deepseek-harness/packages/core/agent-loop/tests/loop.spec.ts

import { describe, expect, it, vi } from 'vitest'
import { Context } from '@deepseek-ai/cordis'
import LlmRuntime, { createUserMessage, ToolCallId, LlmError, ReasoningEffortId, StreamChunk, expandAssistantStream } from '@deepseek-ai/dsh-llm'
import type { GenerateOptions } from '@deepseek-ai/dsh-llm'
import SessionStore, { SessionId, TurnEndReason } from '@deepseek-ai/dsh-session'
import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
import ToolRuntime, { defineContentToolFixture } from '@deepseek-ai/dsh-tools'
import AgentRegistry, { type Agent, type AssistantStreamFrame } from '@deepseek-ai/dsh-agent'

import AgentLoop from '@deepseek-ai/dsh-agent-loop'
import SessionProjectionRegistry from '@deepseek-ai/dsh-session-projection'
import { MockAdapter, maxTokensResponse, textResponse, toolCallResponse } from './mock-adapter.ts'

function driverDone(agent: Agent): Promise<void> {
  return (agent as Agent & { done: Promise<void> }).done
}

async function harness(adapter: MockAdapter, persona = '') {
  const ctx = new Context()
  await ctx.plugin(LlmRuntime)
  await ctx.plugin(SessionStore)
  await ctx.plugin(SessionProjectionRegistry)
  await ctx.plugin(SystemPrompt, { personaPrefix: persona })
  await ctx.plugin(ToolRuntime)
  await ctx.plugin(AgentRegistry)
  await ctx.plugin(AgentLoop, { agents: [] })
  ctx.llm.registerAdapter(['mock'], adapter)
  return ctx
}

/** Wait for the agent's next transition to idle after a waking send. */
function waitForIdle(ctx: Context, agent: Agent): Promise<void> {
  return new Promise((resolve) => {
    const dispose = ctx.on('agent/status', ({ agent: subject, status }) => {
      if (subject === agent && status === 'idle') {
        dispose()
        resolve()
      }
    })
  })
}

function send(agent: Agent, text: string) {
  agent.followup(createUserMessage({ content: [{ type: 'text', text }], source: { kind: 'user' } }))
}

/** The rendered system prompt a request carried as its leading system-role message. */
function systemOf(request: GenerateOptions | undefined): string | undefined {
  const head = request?.messages[0]
  if (head?.role !== 'system') return undefined
  return head.content.flatMap(block => block.type === 'text' ? [block.text] : []).join('')
}

/** All user-message texts recorded in the log (to assert what actually ran). */
function userTexts(agent: Agent): string[] {
  return agent.session.snapshotEvents()
    .filter(e => e.type === 'user/message')
    .flatMap(e => e.type === 'user/message' ? e.data.content : [])
    .flatMap(b => b.type === 'text' ? [b.text] : [])
}

describe('agent loop', () => {
  it('publishes one dense live attempt and commits one v2 message with the exact embedded stream', async () => {
    const ctx = await harness(new MockAdapter([textResponse('live')]))
    const agent = await ctx.agentLoop.create(SessionId('live-assistant-attempt'), {
      provider: 'mock',
      model: 'mock',
    })
    const frames: AssistantStreamFrame[] = []
    let committedAfterMessage = false
    ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
      if (subject !== agent) return
      frames.push(frame)
      if (frame.type === 'end' && frame.outcome.kind === 'committed') {
        committedAfterMessage = agent.session.snapshotEvents().at(-1)?.type === 'assistant/message'
      }
    })

    send(agent, 'stream this')
    await waitForIdle(ctx, agent)

    expect(frames.at(0)?.type).toBe('start')
    const start = frames.at(0)
    if (start?.type === 'start') {
    }
    expect(frames.at(-1)?.type).toBe('end')
    expect(frames.map(frame => frame.revision)).toEqual(
      frames.map((_frame, index) => index + 1),
    )
    const chunks = frames.filter(
      (frame): frame is Extract<AssistantStreamFrame, { type: 'chunk' }> => frame.type === 'chunk',
    )
    expect(chunks.map(frame => frame.index)).toEqual(chunks.map((_frame, index) => index))
    expect(agent.session.snapshotEvents().some(event => (event.type as string) === 'assistant/chunk')).toBe(false)
    const message = agent.session.snapshotEvents().findLast(event => event.type === 'assistant/message')
    expect(message?.type === 'assistant/message'
      ? expandAssistantStream(message.data.stream).map(member => member.chunk)
      : undefined).toStrictEqual(chunks.map(frame => frame.chunk))
    const end = frames.at(-1)
    expect(end).toMatchObject({
      type: 'end',
      index: chunks.length,
      outcome: { kind: 'committed', eventType: 'assistant/message', seq: message?.seq },
    })
    if (end?.type === 'end') {
      expect(committedAfterMessage).toBe(true)
    }
  })

  it('abandons the live row when durable Assistant settlement is rejected', async () => {
    const ctx = await harness(new MockAdapter([textResponse('cannot commit')]))
    const agent = await ctx.agentLoop.create(SessionId('abandoned-assistant-attempt'), {
      provider: 'mock',
      model: 'mock',
    })
    const frames: AssistantStreamFrame[] = []
    ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
      if (subject === agent) frames.push(frame)
    })
    const append = agent.session.append.bind(agent.session)
    Object.defineProperty(agent.session, 'append', {
      configurable: true,
      value: (...args: Parameters<typeof agent.session.append>): ReturnType<typeof agent.session.append> => {
        if (args[0] === 'assistant/message') throw new Error('settlement rejected')
        return Reflect.apply(append, agent.session, args) as ReturnType<typeof agent.session.append>
      },
    })

    send(agent, 'stream this')
    await waitForIdle(ctx, agent)

    expect(frames.at(-1)).toMatchObject({
      type: 'end',
      outcome: { kind: 'abandoned' },
    })
    expect(agent.session.snapshotEvents().some(event => event.type === 'assistant/message')).toBe(false)
  })

  it('abandons a started live attempt when final block assembly fails', async () => {
    const chunks: StreamChunk[] = [
      { type: 'block-start', index: 0, blockType: 'external-block' } as unknown as StreamChunk,
      { type: 'finish', reason: { kind: 'stop' } },
    ]
    const ctx = await harness(new MockAdapter([chunks]))
    const agent = await ctx.agentLoop.create(SessionId('assistant-assembly-failure'), {
      provider: 'mock',
      model: 'mock',
    })
    const frames: AssistantStreamFrame[] = []
    ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
      if (subject === agent) frames.push(frame)
    })

    send(agent, 'produce an incomplete external block')
    await waitForIdle(ctx, agent)

    expect(frames.map(frame => frame.revision)).toEqual([1, 2, 3, 4])
    expect(frames.at(-1)).toMatchObject({
      type: 'end',
      index: 2,
      outcome: { kind: 'abandoned' },
    })
    expect(agent.session.snapshotEvents().some(event => (
      event.type === 'assistant/message' || event.type === 'assistant/attempt'
    ))).toBe(false)
    expect(agent.session.snapshotEvents().at(-1)).toMatchObject({
      type: 'turn/end',
      data: { reason: { kind: 'error' } },
    })
  })

  it('settles a failed attempt before retrying with a new dense attempt', async () => {
    const failed: StreamChunk[] = [
      { type: 'usage', usage: { inputTokens: 1, outputTokens: 0 } },
      { type: 'finish', reason: { kind: 'error', failure: { message: 'retry', code: 'SERVER' } } },
    ]
    const ctx = await harness(new MockAdapter([failed, textResponse('recovered')]))
    const agent = await ctx.agentLoop.create(SessionId('assistant-retry-attempt'), {
      provider: 'mock',
      model: 'mock',
    })
    const frames: AssistantStreamFrame[] = []
    ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
      if (subject === agent) frames.push(frame)
    })
    ctx.on('agent/request-error', async () => ({ kind: 'retry' as const }))

    send(agent, 'retry once')
    await waitForIdle(ctx, agent)

    expect(frames.map(frame => frame.revision)).toEqual(frames.map((_frame, index) => index + 1))
    expect(frames.filter(frame => frame.type === 'start')).toHaveLength(2)
    expect(frames.filter(frame => frame.type === 'end').map(frame => (
      frame.outcome.kind === 'committed' ? frame.outcome.eventType : frame.outcome.kind
    ))).toEqual(['assistant/attempt', 'assistant/message'])
  })

