diff --git a/README.md b/README.md
index b8f8bd8..83cd295 100644
--- a/README.md
+++ b/README.md
@@ -128,20 +128,38 @@ See [`docs/bundles-spec.md`](docs/bundles-spec.md) for the full design.
 
 ## What's isolated, what leaks
 
-A bundle session does **not** see your everyday Claude Code skills, agents, or
-plugins from `~/.claude/`. Some surfaces still pass through, by design or by
-quirk:
-
-| Surface                                       | Visible under a bundle?     | How to control it                                                           |
-|-----------------------------------------------|-----------------------------|-----------------------------------------------------------------------------|
-| `~/.claude/skills`, `agents`, `plugins`       | **No** — fully isolated     | Nothing to do; `--plugin-dir` + `--settings` replace user-scope sources.    |
-| `~/.claude/` oauth, history, projects state   | Yes (shared on purpose)     | Keeps you logged in across bundles. Bundles never write here.               |
-| `<project>/.claude/skills`, `agents`          | **Yes — baseline leak**     | Keep the dir empty, or move those skills into a project-scope bundle (see below). |
-| `<project>/.mcp.json`                         | No, when bundle defines MCPs| Bundle adds `--strict-mcp-config`. Set [`mergeMcp: true`](docs/bundles-spec.md) to additive-merge. |
-| `<project>/.mcp.json`                         | Yes, when bundle has no MCPs| The strict flag is only emitted alongside bundle MCPs.                      |
+By default a bundle is **additive**: `umbel run` launches `claude --plugin-dir
+<cache>`, and in Claude Code `--plugin-dir` only *adds* the bundle on top of
+your normal discovery. So your globally-enabled plugins (`enabledPlugins` in
+`~/.claude/settings.json`) and `~/.claude/skills` still show up in the session.
+Set [`isolate: true`](docs/bundles-spec.md) on the bundle to suppress them.
+
+| Surface                                       | Visible under a bundle?           | How to control it                                                           |
+|-----------------------------------------------|-----------------------------------|-----------------------------------------------------------------------------|
+| `~/.claude/skills`, `agents`, enabled `plugins` | **Yes, by default — leaks**       | `--plugin-dir` only adds the bundle; it does not replace user-scope discovery. Set [`isolate: true`](docs/bundles-spec.md) to launch with `--bare` and load **only** the bundle. |
+| `~/.claude/` oauth, history, projects state   | Yes (shared on purpose)           | Keeps you logged in across bundles. Bundles never write here. `isolate` does not touch this. |
+| `<project>/.claude/skills`, `agents`          | **Yes — baseline leak**           | `isolate: true` (`--bare`) drops these too; otherwise keep the dir empty, or move those skills into a project-scope bundle (see below). |
+| `<project>/.mcp.json`                         | No, when bundle defines MCPs      | Bundle adds `--strict-mcp-config`. Set [`mergeMcp: true`](docs/bundles-spec.md) to additive-merge. |
+| `<project>/.mcp.json`                         | Yes, when bundle has no MCPs      | The strict flag is only emitted alongside bundle MCPs.                      |
 
 So if your goal is "don't show me anything other than what this bundle
-declares", you have to:
+declares", set `isolate: true`:
+
+```yaml
+---
+name: thinking
+isolate: true
+skills: [local/brainstorming, local/tdd]
+---
+```
+
+This launches `claude --bare --plugin-dir <cache> …`, so Claude Code loads
+**only** the bundle's skills/agents — no user plugins, no `~/.claude/skills`,
+no project-scope auto-discovery. The bundle's own `--settings` / `--mcp-config`
+still apply, and shared state (oauth, history) is untouched.
+
+Without `isolate` the bundle stays purely additive (the default). To narrow an
+additive session by hand instead:
 
