FoundationAgents/OpenManus · error · ValueError

Parameters cannot be empty

Error message

Parameters cannot be empty

What it means

Pydantic field_validator (mode='before') on BrowserUseTool.parameters rejects empty input: constructing or validating the tool with parameters={} or parameters=None raises ValueError('Parameters cannot be empty'). The parameters field carries the browser-use action instruction (e.g. {'action': 'go_to_url', 'url': ...}), and an empty value is meaningless, so validation fails at model time, before any browser work starts.

Source

Thrown at app/tool/browser_use_tool.py:138

            "extract_content": ["goal"],
        },
    }

    lock: asyncio.Lock = Field(default_factory=asyncio.Lock)
    browser: Optional[BrowserUseBrowser] = Field(default=None, exclude=True)
    context: Optional[BrowserContext] = Field(default=None, exclude=True)
    dom_service: Optional[DomService] = Field(default=None, exclude=True)
    web_search_tool: WebSearch = Field(default_factory=WebSearch, exclude=True)

    # Context for generic functionality
    tool_context: Optional[Context] = Field(default=None, exclude=True)

    llm: Optional[LLM] = Field(default_factory=LLM)

    @field_validator("parameters", mode="before")
    def validate_parameters(cls, v: dict, info: ValidationInfo) -> dict:
        if not v:
            raise ValueError("Parameters cannot be empty")
        return v

    async def _ensure_browser_initialized(self) -> BrowserContext:
        """Ensure browser and context are initialized."""
        if self.browser is None:
            browser_config_kwargs = {"headless": False, "disable_security": True}

            if config.browser_config:
                from browser_use.browser.browser import ProxySettings

                # handle proxy settings.
                if config.browser_config.proxy and config.browser_config.proxy.server:
                    browser_config_kwargs["proxy"] = ProxySettings(
                        server=config.browser_config.proxy.server,
                        username=config.browser_config.proxy.username,
                        password=config.browser_config.proxy.password,
                    )

View on GitHub (pinned to 52a13f2a57)

Solutions

1. Always supply the required action dict: parameters={'action': 'open_tab', 'url': 'https://example.com'} (fields per the browser-use ActionModel).
2. If the LLM can emit empty tool calls, validate and reject upstream (check the parsed arguments are non-empty) and re-prompt instead of letting pydantic raise.
3. Catch pydantic's ValidationError at the dispatch layer and map it to a user-facing 'missing browser action parameters' message.

Example fix

# before
result = await browser_tool.execute(action=None, parameters={})

# after
params = {"action": "go_to_url", "url": "https://example.com"}
result = await browser_tool.execute(action="", parameters=params)

params = tool_call.get('parameters') or {}
if not params or 'action' not in params:
    raise ValueError('browser tool call needs parameters.action')
result = await browser_tool.execute(action=tool_call.get('action', ''), parameters=params)

from pydantic import ValidationError

def is_valid_browser_params(v: object) -> bool:
    if not isinstance(v, dict) or not v:
        return False
    return isinstance(v.get('action'), str) and bool(v['action'])

try:
    result = await browser_tool.execute(action=a, parameters=params)
except ValidationError as e:
    if 'Parameters cannot be empty' in str(e):
        return 'error: browser action requires parameters (e.g. {"action": "go_to_url", "url": ...})'
    raise

When it happens

Trigger: Building BrowserUseTool via pydantic validation with an omitted/empty parameters dict — typically an LLM emitting a tool call with no arguments, or code instantiating the tool to inspect its schema (model_validate on default empty dict).

Common situations: Agent framework parses an LLM tool call whose arguments JSON was empty ({}); tests instantiating the tool without arguments; schema-registration code that validates a default instance.

Related errors

• Server URL is required for SSE connection
• Command is required for stdio connection
• Unsupported connection type: {self.connection_type}
• Tool calls required but none provided
• Failed to load MCP server config: {e}

AI-assisted analysis of FoundationAgents/OpenManus@52a13f2a57 (2026-08-15). Data as JSON: /api/errors/f2c417c61d12e5ed. Report an issue: GitHub.