  it('does not emit an end frame when prepared dispatch throws before start', async () => {
    const ctx = await harness(new MockAdapter([]))
    const agent = await ctx.agentLoop.create(SessionId('assistant-dispatch-throw'), {
      provider: 'mock',
      model: 'mock',
    })
    const frames: AssistantStreamFrame[] = []
    ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
      if (subject === agent) frames.push(frame)
    })
    const prepareCall = ctx.llm.prepareCall.bind(ctx.llm)
    vi.spyOn(ctx.llm, 'prepareCall').mockImplementation(async (config, signal) => {
      const prepared = await prepareCall(config, signal)
      return {
        ...prepared,
        stream: () => { throw new Error('dispatch failed before start') },
      }
    })

    send(agent, 'fail before streaming')
    await waitForIdle(ctx, agent)

    expect(frames).toEqual([])
    expect(agent.session.snapshotEvents().at(-1)).toMatchObject({
      type: 'turn/end',
      data: { reason: { kind: 'error' } },
    })
  })

  it('preserves the stream failure when its durable attempt settlement is also rejected', async () => {
    const streamFailure = new Error('provider transport failed')
    const settlementFailure = new Error('attempt settlement rejected')
    const ctx = await harness(new MockAdapter([textResponse('partial')]))
    const agent = await ctx.agentLoop.create(SessionId('double-assistant-failure'), {
      provider: 'mock',
      model: 'mock',
    })
    ctx.on('llm/stream', async function* (_options, next) {
      for await (const chunk of next()) {
        yield chunk
        if (chunk.type === 'text-delta') throw streamFailure
      }
    })
    const append = agent.session.append.bind(agent.session)
    Object.defineProperty(agent.session, 'append', {
      configurable: true,
      value: (...args: Parameters<typeof agent.session.append>): ReturnType<typeof agent.session.append> => {
        if (args[0] === 'assistant/attempt') throw settlementFailure
        return Reflect.apply(append, agent.session, args) as ReturnType<typeof agent.session.append>
      },
    })
    const errors: unknown[] = []
    ctx.on('agent/error', ({ agent: subject, error }) => {
      if (subject === agent) errors.push(error)
    })

    send(agent, 'stream this')
    await waitForIdle(ctx, agent)

    const combined = errors.find((error): error is AggregateError => error instanceof AggregateError)
    expect(combined?.errors).toEqual([streamFailure, settlementFailure])
    expect(combined?.cause).toBe(streamFailure)
  })

  it.each([0, -1, 1.5, Number.NaN, Number.MAX_SAFE_INTEGER + 1])(
    'rejects invalid AgentOptions.maxTokens %s before publication',
    async (maxTokens) => {
      const ctx = await harness(new MockAdapter([]))
      await expect(ctx.agentLoop.create(
        SessionId('invalid-max-tokens'),
        { provider: 'mock', model: 'mock', maxTokens },
      )).rejects.toThrow('agent maxTokens must be a positive safe integer')
      expect(ctx.agents.list()).toEqual([])
      expect(ctx.sessions.list()).toEqual([])
    },
  )

  it('seeds a valid AgentOptions.maxTokens into the first model request', async () => {
    const adapter = new MockAdapter([textResponse('bounded')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(
      SessionId('valid-max-tokens'),
      { provider: 'mock', model: 'mock', maxTokens: 256 },
    )

    send(agent, 'use the configured output limit')
    await waitForIdle(ctx, agent)

    expect(adapter.requests[0]?.maxTokens).toBe(256)
  })

  it('seeds an AgentOptions reasoning effort into the first model request', async () => {
    const effort = ReasoningEffortId('high')
    const adapter = new MockAdapter([textResponse('reasoned')], {
      efforts: [{ id: effort, name: 'High' }],
      defaultEffort: effort,
    })
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(
      SessionId('configured-reasoning-effort'),
      { provider: 'mock', model: 'mock', reasoningEffort: effort },
    )

    send(agent, 'use the configured reasoning effort')
    await waitForIdle(ctx, agent)

    expect(adapter.requests[0]?.reasoningEffort).toBe(effort)
  })

  it('validates reasoning effort in declarative agent config', () => {
    const effort = ReasoningEffortId('high')
    expect(AgentLoop.Config({
      agents: [{ id: 'configured-agent', reasoningEffort: effort }],
    }).agents[0]?.reasoningEffort).toBe(effort)
    expect(() => AgentLoop.Config({
      agents: [{ id: 'configured-agent', reasoningEffort: ReasoningEffortId('') }],
    })).toThrow()
  })

  it('rejects concurrent maintenance while one job owns the Agent', async () => {
    const ctx = await harness(new MockAdapter([textResponse('unused')]))
    const agent = await ctx.agentLoop.create(SessionId('concurrent-maintenance'), {
      provider: 'mock', model: 'mock',
    })
    const started = Promise.withResolvers<undefined>()
    const finish = Promise.withResolvers<undefined>()
    const active = agent.runMaintenance(async () => {
      started.resolve(undefined)
      await finish.promise
    })
    await started.promise

    expect(() => agent.runMaintenance(async () => {})).toThrow('already has active work')

    finish.resolve(undefined)
    await active
  })

  it('cancels queued wakeup work together with an active maintenance task', async () => {
    const adapter = new MockAdapter([textResponse('park reply')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('cancel-maintenance-wakeup'), {
      provider: 'mock',
      model: 'mock',
    })
    const started = Promise.withResolvers<undefined>()
    const maintenance = agent.runMaintenance(async (signal) => {
      started.resolve(undefined)
      await new Promise<void>((_resolve, reject) => {
        signal.addEventListener('abort', () => {
          reject(new Error('maintenance aborted', { cause: signal.reason }))
        }, { once: true })
      })
    })
    await started.promise

    send(agent, 'discard this wakeup') // latched behind the live maintenance task
    agent.cancel({ kind: 'user' }) // drops the queue and the latch, aborts maintenance
    send(agent, 'park after cancellation') // newer intent: re-latched, replays at convergence

    await expect(maintenance).rejects.toThrow('maintenance aborted')
    await agent.whenIdle()

    // The pre-cancel wakeup is gone; the post-cancel wake replays at convergence.
    expect(userTexts(agent)).toEqual(['park after cancellation'])
    expect(agent.inbox.nextTurn).toHaveLength(0)
    expect(adapter.requests).toHaveLength(1)
  })

  it('rejects a second maintenance task while one is active', async () => {
    const ctx = await harness(new MockAdapter([]))
    const agent = await ctx.agentLoop.create(SessionId('maintenance-busy'), {
      provider: 'mock',
      model: 'mock',
    })
    const started = Promise.withResolvers<undefined>()
    const finish = Promise.withResolvers<undefined>()
    const maintenance = agent.runMaintenance(async () => {
      started.resolve(undefined)
      await finish.promise
    })
    await started.promise

    expect(() => agent.runMaintenance(async () => {})).toThrow(/already has active work/)

    finish.resolve(undefined)
    await maintenance
    await agent.whenIdle()
  })

  it('replays a wake latched behind maintenance at convergence', async () => {
    const adapter = new MockAdapter([textResponse('wake reply')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('maintenance-wake-replay'), {
      provider: 'mock',
      model: 'mock',
    })
    const started = Promise.withResolvers<undefined>()
    const finish = Promise.withResolvers<undefined>()
    const maintenance = agent.runMaintenance(async () => {
      started.resolve(undefined)
      await finish.promise
    })
    await started.promise

    send(agent, 'wake behind maintenance')
    finish.resolve(undefined)
    await maintenance
    await agent.whenIdle()

    expect(userTexts(agent)).toEqual(['wake behind maintenance'])
    expect(adapter.requests).toHaveLength(1)
  })

  it('suppresses the replay when a latched maintenance wake is removed', async () => {
    const adapter = new MockAdapter([])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('maintenance-wake-removed'), {
      provider: 'mock',
      model: 'mock',
    })
    const started = Promise.withResolvers<undefined>()
    const finish = Promise.withResolvers<undefined>()
    const maintenance = agent.runMaintenance(async () => {
      started.resolve(undefined)
      await finish.promise
    })
    await started.promise

    const wake = createUserMessage({ content: [{ type: 'text', text: 'removed wake' }], source: { kind: 'user' } })
    agent.followup(wake)
    agent.inbox.remove(wake.id)
    finish.resolve(undefined)
    await maintenance
    await agent.whenIdle()

