What causes "Missing required arguments: {missing}" in OpenBB-finance/OpenBB?

Pass every required argument by its exact declared name: prompt.render(arguments={"ticker": "AAPL"})

OpenBB-finance/OpenBB · error · PromptError

Missing required arguments: {missing}

Error message

Missing required arguments: {missing}

What it means

Thrown by Prompt.render (as PromptError) when a prompt declares required arguments but the caller did not supply all of them. Defaults stored in argument_defaults are applied first, then the caller's arguments; the check compares the union of both against the names of arguments flagged required=True.

Source

Thrown at openbb_platform/extensions/mcp_server/openbb_mcp_server/models/prompts.py:30

    content: str
    argument_defaults: dict[str, Any] = {}

    async def render(
        self,
        arguments: dict[str, Any] | None = None,
    ) -> list[PromptMessage]:
        """Render the prompt with arguments."""
        # Start with stored defaults, then overlay caller-supplied values
        args = {**self.argument_defaults, **(arguments or {})}

        # Validate required arguments
        if self.arguments:
            required = {arg.name for arg in self.arguments if arg.required}
            provided = set(args)
            missing = required - provided
            if missing:
                raise PromptError(f"Missing required arguments: {missing}")

        try:
            rendered_content = (
                self.content.format(**args) if self.arguments or args else self.content
            )
            return [
                PromptMessage(
                    role="user", content=TextContent(type="text", text=rendered_content)
                )
            ]
        except KeyError as e:
            raise PromptError(f"Missing argument for formatting: {e}") from e

View on GitHub (pinned to 3e071fcc2c)

Solutions

Pass every required argument by its exact declared name: prompt.render(arguments={"ticker": "AAPL"})
Set a default in the prompt definition (argument_defaults={"ticker": "AAPL"}) or mark the argument required: False if it is genuinely optional
Inspect prompt.arguments (name/required fields) before rendering to build the call dynamically

Example fix

# before
messages = prompt.render(arguments={"period": "1y"})  # ticker missing

# after
messages = prompt.render(arguments={"ticker": "AAPL", "period": "1y"})

Defensive patterns

Strategy: validation

Validate before calling

required = {
    a.name for a in (prompt.arguments or []) if a.required
}
provided = {**prompt.argument_defaults, **(arguments or {})}
missing = required - provided.keys()
if missing:
    raise ValueError(f"supply values for: {missing}")
messages = prompt.render(arguments=arguments)

Type guard

def can_render(prompt, arguments: dict | None) -> bool:
    required = {a.name for a in (prompt.arguments or []) if a.required}
    have = {**prompt.argument_defaults, **(arguments or {})}
    return required <= have.keys()

Try / catch

from openbb_mcp_server.models.prompts import PromptError

try:
    messages = prompt.render(arguments=args)
except PromptError as e:
    if "Missing required arguments" in str(e):
        args.update({name: default_for(name) for name in extract_missing(str(e))})
        messages = prompt.render(arguments=args)
    else:
        raise

Prevention

Build the arguments dict from prompt.arguments metadata so nothing is omitted
Give required arguments sensible argument_defaults when authoring prompts
Match key names exactly — the check is case- and spelling-sensitive

When it happens

Trigger: A prompt declares arguments=[{name: "ticker", required: True}] and the client calls render(arguments={}) or only passes another optional argument. Also when the argument key is misspelled ("ticke") so the required "ticker" is still missing.

Common situations: MCP clients omitting optional-looking parameters, schema drift after a prompt gains a new required argument, spelling/case mismatches between client-supplied keys and declared argument names (matching is exact, not case-insensitive).

Related errors

AI-assisted analysis of OpenBB-finance/OpenBB@3e071fcc2c (2026-08-14). Data as JSON: /api/errors/74f96e170334ff49. Report an issue: GitHub.