diff --git a/.changeset/grok-summarize-adapter-assignability.md b/.changeset/grok-summarize-adapter-assignability.md
new file mode 100644
index 000000000..7f3a327fc
--- /dev/null
+++ b/.changeset/grok-summarize-adapter-assignability.md
@@ -0,0 +1,16 @@
+---
+'@tanstack/ai-grok': patch
+---
+
+Fix `grokSummarize`/`createGrokSummarize` not being assignable to `summarize()`'s
+`adapter` param for any current Grok model (`grok-4.3`, `grok-build-0.1`).
+
+`GrokTextProviderOptions` was declared as an `interface` extending
+`Record<string, unknown>`, giving it an explicit index signature. Under
+`strictFunctionTypes`, the `SummarizeAdapter` constraint is checked
+contravariantly, which requires `object` to be assignable to the provider
+options — but `object` is not assignable to an index-signature type, so the
+check failed (Grok-only; OpenAI's all-optional, no-index-signature options
+passed). The options are now a type-alias intersection matching the OpenAI
+shape, and the text adapter's provider-options constraint is widened to
+`Record<string, any>` to mirror `OpenAITextAdapter`.
diff --git a/packages/ai-grok/src/adapters/text.ts b/packages/ai-grok/src/adapters/text.ts
index 1e8e3fb15..366676a45 100644
--- a/packages/ai-grok/src/adapters/text.ts
+++ b/packages/ai-grok/src/adapters/text.ts
@@ -43,8 +43,11 @@ export type { ExternalTextProviderOptions as GrokTextProviderOptions } from '../
  */
 export class GrokTextAdapter<
   TModel extends (typeof GROK_CHAT_MODELS)[number],
-  TProviderOptions extends Record<string, unknown> =
-    ResolveProviderOptions<TModel>,
+  // Use `Record<string, any>` (not `unknown`) to match the OpenAI text
+  // adapter: the resolved Grok provider options are a type-alias intersection
+  // with no explicit index signature, which is assignable to
+  // `Record<string, any>` but not `Record<string, unknown>`. See issue #821.
+  TProviderOptions extends Record<string, any> = ResolveProviderOptions<TModel>,
   TInputModalities extends ReadonlyArray<Modality> =
     ResolveInputModalities<TModel>,
   TToolCapabilities extends ReadonlyArray<string> =
diff --git a/packages/ai-grok/src/text/text-provider-options.ts b/packages/ai-grok/src/text/text-provider-options.ts
index 9cc319df4..dd430ac96 100644
--- a/packages/ai-grok/src/text/text-provider-options.ts
+++ b/packages/ai-grok/src/text/text-provider-options.ts
@@ -29,11 +29,9 @@ export interface GrokBaseOptions {
 }
 
 /**
- * Grok-specific provider options for text/chat
- * Based on xAI Responses API options
+ * Sampling and response controls for Grok text/chat models.
  */
-export interface GrokTextProviderOptions
-  extends GrokBaseOptions, Record<string, unknown> {
+export interface GrokSamplingOptions {
   /**
    * Temperature for response generation (0-2)
    * Higher values make output more random, lower values more focused
@@ -62,6 +60,21 @@ export interface GrokTextProviderOptions
   reasoning?: GrokReasoning
 }
 
+/**
+ * Grok-specific provider options for text/chat
+ * Based on xAI Responses API options.
+ *
+ * Declared as a type-alias intersection of interfaces with all-optional props
+ * (matching the OpenAI text adapter), NOT an `interface ... extends
+ * Record<string, unknown>`. An explicit index signature makes `object`
+ * un-assignable to these options, which breaks the contravariantly-checked
+ * `summarize()` adapter constraint (`SummarizeAdapter<string, object>`); see
+ * issue #821. Without the index signature these options no longer satisfy a
+ * `Record<string, unknown>` constraint, so the text adapter's provider-options
+ * generic is widened to `Record<string, any>` to match OpenAI.
+ */
+export type GrokTextProviderOptions = GrokBaseOptions & GrokSamplingOptions
+
 export type GrokBuildProviderOptions = Omit<
   GrokTextProviderOptions,
   'reasoning'
diff --git a/packages/ai-grok/tests/grok-adapter.test.ts b/packages/ai-grok/tests/grok-adapter.test.ts
index 4b50cb898..72cbbfba7 100644
--- a/packages/ai-grok/tests/grok-adapter.test.ts
+++ b/packages/ai-grok/tests/grok-adapter.test.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { resolveDebugOption } from '@tanstack/ai/adapter-internals'
-import { EventType } from '@tanstack/ai'
+import { EventType, summarize } from '@tanstack/ai'
 import { createGrokText, grokText } from '../src/adapters/text'
 import { createGrokImage, grokImage } from '../src/adapters/image'
 import { createGrokSummarize, grokSummarize } from '../src/adapters/summarize'
@@ -498,5 +498,18 @@ describe('Grok adapters', () => {
         'XAI_API_KEY is required',
       )
     })
+
+    it('grokSummarize is assignable to summarize() adapter param for every model (#821)', () => {
+      // Type-level regression guard: the SummarizeAdapter constraint only
+      // instantiates at the summarize() call site, so constructing the adapter
+      // (covered above) is not enough. This closure is type-checked but never
+      // executed — passing CI's test:types is the assertion.
+      const _typeCheck = () => {
+        void summarize({ adapter: grokSummarize('grok-4.3'), text: '' })
+        void summarize({ adapter: grokSummarize('grok-build-0.1'), text: '' })
+      }
+
+      expect(_typeCheck).toBeInstanceOf(Function)
+    })
   })
 })