    expect(userTexts(agent)).toEqual([])
    expect(adapter.requests).toEqual([])
    expect(agent.session.snapshotEvents().filter(e => e.type === 'turn/start')).toHaveLength(0)
  })

  it('runs a simple turn: queued message → model → idle, with ordered events', async () => {
    const adapter = new MockAdapter([textResponse('hello there')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    // All boundaries — turn and step — are durable session events on the
    // session/event feed (no agent/* mirror). Record them in fire order to
    // assert the full boundary nesting.
    const order: string[] = []
    ctx.on('session/event', (_session, event) => {
      if (event.type === 'turn/start' || event.type === 'step/start' || event.type === 'step/end' || event.type === 'turn/end') {
        order.push(event.type)
      }
    })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(order).toEqual(['turn/start', 'step/start', 'step/end', 'turn/end'])

    const types = agent.session.snapshotEvents().map(e => e.type)
    // Durable inbox receipt precedes the turn-owned transcript.
    expect(types[0]).toBe('agent/inbox/spliced')
    expect(types).toContain('turn/start')
    expect(types).toContain('user/message')
    expect(types).toContain('assistant/message')
    const assistantMessage = agent.session.snapshotEvents().find(e => e.type === 'assistant/message')
    expect(assistantMessage?.type === 'assistant/message' && assistantMessage.data.usage).toEqual({ inputTokens: 10, outputTokens: 'hello there'.length })
    expect(types.at(-1)).toBe('turn/end')

    // derived history: system prompt (surface node 0) + user + assistant
    const messages = agent.session.deriveMessages()
    expect(messages.map(m => m.role)).toEqual(['system', 'user', 'assistant'])
    expect(messages[2]!.content).toEqual([{ type: 'text', text: 'hello there' }])
  })

  it('round-trips tool calls: model requests tool → executes → result in next request', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', { text: 'ping' }, 'calling echo'),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: 'echo back',
      parameters: { text: { type: 'string' } },
      async execute(args) {
        return [{ type: 'text', text: `echo: ${args.text}` }]
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'use the tool')
    await waitForIdle(ctx, agent)

    // two model calls happened (tool-call step, then final step)
    expect(adapter.requests).toHaveLength(2)

    // the second request's derived history contains the tool result
    const secondMessages = adapter.requests[1]!.messages
    const toolResultMessage = secondMessages.find(m =>
      m.content.some(b => b.type === 'tool-result'))
    expect(toolResultMessage).toBeDefined()
    const block = toolResultMessage!.content.find(b => b.type === 'tool-result')!
    expect(block).toMatchObject({ toolCallId: 'c1', isError: false })
    expect((block).content).toEqual([{ type: 'text', text: 'echo: ping' }])

    // session log records call + result
    const types = agent.session.snapshotEvents().map(e => e.type)
    expect(types).toContain('tool/call')
    expect(types).toContain('tool/result')
  })

  it('renders harness identity, then the persona, then tool guidance — with {{variables}} resolved', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    // The persona is a TEMPLATE: {{model}} is the loop-registered variable
    // projecting this agent's configured model, so the model knows its own name.
    const ctx = await harness(adapter, 'You are a test agent on {{model}}.')
    ctx.systemPrompt.section({ name: 'tool:noop', order: 100, text: 'Use the noop tool wisely.' })
    ctx.tools.register(defineContentToolFixture({
      name: 'noop',
      description: 'does nothing',
      parameters: {},
      async execute() {
        return []
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    const request = adapter.requests[0]
    expect(systemOf(request)).toBe('You are an AI agent powered by DeepSeek Harness.\n\nYou are a test agent on mock.\n\nUse the noop tool wisely.')
    expect(request!.tools?.map(t => t.name)).toEqual(['noop'])
  })

  it('resolves {{cwd}} from the agent session workspace (factory create with meta.cwd)', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter, 'Working in {{cwd}}.')
    const handle = await ctx.agents.create({
      sessionId: SessionId('s-cwd'),
      meta: { cwd: '/work/space' },
      agentOptions: { provider: 'mock', model: 'mock' },
    })

    const agent = handle.agent
    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(systemOf(adapter.requests[0])).toBe('You are an AI agent powered by DeepSeek Harness.\n\nWorking in /work/space.')
  })

  it('contains a strict-variable render failure: the turn errors, the loop keeps serving turns', async () => {
    // A missing cwd variable must fail one turn without preventing a later valid turn.
    const adapter = new MockAdapter([textResponse('ok after rescue')])
    const ctx = await harness(adapter, 'In {{cwd}}.')
    const errors: Error[] = []
    ctx.on('agent/error', ({ error }) => {
      if (error instanceof Error) errors.push(error)
    })
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(0) // the request was never sent
    expect(errors.map(error => error.message)).toEqual([
      'prompt variable "{{cwd}}" has no value for this assembly (section "deployment:persona-prefix")',
    ])
    const turnEnd = agent.session.snapshotEvents().find(e => e.type === 'turn/end')
    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind).toBe('error')
    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
      ? turnEnd.data.reason.error.message
      : '').toContain('no value for this assembly')

    // The loop survived: a waterfall listener rescues {{cwd}} and the SAME
    // agent completes a real model turn.
    ctx.on('system-prompt/assemble', async (assembly, _context, next) => {
      assembly.variables['cwd'] = '/rescued'
      return next()
    })
    send(agent, 'again')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(1)
    expect(systemOf(adapter.requests[0])).toBe('You are an AI agent powered by DeepSeek Harness.\n\nIn /rescued.')
    const turnEnds = agent.session.snapshotEvents().filter(e => e.type === 'turn/end')
    expect(turnEnds).toHaveLength(2)
    expect(turnEnds[1]?.type === 'turn/end' && turnEnds[1].data.reason.kind).toBe('completed')
  })

  it('supports the model-via-agent/request path with a {{model}} persona: the supplier states it via the assemble waterfall', async () => {
    // AgentOptions.model unset: the model arrives in the agent/request
    // waterfall (the loop's documented fallback — see runStep's no-model
    // error). {{model}} renders BEFORE that waterfall, so the SAME plugin
    // states the fact early on system-prompt/assemble — the owner of a
    // late-bound fact owns stating it wherever it is claimed.
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter, 'You run on {{model}}.')
    ctx.on('system-prompt/assemble', async (assembly, _context, next) => {
      assembly.variables['provider'] = 'mock'
      assembly.variables['model'] = 'mock'
      return next()
    })
    ctx.on('agent/request', async (_payload, next) => {
      const config = await next()
      return { ...config, provider: 'mock', model: 'mock' }
    })
    const agent = await ctx.agentLoop.create(SessionId('a-late-model'), {})

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(1)
    expect(adapter.requests[0]!.model).toBe('mock')
    expect(systemOf(adapter.requests[0])).toBe('You are an AI agent powered by DeepSeek Harness.\n\nYou run on mock.')
  })

  it('sends no system message when system-prompt/assemble short-circuits with an empty assembly', async () => {
    // The documented escape valve: a deployment that must drop the harness
    // openers short-circuits the assemble waterfall; the request then carries
    // NO system field at all (not an empty string).
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    ctx.on('system-prompt/assemble', async () => ({ sections: [], contexts: [], tools: [], variables: {} }))
    const agent = await ctx.agentLoop.create(SessionId('a-no-system'), { provider: 'mock', model: 'mock' })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(1)
    expect('system' in adapter.requests[0]!).toBe(false)
    expect(adapter.requests[0]!.messages.map(message => message.role)).toEqual(['user'])
    expect(agent.session.snapshotEvents().filter(event => event.type === 'system/message')).toMatchObject([
      { data: { message: { role: 'system', content: [] } }, surfaceOp: 'append' },
    ])
  })

  it('keeps a later nonempty prompt ahead of history after an initially empty assembly', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    try {
      const emptyAssembly = ctx.on('system-prompt/assemble', async () => ({ sections: [], contexts: [], tools: [], variables: {} }))
      const agent = await ctx.agentLoop.create(SessionId('a-empty-system-head'), { provider: 'mock', model: 'mock' })
      const firstIdle = waitForIdle(ctx, agent)
      send(agent, 'first')
      await firstIdle
      const head = agent.session.snapshotEvents().find(event => event.type === 'system/message')
      expect(adapter.requests[0]?.messages.map(message => message.role)).toEqual(['user'])
      expect(head).toMatchObject({ data: { message: { content: [] } }, surfaceOp: 'append' })
      expect(agent.session.surface.nodes[0]).toBe(head?.seq)

