> Fetch clean Markdown by appending `.md` to any page URL under https://signalwire.com/docs or requesting it with the HTTP header `Accept: text/markdown`. The root index at https://signalwire.com/docs/llms.txt lists the available documentation indexes.

# Client capabilities

> Read the rendering capabilities a browser client declares in its user variables at dial time.

[user-variables]: /docs/server-sdks/reference/python/core/capabilities/user-variables

[declared-capabilities]: /docs/server-sdks/reference/python/core/capabilities/declared-capabilities

[has-capability]: /docs/server-sdks/reference/python/core/capabilities/has-capability

[handoff-router]: /docs/server-sdks/reference/python/agents/handoff-router

A browser client such as the SignalWire address widget declares what it can render in the user
variables it sends at dial time. The `signalwire.core.capabilities` module reads those
declarations so an agent can decide what to offer: whether to push content to a screen, whether to
advertise a text-handoff tool.

```json
{
  "vars": {
    "userVariables": {
      "capabilities": {
        "display_content": true,
        "transcript": true,
        "chat_handoff": false
      }
    }
  }
}
```

> **Declarations, not grants**
>
> A capability says what the client can render. It is never permission to do anything privileged,
> because a caller controls its own user variables. Use these values as hints for what to offer.

Absence means no. Every function resolves errors and missing data to "not declared", because
offering a caller something they can't reach is worse than never mentioning it: a phone caller has no
screen. There is no fixed list of capability names; a client can declare a name this SDK has never
seen and your application can act on it.

```python
from signalwire.core.capabilities import declared_capabilities, has_capability, user_variables
```

## Functions

#### [user\_variables](/docs/server-sdks/reference/python/core/capabilities/user-variables)

Return the user variables from a SWML request body.

#### [declared\_capabilities](/docs/server-sdks/reference/python/core/capabilities/declared-capabilities)

Return the capability names the client declared as truthy.

#### [has\_capability](/docs/server-sdks/reference/python/core/capabilities/has-capability)

Whether the client declared one named capability.

## Example

Offer a screen-only tool to callers who can render it:

```python {8-12}
from signalwire import AgentBase
from signalwire.core.capabilities import has_capability

agent = AgentBase(name="dispatch", route="/dispatch")
agent.set_prompt_text("You are Ada, the dispatcher for Bayview Taxi.")

def configure(query_params, body_params, headers, ephemeral_agent):
    if has_capability(body_params, "display_content"):
        ephemeral_agent.prompt_add_section(
            "Screen",
            body="The caller can see a screen. Offer to show the fare breakdown there.",
        )

agent.add_per_call_config(configure)
agent.serve()
```

For moving a conversation between chat and a call, see [`HandoffRouter`][handoff-router].

> Read the rendering capabilities a browser client declares in its user variables at dial time.

## Docs

- [declared_capabilities](https://signalwire.com/docs/server-sdks/reference/python/core/capabilities/declared-capabilities.md): Return the set of capability names a client declared as truthy.
- [has_capability](https://signalwire.com/docs/server-sdks/reference/python/core/capabilities/has-capability.md): Whether a client explicitly declared one named capability.
- [user_variables](https://signalwire.com/docs/server-sdks/reference/python/core/capabilities/user-variables.md): Return the user variables nested inside a SWML request body.