deepseek-harness/packages/core/agent/src/model-selection.ts

/**
 * Agent-scoped model selection shared by runtime entry points.
 * @module @deepseek-ai/dsh-agent/model-selection
 */

import type { Context } from '@deepseek-ai/cordis'
import {
  boundContextSummary,
  createUserMessage,
  type LlmCallConfig,
  type ReasoningEffortId,
} from '@deepseek-ai/dsh-llm'
import type { PreStepDecision } from './runtime-types.ts'

/** Complete provider, model, and optional reasoning effort selected for one live Agent. */
export interface ModelSelection {
  /** Registered provider route. */
  provider: string
  /** Provider-owned model id. */
  model: string
  /** Adapter-owned reasoning effort, or provider/default behavior when absent. */
  reasoningEffort?: ReasoningEffortId
}

/** Mutable model selection plus the value captured for the current step. */
export interface ModelSelectionRef {
  /** Model selected for the next step that enters prompt assembly. */
  current: ModelSelection | undefined
  /** Selection captured when the current step entered prompt assembly. */
  assembled: ModelSelection | undefined
}

function sameRoute(left: ModelSelection, right: ModelSelection): boolean {
  return left.provider === right.provider && left.model === right.model
}

function routeLabel(route: ModelSelection, other: ModelSelection): string {
  return route.provider === other.provider ? route.model : `${route.provider}/${route.model}`
}

function modelSwitchNotice(previous: ModelSelection, selected: ModelSelection) {
  const from = routeLabel(previous, selected)
  const to = routeLabel(selected, previous)
  return createUserMessage({
    content: [{
      type: 'text' as const,
      text: `[model changed: assistant turns above this point were generated by ${from}; the session continues with ${to}]`,
    }],
    source: {
      kind: 'plugin' as const,
      plugin: 'model-selection',
      form: 'notice' as const,
      summary: boundContextSummary(`${from} → ${to}`),
    },
  })
}

/**
 * Couple one mutable selection to Agent-scoped prompt assembly and request routing.
 * Prompt assembly snapshots the selected model before delegating, then applies
 * its provider/model pair and effort to request config so a
 * concurrent switch takes effect on a later step instead of splitting the two
 * surfaces. An absent selected effort clears any inherited effort, restoring
 * the selected model's provider/default behavior.
 *
 * A provider/model change appends a durable user-role notice to the next
 * admitted request. It compares the assembled selection with the latest
 * request header; effort-only changes and empty no-request decisions add no
 * notice. Failure before header persistence repeats the notice on the next
 * request.
 *
 * @param agentCtx - The selected Agent's scoped context.
 * @param selection - Mutable selection owned by the calling entry point.
 * @returns Disposer for all scoped waterfall listeners.
 */
export function installModelSelection(agentCtx: Context, selection: ModelSelectionRef): () => void {
  const disposeAssembly = agentCtx.on('system-prompt/assemble', async (_assembly, _context, next) => {
    const selected = selection.current
    const assembled = await next()
    selection.assembled = selected
    if (selected === undefined) return assembled
    return {
      ...assembled,
      variables: {
        ...assembled.variables,
        provider: selected.provider,
        model: selected.model,
      },
    }
  })
  const disposeRequest = agentCtx.on(
    'agent/request',
    async (_payload, next): Promise<LlmCallConfig> => {
      const resolved = await next()
      const selected = selection.assembled
      if (selected === undefined) return resolved
      const { reasoningEffort: _inheritedEffort, ...withoutInheritedEffort } = resolved
      return {
        ...withoutInheritedEffort,
        provider: selected.provider,
        model: selected.model,
        ...selected.reasoningEffort === undefined
          ? {}
          : { reasoningEffort: selected.reasoningEffort },
      }
    },
  )
  const disposeNotice = agentCtx.on(
    'agent/pre-step',
    async ({ agent, messages, signal, step }, next): Promise<PreStepDecision> => {
      const decision = await next()
      if (decision.kind === 'reject' || signal.aborted) return decision
      // The loop skips an empty first step and an emptied offered continuation.
      if (decision.messages.length === 0 && (step === 1 || messages.length > 0)) return decision
      const selected = selection.assembled
      const previous = agent.session.requestHeader()?.config
      if (selected === undefined || previous === undefined || sameRoute(selected, previous)) return decision
      return { ...decision, messages: [...decision.messages, modelSwitchNotice(previous, selected)] }
    },
    { prepend: true },
  )
  return () => {
    disposeAssembly()
    disposeRequest()
    disposeNotice()
  }
}