolean } = require('./external-descriptor-trust.cjs');
 
   const cwd = options.cwd || process.cwd();
   const hostVersion = options.hostVersion || readHostVersion();
@@ -748,6 +760,20 @@ export function loadRegistry(options: LoadRegistryOptions = {}): Registry {
         continue;
       }
 
+      // ADR-1239 Phase C-2 (#1681): load-time configHome confinement — reject
+      // (skip + warn) any installed third-party descriptor whose declared
+      // destSubpath escapes the user-approved configHome, BEFORE it is composed.
+      // Defense-in-depth on top of the install-time gate (#1679 AC3).
+      if (typeof options.configHome === 'string' && options.configHome.length > 0) {
+        try {
+          externalDescriptorTrust.assertDescriptorConfined(cap, options.configHome);
+        } catch (confineErr) {
+          acceptedMap.delete(id);
+          skip('configHome confinement rejected: ' + errMessage(confineErr));
+          continue;
+        }
+      }
+
       // Accepted.
       overlayCaps.push(cap);
       acceptedIds.add(id);
diff --git a/tests/external-descriptor-loader-wiring.test.cjs b/tests/external-descriptor-loader-wiring.test.cjs
new file mode 100644
index 000000000..73565c6c0
--- /dev/null
+++ b/tests/external-descriptor-loader-wiring.test.cjs
@@ -0,0 +1,75 @@
+'use strict';
+/**
+ * Integration test: loadRegistry wires the external-descriptor trust gate
+ * (ADR-1239 Phase C-2 / #1681 slice 2). When `configHome` is supplied, an
+ * installed overlay whose declared destSubpath escapes it is rejected
+ * (skip + confinement reason) and NOT composed; a confined overlay composes.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { cleanup } = require('./helpers.cjs');
+const { loadRegistry } = require('../gsd-core/bin/lib/capability-loader.cjs');
+
+const HOST = '1.6.0';
+
+function featureCap(id, extra) {
+  return {
+    id, role: 'feature', version: '1.0.0', title: id, description: 'overlay cap',
+    tier: 'standard', requires: [], engines: { gsd: '>=1.0.0' },
+    runtimeCompat: { supported: ['*'], unsupported: [] },
+    skills: [], agents: [], hooks: [], config: {}, steps: [], contributions: [], gates: [],
+    ...extra,
+  };
+}
+
+// Build a temp GSD home with .gsd/capabilities/<id>/capability.json per cap.
+function makeOverlayHome(caps) {
+  const home = fs.mkdtempSync(path.join(os.tmpdir(), 'cap-trust-'));
+  for (const cap of caps) {
+    const dir = path.join(home, '.gsd', 'capabilities', cap.id);
+    fs.mkdirSync(dir, { recursive: true });
+    fs.writeFileSync(path.join(dir, 'capability.json'), JSON.stringify(cap), 'utf8');
+  }
+  return home;
+}
+
+test('loadRegistry configHome confinement: escaping overlay is skipped with a confinement reason', () => {
+  const home = makeOverlayHome([
+    featureCap('confined-host', { runtime: { artifactLayout: { global: [{ destSubpath: 'skills' }] } } }),
+    featureCap('escape-host', { runtime: { artifactLayout: { global: [{ destSubpath: '../../../etc/passwd' }] } } }),
+  ]);
+  try {
+    const reg = loadRegistry({
+      includeInstalled: true, gsdHome: home, cwd: home, hostVersion: HOST,
+      configHome: path.join(home, '.target'),
+    });
+    const overlayIds = Object.keys(reg.capabilities || {}).filter((id) => id === 'confined-host' || id === 'escape-host');
+    assert.ok(overlayIds.includes('confined-host'), 'confined overlay must be composed');
+    assert.ok(!overlayIds.includes('escape-host'), 'escaping overlay must NOT be composed');
+    const skips = (reg._overlay && reg._overlay.warnings) || [];
+    const confinementSkip = skips.find((s) => /confinement/.test(s.reason || ''));
+    assert.ok(confinementSkip, `an overlay must be skipped with a confinement reason; warnings=${JSON.stringify(skips)}`);
+    assert.match(confinementSkip.reason, /escape-host/, 'the confinement skip must name the escaping descriptor');
+  } finally {
+    cleanup(home);
+  }
+});
+
+test('loadRegistry configHome confinement: omitted configHome = no load-time check (backward-compatible; relies on install-time gate)', () => {
+  // Same escaping overlay, but no configHome passed → it is NOT rejected by the load-time gate.
+  const home = makeOverlayHome([
+    featureCap('escape-host', { runtime: { artifactLayout: { global: [{ destSubpath: '../../../etc' }] } } }),
+  ]);
+  try {
+    const reg = loadRegistry({ includeInstalled: true, gsdHome: home, cwd: home, hostVersion: HOST });
+    const warnings = (reg._overlay && reg._overlay.warnings) || [];
+    const confinementSkip = warnings.find((s) => /confinement/.test(s.reason || ''));
+    assert.ok(!confinementSkip, 'no configHome → no load-time confinement check (backward-compatible)');
+  } finally {
+    cleanup(home);
+  }
+});