createContextSelectionNode

SUPPORT · function · Runtime value

Create an ordinary node that selects reduction steps without executing them.

The node reports the registered step IDs so the graph can route to reduction nodes itself.

Import

TypeScript

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

JavaScript

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

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

Definition

Type parameters

Type parameter	Constraint	Default type
State	—	—
Result	—	—
App	—	undefined

Call

createContextSelectionNode<State, Result, App = undefined>(definition: ContextSelectionNodeDefinition<State, Result, App>): WorkflowNode<State, Result, App>

Create an ordinary node that selects reduction steps without executing them.

The node reports the registered step IDs so the graph can route to reduction nodes itself.

Type parameter	Constraint	Default type
State	—	—
Result	—	—
App	—	undefined

Parameter	Type	Passing	Description
definition	ContextSelectionNodeDefinition<State, Result, App>	Required	—

Fields of definition

Field	Type	Required	Description
id	string	Required	Read-only. Stable node id referenced by routes and error handling.
description	string	Required	Read-only. Required explanation of what the node does; also used in diagnostics.
name	string | undefined	Optional	Read-only. Display name; defaults to id when omitted.
requires	readonly (keyof State & string)[] | undefined	Optional	Read-only. State keys that must be present before this node runs; enforced by the engine.
provides	readonly (keyof State & string)[] | undefined	Optional	Read-only. State keys this node is expected to produce; validated after run.
routes	readonly WorkflowRoute<State, Result, App>[]	Required	Read-only. Candidate successors, evaluated in order. A getter must be pure; registration reads it once.
startsRound	boolean | undefined	Optional	Read-only. Reserves a fresh round on entry. Use it for steps that may legitimately repeat; entering such a node when the round budget is exhausted fails with the configured exhausted error.
onErrorNodeId	string | undefined	Optional	Read-only. Node entered when run/provides/route fails; contract errors always propagate instead.
onErrorRouteId	string | undefined	Optional	Read-only. Route id reported with the recovery transition.
prepared	((state: Readonly<State>, context: WorkflowContext<App>) => PreparedModelCall) | undefined	Optional	Read-only. Optional prebuilt model call, used instead of preparing request.
plan	(state: Readonly<State>, context: WorkflowContext<App>) => ContextPlan	Required	Read-only. Builds the context plan; must be synchronous so budget failures surface before any I/O.
request	(ModelPrepareRequest<string | ModelTransportDefinition<unknown>> & { modelId?: string; }) | ((state: Readonly<State>, context: WorkflowContext<App>) => ModelPrepareRequest<string | ModelTransportDefinition> & { modelId?: string; }) | undefined	Optional	Read-only. Model preparation request, either static or derived from the current node state.
policy	ContextPolicy | undefined	Optional	Read-only. Processing policy; omitted means validate (no reduction, audit checks only).
timeoutMs	number | undefined	Optional	Read-only. Per-node timeout in milliseconds; defaults to the Core call timeout.
compressors	readonly ContextCompressor[] | undefined	Optional	Read-only. Compressors referenced by compression steps; validated against the policy at creation.
selectors	readonly ContextSelector[] | undefined	Optional	Read-only. Selectors referenced by model-driven selection; validated against the policy at creation.
update	(result: ContextSelectionResult, state: Readonly<State>) => Partial<State>	Required	Read-only. Receives the chosen step IDs so the graph can route to reduction nodes.

Returns: WorkflowNode<State, Result, App>.

Return value structure

Field	Type	Required	Description
id	string	Required	Read-only. Stable node id referenced by routes and error handling.
description	string	Required	Read-only. Required explanation of what the node does; also used in diagnostics.
name	string | undefined	Optional	Read-only. Display name; defaults to id when omitted.
report	((update: void | Readonly<Partial<State>>, context: NodeReportingContext) => WorkflowNodeReport) | undefined	Optional	Read-only. Optional application-reported detail, collected only when content logging is enabled.
startsRound	boolean | undefined	Optional	Read-only. Reserves a fresh round on entry. Use it for steps that may legitimately repeat; entering such a node when the round budget is exhausted fails with the configured exhausted error.
onErrorNodeId	string | undefined	Optional	Read-only. Node entered when run/provides/route fails; contract errors always propagate instead.
onErrorRouteId	string | undefined	Optional	Read-only. Route id reported with the recovery transition.
requires	readonly (keyof State & string)[] | undefined	Optional	Read-only. State keys that must be present before this node runs; enforced by the engine.
provides	readonly (keyof State & string)[] | undefined	Optional	Read-only. State keys this node is expected to produce; validated after run.
routes	readonly WorkflowRoute<State, Result, App>[]	Required	Read-only. Candidate successors, evaluated in order. A getter must be pure; registration reads it once.
run	((state: Readonly<State>, context: WorkflowContext<App>) => void | Partial<State> | Promise<void | Partial<State>>) | undefined	Optional	Read-only. Return only changed fields. Core commits them after cancellation and output checks.

Examples

• Context processing is an ordinary Workflow node: Integrate selection, trimming, and compression through normal nodes and routes; Core records and schedules them uniformly.

