diff --git a/src/renderer/store.ts b/src/renderer/store.ts
index c0802bc4..24c55f8a 100644
--- a/src/renderer/store.ts
+++ b/src/renderer/store.ts
@@ -27,9 +27,11 @@
 import { useSyncExternalStore } from 'react'
 import {
   initialState,
+  mergeWireSnapshot,
   rootReducer,
   type AppState,
-  type StateEvent
+  type StateEvent,
+  type WireSnapshotState
 } from '../shared/state'
 import type { LocalTransportHandle, BackendConnection } from './types'
 import { WebSocketClientTransport } from '../shared/transport/transport-websocket'
@@ -53,12 +55,17 @@ class ClientStore {
     for (const l of this.listeners) l()
   }
 
-  setSnapshot(state: AppState): void {
-    // Merge with initialState so a remote backend on an older harness-server
-    // version (missing recently-added slices like snooze, repoConfigs, etc.)
-    // doesn't surface as renderer crashes on the first selector access of
-    // an undefined slice. New renderer + old server is the common skew.
-    this.state = { ...initialState, ...state }
+  setSnapshot(state: WireSnapshotState): void {
+    // Per-slice merge against initialState. This is the wire-side trust
+    // boundary: a remote `harness-server` on an older version may be
+    // missing entire slices (added after it shipped) AND/OR be missing
+    // individual fields inside slices it does send (e.g. v2.9.3 sends
+    // `settings` without `customThemes`, which 99262b2 added). A
+    // top-level shallow merge would only fix the first case; the
+    // per-slice merge fixes both. New renderer + old server is the
+    // common skew. The shared helper enforces — via the AppState return
+    // type — that future slice additions can't silently miss this list.
+    this.state = mergeWireSnapshot(state)
     for (const l of this.listeners) l()
   }
 
diff --git a/src/shared/state/index.ts b/src/shared/state/index.ts
index 8ac6a86f..11c7d062 100644
--- a/src/shared/state/index.ts
+++ b/src/shared/state/index.ts
@@ -262,6 +262,44 @@ export interface StateSnapshot {
   seq: number
 }
 
+/** Snapshot shape as it comes off the wire. Each slice may be entirely
+ *  absent (older server with a not-yet-existing slice) or present but
+ *  missing recently-added fields (older server with the slice but an
+ *  older schema). Consumers must merge with `initialState` defaults via
+ *  `mergeWireSnapshot` before treating the value as a full `AppState`. */
+export type WireSnapshotState = {
+  [K in keyof AppState]?: Partial<AppState[K]>
+}
+
+/** Per-slice shallow merge of a wire snapshot against `initialState`
+ *  defaults. Protects against two version skews:
+ *
+ *   1. Older server is missing an entire slice (e.g. `snooze` added after
+ *      the server shipped) — `initialState[slice]` fills it in.
+ *   2. Older server has the slice but is missing a recently-added field
+ *      (e.g. `settings.customThemes` added in 99262b2 after v2.9.3) —
+ *      the per-field default from `initialState[slice]` fills it in.
+ *
+ *  If a future PR adds a slice to `AppState`/`initialState` and forgets
+ *  to add a line here, TypeScript will fail the build: the object
+ *  literal won't satisfy `AppState`. */
+export function mergeWireSnapshot(state: WireSnapshotState): AppState {
+  return {
+    settings: { ...initialState.settings, ...state.settings },
+    prs: { ...initialState.prs, ...state.prs },
+    onboarding: { ...initialState.onboarding, ...state.onboarding },
+    hooks: { ...initialState.hooks, ...state.hooks },
+    worktrees: { ...initialState.worktrees, ...state.worktrees },
+    terminals: { ...initialState.terminals, ...state.terminals },
+    updater: { ...initialState.updater, ...state.updater },
+    repoConfigs: { ...initialState.repoConfigs, ...state.repoConfigs },
+    costs: { ...initialState.costs, ...state.costs },
+    browser: { ...initialState.browser, ...state.browser },
+    jsonClaude: { ...initialState.jsonClaude, ...state.jsonClaude },
+    snooze: { ...initialState.snooze, ...state.snooze }
+  }
+}
+
 /** Returns a snapshot with `jsonClaude.sessions[*].entries` elided.
  *  Transports call this before serializing the initial-snapshot frame to
  *  keep the wire payload bounded by the rest of the state — entries grow
diff --git a/src/shared/state/wire-merge.test.ts b/src/shared/state/wire-merge.test.ts
new file mode 100644
index 00000000..867b18ae
--- /dev/null
+++ b/src/shared/state/wire-merge.test.ts
@@ -0,0 +1,71 @@
+import { describe, it, expect } from 'vitest'
+import {
+  initialState,
+  mergeWireSnapshot,
+  type AppState,
+  type WireSnapshotState
+} from './index'
+import { EMPTY_CUSTOM_THEMES } from './settings'
+
+describe('mergeWireSnapshot', () => {
+  it('fills in a recently-added field missing from an older servers settings', () => {
+    // Repro of the v2.9.3 server skew: commit 99262b2 added
+    // `customThemes`, so a v2.9.3 snapshot's `settings` object lacks the
+    // field. A top-level shallow merge would clobber initial.settings
+    // wholesale and leave customThemes undefined; the per-slice merge
+    // backfills the default array.
+    const wire: WireSnapshotState = {
+      settings: {
+        themeMode: 'system',
+        themeLight: 'solarized-light',
+        themeDark: 'dark'
+        // no customThemes
+      }
+    }
+    const merged = mergeWireSnapshot(wire)
+    expect(merged.settings.customThemes).toBe(EMPTY_CUSTOM_THEMES)
+    expect(merged.settings.themeMode).toBe('system')
+    expect(merged.settings.themeLight).toBe('solarized-light')
+    expect(merged.settings.themeDark).toBe('dark')
+  })
+
+  it('fills in an entirely-missing slice from initialState', () => {
+    const wire: WireSnapshotState = {}
+    const merged = mergeWireSnapshot(wire)
+    expect(merged.snooze).toEqual(initialState.snooze)
+    expect(merged.repoConfigs).toEqual(initialState.repoConfigs)
+    expect(merged.jsonClaude).toEqual(initialState.jsonClaude)
+    expect(merged.settings).toEqual(initialState.settings)
+  })
+
+  it('preserves server-sent values when the snapshot is complete', () => {
+    const wire: AppState = {
+      ...initialState,
+      settings: {
+        ...initialState.settings,
+        themeMode: 'dark',
+        customThemes: [
+          { id: 'noir', name: 'Noir', mode: 'dark', colors: { bg: '#000' } }
+        ]
+      }
+    }
+    const merged = mergeWireSnapshot(wire)
+    expect(merged.settings.themeMode).toBe('dark')
+    expect(merged.settings.customThemes).toEqual([
+      { id: 'noir', name: 'Noir', mode: 'dark', colors: { bg: '#000' } }
+    ])
+  })
+
+  it('respects an explicit empty array from a mid-version server', () => {
+    const wire: WireSnapshotState = {
+      settings: {
+        themeMode: 'light',
+        themeLight: 'solarized-light',
+        themeDark: 'dark',
+        customThemes: []
+      }
+    }
+    const merged = mergeWireSnapshot(wire)
+    expect(merged.settings.customThemes).toEqual([])
+  })
+})