From b721c94dcbae8167d3fc1ce36ccf2b506f2ff9e9 Mon Sep 17 00:00:00 2001
From: bchanot <bchanot@gmail.fr>
Date: Thu, 8 Oct 2026 16:26:57 +0200
Subject: [PATCH] =?UTF-8?q?feat(mods):=20model-router=20mod,=20wave=201-A?=
 =?UTF-8?q?=20=E2=80=94=20per-request=20model/effort=20routing?=
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Function-hooks plugin under mods/model-router: routes effort (and, behind a
flag, the model) of every main-loop request, sets built-in sub-agents' model
at spawn with full ids, answers Skill(effort-*) itself (single writer, no
pairing rule), exposes the route tool and /route, validates the optional
~/.claude/model-router.json. 11 plugin tests, validate + tsc clean.
Contract .claude/tasks/contracts/2026-10-08-model-router-w1a-1533.md.
---
 mods/model-router/.claude-plugin/plugin.json |   8 +
 mods/model-router/hooks/hooks.json           |   1 +
 mods/model-router/hooks/register.test.ts     | 202 +++++
 mods/model-router/hooks/register.ts          | 834 +++++++++++++++++++
 4 files changed, 1045 insertions(+)
 create mode 100644 mods/model-router/.claude-plugin/plugin.json
 create mode 100644 mods/model-router/hooks/hooks.json
 create mode 100644 mods/model-router/hooks/register.test.ts
 create mode 100644 mods/model-router/hooks/register.ts

diff --git a/mods/model-router/.claude-plugin/plugin.json b/mods/model-router/.claude-plugin/plugin.json
new file mode 100644
index 0000000..548269f
--- /dev/null
+++ b/mods/model-router/.claude-plugin/plugin.json
@@ -0,0 +1,8 @@
+{
+  "name": "model-router",
+  "version": "0.1.0",
+  "description": "Routes every model request (main loop and sub-agents) to the model and effort its phase deserves, from a user config, a route tool, /route and skill or prompt rules.",
+  "author": {
+    "name": "bchanot"
+  }
+}
diff --git a/mods/model-router/hooks/hooks.json b/mods/model-router/hooks/hooks.json
new file mode 100644
index 0000000..5fcba7c
--- /dev/null
+++ b/mods/model-router/hooks/hooks.json
@@ -0,0 +1 @@
+{ "modules": ["./register.ts"] }
diff --git a/mods/model-router/hooks/register.test.ts b/mods/model-router/hooks/register.test.ts
new file mode 100644
index 0000000..5c0c634
--- /dev/null
+++ b/mods/model-router/hooks/register.test.ts
@@ -0,0 +1,202 @@
+import { test, expect } from 'claude-code/testing'
+import type { Engine } from 'claude-code/testing'
+import type { On } from 'claude-code'
+
+/** Fires session.start so the mod loads its config and registers /route. */
+async function boot($: Engine, on: On): Promise<void> {
+  on('session.start', ($, e) => ({ cwd: e.cwd }))
+  await $.session.start({ cwd: '/tmp', surface: null, isInteractive: false })
+}
+
+/** Runs `/route <args>` as the user would type it. */
+async function route($: Engine, args: string): Promise<string> {
+  const out = await $.command.run({
+    command: 'route',
+    args,
+    origin: { kind: 'composer' },
+    presentation: { isFullscreen: false, columns: 80 },
+  })
+  return out.text ?? ''
+}
+
+/** The `main:` line of `show`: the phases listing holds every id and level. */
+function mainLine(text: string): string {
+  return text.split('\n').find(l => l.startsWith('main:')) ?? ''
+}
+
+const spawnInput = (model?: string) => ({
+  tool_use_id: 't1',
+  prompt: 'x',
+  description: 'd',
+  subagentType: 'Explore',
+  provider: { plugin: 'engine', tier: 'core' as const },
+  parentModel: 'claude-fable-5-1',
+  background: false,
+  fork: false,
+  ...(model === undefined ? {} : { model }),
+})
+
+test('Skill(effort-low) is answered without next, route shows low', async (
+  $, on) => {
+  let reached = false
+  on('tool.call', { tool: 'Skill' }, () => {
+    reached = true
+    return { result: { success: true, commandName: 'bottom' } }
+  })
+  await boot($, on)
+  const out = await $.tool.call({ tool: 'Skill', skill: 'effort-low' })
+  expect(out).toMatchObject({
+    result: { success: true, commandName: 'effort-low' },
+  })
+  expect(reached).toBe(false)
+  const line = mainLine(await route($, 'show'))
+  expect(line).toContain('skill effort-low')
+  expect(line).toContain('effort low')
+})
+
+test('route tool with phase orchestrate sets medium on main', async (
+  $, on) => {
+  await boot($, on)
+  const out = await $.tool.call({
+    tool: 'mcp__model-router__route',
+    phase: 'orchestrate',
+  })
+  expect(out).toHaveProperty('result')
+  const line = mainLine(await route($, 'show'))
+  expect(line).toContain('model orchestrate')
+  expect(line).toContain('effort medium')
+})
+
+test('/route clear drops the route', async ($, on) => {
+  await boot($, on)
