diff --git a/.changeset/healthy-laws-divide.md b/.changeset/healthy-laws-divide.md
new file mode 100644
index 0000000000..4ab29e6426
--- /dev/null
+++ b/.changeset/healthy-laws-divide.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+Implement `toString()` and `toJSON()` for extension instances.
diff --git a/packages/frontend-app-api/src/wiring/createApp.test.tsx b/packages/frontend-app-api/src/wiring/createApp.test.tsx
index 14e52af469..abd0fe9b00 100644
--- a/packages/frontend-app-api/src/wiring/createApp.test.tsx
+++ b/packages/frontend-app-api/src/wiring/createApp.test.tsx
@@ -25,6 +25,7 @@ import { screen } from '@testing-library/react';
 import { MockConfigApi, renderWithEffects } from '@backstage/test-utils';
 import React from 'react';
 import { createRouteRef } from '@backstage/core-plugin-api';
+import { createExtensionInstance } from './createExtensionInstance';
 
 describe('createInstances', () => {
   it('throws an error when a root extension is parametrized', () => {
@@ -132,4 +133,112 @@ describe('createApp', () => {
 
     await expect(screen.findByText('Derp')).resolves.toBeInTheDocument();
   });
+
+  it('should log an app', () => {
+    const { rootInstances } = createInstances({
+      config: new MockConfigApi({}),
+      plugins: [],
+    });
+    const root = createExtensionInstance({
+      extension: createExtension({
+        id: 'root',
+        attachTo: { id: '', input: '' },
+        output: {},
+        factory() {},
+      }),
+      config: undefined,
+      attachments: new Map([['children', rootInstances]]),
+    });
+
+    expect(String(root)).toMatchInlineSnapshot(`
+      "<root>
+        children [
+          <core>
+            themes [
+              <themes.light out=[core.theme] />
+              <themes.dark out=[core.theme] />
+            ]
+          </core>
+          <core.layout out=[core.reactElement]>
+            content [
+              <core.routes out=[core.reactElement] />
+            ]
+            nav [
+              <core.nav out=[core.reactElement] />
+            ]
+          </core.layout>
+        ]
+      </root>"
+    `);
+  });
+
+  it('should serialize an app as JSON', () => {
+    const { rootInstances } = createInstances({
+      config: new MockConfigApi({}),
+      plugins: [],
+    });
+    const root = createExtensionInstance({
+      extension: createExtension({
+        id: 'root',
+        attachTo: { id: '', input: '' },
+        output: {},
+        factory() {},
+      }),
+      config: undefined,
+      attachments: new Map([['children', rootInstances]]),
+    });
+
+    expect(JSON.parse(JSON.stringify(root))).toMatchInlineSnapshot(`
+      {
+        "attachments": {
+          "children": [
+            {
+              "attachments": {
+                "themes": [
+                  {
+                    "id": "themes.light",
+                    "output": [
+                      "core.theme",
+                    ],
+                  },
+                  {
+                    "id": "themes.dark",
+                    "output": [
+                      "core.theme",
+                    ],
+                  },
+                ],
+              },
+              "id": "core",
+            },
+            {
+              "attachments": {
+                "content": [
+                  {
+                    "id": "core.routes",
+                    "output": [
+                      "core.reactElement",
+                    ],
+                  },
+                ],
+                "nav": [
+                  {
+                    "id": "core.nav",
+                    "output": [
+                      "core.reactElement",
+                    ],
+                  },
+                ],
+              },
+              "id": "core.layout",
+              "output": [
+                "core.reactElement",
+              ],
+            },
+          ],
+        },
+        "id": "root",
+      }
+    `);
+  });
 });
diff --git a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
index 18551eb8da..9aec3078bc 100644
--- a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
+++ b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
@@ -90,6 +90,68 @@ function resolveInputs(
   });
 }
 
+function indent(str: string) {
+  return str.replace(/^/gm, '  ');
+}
+
+class ExtensionInstanceImpl implements ExtensionInstance {
+  readonly $$type = '@backstage/ExtensionInstance';
+
+  readonly id: string;
+  readonly #extensionData: Map<string, unknown>;
+  readonly attachments: Map<string, ExtensionInstance[]>;
+  readonly source?: BackstagePlugin;
+
+  constructor(
+    id: string,
+    extensionData: Map<string, unknown>,
+    attachments: Map<string, ExtensionInstance[]>,
+    source: BackstagePlugin | undefined,
+  ) {
+    this.id = id;
+    this.#extensionData = extensionData;
+    this.attachments = attachments;
+    this.source = source;
+  }
+
+  getData<T>(ref: ExtensionDataRef<T>): T | undefined {
+    return this.#extensionData.get(ref.id) as T | undefined;
+  }
+
+  toJSON() {
+    return {
+      id: this.id,
+      output:
+        this.#extensionData.size > 0
+          ? [...this.#extensionData.keys()]
+          : undefined,
+      attachments:
+        this.attachments.size > 0
+          ? Object.fromEntries(this.attachments)
+          : undefined,
+    };
+  }
+
+  toString() {
+    const out =
+      this.#extensionData.size > 0
+        ? ` out=[${[...this.#extensionData.keys()].join(', ')}]`
+        : '';
+
+    if (this.attachments.size === 0) {
+      return `<${this.id}${out} />`;
+    }
+
+    return [
+      `<${this.id}${out}>`,
+      ...[...this.attachments.entries()].map(([k, v]) =>
+        indent([`${k} [`, ...v.map(e => indent(e.toString())), `]`].join('\n')),
+      ),
+      `</${this.id}>`,
+    ].join('\n');
+  }
+}
+
 /** @internal */
 export function createExtensionInstance(options: {
   extension: Extension<unknown>;
@@ -137,13 +199,10 @@ export function createExtensionInstance(options: {
     );
   }
 
-  return {
-    $$type: '@backstage/ExtensionInstance',
-    id: options.extension.id,
-    getData<T>(ref: ExtensionDataRef<T>): T | undefined {
-      return extensionData.get(ref.id) as T | undefined;
-    },
-    source,
+  return new ExtensionInstanceImpl(
+    options.extension.id,
+    extensionData,
     attachments,
-  };
+    source,
+  );
 }