/**
 * Loop-level reconstructability: every request the loop sends is a pure function of the
 * session log — messages derive at the step/start boundary and the header is the latest
 * request/header snapshot. Each request extends its predecessor unless a logged compaction
 * replacement or header change explains the difference.
 */

import { describe, expect, it } from 'vitest'
import { Context } from '@deepseek-ai/cordis'
import LlmRuntime, { createUserMessage, LlmError, ReasoningEffortId  } from '@deepseek-ai/dsh-llm'
import type { GenerateOptions, LlmModelReasoningInfo, LlmResolvedModelInfo, StreamChunk } from '@deepseek-ai/dsh-llm'
import SessionStore, { Session, SessionId, foldRequestHeader } 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 } from '@deepseek-ai/dsh-agent'

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

async function harness(adapter: MockAdapter, persona = 'stable base') {
  return harnessRoutes([['mock', adapter]], persona)
}

async function harnessRoutes(
  adapters: readonly (readonly [provider: string, adapter: MockAdapter])[],
  persona = 'stable base',
) {
  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: [] })
  for (const [provider, adapter] of adapters) ctx.llm.registerAdapter([provider], adapter)
  return ctx
}

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' } }))
}

/** Assert `previous` is a strict value-prefix of `current`. */
function expectPrefixExtension(previous: GenerateOptions, current: GenerateOptions) {
  expect(current.messages.length).toBeGreaterThan(previous.messages.length)
  expect(current.messages.slice(0, previous.messages.length)).toEqual([...previous.messages])
  expect(current.system).toBeUndefined()
  expect(current.tools).toEqual(previous.tools)
}

function registerEcho(ctx: Context) {
  ctx.tools.register(defineContentToolFixture({
    name: 'echo',
    description: 'echo back',
    parameters: { text: { type: 'string' } },
    async execute(args) {
      return [{ type: 'text', text: `echo: ${String(args.text)}` }]
    },
  }))
}

describe('request stability across the loop', () => {
  it('each step request within a turn append-extends the previous, frozen end to end', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', { text: 'one' }, 'first'),
      toolCallResponse('c2', 'echo', { text: 'two' }, 'second'),
      textResponse('done'),
    ])
    const ctx = await harness(adapter)
    registerEcho(ctx)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

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

    expect(adapter.requests).toHaveLength(3)
    expectPrefixExtension(adapter.requests[0]!, adapter.requests[1]!)
    expectPrefixExtension(adapter.requests[1]!, adapter.requests[2]!)
    for (const request of adapter.requests) {
      expect(Object.isFrozen(request)).toBe(true)
      expect(Object.isFrozen(request.messages)).toBe(true)
    }
    // One anchoring header snapshot; no further header events (nothing changed).
    const headerEvents = agent.session.snapshotEvents().filter(e => e.type === 'request/header')
    expect(headerEvents).toHaveLength(1)
    expect(headerEvents[0]?.type === 'request/header' && headerEvents[0].data.reason).toBe('initial')
  })

  it('a later turn append-extends the previous turn (one conversation, one log)', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

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

    expect(adapter.requests).toHaveLength(2)
    expectPrefixExtension(adapter.requests[0]!, adapter.requests[1]!)
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial'])
  })

  it('starts a new request series only when the admitted step explicitly asks for one', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    ctx.on('agent/pre-step', async ({ turn }, next) => {
      const decision = await next()
      return decision.kind === 'enter' && turn === 2
        ? { ...decision, startsRequestSeries: true }
        : decision
    })

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

    expectPrefixExtension(adapter.requests[0]!, adapter.requests[1]!)
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])
  })

  it('retains the explicit series boundary when that request also changes its header', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    ctx.on('agent/pre-step', async ({ turn }, next) => {
      const decision = await next()
      return decision.kind === 'enter' && turn === 2
        ? { ...decision, startsRequestSeries: true }
        : decision
    })
    ctx.on('agent/request', async ({ turn }, next) => {
      const config = await next()
      return turn === 2 ? { ...config, maxTokens: 1_024 } : config
    })