+  await $.tool.call({ tool: 'mcp__model-router__route', phase: 'plan' })
+  expect(mainLine(await route($, 'show'))).toContain('model plan')
+  await route($, 'clear')
+  expect(mainLine(await route($, 'show'))).toContain('session defaults')
+})
+
+test('/route bogus names the phases', async ($, on) => {
+  await boot($, on)
+  const text = await route($, 'bogus')
+  expect(text).toContain('unknown')
+  for (const phase of ['plan', 'judge', 'explore', 'mechanical']) {
+    expect(text).toContain(phase)
+  }
+})
+
+test('ultrathink in a prompt sets escalate on main', async ($, on) => {
+  on('prompt.submit', ($, e) => ({ text: e.text }))
+  await boot($, on)
+  await $.prompt.submit({
+    text: 'ultrathink please',
+    wait: false,
+    origin: { kind: 'composer' },
+  })
+  expect(mainLine(await route($, 'show'))).toContain('prompt escalate')
+})
+
+test('/route model: alias resolved, id passed, typo refused', async (
+  $, on) => {
+  await boot($, on)
+  const alias = await route($, 'model=sonnet')
+  expect(mainLine(alias)).toContain('claude-sonnet-5-5')
+  const id = await route($, 'model=claude-x-9')
+  expect(mainLine(id)).toContain('claude-x-9')
+  expect(await route($, 'model=sonet')).toContain('unknown')
+})
+
+test('Explore spawn gets the sonnet id; an explicit model wins', async (
+  $, on) => {
+  const seen: (string | undefined)[] = []
+  on('agent.spawn', ($, e) => {
+    seen.push(e.model)
+    return { model: e.model ?? e.parentModel, agentId: 'a1' }
+  })
+  await boot($, on)
+  await $.agent.spawn(spawnInput())
+  await $.agent.spawn(spawnInput('opus'))
+  expect(seen).toEqual(['claude-sonnet-5-5', 'opus'])
+})
+
+test('a sticky /route wins over a model-declared route', async ($, on) => {
+  await boot($, on)
+  await route($, 'judge')
+  const out = await $.tool.call({
+    tool: 'mcp__model-router__route',
+    phase: 'mechanical',
+  })
+  expect(JSON.stringify(out)).toContain('sticky')
+  expect(mainLine(await route($, 'show'))).toContain('user judge')
+})
+
+test('/route off makes the route tool a no-op', async ($, on) => {
+  await boot($, on)
+  await route($, 'off')
+  const out = await $.tool.call({
+    tool: 'mcp__model-router__route',
+    phase: 'plan',
+  })
+  expect(JSON.stringify(out)).toContain('off')
+  expect(mainLine(await route($, 'show'))).toContain('session defaults')
+})
+
+type Seen = { model: string; effort: unknown }
+
+/** A bottom turn.step hook that records what reaches the model. */
+function recordSteps(on: On, seen: Seen[]): void {
+  on('turn.step', async function* ($, e) {
+    seen.push({ model: e.model, effort: e.effort })
+    return {
+      turnId: e.turnId,
+      index: e.index,
+      answer: '',
+      toolUses: [],
+      stopReason: 'end_turn' as const,
+      usage: null,
+    }
+  })
+}
+
+const stepInput = (agentId?: string) => ({
+  turnId: 'u1',
+  index: 0,
+  model: 'claude-fable-5-1',
+  effort: 'low' as const,
+  messageCount: 1,
+  ...(agentId === undefined ? {} : { agentId }),
+})
+
+/** Streams one turn.step to its end; the hooks run as the chunks flow. */
+async function runStep(
+  $: Engine,
+  input: ReturnType<typeof stepInput>,
+): Promise<void> {
+  const stream = $.turn.step(input)
+  for await (const _chunk of stream) {
+    // chunks are not under test
+  }
+  await stream.result
+}
+
+test('the main step carries the routed effort, model untouched', async (
+  $, on) => {
+  const seen: Seen[] = []
+  recordSteps(on, seen)
+  await boot($, on)
+  await $.tool.call({ tool: 'mcp__model-router__route', phase: 'plan' })
+  await runStep($, stepInput())
+  expect(seen).toEqual([{ model: 'claude-fable-5-1', effort: 'xhigh' }])
+})
+
+test('a tabled agent steps at its table effort', async ($, on) => {
+  const seen: Seen[] = []
+  recordSteps(on, seen)
+  on('agent.spawn', ($, e) => ({
+    model: e.model ?? e.parentModel,
+    agentId: 'a1',
+  }))
+  await boot($, on)
+  await $.agent.spawn(spawnInput())
+  await runStep($, stepInput('a1'))
+  expect(seen).toEqual([{ model: 'claude-fable-5-1', effort: 'medium' }])
+})
diff --git a/mods/model-router/hooks/register.ts b/mods/model-router/hooks/register.ts
new file mode 100644
index 0000000..9bf4ae4
--- /dev/null
+++ b/mods/model-router/hooks/register.ts
@@ -0,0 +1,834 @@
+// model-router: routes each model request (main loop and sub-agents) to the
+// model and effort its phase deserves. Phases, agents, skills and prompt
+// rules come from DEFAULT_CONFIG, overridable by ~/.claude/model-router.json.
+// One writer per concern: the Agent tool's own params are never rewritten,
+// the spawn hook sets an agent's model once, turn.step sets efforts.
