diff --git a/.changeset/run-input-protocol-version.md b/.changeset/run-input-protocol-version.md
new file mode 100644
index 0000000000..89d392b8b5
--- /dev/null
+++ b/.changeset/run-input-protocol-version.md
@@ -0,0 +1,6 @@
+---
+'@tanstack/ai': patch
+'@tanstack/ai-client': patch
+---
+
+Connection adapters now send the AG-UI `protocolVersion` (`"1.0"`) in the `RunAgentInput` request body, as the AG-UI spec asks. `@tanstack/ai/client` re-exports `PROTOCOL_VERSION`.
diff --git a/packages/ai-client/src/connection-adapters.ts b/packages/ai-client/src/connection-adapters.ts
index 00d7b8551d..124c055cd7 100644
--- a/packages/ai-client/src/connection-adapters.ts
+++ b/packages/ai-client/src/connection-adapters.ts
@@ -1,5 +1,6 @@
 import {
   EventType,
+  PROTOCOL_VERSION,
   getChunkRunId as getNormalizedChunkRunId,
   restoreInboundChunk,
   uiMessagesToWire,
@@ -1416,6 +1417,7 @@ function buildRunAgentInputBody(
   return {
     threadId: runContext.threadId,
     runId: runContext.runId,
+    protocolVersion: PROTOCOL_VERSION,
     ...(runContext.parentRunId !== undefined && {
       parentRunId: runContext.parentRunId,
     }),
diff --git a/packages/ai-client/tests/connection-adapters.test.ts b/packages/ai-client/tests/connection-adapters.test.ts
index c70c077014..786d807f94 100644
--- a/packages/ai-client/tests/connection-adapters.test.ts
+++ b/packages/ai-client/tests/connection-adapters.test.ts
@@ -575,6 +575,31 @@ describe('connection-adapters', () => {
       expect(body.forwardedProps).toMatchObject({ key: 'value' })
     })
 
+    it('should send the AG-UI protocolVersion in the request body', async () => {
+      fetchMock.mockResolvedValue({
+        ok: true,
+        body: {
+          getReader: () => ({
+            read: vi.fn().mockResolvedValue({ done: true, value: undefined }),
+            releaseLock: vi.fn(),
+          }),
+        },
+      } as any)
+
+      const adapter = fetchServerSentEvents('/api/chat')
+      for await (const _ of adapter.connect(
+        [{ role: 'user', content: 'Hello' }],
+        undefined,
+        undefined,
+        RUN_CONTEXT,
+      )) {
+        // Consume
+      }
+
+      const body = JSON.parse(fetchMock.mock.calls[0]?.[1]?.body as string)
+      expect(body.protocolVersion).toBe('1.0')
+    })
+
     it('should use custom fetchClient when provided', async () => {
       const customFetch = vi.fn()
       const mockReader = {
diff --git a/packages/ai/src/client.ts b/packages/ai/src/client.ts
index f1849d3224..c74ccd340d 100644
--- a/packages/ai/src/client.ts
+++ b/packages/ai/src/client.ts
@@ -157,7 +157,7 @@ export async function generationParamsFromRequest<TKind extends GenerationKind>(
   return generationParamsFromBody(kind, body)
 }
 
-export { EventType } from '@ag-ui/core'
+export { EventType, PROTOCOL_VERSION } from '@ag-ui/core'
 export type { RunAgentInput } from '@ag-ui/core'
 
 export {
diff --git a/testing/e2e/tests/ag-ui-compliance.spec.ts b/testing/e2e/tests/ag-ui-compliance.spec.ts
index 5bb87f0a59..df3da51152 100644
--- a/testing/e2e/tests/ag-ui-compliance.spec.ts
+++ b/testing/e2e/tests/ag-ui-compliance.spec.ts
@@ -39,6 +39,8 @@ test.describe('AG-UI client-to-server compliance', () => {
       expect(body).toHaveProperty('tools')
       expect(body).toHaveProperty('context')
       expect(body).toHaveProperty('forwardedProps')
+      // AG-UI 1.0: the client states its protocol version in-band.
+      expect(body.protocolVersion).toBe('1.0')
       expect(Array.isArray(body.messages)).toBe(true)
       expect(Array.isArray(body.tools)).toBe(true)
     }