StructuredTool schema does not work with optional argument

[yos1p]

Hi,

I'm using Langchain JS 0.2, following this documentation: https://js.langchain.com/v0.2/docs/how_to/custom_tools/

I have a function to call, with parameter (a,b,c), but the parameter c is optional. I have defined it in Zod schema.
What I noticed is that AgentExecutor will only call the function when all arguments are given, but when only (a,b) are given, the function was not called.

Any reason why?