 1. Author the bundle (user-scope or project-scope — bundles themselves are
    not the leak).
diff --git a/docs/bundles-spec.md b/docs/bundles-spec.md
index 7bbde27..fff5598 100644
--- a/docs/bundles-spec.md
+++ b/docs/bundles-spec.md
@@ -83,6 +83,7 @@ in DuckDB / pandas.
 | `hooks`          | no       | list of qualified refs | Each entry is `<source>/<leaf>`. Resolved against `~/.config/umbel/hooks/<source>/<leaf>/HOOK.md` + sidecars. |
 | `mcps`           | no       | list of qualified refs | Each entry is `<source>/<leaf>`. Resolved against `~/.config/umbel/mcps/<source>/<leaf>/MCP.md` + sidecars. |
 | `mergeMcp`       | no       | bool, default false | When true, omit `--strict-mcp-config`; bundle MCPs add to project. |
+| `isolate`        | no       | bool, default false | When true, launch adds `--bare` so the session loads **only** the bundle's artifacts — user plugins, `~/.claude/skills`, and project-scope auto-discovery are skipped. Default keeps the additive behaviour where `--plugin-dir` layers the bundle on top of normal discovery. |
 | `settings`       | no       | object              | Whitelisted keys only (see [Settings whitelist](#settings-whitelist)). |
 
 Unknown fields → build warning, ignored.
@@ -140,7 +141,7 @@ subfolders). Per-artifact root override is not in scope.
    - **Lists** (`skills`, `agents`, `hooks`, `mcps`):
      concat parent's then child's, dedupe by ref with **child winning**.
    - **Maps** (`settings`): deep-merge with child keys overriding.
-   - **Scalars** (`description`, `mergeMcp`): child overrides parent.
+   - **Scalars** (`description`, `mergeMcp`, `isolate`): child overrides parent.
 4. `extends:` itself is not inherited (each bundle declares its own parents).
 
 Missing parent name → build error.
@@ -264,6 +265,11 @@ Rules for the `## Invocation` block:
 - `--mcp-config` line present only when `.mcp.json` was emitted.
 - `--strict-mcp-config` line present only when previous AND `mergeMcp` is
   not `true`.
+- `--bare` line present (before `--plugin-dir`) only when `isolate` is `true`.
+  `--plugin-dir` alone is additive — Claude Code layers the bundle on top of
+  the user's enabled plugins and `~/.claude/skills`; `--bare` makes it load
+  only the bundle plugin while still honouring the `--settings` / `--mcp-config`
+  emitted below.
 - Future bundle frontmatter fields that map to claude flags add new lines
   here. This is the **single source of truth** for the bundle-features →
   claude-flags mapping; consumers do not reimplement it.
diff --git a/src/bundle/claude-args.ts b/src/bundle/claude-args.ts
index 366be57..27f3dc2 100644
--- a/src/bundle/claude-args.ts
+++ b/src/bundle/claude-args.ts
@@ -8,7 +8,17 @@ import type { ResolvedBundle } from "./compose.ts";
  * cache directory path — no filesystem inspection.
  */
 export function computeClaudeArgs(bundle: ResolvedBundle, cacheDir: string): string[] {
-  const args: string[] = ["--plugin-dir", cacheDir];
+  const args: string[] = [];
+
+  // Opt-in full isolation. `--plugin-dir` only ADDS the bundle on top of the
+  // user's normal discovery, so globally-enabled plugins and ~/.claude skills
+  // still leak in. `--bare` makes Claude Code load ONLY the explicitly-passed
+  // plugin (and its skills/agents) — it skips user plugins, ~/.claude skills,
+  // and project-scope auto-discovery, while still honouring the bundle's own
+  // --settings / --mcp-config below.
+  if (bundle.isolate === true) args.push("--bare");
+
+  args.push("--plugin-dir", cacheDir);
 