      emptyAssembly()
      const secondIdle = waitForIdle(ctx, agent)
      send(agent, 'second')
      await secondIdle
      expect(adapter.requests).toHaveLength(2)
      expect(systemOf(adapter.requests[1])).toBe('You are an AI agent powered by DeepSeek Harness.')
      expect(adapter.requests[1]?.messages.map(message => message.role)).toEqual(['system', 'user', 'assistant', 'user'])
      const replacement = agent.session.snapshotEvents().findLast(event => event.type === 'system/message')
      expect(replacement).toMatchObject({
        surfaceOp: { op: 'replace', startSeq: head?.seq, endSeq: head?.seq },
        sourceEventSeqs: [head?.seq],
      })
      expect(agent.session.surface.nodes[0]).toBe(replacement?.seq)
      expect(agent.session.snapshotEvents().flatMap(event =>
        event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])
    } finally {
      await ctx.fiber.dispose()
    }
  })

  it('materializes changed runtime context at the history tail without rewriting the system header', async () => {
    const adapter = new MockAdapter([
      textResponse('one'),
      textResponse('two'),
      textResponse('three'),
      textResponse('four'),
      textResponse('five'),
    ])
    const ctx = await harness(adapter)
    let mode = 'read-only'
    const dispose = ctx.systemPrompt.context({ name: 'policy', order: 0, text: () => `Mode: ${mode}.` })
    const agent = await ctx.agentLoop.create(SessionId('a-runtime-context'), { provider: 'mock', model: 'mock' })
    const contextEvents = () => agent.session.snapshotEvents().flatMap(event =>
      event.type === 'user/message'
        && event.data.source.kind === 'plugin'
        && event.data.source.plugin === '@deepseek-ai/dsh-system-prompt'
        ? [event]
        : [])

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    expect(contextEvents()).toHaveLength(1)
    expect(contextEvents()[0]?.data.content).toEqual([{
      type: 'text',
      text: 'Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nMode: read-only.',
    }])

    send(agent, 'unchanged')
    await waitForIdle(ctx, agent)
    expect(contextEvents()).toHaveLength(1)

    mode = 'danger-full-access'
    send(agent, 'changed')
    await waitForIdle(ctx, agent)
    expect(contextEvents()).toHaveLength(2)
    const changedBlock = contextEvents()[1]?.data.content[0]
    expect(changedBlock?.type).toBe('text')
    if (changedBlock?.type !== 'text') throw new Error('changed runtime context is not text')
    expect(changedBlock.text).toContain('danger-full-access')

    dispose()
    send(agent, 'cleared')
    await waitForIdle(ctx, agent)
    expect(contextEvents()).toHaveLength(3)
    expect(contextEvents()[2]?.data.content).toEqual([{
      type: 'text',
      text: 'Current runtime context: none. Earlier runtime-context snapshots no longer apply.',
    }])

    send(agent, 'still clear')
    await waitForIdle(ctx, agent)
    expect(contextEvents()).toHaveLength(3)
    expect(adapter.requests.map(systemOf)).toEqual(Array(5).fill(systemOf(adapter.requests[0])))
    expect(agent.session.snapshotEvents().filter(event => event.type === 'system/message')).toHaveLength(1)
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial'])
  })

  it('re-emits unchanged runtime context when a surface replacement removed the retained snapshot', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    ctx.systemPrompt.context({ name: 'policy', order: 0, text: 'Mode: read-only.' })
    const agent = await ctx.agentLoop.create(SessionId('a-runtime-context-compacted'), { provider: 'mock', model: 'mock' })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    const contextEvent = agent.session.snapshotEvents().find(event =>
      event.type === 'user/message'
      && event.data.source.kind === 'plugin'
      && event.data.source.plugin === '@deepseek-ai/dsh-system-prompt')
    if (contextEvent?.type !== 'user/message') throw new Error('first turn did not materialize runtime context')
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'compacted summary' }],
      source: { kind: 'plugin', plugin: 'test-compaction' },
    }), {
      surfaceOp: { op: 'replace', startSeq: contextEvent.seq, endSeq: contextEvent.seq },
      sourceEventSeqs: [contextEvent.seq],
    })

    send(agent, 'after compaction')
    await waitForIdle(ctx, agent)
    const runtimeContexts = agent.session.snapshotEvents().flatMap(event =>
      event.type === 'user/message'
        && event.data.source.kind === 'plugin'
        && event.data.source.plugin === '@deepseek-ai/dsh-system-prompt'
        ? [event]
        : [])
    expect(runtimeContexts).toHaveLength(2)
    expect(adapter.requests[1]?.messages.some(message =>
      message.source.kind === 'plugin'
      && message.source.plugin === '@deepseek-ai/dsh-system-prompt')).toBe(true)
  })

  it('clears compacted runtime context after the active set becomes empty', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const dispose = ctx.systemPrompt.context({ name: 'policy', order: 0, text: 'Mode: read-only.' })
    const agent = await ctx.agentLoop.create(SessionId('a-runtime-context-compacted-clear'), { provider: 'mock', model: 'mock' })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    const contextEvent = agent.session.snapshotEvents().find(event =>
      event.type === 'user/message'
      && event.data.source.kind === 'plugin'
      && event.data.source.plugin === '@deepseek-ai/dsh-system-prompt')
    if (contextEvent?.type !== 'user/message') throw new Error('first turn did not materialize runtime context')
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'summary retaining old mode: read-only' }],
      source: { kind: 'plugin', plugin: 'test-compaction' },
    }), {
      surfaceOp: { op: 'replace', startSeq: contextEvent.seq, endSeq: contextEvent.seq },
      sourceEventSeqs: [contextEvent.seq],
    })
    dispose()

    send(agent, 'after compaction')
    await waitForIdle(ctx, agent)
    const clearing = adapter.requests[1]?.messages.find(message =>
      message.role === 'user'
      && message.source.kind === 'plugin'
      && message.source.plugin === '@deepseek-ai/dsh-system-prompt')
    expect(clearing?.content).toEqual([{
      type: 'text',
      text: 'Current runtime context: none. Earlier runtime-context snapshots no longer apply.',
    }])
  })

  it('does not clear runtime context after an unrelated replacement', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a-runtime-context-unrelated-compaction'), { provider: 'mock', model: 'mock' })
    const original = agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'old context' }],
      source: { kind: 'plugin', plugin: 'test-context' },
    }), { surfaceOp: 'append' })
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'compacted summary' }],
      source: { kind: 'plugin', plugin: 'test-compaction' },
    }), {
      surfaceOp: { op: 'replace', startSeq: original.seq, endSeq: original.seq },
      sourceEventSeqs: [original.seq],
    })

    send(agent, 'after compaction')
    await waitForIdle(ctx, agent)
    expect(adapter.requests[0]?.messages.some(message =>
      message.role === 'user'
      && message.source.kind === 'plugin'
      && message.source.plugin === '@deepseek-ai/dsh-system-prompt')).toBe(false)
  })

  it('replaces a malformed retained runtime-context message with the current complete snapshot', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    ctx.systemPrompt.context({ name: 'policy', order: 0, text: 'Mode: read-only.' })
    const agent = await ctx.agentLoop.create(SessionId('a-runtime-context-malformed'), { provider: 'mock', model: 'mock' })
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'broken' }, { type: 'text', text: 'snapshot' }],
      source: { kind: 'plugin', plugin: '@deepseek-ai/dsh-system-prompt' },
    }), { surfaceOp: 'append' })

    send(agent, 'repair context')
    await waitForIdle(ctx, agent)
    const runtimeContexts = agent.session.snapshotEvents().flatMap(event =>
      event.type === 'user/message'
        && event.data.source.kind === 'plugin'
        && event.data.source.plugin === '@deepseek-ai/dsh-system-prompt'
        ? [event]
        : [])
    expect(runtimeContexts).toHaveLength(2)
    expect(runtimeContexts[1]?.data.content).toEqual([{
      type: 'text',
      text: 'Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nMode: read-only.',
    }])
  })

  it('records exact replay chunks inside the durable assistant message', async () => {
    const adapter = new MockAdapter([textResponse('abc')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    const message = agent.session.snapshotEvents().find(e => e.type === 'assistant/message')
    const chunks = message?.type === 'assistant/message' ? expandAssistantStream(message.data.stream) : []
    // textResponse('abc') = block-start + 3 deltas + block-end + usage + finish = 7
    expect(chunks).toHaveLength(7)
    const deltaText = chunks
      .map(member => member.chunk)
      .filter((c: StreamChunk): c is Extract<StreamChunk, { type: 'text-delta' }> => c.type === 'text-delta')
      .map(c => c.text)
      .join('')
    expect(deltaText).toBe('abc')
  })

  it('injects steering between steps and continues the turn', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'slow', {}),
      textResponse('addressed the steering'),
    ])
    const ctx = await harness(adapter)

