diff --git a/packages/@sanity/cli/src/actions/dev/registration/__tests__/startDevServerRegistration.test.ts b/packages/@sanity/cli/src/actions/dev/registration/__tests__/startDevServerRegistration.test.ts
index d68bfda95..63b833de0 100644
--- a/packages/@sanity/cli/src/actions/dev/registration/__tests__/startDevServerRegistration.test.ts
+++ b/packages/@sanity/cli/src/actions/dev/registration/__tests__/startDevServerRegistration.test.ts
@@ -16,7 +16,10 @@ vi.mock('@sanity/cli-core', async (importOriginal) => ({
   ...(await importOriginal<typeof import('@sanity/cli-core')>()),
   getCliConfigUncached: mockGetCliConfigUncached,
 }))
-vi.mock('@sanity/workbench-cli/dev', () => ({
+// Only the registry write is mocked; `deriveInterfaces`/`trackInterfaceSet` are
+// pure and exercised for real, as they were before moving into workbench-cli.
+vi.mock('@sanity/workbench-cli/dev', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('@sanity/workbench-cli/dev')>()),
   registerDevServer: mockRegisterDevServer,
 }))
 vi.mock('../startDevManifestWatcher.js', () => ({
diff --git a/packages/@sanity/cli/src/actions/dev/registration/deriveInterfaces.ts b/packages/@sanity/cli/src/actions/dev/registration/deriveInterfaces.ts
deleted file mode 100644
index 4e493ac77..000000000
--- a/packages/@sanity/cli/src/actions/dev/registration/deriveInterfaces.ts
+++ /dev/null
@@ -1,51 +0,0 @@
-import {type CliConfig, isWorkbenchApp} from '@sanity/cli-core'
-import {type DevServerManifest} from '@sanity/workbench-cli/dev'
-
-/** One forwarded interface record on the dev-server registry entry. */
-export type DevServerInterface = NonNullable<DevServerManifest['interfaces']>[number]
-
-/**
- * Derive the workbench `interfaces[]` an app forwards to the dev-server
- * registry from its `unstable_defineApp` config: `views` → `panel`s,
- * `services` → `worker`s, and (SDK apps) `entry` → the navigable `app` view.
- * `entry_point` is the declared `src` — the raw value, not a resolved URL.
- *
- * Returns `undefined` for a non-branded app (no `unstable_defineApp`). A studio
- * that declares `entry` reaches the not-yet-implemented studio app-view path and
- * is rejected (FR-026).
- *
- * Shared by the initial registration and the dev config watcher so editing
- * `views`/`services`/`entry` in `sanity.cli.ts` re-pushes the same shape live,
- * the way `title`/`icon` already re-sync (FR-024).
- */
-export function deriveInterfaces(
-  app: CliConfig['app'],
-  options: {isApp: boolean},
-): DevServerInterface[] | undefined {
-  if (!isWorkbenchApp(app)) return undefined
-
-  // sanity-io/workbench spec 002-workbench-extension-api, US5 — studio app views are not implemented yet. A studio (not an SDK app)
-  // that declares `entry` reaches the app-view path; reject with a clear error
-  // rather than deriving an `app` interface for it.
-  if (!options.isApp && app.entry !== undefined) {
-    throw new Error('App views for studios are not implemented yet')
-  }
-
-  return [
-    ...(app.views?.map((view) => ({
-      entry_point: view.src,
-      interface_type: view.type,
-      name: view.name,
-    })) ?? []),
-    ...(app.services?.map((service) => ({
-      entry_point: service.src,
-      interface_type: service.type,
-      name: service.name,
-    })) ?? []),
