diff --git a/src/tools/README.md b/src/tools/README.md
new file mode 100644
index 0000000..d7c6380
--- /dev/null
+++ b/src/tools/README.md
@@ -0,0 +1,383 @@
+# Tools
+
+Tools allow the agent to perform actions and fetch data. Each tool defines a **schema** (what arguments it accepts) and an **execution function** (what it does).
+
+## Quick Start
+
+Add a new tool by creating a `.jl` file in `src/tools/`. The file must define a `getTool()` function that returns an `agentTool`:
+
+```julia
+# src/tools/my_tool.jl
+
+function executeTool(toolCallId::String, args::Dict{String,Any}, signal::Union{Nothing,abortSignal}, onPartialResult::Function)::agentToolResult
+    city = args["city"]
+    return agentToolResult(
+        [textContent("Hello from $(city)!")],
+        Dict{Any,Any}(), nothing, false
+    )
+end
+
+function getTool()::agentTool
+    return agentTool(
+        name = "my_tool",
+        label = "My Tool",
+        description = "Says hello to a city.",
+        inputSchema = Dict{String,Any}(
+            "type" => "object",
+            "properties" => Dict(
+                "city" => Dict("type" => "string", "description" => "City name")
+            ),
+            "required" => ["city"]
+        ),
+        execute = executeTool,
+        prepareArguments = nothing,
+        validateRequiredArgs = nothing,
+        parallelToolExecute = false
+    )
+end
+```
+
+When `loadTools()` or `registerTool()` is called, the tool becomes available to the agent.
+
+## Tool Anatomy
+
+Each tool has 3 main parts:
+
+### 1. Schema (`inputSchema`)
+
+JSON Schema (MCP format) describing the tool's arguments. The `"required"` array lists mandatory fields:
+
+```julia
+inputSchema = Dict{String,Any}(
+    "type" => "object",
+    "properties" => Dict(
+        "city" => Dict("type" => "string", "description" => "City name"),
+        "units" => Dict("type" => "string", "enum" => ["celsius", "fahrenheit"], "default" => "celsius")
+    ),
+    "required" => ["city"]
+)
+```
+
+### 2. Execution Function (`execute`)
+
+A function with the signature:
+
+```julia
+execute(toolCallId::String, args::Dict{String,Any}, signal::Union{Nothing,abortSignal}, onPartialResult::Function)::agentToolResult
+```
+
+- **`toolCallId`** — unique ID for this invocation (from the LLM's tool call)
+- **`args`** — validated arguments provided by the LLM
+- **`signal`** — abort signal for cancellable operations
+- **`onPartialResult`** — callback for streaming progress updates
+- **Returns** — `agentToolResult` with content, details, usage, and termination flag
+
+```julia
+function executeTool(toolCallId::String, args::Dict{String,Any}, signal::Union{Nothing,abortSignal}, onPartialResult::Function)::agentToolResult
+    # Optional: stream progress updates
+    onPartialResult(Dict("status" => "Fetching data..."))
+    
+    # Do work
+    result = "Weather in $(args["city"]): Sunny, 22°C"
+    
+    # Return result
+    return agentToolResult(
+        [textContent(result)],
+        Dict{Any,Any}(),  # details
+        nothing,           # usage
+        false              # terminate (true to stop agent loop)
+    )
+end
+```
+
+### 3. Tool Definition (`getTool()`)
+
+Returns an `agentTool` struct:
+
+| Field | Type | Description |
+|---|---|---|
+| `name` | `String` | Unique identifier (e.g. `"getWeather"`) |
+| `label` | `String` | Human-readable name (e.g. `"Weather Lookup"`) |
+| `description` | `String` | What the tool does (shown to the LLM) |
+| `inputSchema` | `Any` | JSON Schema (MCP format) |
+| `execute` | `Function` | The execution function |
+| `prepareArguments` | `Union{Function,Nothing}` | Optional argument transform before validation |
+| `validateRequiredArgs` | `Union{Function,Nothing}` | Optional custom validation |
+| `parallelToolExecute` | `Bool` | Run this tool in parallel with others |
+
+## Argument Validation
+
+Validation happens **before** tool execution, in the `prepareToolCall` phase. Invalid calls return an error immediately without invoking `execute`, `beforeToolCall`, or logging `toolExecutionStart`.
+
+### Default: JSON Schema Required Fields
+
+Set `validateRequiredArgs = nothing` to use the default validator, which checks that all fields in `inputSchema["required"]` are present:
+
+```julia
+# src/tools/getWeather.jl — uses default validation
+function getTool()::agentTool
+    return agentTool(
+        name = "getWeather",
+        # ...
+        validateRequiredArgs = nothing,  # uses default
+    )
+end
+```
+
+### Custom Validation Hook
+
+Override `validateRequiredArgs` when you need:
+- **Cross-field constraints** (e.g. "at least one of X or Y")
+- **Format validation** (e.g. regex patterns, date parsing)
+- **Domain rules** (e.g. value ranges, business logic)
+
+The hook signature takes only `args`:
+
+```julia
+function validateRequiredArgs(args::Dict{String,Any})::Union{Nothing,String}
