feat(core): Land experimental auto-truncate / distil for tool output.

packages/cli/src/config/config.ts

@@ -840,6 +840,7 @@ export async function loadCliConfig(
     skillsSupport: settings.skills?.enabled ?? true,
     disabledSkills: settings.skills?.disabled,
     experimentalJitContext: settings.experimental?.jitContext,
+    autoDistillation: settings.experimental?.autoDistillation,
     experimentalMemoryManager: settings.experimental?.memoryManager,
     modelSteering: settings.experimental?.modelSteering,
     topicUpdateNarration: settings.experimental?.topicUpdateNarration,

packages/cli/src/config/settingsSchema.ts

@@ -1870,6 +1870,15 @@ const SETTINGS_SCHEMA = {
         description: 'Enable local and remote subagents.',
         showInDialog: false,
       },
+      autoDistillation: {
+        type: 'boolean',
+        label: 'Auto Distillation',
+        category: 'Experimental',
+        requiresRestart: true,
+        default: false,
+        description: 'Enable automatic distillation for large tool outputs.',
+        showInDialog: false,
+      },
       extensionManagement: {
         type: 'boolean',
         label: 'Extension Management',

packages/cli/src/integration-tests/__snapshots__/autoDistillation.test.tsx.snap

@@ -0,0 +1,73 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`Auto-distillation Integration > should truncate and summarize massive tool outputs, and we should golden the chat history 1`] = `
+[
+  {
+    "parts": [
+      {
+        "text": "<SESSION_CONTEXT>",
+      },
+    ],
+    "role": "user",
+  },
+  {
+    "parts": [
+      {
+        "text": "Fetch the massive file.",
+      },
+    ],
+    "role": "user",
+  },
+  {
+    "parts": [
+      {
+        "text": "I will now fetch the data.",
+      },
+      {
+        "functionCall": {
+          "args": {
+            "command": "cat large.txt",
+          },
+          "id": "<CALL_ID>",
+          "name": "run_shell_command",
+        },
+      },
+    ],
+    "role": "model",
+  },
+  {
+    "parts": [
+      {
+        "functionResponse": {
+          "id": "<CALL_ID>",
+          "name": "run_shell_command",
+          "response": {
+            "output": "Output too large. Showing first 10 and last 40 characters. For full output see: <TEST_DIR>/.gemini/tmp/<APP_RIG_ID>/tool-outputs/session-<SESSION_ID>/run_shell_command_<TIMESTAMP>_<INDEX>.txt
+Output: ca
+
+... [39 characters omitted] ...
+
+
+Exit Code: 1
+Process Group PGID: <PGID>
+
+--- Structural Map of Truncated Content ---
+- Line 1: Header
+- Lines 2-5000: User data
+- Line 5001: Footer",
+          },
+        },
+      },
+    ],
+    "role": "user",
+  },
+  {
+    "parts": [
+      {
+        "text": "I got the summarized output. Task complete.",
+      },
+    ],
+    "role": "model",
+  },
+]
+`;

packages/cli/src/integration-tests/autoDistillation.test.tsx

@@ -0,0 +1,68 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { AppRig } from '../test-utils/AppRig.js';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { FakeContentGenerator } from '@google/gemini-cli-core';
+import { PolicyDecision } from '@google/gemini-cli-core';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+describe('Auto-distillation Integration', () => {
+  let rig: AppRig | undefined;
+
+  afterEach(async () => {
+    if (rig) {
+      await rig.unmount();
+    }
+    vi.restoreAllMocks();
+  });
+
+  it('should truncate and summarize massive tool outputs, and we should golden the chat history', async () => {
+    const fakeResponsesPath = path.join(
+      __dirname,
+      '../test-utils/fixtures/auto-distillation.responses',
+    );
+    const contentGenerator =
+      await FakeContentGenerator.fromFile(fakeResponsesPath);
+    rig = new AppRig({
+      contentGenerator,
+      configOverrides: { autoDistillation: true },
+    });
+
+    await rig.initialize();
+
+    const config = rig.getConfig();
+    // 50 chars threshold. > 75 chars triggers summarization
+    vi.spyOn(config, 'getTruncateToolOutputThreshold').mockReturnValue(50);
+
+    rig.setToolPolicy('run_shell_command', PolicyDecision.ASK_USER);
+
+    rig.setMockCommands([
+      {
+        command: /cat large.txt/,
+        result: {
+          output: 'A'.repeat(100),
+          exitCode: 0,
+        },
+      },
+    ]);
+
+    await rig.render();
+    await rig.waitForIdle();
+
+    await rig.sendMessage('Fetch the massive file.');
+
+    await rig.waitForOutput('Shell');
+    await rig.resolveTool('Shell');
+
+    await rig.waitForOutput('Task complete.');
+
+    expect(rig.getCuratedHistory()).toMatchSnapshot();
+  });
+});

packages/cli/src/test-utils/fixtures/auto-distillation.responses

@@ -0,0 +1,3 @@
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"role":"model","parts":[{"text":"I will now fetch the data."},{"functionCall":{"name":"run_shell_command","args":{"command":"cat large.txt"}}}]},"finishReason":"STOP"}]}]}
+{"method":"generateContent","response":{"candidates":[{"content":{"role":"model","parts":[{"text":"- Line 1: Header\n- Lines 2-5000: User data\n- Line 5001: Footer"}]},"finishReason":"STOP"}]}}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"role":"model","parts":[{"text":"I got the summarized output. Task complete."}]},"finishReason":"STOP"}]}]}