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

    expectPrefixExtension(adapter.requests[0]!, adapter.requests[1]!)
    expect(agent.session.snapshotEvents().flatMap(event => event.type === 'request/header'
      ? [{ reason: event.data.reason, startsSeries: event.data.startsSeries }]
      : [])).toEqual([
      { reason: 'initial', startsSeries: undefined },
      { reason: 'change', startsSeries: true },
    ])
  })

  it('keeps the series declaration when an outer listener rebuilds the enter decision', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    // Context-appending wrapper in the tool-cordis / session-reference shape:
    // it rebuilds the downstream decision, so it must spread it to keep fields
    // it does not own — a bare `{ kind: 'enter', messages }` drops the series.
    ctx.on('agent/pre-step', async (_payload, next) => {
      const decision = await next()
      if (decision.kind === 'reject') return decision
      const appended = createUserMessage({
        content: [{ type: 'text', text: 'appended reference context' }],
        source: { kind: 'plugin', plugin: 'outer-wrapper' },
      })
      return { ...decision, messages: [...decision.messages, appended] }
    }, { prepend: true })
    ctx.on('agent/pre-step', async ({ turn }, next) => {
      const decision = await next()
      return decision.kind === 'enter' && turn === 2
        ? { ...decision, startsRequestSeries: true }
        : decision
    })

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

    expectPrefixExtension(adapter.requests[0]!, adapter.requests[1]!)
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])
  })

  it('logs adapter defaults, supports per-turn effort changes, and restores the effective value', async () => {
    const reasoning = {
      efforts: [
        { id: ReasoningEffortId('high'), name: 'High' },
        { id: ReasoningEffortId('max'), name: 'Max' },
      ],
      defaultEffort: ReasoningEffortId('high'),
    }
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')], reasoning)
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('effort'), { provider: 'mock', model: 'mock' })
    ctx.on('agent/request', async ({ turn }, next) => {
      const config = await next()
      return turn === 2 ? { ...config, reasoningEffort: ReasoningEffortId('max') } : config
    })

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

    expect(adapter.requests.map(request => request.reasoningEffort)).toEqual([
      ReasoningEffortId('high'),
      ReasoningEffortId('max'),
    ])
    const headers = agent.session.snapshotEvents().filter(event => event.type === 'request/header')
    expect(headers.map(event => event.data.header.config.reasoningEffort)).toEqual([
      ReasoningEffortId('high'),
      ReasoningEffortId('max'),
    ])
    expect(headers.map(event => event.data.header.adapterDefaults)).toEqual([
      { reasoningEffort: true },
      undefined,
    ])
    expect(headers.map(event => event.data.reason)).toEqual(['initial', 'change'])

    for (const [model, effort] of [
      ['mock', ReasoningEffortId('max')],
      ['replacement', ReasoningEffortId('high')],
    ] as const) {
      const resumedAdapter = new MockAdapter([textResponse('resumed')], reasoning)
      const resumedCtx = await harness(resumedAdapter)
      const resumedHandle = await resumedCtx.agents.create({
        sessionId: SessionId(`effort-${model}`),
        seed: structuredClone(agent.session.snapshotEvents()),
        agentOptions: { provider: 'mock', model },
      })
      send(resumedHandle.agent, 'resumed')
      await waitForIdle(resumedCtx, resumedHandle.agent)

      expect(resumedAdapter.requests[0]?.model).toBe(model)
      expect(resumedAdapter.requests[0]?.reasoningEffort).toBe(effort)
      const resumedHeaders = resumedHandle.agent.session.snapshotEvents().filter(event => event.type === 'request/header')
      expect(resumedHeaders.at(-1)?.data.header.config.reasoningEffort).toBe(effort)
      expect(resumedHeaders.at(-1)?.data.reason).toBe('resume')
    }
  })

  it('logs an adapter-owned maxTokens default before dispatch', async () => {
    const adapter = new MockAdapter([textResponse('bounded')], undefined, 256_000)
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('adapter-max-tokens'), {
      provider: 'mock',
      model: 'mock',
    })

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

    expect(adapter.requests[0]?.maxTokens).toBe(256_000)
    const header = agent.session.snapshotEvents().find(event => event.type === 'request/header')
    expect(header?.type === 'request/header' && header.data.header.config.maxTokens).toBe(256_000)
    expect(header?.type === 'request/header' && header.data.header.adapterDefaults)
      .toEqual({ maxTokens: true })
  })

  it('rematerializes the selected adapter maxTokens default after a provider switch', async () => {
    const deepseek = new MockAdapter([textResponse('deepseek')], undefined, 256_000)
    const other = new MockAdapter([textResponse('other')], undefined, 8_192)
    const ctx = await harnessRoutes([
      ['deepseek', deepseek],
      ['other', other],
    ])
    const agent = await ctx.agentLoop.create(SessionId('adapter-max-tokens-switch'), {
      provider: 'deepseek',
      model: 'deepseek-model',
    })
    ctx.on('agent/request', async ({ turn }, next) => {
      const config = await next()
      return turn === 2
        ? { ...config, provider: 'other', model: 'other-model' }
        : config
    })