+    tz = get(args, "timezone", nothing)
+    city = get(args, "city", "")
+    
+    if !haskey(args, "timezone") && isempty(city)
+        return "Missing required argument: provide at least one of 'timezone' or 'city'"
+    end
+    
+    if tz !== nothing
+        tz_str = string(tz)
+        if !occursin(r"^[A-Za-z]+\/[A-Za-z]+(/[A-Za-z]+)*$", tz_str)
+            return "Invalid timezone format: '$tz_str'. Use IANA format, e.g. 'America/New_York'"
+        end
+    end
+    
+    return nothing
+end
+```
+
+Return `nothing` to pass, or an error `String` to fail. The error is fed back to the LLM so it can retry with corrected arguments.
+
+## Tool Call Lifecycle
+
+```
+LLM requests tool call
+  └── prepareToolCall (agentCore.jl:511)
+       ├── Tool lookup by name
+       ├── prepareArguments (tool-specific transform, if defined)
+       ├── validateToolArguments (validateRequiredArgs hook or default)
+       │     └── on failure → immediateOutcome (no execution)
+       ├── beforeToolCall hook (if defined)
+       │     └── on block → immediateOutcome (no execution)
+       └── returns preparedToolCall
+
+executed by executeToolCallsSequential or executeToolCallsParallel
+  └── executePreparedToolCall (agentCore.jl:589)
+       ├── emit toolExecutionStart
+       ├── call tool.execute()
+       │     └── on error → executedOutcome(isError=true)
+       └── returns executedOutcome
+
+finalizeExecutedToolCall (agentCore.jl:675)
+  ├── afterToolCall hook (if defined)
+  │     └── can mutate result content, usage, terminate, isError
+  └── returns finalizedOutcome
+
+emit toolExecutionEnd
+  └── createToolResultMessage → added to conversation history
+```
+
+## Execution Modes
+
+### Sequential
+
+Tools execute one at a time in order. Required when:
+- Tools have implicit dependencies
+- Tools share state (e.g. writing to the same file)
+- Tools have `parallelToolExecute = false`
+
+Set globally via `agentLoopConfig.toolExecution = "sequential"`, or per-tool via `parallelToolExecute = false`.
+
+### Parallel
+
+Tools execute concurrently when all are independent. Reduces wall-clock time. Set `parallelToolExecute = true` on individual tools, or set `agentLoopConfig.toolExecution = "parallel"`.
+
+## Streaming Partial Results
+
+For long-running tools (API calls, file uploads, training), use `onPartialResult` to stream progress:
+
+```julia
+function executeTool(toolCallId::String, args::Dict{String,Any}, signal::Union{Nothing,abortSignal}, onPartialResult::Function)::agentToolResult
+    onPartialResult(Dict("status" => "Step 1: Fetching data..."))
+    sleep(1)
+    
+    onPartialResult(Dict("status" => "Step 2: Processing..."))
+    sleep(1)
+    
+    return agentToolResult(
+        [textContent("Done!")],
+        Dict{Any,Any}(), nothing, false
+    )
+end
+```
+
+UI listeners and the TUI consume these events in real time via `toolExecutionUpdate`.
+
+## Loading Tools
+
+### Auto-load from Directory
+
+```julia
+using .toolRegistry
+
+tools = loadTools("src/tools")  # scans for *.jl files with getTool()
+```
+
+Files are loaded alphabetically for deterministic registration order.
+
+### Manual Registration
+
+```julia
+tool = getTool()  # from your tool module
+registerTool(tool)
+```
+
+## Using Tools with an Agent
+
+Loading tools only registers them — you must pass them to the `yiemAgent` and provide an `llmCall` function. Here is the complete flow:
+
+```julia
+using .YiemAgent
+using .toolRegistry
+
+# 1. Load tools from the tools directory
+tools = loadTools("src/tools")
+# [toolRegistry] Loading tool from: src/tools/getTime.jl
+# [toolRegistry] Loaded tool: getTime — Time Lookup
+# [toolRegistry] Loading tool from: src/tools/getWeather.jl
+# [toolRegistry] Loaded tool: getWeather — Weather Lookup
+
+# 2. Define your LLM call function
+function my_llm_call(messages::Dict)::assistantMessage
+    # Call your LLM API here (OpenAI, Anthropic, local model, etc.)
+    # Return an assistantMessage with the response content
+    # If the LLM wants to call a tool, include tool_call content blocks
+    ...
+end
+
+# 3. Define your event sink (optional, for logging/debugging)
+function my_event_sink(event)
+    if event isa toolExecStartEvent
+        println("[EVENT] Tool start: $(event.toolName)")
+    elseif event isa toolExecEndEvent
+        status = event.isError ? "ERROR" : "OK"
+        println("[EVENT] Tool end: $(event.toolName) — $status")
+    end
+end
+
+# 4. Create the agent with tools
+agent = yiemAgent(
+    systemPrompt = "You are a helpful assistant that can check weather and time.",
+    model = my_model,
+    tools = tools,                        # pass loaded tools
+    llmCall = my_llm_call,                # your LLM function
+    agentEventSink = my_event_sink,       # event handler
+)
+
+# 5. Send a message and get a response
+run_agent(agent, "What's the weather in Tokyo?")
