diff --git a/evidence/hosted-hardening/mutation.txt b/evidence/hosted-hardening/mutation.txt
new file mode 100644
index 00000000..44bea48d
--- /dev/null
+++ b/evidence/hosted-hardening/mutation.txt
@@ -0,0 +1,44 @@
+# Mutation 1: make the Surface entry resolve lazily again (revert fix A)
+$ npx vitest run tests/hosted-hardening.test.ts -t initializes
+   × hosted extension hardening > resolves the Surface entry while this module initializes, before authored code can steer the loader 25ms
+     → expected '/private/var/folders/l7/1p99_dl16pj5x…' to be '/Users/khaliqgant/Projects/AgentWorkf…' // Object.is equality
+      Tests  1 failed | 10 skipped (11)
+$ shasum -a 256 original restored
+298a611328194a969330551de0cccdccee2221fe51d958c5e7e607d4edaf5dd2
+298a611328194a969330551de0cccdccee2221fe51d958c5e7e607d4edaf5dd2
+$ npx vitest run tests/hosted-hardening.test.ts -t initializes
+ ✓ tests/hosted-hardening.test.ts (11 tests | 10 skipped) 8ms
+      Tests  1 passed | 10 skipped (11)
+
+# Mutation 2: drop the hosted lock name pattern check (revert fix B)
+$ npx vitest run tests/hosted-hardening.test.ts -t not one safe path component
+   × hosted extension hardening > refuses a hosted lock whose plugin name "../escape" is not one safe path component 29ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+   × hosted extension hardening > refuses a hosted lock whose plugin name "a/b" is not one safe path component 6ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+   × hosted extension hardening > refuses a hosted lock whose plugin name "." is not one safe path component 5ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+   × hosted extension hardening > refuses a hosted lock whose plugin name ".." is not one safe path component 5ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+   × hosted extension hardening > refuses a hosted lock whose plugin name "Babysitter" is not one safe path component 5ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+   × hosted extension hardening > refuses a hosted lock whose plugin name "babysitter/../../etc" is not one safe path component 5ms
+     → expected Error: /private/var/folders/l7/1p99_dl16p… { code: '…' } to match object { code: 'plugin_lock_invalid' }
+$ shasum -a 256 original restored
+b1c276859dc837aecd4fb2d5578d30c0f9913ad9a72653a6c655fae2a5324646
+b1c276859dc837aecd4fb2d5578d30c0f9913ad9a72653a6c655fae2a5324646
+$ npx vitest run tests/hosted-hardening.test.ts -t not one safe path component
+ ✓ tests/hosted-hardening.test.ts (11 tests | 4 skipped) 36ms
+      Tests  7 passed | 4 skipped (11)
+
+# Mutation 3: restore last-match compat.base selection (revert fix C)
+$ npx vitest run tests/hosted-hardening.test.ts -t first compat.base entry
+   × hosted extension hardening > matches the first compat.base entry for a name, not the last 17ms
+     → expected [Function] to not throw an error but 'Error: babysitter requires software-f…' was thrown
+      Tests  1 failed | 10 skipped (11)
+$ shasum -a 256 original restored
+3f4069271d2e16cd35dc6ce78ef2ff24067d62ca7fc1d7a0ea8bb3b53a3dd987
+3f4069271d2e16cd35dc6ce78ef2ff24067d62ca7fc1d7a0ea8bb3b53a3dd987
+$ npx vitest run tests/hosted-hardening.test.ts -t first compat.base entry
+ ✓ tests/hosted-hardening.test.ts (11 tests | 10 skipped) 9ms
+      Tests  1 passed | 10 skipped (11)
diff --git a/evidence/hosted-hardening/tests.txt b/evidence/hosted-hardening/tests.txt
new file mode 100644
index 00000000..4145acc6
--- /dev/null
+++ b/evidence/hosted-hardening/tests.txt
@@ -0,0 +1,46 @@
+$ (cd packages/sdk && npm run typecheck)
+> @relayflows/sdk@2.0.32 typecheck
+> tsc --noEmit && tsc -p tsconfig.type-tests.json
+
+Exit: 0
+
+$ (cd packages/sdk && npm run build)
+> tsc && node scripts/make-cli-executable.mjs
+
+Exit: 0
+
+$ (cd packages/sdk && npm run typecheck:tests)
+> @relayflows/sdk@2.0.32 typecheck:tests
+> tsc -p tsconfig.tests.json
+
+Exit: 0
+
+$ (cd packages/sdk && npx vitest run tests/hosted-hardening.test.ts tests/hosted-extension-isolation.test.ts tests/hosted-extension-protocol.test.ts tests/hosted-extension-routing.test.ts tests/hosted-base-snapshot.test.ts tests/plugin-store-bounds.test.ts tests/babysitter-native-extension.test.ts tests/flow-extension-compose.test.ts tests/plugin-extension.test.ts tests/authored-flow.test.ts)  # passing lines elided
+ ❯ runTest node_modules/@vitest/runner/dist/index.js:1056:11
+ ❯ runSuite node_modules/@vitest/runner/dist/index.js:1205:15
+ ❯ runSuite node_modules/@vitest/runner/dist/index.js:1205:15
+ ❯ runFiles node_modules/@vitest/runner/dist/index.js:1262:5
+ ❯ startTests node_modules/@vitest/runner/dist/index.js:1271:3
+
+⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+Serialized Error: { code: 'plugin_unsupported' }
+This error originated in "tests/hosted-extension-protocol.test.ts" test file. It doesn't mean the error was thrown inside the file itself, but while it was running.
