sgl-project/sglang · error · ValueError

prompt event payload must be a non-empty string

Error message

prompt event payload must be a non-empty string

What it means

_validate_prompt_payload requires the prompt payload (from a prompt event or a composite_input 'prompt' item) to be a non-empty string; None, non-strings, or empty strings are rejected.

Source

Thrown at python/sglang/multimodal_gen/runtime/entrypoints/openai/realtime/adapters/lingbot_world_realtime_adapter.py:184

        return state.receive_camera_control_event_payload(
            payload,
            event_id=event_id,
        )

    def _ingest_prompt(
        self,
        state: LingBotWorldRealtimeState,
        payload: Any,
        event_id: int | None,
    ) -> str:
        prompt = self._validate_prompt_payload(payload)
        state.receive_prompt(prompt, event_id=event_id)
        return f"kind=prompt, prompt_len={len(prompt)}"

    @staticmethod
    def _validate_prompt_payload(payload: Any) -> str:
        if not isinstance(payload, str) or not payload:
            raise ValueError("prompt event payload must be a non-empty string")
        return payload

    def _ingest_composite_input(
        self,
        state: LingBotWorldRealtimeState,
        payload: Any,
        event_id: int | None,
    ) -> str:
        if not isinstance(payload, dict):
            raise ValueError("composite_input event payload must be a map")
        input_types = payload.get("input_types")
        if not isinstance(input_types, list) or not input_types:
            raise ValueError(
                "composite_input event payload requires non-empty input_types"
            )

        parsed_inputs = []
        for input_type in input_types:

View on GitHub (pinned to 0132848349)

Solutions

1. Always include a non-empty prompt string when the event/item declares prompt
2. Default to a placeholder prompt (e.g. 'continue') if your flow allows empty intent
3. Skip sending the prompt event entirely when there is no prompt

Example fix

// before
{kind: "prompt", payload: ""}
// after
{kind: "prompt", payload: "continue"}

if not isinstance(prompt, str) or not prompt: prompt = 'continue'

def is_nonempty_str(x: object) -> bool: return isinstance(x, str) and bool(x)

except ValueError as e: if 'non-empty string' in str(e): default the prompt and resend

When it happens

Trigger: Sending a prompt event with payload "" or null, or a composite_input item declaring input_types: ["prompt"] without a non-empty prompt value.

Common situations: Client sends empty prompt to 'kick off' a session; optional prompt field serialized as None by a Python client; whitespace-only is fine but truly empty strings are not.

Related errors

• prompt event payload must be a string
• camera_actions event payload must be list[list[str]]
• composite_input event payload must be a map
• composite_input event payload requires non-empty input_types
• composite_input input_types must contain non-empty strings

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/04f43845f3c20fe1. Report an issue: GitHub.