+response = take_response(agent)
+
+# response.content contains the LLM's reply (with tool results if applicable)
+println(response.content)
+
+# 6. When done, stop the agent
+stop_agent(agent)
+```
+
+### How It Works
+
+1. **User sends a message** via `run_agent(agent, "What's the weather in Tokyo?")`. The message goes into `inputChannel`.
+
+2. **Agent loop** (`_agent_loop`) picks it up, converts it to a `userMessage`, and adds it to `agent._state.messages`.
+
+3. **LLM is called** via `agent.llmCall(formatted_messages)`. The LLM sees the system prompt, conversation history, and the tool definitions in the prompt (via `formatMsgForLLM`).
+
+4. **If the LLM uses a tool**, it returns a response with `tool_call` content blocks. The agent:
+   - Extracts each tool call (name, arguments)
+   - Runs validation (`validateRequiredArgs` or default)
+   - Executes the tool (or returns an error if validation fails)
+   - Feeds the result back as a `toolResultMessage` in the conversation
+
+5. **LLM is called again** with the tool results. This repeats until the LLM returns a text response with no tool calls.
+
+6. **Final response** is sent to `outputChannel` — retrieve it with `take_response(agent)`.
+
+### Minimal Working Example
+
+```julia
+using .YiemAgent
+using .toolRegistry
+
+# Load tools
+tools = loadTools("src/tools")
+
+# Mock LLM that echoes back a tool call, then a text response
+call_count = 0
+function mock_llm_call(messages::Dict)::assistantMessage
+    global call_count += 1
+    if call_count == 1
+        # First call: LLM decides to use getWeather
+        return assistantMessage(
+            content=[
+                Dict("type" => "tool_calls", 
+                     "tool_calls" => [Dict("id" => "call_1", "name" => "getWeather",
+                                           "arguments" => Dict("city" => "Tokyo"))])
+            ],
+            model = "mock",
+            usage = llmUsage(0, 0)
+        )
+    else
+        # Second call: LLM returns text (after tool result)
+        return assistantMessage(
+            content = [textContent("The weather in Tokyo is sunny, 22°C.")],
+            model = "mock",
+            usage = llmUsage(0, 0)
+        )
+    end
+end
+
+# Create agent
+agent = yiemAgent(
+    systemPrompt = "You are a helpful assistant.",
+    tools = tools,
+    llmCall = mock_llm_call,
+    agentEventSink = e -> nothing,  # no events
+)
+
+# Run
+run_agent(agent, "What's the weather in Tokyo?")
+response = take_response(agent)
+
+stop_agent(agent)
+```
+
+## Available Tools
+
+| Tool | Description | Validation |
+|---|---|---|
+| `getWeather` | Fetch weather for a city | Default (JSON Schema required) |
+| `getTime` | Get current time for a timezone or city | Custom (cross-field + format) |
+
+## Example: Error Flow
+
+When the LLM calls a tool with invalid arguments:
+
+```
+User: "What's the weather?"
+  └── LLM: call getWeather() with no arguments
+       └── prepareToolCall → validateRequiredArgs → "Missing required arguments: city"
+            └── immediateOutcome → error tool result
+                 └── LLM sees: "Missing required arguments: city"
+                      └── LLM retries: call getWeather(city="Tokyo")
+                           └── executeTool → "Weather in Tokyo: Sunny, 22°C"
+```
+
+The agent feeds the error back to the LLM as a tool result message, allowing it to self-correct.
diff --git a/src/tools/getTime.jl b/src/tools/getTime.jl
index d100f9b..97636ab 100644
--- a/src/tools/getTime.jl
+++ b/src/tools/getTime.jl
@@ -1,5 +1,43 @@
 """
-Execute the get_time tool.
+Validate required arguments for the getTime tool.
+
+Demonstrates custom validation beyond simple required-field checking:
+- Ensures at least one time source (timezone or city) is provided
+- Validates timezone is in IANA format if specified
+- Validates city name is not empty if specified
+
+# Arguments
+- `args::Dict{String,Any}`: Arguments from the LLM
+
+# Returns
+- `nothing` if validation passes
+- `String` error message if validation fails
+"""
+function validateRequiredArgs(args::Dict{String,Any})::Union{Nothing,String}
+    tz = get(args, "timezone", nothing)
+    city = get(args, "city", "")
+    
+    hasTz = tz !== nothing && !isempty(tz)
+    hasCity = !isempty(city)
+    
+    # At least one of timezone or city is required
+    if !hasTz && !hasCity
+        return "Missing required argument: provide at least one of 'timezone' or 'city'"
+    end
+    
+    # Validate timezone format (IANA tz database: "Continent/City" or "Continent/City/SubCity")
+    if hasTz
+        tz_str = string(tz)
+        if !occursin(r"^[A-Za-z]+\/[A-Za-z]+(/[A-Za-z]+)*$", tz_str)
+            return "Invalid timezone format: '$tz_str'. Use IANA format, e.g. 'America/New_York' or 'Asia/Tokyo'"
+        end
+    end
+    
+    return nothing
+end
+
+"""
+Execute the getTime tool.
 