    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    ctx.tools.register(defineContentToolFixture({
      name: 'slow',
      description: '',
      parameters: {},
      async execute() {
        // steer while the turn is running (during tool execution)
        agent.steer(createUserMessage({ content: [{ type: 'text', text: 'change of plans' }], source: { kind: 'user' } }))
        return [{ type: 'text', text: 'tool done' }]
      },
    }))

    send(agent, 'start')
    await waitForIdle(ctx, agent)

    const steering = agent.session.snapshotEvents().find(e =>
      e.type === 'user/message' && JSON.stringify(e.data.content).includes('change of plans'))
    expect(steering).toBeDefined()
    // The entered batch is appended after the second step opens and before its
    // request derives history.
    const steeringSeq = steering!.seq
    const secondStepStart = agent.session.snapshotEvents().filter(e => e.type === 'step/start')[1]
    expect(secondStepStart).toBeDefined()
    expect(steeringSeq).toBeGreaterThan(secondStepStart!.seq)

    // the second model request saw the steering content
    const secondRequest = adapter.requests[1]
    const flat = JSON.stringify(secondRequest!.messages)
    expect(flat).toContain('change of plans')
  })

  it('starts idle steering synchronously and enters later steering at the next step', async () => {
    const adapter = new MockAdapter([textResponse('first'), textResponse('second')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const idle = waitForIdle(ctx, agent)
    agent.steer(createUserMessage({ content: [{ type: 'text', text: 'first idle steer' }], source: { kind: 'user' } }))
    expect(agent.status).toBe('running')
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(1)
    agent.steer(createUserMessage({ content: [{ type: 'text', text: 'second idle steer' }], source: { kind: 'user' } }))
    await idle

    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(1)
    expect(agent.session.snapshotEvents()
      .filter(event => event.type === 'user/message')
      .map(event => event.data.content)).toEqual([
      [{ type: 'text', text: 'first idle steer' }],
      [{ type: 'text', text: 'second idle steer' }],
    ])
    expect(adapter.requests).toHaveLength(2)
    expect(JSON.stringify(adapter.requests[0]?.messages)).toContain('first idle steer')
    expect(JSON.stringify(adapter.requests[0]?.messages)).not.toContain('second idle steer')
    expect(JSON.stringify(adapter.requests[1]?.messages)).toContain('second idle steer')
  })

  it('stops after a throwing pre-step listener and retains later steering until a wakeup', async () => {
    const adapter = new MockAdapter([textResponse('recovered')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('failed-steering'), { provider: 'mock', model: 'mock' })
    let fail = true
    ctx.on('agent/pre-step', ({ agent: subject }, next) => {
      if (subject !== agent || !fail) return next()
      fail = false
      subject.steer(createUserMessage({ content: [{ type: 'text', text: 'pending steering' }], source: { kind: 'user' } }))
      throw new Error('pre-step failed')
    })

    send(agent, 'prompt')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(0)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(1)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/end')).toHaveLength(1)
    expect(agent.inbox.nextStep).toHaveLength(1)

    send(agent, 'resume')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(1)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(2)
    expect(JSON.stringify(adapter.requests[0]?.messages)).toContain('pending steering')
  })

  it('inject() while idle durably stages context without opening a turn', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    agent.inject(createUserMessage({ content: [{ type: 'text', text: 'file changed: a.ts' }], source: { kind: 'plugin', plugin: 'watcher' } }))
    expect(agent.status).toBe('idle')
    expect(adapter.requests).toHaveLength(0)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(0)
    expect(agent.session.snapshotEvents().at(-1)).toMatchObject({
      type: 'agent/inbox/spliced',
      data: {
        target: 'next-step',
        inserted: [{
          role: 'user',
          content: [{ type: 'text', text: 'file changed: a.ts' }],
          source: { kind: 'plugin', plugin: 'watcher' },
        }],
      },
    })

    send(agent, 'go')
    await waitForIdle(ctx, agent)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(1)
    const flat = JSON.stringify(adapter.requests[0]!.messages)
    expect(flat).toContain('file changed: a.ts')
    expect(flat).not.toContain('<context source=')
  })

  it('inject() persists structured context content verbatim with durable source', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('raw-context'), { provider: 'mock', model: 'mock' })
    const text = '<system-reminder>Additional instructions from: pkg/AGENTS.md</system-reminder>'
    agent.inject(createUserMessage({ content: [{ type: 'text', text }], source: { kind: 'plugin', plugin: 'agent-instructions' } }))
    send(agent, 'go')
    await waitForIdle(ctx, agent)

    const contextEvent = agent.session.snapshotEvents().find(event => event.type === 'user/message' && event.data.source.kind === 'plugin')
    expect(contextEvent?.type === 'user/message' && contextEvent.data.source)
      .toEqual({ kind: 'plugin', plugin: 'agent-instructions' })
    const requestText = JSON.stringify(adapter.requests[0]!.messages)
    expect(requestText).toContain('Additional instructions from: pkg/AGENTS.md')
    expect(requestText).not.toContain('<context source=')
  })

  it('defers inject() during tool execution until after the tool result', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'noticer', {}, 'calling'),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    let visibleDuringTool = false
    ctx.tools.register(defineContentToolFixture({
      name: 'noticer',
      description: 'injects a notice',
      parameters: {},
      async execute() {
        await Promise.resolve()
        const first = { type: 'text' as const, text: 'mid-turn notice' }
        agent.inject(createUserMessage({ content: [first], source: { kind: 'plugin', plugin: 'x' } }))
        first.text = 'mutated after inject'
        agent.inject(createUserMessage({ content: [{ type: 'text', text: 'second notice' }], source: { kind: 'plugin', plugin: 'x' } }))
        visibleDuringTool = agent.session.snapshotEvents().some(e => e.type === 'user/message' && e.data.source.kind === 'plugin')
        return [{ type: 'text', text: 'ok' }]
      },
    }))

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(visibleDuringTool).toBe(false)

    // The injection stays in the open turn, but its user-role context cannot
    // split the assistant tool call from the provider's tool-result message.
    const turnStarts = agent.session.snapshotEvents().filter(e => e.type === 'turn/start')
    expect(turnStarts).toHaveLength(1)
    const result = agent.session.snapshotEvents().find(e => e.type === 'tool/result')!
    const contexts = agent.session.snapshotEvents().filter(e => e.type === 'user/message' && e.data.source.kind === 'plugin')
    expect(contexts).toHaveLength(2)
    expect(result.seq).toBeLessThan(contexts[0]!.seq)
    expect(contexts.flatMap(event => event.type === 'user/message' ? event.data.content : []))
      .toEqual([
        { type: 'text', text: 'mid-turn notice' },
        { type: 'text', text: 'second notice' },
      ])

    const secondRequest = adapter.requests[1]!.messages
    const resultIndex = secondRequest.findIndex(message =>
      message.content.some(block => block.type === 'tool-result'))
    const contextIndexes = secondRequest.flatMap((message, index) =>
      message.content.some(block => block.type === 'text'
        && (block.text.includes('mid-turn notice') || block.text.includes('second notice')))
        ? [index]
        : [])
    expect(resultIndex).toBeGreaterThanOrEqual(0)
    expect(contextIndexes).toHaveLength(2)
    expect(contextIndexes.every(index => index > resultIndex)).toBe(true)
  })

  it('rejects non-JSON context before it enters the active tool-batch FIFO', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'invalid-injector', {}, 'calling'),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('invalid-context'), { provider: 'mock', model: 'mock' })
    ctx.tools.register(defineContentToolFixture({
      name: 'invalid-injector',
      description: 'attempts an invalid context injection',
      parameters: {},
      async execute() {
        expect(() => {
          agent.inject(createUserMessage({ content: [{ type: 'text', text: 'invalid' }], source: { kind: 'plugin', plugin: 'test', bigint: 1n } as never }))
        }).toThrow('agent context must be losslessly JSON-serializable')
        return [{ type: 'text', text: 'rejected invalid context' }]
      },
    }))