+The latest test that might've caused the error is "rejects two forged calls after the authoritative first outcome settles". It might mean one of the following:
+- The error was thrown, while Vitest was running this test.
+- If the error occurred after the test had been completed, this was the last documented test before it was thrown.
+⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+
+ Test Files  4 failed | 6 passed (10)
+      Tests  34 failed | 199 passed | 47 skipped (280)
+     Errors  1 error
+   Start at  22:55:46
+   Duration  51.29s (transform 11.41s, setup 783ms, collect 60.44s, tests 63.80s, environment 7ms, prepare 5.61s)
+
+Exit: 1
+
+# Baseline: the same suites on this branch with all changes stashed (git stash push -u), macOS
+$ (cd packages/sdk && npx vitest run tests/hosted-extension-isolation.test.ts -t "resolves locked artifacts")
+   × hosted extension capability isolation > resolves locked artifacts without importing extension top-level code
+     → expected [ { …(6) } ] to deeply equal [ { …(6) } ]
+      Tests  1 failed | 21 skipped (22)
+The 34 failures in the run above are all pre-existing on this host: 33 are 'hosted extension isolation requires Linux' (these suites need Linux + bubblewrap) and the deep-equal one reproduces with the changes stashed, as captured here. CI on Linux is the authority.
diff --git a/packages/sdk/src/flow-extension-compat.ts b/packages/sdk/src/flow-extension-compat.ts
index ad38f3db..8d408075 100644
--- a/packages/sdk/src/flow-extension-compat.ts
+++ b/packages/sdk/src/flow-extension-compat.ts
@@ -42,7 +42,9 @@ function assertRuntimeRange(
  */
 export function assertBaseCompatible(manifest: FlowExtensionManifest, base: { readonly name: string; readonly version?: string }): void {
   let entry: FlowExtensionManifest['compat']['base'][number] | undefined;
-  for (let index = 0; index < manifest.compat.base.length; index += 1) {
+  // First match, as `.find()` did before the intrinsic-capture rewrite: a
+  // later duplicate name must not widen or narrow the range that decides.
+  for (let index = 0; index < manifest.compat.base.length && entry === undefined; index += 1) {
     if (manifest.compat.base[index]!.name === base.name) entry = manifest.compat.base[index];
   }
   if (entry === undefined) {
diff --git a/packages/sdk/src/hosted-extension-declarations.ts b/packages/sdk/src/hosted-extension-declarations.ts
index d3571407..b9ffa89f 100644
--- a/packages/sdk/src/hosted-extension-declarations.ts
+++ b/packages/sdk/src/hosted-extension-declarations.ts
@@ -67,6 +67,9 @@ const STRING_STARTS_WITH = Function.prototype.call.bind(
 ) as (value: string, search: string) => boolean;
 const LOCK_HEX64 = /^[0-9a-f]{64}$/;
 const LOCK_SHA = /^[0-9a-f]{40}$/;
+// `name` reaches pluginStoreDirectory as a path segment, so it stays one
+// lowercase kebab-case component: no separators, no `.`/`..`, no traversal.