 # Arguments
 - `toolCallId::String`: Unique identifier for this tool call
@@ -8,42 +46,44 @@ Execute the get_time tool.
 - `onPartialResult::Function`: Callback for streaming partial results
 
 # Returns
-- `agentToolResult`: Result content with current time
+- `agentToolResult`: Result content with current time data
 """
 function executeTool(toolCallId::String, args::Dict{String,Any}, signal::Union{Nothing,abortSignal}, onPartialResult::Function)::agentToolResult
-    tz = get(args, "timezone", "local")
+    tz = get(args, "timezone", nothing)
+    city = get(args, "city", "")
     
-    t = now()
-    
-    if tz == "local"
-        timeStr = string(t)
+    # Simulate time lookup — replace with actual timezone API call
+    if tz !== nothing
+        result = "Current time in $(tz): $(now())"
     else
-        timeStr = string(t)
+        result = "Current time in $(city): $(now())"
     end
     
     return agentToolResult(
-        [textContent("Current time: $(timeStr)")],
+        [textContent(result)],
         Dict{Any,Any}(), nothing, false
     )
 end
 
 """
-Define and return the get_time agentTool.
+Define and return the getTime agentTool.
 """
 function getTool()::agentTool
     return agentTool(
-        name = "get_time",
-        label = "Get Current Time",
-        description = "Get the current date and time.",
+        name = "getTime",
+        label = "Time Lookup",
+        description = "Get current local time for a timezone or city.",
         inputSchema = Dict{String,Any}(
             "type" => "object",
             "properties" => Dict(
-                "timezone" => Dict("type" => "string", "description" => "Timezone (currently only 'local' is supported)")
+                "timezone" => Dict("type" => "string", "description", "IANA timezone, e.g. 'America/New_York'"),
+                "city" => Dict("type" => "string", "description", "City name as fallback")
             ),
             "required" => []
         ),
         execute = executeTool,
         prepareArguments = nothing,
+        validateRequiredArgs = validateRequiredArgs,
         parallelToolExecute = false
     )
 end
diff --git a/src/tools/getWeather.jl b/src/tools/getWeather.jl
index f62cb80..0411841 100644
--- a/src/tools/getWeather.jl
+++ b/src/tools/getWeather.jl
@@ -14,14 +14,6 @@ function executeTool(toolCallId::String, args::Dict{String,Any}, signal::Union{N
     city = get(args, "city", "")
     units = get(args, "units", "celsius")
     
