feat(compact): add compactModel config to override compaction model

src/services/compact/compact.ts

@@ -41,7 +41,7 @@ import {
   getDeferredToolsDeltaAttachment,
   getMcpInstructionsDeltaAttachment,
 } from '../../utils/attachments.js'
-import { getMemoryPath } from '../../utils/config.js'
+import { getGlobalConfig, getMemoryPath } from '../../utils/config.js'
 import { COMPACT_MAX_OUTPUT_TOKENS } from '../../utils/context.js'
 import {
   analyzeContext,
@@ -83,6 +83,7 @@ import {
 } from '../../utils/sessionStorage.js'
 import { sleep } from '../../utils/sleep.js'
 import { jsonStringify } from '../../utils/slowOperations.js'
+import { parseUserSpecifiedModel } from '../../utils/model/model.js'
 /* eslint-enable @typescript-eslint/no-require-imports */
 import { asSystemPrompt } from '../../utils/systemPromptType.js'
 import { getTaskOutputPath } from '../../utils/task/diskOutput.js'
@@ -1154,10 +1155,15 @@ async function streamCompactSummary({
   // main conversation's cached prefix (system prompt, tools, context messages).
   // Falls back to regular streaming path on failure.
   // 3P default: true — see comment at the other tengu_compact_cache_prefix read above.
-  const promptCacheSharingEnabled = getFeatureValue_CACHED_MAY_BE_STALE(
-    'tengu_compact_cache_prefix',
-    true,
-  )
+  // Disabled when compactModel overrides mainLoopModel: different model = different
+  // cache key, so the forked-agent cache-sharing path would be a guaranteed miss.
+  const rawCompactModel = getGlobalConfig().compactModel
+  const compactModel = rawCompactModel
+    ? parseUserSpecifiedModel(rawCompactModel)
+    : context.options.mainLoopModel
+  const promptCacheSharingEnabled =
+    compactModel === context.options.mainLoopModel &&
+    getFeatureValue_CACHED_MAY_BE_STALE('tengu_compact_cache_prefix', true)
   // Send keep-alive signals during compaction to prevent remote session
   // WebSocket idle timeouts from dropping bridge connections. Compaction
   // API calls can take 5-10+ seconds, during which no other messages
@@ -1262,10 +1268,10 @@ async function streamCompactSummary({
       let response: AssistantMessage | undefined
       context.setResponseLength?.(() => 0)
 
-      // Check if tool search is enabled using the main loop's tools list.
+      // Check if tool search is enabled using the model that will actually compact.
       // context.options.tools includes MCP tools merged via useMergedTools.
       const useToolSearch = await isToolSearchEnabled(
-        context.options.mainLoopModel,
+        compactModel,
         context.options.tools,
         async () => appState.toolPermissionContext,
         context.options.agentDefinitions.activeAgents,
@@ -1312,13 +1318,13 @@ async function streamCompactSummary({
             const appState = context.getAppState()
             return appState.toolPermissionContext
           },
-          model: context.options.mainLoopModel,
+          model: compactModel,
           toolChoice: undefined,
           isNonInteractiveSession: context.options.isNonInteractiveSession,
           hasAppendSystemPrompt: !!context.options.appendSystemPrompt,
           maxOutputTokensOverride: Math.min(
             COMPACT_MAX_OUTPUT_TOKENS,
-            getMaxOutputTokensForModel(context.options.mainLoopModel),
+            getMaxOutputTokensForModel(compactModel),
           ),
           querySource: 'compact',
           agents: context.options.agentDefinitions.activeAgents,