-    // sanity-io/workbench spec 002-workbench-extension-api, US5 — with no `entry` the app has no `app` view and isn't reachable as a
-    // full-page app; with one, forward it so the workbench gates navigability.
-    ...(app.entry === undefined
-      ? []
-      : [{entry_point: app.entry, interface_type: 'app' as const, name: app.name}]),
-  ]
-}
diff --git a/packages/@sanity/cli/src/actions/dev/registration/interfaceSetId.ts b/packages/@sanity/cli/src/actions/dev/registration/interfaceSetId.ts
deleted file mode 100644
index 2a313c966..000000000
--- a/packages/@sanity/cli/src/actions/dev/registration/interfaceSetId.ts
+++ /dev/null
@@ -1,46 +0,0 @@
-import {type DevServerInterface} from './deriveInterfaces.js'
-
-/**
- * The identity of an app's declared interface set — an order-independent key
- * over its forwarded Interface records (interface_type, name, entry_point).
- * Two sets that differ only in declaration order share an id, so reordering
- * `views`/`services` in `sanity.cli.ts` is not a change; adding, removing,
- * renaming, or repointing a view/service is. An `undefined` set (project types
- * that declare no interfaces, e.g. studios) gets the same id as the empty set.
- *
- * Both detection sites compare this id against their own last-seen value across
- * the dev-server registry seam: the app dev server rebuilds the federation
- * remote when its set changes, and the workbench dev server reloads the page.
- * Editing a view's/service's *source file* doesn't change the id, so it stays
- * on the HMR path.
- */
-export function interfaceSetId(interfaces: readonly DevServerInterface[] | undefined): string {
-  if (!interfaces || interfaces.length === 0) return ''
-  return interfaces
-    .map((iface) => [iface.interface_type, iface.name, iface.entry_point].join('::'))
-    .toSorted()
-    .join('|')
-}
-
-/**
- * Track the declared interface *set* across config reloads — an added, removed,
- * renamed, or repointed view/service. `changed` reports whether a set differs
- * from the last *committed* one (a reorder or manifest-only/source-file edit
- * leaves it unchanged, so HMR handles it) without advancing the committed set;
- * `commit` advances it. Splitting the two lets the caller commit only after the
- * rebuild that depends on the new set has succeeded — a thrown rebuild leaves
- * the set uncommitted, so the next config save retries it instead of skipping.
- * Seed it with the initially registered set.
- */
-export function trackInterfaceSet(initial: readonly DevServerInterface[] | undefined): {
-  changed: (interfaces: readonly DevServerInterface[] | undefined) => boolean
-  commit: (interfaces: readonly DevServerInterface[] | undefined) => void
-} {
-  let lastId = interfaceSetId(initial)
-  return {
-    changed: (interfaces) => interfaceSetId(interfaces) !== lastId,
-    commit: (interfaces) => {
-      lastId = interfaceSetId(interfaces)
-    },
-  }
-}
diff --git a/packages/@sanity/cli/src/actions/dev/registration/startDevManifestWatcher.ts b/packages/@sanity/cli/src/actions/dev/registration/startDevManifestWatcher.ts
index c5c1255c8..d9c1dc568 100644
--- a/packages/@sanity/cli/src/actions/dev/registration/startDevManifestWatcher.ts
+++ b/packages/@sanity/cli/src/actions/dev/registration/startDevManifestWatcher.ts
@@ -2,10 +2,9 @@ import {watch} from 'node:fs'
 import {basename, dirname} from 'node:path'
 
