agentscope-ai/agentscope · error · ValueError

Either client_gen or session must be provided, but not both.

Error message

Either client_gen or session must be provided, but not both.

What it means

This adapter class (MCP-related, in tool/_adapters.py) requires exactly one of client_gen or session in __init__; providing neither or both raises ValueError.

Source

Thrown at src/agentscope/tool/_adapters.py:283

        self.is_read_only = False
        if tool.annotations and hasattr(tool.annotations, "readOnlyHint"):
            self.is_read_only = tool.annotations.readOnlyHint or False

        # Store MCP tool and connection info
        self._tool = tool
        self._client_gen = client_gen
        self._session = session

        if timeout:
            self._timeout = timedelta(seconds=timeout)
        else:
            self._timeout = None

        # Validate that either client_gen or session is provided
        if (client_gen is None and session is None) or (
            client_gen is not None and session is not None
        ):
            raise ValueError(
                "Either client_gen or session must be provided, but not both.",
            )

    async def check_permissions(
        self,
        *_args: Any,
        **_kwargs: Any,
    ) -> PermissionDecision:
        """Check permissions for the MCP tool usage.

        Default implementation allows all operations.

        Returns:
            `PermissionDecision`:
                Permission decision (default: ask for confirmation).
        """
        if self.is_read_only:
            return PermissionDecision(

View on GitHub (pinned to e90f1c7592)

Solutions

1. Pass only client_gen (an async factory producing an async-exit-stack pair) for lazily managed connections
2. Or pass only a pre-established session for externally managed lifetimes
3. Remove the redundant argument from your constructor call

Example fix

# before
adapter = Adapter(client_gen=gen, session=sess)
# after
adapter = Adapter(client_gen=gen)

assert (client_gen is None) != (session is None)

When it happens

Trigger: Constructing the adapter with both an AsyncExitStack-managed session and a client generator, or with neither (relying on some other connection mechanism).

Common situations: Migrating MCP client setup code where a session was previously created externally, then also passing client_gen; or refactoring removed the client_gen argument entirely.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• STDIO MCP does not support ephemeral mode. Use 'shared' or '
• Invalid values for MCP {card.name!r}: {e.message}
• MCP {card.name!r} needs a value for: {', '.join(sorted(missi
• MCP {card.name!r} produced an invalid client: {e}
• Exactly one of `data` or `url` must be provided.

AI-assisted analysis of agentscope-ai/agentscope@e90f1c7592 (2026-08-28). Data as JSON: /api/errors/8196722d0439338d. Report an issue: GitHub.