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

    expect(deepseek.requests[0]?.maxTokens).toBe(256_000)
    expect(other.requests[0]?.maxTokens).toBe(8_192)
    const headers = agent.session.snapshotEvents().filter(event => event.type === 'request/header')
    expect(headers.map(event => event.data.header.config.maxTokens)).toEqual([256_000, 8_192])
    expect(headers.map(event => event.data.header.adapterDefaults)).toEqual([
      { maxTokens: true },
      { maxTokens: true },
    ])
  })

  it('preserves an explicit agent maxTokens cap across a provider switch', async () => {
    const deepseek = new MockAdapter([textResponse('deepseek')], undefined, 256_000)
    const other = new MockAdapter([textResponse('other')], undefined, 8_192)
    const ctx = await harnessRoutes([
      ['deepseek', deepseek],
      ['other', other],
    ])
    const agent = await ctx.agentLoop.create(SessionId('explicit-max-tokens-switch'), {
      provider: 'deepseek',
      model: 'deepseek-model',
      maxTokens: 4_096,
    })
    ctx.on('agent/request', async ({ turn }, next) => {
      const config = await next()
      return turn === 2
        ? { ...config, provider: 'other', model: 'other-model' }
        : config
    })

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

    expect(deepseek.requests[0]?.maxTokens).toBe(4_096)
    expect(other.requests[0]?.maxTokens).toBe(4_096)
    const headers = agent.session.snapshotEvents().filter(event => event.type === 'request/header')
    expect(headers.map(event => event.data.header.config.maxTokens)).toEqual([4_096, 4_096])
    expect(headers.map(event => event.data.header.adapterDefaults)).toEqual([undefined, undefined])
  })

  it('keeps exact-model resolution, request logging, and dispatch on one adapter registration', async () => {
    const ctx = new Context()
    await ctx.plugin(LlmRuntime)
    await ctx.plugin(SessionStore)
    await ctx.plugin(SessionProjectionRegistry)
    await ctx.plugin(SystemPrompt, { personaPrefix: 'stable base' })
    await ctx.plugin(ToolRuntime)
    await ctx.plugin(AgentRegistry)
    await ctx.plugin(AgentLoop, { agents: [] })
    const started = Promise.withResolvers<undefined>()
    const reasoning = Promise.withResolvers<LlmModelReasoningInfo>()
    const first = new class extends MockAdapter {
      override async resolveModel(
        provider: string,
        model: string,
        _signal?: AbortSignal,
      ): Promise<LlmResolvedModelInfo> {
        started.resolve(undefined)
        return {
          provider,
          id: model,
          name: model,
          reasoning: await reasoning.promise,
        }
      }
    }([textResponse('first')])
    const second = new MockAdapter([textResponse('second')], {
      efforts: [{ id: ReasoningEffortId('max'), name: 'Max' }],
      defaultEffort: ReasoningEffortId('max'),
    })
    const disposeFirst = ctx.llm.registerAdapter(['mock'], first)
    const agent = await ctx.agentLoop.create(SessionId('effort-hmr'), { provider: 'mock', model: 'mock' })

    send(agent, 'go')
    await started.promise
    disposeFirst()
    ctx.llm.registerAdapter(['mock'], second)
    reasoning.resolve({
      efforts: [{ id: ReasoningEffortId('high'), name: 'High' }],
      defaultEffort: ReasoningEffortId('high'),
    })
    await waitForIdle(ctx, agent)

    expect(first.requests.map(request => request.reasoningEffort)).toEqual([
      ReasoningEffortId('high'),
    ])
    expect(second.requests).toHaveLength(0)
    const headers = agent.session.snapshotEvents().filter(event => event.type === 'request/header')
    expect(headers.at(-1)?.data.header.config.reasoningEffort).toBe(ReasoningEffortId('high'))
  })