 import {findProjectRoot, type Output} from '@sanity/cli-core'
-import {canonicalizeWatchDir} from '@sanity/workbench-cli/dev'
+import {canonicalizeWatchDir, type DevServerInterface} from '@sanity/workbench-cli/dev'
 
 import {devDebug} from '../devDebug.js'
-import {type DevServerInterface} from './deriveInterfaces.js'
 
 /**
  * Debounce window between config file events and the next manifest
diff --git a/packages/@sanity/cli/src/actions/dev/registration/startDevServerRegistration.ts b/packages/@sanity/cli/src/actions/dev/registration/startDevServerRegistration.ts
index 559ac8d93..65e5f20a2 100644
--- a/packages/@sanity/cli/src/actions/dev/registration/startDevServerRegistration.ts
+++ b/packages/@sanity/cli/src/actions/dev/registration/startDevServerRegistration.ts
@@ -1,12 +1,10 @@
 import {type CliConfig, getCliConfigUncached, type Output} from '@sanity/cli-core'
-import {registerDevServer} from '@sanity/workbench-cli/dev'
+import {deriveInterfaces, registerDevServer, trackInterfaceSet} from '@sanity/workbench-cli/dev'
 import {type ViteDevServer} from 'vite'
 
 import {checkForDeprecatedAppId, getAppId} from '../../../util/appId.js'
 import {extractCoreAppManifest} from '../../manifest/extractCoreAppManifest.js'
-import {deriveInterfaces} from './deriveInterfaces.js'
 import {extractStudioManifest} from './extractDevServerManifest.js'
-import {trackInterfaceSet} from './interfaceSetId.js'
 import {startDevManifestWatcher} from './startDevManifestWatcher.js'
 
 interface DevServerRegistrationOptions {
diff --git a/packages/@sanity/cli/src/actions/dev/workbench/__tests__/startWorkbenchDevServer.test.ts b/packages/@sanity/cli/src/actions/dev/workbench/__tests__/startWorkbenchDevServer.test.ts
index c5a8f2ec2..13d6f14ea 100644
--- a/packages/@sanity/cli/src/actions/dev/workbench/__tests__/startWorkbenchDevServer.test.ts
+++ b/packages/@sanity/cli/src/actions/dev/workbench/__tests__/startWorkbenchDevServer.test.ts
@@ -28,7 +28,10 @@ vi.mock('@vitejs/plugin-react', () => ({default: vi.fn(() => [])}))
 vi.mock('../writeWorkbenchRuntime.js', () => ({
   writeWorkbenchRuntime: mockWriteWorkbenchRuntime,
 }))
-vi.mock('@sanity/workbench-cli/dev', () => ({
+// Registry/lock I/O is mocked; the pure `createInterfacesTracker` runs for
+// real so the watcher's reload-vs-reconcile decision is exercised, not stubbed.
+vi.mock('@sanity/workbench-cli/dev', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('@sanity/workbench-cli/dev')>()),
   acquireWorkbenchLock: mockAcquireWorkbenchLock,
   getRegisteredServers: mockGetRegisteredServers,
   readWorkbenchLock: mockReadWorkbenchLock,
diff --git a/packages/@sanity/cli/src/actions/dev/workbench/startWorkbenchDevServer.ts b/packages/@sanity/cli/src/actions/dev/workbench/startWorkbenchDevServer.ts
index f85c726c2..1edee30ec 100644
--- a/packages/@sanity/cli/src/actions/dev/workbench/startWorkbenchDevServer.ts
+++ b/packages/@sanity/cli/src/actions/dev/workbench/startWorkbenchDevServer.ts
@@ -2,6 +2,7 @@ import {SANITY_CACHE_DIR} from '@sanity/cli-build/_internal/build'
 import {isWorkbenchApp, resolveLocalPackage} from '@sanity/cli-core'
 import {
   acquireWorkbenchLock,
+  createInterfacesTracker,
   type DevServerManifest,
   getRegisteredServers,
   readWorkbenchLock,
@@ -13,15 +14,11 @@ import {z} from 'zod/mini'
 
 import {resolveReactStrictMode} from '../../../util/resolveReactStrictMode.js'
 import {devDebug} from '../devDebug.js'
-import {interfaceSetId} from '../registration/interfaceSetId.js'
 import {type DevActionOptions} from '../types.js'
 import {writeWorkbenchRuntime} from './writeWorkbenchRuntime.js'
 
 const noop = async () => {}
 
-/** Stable per-app key for the registry-watch interface diff. */
-const serverKey = (s: DevServerManifest) => `${s.id ?? ''}@${s.host ?? ''}:${s.port}`
-
 const toApplicationsPayload = (servers: DevServerManifest[]) => ({
   applications: servers.map(({host, id, interfaces, manifest, port, projectId, type}) => ({
     host,
@@ -234,22 +231,12 @@ async function createWorkbenchViteServer(
     )
   })
 