+const LOCK_NAME = /^[a-z0-9]+(?:-[a-z0-9]+)*$/;
 const LOCK_OWNER = /^[A-Za-z0-9](?:[A-Za-z0-9-]{0,38})$/;
 const LOCK_REPO = /^[A-Za-z0-9_.-]{1,100}$/;
 const HTTPS_GITHUB = /^https:\/\/github\.com\//;
@@ -240,6 +243,7 @@ function parseHostedPluginLock(input: unknown): {
       ]) ||
       entry.kind !== "flow-extension" ||
       typeof entry.name !== "string" ||
+      !REGEXP_TEST(LOCK_NAME, entry.name) ||
       typeof entry.version !== "string" ||
       typeof entry.digest !== "string" ||
       !REGEXP_TEST(LOCK_HEX64, entry.digest) ||
diff --git a/packages/sdk/src/hosted-extension-sandbox.ts b/packages/sdk/src/hosted-extension-sandbox.ts
index 214c4728..c7564909 100644
--- a/packages/sdk/src/hosted-extension-sandbox.ts
+++ b/packages/sdk/src/hosted-extension-sandbox.ts
@@ -53,6 +53,22 @@ const PATH_RESOLVE = resolve;
 const SPAWN = spawn;
 const PROCESS_EXEC_PATH = process.execPath;
 const PROCESS_NODE_VERSION = process.versions.node;
+
+/**
+ * The host's own `@relayflows/surface` entry, resolved once while this module
+ * initializes. `require.resolve` consults `Module._resolveFilename`, which is
+ * writable: resolving lazily would run after authored flow code has had a
+ * chance to replace it and could hand the sandbox a different package. The
+ * lookup therefore happens before any authored module can load, and a failure
+ * is reported when a caller actually needs the root.
+ */
+const SURFACE_ENTRY: string | undefined = (() => {
+  try { return REALPATH_SYNC(CREATE_REQUIRE(import.meta.url).resolve('@relayflows/surface')); }
+  catch { return undefined; }
+})();
+
+/** @internal Seam for the regression that pins initialization-time capture. */
+export function capturedSurfaceEntry(): string | undefined { return SURFACE_ENTRY; }
 const PROCESS_PLATFORM = process.platform;
 const PROMISE = Promise;
 const EVENT_ON = Function.prototype.call.bind(EventEmitter.prototype.on) as (
@@ -250,9 +266,8 @@ export async function runHostedExtensionSandbox(
 
 async function resolveSurfaceRoot(expectedVersion: string, override?: string): Promise<string> {
   if (override !== undefined) return await checkedSurfaceRoot(override, expectedVersion);
-  let resolved: string;
-  try { resolved = REALPATH_SYNC(CREATE_REQUIRE(import.meta.url).resolve('@relayflows/surface')); }
-  catch { return unsupported('hosted extension cannot resolve @relayflows/surface'); }
+  const resolved = SURFACE_ENTRY;
+  if (resolved === undefined) return unsupported('hosted extension cannot resolve @relayflows/surface');
   let directory = PATH_DIRNAME(resolved);
   const root = PATH_PARSE(directory).root;
   while (directory !== root) {
diff --git a/packages/sdk/tests/hosted-hardening.test.ts b/packages/sdk/tests/hosted-hardening.test.ts
new file mode 100644
index 00000000..3e15b0de
--- /dev/null
+++ b/packages/sdk/tests/hosted-hardening.test.ts
@@ -0,0 +1,109 @@
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import { tmpdir } from 'node:os';
+import { join, resolve } from 'node:path';
+import { afterEach, describe, expect, it } from 'vitest';
+import { assertBaseCompatible } from '../src/flow-extension-compat.js';
+import type { FlowExtensionManifest } from '../src/flow-extension-manifest.js';
+import { capturedSurfaceEntry } from '../src/hosted-extension-sandbox.js';
+import { loadHostedExtensionArtifacts } from '../src/hosted-extension-runtime.js';
+
+const roots: string[] = [];
+afterEach(() => roots.splice(0).forEach(path => rmSync(path, { recursive: true, force: true })));
+
+/** A project whose lockfile names one extension, with no artifact materialized. */
+function project(name: string): string {
+  const root = mkdtempSync(join(tmpdir(), 'hosted-hardening-'));
+  roots.push(root);
+  writeFileSync(join(root, 'package.json'), '{"type":"module"}');
+  writeFileSync(join(root, 'flows.json'), JSON.stringify({
+    plugins: [`github:AgentWorkforce/flows@${'a'.repeat(40)}#extensions/babysitter`],
+  }));
+  writeFileSync(join(root, 'flows.lock.json'), JSON.stringify({
+    version: 2,
+    plugins: [{
+      name, kind: 'flow-extension', version: '0.2.0',
+      source: {
+        host: 'github', owner: 'AgentWorkforce', repo: 'flows',
+        sha: 'a'.repeat(40), path: 'extensions/babysitter',
+      },
+      digest: 'b'.repeat(64), manifestSha256: 'c'.repeat(64),
+      order: 1, resolvedAt: '2026-09-22T12:00:00.000Z',
+    }],
+  }));
+  writeFileSync(join(root, 'software-factory.flow.ts'), 'export default {};');
+  return join(root, 'software-factory.flow.ts');
+}
+
+function manifest(base: FlowExtensionManifest['compat']['base']): FlowExtensionManifest {
+  return {
+    schema: 2, kind: 'flow-extension', name: 'babysitter', version: '0.2.0',
+    compat: { surface: '*', sdk: '*', base },
+    entry: 'babysitter.flow.ts',
+    extends: { handlers: true, hooks: [] },
+    triggers: [],
+    permissions: { integrations: [], harnesses: [], mcp: [], writes: [] },
+    preflight: { credentials: [], servers: [] },
+  } as FlowExtensionManifest;
+}
+
+describe('hosted extension hardening', () => {
+  it('resolves the Surface entry while this module initializes, before authored code can steer the loader', () => {
+    const before = capturedSurfaceEntry();
+    // A realpath: a linked workspace copy resolves outside node_modules.