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(agent.session.snapshotEvents().some(event => event.type === 'user/message' && event.data.source.kind === 'plugin')).toBe(false)
  })

  it('agent/turn-stopping can steer another step (/loop pattern)', async () => {
    const adapter = new MockAdapter([
      textResponse('step 1'),
      textResponse('step 2'),
      textResponse('step 3'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let steps = 0
    ctx.on('session/event', (_session, event) => { if (event.type === 'step/end') steps++ })
    ctx.on('agent/turn-stopping', ({ agent: subject }) => {
      if (steps < 3) {
        subject.steer(createUserMessage({ content: [{ type: 'text', text: 'continue' }], source: { kind: 'plugin', plugin: 'loop-test' } }))
      }
    })

    send(agent, 'go')
    await waitForIdle(ctx, agent)
    expect(steps).toBe(3)
    expect(adapter.requests).toHaveLength(3)
  })

  it('a tool can conclude the turn despite owing a follow-up request', async () => {
    const adapter = new MockAdapter([toolCallResponse('c1', 'echo', { text: 'x' })])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: '',
      parameters: { text: { type: 'string' } },
      async execute(args, exec) {
        exec.concludeTurn()
        return [{ type: 'text', text: String(args.text) }]
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'go')
    await waitForIdle(ctx, agent)
    // only one model call despite the tool call requesting a follow-up
    expect(adapter.requests).toHaveLength(1)
    // The tool still executes and durably records its result.
    expect(agent.session.snapshotEvents().some(e => e.type === 'tool/result')).toBe(true)
  })

  it('continues for steering that arrived during a concluding tool step', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'finalize', {}),
      textResponse('next turn reply'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    ctx.tools.register(defineContentToolFixture({
      name: 'finalize',
      description: '',
      parameters: {},
      async execute(_args, exec) {
        // Steering lands while the concluding tool is still executing.
        agent.steer(createUserMessage({ content: [{ type: 'text', text: 'late steering' }], source: { kind: 'user' } }))
        exec.concludeTurn()
        return [{ type: 'text', text: 'final' }]
      },
    }))

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(2)
    const events = agent.session.snapshotEvents().map(event => event.type)
    expect(events.filter(type => type === 'turn/end')).toHaveLength(1)
    expect(JSON.stringify(adapter.requests[1]?.messages)).toContain('late steering')
    const texts = adapter.requests[1]!.messages
      .flatMap(message => message.content)
      .filter(block => block.type === 'text')
      .map(block => block.text)
    expect(texts).toContain('late steering')
  })

  it('agent/request waterfall switches models by returning a replacement config; the switch is logged', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    ctx.on('agent/request', async (_payload, next) => {
      const config = await next()
      // The seed is frozen — config is not a mutable per-call knob; a switch
      // is proposed by returning a replacement, and the loop logs it.
      expect(Object.isFrozen(config)).toBe(true)
      expect(() => { (config as { model: string }).model = 'other-model' }).toThrow(TypeError)
      return { ...config, model: 'other-model' }
    })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)
    expect(adapter.requests[0]!.model).toBe('other-model')
    // The header event records what the request ACTUALLY used — the switch is
    // a reconstructable fact, not silent drift.
    const headerEvent = agent.session.snapshotEvents().find(e => e.type === 'request/header')
    expect(headerEvent?.type === 'request/header' && headerEvent.data.header.config.model).toBe('other-model')
  })

  it('agent/pre-step fires once per proposed step before the step is opened', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', {}, 'calling echo'),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo', description: 'echo', parameters: {},
      async execute() { return [{ type: 'text', text: 'echoed' }] },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const fires: { turn: number; step: number; signal: AbortSignal }[] = []
    ctx.on('agent/pre-step', ({ agent: subject, turn, step, signal }, next) => {
      if (subject === agent) fires.push({ turn, step, signal })
      return next()
    })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(fires.map(({ turn, step }) => ({ turn, step }))).toEqual([
      { turn: 1, step: 1 },
      { turn: 1, step: 2 },
    ])
    expect(fires.every(({ signal }) => signal instanceof AbortSignal)).toBe(true)
  })

  it('agent/pre-step fires before its step boundary opens and before the request', async () => {
    const adapter = new MockAdapter([textResponse('ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let boundaryOpen = true
    ctx.on('agent/pre-step', ({ agent: subject }, next) => {
      if (subject === agent) boundaryOpen = subject.session.snapshotEvents().at(-1)?.type === 'step/start'
      return next()
    })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(boundaryOpen).toBe(false)
    expect(adapter.requests).toHaveLength(1)
  })

  it('a throwing agent/pre-step listener fails the proposal, not the loop', async () => {
    const adapter = new MockAdapter([textResponse('second turn ok')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let throwOnce = true
    ctx.on('agent/pre-step', (_payload, next) => {
      if (throwOnce) { throwOnce = false; throw new Error('boom in pre-step') }
      return next()
    })

    const errors: Error[] = []
    ctx.on('agent/error', ({ error }) => {
      if (error instanceof Error) errors.push(error)
    })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    // The first proposal failed inside a balanced turn without calling the model.
    expect(errors.map(error => error.message)).toEqual(['boom in pre-step'])
    expect(adapter.requests.length).toBe(0)
    expect(agent.session.snapshotEvents().some(event => event.type === 'turn/start')).toBe(true)
    expect(agent.session.snapshotEvents().some(event => event.type === 'turn/end')).toBe(true)

    // The loop survived: a second prompt runs a normal completed turn.
    send(agent, 'second')
    await waitForIdle(ctx, agent)
    expect(adapter.requests.length).toBe(1)
    const lastTurnEnd = agent.session.snapshotEvents().findLast(e => e.type === 'turn/end')
    expect(lastTurnEnd?.type === 'turn/end' && lastTurnEnd.data.reason).toEqual({ kind: 'completed' })
  })

  it('cancel() mid-stream ends the turn with reason aborted', async () => {
    const adapter = new MockAdapter(['hang'])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    const frames: AssistantStreamFrame[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })
    ctx.on('agent/assistant-stream', ({ frame }) => { frames.push(frame) })

    send(agent, 'go')
    // wait until the stream is hanging, then cancel
    await new Promise(r => setTimeout(r, 30))
    expect(agent.status).toBe('running')
    agent.cancel({ kind: 'user' })
    await waitForIdle(ctx, agent)

    expect(reasons).toEqual([{ kind: 'aborted', reason: { kind: 'user' } }])
    const chunks = frames.filter(frame => frame.type === 'chunk')
    expect(frames.at(-1)).toMatchObject({
      type: 'end',
      index: chunks.length,
      outcome: { kind: 'committed', eventType: 'assistant/message' },
    })
  })

  it('surfaces max-tokens as the turn-end reason when the last step is cut off', async () => {
    // A single step that ends with a max-tokens finish (no tool calls): the
    // turn stops by default and ends max-tokens, not completed.
    const adapter = new MockAdapter([maxTokensResponse('truncat')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(1)
    expect(reasons).toEqual([{ kind: 'max-tokens' }])
    // Assert the durable row, not only the live listener.
    const turnEnd = agent.session.snapshotEvents().findLast(e => e.type === 'turn/end')
    expect(turnEnd!.data.reason).toEqual({ kind: 'max-tokens' })
  })

  it('a max-tokens step earlier in a turn still surfaces as max-tokens after a later completed step', async () => {
    // Step 1 is cut off (max-tokens, no tool calls → would stop by default), so continuation
    // must be FORCED to reach step 2 which finishes normally (stop).
    const adapter = new MockAdapter([
      maxTokensResponse('first half'),
      textResponse('second half'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let steps = 0
    ctx.on('session/event', (_session, event) => { if (event.type === 'step/end') steps++ })
    // Force exactly one continuation (step 1 → step 2), then defer to default
    // (step 2 is a plain stop with no tool calls → stops).
    ctx.on('agent/turn-stopping', ({ agent: subject }) => {
      if (steps < 2) {
        subject.steer(createUserMessage({ content: [{ type: 'text', text: 'continue after truncation' }], source: { kind: 'plugin', plugin: 'max-tokens-test' } }))
      }
    })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(steps).toBe(2)
    expect(adapter.requests).toHaveLength(2)
    expect(adapter.requests[1]!.messages.slice(1)).toEqual([
      {
        id: expect.any(String) as unknown,
        role: 'user',
        content: [{ type: 'text', text: 'go' }],
        source: { kind: 'user' },
      },
      {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [{ type: 'text', text: 'first half' }],
        source: { kind: 'model', provider: 'mock', model: 'mock' },
      },
      {
        id: expect.any(String) as unknown,
        role: 'user',
        content: [{ type: 'text', text: 'continue after truncation' }],
        source: { kind: 'plugin', plugin: 'max-tokens-test' },
      },
    ])
    // A max-token step is sticky: the later completed step must not
    // downgrade the turn outcome.
    expect(reasons).toEqual([{ kind: 'max-tokens' }])
  })