+import type { EngineInterface, On, Register, TurnStepInput } from 'claude-code'
+
+type Api = EngineInterface
+type Level = 'low' | 'medium' | 'high' | 'xhigh' | 'max'
+type Route = { model?: string; effort?: Level } // model: alias or full id
+type PromptRule = { pattern: string; phase: string }
+type Config = {
+  models: Record<string, string> // alias -> full id
+  windows: Record<string, number> // full id -> context window (tokens)
+  phases: Record<string, Route>
+  agents: Record<string, string> // built-in subagentType -> phase
+  skills: Record<string, string> // skill name -> phase
+  prompt: PromptRule[]
+  mainModelSwitch: boolean
+  verbose: boolean
+  spinner: boolean
+}
+type Rule = { re: RegExp; phase: string }
+type Source = 'user' | 'model' | 'skill' | 'prompt' | 'slash'
+type Routed = { phase: string; route: Route; source: Source }
+type Loop = {
+  effort?: Level
+  model?: string // routed by the table or an in-agent call
+  spawnModel: string // the engine's model at spawn
+  frozen: boolean // fork or workflow agent: never re-modelled
+  explicitModel: boolean // Agent call gave a model: axis frozen
+  explicitEffort: boolean // Agent call gave an effort: axis frozen
+}
+type State = {
+  cfg: Config
+  rules: Rule[]
+  source: string // 'defaults' or the override path
+  userMain: Routed | null // /route by the user, sticky until /route clear
+  turnMain: Routed | null // tool, skill, slash, prompt; dropped at turn end
+  pendingPrompt: Routed | null // typed mid-turn, promoted next turn
+  loops: Map<string, Loop> // agentId -> that loop's routing
+  explicitEffort: Map<string, Level> // Agent tool_use_id -> effort param
+  skillCalls: number // Skill tool calls in flight
+  off: boolean // /route off: every hook passes through
+  lastMain: string // "model/effort" of the last main step (spinner)
+  windowWarned: boolean // context-window warning already logged this turn
+}
+type Log = (text: string) => void
+type StepIn = Readonly<TurnStepInput>
+type Plan = { model: string; effort: StepIn['effort'] }
+type RouteInput = {
+  phase?: unknown
+  effort?: unknown
+  clear?: unknown
+  agentId?: string
+}
+type Picked = { phase: string; route: Route }
+
+const LEVELS: readonly Level[] = ['low', 'medium', 'high', 'xhigh', 'max']
+const MODEL_ID = /^claude-[a-z0-9.-]+$/
+const TOOL = 'mcp__model-router__route'
+const EFFORT_SKILL = /^effort-(low|medium|high|xhigh|max)$/
+const OVERRIDE = '.claude/model-router.json'
+const HAIKU = 'claude-haiku'
+
+const DEFAULT_CONFIG: Config = {
+  models: {
+    haiku: 'claude-haiku-4-5-20251001',
+    sonnet: 'claude-sonnet-5-5',
+    opus: 'claude-opus-5-5',
+    fable: 'claude-fable-5-1',
+  },
+  windows: { 'claude-haiku-4-5-20251001': 200000 },
+  phases: {
+    plan: { effort: 'xhigh' },
+    reflect: { effort: 'high' },
+    orchestrate: { effort: 'medium' },
+    escalate: { effort: 'max' },
+    judge: { model: 'opus', effort: 'xhigh' },
+    implement: { model: 'sonnet', effort: 'medium' },
+    write: { model: 'sonnet', effort: 'medium' },
+    verify: { model: 'sonnet', effort: 'xhigh' },
+    explore: { model: 'sonnet', effort: 'medium' },
+    mechanical: { model: 'haiku', effort: 'low' },
+  },
+  // Built-ins only: repo agents keep their frontmatter pin (wave 2).
+  agents: { Explore: 'explore', Plan: 'judge' },
+  skills: {},
+  prompt: [{ pattern: '\\bultrathink\\b', phase: 'escalate' }],
+  mainModelSwitch: false,
+  verbose: false,
+  spinner: true,
+}
+
+// ---- config ----------------------------------------------------------
+
+const isRecord = (v: unknown): v is Record<string, unknown> =>
+  typeof v === 'object' && v !== null && !Array.isArray(v)
+const isLevel = (v: unknown): v is Level => LEVELS.some(l => l === v)
+const isFullId = (v: unknown): v is string =>
+  typeof v === 'string' && MODEL_ID.test(v)
+const hasKey = (table: object, key: string): boolean =>
+  Object.hasOwn(table, key)
+
+function isModelName(models: Record<string, string>, v: unknown): v is string {
+  return typeof v === 'string' && (hasKey(models, v) || isFullId(v))
+}
+
+function resolveModel(cfg: Config, name: string): string {
+  return hasKey(cfg.models, name) ? (cfg.models[name] ?? name) : name
+}
+
+function phaseRoute(cfg: Config, phase: string): Route | undefined {
+  return hasKey(cfg.phases, phase) ? cfg.phases[phase] : undefined
+}
+
+/** Merges a user table over a default one, dropping invalid entries. */
+function mergeTable<T>(
+  base: Record<string, T>,
+  user: unknown,
+  name: string,
+  accept: (key: string, value: unknown) => T | undefined,
+  log: Log,
+): Record<string, T> {
+  const out = { ...base }
+  if (!isRecord(user)) return out
+  for (const [key, value] of Object.entries(user)) {
+    const ok = key === '__proto__' ? undefined : accept(key, value)
+    if (ok === undefined) log(`model-router: config ${name}.${key} ignored`)
+    else out[key] = ok
+  }
+  return out
+}
+
