web: drive control limits from the served bounds and clamp, not 400

The store fetches {config, bounds} at startup and snaps values into
range everywhere they enter: typed fields, sliders, panel overrides,
and shared links (a ?edgesPerNode=150 link now runs at 8 instead of
showing a validation error). Shrinking numStudents pulls numEducated
and origin down with it. If the bounds fetch fails the app still
starts and the engine's validation backstops. Native min/max
attributes follow the same numbers, so spinners stop at the bounds.

web/src/components/AdvancedFields.vue

@@ -14,15 +14,46 @@ interface NumberFieldDef {
   field: keyof Config
   label: string
   min: number
+  max: number | undefined
   step: number
 }
 
-const numberFields: NumberFieldDef[] = [
-  { field: 'numStudents', label: CONFIG_FIELD_LABELS.numStudents, min: 2, step: 1 },
-  { field: 'edgesPerNode', label: CONFIG_FIELD_LABELS.edgesPerNode, min: 1, step: 1 },
-  { field: 'triangleProb', label: CONFIG_FIELD_LABELS.triangleProb, min: 0, step: 0.05 },
-  { field: 'origin', label: CONFIG_FIELD_LABELS.origin, min: 0, step: 1 },
+// min/max come from the engine's served bounds; until they arrive the
+// inputs are unbounded and the store clamps nothing (the engine's own
+// validation backstops). origin's ceiling tracks numStudents.
+const numberFields = computed<NumberFieldDef[]>(() => {
+  const bounds = store.state.bounds
+  return [
+    {
+      field: 'numStudents',
+      label: CONFIG_FIELD_LABELS.numStudents,
+      min: bounds?.numStudents.min ?? 2,
+      max: bounds?.numStudents.max,
+      step: 1,
+    },
+    {
+      field: 'edgesPerNode',
+      label: CONFIG_FIELD_LABELS.edgesPerNode,
+      min: bounds?.edgesPerNode.min ?? 1,
+      max: bounds?.edgesPerNode.max,
+      step: 1,
+    },
+    {
+      field: 'triangleProb',
+      label: CONFIG_FIELD_LABELS.triangleProb,
+      min: bounds?.triangleProb.min ?? 0,
+      max: bounds?.triangleProb.max,
+      step: 0.05,
+    },
+    {
+      field: 'origin',
+      label: CONFIG_FIELD_LABELS.origin,
+      min: 0,
+      max: store.state.base.numStudents - 1,
+      step: 1,
+    },
   ]
+})
 
 interface SeedFieldDef {
   field: keyof Config
@@ -39,7 +70,7 @@ const seedFields: SeedFieldDef[] = [
 const offendingField = computed(() => {
   const validationError = store.state.validationError
   if (!validationError) return null
-  const allFields = [...numberFields, ...seedFields].map(({ field }) => field)
+  const allFields = [...numberFields.value, ...seedFields].map(({ field }) => field)
   return allFields.find((field) => validationError.includes(field)) ?? null
 })
 
@@ -82,6 +113,7 @@ function rerollWorld() {
         <input
           type="number"
           :min="def.min"
+          :max="def.max"
           :step="def.step"
           :value="store.state.base[def.field]"
           :aria-invalid="offendingField === def.field || undefined"

web/src/components/ControlsCard.vue

@@ -36,8 +36,8 @@ const offendsNumEducated = computed(
       label="Chance to forward"
       hint="per friendship, per round"
       :value="base.forwardProb"
-      :min="0"
-      :max="1"
+      :min="store.state.bounds?.forwardProb.min ?? 0"
+      :max="store.state.bounds?.forwardProb.max ?? 1"
       :step="0.01"
       :display-value="formatPct(base.forwardProb * 100)"
       :invalid="offendsForwardProb"

web/src/components/PanelEditorPopover.vue

@@ -40,6 +40,45 @@ function setOverride(field: keyof Config, event: Event) {
   if (Number.isFinite(nextValue)) store.setPanelOverride(props.panel.id, field, nextValue)
 }
 
