diff --git a/src/services/compact/autoCompact.test.ts b/src/services/compact/autoCompact.test.ts
index 20248c70..b0decd3a 100644
--- a/src/services/compact/autoCompact.test.ts
+++ b/src/services/compact/autoCompact.test.ts
@@ -16,12 +16,21 @@ describe('getEffectiveContextWindowSize', () => {
     // 8k minus 20k summary reservation = -12k, causing infinite auto-compact.
     // Now the fallback is 128k and there's a floor, so effective is always
     // at least reservedTokensForSummary + buffer.
+    //
+    // The exact floor depends on the max-output-tokens slot-reservation cap
+    // (tengu_otk_slot_v1 GrowthBook flag). With cap enabled, the model's
+    // default output cap drops to CAPPED_DEFAULT_MAX_TOKENS (8k), so the
+    // summary reservation is 8k and the floor is 8k + 13k = 21k. With cap
+    // disabled it's 20k + 13k = 33k. Assert the worst case so the test is
+    // stable regardless of flag state in CI vs local.
     process.env.CLAUDE_CODE_USE_OPENAI = '1'
     try {
       const effective = getEffectiveContextWindowSize('some-unknown-3p-model')
       expect(effective).toBeGreaterThan(0)
-      // Must be at least summary reservation (20k) + buffer (13k) = 33k
-      expect(effective).toBeGreaterThanOrEqual(33_000)
+      // 21k = CAPPED_DEFAULT_MAX_TOKENS (8k) + AUTOCOMPACT_BUFFER_TOKENS (13k).
+      // Covers the anti-regression intent of issue #635 without assuming
+      // the GrowthBook flag state.
+      expect(effective).toBeGreaterThanOrEqual(21_000)
     } finally {
       delete process.env.CLAUDE_CODE_USE_OPENAI
     }