  it('aborts a blocked reasoning lookup before quiescent disposal completes', async () => {
    const started = Promise.withResolvers<AbortSignal>()
    const adapter = new class extends MockAdapter {
      override resolveModel(
        _provider: string,
        _model: string,
        signal?: AbortSignal,
      ): Promise<never> {
        if (signal === undefined) return Promise.reject(new Error('missing reasoning signal'))
        started.resolve(signal)
        return new Promise((_resolve, reject) => {
          if (signal.aborted) {
            reject(signal.reason instanceof Error ? signal.reason : new Error('reasoning aborted'))
            return
          }
          signal.addEventListener('abort', () => {
            reject(signal.reason instanceof Error ? signal.reason : new Error('reasoning aborted'))
          }, { once: true })
        })
      }
    }([])
    const ctx = await harness(adapter)
    const handle = await ctx.agents.create({
      sessionId: SessionId('reasoning-dispose'),
      agentOptions: { provider: 'mock', model: 'mock' },
    })

    send(handle.agent, 'go')
    const signal = await started.promise
    await handle.dispose()

    expect(signal.aborted).toBe(true)
    expect(handle.agent.status).toBe('idle')
    expect(adapter.requests).toHaveLength(0)
    expect(handle.agent.session.snapshotEvents().some(event => event.type === 'request/header')).toBe(false)
  })

  it.each(['plain error', 'LLM error'] as const)(
    'does not swallow a %s from exact-model resolution',
    async (kind) => {
      const failure = kind === 'plain error'
        ? new Error('reasoning metadata failed')
        : new LlmError('unsupported effort', 'UNSUPPORTED_REASONING_EFFORT')
      const adapter = new class extends MockAdapter {
        override resolveModel(): Promise<never> {
          return Promise.reject(failure)
        }
      }([])
      const ctx = await harness(adapter)
      const agent = await ctx.agentLoop.create(SessionId(`reasoning-${kind}`), {
        provider: 'mock',
        model: 'mock',
      })

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

      expect(agent.session.snapshotEvents().findLast(event => event.type === 'turn/end')).toMatchObject({
        data: {
          reason: failure instanceof LlmError
            ? { kind: 'error', error: failure.failure }
            : { kind: 'error', error: { message: failure.message, code: 'UNKNOWN' } },
        },
      })
      expect(adapter.requests).toHaveLength(0)
    },
  )

  it('lets a short-circuiting llm/stream listener own an unregistered route', async () => {
    const ctx = new Context()
    await ctx.plugin(LlmRuntime)
    await ctx.plugin(SessionStore)
    await ctx.plugin(SessionProjectionRegistry)
    await ctx.plugin(SystemPrompt, { personaPrefix: 'stable base' })
    await ctx.plugin(ToolRuntime)
    await ctx.plugin(AgentRegistry)
    await ctx.plugin(AgentLoop, { agents: [] })
    let observed: GenerateOptions | undefined
    ctx.on('llm/stream', (options) => {
      observed = options
      return (async function* () {
        yield* textResponse('owned')
      })()
    })
    const agent = await ctx.agentLoop.create(SessionId('listener-owned'), {
      provider: 'listener',
      model: 'virtual',
    })

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

    expect(observed).toMatchObject({ provider: 'listener', model: 'virtual' })
    expect(agent.session.requestHeader()?.config).toEqual({
      provider: 'listener',
      model: 'virtual',
    })
    expect(agent.session.deriveMessages().at(-1)?.content).toContainEqual({
      type: 'text',
      text: 'owned',
    })
  })

  it('a compaction replace rewrites the resend, and the log explains it', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    ctx.on('agent/request', async ({ turn }, next) => {
      const config = await next()
      return turn === 2 ? { ...config, maxTokens: 1_024 } : config
    })

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

    // Node 0 is the system prompt; the compaction range starts after it.
    const nodes = agent.session.surface.nodes
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: '[summary of turn 1]' }],
      source: { kind: 'plugin', plugin: 'test-compact' },
    }), {
      surfaceOp: { op: 'replace', startSeq: nodes[1]!, endSeq: nodes[2]! },
      sourceEventSeqs: [nodes[1]!, nodes[2]!],
    })