-  // A running app's declared interface set changing (a view/service added or
-  // removed in `sanity.cli.ts`) means its remote was rebuilt with new exposes.
-  // Module federation has the old remote-entry cached, so an in-place reconcile
-  // would load a stale remote (empty panel / no worker) — the page must reload
-  // to re-fetch it. A new/removed app, or a manifest-only edit (title/icon),
-  // reconciles softly as before. Source-file edits don't change the set, so they
-  // stay on the HMR path and never trip a reload here.
-  let knownInterfaces = new Map<string, string>()
+  // A changed interface set means the remote was rebuilt with new exposes —
+  // full-reload so the page drops the stale remote-entry; otherwise rebroadcast
+  // the app list for a soft reconcile.
+  const setTracker = createInterfacesTracker()
   const registryWatcher = watchRegistry((servers) => {
-    const rebuiltApp = servers.some((s) => {
-      const key = serverKey(s)
-      return knownInterfaces.has(key) && knownInterfaces.get(key) !== interfaceSetId(s.interfaces)
-    })
-    knownInterfaces = new Map(servers.map((s) => [serverKey(s), interfaceSetId(s.interfaces)]))
-
-    if (rebuiltApp) {
+    if (setTracker.hasChanged(servers)) {
       server.ws.send({type: 'full-reload'})
       return
     }
diff --git a/packages/@sanity/workbench-cli/src/_exports/dev.ts b/packages/@sanity/workbench-cli/src/_exports/dev.ts
index 873fa8fd3..da02a54c1 100644
--- a/packages/@sanity/workbench-cli/src/_exports/dev.ts
+++ b/packages/@sanity/workbench-cli/src/_exports/dev.ts
@@ -1,10 +1,13 @@
-// Node-only dev entry: the workbench dev-server registry the CLI's dev
-// orchestration drives. It tracks running studio/app dev servers (single
-// instance via a lock + PID-liveness), and the workbench host watches it to
-// render local panels/services without a deploy. The CLI owns the orchestration
-// (starting servers, extracting manifests); this package owns the registry it
-// registers into and watches.
+// Node-only dev entry: the dev-server registry the CLI drives and the workbench
+// host watches (singleton lock + PID-liveness), plus the interface model derived
+// from `unstable_defineApp`.
 export {canonicalizeWatchDir} from '../actions/dev/canonicalizeWatchDir.js'
+export {deriveInterfaces, type DevServerInterface} from '../actions/dev/deriveInterfaces.js'
+export {
+  createInterfacesTracker,
+  interfaceSetId,
+  trackInterfaceSet,
+} from '../actions/dev/interfaceSetId.js'
 export {
   acquireWorkbenchLock,
   type DevServerManifest,
diff --git a/packages/@sanity/cli/src/actions/dev/registration/__tests__/deriveInterfaces.test.ts b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/deriveInterfaces.test.ts
similarity index 97%
rename from packages/@sanity/cli/src/actions/dev/registration/__tests__/deriveInterfaces.test.ts
rename to packages/@sanity/workbench-cli/src/actions/dev/__tests__/deriveInterfaces.test.ts
index 0b22461f4..23e2ee60c 100644
--- a/packages/@sanity/cli/src/actions/dev/registration/__tests__/deriveInterfaces.test.ts
+++ b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/deriveInterfaces.test.ts
@@ -1,8 +1,8 @@
 import {type CliConfig} from '@sanity/cli-core'
 import {describe, expect, test} from 'vitest'
 
-import {workbenchApp} from '../../__tests__/testHelpers.js'
 import {deriveInterfaces} from '../deriveInterfaces.js'
+import {workbenchApp} from './devTestHelpers.js'
 