+// Native min/max hints from the served bounds; the store clamps anyway,
+// these just make the spinners stop at the right place. Relational
+// ceilings follow this panel's effective student count.
+function fieldMin(field: keyof Config): number | undefined {
+  const bounds = store.state.bounds
+  switch (field) {
+    case 'numStudents':
+      return bounds?.numStudents.min
+    case 'edgesPerNode':
+      return bounds?.edgesPerNode.min
+    case 'triangleProb':
+      return bounds?.triangleProb.min
+    case 'forwardProb':
+      return bounds?.forwardProb.min
+    default:
+      return 0
+  }
+}
+
+function fieldMax(field: keyof Config): number | undefined {
+  const bounds = store.state.bounds
+  switch (field) {
+    case 'numStudents':
+      return bounds?.numStudents.max
+    case 'edgesPerNode':
+      return bounds?.edgesPerNode.max
+    case 'triangleProb':
+      return bounds?.triangleProb.max
+    case 'forwardProb':
+      return bounds?.forwardProb.max
+    case 'numEducated':
+      return effectiveValue('numStudents')
+    case 'origin':
+      return effectiveValue('numStudents') - 1
+    default:
+      return undefined
+  }
+}
+
 function rename(event: Event) {
   const label = (event.target as HTMLInputElement).value.trim()
   if (label) store.renamePanel(props.panel.id, label)
@@ -123,6 +162,8 @@ onBeforeUnmount(() => {
         <input
           :id="`${panel.id}-${field}`"
           type="number"
+          :min="fieldMin(field)"
+          :max="fieldMax(field)"
           :value="effectiveValue(field)"
           :aria-invalid="offendingField === field || undefined"
           @change="setOverride(field, $event)"

web/src/composables/__tests__/useSimStore.spec.ts

@@ -1,14 +1,29 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { createSimStore, RUN_DEBOUNCE_MS } from '../useSimStore'
-import { ApiError, runScenario } from '@/lib/api'
+import { ApiError, fetchDefaultConfig, runScenario } from '@/lib/api'
 import type { ScenarioRequest, ScenarioResponse } from '@/types/api'
+import type { Bounds } from '@/types/engine'
 
 vi.mock('@/lib/api', async (importOriginal) => {
   const actual = await importOriginal<typeof import('@/lib/api')>()
-  return { ...actual, runScenario: vi.fn<typeof actual.runScenario>() }
+  return {
+    ...actual,
+    runScenario: vi.fn<typeof actual.runScenario>(),
+    fetchDefaultConfig: vi.fn<typeof actual.fetchDefaultConfig>(),
+  }
 })
 
 const runScenarioMock = vi.mocked(runScenario)
+const fetchDefaultConfigMock = vi.mocked(fetchDefaultConfig)
+
+// Stand-in bounds matching the engine's real ones closely enough for the
+// clamping tests; the store treats them as opaque numbers either way.
+const TEST_BOUNDS: Bounds = {
+  numStudents: { min: 10, max: 500 },
+  edgesPerNode: { min: 1, max: 8 },
+  triangleProb: { min: 0, max: 1 },
+  forwardProb: { min: 0, max: 0.9 },
+}
 
 // The fake engine: numReached mirrors numEducated so tests can tell which
 // config produced a result, and the no-program panel runs the longest.
@@ -40,6 +55,11 @@ beforeEach(() => {
   vi.useFakeTimers()
   runScenarioMock.mockReset()
   runScenarioMock.mockImplementation(async (request) => fakeResponse(request))
+  fetchDefaultConfigMock.mockReset()
+  fetchDefaultConfigMock.mockImplementation(async () => {
+    const { deepfakeSchoolPreset } = await import('@/presets/deepfake-school')
+    return { config: deepfakeSchoolPreset.base, bounds: TEST_BOUNDS }
+  })
   window.history.replaceState(null, '', '/')
 })
 