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

    const second = adapter.requests[1]!
    // The rewritten history: summary replaces turn 1's user+assistant pair behind the system prompt.
    expect(second.messages[0]!.role).toBe('system')
    expect(second.messages[1]!.content.some(b => b.type === 'text' && b.text.includes('[summary of turn 1]'))).toBe(true)
    expect(agent.session.snapshotEvents().flatMap(event => event.type === 'request/header'
      ? [{ reason: event.data.reason, startsSeries: event.data.startsSeries }]
      : [])).toEqual([
      { reason: 'initial', startsSeries: undefined },
      { reason: 'change', startsSeries: true },
    ])
  })

  it('starts a new request series when compaction rewrites a retry in the same step', async () => {
    const adapter = new MockAdapter([
      () => { throw new LlmError('request is too large', 'CONTEXT_LENGTH') },
      textResponse('recovered'),
    ])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('same-step-compaction'), {
      provider: 'mock',
      model: 'mock',
    })
    ctx.on('agent/request-error', async ({ agent: subject }) => {
      const first = subject.session.surface.nodes[1]
      if (first === undefined) throw new Error('request has no surface message to compact')
      subject.session.append('user/message', createUserMessage({
        content: [{ type: 'text', text: '[summary for retry]' }],
        source: { kind: 'plugin', plugin: 'test-compact' },
      }), {
        surfaceOp: { op: 'replace', startSeq: first, endSeq: first },
        sourceEventSeqs: [first],
      })
      return { kind: 'retry' }
    })

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

    expect(adapter.requests).toHaveLength(2)
    expect(adapter.requests[1]?.messages[1]?.content).toContainEqual({
      type: 'text', text: '[summary for retry]',
    })
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])
  })

  it('a system-prompt change replaces surface node 0 and starts a new series under the same header', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two'), textResponse('three')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    send(agent, 'second')
    await waitForIdle(ctx, agent)
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial'])

    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'new guidance' })
    send(agent, 'third')
    await waitForIdle(ctx, agent)

    const snapshots = agent.session.snapshotEvents().filter(e => e.type === 'request/header')
    expect(snapshots.map(event => event.data.reason)).toEqual(['initial', 'series'])
    const systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(2)
    expect(systemNodes[1]?.surfaceOp).toEqual({ op: 'replace', startSeq: systemNodes[0]?.seq, endSeq: systemNodes[0]?.seq })
    expect(systemNodes[1]?.sourceEventSeqs).toEqual([systemNodes[0]?.seq])
    const head = adapter.requests[2]!.messages[0]!
    expect(head.role).toBe('system')
    expect(head.content).toContainEqual({ type: 'text', text: expect.stringContaining('new guidance') as unknown })
    expect(agent.session.surface.nodes[0]).toBe(systemNodes[1]?.seq)
    // History is preserved across the change — only node 0 moved.
    expect(adapter.requests[2]!.messages.length).toBeGreaterThan(adapter.requests[1]!.messages.length)
  })

  it('on an in-history route a system-prompt change appends after the cached history under the same header', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two'), textResponse('three'), textResponse('four')])
    adapter.systemPromptUpdate = 'in-history'
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    expect(agent.session.requestContext()).toEqual({ provider: 'mock', model: 'mock', systemPromptUpdate: 'in-history' })
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'new guidance' })
    send(agent, 'third')
    await waitForIdle(ctx, agent)

    // No new series: the header stays, node 0 stays, and the prompt update follows the cached prefix.
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial'])
    const systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(2)
    expect(systemNodes[1]?.surfaceOp).toBe('append')
    expect(agent.session.surface.nodes[0]).toBe(systemNodes[0]?.seq)
    expectPrefixExtension(adapter.requests[1]!, adapter.requests[2]!)
    const appended = adapter.requests[2]!.messages.slice(adapter.requests[1]!.messages.length)
    expect(appended.map(message => message.role)).toEqual(['assistant', 'system', 'user'])
    expect(appended[1]?.content).toContainEqual({ type: 'text', text: expect.stringContaining('new guidance') as unknown })
    expect(adapter.requests[2]!.messages[0]?.content).not.toContainEqual({ type: 'text', text: expect.stringContaining('new guidance') as unknown })

    // An unchanged prompt adds nothing on the next step.
    send(agent, 'fourth')
    await waitForIdle(ctx, agent)
    expect(agent.session.snapshotEvents().filter(e => e.type === 'system/message')).toHaveLength(2)
    expectPrefixExtension(adapter.requests[2]!, adapter.requests[3]!)
  })

  it('on an in-history route a series start folds a prompt change into node 0 and empties later system nodes', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two'), textResponse('three'), textResponse('four')])
    adapter.systemPromptUpdate = 'in-history'
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
    let startSeries = false
    ctx.on('agent/pre-step', async (_payload, next) => {
      const decision = await next()
      return decision.kind === 'enter' && startSeries ? { ...decision, startsRequestSeries: true } : decision
    })