-    # Validate required arguments
-    if isempty(city)
-        return agentToolResult(
-            [textContent("Error: 'city' argument is required.")],
-            Dict{Any,Any}(), nothing, false
-        )
-    end
-    
     # Simulate weather fetch — replace with actual API call
     # You can call onPartialResult() here for streaming progress updates:
     #   onPartialResult(Dict("status" => "Fetching weather data..."))
@@ -54,6 +46,7 @@ function getTool()::agentTool
         ),
         execute = executeTool,  # reference the function defined above
         prepareArguments = nothing,
+        validateRequiredArgs = nothing,
         parallelToolExecute = false
     )
 end
diff --git a/src/type.jl b/src/type.jl
index 5146870..60ae3c5 100644
--- a/src/type.jl
+++ b/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
 
diff --git a/src/utils.jl b/src/utils.jl
index 0c92f43..db3e51d 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -1,6 +1,6 @@
 module utils
 
-export clearhistory, availableWineToText, prepareContext, formatMsgForLLM, _userMessageToOpenAI, 
+export clearhistory, availableWineToText, prepareContext, formatMsgForLLM, validateRequiredArgs, validateToolArguments, _userMessageToOpenAI, 
         _assistantMessageToOpenAI, _toolResultMessageToOpenAI, _messageContentToBlocks
 
 using UUIDs, Dates, DataStructures, HTTP, JSON
@@ -275,10 +275,109 @@ function _messageContentToBlocks(contents::Vector{messageContent})::Vector{Dict{
     end
   end
 
-  return blocks
+    return blocks
 end
 
 
+"""
+    validateRequiredArgs(args::Dict{String,Any}, inputSchema::Dict{String,Any}) -> Union{Nothing,String}
+
+Validates that all required fields listed in the tool's JSON Schema are present
+in `args`. Returns `nothing` if validation passes, or a descriptive error string
+listing the missing required fields.
+
+This is the default `validateRequiredArgs` hook. Tool authors can override it
+with a custom validation function that performs additional checks (e.g. type
+coercion, format validation, cross-field constraints).
+
+# Arguments
+- `args::Dict{String,Any}`: The arguments provided by the LLM
+- `inputSchema::Dict{String,Any}`: The tool's `inputSchema` (JSON Schema format)
+
+# Returns
+- `nothing` if all required args are present
+- `String` error message listing missing fields otherwise
+
+# Examples
+```julia
+schema = Dict("required" => ["city"])
+args = Dict{String,Any}()
+validateRequiredArgs(args, schema)  # => "Missing required arguments: city"
+
+args2 = Dict("city" => "Tokyo")
+validateRequiredArgs(args2, schema)  # => nothing
+```
+"""
+function validateRequiredArgs(args::Dict{String,Any}, inputSchema::Dict{String,Any})::Union{Nothing,String}
+    required = get(inputSchema, "required", Any[])
+    if isempty(required)
+        return nothing
+    end
+    
+    missing = String[]
+    for field in required
+        if !(field in keys(args))
+            push!(missing, field)
+        end
+    end
+    
+    if !isempty(missing)
+        return "Missing required arguments: $(join(missing, ", "))"
+    end
+    
+    return nothing
+end
+
+
+"""
+    validateToolArguments(tool::agentTool, prepared::agentToolCall) -> Dict{String,Any}
+
+Validates the prepared tool call arguments by calling the tool's
+`validateRequiredArgs` hook (or the default implementation). If validation
+fails, returns a modified `agentToolCall` with an empty arguments dict
+so downstream code can detect the failure. If the hook exists on the tool
+and returns an error string, that error is returned.
+
+This runs **before** the `beforeToolCall` hook, allowing the agent to
+reject invalid calls without invoking lifecycle callbacks or logging
+false `toolExecutionStart` events.
+
+# Arguments
+- `tool::agentTool`: The resolved tool definition
+- `prepared::agentToolCall`: The prepared tool call with potentially transformed arguments
+
+# Returns
+- `Dict{String,Any}`: The validated arguments if successful
+
+# Errors
+- Throws `ArgumentError` if validation fails — this is caught by `prepareToolCall`
+  and converted to an `immediateOutcome`
+
+# Examples
+```julia
+# With validateRequiredArgs hook set on the tool
+validateToolArguments(toolWithHook, tc)  # => validated args or throws
+
+# With default validation (nothing on tool)
+validateToolArguments(toolDefault, tc)  # => args or throws
+```
+"""
+function validateToolArguments(tool::agentTool, prepared::agentToolCall)::Dict{String,Any}
+    # Use default (2-arg: args + schema) or tool-specific hook (1-arg: args only)
+    if isnothing(tool.validateRequiredArgs)
+        result = validateRequiredArgs(prepared.arguments, tool.inputSchema)
+    else
+        result = tool.validateRequiredArgs(prepared.arguments)
+    end
+    
+    if result !== nothing
+        throw(ArgumentError(result))
+    end
+    
+    return prepared.arguments
+end
+
+