@@ -271,3 +291,52 @@ describe('useSimStore panel management', () => {
     expect(window.location.search).not.toContain('focus=')
   })
 })
+
+describe('bounds clamping', () => {
+  it('stores the served bounds on initialize', async () => {
+    const store = createSimStore()
+    await store.initialize('')
+    expect(store.state.bounds).toEqual(TEST_BOUNDS)
+  })
+
+  it('clamps wild shared-link values instead of running them', async () => {
+    const store = createSimStore()
+    await store.initialize('?numStudents=5000&edgesPerNode=150&forwardProb=1&panel=Wild~random~numEducated:9999')
+
+    expect(store.state.base.numStudents).toBe(500)
+    expect(store.state.base.edgesPerNode).toBe(8)
+    expect(store.state.base.forwardProb).toBe(0.9)
+    expect(store.state.panels[0]!.overrides.numEducated).toBe(500)
+  })
+
+  it('clamps typed base values to the bounds', async () => {
+    const store = createSimStore()
+    await store.initialize('')
+
+    store.setBaseField('forwardProb', 1)
+    expect(store.state.base.forwardProb).toBe(0.9)
+    store.setBaseField('edgesPerNode', 150)
+    expect(store.state.base.edgesPerNode).toBe(8)
+  })
+
+  it('pulls relational fields down when numStudents shrinks', async () => {
+    const store = createSimStore()
+    await store.initialize('')
+    store.setBaseField('origin', 100)
+    store.setBaseField('numEducated', 110)
+
+    store.setBaseField('numStudents', 50)
+    expect(store.state.base.origin).toBe(49)
+    expect(store.state.base.numEducated).toBe(50)
+  })
+
+  it('starts without clamping when the bounds fetch fails', async () => {
+    fetchDefaultConfigMock.mockRejectedValue(new Error('offline'))
+    const store = createSimStore()
+    await store.initialize('')
+
+    expect(store.state.bounds).toBeNull()
+    store.setBaseField('forwardProb', 1)
+    expect(store.state.base.forwardProb).toBe(1) // the engine 400 backstops
+  })
+})

web/src/composables/useSimStore.ts

@@ -1,11 +1,12 @@
 import { reactive } from 'vue'
-import { ApiError, runScenario } from '@/lib/api'
+import { ApiError, fetchDefaultConfig, runScenario } from '@/lib/api'
+import { clampConfig, clampConfigField } from '@/lib/bounds'
 import { roundPct } from '@/lib/format'
 import { graphKey } from '@/lib/graph'
 import { parseUrlState, serializeUrlState } from '@/lib/urlState'
 import { deepfakeSchoolPreset } from '@/presets/deepfake-school'
 import { MAX_PANELS, type PanelSpec, type StudyPreset } from '@/presets/types'
-import type { Config, Result, Strategy } from '@/types/engine'
+import type { Bounds, Config, Result, Strategy } from '@/types/engine'
 import { StrategyNone } from '@/types/engine'
 
 // The one store (spec section 4): a plain reactive module singleton, no
@@ -21,6 +22,10 @@ export type PlaybackSpeed = 0.5 | 1 | 2
 
 export interface SimState {
   base: Config
+  // The engine's field bounds, fetched at startup; null until they arrive
+  // (or when the fetch failed, in which case the engine's own validation
+  // is the backstop and nothing clamps client-side).
+  bounds: Bounds | null
   panels: PanelSpec[]
   resultsByPanelId: Record<string, Result>
   edgesByGraphHash: Record<string, number[][]>
@@ -52,6 +57,7 @@ function clonePanel(panel: PanelSpec): PanelSpec {
 export function createSimStore(preset: StudyPreset = deepfakeSchoolPreset) {
   const state: SimState = reactive({
     base: { ...preset.base },
+    bounds: null,
     panels: preset.panels.map(clonePanel),
     resultsByPanelId: {},
     edgesByGraphHash: {},
@@ -181,10 +187,31 @@ export function createSimStore(preset: StudyPreset = deepfakeSchoolPreset) {
     field: FieldName,
     value: Config[FieldName],
   ) {
+    if (state.bounds) {
+      value = clampConfigField(field, value, state.base.numStudents, state.bounds)
+    }
     state.base[field] = value
+    if (field === 'numStudents') clampEverythingToBounds()
     scheduleRun()
   }
 
