diff --git a/.changeset/fix-grok-provider-options-index-sig.md b/.changeset/fix-grok-provider-options-index-sig.md
new file mode 100644
index 000000000..672e0a501
--- /dev/null
+++ b/.changeset/fix-grok-provider-options-index-sig.md
@@ -0,0 +1,5 @@
+---
+"@tanstack/ai-grok": patch
+---
+
+Remove `Record<string, unknown>` index signature from `GrokTextProviderOptions` so that `grokSummarize` adapters are assignable to `SummarizeAdapter<string, object>`. Under `strictFunctionTypes`, the index signature caused `object` to be un-assignable to `GrokTextProviderOptions` (contravariant parameter check), making `grokSummarize('grok-4.3')` a type error at every `summarize()` call site. All fields on `GrokTextProviderOptions` are explicitly typed optional members, so the index signature was unnecessary.
diff --git a/packages/ai-grok/src/text/text-provider-options.ts b/packages/ai-grok/src/text/text-provider-options.ts
index 9cc319df4..69f410a47 100644
--- a/packages/ai-grok/src/text/text-provider-options.ts
+++ b/packages/ai-grok/src/text/text-provider-options.ts
@@ -32,8 +32,7 @@ export interface GrokBaseOptions {
  * Grok-specific provider options for text/chat
  * Based on xAI Responses API options
  */
-export interface GrokTextProviderOptions
-  extends GrokBaseOptions, Record<string, unknown> {
+export interface GrokTextProviderOptions extends GrokBaseOptions {
   /**
    * Temperature for response generation (0-2)
    * Higher values make output more random, lower values more focused