📄 ai-sdk/docs/troubleshooting/ontoolcall-type-narrowing
File: ontoolcall-type-narrowing.md | Updated: 11/15/2025
Source: https://ai-sdk.dev/docs/troubleshooting/ontoolcall-type-narrowing
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When using the onToolCall callback with TypeScript, you may encounter type errors when trying to pass tool properties directly to addToolOutput.
TypeScript cannot automatically narrow the type of toolCall.toolName when you have both static and dynamic tools, leading to type errors:
// ❌ This causes a TypeScript errorconst { messages, sendMessage, addToolOutput } = useChat({ async onToolCall({ toolCall }) { addToolOutput({ tool: toolCall.toolName, // Type 'string' is not assignable to type '"yourTool" | "yourOtherTool"' toolCallId: toolCall.toolCallId, output: someOutput, }); },});
The error occurs because:
- Static tools have specific literal types for their names (e.g.,
"getWeatherInformation") - Dynamic tools have
toolNameas a genericstring - TypeScript can't guarantee that
toolCall.toolNamematches your specific tool names
Check if the tool is dynamic first to enable proper type narrowing:
// ✅ Correct approach with type narrowingconst { messages, sendMessage, addToolOutput } = useChat({ async onToolCall({ toolCall }) { // Check if it's a dynamic tool first if (toolCall.dynamic) { return; }
// Now TypeScript knows this is a static tool with the correct type addToolOutput({ tool: toolCall.toolName, // No type error! toolCallId: toolCall.toolCallId, output: someOutput, }); },});
If you're still using the deprecated addToolResult method, this solution applies the same way. Consider migrating to addToolOutput for consistency with the latest API.
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