   if (hasSettings(bundle)) {
     args.push("--settings", join(cacheDir, "settings.json"));
diff --git a/src/bundle/compile.ts b/src/bundle/compile.ts
index 684dd50..b23d8b8 100644
--- a/src/bundle/compile.ts
+++ b/src/bundle/compile.ts
@@ -362,6 +362,7 @@ function resolvedFrontmatter(bundle: ResolvedBundle, hash: string): Record<strin
     if (v !== undefined && v.length > 0) out[f] = v;
   }
   if (bundle.mergeMcp !== undefined) out.mergeMcp = bundle.mergeMcp;
+  if (bundle.isolate !== undefined) out.isolate = bundle.isolate;
   if (bundle.settings && Object.keys(bundle.settings).length > 0) {
     out.settings = bundle.settings;
   }
diff --git a/src/bundle/compose.ts b/src/bundle/compose.ts
index 841525a..3cc601c 100644
--- a/src/bundle/compose.ts
+++ b/src/bundle/compose.ts
@@ -67,6 +67,7 @@ function mergePair(parent: ResolvedBundle, child: BundleManifest): ResolvedBundl
   if (childPlain.body !== undefined) out.body = childPlain.body;
   if (childPlain.description !== undefined) out.description = childPlain.description;
   if (childPlain.mergeMcp !== undefined) out.mergeMcp = childPlain.mergeMcp;
+  if (childPlain.isolate !== undefined) out.isolate = childPlain.isolate;
 
   for (const f of ARTIFACT_KINDS) {
     const merged = mergeStringList(parent[f], childPlain[f]);
diff --git a/src/bundle/manifest.ts b/src/bundle/manifest.ts
index 58627b8..24baced 100644
--- a/src/bundle/manifest.ts
+++ b/src/bundle/manifest.ts
@@ -51,6 +51,14 @@ export interface BundleManifest {
   /** List of qualified `<source>/<leaf>` refs. */
   mcps?: string[];
   mergeMcp?: boolean;
+  /**
+   * Opt-in full isolation. When true, the session loads ONLY the bundle's own
+   * artifacts: the launch adds `--bare`, so Claude Code skips the user's
+   * globally-enabled plugins, `~/.claude/skills`, and project-scope
+   * auto-discovery. Default (false/absent) keeps today's additive behaviour —
+   * `--plugin-dir` layers the bundle on top of whatever is already enabled.
+   */
+  isolate?: boolean;
   settings?: BundleSettings;
   body: string;
   sourcePath: string;
@@ -111,6 +119,12 @@ export function loadManifest(path: string): ManifestResult {
   if (data.mergeMcp !== undefined) {
     manifest.mergeMcp = data.mergeMcp as boolean;
   }
+  if (data.isolate !== undefined) {
+    if (typeof data.isolate !== "boolean") {
+      throw new UsageError(`bundle ${path}: 'isolate' must be a boolean`);
+    }
+    manifest.isolate = data.isolate;
+  }
   if (data.settings !== undefined) {
     const settings = data.settings as Record<string, unknown>;
     for (const key of Object.keys(settings)) {
@@ -144,6 +158,7 @@ const KNOWN_FIELDS = new Set([
   "hooks",
   "mcps",
   "mergeMcp",
+  "isolate",
   "settings",
 ]);
 
diff --git a/test/unit/bundle/claude-args.test.ts b/test/unit/bundle/claude-args.test.ts
index 505e2be..db505e1 100644
--- a/test/unit/bundle/claude-args.test.ts
+++ b/test/unit/bundle/claude-args.test.ts
@@ -46,6 +46,58 @@ describe("computeClaudeArgs", () => {
   });
 });
 
