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

[corners99]

Summary

• Fixes tool-call message conversion in convertAGUIMessagesToVoltMessages() — renames args to input on tool-call parts to match the AI SDK's ToolCallPart interface

Problem

When CopilotKit replays assistant messages containing tool calls, the ag-ui adapter creates tool-call parts with an args field. The AI SDK's ToolCallPart interface (from @ai-sdk/provider-utils) expects input. When the Anthropic provider builds the API request, it reads part.input which is undefined, causing:

AI_APICallError: messages.N.content.N.tool_use.input: Input should be a valid dictionary

This breaks any multi-turn conversation where the assistant previously called a tool (the replayed message has the wrong field name).

Root Cause

In packages/ag-ui/src/voltagent-agent.ts, convertAGUIMessagesToVoltMessages() line 314-321:

// Before (broken)
const args = safelyParseJson(call.function.arguments);
parts.push({ type: "tool-call", toolCallId: call.id, toolName: call.function.name, args });

// After (fixed)
const input = safelyParseJson(call.function.arguments);
parts.push({ type: "tool-call", toolCallId: call.id, toolName: call.function.name, input });

The AI SDK ToolCallPart type:

interface ToolCallPart {
  type: 'tool-call';
  toolCallId: string;
  toolName: string;
  input: unknown;  // <-- expects "input", not "args"
}

Test plan

• Verified ToolCallPart interface in @ai-sdk/provider-utils expects input field
• Verified Anthropic provider reads part.input to construct tool_use blocks
• Multi-turn conversation with tool calls no longer throws AI_APICallError

🤖 Generated with Claude Code

---

Summary by cubic

Use input instead of args for tool-call parts in @voltagent/ag-ui message conversion. This matches @ai-sdk/provider-utils ToolCallPart and prevents Anthropic requests from failing when replaying tool calls.

^{Written for commit 27387c6. Summary will update on new commits.}

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Fixed an issue where tool call arguments were incorrectly formatted in message conversions, causing API rejections. Tool calls now use the expected input field format.

[changeset-bot]

🦋 Changeset detected

Latest commit: 27387c6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@voltagent/ag-ui	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[coderabbitai]

📝 Walkthrough

Walkthrough

The change renames the tool-call payload field from args to input in the VoltUI protocol to align with Anthropic provider expectations, preventing API rejections from undefined tool_use.input values.

Changes

Cohort / File(s)	Summary
Documentation .changeset/fix-ag-ui-tool-call-input.md	Added patch note documenting the fix for tool-call input field naming to prevent Anthropic API rejection errors.
Protocol Definition & Implementation packages/ag-ui/src/voltagent-agent.ts	Updated VoltUIPart type to use input?: unknown instead of args?: unknown for tool-call parts; renamed local variable and field reference from args to input in message conversion logic.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A rabbit hops with glee so bright,
Renaming args to input right,
The Anthropic API now sees clear,
No more undefined errors here! 🐇✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: renaming the field from args to input for tool-call parts to match the AI SDK interface.
Description check	✅ Passed	The description comprehensively covers the problem, root cause, solution, and test plan, though some checklist items are not marked as completed.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[cubic-dev-ai]

No issues found across 2 files

[coderabbitai]

Actionable comments posted: 1

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 5368cdb7-c75b-40a5-91cd-fa06e7044432

📥 Commits

Reviewing files that changed from the base of the PR and between a1b68cc and 27387c6.

📒 Files selected for processing (2)

• .changeset/fix-ag-ui-tool-call-input.md
• packages/ag-ui/src/voltagent-agent.ts

[coderabbitai]

⚠️ 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:

• 1: https://docs.anthropic.com/en/docs/agents-and-tools/tool-use/implement-tool-use
• 2: https://app.unpkg.com/ai%406.0.116/files/dist/internal/index.js.map

---

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.