  it('a completed step after no max-tokens keeps the turn completed (max-tokens does not leak across turns)', async () => {
    // Two consecutive turns: turn 1 is cut off (max-tokens), turn 2 is a clean
    // stop. The per-turn reason must be independent — turn 2 ends completed.
    const adapter = new MockAdapter([maxTokensResponse('cut'), textResponse('clean')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    expect(reasons).toEqual([{ kind: 'max-tokens' }, { kind: 'completed' }])
  })

  it('does not dispatch tool calls from a max-tokens-truncated step', async () => {
    const callId = ToolCallId('c1')
    const adapter = new MockAdapter([[
      { type: 'block-start', index: 0, blockType: 'tool-call' },
      { type: 'tool-call-delta', index: 0, id: callId, name: 'echo', argumentsDelta: '{"text":"x"}' },
      { type: 'block-end', index: 0, block: { type: 'tool-call', id: callId, name: 'echo', arguments: '{"text":"x"}' } },
      { type: 'usage', usage: { inputTokens: 10, outputTokens: 5 } },
      { type: 'finish', reason: { kind: 'max-tokens' } },
    ]])
    const ctx = await harness(adapter)
    let executions = 0
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: '',
      parameters: { text: { type: 'string' } },
      async execute() {
        executions += 1
        return [{ type: 'text', text: 'should not run' }]
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(executions).toBe(0)
    expect(agent.session.snapshotEvents().some(e => e.type === 'tool/call')).toBe(false)
    expect(agent.session.deriveMessages().slice(1)).toEqual([{
      id: expect.any(String) as unknown,
      role: 'user',
      content: [{ type: 'text', text: 'go' }],
      source: { kind: 'user' },
    }])
    expect(reasons).toEqual([{ kind: 'max-tokens' }])
    // Empty content still needs an assistant/message to carry usage; derivation
    // skips that host so it does not create a spurious assistant turn.
    const assistantMessage = agent.session.snapshotEvents().find(e => e.type === 'assistant/message')
    expect(assistantMessage?.type === 'assistant/message' && assistantMessage.data).toMatchObject({
      turn: 1,
      step: 1,
      message: {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [],
        source: { kind: 'model', provider: 'mock', model: 'mock' },
      },
      usage: { inputTokens: 10, outputTokens: 5 },
    })
  })

  it('appends an empty completion anchor for a max-tokens step with no usage', async () => {
    // The truncated tool call is dropped from durable content, while the
    // successful provider call still needs an exact replay anchor.
    const callId = ToolCallId('c1')
    const adapter = new MockAdapter([[
      { type: 'block-start', index: 0, blockType: 'tool-call' },
      { type: 'tool-call-delta', index: 0, id: callId, name: 'echo', argumentsDelta: '{"text":"x"}' },
      { type: 'block-end', index: 0, block: { type: 'tool-call', id: callId, name: 'echo', arguments: '{"text":"x"}' } },
      { type: 'finish', reason: { kind: 'max-tokens' } },
    ]])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: '',
      parameters: { text: { type: 'string' } },
      async execute() { return [{ type: 'text', text: 'should not run' }] },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(reasons).toEqual([{ kind: 'max-tokens' }])
    const assistant = agent.session.snapshotEvents().find(e => e.type === 'assistant/message')!
    expect(assistant.type === 'assistant/message' && assistant.data).toMatchObject({
      turn: 1,
      step: 1,
      message: {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [],
        source: { kind: 'model', provider: 'mock', model: 'mock' },
      },
    })
    expect(assistant.sourceEventSeqs).toBeUndefined()
    expect(assistant.type === 'assistant/message' ? assistant.data.stream.length : 0).toBeGreaterThan(0)
    expect(agent.session.deriveMessages().slice(1)).toEqual([{
      id: expect.any(String) as unknown,
      role: 'user',
      content: [{ type: 'text', text: 'go' }],
      source: { kind: 'user' },
    }])
  })

  it('appends an empty completion anchor for a normal stop with no usage', async () => {
    // A clean content-less call stays absent from derived messages but remains
    // a durable successful-call boundary for replay consumers.
    const adapter = new MockAdapter([[{ type: 'finish', reason: { kind: 'stop' } }]])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const reasons: TurnEndReason[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(reasons).toEqual([{ kind: 'completed' }])
    const assistant = agent.session.snapshotEvents().find(e => e.type === 'assistant/message')!
    expect(assistant.type === 'assistant/message' && assistant.data).toMatchObject({
      turn: 1,
      step: 1,
      message: {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [],
        source: { kind: 'model', provider: 'mock', model: 'mock' },
      },
    })
    expect(assistant.sourceEventSeqs).toBeUndefined()
    expect(assistant.type === 'assistant/message' ? assistant.data.stream.length : 0).toBe(1)
    expect(agent.session.deriveMessages().slice(1)).toEqual([{
      id: expect.any(String) as unknown,
      role: 'user',
      content: [{ type: 'text', text: 'go' }],
      source: { kind: 'user' },
    }])
  })

  it('keeps safe max-tokens assistant content while dropping truncated tool calls', async () => {
    const callId = ToolCallId('c1')
    const adapter = new MockAdapter([[
      { type: 'block-start', index: 0, blockType: 'text' },
      { type: 'text-delta', index: 0, text: 'partial text' },
      { type: 'block-end', index: 0, block: { type: 'text', text: 'partial text' } },
      { type: 'block-start', index: 1, blockType: 'tool-call' },
      { type: 'tool-call-delta', index: 1, id: callId, name: 'echo', argumentsDelta: '{"text"' },
      {
        type: 'finish',
        reason: { kind: 'max-tokens' },
        replayState: { response: { responseId: 'resp-1' }, blocks: ['text-meta', 'tool-meta'] },
      },
    ], textResponse('continued')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'go')
    await waitForIdle(ctx, agent)
    send(agent, 'continue')
    await waitForIdle(ctx, agent)

    expect(agent.session.snapshotEvents().some(e => e.type === 'tool/call')).toBe(false)
    // The follow-up request replays the truncated message with its replay
    // metadata pruned in step with the dropped tool call.
    expect(adapter.requests[1]?.messages[2]?.source).toEqual({
      kind: 'model',
      provider: 'mock',
      model: 'mock',
      replayState: { response: { responseId: 'resp-1' }, blocks: ['text-meta'] },
    })
    expect(agent.session.deriveMessages().slice(1)).toEqual([
      {
        id: expect.any(String) as unknown,
        role: 'user',
        content: [{ type: 'text', text: 'go' }],
        source: { kind: 'user' },
      },
      {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [{ type: 'text', text: 'partial text' }],
        source: {
          kind: 'model',
          provider: 'mock',
          model: 'mock',
          replayState: { response: { responseId: 'resp-1' }, blocks: ['text-meta'] },
        },
      },
      {
        id: expect.any(String) as unknown,
        role: 'user',
        content: [{ type: 'text', text: 'continue' }],
        source: { kind: 'user' },
      },
      {
        id: expect.any(String) as unknown,
        role: 'assistant',
        content: [{ type: 'text', text: 'continued' }],
        source: { kind: 'model', provider: 'mock', model: 'mock' },
      },
    ])
  })

  it('contains a step/end observer failure without changing continuation', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', { text: 'x' }),
      textResponse('continued after tool call'),
    ])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: '',
      parameters: { text: { type: 'string' } },
      async execute(args) {
        return [{ type: 'text', text: String(args.text) }]
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    let threw = false
    // Post-commit session observers cannot control the loop. The tool call still
    // drives the second model request, and the turn completes normally.
    ctx.on('session/event', (_session, event) => {
      if (event.type === 'step/end' && !threw) { threw = true; throw new Error('bad step/end listener') }
    })

