createContextCompressor

SUPPORT · function · Runtime value

Build a compressor that asks a model to rewrite one fragment.

The request pairs a system instruction with a user message carrying the fragment ID, its current value and the target character count. Only the model's parsed value is returned; the caller decides whether the result fits the reduction contract.

Import

TypeScript

import { createContextCompressor } from '@jangts/agent-core/support';

JavaScript

import { createContextCompressor } from '@jangts/agent-core/support';

Other public entry points:@jangts/agent-core/support, @jangts/agent-core/support/context.

Definition

Call

createContextCompressor(options: { id: string; instruction: string; modelId?: string; transport: string; maxOutputTokens: number; parse(value: unknown): ContextFragment["value"]; }): ContextCompressor

Build a compressor that asks a model to rewrite one fragment.

The request pairs a system instruction with a user message carrying the fragment ID, its current value and the target character count. Only the model's parsed value is returned; the caller decides whether the result fits the reduction contract.

Parameter	Type	Passing	Description
options	{ id: string; instruction: string; modelId?: string; transport: string; maxOutputTokens: number; parse(value: unknown): ContextFragment["value"]; }	Required	—

Fields of options

Field	Type	Required	Description
id	string	Required	See the type definition and usage context.
instruction	string	Required	See the type definition and usage context.
modelId	string | undefined	Optional	See the type definition and usage context.
transport	string	Required	See the type definition and usage context.
maxOutputTokens	number	Required	See the type definition and usage context.
parse	(value: unknown) => ContextFragment["value"]	Required	See the type definition and usage context.

Returns: ContextCompressor.

Return value structure

Field	Type	Required	Description
id	string	Required	See the type definition and usage context.
maxCalls	number | undefined	Optional	Maximum model calls through callModel, default 1; declare 0 for local compression.
compress	(call: ContextCompressionCall, context: ExecutionContext) => Promise<ContextFragment["value"]>	Required	Produces a replacement value no larger than the requested target where possible.

Examples

• Compressors and selectors: recommended implementations and custom algorithms: Bind prompts, output parsing, and algorithms as handlers; model I/O goes through callModel of the same request.

Bind a text compression handler

Beginner · Node.js

TypeScript

import { Support } from '@jangts/agent-core';
const compressor = Support.createContextCompressor({
  id: 'history-summary', instruction: '保留事实、时间和未完成事项，精简重复表达。',
  transport: 'core:plain-text', maxOutputTokens: 500,
  parse(value: unknown) { if (typeof value !== 'string' || !value.trim()) throw new TypeError('需要非空摘要'); return value; },
});
console.log(compressor.id);

JavaScript

import { Support } from '@jangts/agent-core';
const compressor = Support.createContextCompressor({
    id: 'history-summary', instruction: '保留事实、时间和未完成事项，精简重复表达。',
    transport: 'core:plain-text', maxOutputTokens: 500,
    parse(value) { if (typeof value !== 'string' || !value.trim())
        throw new TypeError('需要非空摘要'); return value; },
});
console.log(compressor.id);

A selector explicitly controls the visible excerpt range

Configuration · Node.js

selector-json must be registered by the App and return value={stepIds:string[]}; an ordinary text transport must not be treated as already parsed JSON. This example only shows handler assembly and sends no request.

TypeScript

import { Support } from '@jangts/agent-core';
const selector = Support.createContextSelector({ id: 'select-reduction',
  transport: 'selector-json', maxOutputTokens: 200, maxExcerptChars: 1000,
});
console.log(selector.id);

JavaScript

import { Support } from '@jangts/agent-core';
const selector = Support.createContextSelector({ id: 'select-reduction',
    transport: 'selector-json', maxOutputTokens: 200, maxExcerptChars: 1000,
});
console.log(selector.id);

Implement the same selection contract with a local class

Extension · Node.js

This policy does not call a model. It only suggests registered candidates, and the node still performs execution and checks.

TypeScript

import type { ContextSelector, ContextSelectionCall } from '@jangts/agent-core/support/context';
class TrimFirst implements ContextSelector {
  readonly id = 'trim-first';
  readonly maxCalls = 0;
  async select(call: ContextSelectionCall) {
    const candidate = call.candidates.find(item => item.kind === 'trim');
    return candidate ? [candidate.id] : [];
  }
}
const selector = new TrimFirst();
console.log(selector.id, selector.maxCalls);

JavaScript

class TrimFirst {
    id = 'trim-first';
    maxCalls = 0;
    async select(call) {
        const candidate = call.candidates.find(item => item.kind === 'trim');
        return candidate ? [candidate.id] : [];
    }
}
const selector = new TrimFirst();
console.log(selector.id, selector.maxCalls);
export {};

Related policy source

• Default selection instruction and excerpt range
• Compression request and the real callModel contract

Related definitions

• ContextCompressor
• ContextFragment