+  // Re-clamp the base config and every panel override; needed when
+  // numStudents changes (the relational ceilings move) and when the
+  // bounds first arrive while the URL may have carried wild values.
+  function clampEverythingToBounds() {
+    const bounds = state.bounds
+    if (!bounds) return
+    state.base = clampConfig(state.base, bounds)
+    for (const panel of state.panels) {
+      const panelStudents = panel.overrides.numStudents ?? state.base.numStudents
+      for (const field of Object.keys(panel.overrides) as (keyof Config)[]) {
+        const override = panel.overrides[field]
+        if (override === undefined) continue
+        panel.overrides[field] = clampConfigField(field, override, panelStudents, bounds)
+      }
+    }
+  }
+
   function addPanel(): PanelSpec | null {
     if (state.panels.length >= MAX_PANELS) return null
     const panel: PanelSpec = {
@@ -253,6 +280,11 @@ export function createSimStore(preset: StudyPreset = deepfakeSchoolPreset) {
   ) {
     const panel = panelById(panelId)
     if (panel === undefined) return
+    if (state.bounds) {
+      const panelStudents =
+        field === 'numStudents' ? value : (panel.overrides.numStudents ?? state.base.numStudents)
+      value = clampConfigField(field, value, panelStudents, state.bounds)
+    }
     if (value === state.base[field]) {
       // Typing the base value back is "no override" (spec 5.7).
       delete panel.overrides[field]
@@ -281,10 +313,18 @@ export function createSimStore(preset: StudyPreset = deepfakeSchoolPreset) {
   }
 
   // Parse the opened URL (or fall back to the preset) and run every panel.
-  // The initial run is not debounced: nothing is on screen yet.
-  function initialize(
+  // The initial run is not debounced: nothing is on screen yet. The bounds
+  // arrive first so a shared link's wild values clamp instead of 400ing;
+  // if that fetch fails the app still starts, with the engine's own
+  // validation as the backstop.
+  async function initialize(
     search: string = typeof window === 'undefined' ? '' : window.location.search,
   ): Promise<void> {
+    try {
+      state.bounds = (await fetchDefaultConfig()).bounds
+    } catch {
+      state.bounds = null
+    }
     const parsed = parseUrlState(search, preset, createPanelId)
     if (parsed === null) {
       state.urlStateInvalid = true // preset defaults are already loaded
@@ -294,6 +334,7 @@ export function createSimStore(preset: StudyPreset = deepfakeSchoolPreset) {
       state.focusPanelId =
         parsed.focusIndex !== null ? (parsed.panels[parsed.focusIndex]?.id ?? null) : null
     }
+    clampEverythingToBounds()
     return runPanels(null)
   }
 

web/src/lib/__tests__/bounds.spec.ts

@@ -0,0 +1,70 @@
+import { describe, expect, it } from 'vitest'
+import { clampConfig, clampConfigField } from '../bounds'
+import type { Bounds, Config } from '@/types/engine'
+
+// Bounds values here are test stand-ins; the real ones come from the API.
+const bounds: Bounds = {
+  numStudents: { min: 10, max: 500 },
+  edgesPerNode: { min: 1, max: 8 },
+  triangleProb: { min: 0, max: 1 },
+  forwardProb: { min: 0, max: 0.9 },
+}
+
+const config: Config = {
+  numStudents: 120,
+  edgesPerNode: 3,
+  triangleProb: 0.45,
+  forwardProb: 0.38,
+  numEducated: 36,
+  origin: 0,
+  graphSeed: 17,
+  thresholdSeed: 2,
+  educationSeed: 1,
+}
+
+describe('clampConfigField', () => {
+  it('snaps absolute fields to their bounds', () => {
+    expect(clampConfigField('edgesPerNode', 150, 120, bounds)).toBe(8)
+    expect(clampConfigField('edgesPerNode', 0, 120, bounds)).toBe(1)
+    expect(clampConfigField('forwardProb', 1, 120, bounds)).toBe(0.9)
+    expect(clampConfigField('numStudents', 5000, 120, bounds)).toBe(500)
+    expect(clampConfigField('numStudents', 2, 120, bounds)).toBe(10)
+  })
+
+  it('clamps relational fields against numStudents', () => {
+    expect(clampConfigField('numEducated', 200, 120, bounds)).toBe(120)
+    expect(clampConfigField('origin', 120, 120, bounds)).toBe(119)
+    expect(clampConfigField('origin', -3, 120, bounds)).toBe(0)
+  })
+
+  it('rounds integer fields and keeps seeds non-negative', () => {
+    expect(clampConfigField('numStudents', 99.7, 120, bounds)).toBe(100)
+    expect(clampConfigField('graphSeed', -5, 120, bounds)).toBe(0)
+    expect(clampConfigField('triangleProb', 0.45, 120, bounds)).toBe(0.45)
+  })
+
+  it('leaves in-range values untouched', () => {
+    expect(clampConfigField('forwardProb', 0.38, 120, bounds)).toBe(0.38)
+    expect(clampConfigField('numEducated', 36, 120, bounds)).toBe(36)
+  })
+})
+
+describe('clampConfig', () => {
+  it('returns an equal config when everything is in range', () => {
+    expect(clampConfig(config, bounds)).toEqual(config)
+  })
+
+  it('clamps numStudents first so relational fields follow the corrected value', () => {
+    const wild = { ...config, numStudents: 5000, numEducated: 1000, origin: 999 }
+    const clamped = clampConfig(wild, bounds)
+    expect(clamped.numStudents).toBe(500)
+    expect(clamped.numEducated).toBe(500)
+    expect(clamped.origin).toBe(499)
+  })
+
+  it('does not mutate its input', () => {
+    const wild = { ...config, forwardProb: 2 }
+    clampConfig(wild, bounds)
+    expect(wild.forwardProb).toBe(2)
+  })
+})