 describe('deriveInterfaces', () => {
   test('returns undefined for a non-branded app (no unstable_defineApp)', () => {
diff --git a/packages/@sanity/workbench-cli/src/actions/dev/__tests__/devTestHelpers.ts b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/devTestHelpers.ts
new file mode 100644
index 000000000..a5fdc2219
--- /dev/null
+++ b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/devTestHelpers.ts
@@ -0,0 +1,13 @@
+import {type CliConfig} from '@sanity/cli-core'
+
+import {unstable_defineApp} from '../../../defineApp.js'
+
+/** A CliConfig `app` from a branded `unstable_defineApp(...)` — the workbench opt-in. */
+export function workbenchApp(overrides: Record<string, unknown> = {}): CliConfig['app'] {
+  return unstable_defineApp({
+    name: 'test-app',
+    organizationId: 'org-123',
+    title: 'Test App',
+    ...overrides,
+  }) as unknown as CliConfig['app']
+}
diff --git a/packages/@sanity/cli/src/actions/dev/registration/__tests__/interfaceSetId.test.ts b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/interfaceSetId.test.ts
similarity index 66%
rename from packages/@sanity/cli/src/actions/dev/registration/__tests__/interfaceSetId.test.ts
rename to packages/@sanity/workbench-cli/src/actions/dev/__tests__/interfaceSetId.test.ts
index 513c257b4..e310dfac7 100644
--- a/packages/@sanity/cli/src/actions/dev/registration/__tests__/interfaceSetId.test.ts
+++ b/packages/@sanity/workbench-cli/src/actions/dev/__tests__/interfaceSetId.test.ts
@@ -1,7 +1,8 @@
 import {describe, expect, test} from 'vitest'
 
 import {type DevServerInterface} from '../deriveInterfaces.js'
-import {interfaceSetId, trackInterfaceSet} from '../interfaceSetId.js'
+import {createInterfacesTracker, interfaceSetId, trackInterfaceSet} from '../interfaceSetId.js'
+import {type DevServerManifest} from '../registry.js'
 
 const panel = (name: string, src = `./src/${name}.tsx`): DevServerInterface => ({
   entry_point: src,
@@ -13,6 +14,17 @@ const worker = (name: string, src = `./src/${name}.ts`): DevServerInterface => (
   interface_type: 'worker',
   name,
 })
+const server = (id: string, port: number, interfaces: DevServerInterface[]): DevServerManifest => ({
+  host: 'localhost',
+  id,
+  interfaces,
+  pid: 1,
+  port,
+  startedAt: '2026-01-01T00:00:00.000Z',
+  type: 'coreApp',
+  version: 1,
+  workDir: '/tmp/app',
+})
 