+const acceptModel = (_key: string, v: unknown): string | undefined =>
+  isFullId(v) ? v : undefined
+
+const acceptWindow = (key: string, v: unknown): number | undefined =>
+  isFullId(key) && typeof v === 'number' && Number.isInteger(v) && v > 0
+    ? v
+    : undefined
+
+function acceptPhase(models: Record<string, string>) {
+  return (_key: string, v: unknown): Route | undefined => {
+    if (!isRecord(v)) return undefined
+    const route: Route = {}
+    if (v.effort !== undefined) {
+      if (!isLevel(v.effort)) return undefined
+      route.effort = v.effort
+    }
+    if (v.model !== undefined) {
+      if (!isModelName(models, v.model)) return undefined
+      route.model = v.model
+    }
+    return route.effort || route.model ? route : undefined
+  }
+}
+
+function acceptPhaseRef(phases: Record<string, Route>) {
+  return (_key: string, v: unknown): string | undefined =>
+    typeof v === 'string' && hasKey(phases, v) ? v : undefined
+}
+
+function compiles(pattern: string): boolean {
+  try {
+    new RegExp(pattern, 'i')
+    return true
+  } catch {
+    return false
+  }
+}
+
+function acceptRule(phases: Record<string, Route>, v: unknown) {
+  if (!isRecord(v)) return undefined
+  const { pattern, phase } = v
+  if (typeof pattern !== 'string' || typeof phase !== 'string') return undefined
+  return hasKey(phases, phase) && compiles(pattern)
+    ? { pattern, phase }
+    : undefined
+}
+
+/** A user prompt array replaces the default rules; bad rules are dropped. */
+function mergePrompt(
+  base: PromptRule[],
+  user: unknown,
+  phases: Record<string, Route>,
+  log: Log,
+): PromptRule[] {
+  if (!Array.isArray(user)) return base
+  const rules: PromptRule[] = []
+  for (const item of user as unknown[]) {
+    const rule = acceptRule(phases, item)
+    if (rule) rules.push(rule)
+    else log('model-router: config prompt rule ignored')
+  }
+  return rules
+}
+
+const pickBool = (v: unknown, fallback: boolean): boolean =>
+  typeof v === 'boolean' ? v : fallback
+
+/** Defaults overlaid with the user's entries, each validated first. */
+function mergeConfig(user: unknown, log: Log): Config {
+  const base = structuredClone(DEFAULT_CONFIG)
+  if (!isRecord(user)) return base
+  const models = mergeTable(
+    base.models, user.models, 'models', acceptModel, log)
+  const phases = mergeTable(
+    base.phases, user.phases, 'phases', acceptPhase(models), log)
+  const ref = acceptPhaseRef(phases)
+  return {
+    models,
+    windows: mergeTable(
+      base.windows, user.windows, 'windows', acceptWindow, log),
+    phases,
+    agents: mergeTable(base.agents, user.agents, 'agents', ref, log),
+    skills: mergeTable(base.skills, user.skills, 'skills', ref, log),
+    prompt: mergePrompt(base.prompt, user.prompt, phases, log),
+    mainModelSwitch: pickBool(user.mainModelSwitch, base.mainModelSwitch),
+    verbose: pickBool(user.verbose, base.verbose),
+    spinner: pickBool(user.spinner, base.spinner),
+  }
+}
+
+async function readOverride(
+  $: Api,
+  log: Log,
+): Promise<{ path: string; data: unknown } | undefined> {
+  try {
+    const home = await $.env.get('HOME')
+    if (!home) return undefined
+    const path = `${home}/${OVERRIDE}`
+    if (!(await $.fs.exists(path))) return undefined
+    return { path, data: JSON.parse(await $.fs.read(path)) }
+  } catch (err) {
+    log(`model-router: ${OVERRIDE} unreadable (${String(err)}); defaults`)
+    return undefined
+  }
+}
+
+/** Never throws; a failed read or parse leaves the defaults. */
+async function loadConfig(
+  $: Api,
+  log: Log,
+): Promise<{ cfg: Config; source: string }> {
+  const found = await readOverride($, log)
+  if (!found) return { cfg: mergeConfig(undefined, log), source: 'defaults' }
+  return { cfg: mergeConfig(found.data, log), source: found.path }
+}
+
+function compileRules(cfg: Config): Rule[] {
+  return cfg.prompt.map(r => ({
+    re: new RegExp(r.pattern, 'i'),
+    phase: r.phase,
+  }))
+}
+
+// ---- state -----------------------------------------------------------
+
+function newState(cfg: Config, source: string): State {
+  return {
+    cfg,
+    rules: compileRules(cfg),
+    source,
+    userMain: null,
+    turnMain: null,
+    pendingPrompt: null,
+    loops: new Map(),
+    explicitEffort: new Map(),
+    skillCalls: 0,
+    off: false,
+    lastMain: '',
+    windowWarned: false,
+  }
+}
+
+/** The main loop's effective route: user /route > latest turn route. */
+const mainRoute = (st: State): Routed | null => st.userMain ?? st.turnMain
+
+function loopOf(st: State, agentId: string): Loop {
+  const known = st.loops.get(agentId)
+  if (known) return known
+  const fresh: Loop = {
+    spawnModel: '',
+    frozen: false,
+    explicitModel: false,
+    explicitEffort: false,
+  }
+  st.loops.set(agentId, fresh)
+  return fresh
+}
+
+/** Writes a route on an agent loop, never on an axis given explicitly. */
+function writeLoop(loop: Loop, route: Route): void {
