diff --git a/crates/punktfunk-host/src/plugins.rs b/crates/punktfunk-host/src/plugins.rs
index 3811aaa2..72ece455 100644
--- a/crates/punktfunk-host/src/plugins.rs
+++ b/crates/punktfunk-host/src/plugins.rs
@@ -101,6 +101,20 @@ NOTES:
 /// Locate the runner and hand it the argv verbatim, inheriting stdio so bun's progress output goes
 /// straight to the user's terminal. Exits with the runner's own status code.
 fn forward_to_runner(args: &[String]) -> Result<()> {
+    // `bun add` installs into the nearest ancestor `package.json`, not into its working directory,
+    // so the plugins dir has to own one before the runner runs or a stray `~/package.json` captures
+    // the install — silently, exit 0 (see `store::ensure_plugin_root`). The runner seeds it too, but
+    // the installed scripting package can predate this binary, so do it on this side as well.
+    if args.first().map(String::as_str) == Some("add") {
+        let dir = args
+            .iter()
+            .position(|a| a == "--plugins")
+            .and_then(|i| args.get(i + 1))
+            .map(std::path::PathBuf::from)
+            .unwrap_or_else(crate::store::plugins_dir);
+        crate::store::ensure_plugin_root(&dir)
+            .with_context(|| format!("prepare {}", dir.display()))?;
+    }
     let (program, prefix) = runner_command()?;
     let status = Command::new(&program)
         .args(&prefix)
diff --git a/crates/punktfunk-host/src/store.rs b/crates/punktfunk-host/src/store.rs
index b79365d9..005bd7f7 100644
--- a/crates/punktfunk-host/src/store.rs
+++ b/crates/punktfunk-host/src/store.rs
@@ -199,6 +199,48 @@ pub(crate) fn ensure_bunfig_scope(dir: &Path, scope: &str, url: &str) -> Result<
     Ok(())
 }
 
+/// Anchor bun's install root at the plugins dir by giving it a `package.json`.
+///
+/// **This is load-bearing.** `bun add` does not install into its working directory — it walks *up*
+/// to the nearest ancestor `package.json` and installs into THAT tree. A fresh plugins dir has no
+/// `package.json`, so any stray one above it (a `~/package.json` from someone's one-off `bun add`
+/// or `npm init`) silently captures the install: bun reports success and **exits 0**, the packages
+/// land in the ancestor's `node_modules`, and the plugins dir gets nothing. Reproduced on-glass
+/// 2026-07-31 (Nobara 44, host + runner 0.22.3): the runner exits 0 in ~100 ms and the install then
+/// fails the presence check with the unhelpful "not present after install" — a user report that
+/// looked like a broken store and was a stray `~/package.json` all along.
+///
+/// The host writes this itself rather than leaving it to the runner, for the same reason as
+/// [`ensure_bunfig_scope`]: the installed scripting package can be older than this binary.
+///
+/// Only seeds a tree that has no `node_modules` yet. A dir with packages but no `package.json` is
+/// hand-assembled or an older layout, and [`installed_packages`] deliberately falls back to the
+/// naming convention there — dropping an empty `dependencies` on it would make every plugin already
+/// installed vanish from the store.
+pub(crate) fn ensure_plugin_root(dir: &Path) -> Result<()> {
+    std::fs::create_dir_all(dir).with_context(|| format!("create {}", dir.display()))?;
+    let path = dir.join("package.json");
+    if path.exists() || dir.join("node_modules").exists() {
+        return Ok(());
+    }
+    std::fs::write(
+        &path,
+        "{\n  \"name\": \"punktfunk-plugins\",\n  \"private\": true\n}\n",
+    )
+    .with_context(|| format!("write {}", path.display()))?;
+    Ok(())
+}
+
+/// The nearest `package.json` **above** `dir`, if any — the thing that would capture a `bun add`
+/// run inside `dir` (see [`ensure_plugin_root`]). Used to turn a silent mis-install into an error
+/// the operator can act on.
+pub(crate) fn capturing_ancestor(dir: &Path) -> Option<PathBuf> {
+    dir.ancestors()
+        .skip(1)
+        .map(|a| a.join("package.json"))
+        .find(|p| p.exists())
+}
+
 /// Is `pkg` a name the runner would supervise? Guards the uninstall route so a stray
 /// `POST /store/uninstall {"pkg": "effect"}` can't rip a shared dependency out of the tree.
 pub(crate) fn valid_installed_pkg(pkg: &str) -> Result<()> {
@@ -495,6 +537,72 @@ mod tests {
         assert_eq!(installed_packages(dir.path()).len(), 1);
     }
 