+// Opt-in isolation (`isolate: true`).
+//
+// `umbel run <bundle>` launches `claude --plugin-dir <cache>`. In Claude Code
+// 2.1.x `--plugin-dir` ADDS the bundle plugin on top of normal discovery — it
+// does NOT suppress the user's globally-enabled plugins (`enabledPlugins` in
+// ~/.claude/settings.json) nor ~/.claude/skills. So those skills leak into a
+// bundle session. `isolate: true` opts a bundle into `--bare`, the documented
+// lever that loads ONLY the --plugin-dir plugin (and its own skills/agents).
+// Isolation is opt-in to preserve today's additive default behaviour.
+describe("computeClaudeArgs — opt-in isolation (isolate: true)", () => {
+  const cache = "/abs/cache";
+
+  const isolatesUserPlugins = (args: string[]): boolean => {
+    if (args.includes("--bare")) return true;
+    const i = args.indexOf("--setting-sources");
+    if (i === -1) return false;
+    const sources = (args[i + 1] ?? "").split(",").map((s) => s.trim());
+    return sources.length > 0 && !sources.includes("user");
+  };
+
+  it("does NOT isolate by default — additive behaviour preserved", () => {
+    const args = computeClaudeArgs(bundle({ skills: ["superpowers/brainstorming"] }), cache);
+    expect(isolatesUserPlugins(args)).toBe(false);
+    expect(args).toEqual(["--plugin-dir", cache]);
+  });
+
+  it("isolate: true drops the user's globally-enabled plugins", () => {
+    const args = computeClaudeArgs(
+      bundle({ skills: ["superpowers/brainstorming"], isolate: true }),
+      cache,
+    );
+    expect(isolatesUserPlugins(args)).toBe(true);
+    expect(args).toContain("--plugin-dir");
+    expect(args).toContain(cache);
+  });
+
+  it("isolate keeps the bundle's own settings flowing (still passes --settings)", () => {
+    const args = computeClaudeArgs(
+      bundle({ skills: ["local/tdd"], isolate: true, settings: { model: "claude-opus-4-7" } }),
+      cache,
+    );
+    expect(isolatesUserPlugins(args)).toBe(true);
+    expect(args).toContain("--settings");
+    expect(args).toContain(`${cache}/settings.json`);
+  });
+
+  it("isolate: false behaves like the default (no isolation)", () => {
+    const args = computeClaudeArgs(bundle({ skills: ["local/tdd"], isolate: false }), cache);
+    expect(isolatesUserPlugins(args)).toBe(false);
+  });
+});
+
 describe("formatClaudeInvocation", () => {
   it("renders a flag-only argv as backslash-continued bash", () => {
     const out = formatClaudeInvocation(["--plugin-dir", "/abs"]);
diff --git a/test/unit/bundle/compose.test.ts b/test/unit/bundle/compose.test.ts
index cad753a..9eabfef 100644
--- a/test/unit/bundle/compose.test.ts
+++ b/test/unit/bundle/compose.test.ts
@@ -47,6 +47,16 @@ describe("compose", () => {
     expect(r.mergeMcp).toBe(true);
   });
 
+  it("inherits isolate from parent; child can override it off", () => {
+    const ix = index(
+      m("base", { isolate: true }),
+      m("child", { extends: ["base"], skills: ["c"] }),
+      m("opt-out", { extends: ["base"], isolate: false }),
+    );
+    expect(compose("child", ix).isolate).toBe(true);
+    expect(compose("opt-out", ix).isolate).toBe(false);
+  });
+
   it("deep-merges settings env across parent + child", () => {
     const ix = index(
       m("base", { settings: { model: "old", env: { A: "1" } } }),
diff --git a/test/unit/bundle/manifest.test.ts b/test/unit/bundle/manifest.test.ts
index cd7e74e..3a658e4 100644
--- a/test/unit/bundle/manifest.test.ts
+++ b/test/unit/bundle/manifest.test.ts
@@ -102,6 +102,21 @@ Bundle for ad-hoc analysis.
     expect(warnings).toEqual([]);
   });
 
+  it.each([
+    ["true", true],
+    ["false", false],
+  ])("parses 'isolate: %s' as a known boolean field", (literal, expected) => {
+    const path = writeBundle("iso", `---\nname: iso\nisolate: ${literal}\n---\n`);
+    const { manifest, warnings } = loadManifest(path);
+    expect(manifest.isolate).toBe(expected);
+    expect(warnings).toEqual([]);
+  });
+
+  it("rejects non-boolean 'isolate'", () => {
+    const path = writeBundle("bad-iso", "---\nname: bad-iso\nisolate: yes-please\n---\n");
+    expect(() => loadManifest(path)).toThrow(/isolate.*boolean/i);
+  });
+
   it("rejects settings keys outside the whitelist", () => {
     const path = writeBundle(
       "bad-settings",