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

    // Series start with only node 0: the change rewrites node 0 (the cache is lost anyway).
    let disposeSection = ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'new guidance' })
    startSeries = true
    send(agent, 'second')
    await waitForIdle(ctx, agent)
    let systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(2)
    expect(systemNodes[1]?.surfaceOp).toEqual({ op: 'replace', startSeq: systemNodes[0]?.seq, endSeq: systemNodes[0]?.seq })
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])

    // A continuing series appends a mid-history node…
    startSeries = false
    disposeSection()
    disposeSection = ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'newer guidance' })
    send(agent, 'third')
    await waitForIdle(ctx, agent)
    systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(3)
    expect(systemNodes[2]?.surfaceOp).toBe('append')

    // A broken series normalizes every active prompt version, preserving intervening messages.
    startSeries = true
    disposeSection()
    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'newest guidance' })
    send(agent, 'fourth')
    await waitForIdle(ctx, agent)
    systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(5)
    expect(systemNodes[3]?.data.message.content).toEqual([])
    expect(systemNodes[3]?.surfaceOp).toEqual({ op: 'replace', startSeq: systemNodes[2]?.seq, endSeq: systemNodes[2]?.seq })
    expect(agent.session.surface.nodes[0]).toBe(systemNodes[4]?.seq)
    const systemTexts = adapter.requests[3]!.messages.flatMap(message => message.role === 'system' ? [message.content[0]] : [])
    expect(systemTexts).toEqual([
      { type: 'text', text: expect.stringContaining('newest guidance') as unknown },
    ])
  })

  it('on an in-history route a compaction replace since the last request re-baselines node 0', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    adapter.systemPromptUpdate = 'in-history'
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

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

    // Node 0 is the system prompt; the compaction range starts after it.
    const nodes = agent.session.surface.nodes
    agent.session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: '[summary of turn 1]' }],
      source: { kind: 'plugin', plugin: 'test-compact' },
    }), {
      surfaceOp: { op: 'replace', startSeq: nodes[1]!, endSeq: nodes[2]! },
      sourceEventSeqs: [nodes[1]!, nodes[2]!],
    })
    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'new guidance' })
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    const systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(2)
    expect(systemNodes[1]?.surfaceOp).toEqual({ op: 'replace', startSeq: systemNodes[0]?.seq, endSeq: systemNodes[0]?.seq })
    expect(adapter.requests[1]!.messages.map(message => message.role)).toEqual(['system', 'user', 'user'])
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial', 'series'])
  })

  it('on an in-history route a tool-schema change re-baselines node 0 together with the changed header', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    adapter.systemPromptUpdate = 'in-history'
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

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

    registerEcho(ctx)
    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'new guidance' })
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    const headers = agent.session.snapshotEvents().filter(e => e.type === 'request/header')
    expect(headers.map(event => [event.data.reason, event.data.startsSeries])).toEqual([['initial', undefined], ['change', true]])
    const systemNodes = agent.session.snapshotEvents().filter(e => e.type === 'system/message')
    expect(systemNodes).toHaveLength(2)
    expect(systemNodes[1]?.surfaceOp).toEqual({ op: 'replace', startSeq: systemNodes[0]?.seq, endSeq: systemNodes[0]?.seq })
    expect(adapter.requests[1]!.messages.filter(message => message.role === 'system')).toHaveLength(1)
  })

  it('an inject() during the agent/request waterfall joins the NEXT request (the step/start boundary)', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    let injected = false
    ctx.on('agent/request', async (_payload, next) => {
      if (!injected) {
        injected = true
        agent.inject(createUserMessage({ content: [{ type: 'text', text: '[late context]' }], source: { kind: 'plugin', plugin: 'test' } }))
      }
      return next()
    })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    const first = adapter.requests[0]!
    // The inject landed in the log after the boundary: not in THIS request…
    expect(first.messages.some(m => m.content.some(b => b.type === 'text' && b.text.includes('[late context]')))).toBe(false)
    expect(agent.session.snapshotEvents().some(e => e.type === 'user/message' && e.data.source.kind === 'plugin')).toBe(true)

    send(agent, 'second')
    await waitForIdle(ctx, agent)
    // …but in the next one, at its logged position.
    const second = adapter.requests[1]!
    expect(second.messages.some(m => m.content.some(b => b.type === 'text' && b.text.includes('[late context]')))).toBe(true)
  })

  it('a mutation attempt on the frozen request content throws into the step (loud, not silent)', async () => {
    const adapter = new MockAdapter([textResponse('one')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    ctx.on('llm/stream', (options, next) => {
      // The historical failure mode this design kills: a listener rewriting
      // request content in place. The freeze turns it into a loud error.
      options.messages.push(createUserMessage({
        content: [{ type: 'text', text: 'sneaky' }],
        source: { kind: 'plugin', plugin: 'test' },
      }))
      return next()
    })

