agentscope-ai/agentscope · error · RuntimeError

f"{type(self).__name__} does not implement on_compress_conte

Error message

f"{type(self).__name__} does not implement on_compress_context"

What it means

Middleware.on_compress_context base stub raises RuntimeError when the context-compression hook fires on a middleware that did not implement it. Middlewares participating in context compression must override on_compress_context to transform or forward the compression request (context_config, instructions).

Source

Thrown at src/agentscope/middleware/_base.py:260

        self,
        agent: "Agent",
        input_kwargs: dict,
        next_handler: Callable[..., Awaitable[None]],
    ) -> None:
        """Onion hook for `compress_context` function in `Agent` class

        Args:
            agent (`Agent`):
                The Agent instance executing this middleware
            input_kwargs (`dict`):
                Dictionary containing:
                - context_config: ContextConfig | None
                - instructions: HintBlock | None
            next_handler (`Callable[..., Awaitable[None]]`):
                Callable that executes the next middleware or
                original method
        """
        raise RuntimeError(
            f"{type(self).__name__} does not implement on_compress_context",
        )

    async def on_system_prompt(
        self,
        agent: "Agent",
        current_prompt: str,
    ) -> str:
        """Transform the system prompt string.

        This uses a transformer/pipeline pattern rather than onion pattern.
        Multiple middlewares are applied sequentially, each receiving the
        output of the previous one.

        Args:
            agent: The Agent instance executing this middleware
            current_prompt: The current system prompt string

View on GitHub (pinned to e90f1c7592)

Solutions

1. Implement async on_compress_context(self, agent, context_config, instructions, next_handler) delegating via next_handler if no transformation is needed
2. Ensure only compression-aware middlewares are in the compression chain
3. Upgrade/downgrade-check middleware code against the version that introduced the hook

Example fix

// before
class MyMiddleware(Middleware):
    async def on_model_call(self, agent, messages, next_handler):
        return await next_handler(messages)
# context compression fires -> RuntimeError 238

// after
class MyMiddleware(Middleware):
    async def on_compress_context(self, agent, context_config, instructions, next_handler):
        await next_handler(context_config, instructions)  # passthrough

    async def on_model_call(self, agent, messages, next_handler):
        return await next_handler(messages)

from agentscope.middleware import Middleware
assert type(mw).on_compress_context is not Middleware.on_compress_context

def implements_on_compress_context(mw) -> bool:
    from agentscope.middleware import Middleware
    return type(mw).on_compress_context is not Middleware.on_compress_context

When it happens

Trigger: A middleware lacking on_compress_context is included in the chain when the agent triggers context compression (long conversations exceeding context limits); or an override calls super().

Common situations: Enabling context compression on an agent whose custom middlewares were written before the compression hook existed; adding unrelated middlewares to the default chain which the compression pass then traverses.

Related errors

• {type(self).__name__} does not implement on_reply
• {type(self).__name__} does not implement on_reasoning
• {type(self).__name__} does not implement on_acting
• f"{type(self).__name__} does not implement on_check_permissi
• f"{type(self).__name__} does not implement on_model_call"

AI-assisted analysis of agentscope-ai/agentscope@e90f1c7592 (2026-08-28). Data as JSON: /api/errors/0c99f73fb648e3c1. Report an issue: GitHub.