update

src/type.jl

@@ -8,8 +8,8 @@
            textContent, imageContent,
            # Message types
            userMessage, assistantMessage, toolResultMessage,
-           # Tool types
-           agentTool,
+            # Tool types
+            agentTool, validateRequiredArgs
            # Context types
            agentContext, agentState, agentToolCall, prepareNextTurnContext,
            # Loop & execution types
@@ -207,6 +207,8 @@ Maps MCP server tool definitions to an executable Julia tool.
 - `execute::Function`: Tool execution function, signature:
     `execute(toolCallId::String, args::Dict, signal::Union{Nothing,AbortSignal}, onPartialResult::Function)`
 - `prepareArguments::Union{Function, Nothing}`: Optional argument preparation callback
+- `validateRequiredArgs::Union{Function, Nothing}`: Optional validation hook, signature:
+    `validateRequiredArgs(args::Dict) -> Union{Nothing, String}` where `String` is an error message
 - `parallelToolExecute::Bool`: Override: run tool calls sequentially or in parallel
 
 # Returns
@@ -249,7 +251,10 @@ tool = agentTool(
             [textContent("Sunny, 22C in $(city)")],
             Dict{Any,Any}(), nothing, false
         )
-    end
+    end,
+    prepareArguments = nothing,
+    validateRequiredArgs = nothing,
+    parallelToolExecute = false
 )
 ```
 """
@@ -260,6 +265,7 @@ struct agentTool                              # A tool available to the agent
     inputSchema::Any                          # Tool parameters schema (JSON schema, MCP inputSchema format)
     execute::Function                         # Tool execution function
     prepareArguments::Union{Function, Nothing}  # Optional argument preparation callback
+    validateRequiredArgs::Union{Function, Nothing}  # Optional validation hook for required args
     parallelToolExecute::Bool  # Override: run tool calls sequentially or in parallel
 end