diff --git a/packages/opencode/src/session/overflow.ts b/packages/opencode/src/session/overflow.ts
index 3374d1c9f8d4..c8ea09f8f940 100644
--- a/packages/opencode/src/session/overflow.ts
+++ b/packages/opencode/src/session/overflow.ts
@@ -1,4 +1,4 @@
-import type { Config } from "@/config/config"
+import type { Config } from "@/config/config"
 import { ConfigV1 } from "@opencode-ai/core/v1/config/config"
 import { SessionV1 } from "@opencode-ai/core/v1/session"
 import type { Provider } from "@/provider/provider"
@@ -11,12 +11,11 @@ export function usable(input: { cfg: ConfigV1.Info; model: Provider.Model; outpu
   const context = input.model.limit.context
   if (context === 0) return 0
 
-  const reserved =
-    input.cfg.compaction?.reserved ??
-    Math.min(COMPACTION_BUFFER, ProviderTransform.maxOutputTokens(input.model, input.outputTokenMax))
+  const reserved = input.cfg.compaction?.reserved
+  const output = ProviderTransform.maxOutputTokens(input.model, input.outputTokenMax)
   return input.model.limit.input
-    ? Math.max(0, input.model.limit.input - reserved)
-    : Math.max(0, context - ProviderTransform.maxOutputTokens(input.model, input.outputTokenMax))
+    ? Math.max(0, input.model.limit.input - (reserved ?? Math.min(COMPACTION_BUFFER, output)))
+    : Math.max(0, context - (reserved ?? output))
 }
 
 export function isOverflow(input: {