 describe('interfaceSetId', () => {
   test('undefined and empty both id to the empty set', () => {
@@ -69,3 +81,30 @@ describe('trackInterfaceSet', () => {
     expect(trackInterfaceSet(undefined).changed([])).toBe(false)
   })
 })
+
+describe('createInterfacesTracker', () => {
+  test('the first snapshot is never a rebuild — no app is known yet', () => {
+    const tracker = createInterfacesTracker()
+    expect(tracker.hasChanged([server('a', 1, [panel('x')])])).toBe(false)
+  })
+
+  test('a known app whose set changed signals a rebuild', () => {
+    const tracker = createInterfacesTracker()
+    tracker.hasChanged([server('a', 1, [panel('x')])])
+    expect(tracker.hasChanged([server('a', 1, [panel('x'), panel('y')])])).toBe(true)
+  })
+
+  test('a newly appearing app is not a rebuild — it reconciles softly', () => {
+    const tracker = createInterfacesTracker()
+    tracker.hasChanged([server('a', 1, [panel('x')])])
+    expect(tracker.hasChanged([server('a', 1, [panel('x')]), server('b', 2, [panel('z')])])).toBe(
+      false,
+    )
+  })
+
+  test('a reorder of a known app is not a rebuild', () => {
+    const tracker = createInterfacesTracker()
+    tracker.hasChanged([server('a', 1, [panel('x'), worker('y')])])
+    expect(tracker.hasChanged([server('a', 1, [worker('y'), panel('x')])])).toBe(false)
+  })
+})
diff --git a/packages/@sanity/workbench-cli/src/actions/dev/deriveInterfaces.ts b/packages/@sanity/workbench-cli/src/actions/dev/deriveInterfaces.ts
new file mode 100644
index 000000000..6c8638421
--- /dev/null
+++ b/packages/@sanity/workbench-cli/src/actions/dev/deriveInterfaces.ts
@@ -0,0 +1,40 @@
+import {type CliConfig, isWorkbenchApp} from '@sanity/cli-core'
+
+import {type DevServerManifest} from './registry.js'
+
+/** One forwarded interface record on the dev-server registry entry. */
+export type DevServerInterface = NonNullable<DevServerManifest['interfaces']>[number]
+
+/**
+ * Map an app's `unstable_defineApp` config to the interface records forwarded on
+ * its registry entry: `views` → panels, `services` → workers, `entry` → the
+ * navigable `app` view (`entry_point` is the raw `src`, not a resolved URL).
+ * `undefined` for a non-branded app; a studio that declares `entry` is rejected
+ * (FR-026, studio app views are not implemented yet).
+ */
+export function deriveInterfaces(
+  app: CliConfig['app'],
+  options: {isApp: boolean},
+): DevServerInterface[] | undefined {
+  if (!isWorkbenchApp(app)) return undefined
+
+  if (!options.isApp && app.entry !== undefined) {
+    throw new Error('App views for studios are not implemented yet')
+  }
+
+  return [
+    ...(app.views?.map((view) => ({
+      entry_point: view.src,
+      interface_type: view.type,
+      name: view.name,
+    })) ?? []),
+    ...(app.services?.map((service) => ({
+      entry_point: service.src,
+      interface_type: service.type,
+      name: service.name,
+    })) ?? []),
+    ...(app.entry === undefined
+      ? []
+      : [{entry_point: app.entry, interface_type: 'app' as const, name: app.name}]),
+  ]
+}
diff --git a/packages/@sanity/workbench-cli/src/actions/dev/interfaceSetId.ts b/packages/@sanity/workbench-cli/src/actions/dev/interfaceSetId.ts
new file mode 100644
index 000000000..9392ead78
--- /dev/null
+++ b/packages/@sanity/workbench-cli/src/actions/dev/interfaceSetId.ts
@@ -0,0 +1,62 @@
+import {type DevServerInterface} from './deriveInterfaces.js'
+import {type DevServerManifest} from './registry.js'
+
+/**
+ * Order-independent identity of an app's interface set. Reordering
+ * `views`/`services` keeps the same id (not a change); adding, removing,
+ * renaming, or repointing one changes it. `undefined` ids to the empty set.
+ */
+export function interfaceSetId(interfaces: readonly DevServerInterface[] | undefined): string {
+  if (!interfaces || interfaces.length === 0) return ''
+  return interfaces
+    .map((iface) => [iface.interface_type, iface.name, iface.entry_point].join('::'))
+    .toSorted()
+    .join('|')
+}
+
+/**
+ * Tracks one app's interface set across config reloads. `changed` and `commit`
+ * are split so the caller commits only after the rebuild that depends on the new
+ * set succeeds — a thrown rebuild leaves it uncommitted, so the next save retries
+ * instead of skipping. Seed with the initially registered set.
+ */
+export function trackInterfaceSet(initial: readonly DevServerInterface[] | undefined): {
+  changed: (interfaces: readonly DevServerInterface[] | undefined) => boolean
+  commit: (interfaces: readonly DevServerInterface[] | undefined) => void
+} {
+  let lastId = interfaceSetId(initial)
+  return {
+    changed: (interfaces) => interfaceSetId(interfaces) !== lastId,
+    commit: (interfaces) => {
+      lastId = interfaceSetId(interfaces)
+    },
+  }
+}
+
+const serverKey = (server: DevServerManifest): string =>
+  `${server.id ?? ''}@${server.host ?? ''}:${server.port}`
+
+/**
+ * Tracks every registered app's interface set across registry snapshots (the
+ * multi-app counterpart to {@link trackInterfaceSet}). `hasChanged` is true when
+ * a *known* app's set changed since the last call — its remote was rebuilt with
+ * new exposes, so the workbench must full-reload to drop the stale remote-entry.
+ * A new/removed app or manifest-only edit isn't a rebuild.
+ */
+export function createInterfacesTracker(): {
+  hasChanged: (servers: readonly DevServerManifest[]) => boolean
+} {
+  let known = new Map<string, string>()
+  return {
+    hasChanged(servers) {
+      const rebuilt = servers.some((server) => {
+        const key = serverKey(server)
+        return known.has(key) && known.get(key) !== interfaceSetId(server.interfaces)
+      })
+      known = new Map(
+        servers.map((server) => [serverKey(server), interfaceSetId(server.interfaces)]),
+      )
+      return rebuilt
+    },
+  }
+}