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fix(ag-ui): use input instead of args for tool-call parts #1149

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Merged
omeraplak merged 1 commit into from
Mar 15, 2026
Merged

fix(ag-ui): use input instead of args for tool-call parts #1149

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12 changes: 12 additions & 0 deletions .changeset/fix-ag-ui-tool-call-input.md
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Original file line number Diff line number Diff line change
@@ -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"
6 changes: 3 additions & 3 deletions packages/ag-ui/src/voltagent-agent.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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 };
Comment on lines +279 to 280
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@coderabbitai coderabbitai bot Mar 12, 2026

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⚠️ Potential issue | 🟠 Major

🧩 Analysis chain

🌐 Web query:

In the current AI SDK / Anthropic tool-use contract, is tool_use.inputrequired to be a JSON object/dictionary, and what is the TypeScript shape ofToolCallPart.input?

💡 Result:

Anthropic tool-use contract (tool_use.input)

Yes—tool_use.input is required, and it is a parsed JSON object (i.e., a dictionary) containing the arguments for the tool, conforming to the tool’s input_schema. (docs.anthropic.com)

AI SDK TypeScript shape (ToolCallPart.input)

In the AI SDK, ToolCallPart.input is typed as unknown (i.e., not restricted to an object at the type level). The shape is: (app.unpkg.com)

export interface ToolCallPart {
  type: 'tool-call';
  toolCallId: string;
  toolName: string;
  input: unknown;
  providerOptions?: ProviderOptions;
  providerExecuted?: boolean;
}

Citations:

Normalize tool-call input to an object, not unknown.

The Anthropic API requires tool_use.input to be a JSON object, but the current code at line 314 can produce a string, array, primitive, or null via safelyParseJson(), and line 279 allows that invalid value through unchecked with the input?: unknown type. This will cause requests to fail at runtime.

Proposed fix 
+type VoltToolInput = Record<string, unknown>;
+
 type VoltUIPart =
   | { type: "text"; text: string }
-  | { type: "tool-call"; toolCallId: string; toolName: string; input?: unknown }
+  | { type: "tool-call"; toolCallId: string; toolName: string; input: VoltToolInput }
   | { type: "tool-result"; toolCallId?: string; toolName?: string; output?: unknown };
@@
-        const input = safelyParseJson(call.function.arguments);
+        const parsedInput = safelyParseJson(call.function.arguments);
+        if (
+          parsedInput === null ||
+          typeof parsedInput !== "object" ||
+          Array.isArray(parsedInput)
+        ) {
+          throw new Error(
+            `Invalid tool-call arguments for "${call.function.name}": expected a JSON object`,
+          );
+        }
         toolNameById.set(call.id, call.function.name);
         parts.push({
           type: "tool-call",
           toolCallId: call.id,
           toolName: call.function.name,
-          input,
+          input: parsedInput,
         });

Lines 279–280, 314–320.


type VoltUIMessage = {
Expand Down Expand Up @@ -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,
});
}

Expand Down
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