diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 8cdc48e243..bab056dae7 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -832,7 +832,16 @@ export namespace ProviderTransform {
     if (input.model.api.id.includes("gpt-5") && !input.model.api.id.includes("gpt-5-chat")) {
       if (!input.model.api.id.includes("gpt-5-pro")) {
         result["reasoningEffort"] = "medium"
-        result["reasoningSummary"] = "auto"
+        // Only inject reasoningSummary for providers that support it natively.
+        // @ai-sdk/openai-compatible proxies (e.g. LiteLLM) do not understand this
+        // parameter and return "Unknown parameter: 'reasoningSummary'".
+        if (
+          input.model.api.npm === "@ai-sdk/openai" ||
+          input.model.api.npm === "@ai-sdk/azure" ||
+          input.model.api.npm === "@ai-sdk/github-copilot"
+        ) {
+          result["reasoningSummary"] = "auto"
+        }
       }
 
       // Only set textVerbosity for non-chat gpt-5.x models