+  if (!loop.explicitEffort) loop.effort = route.effort
+  if (!loop.explicitModel) loop.model = route.model
+}
+
+function clearRoutes(st: State): void {
+  st.userMain = null
+  st.turnMain = null
+  st.pendingPrompt = null
+}
+
+// ---- text ------------------------------------------------------------
+
+const modelText = (cfg: Config, model: string | undefined): string =>
+  model === undefined ? '-' : resolveModel(cfg, model)
+
+function mainText(st: State): string {
+  const r = mainRoute(st)
+  if (!r) return 'main: session defaults'
+  const model = modelText(st.cfg, r.route.model)
+  return `main: ${r.source} ${r.phase} · model ${model} · effort ${
+    r.route.effort ?? '-'}`
+}
+
+function phasesText(cfg: Config): string {
+  const entries = Object.entries(cfg.phases).map(([name, r]) =>
+    `${name}=${r.model ? resolveModel(cfg, r.model) : 'session'}/${
+      r.effort ?? 'session'}`)
+  return `phases: ${entries.join(' ')}`
+}
+
+function show(st: State): string {
+  const c = st.cfg
+  const flag = (b: boolean) => (b ? 'on' : 'off')
+  return [
+    mainText(st),
+    `router: ${st.off ? 'off' : 'on'} · switch: ${flag(c.mainModelSwitch)} · ` +
+      `verbose: ${flag(c.verbose)} · spinner: ${flag(c.spinner)}`,
+    `live loops: ${st.loops.size}`,
+    phasesText(c),
+    `config: ${st.source}`,
+  ].join('\n')
+}
+
+function statusLine(st: State): string {
+  const r = mainRoute(st)
+  const now = st.off ? 'off' : r ? `${r.source} ${r.phase}` : 'session defaults'
+  return `route: ${now}${st.cfg.mainModelSwitch ? ' · switch on' : ''}`
+}
+
+const refresh = ($: Api, st: State): void => $.ui.status(statusLine(st))
+
+function vlog($: Api, st: State, text: string): void {
+  if (st.cfg.verbose) $.ui.log(text)
+}
+
+const unknownText = (cfg: Config, token: string): string =>
+  `unknown token "${token}"; phases: ${Object.keys(cfg.phases).join(' ')}; ` +
+  `levels: ${LEVELS.join(' ')}; models: ${Object.keys(cfg.models).join(' ')} ` +
+  'or a full claude-* id'
+
+// ---- /route command --------------------------------------------------
+
+/** One token: `model=x`, `effort=y`, a bare alias, id or level. */
+function applyToken(cfg: Config, route: Route, token: string): boolean {
+  const eq = token.indexOf('=')
+  const key = eq < 0 ? '' : token.slice(0, eq)
+  const value = eq < 0 ? token : token.slice(eq + 1)
+  if (key !== '' && key !== 'model' && key !== 'effort') return false
+  if (key !== 'model' && isLevel(value)) route.effort = value
+  else if (key !== 'effort' && isModelName(cfg.models, value)) {
+    route.model = value
+  } else return false
+  return true
+}
+
+function parseRoute(cfg: Config, args: string): Routed | string {
+  const words = args.trim().split(/\s+/)
+  const only = words.length === 1 ? words[0] : undefined
+  const named = only === undefined ? undefined : phaseRoute(cfg, only)
+  if (only !== undefined && named) {
+    return { phase: only, route: { ...named }, source: 'user' }
+  }
+  const route: Route = {}
+  for (const word of words) {
+    if (!applyToken(cfg, route, word)) return unknownText(cfg, word)
+  }
+  return { phase: 'custom', route, source: 'user' }
+}
+
+function toggle(st: State, what: string, arg: string | undefined): string {
+  if (arg !== 'on' && arg !== 'off') return `usage: /route ${what} on|off`
+  if (what === 'switch') st.cfg.mainModelSwitch = arg === 'on'
+  else st.cfg.verbose = arg === 'on'
+  return show(st)
+}
+
+function setUserRoute($: Api, st: State, args: string): string {
+  const parsed = parseRoute(st.cfg, args)
+  if (typeof parsed === 'string') return parsed
+  st.userMain = parsed
+  refresh($, st)
+  return show(st)
+}
+
+/** (Re)loads the config into the state and re-registers the route tool. */
+async function reloadConfig($: Api, st: State): Promise<void> {
+  const loaded = await loadConfig($, text => $.ui.log(text))
+  st.cfg = loaded.cfg
+  st.rules = compileRules(loaded.cfg)
+  st.source = loaded.source
+  await registerTool($, st)
+}
+
+async function handleCommand($: Api, st: State, args: string): Promise<string> {
+  const [head = '', ...rest] = args.trim().split(/\s+/)
+  switch (head) {
+    case '':
+    case 'show':
+      return show(st)
+    case 'clear':
+      clearRoutes(st)
+      refresh($, st)
+      return 'route cleared\n' + show(st)
+    case 'on':
+    case 'off':
+      st.off = head === 'off'
+      refresh($, st)
+      return show(st)
+    case 'reload':
+      await reloadConfig($, st)
+      refresh($, st)
+      return 'config reloaded\n' + show(st)
+    case 'switch':
+    case 'verbose':
+      return toggle(st, head, rest[0])
+    default:
+      return setUserRoute($, st, args)
+  }
+}
+
+// ---- route tool ------------------------------------------------------
+
+async function registerTool($: Api, st: State): Promise<void> {
+  try {
+    await $.tool.register({
+      name: 'route',
+      description:
+        'Declare the phase of the work ahead so the next model requests ' +
