diff --git a/llama_toolchain/agentic_system/api/datatypes.py b/llama_toolchain/agentic_system/api/datatypes.py
index 1dda64834..825859b63 100644
--- a/llama_toolchain/agentic_system/api/datatypes.py
+++ b/llama_toolchain/agentic_system/api/datatypes.py
@@ -110,6 +110,12 @@ class Session(BaseModel):
     started_at: datetime
 
 
+@json_schema_type
+class ToolPromptFormat(Enum):
+    json = "json"
+    function_tag = "function_tag"
+
+
 @json_schema_type
 class AgenticSystemInstanceConfig(BaseModel):
     instructions: str
@@ -127,6 +133,9 @@ class AgenticSystemInstanceConfig(BaseModel):
     # if you completely want to replace the messages prefixed by the system,
     # this is debug only
     debug_prefix_messages: Optional[List[Message]] = Field(default_factory=list)
+    tool_prompt_format: Optional[ToolPromptFormat] = Field(
+        default=ToolPromptFormat.function_tag
+    )
 
 
 class AgenticSystemTurnResponseEventType(Enum):
diff --git a/llama_toolchain/agentic_system/client.py b/llama_toolchain/agentic_system/client.py
index 597872e30..6b6dc6106 100644
--- a/llama_toolchain/agentic_system/client.py
+++ b/llama_toolchain/agentic_system/client.py
@@ -30,6 +30,7 @@ from .api import (
     AgenticSystemToolDefinition,
     AgenticSystemTurnCreateRequest,
     AgenticSystemTurnResponseStreamChunk,
+    ToolPromptFormat,
 )
 
 
@@ -132,6 +133,7 @@ async def run_main(host: str, port: int):
             output_shields=[],
             quantization_config=None,
             debug_prefix_messages=[],
+            tool_prompt_format=ToolPromptFormat.json,
         ),
     )
 
diff --git a/llama_toolchain/agentic_system/meta_reference/agent_instance.py b/llama_toolchain/agentic_system/meta_reference/agent_instance.py
index 8e4555cb4..69784504f 100644
--- a/llama_toolchain/agentic_system/meta_reference/agent_instance.py
+++ b/llama_toolchain/agentic_system/meta_reference/agent_instance.py
@@ -10,6 +10,8 @@ import uuid
 from datetime import datetime
 from typing import AsyncGenerator, List, Optional
 
+from termcolor import cprint
+
 from llama_toolchain.agentic_system.api.datatypes import (
     AgenticSystemInstanceConfig,
     AgenticSystemTurnResponseEvent,
@@ -24,6 +26,7 @@ from llama_toolchain.agentic_system.api.datatypes import (
     ShieldCallStep,
     StepType,
     ToolExecutionStep,
+    ToolPromptFormat,
     Turn,
 )
 
@@ -51,7 +54,6 @@ from llama_toolchain.safety.api.datatypes import (
     ShieldDefinition,
     ShieldResponse,
 )
-from termcolor import cprint
 from llama_toolchain.agentic_system.api.endpoints import *  # noqa
 
 from .safety import SafetyException, ShieldRunnerMixin
@@ -74,6 +76,7 @@ class AgentInstance(ShieldRunnerMixin):
         output_shields: List[ShieldDefinition],
         max_infer_iters: int = 10,
         prefix_messages: Optional[List[Message]] = None,
+        tool_prompt_format: Optional[ToolPromptFormat] = ToolPromptFormat.function_tag,
     ):
         self.system_id = system_id
         self.instance_config = instance_config
@@ -86,7 +89,9 @@ class AgentInstance(ShieldRunnerMixin):
             self.prefix_messages = prefix_messages
         else:
             self.prefix_messages = get_agentic_prefix_messages(
-                builtin_tools, custom_tool_definitions
+                builtin_tools,
+                custom_tool_definitions,
+                tool_prompt_format,
             )
 
         for m in self.prefix_messages:
diff --git a/llama_toolchain/agentic_system/meta_reference/agentic_system.py b/llama_toolchain/agentic_system/meta_reference/agentic_system.py
index 5db8d6168..ae1d282aa 100644
--- a/llama_toolchain/agentic_system/meta_reference/agentic_system.py
+++ b/llama_toolchain/agentic_system/meta_reference/agentic_system.py
@@ -108,6 +108,7 @@ class MetaReferenceAgenticSystemImpl(AgenticSystem):
             input_shields=cfg.input_shields,
             output_shields=cfg.output_shields,
             prefix_messages=cfg.debug_prefix_messages,
+            tool_prompt_format=cfg.tool_prompt_format,
         )
 
         return AgenticSystemCreateResponse(
diff --git a/llama_toolchain/agentic_system/meta_reference/system_prompt.py b/llama_toolchain/agentic_system/meta_reference/system_prompt.py
index d51e53a82..a6fa8e638 100644
--- a/llama_toolchain/agentic_system/meta_reference/system_prompt.py
+++ b/llama_toolchain/agentic_system/meta_reference/system_prompt.py
@@ -8,6 +8,8 @@ import json
 from datetime import datetime
 from typing import List
 
+from llama_toolchain.agentic_system.api.datatypes import ToolPromptFormat
+
 from llama_toolchain.inference.api import (
     BuiltinTool,
     Message,
@@ -19,7 +21,9 @@ from .tools.builtin import SingleMessageBuiltinTool
 
 
 def get_agentic_prefix_messages(
-    builtin_tools: List[SingleMessageBuiltinTool], custom_tools: List[ToolDefinition]
+    builtin_tools: List[SingleMessageBuiltinTool],
+    custom_tools: List[ToolDefinition],
+    tool_prompt_format: ToolPromptFormat,
 ) -> List[Message]:
     messages = []
     content = ""
@@ -44,14 +48,15 @@ Today Date: {formatted_date}\n"""
     content += date_str
 
     if custom_tools:
-        custom_message = get_system_prompt_for_custom_tools(custom_tools)
-        content += custom_message
+        if tool_prompt_format == ToolPromptFormat.function_tag:
+            custom_message = get_system_prompt_for_custom_tools(custom_tools)
+            content += custom_message
+            messages.append(SystemMessage(content=content))
+        else:
+            raise NotImplementedError(
+                f"Tool prompt format {tool_prompt_format} is not supported"
+            )
 
-    # TODO: Replace this hard coded message with instructions coming in the request
-    if False:
-        content += "\nYou are a helpful Assistant."
-
-    messages.append(SystemMessage(content=content))
     return messages