+    expect(before).toMatch(/surface[/\\]dist[/\\]index\.js$/);
+    // What an authored module could do to the resolver once it is imported.
+    const moduleExports = createRequire(import.meta.url)('node:module') as {
+      _resolveFilename: (...args: unknown[]) => string;
+    };
+    const original = moduleExports._resolveFilename;
+    const hostile = mkdtempSync(join(tmpdir(), 'hosted-hostile-surface-'));
+    roots.push(hostile);
+    mkdirSync(join(hostile, 'dist'), { recursive: true });
+    writeFileSync(join(hostile, 'package.json'), JSON.stringify({ name: '@relayflows/surface', version: '0.0.0' }));
+    writeFileSync(join(hostile, 'dist', 'index.js'), 'export {};\n');
+    try {
+      moduleExports._resolveFilename = function poisoned(request: unknown, ...rest: unknown[]): string {
+        if (request === '@relayflows/surface') return join(hostile, 'dist', 'index.js');
+        return original.call(this, request, ...rest);
+      };
+      // The capture already happened, so the poisoned resolver cannot be consulted.
+      expect(capturedSurfaceEntry()).toBe(before);
+      expect(capturedSurfaceEntry()).not.toContain(hostile);
+    } finally {
+      moduleExports._resolveFilename = original;
+    }
+  });
+
+  it.each(['../escape', 'a/b', '.', '..', 'Babysitter', 'babysitter/../../etc', ''])(
+    'refuses a hosted lock whose plugin name %j is not one safe path component',
+    async name => {
+      await expect(loadHostedExtensionArtifacts(project(name)))
+        .rejects.toMatchObject({ code: 'plugin_lock_invalid' });
+    },
+  );
+
+  it('still accepts an ordinary kebab-case lock name, failing later on the absent artifact', async () => {
+    // Not plugin_lock_invalid: the name parses, and verification fails on bytes.
+    await expect(loadHostedExtensionArtifacts(project('babysitter')))
+      .rejects.not.toMatchObject({ code: 'plugin_lock_invalid' });
+  });
+
+  it('matches the first compat.base entry for a name, not the last', () => {
+    const duplicated = manifest([
+      Object.freeze({ name: 'software-factory', version: '2.0.22' }),
+      Object.freeze({ name: 'software-factory', version: '9.9.9' }),
+    ]);
+    // First entry decides: 2.0.22 satisfies it, and the wider later duplicate cannot rescue 9.9.9.
+    expect(() => assertBaseCompatible(duplicated, { name: 'software-factory', version: '2.0.22' })).not.toThrow();
+    expect(() => assertBaseCompatible(duplicated, { name: 'software-factory', version: '9.9.9' }))
+      .toThrow(expect.objectContaining({ code: 'plugin_incompatible' }));
+  });
+
+  it('keeps reporting every declared base name when none matches', () => {
+    expect(() => assertBaseCompatible(
+      manifest([Object.freeze({ name: 'software-factory', version: '*' }), Object.freeze({ name: 'garden', version: '*' })]),
+      { name: 'release-manager', version: '1.0.0' },
+    )).toThrow(/extends software-factory, garden, not "release-manager"/);
+  });
+});