src/tools/ConfigTool/ConfigTool.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, test } from 'bun:test'
+import { ConfigTool } from './ConfigTool.js'
+
+// ---------------------------------------------------------------------------
+// mapToolResultToToolResultBlockParam — rendering of set/clear results
+// ---------------------------------------------------------------------------
+
+describe('ConfigTool.mapToolResultToToolResultBlockParam', () => {
+  const toolUseID = 'test-id'
+
+  test('renders set result using newValue', () => {
+    const result = ConfigTool.mapToolResultToToolResultBlockParam(
+      {
+        success: true,
+        operation: 'set',
+        setting: 'compactModel',
+        newValue: 'claude-sonnet-4-6',
+      },
+      toolUseID,
+    )
+    expect(result.content).toBe('Set compactModel to "claude-sonnet-4-6"')
+  })
+
+  test('clearable "default" result renders Set…to null, not undefined', () => {
+    // Regression: clearable branch previously returned value:null without newValue,
+    // causing mapToolResultToToolResultBlockParam to render "Set compactModel to undefined".
+    const result = ConfigTool.mapToolResultToToolResultBlockParam(
+      {
+        success: true,
+        operation: 'set',
+        setting: 'compactModel',
+        newValue: null,
+      },
+      toolUseID,
+    )
+    expect(result.content).toBe('Set compactModel to null')
+    expect(result.content).not.toContain('undefined')
+  })
+
+  test('renders get result using value', () => {
+    const result = ConfigTool.mapToolResultToToolResultBlockParam(
+      {
+        success: true,
+        operation: 'get',
+        setting: 'compactModel',
+        value: 'claude-opus-4-8',
+      },
+      toolUseID,
+    )
+    expect(result.content).toBe('compactModel = "claude-opus-4-8"')
+  })
+
+  test('renders error result', () => {
+    const result = ConfigTool.mapToolResultToToolResultBlockParam(
+      { success: false, error: 'Unknown setting: "foo"' },
+      toolUseID,
+    )
+    expect(result.content).toBe('Error: Unknown setting: "foo"')
+    expect(result.is_error).toBe(true)
+  })
+})

src/tools/ConfigTool/ConfigTool.ts

@@ -179,6 +179,39 @@ export const ConfigTool = buildTool({
       }
     }
 
+    // Handle "default" for clearable settings — delete the key to restore the built-in default.
+    if (
+      config.clearable &&
+      typeof value === 'string' &&
+      value.toLowerCase().trim() === 'default'
+    ) {
+      const key = path[0]
+      if (!key) {
+        return {
+          data: {
+            success: false,
+            operation: 'set',
+            setting,
+            error: 'Invalid setting path',
+          },
+        }
+      }
+      saveGlobalConfig(prev => {
+        if (!(key in prev)) return prev
+        const next = { ...prev }
+        delete (next as Record<string, unknown>)[key]
+        return next
+      })
+      return {
+        data: {
+          success: true,
+          operation: 'set',
+          setting,
+          newValue: null,
+        },
+      }
+    }
+
     let finalValue: unknown = value
 
     // Coerce and validate boolean values

src/tools/ConfigTool/supportedSettings.ts

@@ -27,6 +27,8 @@ type SettingConfig = {
   validateOnWrite?: (v: unknown) => Promise<{ valid: boolean; error?: string }>
   /** Format value when reading/getting for display */
   formatOnRead?: (v: unknown) => unknown
+  /** Whether setting value "default" should delete the key and restore the default */
+  clearable?: boolean
 }
 
 export const SUPPORTED_SETTINGS: Record<string, SettingConfig> = {
@@ -59,6 +61,13 @@ export const SUPPORTED_SETTINGS: Record<string, SettingConfig> = {
     type: 'boolean',
     description: 'Auto-compact when context is full',
   },
+  compactModel: {
+    source: 'global',
+    type: 'string',
+    description: 'Model used for compaction (defaults to main model); set "default" to clear',
+    validateOnWrite: async (v: unknown) => validateModel(v as string),
+    clearable: true,
+  },
   autoMemoryEnabled: {
     source: 'settings',
     type: 'boolean',

src/utils/config.ts

@@ -254,6 +254,7 @@ export type GlobalConfig = {
   bypassPermissionsModeAccepted?: boolean
   hasUsedBackslashReturn?: boolean
   autoCompactEnabled: boolean // Controls whether auto-compact is enabled
+  compactModel?: string // Override model used for compaction (defaults to mainLoopModel)
   toolHistoryCompressionEnabled: boolean // Compress old tool_result content for small-context providers
   showTurnDuration: boolean // Controls whether to show turn duration message (e.g., "Cooked for 1m 6s")
   // Controls whether to show per-query cache hit/miss stats at the end of each turn.
@@ -704,6 +705,7 @@ export const GLOBAL_CONFIG_KEYS = [
   'editorMode',
   'hasUsedBackslashReturn',
   'autoCompactEnabled',
+  'compactModel',
   'toolHistoryCompressionEnabled',
   'showTurnDuration',
   'showCacheStats',