+        'run at the effort (and model) it deserves. Call it before a span ' +
+        'of work changes nature (planning, orchestrating, mechanical ' +
+        'work). It acts on the calling loop only; no model choice here. ' +
+        `Phases: ${Object.keys(st.cfg.phases).join(', ')}.`,
+      inputSchema: {
+        type: 'object',
+        properties: {
+          phase: { type: 'string', enum: Object.keys(st.cfg.phases) },
+          effort: { type: 'string', enum: [...LEVELS] },
+          clear: { type: 'boolean', description: 'drop this loop\'s route' },
+        },
+      },
+    })
+  } catch (err) {
+    $.ui.log(`model-router: route tool not registered: ${String(err)}`)
+  }
+}
+
+async function registerCommand($: Api): Promise<void> {
+  try {
+    await $.command.register({
+      name: 'route',
+      description: 'model-router: show or set the model and effort route',
+      argumentHint:
+        '[show|clear|off|on|reload|<phase>|model=<alias|id> effort=<level>' +
+        '|switch on|off|verbose on|off]',
+      immediate: true,
+    })
+  } catch (err) {
+    $.ui.log(`model-router: /route not registered: ${String(err)}`)
+  }
+}
+
+function pickRoute(cfg: Config, phase: unknown, effort: unknown) {
+  const phases = Object.keys(cfg.phases).join(', ')
+  const named = typeof phase === 'string' ? phaseRoute(cfg, phase) : undefined
+  if (phase !== undefined && !named) {
+    return `unknown phase "${String(phase)}"; phases: ${phases}`
+  }
+  if (effort !== undefined && !isLevel(effort)) {
+    return `unknown effort "${String(effort)}"; levels: ${LEVELS.join(', ')}`
+  }
+  if (phase === undefined && effort === undefined) {
+    return `give a phase (${phases}), an effort, or clear`
+  }
+  const route: Route = { ...named, ...(isLevel(effort) ? { effort } : {}) }
+  const name = typeof phase === 'string' ? phase : `effort-${String(effort)}`
+  return { phase: name, route }
+}
+
+function applyRoute(st: State, agentId: string | undefined, p: Picked): void {
+  if (agentId === undefined) {
+    st.turnMain = { phase: p.phase, route: p.route, source: 'model' }
+  } else {
+    writeLoop(loopOf(st, agentId), p.route)
+  }
+}
+
+function clearLoop(st: State, agentId: string | undefined): string {
+  if (agentId === undefined) {
+    st.turnMain = null
+    return 'route cleared for main'
+  }
+  const loop = st.loops.get(agentId)
+  if (loop) writeLoop(loop, {})
+  return 'route cleared for this agent'
+}
+
+/** Truthful answer: states what the calling loop will actually do. */
+function routedText(st: State, agentId: string | undefined, p: Picked): string {
+  if (agentId === undefined && st.userMain) {
+    return `recorded ${p.phase} for this turn, but a sticky /route ` +
+      `${st.userMain.phase} is in force; it wins until /route clear`
+  }
+  const loop = agentId === undefined ? undefined : st.loops.get(agentId)
+  const effort = loop?.explicitEffort ? undefined : p.route.effort
+  const model = agentId === undefined && !st.cfg.mainModelSwitch
+    ? undefined
+    : loop?.explicitModel ? undefined : p.route.model
+  const modelNote = p.route.model && model === undefined ? ' (switch off)' : ''
+  return `routed ${agentId === undefined ? 'main' : 'this agent'} to ` +
+    `${p.phase}: effort ${effort ?? 'unchanged'}, model ` +
+    `${model === undefined ? 'unchanged' : resolveModel(st.cfg, model)}` +
+    modelNote
+}
+
+function handleRouteTool(st: State, e: RouteInput) {
+  if (st.off) {
+    return {
+      result: 'model-router is off (/route on to resume); nothing routed',
+    }
+  }
+  if (e.clear === true) return { result: clearLoop(st, e.agentId) }
+  const picked = pickRoute(st.cfg, e.phase, e.effort)
+  if (typeof picked === 'string') return { deny: picked }
+  applyRoute(st, e.agentId, picked)
+  return { result: routedText(st, e.agentId, picked) }
+}
+
+// ---- skills ----------------------------------------------------------
+
+const skillResult = (skill: string, line: string) => ({
+  result: { success: true, commandName: skill, status: 'inline' as const },
+  context: [line],
+})
+
+/** Answers Skill(effort-<l>) in place: one writer, the skill never loads. */
+function effortBridge(st: State, agentId: string | undefined, skill: string,
+  level: Level) {
+  if (agentId === undefined) {
+    const route = { ...st.turnMain?.route, effort: level }
+    st.turnMain = { phase: skill, route, source: 'skill' }
+    return skillResult(skill, st.userMain
+      ? `model-router: ${skill} recorded, but a sticky /route ` +
+        `${st.userMain.phase} is in force and wins until /route clear.`
+      : `model-router: effort → ${level} for this loop from the next ` +
+        `request on; the ${skill} skill text was not loaded.`)
+  }
+  const loop = loopOf(st, agentId)
+  if (loop.explicitEffort) {
+    return skillResult(skill, 'model-router: this agent was dispatched with ' +
+      'an explicit effort; the shift does not apply.')