+    /// A fresh plugins dir must own its install root, or `bun add` installs somewhere else.
+    ///
+    /// Field bug 2026-07-31: with no `package.json` here, bun walked up to the operator's stray
+    /// `~/package.json`, installed the plugin into `~/node_modules`, and exited 0 — the store then
+    /// failed the presence check on an empty dir.
+    #[test]
+    fn a_fresh_dir_is_seeded_as_its_own_install_root() {
+        let dir = tempfile::tempdir().unwrap();
+        let root = dir.path().join("plugins");
+        ensure_plugin_root(&root).unwrap();
+        let seeded = std::fs::read_to_string(root.join("package.json")).unwrap();
+        assert!(seeded.contains("punktfunk-plugins"), "{seeded}");
+        // Seeding must not make the dir look like it has installs, nor lose the ones it gets.
+        assert!(installed_packages(&root).is_empty());
+    }
+
+    #[test]
+    fn seeding_never_touches_an_existing_package_json() {
+        let dir = tempfile::tempdir().unwrap();
+        touch_pkg(dir.path(), "@punktfunk/plugin-rom-manager", "0.3.1");
+        let manifest = r#"{"dependencies":{"@punktfunk/plugin-rom-manager":"0.3.1"}}"#;
+        std::fs::write(dir.path().join("package.json"), manifest).unwrap();
+        ensure_plugin_root(dir.path()).unwrap();
+        assert_eq!(
+            std::fs::read_to_string(dir.path().join("package.json")).unwrap(),
+            manifest
+        );
+    }
+
+    /// The one tree we must NOT seed: packages present, no `package.json`. `installed_packages`
+    /// falls back to the naming convention there, and a seeded empty `dependencies` would report
+    /// every plugin the operator already runs as uninstalled (see
+    /// `an_emptied_dependency_list_means_nothing_is_installed`).
+    #[test]
+    fn seeding_skips_a_tree_that_already_has_packages() {
+        let dir = tempfile::tempdir().unwrap();
+        touch_pkg(dir.path(), "punktfunk-plugin-legacy", "0.1.0");
+        ensure_plugin_root(dir.path()).unwrap();
+        assert!(!dir.path().join("package.json").exists());
+        assert_eq!(installed_packages(dir.path()).len(), 1);
+    }
+
+    #[test]
+    fn capturing_ancestor_looks_strictly_upwards() {
+        let dir = tempfile::tempdir().unwrap();
+        let plugins = dir.path().join("config/punktfunk/plugins");
+        std::fs::create_dir_all(&plugins).unwrap();
+        std::fs::write(dir.path().join("package.json"), "{}").unwrap();
+        assert_eq!(
+            capturing_ancestor(&plugins),
+            Some(dir.path().join("package.json"))
+        );
+
+        // The nearest one wins — that is the tree bun would pick.
+        let nearer = dir.path().join("config/package.json");
+        std::fs::write(&nearer, "{}").unwrap();
+        assert_eq!(capturing_ancestor(&plugins), Some(nearer));
+
+        // The dir's OWN manifest is not a capture — it is the anchor that prevents one.
+        std::fs::write(plugins.join("package.json"), "{}").unwrap();
+        assert_ne!(
+            capturing_ancestor(&plugins),
+            Some(plugins.join("package.json"))
+        );
+    }
+
     /// Uninstalling the last plugin must not resurrect its library as an installed plugin.
     ///
     /// `bun remove` drops the `dependencies` key entirely once it empties, while orphaned
diff --git a/crates/punktfunk-host/src/store/jobs.rs b/crates/punktfunk-host/src/store/jobs.rs
index 960dcba8..a6260dbe 100644
--- a/crates/punktfunk-host/src/store/jobs.rs
+++ b/crates/punktfunk-host/src/store/jobs.rs
@@ -353,6 +353,10 @@ fn run_install(id: &str, plan: Plan) -> Result<()> {
 
     // ---- install ------------------------------------------------------------------------------
     set_phase(id, "installing");
+    // Before anything else: make this dir bun's install root. Without a `package.json` here, `bun
+    // add` walks up and installs into the nearest ancestor that has one — successfully, exit 0,
+    // into somebody else's tree (see `ensure_plugin_root`).
+    super::ensure_plugin_root(&dir).with_context(|| format!("prepare {}", dir.display()))?;
     let before = super::installed_packages(&dir);
     // Map the entry's scope to its registry ourselves rather than through a runner flag: the
     // installed scripting package can be older than this binary, and an older runner would read an
@@ -397,10 +401,22 @@ fn run_install(id: &str, plan: Plan) -> Result<()> {
             "the install finished but no new plugin package appeared — is this package a \
              punktfunk plugin? (it must be named `@scope/plugin-*` or `punktfunk-plugin-*`)",
         )?;
-    let installed = after
-        .iter()
-        .find(|p| p.pkg == pkg)
-        .with_context(|| format!("{pkg} is not present after install"))?;
+    let installed = after.iter().find(|p| p.pkg == pkg).with_context(|| {
+        // The runner said it succeeded and the package still isn't here. The one field cause is a
+        // capturing ancestor `package.json` (`ensure_plugin_root`) — which this job now seeds
+        // against, so reaching here means a tree we deliberately don't seed (packages present, no
+        // `package.json`). Name the file rather than leaving the operator with a dead end.
+        match super::capturing_ancestor(&dir) {
+            Some(p) => format!(
+                "the runner reported success but {pkg} is not in {} — `{}` is capturing the \
+                 install (bun installs into the nearest package.json ABOVE the working directory). \
+                 Move or delete it, or add a package.json to the plugins dir.",
+                dir.display(),
+                p.display()
+            ),
+            None => format!("{pkg} is not present after install"),
+        }
+    })?;
 