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

    const turnEnd = agent.session.snapshotEvents().findLast(event => event.type === 'turn/end')
    expect(turnEnd).toMatchObject({ data: { reason: { kind: 'error' } } })
    if (turnEnd?.type !== 'turn/end' || turnEnd.data.reason.kind !== 'error') throw new Error()
    expect(turnEnd.data.reason.error.message).toMatch(/not extensible|frozen|read only|readonly/i)
  })

  it('a fresh loop instance over a seeded log anchors with a resume snapshot and stays cache-aligned', async () => {
    const adapter = new MockAdapter([textResponse('one')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('gen1'), { provider: 'mock', model: 'mock' })
    send(agent, 'first')
    await waitForIdle(ctx, agent)

    // Second generation: a new agent whose session is seeded with the first
    // one's full log (the resume/fork path).
    const adapter2 = new MockAdapter([textResponse('two')])
    const ctx2 = await harness(adapter2)
    const handle = await ctx2.agents.create({
      sessionId: SessionId('gen2-session'),
      seed: agent.session.snapshotEvents(),
      agentOptions: { provider: 'mock', model: 'mock' },
    })
    const agent2 = handle.agent
    send(agent2, 'second')
    await waitForIdle(ctx2, agent2)

    const snapshots = agent2.session.snapshotEvents().filter(e => e.type === 'request/header')
    expect(snapshots).toHaveLength(2)
    expect(snapshots[1]?.data.reason).toBe('resume')
    // Identical header and an unchanged system node across the restart: byte-identical continuation.
    expect(adapter2.requests[0]!.messages[0]).toEqual(adapter.requests[0]!.messages[0])
    expect(agent2.session.snapshotEvents().filter(event => event.type === 'system/message')).toHaveLength(1)
    expectPrefixExtension(adapter.requests[0]!, adapter2.requests[0]!)
  })

  it('a delegating listener cannot mutate the seed through next() — the fold stays log-true', async () => {
    const adapter = new MockAdapter([textResponse('one'), textResponse('two')])
    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()
      // next() resolves the SAME frozen seed — in-place shaping after
      // delegation is unrepresentable, so a "mutate what next() returned"
      // listener cannot desync the log from the request (nor reach the
      // session's cached header fold, which is deep-cloned away and itself
      // frozen).
      expect(Object.isFrozen(config)).toBe(true)
      expect(() => { (config as { temperature?: number }).temperature = 0.9 }).toThrow(TypeError)
      return config
    })

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

    // The second turn reuses the same series and header; the session's own
    // fold remains immutable state.
    expect(agent.session.snapshotEvents().flatMap(event =>
      event.type === 'request/header' ? [event.data.reason] : [])).toEqual(['initial'])
    expect(Object.isFrozen(agent.session.requestHeader())).toBe(true)
    expect(adapter.requests[1]!.temperature).toBeUndefined()
  })

  it('THEOREM: every request rebuilds byte-equal from the session log alone', async () => {
    const adapter = new MockAdapter([
      toolCallResponse('c1', 'echo', { text: 'one' }, 'calling'),
      textResponse('done'),
      textResponse('after change'),
    ])
    const ctx = await harness(adapter)
    registerEcho(ctx)
    const agent = await ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })

    send(agent, 'go')
    await waitForIdle(ctx, agent)
    ctx.systemPrompt.section({ name: 'extra', order: 2, text: 'now with guidance' })
    ctx.on('agent/request', async (_payload, next) => ({
      ...await next(), temperature: 0.5, maxTokens: 99, stop: ['<END>'],
    }))
    send(agent, 'again')
    await waitForIdle(ctx, agent)

    expect(adapter.requests).toHaveLength(3)
    const events = agent.session.snapshotEvents()
    const stepStarts = events.filter(e => e.type === 'step/start')
    expect(stepStarts).toHaveLength(3)

    adapter.requests.forEach((request, index) => {
      const stepStart = stepStarts[index]!
      const settlement = events.find(e =>
        (e.type === 'assistant/message' || e.type === 'assistant/attempt')
        && e.data.turn === stepStart.data.turn
        && e.data.step === stepStart.data.step,
      )!
      // Messages: the entered batch is logged after step/start, so rebuild the
      // complete dispatch prefix through a completely fresh Session.
      const rebuilt = Session.create(SessionId(`rebuild-${index}`), structuredClone(events.slice(0, settlement.seq)))
      expect(structuredClone(request.messages)).toEqual(rebuilt.deriveMessages())