+  }
+  loop.effort = level
+  return skillResult(skill, `model-router: effort → ${level} for this ` +
+    `loop from the next request on; the ${skill} skill text was not loaded.`)
+}
+
+/** A non-effort skill load: resets the loop's route, applies its table row. */
+function onSkillLoad(st: State, skill: string, agentId: string | undefined) {
+  const table = hasKey(st.cfg.skills, skill) ? st.cfg.skills[skill] : undefined
+  const route = table === undefined ? undefined : phaseRoute(st.cfg, table)
+  if (agentId === undefined) {
+    if (st.turnMain && st.turnMain.source !== 'prompt') st.turnMain = null
+    if (table !== undefined && route) {
+      st.turnMain = { phase: table, route, source: 'skill' }
+    }
+    return
+  }
+  const loop = route ? loopOf(st, agentId) : st.loops.get(agentId)
+  if (loop) writeLoop(loop, { effort: route?.effort })
+}
+
+/** A user-typed /effort-<l>: prepends one line, args ride in the text. */
+function slashEffort(st: State, skill: string, text: string) {
+  const level = EFFORT_SKILL.exec(skill)?.[1]
+  if (!isLevel(level)) return undefined
+  st.turnMain = { phase: skill, route: { effort: level }, source: 'slash' }
+  const line = st.userMain
+    ? `Effort ${level} recorded; the sticky /route ${st.userMain.phase} ` +
+      'wins until /route clear.'
+    : `Effort shifted to ${level} by model-router for this turn.`
+  return { text: line + '\n' + text }
+}
+
+// ---- agents ----------------------------------------------------------
+
+type SpawnIn = {
+  tool_use_id: string
+  subagentType: string
+  provider: { plugin: string }
+  model?: string
+  fork: boolean
+  workflow?: unknown
+}
+
+/**
+ * The table row of a built-in agent. Known limit: provider.plugin ===
+ * 'engine' is the best built-in test at spawn; a user agent named Explore
+ * in a foreign project also matches (wave 1: sonnet/medium on it).
+ */
+function spawnRoute(cfg: Config, e: SpawnIn, frozen: boolean) {
+  if (frozen || e.provider.plugin !== 'engine') return undefined
+  if (!hasKey(cfg.agents, e.subagentType)) return undefined
+  const phase = cfg.agents[e.subagentType]
+  return phase === undefined ? undefined : phaseRoute(cfg, phase)
+}
+
+function trackLoop(st: State, e: SpawnIn, started: {
+  model: string
+  agentId?: string
+}, route: Route | undefined, frozen: boolean): void {
+  const given = st.explicitEffort.get(e.tool_use_id)
+  st.explicitEffort.delete(e.tool_use_id)
+  if (started.agentId === undefined) return
+  st.loops.set(started.agentId, {
+    spawnModel: started.model,
+    frozen,
+    explicitModel: e.model !== undefined,
+    explicitEffort: given !== undefined,
+    effort: given ? undefined : route?.effort,
+  })
+}
+
+// ---- turn steps ------------------------------------------------------
+
+function agentPlan(st: State, e: StepIn): Plan {
+  const loop = e.agentId === undefined ? undefined : st.loops.get(e.agentId)
+  const wanted = loop?.model
+  const reroute = loop && wanted !== undefined && !loop.frozen &&
+    e.model === loop.spawnModel
+  return {
+    model: reroute ? resolveModel(st.cfg, wanted) : e.model,
+    effort: loop?.effort ?? e.effort,
+  }
+}
+
+/** True when the context still fits the target model's known window. */
+async function windowOk($: Api, st: State, id: string): Promise<boolean> {
+  const limit = hasKey(st.cfg.windows, id) ? st.cfg.windows[id] : undefined
+  if (limit === undefined) return true
+  let tokens: number | undefined
+  try {
+    tokens = (await $.session.usage()).context.tokens
+  } catch {
+    tokens = undefined
+  }
+  if (typeof tokens === 'number' && tokens < limit) return true
+  if (!st.windowWarned) {
+    st.windowWarned = true
+    $.ui.log(`model-router: no switch to ${id}: context not known to fit`)
+  }
+  return false
+}
+
+async function mainPlan($: Api, st: State, e: StepIn): Promise<Plan> {
+  const set = mainRoute(st)
+  const effort = set?.route.effort ?? e.effort
+  const wanted = set?.route.model
+  if (wanted === undefined || !st.cfg.mainModelSwitch) {
+    return { model: e.model, effort }
+  }
+  const id = resolveModel(st.cfg, wanted)
+  return { model: (await windowOk($, st, id)) ? id : e.model, effort }
+}
+
+async function planStep($: Api, st: State, e: StepIn): Promise<Plan> {
+  const plan = e.agentId === undefined
+    ? await mainPlan($, st, e)
+    : agentPlan(st, e)
+  // Haiku takes no effort: omit it rather than send a hook-set value.
