diff --git a/src/services/compact/compact.ts b/src/services/compact/compact.ts
index f46194ffb..a0d5903d1 100644
--- a/src/services/compact/compact.ts
+++ b/src/services/compact/compact.ts
@@ -103,6 +103,7 @@ import {
   getMaxOutputTokensForModel,
   queryModelWithStreaming,
 } from '../api/claude.js'
+import { getCompactModel } from '../../utils/model/model.js'
 import {
   getPromptTooLongTokenGap,
   PROMPT_TOO_LONG_ERROR_MESSAGE,
@@ -1314,13 +1315,13 @@ async function streamCompactSummary({
             const appState = context.getAppState()
             return appState.toolPermissionContext
           },
-          model: context.options.mainLoopModel,
+          model: getCompactModel(context.options.mainLoopModel),
           toolChoice: undefined,
           isNonInteractiveSession: context.options.isNonInteractiveSession,
           hasAppendSystemPrompt: !!context.options.appendSystemPrompt,
           maxOutputTokensOverride: Math.min(
             COMPACT_MAX_OUTPUT_TOKENS,
-            getMaxOutputTokensForModel(context.options.mainLoopModel),
+            getMaxOutputTokensForModel(getCompactModel(context.options.mainLoopModel)),
           ),
           querySource: 'compact',
           agents: context.options.agentDefinitions.activeAgents,
diff --git a/src/utils/model/model.ts b/src/utils/model/model.ts
index 791daeb6b..1aa3c79a6 100644
--- a/src/utils/model/model.ts
+++ b/src/utils/model/model.ts
@@ -47,6 +47,28 @@ export function getSmallFastModel(): ModelName {
   return process.env.ANTHROPIC_SMALL_FAST_MODEL || getDefaultHaikuModel()
 }
 
+/**
+ * Get the model to use for compaction, one tier below the current model.
+ * Opus → Sonnet, Sonnet → Haiku, Haiku → Haiku (already lowest).
+ * Preserves [1m] suffix only if the target family supports it (sonnet/opus).
+ */
+export function getCompactModel(model: ModelName): ModelName {
+  const has1m = model.endsWith('[1m]')
+  const baseModel = has1m ? model.slice(0, -4) : model
+  const canonical = getCanonicalName(baseModel)
+
+  // Opus family → Sonnet
+  if (canonical.includes('opus')) {
+    return getDefaultSonnetModel()
+  }
+  // Sonnet family → Haiku
+  if (canonical.includes('sonnet')) {
+    return getDefaultHaikuModel()
+  }
+  // Haiku or unknown → Haiku (lowest tier)
+  return getDefaultHaikuModel()
+}
+
 export function isNonCustomOpusModel(model: ModelName): boolean {
   return (
     model === getModelStrings().opus40 ||