      // Header: the latest request/header snapshot up to this step's dispatch
      // (its header event sits between step/start and the Assistant settlement).
      const header = foldRequestHeader(events.slice(0, settlement.seq))!
      expect(request.model).toBe(header.config.model)
      expect(request.reasoningEffort).toBe(header.config.reasoningEffort)
      expect(request.system).toBeUndefined()
      expect(structuredClone(request.tools ?? [])).toEqual(structuredClone(header.tools ?? []))
      expect(request.temperature).toBe(header.config.temperature)
      expect(request.maxTokens).toBe(header.config.maxTokens)
      expect(request.stop).toEqual(header.config.stop)
    })
  })
})

describe('request/context capacity records', () => {
  /** Adapter advertising a per-model capacity, keyed by model id. */
  function capacityAdapter(windows: Record<string, number>, script: StreamChunk[][]): MockAdapter {
    return new class extends MockAdapter {
      override resolveModel(provider: string, model: string): Promise<LlmResolvedModelInfo> {
        const contextWindow = windows[model]
        return Promise.resolve({
          provider,
          id: model,
          name: model,
          ...contextWindow === undefined ? {} : { context: { contextWindow } },
        })
      }
    }(script)
  }

  it('records capacity once and skips it while the route is unchanged', async () => {
    const adapter = capacityAdapter({ mock: 128_000 }, [textResponse('a'), textResponse('b')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('capacity-dedup'), { provider: 'mock', model: 'mock' })

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

    const records = agent.session.snapshotEvents().filter(event => event.type === 'request/context')
    expect(records).toHaveLength(1)
    expect(records[0]?.data).toEqual({ provider: 'mock', model: 'mock', contextWindow: 128_000 })
    // Log-only: not a SurfaceEventType, so it can never reach a model request
    // (the type system rejects a surfaceOp here; the session invariant also
    // requires the record to sit inside its open turn).
    expect(agent.session.surface.nodes).not.toContain(records[0]?.seq)
  })

  it('records a second capacity when the route changes mid-session', async () => {
    const adapter = capacityAdapter(
      { small: 64_000, large: 256_000 },
      [textResponse('a'), textResponse('b')],
    )
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('capacity-switch'), { provider: 'mock', model: 'small' })

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    ctx.on('agent/request', ({ agent: subject }, next) => subject === agent
      ? Promise.resolve({ provider: 'mock', model: 'large' })
      : next())
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    expect(agent.session.snapshotEvents()
      .filter(event => event.type === 'request/context')
      .map(event => event.data.contextWindow)).toEqual([64_000, 256_000])
  })

  it('records and deduplicates a route whose adapter advertises no capacity', async () => {
    const ctx = await harness(new MockAdapter([textResponse('a'), textResponse('b')]))
    const agent = await ctx.agentLoop.create(SessionId('capacity-absent'), { provider: 'mock', model: 'mock' })
    send(agent, 'first')
    await waitForIdle(ctx, agent)
    send(agent, 'second')
    await waitForIdle(ctx, agent)
    expect(agent.session.snapshotEvents()
      .filter(event => event.type === 'request/context')
      .map(event => event.data)).toEqual([{ provider: 'mock', model: 'mock' }])
  })

  it('clears a previous capacity when the next route advertises none', async () => {
    const adapter = capacityAdapter({ known: 64_000 }, [textResponse('a'), textResponse('b')])
    const ctx = await harness(adapter)
    const agent = await ctx.agentLoop.create(SessionId('capacity-clear'), { provider: 'mock', model: 'known' })
    let model = 'known'
    ctx.on('agent/request', ({ agent: subject }, next) => subject === agent
      ? Promise.resolve({ provider: 'mock', model })
      : next())

    send(agent, 'first')
    await waitForIdle(ctx, agent)
    model = 'unknown'
    send(agent, 'second')
    await waitForIdle(ctx, agent)

    expect(agent.session.snapshotEvents()
      .filter(event => event.type === 'request/context')
      .map(event => event.data)).toEqual([
      { provider: 'mock', model: 'known', contextWindow: 64_000 },
      { provider: 'mock', model: 'unknown' },
    ])
  })
})