+  return plan.model.startsWith(HAIKU) ? { ...plan, effort: undefined } : plan
+}
+
+function withPlan(e: StepIn, plan: Plan): StepIn {
+  const { effort: _replaced, ...rest } = e
+  const base = { ...rest, model: plan.model }
+  return plan.effort === undefined ? base : { ...base, effort: plan.effort }
+}
+
+function stepLog(e: StepIn, plan: Plan): string {
+  const id = e.agentId
+  const loop = id === undefined ? 'main' : `agent ${id.slice(0, 8)}`
+  return `step ${e.index} ${loop}: ${e.model}/${String(e.effort)} → ` +
+    `${plan.model}/${String(plan.effort)}`
+}
+
+function noteMain($: Api, st: State, plan: Plan): void {
+  st.lastMain = `${plan.model.replace(/^claude-/, '')}/${plan.effort ?? '-'}`
+  refresh($, st)
+}
+
+function endMainTurn($: Api, st: State): void {
+  st.turnMain = st.pendingPrompt
+  st.pendingPrompt = null
+  st.explicitEffort.clear()
+  st.lastMain = ''
+  st.windowWarned = false
+  refresh($, st)
+}
+
+// ---- registration ----------------------------------------------------
+
+function registerSession(on: On, st: State): void {
+  on('session.start', async ($, e, next) => {
+    await reloadConfig($, st)
+    await registerCommand($)
+    refresh($, st)
+    return next(e)
+  }).catch(($, e, next) => next(e))
+  on('session.end', async ($, e, next) => {
+    Object.assign(st, newState(st.cfg, st.source))
+    return next(e)
+  }).catch(($, e, next) => next(e))
+  on('command.run', { command: 'route' }, async ($, e) => ({
+    text: await handleCommand($, st, e.args),
+  })).catch(($, e, next) => ({ text: `route failed (${next.error.kind})` }))
+}
+
+function registerRouteTool(on: On, st: State): void {
+  on('tool.call', { tool: TOOL }, async ($, e) => {
+    const out = handleRouteTool(st, e)
+    refresh($, st)
+    vlog($, st, `route ${e.agentId ?? 'main'}: ${JSON.stringify(out)}`)
+    return out
+  }).catch(($, e, next) => ({
+    result: `route failed (${next.error.kind}); nothing routed`,
+  }))
+}
+
+function registerSkills(on: On, st: State): void {
+  on('tool.call', { tool: 'Skill' }, async ($, e, next) => {
+    if (st.off) return next(e)
+    const level = EFFORT_SKILL.exec(e.skill)?.[1]
+    if (isLevel(level)) return effortBridge(st, e.agentId, e.skill, level)
+    st.skillCalls += 1
+    try {
+      onSkillLoad(st, e.skill, e.agentId)
+      return await next(e)
+    } finally {
+      st.skillCalls -= 1
+    }
+  }).catch(($, e, next) => next(e))
+  on('skill.prompt', async ($, e, next) => {
+    if (st.off || st.skillCalls > 0) return next(e)
+    return slashEffort(st, e.skill, e.text) ?? next(e)
+  }).catch(($, e, next) => next(e))
+}
+
+function registerAgents(on: On, st: State): void {
+  on('tool.call', { tool: 'Agent' }, async ($, e, next) => {
+    if (!st.off && isLevel(e.effort) && typeof e.tool_use_id === 'string') {
+      st.explicitEffort.set(e.tool_use_id, e.effort)
+    }
+    return next(e)
+  }).catch(($, e, next) => next(e))
+  on('agent.spawn', async ($, e, next) => {
+    if (st.off) return next(e)
+    const frozen = e.fork || e.workflow !== undefined
+    const route = spawnRoute(st.cfg, e, frozen)
+    // An explicit model param on the Agent call always wins.
+    const wanted = e.model === undefined ? route?.model : undefined
+    const started = await next(wanted === undefined
+      ? e
+      : { ...e, model: resolveModel(st.cfg, wanted) })
+    if (started.deny !== undefined) return started
+    trackLoop(st, e, started, route, frozen)
+    vlog($, st, `spawn ${e.subagentType}: ${e.model ?? '-'} → ${started.model}`)
+    return started
+  }).catch(($, e, next) => next(e))
+}
+
+function registerTurns(on: On, st: State): void {
+  on('turn.step', async function* ($, e, next) {
+    if (st.off) return yield* next(e)
+    const plan = await planStep($, st, e)
+    const changed = plan.model !== e.model || plan.effort !== e.effort
+    if (e.agentId === undefined) noteMain($, st, plan)
+    vlog($, st, stepLog(e, plan))
+    const result = yield* next(changed ? withPlan(e, plan) : e)
+    vlog($, st, `step ${e.index} answered by ${result.usage?.model ?? '?'}`)
+    return result
+  }).catch(async function* ($, e, next) {
+    return yield* next(e)
+  })
+  on('turn.complete', async ($, e, next) => {
+    if (e.agentId !== undefined) st.loops.delete(e.agentId)
+    else endMainTurn($, st)
+    return next(e)
+  }).catch(($, e, next) => next(e))
+}
+
+function registerPrompt(on: On, st: State): void {
+  on('prompt.submit', async ($, e, next) => {
+    if (st.off || e.origin.kind !== 'composer') return next(e)
+    const rule = st.rules.find(r => r.re.test(e.text))
+    const route = rule ? phaseRoute(st.cfg, rule.phase) : undefined
+    if (rule && route) {
+      const routed: Routed = { phase: rule.phase, route, source: 'prompt' }
+      // Typed mid-turn and asked to wait: it belongs to the NEXT turn.
+      if (e.turnId !== undefined && e.wait) st.pendingPrompt = routed
+      else st.turnMain = routed
+      refresh($, st)
+    }
+    return next(e)
+  }).catch(($, e, next) => next(e))
+  on('ui.render', { component: 'Spinner' }, async ($, e, next) => {
+    if (st.off || !st.cfg.spinner || !st.lastMain) return next(e)
+    const suffix = ` · ${st.lastMain}…`
+    return next({ ...e, props: { ...e.props, suffix } })
+  }).catch(($, e, next) => next(e))
+}
+
+export const register: Register = on => {
+  const st = newState(mergeConfig(undefined, () => undefined), 'defaults')
+  registerSession(on, st)
+  registerRouteTool(on, st)
+  registerSkills(on, st)
+  registerAgents(on, st)
+  registerTurns(on, st)
+  registerPrompt(on, st)
+}