    send(agent, 'go')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(2)
    const turnEnd = agent.session.snapshotEvents().findLast(e => e.type === 'turn/end')
    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind).toBe('completed')
  })

  it('contains a reentrant send attempted during durable inbox publication', async () => {
    const adapter = new MockAdapter([textResponse('first')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let nested = false
    ctx.on('session/event', (session, event) => {
      if (session !== agent.session || event.type !== 'agent/inbox/spliced'
        || event.data.inserted.length === 0 || nested) return
      nested = true
      send(agent, 'queued listener message')
    })

    const idle = waitForIdle(ctx, agent)
    send(agent, 'outer message')
    await idle

    const turns = agent.session.snapshotEvents().filter(event => event.type === 'turn/start')
    const messages = agent.session.snapshotEvents()
      .filter(event => event.type === 'user/message')
      .map(event => event.data.content)
    expect(turns).toHaveLength(1)
    expect(messages).toEqual([[{ type: 'text', text: 'outer message' }]])
  })

  it('preserves independent turn sources across an adjacent microtask send', async () => {
    const adapter = new MockAdapter([textResponse('first'), textResponse('second')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const idle = waitForIdle(ctx, agent)
    agent.followup(createUserMessage({ content: [{ type: 'text', text: 'user message' }], source: { kind: 'user' } }))
    await Promise.resolve()
    agent.followup(createUserMessage({ content: [{ type: 'text', text: 'plugin message' }], source: { kind: 'plugin', plugin: 'test' } }))
    await idle

    const turns = agent.session.snapshotEvents().filter(event => event.type === 'turn/start')
    const sources = agent.session.snapshotEvents()
      .filter(event => event.type === 'user/message')
      .map(event => event.data.source)
    expect(turns).toHaveLength(2)
    expect(sources).toEqual([
      { kind: 'user' },
      { kind: 'plugin', plugin: 'test' },
    ])
  })

  it('keeps a session-listener send after dequeue in the following turn', async () => {
    const adapter = new MockAdapter([textResponse('first'), textResponse('second')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const turns: number[] = []
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/start') turns.push(event.data.turn) })

    // Queue the second from the first turn's durable Assistant settlement.
    let queued = false
    ctx.on('session/event', (_s, event) => {
      if (event.type === 'assistant/message' && !queued) {
        queued = true
        queueMicrotask(() => { send(agent, 'second message') })
      }
    })

    send(agent, 'first message')
    await waitForIdle(ctx, agent)

    expect(turns).toEqual([1, 2])
    expect(adapter.requests).toHaveLength(2)
    expect(JSON.stringify(adapter.requests[1]!.messages)).toContain('first')
    expect(JSON.stringify(adapter.requests[1]!.messages)).toContain('second message')
  })

  it('keeps a model-adapter callback send in the following turn', async () => {
    const agentRef: { current?: Agent } = {}
    const adapter = new MockAdapter([
      () => {
        const agent = agentRef.current
        if (agent === undefined) throw new Error('model callback ran before agent setup')
        send(agent, 'model callback message')
        return textResponse('first')
      },
      textResponse('second'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    agentRef.current = agent

    const idle = waitForIdle(ctx, agent)
    send(agent, 'outer message')
    await idle

    const messages = agent.session.snapshotEvents()
      .filter(event => event.type === 'user/message')
      .map(event => event.data.content)
    expect(agent.session.snapshotEvents().filter(event => event.type === 'turn/start')).toHaveLength(2)
    expect(messages).toEqual([
      [{ type: 'text', text: 'outer message' }],
      [{ type: 'text', text: 'model callback message' }],
    ])
  })

  it('records normalized model errors on the turn boundary', async () => {
    const adapter = new MockAdapter([]) // script exhausted → throws
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    const errors: unknown[] = []
    const reasons: TurnEndReason[] = []
    ctx.on('agent/error', ({ error }) => {
      errors.push(error)
    })
    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })

    send(agent, 'hi')
    await waitForIdle(ctx, agent)

    expect(errors).toHaveLength(1)
    expect(errors[0]).toBeInstanceOf(LlmError)
    expect((errors[0] as LlmError).failure).toEqual({
      message: 'MockAdapter: script exhausted',
      code: 'UNKNOWN',
    })
    expect(reasons[0]).toMatchObject({ kind: 'error' })
    // The durable failure and live relay describe the same failed turn.
    const turnEnd = agent.session.snapshotEvents().find(e => e.type === 'turn/end')
    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toMatchObject({ kind: 'error' })
  })

  it('disposing the loop fiber mid-turn stops the loop (HMR safety)', async () => {
    const adapter = new MockAdapter(['hang'])
    const ctx = await harness(adapter)

    let agent!: Agent
    const fiber = await ctx.plugin(Object.assign(async (inner: Context) => {
      agent = await inner.agentLoop.create(SessionId('scoped'), { provider: 'mock', model: 'mock' })
    }, { inject: ['agentLoop'] }))

    expect(ctx.agents.get(SessionId('scoped'))).toBe(agent)
    send(agent, 'go')
    await new Promise(r => setTimeout(r, 30))
    expect(agent.status).toBe('running')

    await fiber.dispose()
    await driverDone(agent)

    expect(ctx.agents.get(SessionId('scoped'))).toBeUndefined()
  })

  it('creates agents from config on startup', async () => {
    const effort = ReasoningEffortId('high')
    const adapter = new MockAdapter([textResponse('from config')], {
      efforts: [{ id: effort, name: 'High' }],
      defaultEffort: effort,
    })
    const ctx = new Context()
    await ctx.plugin(LlmRuntime)
    await ctx.plugin(SessionStore)
    await ctx.plugin(SessionProjectionRegistry)
    await ctx.plugin(SystemPrompt)
    await ctx.plugin(ToolRuntime)
    await ctx.plugin(AgentRegistry)
    await ctx.plugin(AgentLoop, {
      agents: [{ id: SessionId('config-agent'), provider: 'mock', model: 'mock', reasoningEffort: effort }],
    })
    ctx.llm.registerAdapter(['mock'], adapter)

    const agent = ctx.agents.list()[0]!
    expect(agent).toBeDefined()
    expect(agent.id).toBe(agent.session.id)
    expect(agent.id).toMatch(/^config-agent-session-/)
    expect(agent.options.model).toBe('mock')

    // the agent is alive: send triggers a turn
    send(agent, 'hi')
    await waitForIdle(ctx, agent)
    expect(adapter.requests).toHaveLength(1)
    expect(adapter.requests[0]?.reasoningEffort).toBe(effort)
    const header = agent.session.snapshotEvents().find(event => event.type === 'request/header')
    expect(header?.type === 'request/header' && header.data.header.config.reasoningEffort).toBe(effort)
  })

  it('attaches config agent cwd to the fresh session header', async () => {
    const ctx = new Context()
    await ctx.plugin(LlmRuntime)
    await ctx.plugin(SessionStore)
    await ctx.plugin(SessionProjectionRegistry)
    await ctx.plugin(SystemPrompt)
    await ctx.plugin(ToolRuntime)
    await ctx.plugin(AgentRegistry)
    await ctx.plugin(AgentLoop, {
      agents: [{ id: SessionId('config-agent'), provider: 'mock', model: 'mock', cwd: '/work/project' }],
    })

    const agent = ctx.agents.list()[0]!
    expect(agent.session.header.cwd).toBe('/work/project')
  })

  it('replays a session log into an identical derived history', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', { text: 'x' }),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    ctx.tools.register(defineContentToolFixture({
      name: 'echo',
      description: '',
      parameters: { text: { type: 'string' } },
      async execute(args) {
        return [{ type: 'text', text: String(args.text) }]
      },
    }))
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    send(agent, 'run')
    await waitForIdle(ctx, agent)

    const replayed = ctx.sessions.create(SessionId('replayed'), { seed: [...agent.session.snapshotEvents()] })
    expect(replayed.deriveMessages()).toEqual(agent.session.deriveMessages())
    // event-by-event identity of types over the inherited prefix
    expect(replayed.snapshotEvents().slice(0, agent.session.seq).map(e => e.type)).toEqual(
      agent.session.snapshotEvents().map(e => e.type))
    expect(replayed.snapshotEvents().at(-1)?.type).toBe('session/end-seed')
  })
})