     if let (Some(want), Some(got)) = (plan.version.as_deref(), installed.version.as_deref()) {
         if want != got {
diff --git a/sdk/src/plugins.ts b/sdk/src/plugins.ts
index 50628588..0e4aee01 100644
--- a/sdk/src/plugins.ts
+++ b/sdk/src/plugins.ts
@@ -54,9 +54,27 @@ export const resolvePackage = (
 /** Does this resolved package name install from Punktfunk's own (Gitea) registry? */
 const isFirstParty = (pkg: string): boolean => pkg.startsWith("@punktfunk/");
 
-/** Create the plugins dir (and parents) if needed. On Windows the ACL lockdown is the host's job. */
+/**
+ * Create the plugins dir (and parents) if needed, and make it bun's install ROOT. On Windows the
+ * ACL lockdown is the host's job.
+ *
+ * The `package.json` is load-bearing, not decoration: `bun add` installs into the nearest ancestor
+ * `package.json`, not into its working directory. Without one here, a stray `~/package.json` — one
+ * old `bun add`/`npm init` in a home dir — silently captures every plugin install. bun reports
+ * success and exits 0, the packages land in that tree, and the plugins dir stays empty (reproduced
+ * on-glass 2026-07-31; it presented as a plugin store that installs nothing).
+ *
+ * Only seeds a tree with no `node_modules`. A dir with packages but no `package.json` is
+ * hand-assembled or an older layout, and both this module's [`listInstalled`] and the host's
+ * installed-package scan fall back to the naming convention there; an empty `dependencies` would
+ * make the host report every plugin already installed as gone.
+ */
 export const ensurePluginsDir = (dir = pluginsDirDefault()): string => {
 	fs.mkdirSync(dir, { recursive: true });
+	const manifest = path.join(dir, "package.json");
+	if (!fs.existsSync(manifest) && !fs.existsSync(path.join(dir, "node_modules"))) {
+		fs.writeFileSync(manifest, '{\n  "name": "punktfunk-plugins",\n  "private": true\n}\n');
+	}
 	return dir;
 };
 
diff --git a/sdk/test/plugins.test.ts b/sdk/test/plugins.test.ts
index a2fae353..175ab632 100644
--- a/sdk/test/plugins.test.ts
+++ b/sdk/test/plugins.test.ts
@@ -194,4 +194,38 @@ describe("ensurePluginsDir", () => {
 		expect(fs.statSync(dir).isDirectory()).toBe(true);
 		ensurePluginsDir(dir); // idempotent
 	});
+
+	// Field bug 2026-07-31: `bun add` installs into the nearest ancestor package.json, not into
+	// its working dir. With none here, a stray ~/package.json captured every plugin install — bun
+	// exited 0, the packages landed in the home dir, and the plugins dir stayed empty.
+	test("seeds a package.json so bun cannot install into an ancestor", () => {
+		const dir = path.join(tmp("ensure-root"), "plugins");
+		ensurePluginsDir(dir);
+		const seeded = JSON.parse(
+			fs.readFileSync(path.join(dir, "package.json"), "utf8"),
+		) as { name: string; private: boolean };
+		expect(seeded.name).toBe("punktfunk-plugins");
+		expect(seeded.private).toBe(true);
+	});
+
+	test("never overwrites an existing package.json", () => {
+		const dir = tmp("ensure-keep");
+		const manifest = '{"dependencies":{"@punktfunk/plugin-playnite":"0.3.0"}}';
+		fs.writeFileSync(path.join(dir, "package.json"), manifest);
+		ensurePluginsDir(dir);
+		expect(fs.readFileSync(path.join(dir, "package.json"), "utf8")).toBe(manifest);
+	});
+
+	// The one tree we must not seed: packages present, no package.json. Discovery falls back to
+	// the naming convention there, and an empty `dependencies` would report every installed plugin
+	// as gone (the host's installed-package scan narrows to that list).
+	test("leaves a tree that already has packages alone", () => {
+		const dir = tmp("ensure-existing");
+		writePkg(dir, "punktfunk-plugin-legacy", "0.1.0");
+		ensurePluginsDir(dir);
+		expect(fs.existsSync(path.join(dir, "package.json"))).toBe(false);
+		expect(listInstalled(dir).map((p) => p.pkg)).toEqual([
+			"punktfunk-plugin-legacy",
+		]);
+	});
 });