diff --git a/.changeset/fix-ag-ui-tool-call-input.md b/.changeset/fix-ag-ui-tool-call-input.md
new file mode 100644
index 000000000..1155e82b1
--- /dev/null
+++ b/.changeset/fix-ag-ui-tool-call-input.md
@@ -0,0 +1,12 @@
+---
+"@voltagent/ag-ui": patch
+---
+
+fix: use `input` instead of `args` for tool-call parts in message conversion
+
+When converting CopilotKit assistant messages with tool calls to VoltAgent format,
+the adapter was setting `args` on tool-call parts. The AI SDK's `ToolCallPart`
+interface expects `input`, causing the Anthropic provider to send `undefined` as
+the tool_use input — rejected by the API with:
+
+  "messages.N.content.N.tool_use.input: Input should be a valid dictionary"
diff --git a/packages/ag-ui/src/voltagent-agent.ts b/packages/ag-ui/src/voltagent-agent.ts
index 794d4da34..d9919aa42 100644
--- a/packages/ag-ui/src/voltagent-agent.ts
+++ b/packages/ag-ui/src/voltagent-agent.ts
@@ -276,7 +276,7 @@ const isToolMessage = (message: Message): message is ToolMessage => message.role
 
 type VoltUIPart =
   | { type: "text"; text: string }
-  | { type: "tool-call"; toolCallId: string; toolName: string; args?: unknown }
+  | { type: "tool-call"; toolCallId: string; toolName: string; input?: unknown }
   | { type: "tool-result"; toolCallId?: string; toolName?: string; output?: unknown };
 
 type VoltUIMessage = {
@@ -311,13 +311,13 @@ function convertAGUIMessagesToVoltMessages(messages: Message[]): VoltUIMessage[]
       }
 
       for (const call of msg.toolCalls ?? []) {
-        const args = safelyParseJson(call.function.arguments);
+        const input = safelyParseJson(call.function.arguments);
         toolNameById.set(call.id, call.function.name);
         parts.push({
           type: "tool-call",
           toolCallId: call.id,
           toolName: call.function.name,
-          args,
+          input,
         });
       }