web/src/lib/api.ts

@@ -1,5 +1,10 @@
 import type { Config } from '@/types/engine'
-import type { ComparisonResponse, ScenarioRequest, ScenarioResponse } from '@/types/api'
+import type {
+  ComparisonResponse,
+  DefaultConfigResponse,
+  ScenarioRequest,
+  ScenarioResponse,
+} from '@/types/api'
 
 // Thin typed wrappers around the JSON API. The types come from
 // src/types/, which is generated from the Go structs (single source of
@@ -44,8 +49,8 @@ async function requestJSON<T>(input: string, init?: RequestInit): Promise<T> {
   return response.json() as Promise<T>
 }
 
-export function fetchDefaultConfig(): Promise<Config> {
-  return requestJSON<Config>('/api/config/default')
+export function fetchDefaultConfig(): Promise<DefaultConfigResponse> {
+  return requestJSON<DefaultConfigResponse>('/api/config/default')
 }
 
 export function runComparison(config: Config): Promise<ComparisonResponse> {

web/src/lib/bounds.ts

@@ -0,0 +1,61 @@
+import type { Bounds, Config } from '@/types/engine'
+
+// Frontend mirror of the engine's ValidateConfig, but clamping instead of
+// rejecting (decided 2026-06-11): typed values snap to the nearest bound
+// and out-of-range shared links keep working. The numbers themselves are
+// never duplicated here; they arrive from GET /api/config/default.
+
+const INTEGER_FIELDS: ReadonlySet<keyof Config> = new Set([
+  'numStudents',
+  'edgesPerNode',
+  'numEducated',
+  'origin',
+  'graphSeed',
+  'thresholdSeed',
+  'educationSeed',
+])
+
+function clampNumber(value: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, value))
+}
+
+// clampConfigField snaps one field's value into range. Relational fields
+// (numEducated, origin) clamp against numStudents, which callers pass from
+// whatever config the field is about to land in; seeds only need to be
+// non-negative integers.
+export function clampConfigField(
+  field: keyof Config,
+  value: number,
+  numStudents: number,
+  bounds: Bounds,
+): number {
+  const rounded = INTEGER_FIELDS.has(field) ? Math.round(value) : value
+  switch (field) {
+    case 'numStudents':
+      return clampNumber(rounded, bounds.numStudents.min, bounds.numStudents.max)
+    case 'edgesPerNode':
+      return clampNumber(rounded, bounds.edgesPerNode.min, bounds.edgesPerNode.max)
+    case 'triangleProb':
+      return clampNumber(rounded, bounds.triangleProb.min, bounds.triangleProb.max)
+    case 'forwardProb':
+      return clampNumber(rounded, bounds.forwardProb.min, bounds.forwardProb.max)
+    case 'numEducated':
+      return clampNumber(rounded, 0, numStudents)
+    case 'origin':
+      return clampNumber(rounded, 0, numStudents - 1)
+    default:
+      return Math.max(0, rounded)
+  }
+}
+
+// clampConfig snaps a whole config into range, numStudents first so the
+// relational fields clamp against the corrected value.
+export function clampConfig(config: Config, bounds: Bounds): Config {
+  const clamped = { ...config }
+  clamped.numStudents = clampConfigField('numStudents', config.numStudents, 0, bounds)
+  for (const field of Object.keys(config) as (keyof Config)[]) {
+    if (field === 'numStudents') continue
+    clamped[field] = clampConfigField(field, config[field], clamped.numStudents, bounds)
+  }
+  return clamped
+}