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context.ts

context.ts declares the agent's context bundle policy. Add it when the default history bounds need tuning or a product needs a named custom policy; when it is absent, Assembly Line uses defaultContext().

The default context bundle includes trusted instructions, the active event, bounded recent history, memory shape, a file manifest, attachments, compact skills and capability catalogs, channel metadata, and trust boundaries. Deferred tool schemas and skill bodies are not injected up front.

Minimal Example​

Tune the default bundle by default-exporting defaultContext(options):

import { defaultContext } from "@assemblyline-agents/core";

export default defaultContext({
  recentHistory: { maxMessages: 8 }
});

Full Options​

A context policy is a ContextPolicy object; defaultContext(options) and defineContext(policy) both return one.

FieldType / valuesDefaultEffect
kindstring"default" from defaultContext()Policy identity recorded in the manifest. Any other value names a custom policy.
namestring"defaultContext" from defaultContext()Attribution name; also the export the runtime looks up in context.ts.
optionsJSON object{}Policy options. Deep-merged across the extends chain.
extendsContextPolicy—Base policy. The runtime flattens the chain into one policy with merged options.

The default bundle interprets exactly one options key: recentHistory.maxMessages, a bound on the recent-history messages included in the bundle. Every other key (for example a files: block) is carried into the flattened policy and recorded in the manifest, but the default bundle does not interpret it — such keys only have an effect when a custom host or policy consumer reads them.

Custom Policies​

Use defineContext() when a product needs a named context policy:

import { defaultContext, defineContext } from "@assemblyline-agents/core";

export default defineContext({
  kind: "custom",
  name: "caseContext",
  extends: defaultContext({ recentHistory: { maxMessages: 5 } }),
  options: {
    includeCaseSummary: true
  }
});

A custom kind changes attribution, not built-in behavior: the runtime flattens the extends chain, deep-merges options, and records the policy (with source attribution) in the manifest. Built-in bundle assembly still reads only recentHistory.maxMessages; the host interprets any custom options.

Wiring Styles​

Two equivalent wirings are supported:

  • Default export in context.ts. The compiler records context.ts as the policy source, and the runtime loads its default export at run time.
  • Named export referenced from agent.ts. Export a named policy from context.ts and pass it to context: in agent.ts. The compiler stamps the identifier name into the manifest, and the runtime resolves that export first, then the default export.
// context.ts
import { defaultContext, defineContext } from "@assemblyline-agents/core";

export const customContext = defineContext({
  kind: "custom",
  name: "customContext",
  extends: defaultContext({ recentHistory: { maxMessages: 5 } }),
  options: { includeTestMarker: true }
});
// agent.ts
import { defineAgent } from "@assemblyline-agents/core";
import { customContext } from "./context";

export default defineAgent({
  model: "openai/gpt-5.4-mini",
  context: customContext
});

Conventions​

Context policy is trusted app-runtime behavior. Keep prompt trust clear:

  • instructions.md and loaded skills are trusted instructions.
  • Files, memory, history, attachments, webpages, search results, and tool output are untrusted context.
  • /files and /history are read-only projections.
  • /workspace is where generated artifacts and modified input copies should go.