Validate and assemble only, without compression yet

Beginner · Node.js

TypeScript

import { AgentCoreEngine, defineWorkflow, route } from '@jangts/agent-core/engine';
import { Support } from '@jangts/agent-core';
import type { BuiltContext, PreparedModelCall } from '@jangts/agent-core/types';
type State = { question: string; input?: BuiltContext; prepared?: PreparedModelCall; answer?: string };
const builder = Support.defineContextFragments<State>([
  { id: 'question', sourceId: 'request', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 1000, format: state => state.question },
], { maxInputChars: 2000 });
const prepare = Support.createContextProcessingNode<State, string>({
  id: 'prepare', description: '装配并校验输入', startsRound: true,
  plan: state => builder.plan({ input: state }),
  update: result => ({ input: result.input, prepared: result.prepared }),
  routes: [route.to('reply')],
});
const workflow = defineWorkflow<State, string>({ id: 'chat', nodes: [prepare, {
  id: 'reply', description: '使用同次准备回答', requires: ['input', 'prepared'],
  async run(state, { model }) { const { value } = await model.invoke({ input: state.input!, prepared: state.prepared! }); return { answer: String(value) }; },
  routes: [route.deliver('answer')],
}] });
const agent = new AgentCoreEngine({ workflow, model: { provider: { async *stream() { yield '服务正常'; } } } });
console.log(await agent.run({ question: '状态？' }));

JavaScript

import { AgentCoreEngine, defineWorkflow, route } from '@jangts/agent-core/engine';
import { Support } from '@jangts/agent-core';
const builder = Support.defineContextFragments([
    { id: 'question', sourceId: 'request', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 1000, format: state => state.question },
], { maxInputChars: 2000 });
const prepare = Support.createContextProcessingNode({
    id: 'prepare', description: '装配并校验输入', startsRound: true,
    plan: state => builder.plan({ input: state }),
    update: result => ({ input: result.input, prepared: result.prepared }),
    routes: [route.to('reply')],
});
const workflow = defineWorkflow({ id: 'chat', nodes: [prepare, {
            id: 'reply', description: '使用同次准备回答', requires: ['input', 'prepared'],
            async run(state, { model }) { const { value } = await model.invoke({ input: state.input, prepared: state.prepared }); return { answer: String(value) }; },
            routes: [route.deliver('answer')],
        }] });
const agent = new AgentCoreEngine({ workflow, model: { provider: { async *stream() { yield '服务正常'; } } } });
console.log(await agent.run({ question: '状态？' }));

Add a history trimming policy to the same node contract

Configuration · Node.js

Put this node back into the same Workflow as the previous example; each run just adds history to State, and the model reply node does not change its interface. In a complete flow the reply target must exist.

TypeScript

import { Support } from '@jangts/agent-core';
import { route } from '@jangts/agent-core/engine';
import type { BuiltContext, PreparedModelCall } from '@jangts/agent-core/types';
type State = { question: string; history: { role: string; content: string }[]; input?: BuiltContext; prepared?: PreparedModelCall };
const builder = Support.defineContextFragments<State>([
  { id: 'question', sourceId: 'request', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 1000, format: state => state.question },
  { id: 'history', sourceId: 'conversation', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 10000, format: state => ({ messages: state.history }) },
], { maxInputChars: 1200 });
const prepare = Support.createContextProcessingNode<State, string>({
  id: 'prepare', description: '按需要裁切旧历史', startsRound: true,
  plan: state => builder.plan({ input: state }),
  policy: { mode: 'reduce', steps: [{ id: 'recent', kind: 'trim', fragmentId: 'history', field: 'messages',
    targetChars: 700, grouping: { kind: 'user-turn' }, keepGroups: 1 }] },
  update: result => ({ input: result.input, prepared: result.prepared }),
  routes: [route.to('reply')],
});
console.log(prepare.id);

JavaScript

import { Support } from '@jangts/agent-core';
import { route } from '@jangts/agent-core/engine';
const builder = Support.defineContextFragments([
    { id: 'question', sourceId: 'request', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 1000, format: state => state.question },
    { id: 'history', sourceId: 'conversation', trust: 'user', lifetime: 'turn', role: 'user', maxChars: 10000, format: state => ({ messages: state.history }) },
], { maxInputChars: 1200 });
const prepare = Support.createContextProcessingNode({
    id: 'prepare', description: '按需要裁切旧历史', startsRound: true,
    plan: state => builder.plan({ input: state }),
    policy: { mode: 'reduce', steps: [{ id: 'recent', kind: 'trim', fragmentId: 'history', field: 'messages',
                targetChars: 700, grouping: { kind: 'user-turn' }, keepGroups: 1 }] },
    update: result => ({ input: result.input, prepared: result.prepared }),
    routes: [route.to('reply')],
});
console.log(prepare.id);

Related policy source

• Configuration binding and stage execution for ordinary nodes

Related definitions

• ContextProcessingNodeDefinition
• ContextSelectionNodeDefinition
• WorkflowNode