ContextEditingMiddleware

Trims or filters the conversation context before each model call. Use this to keep the context window manageable when full summarization is unnecessary -- for example, dropping old messages or stripping tool call noise from the history.

Constructor

The middleware accepts a ContextStrategy that defines how messages are edited:

use synaptic::middleware::{ContextEditingMiddleware, ContextStrategy};

// Keep only the last 10 non-system messages
let mw = ContextEditingMiddleware::new(ContextStrategy::LastN(10));

// Remove tool call/result pairs, keeping only human/AI content messages
let mw = ContextEditingMiddleware::new(ContextStrategy::StripToolCalls);

// Strip tool calls first, then keep last N
let mw = ContextEditingMiddleware::new(ContextStrategy::StripAndTruncate(10));

Convenience Constructors

let mw = ContextEditingMiddleware::last_n(10);
let mw = ContextEditingMiddleware::strip_tool_calls();

Strategies

Strategy	Behavior
`LastN(n)`	Keeps leading system messages, then the last `n` non-system messages
`StripToolCalls`	Removes `Tool` messages and AI messages that contain only tool calls (no text)
`StripAndTruncate(n)`	Applies `StripToolCalls` first, then `LastN(n)`

Usage with `create_agent`

use std::sync::Arc;
use synaptic::graph::{create_agent, AgentOptions};
use synaptic::middleware::ContextEditingMiddleware;

let options = AgentOptions {
    middleware: vec![
        Arc::new(ContextEditingMiddleware::last_n(20)),
    ],
    ..Default::default()
};

let graph = create_agent(model, tools, options)?;

How It Works

Lifecycle hook: before_model
Before each model call, the middleware applies the configured strategy to request.messages.
LastN: System messages at the start of the list are always preserved. From the remaining messages, only the last n are kept. Earlier messages are dropped.
StripToolCalls: Messages with is_tool() == true are removed. AI messages that have tool calls but empty text content are also removed. This cleans up the tool-call/tool-result pairs while preserving the conversational content.
StripAndTruncate: Runs both filters in sequence -- first strips tool calls, then truncates to the last N.

The original message list in the agent state is not modified; only the request sent to the model is trimmed.

Example: Combining with Summarization

For maximum context efficiency, strip tool calls first, then summarize what remains:

let options = AgentOptions {
    middleware: vec![
        Arc::new(ContextEditingMiddleware::strip_tool_calls()),
        Arc::new(SummarizationMiddleware::new(model.clone(), 4000, |msg| msg.content().len() / 4)),
    ],
    ..Default::default()
};

let graph = create_agent(model, tools, options)?;

The context editor removes tool noise before summarization runs, producing cleaner summaries.

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