> ## Documentation Index
> Fetch the complete documentation index at: https://docs.veadk.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# System prompts

System prompts define an agent's task, response style, and behavioral boundaries. Set `Agent.instruction` to fixed text, a template that reads session state, or a function that builds instructions from the current context

Complete [installation and model configuration](/productions/veadk/preview/en/get-started/quickstart) before running these examples. Save each script with the indicated filename and run it with `python filename.py`

## Static prompts

Use a string for a fixed task. Specify the task, information to clarify, and data the agent must not request

```python main.py lines theme={null}
import asyncio
from veadk import Agent, Runner

agent = Agent(
    name="support_assistant",
    instruction=(
        "You help users troubleshoot billing issues. "
        "Ask for missing details before suggesting a solution. "
        "Do not ask for passwords or payment card numbers."
    ),
)

async def main():
    result = await Runner(agent=agent).run(
        messages="My invoice total is different from last month.",
        session_id="prompt-demo",
    )
    print(result)

if __name__ == "__main__":
    asyncio.run(main())
```

The script prints a response to the billing question. The exact wording depends on the model

## Dynamic prompts

### Use session state

A `{name}` placeholder in a string instruction reads the matching key from session state. This example creates the session with an initial name and language before running the agent

```python state_prompt.py lines theme={null}
import asyncio
from veadk import Agent, Runner

agent = Agent(
    name="personal_assistant",
    instruction="Address the user as {user_name}. Answer in {language}.",
)

async def main():
    runner = Runner(agent=agent, app_name="prompt_demo", user_id="demo-user")
    await runner.session_service.create_session(
        app_name="prompt_demo",
        user_id="demo-user",
        session_id="state-demo",
        state={"user_name": "Alex", "language": "English"},
    )
    print(await runner.run(messages="Introduce yourself.", session_id="state-demo"))

if __name__ == "__main__":
    asyncio.run(main())
```

| Syntax | Behavior |
| :- | :- |
| `{user_name}` | Reads a required key; a missing key causes the run to fail |
| `{user_name?}` | Reads an optional key; a missing key becomes an empty string |
| `{app:language}`, `{user:language}`, `{temp:language}` | Reads a state key in the corresponding namespace |

A state value of `None` also becomes an empty string. Tools and other agents can update state during a run. See [session management](/productions/veadk/preview/en/components/session/index)

### Use an InstructionProvider

Pass a function to `instruction` when you need defaults, conditional logic, or several state values. The function receives a `ReadonlyContext` and returns a string. Async functions defined with `async def` are also supported

```python dynamic_prompt.py lines theme={null}
import asyncio
from google.adk.agents.readonly_context import ReadonlyContext
from veadk import Agent, Runner

def build_instruction(context: ReadonlyContext) -> str:
    language = context.state.get("language", "English")
    return f"Answer in {language}. Keep the explanation concise and factual."

agent = Agent(name="assistant", instruction=build_instruction)

async def main():
    print(await Runner(agent=agent).run(
        messages="What is a session?", session_id="provider-demo"
    ))

if __name__ == "__main__":
    asyncio.run(main())
```

`context.state` is read-only. Other useful fields include `user_id`, `user_content`, `session`, and `agent_name`. Placeholders in a function's return value are not automatically substituted: build the string in the function. To reuse ADK template substitution, call `await inject_session_state(template, context)` from an async function, importing it from `google.adk.utils.instructions_utils`

## instruction and description

| Parameter | Type | Default | Purpose |
| :- | :- | :- | :- |
| `instruction` | `str` or `InstructionProvider` | VeADK general task instructions | Guides the current agent's task and response |
| `description` | `str` | VeADK general capability description | Helps other agents select a transfer target |
| `prompt_manager` | `BasePromptManager \| None` | `None` | Provides `instruction` through a manager, overriding explicitly supplied instructions |

Give each agent specific instructions and a concise description in a multi-agent application. To maintain prompts in an external service, see [prompt management](/productions/veadk/preview/en/components/agent/prompt-management)