diff --git a/docs/reference/modernization-backlog.md b/docs/reference/modernization-backlog.md
index 4a0b82670..c57c6ab3d 100644
--- a/docs/reference/modernization-backlog.md
+++ b/docs/reference/modernization-backlog.md
@@ -241,8 +241,8 @@ Survey scope: Go 1.18 through 1.26. Re-run Part A on a 1.27+ toolchain bump.
 | 5 | `✓ shipped #1589` | 3a#6 | `refactor(frontend): Object.groupBy for LockGroup membersByGroup` (2 sites; needs #1) |
 | 6 | `skipped (retired — FP)` | 3a#5 | `refactor(frontend): error cause chaining on catch-rethrow` — retired, see Retired table (0 wrap-rethrow-without-cause sites) |
 | 7 | `✓ shipped #1591` | 3a#9 | `refactor(frontend): satisfies on config/route maps` (targeted) |
-| 8 | `PR open #1592` (graph.ts slice) | 3a#8 | `refactor(frontend): readonly on cache/store/API types` (type-level, large surface; split per-shape — `graph.ts` slice = #1592, `app-types.ts` composites still open) |
-| 9 | | 3a#7 | `refactor(frontend): React Query queryOptions + skipToken` (HIGH value, largest/most semantic — split as needed) |
+| 8 | `✓ shipped #1592` (graph.ts) `+ app-types` | 3a#8 | `refactor(frontend): readonly on cache/store/API types` — done in 2 slices: `graph.ts` = #1592, `app-types.ts` composites (Camper/BunkRequest/Constraint/SolverRun/DragItem/BunkWithCampers) = this PR. Both ~0 cascade (graph 0, app-types 2 trivial: `buildCamperRows` param widen + `RequestForm` useState copy) |
+| 9 | `next` | 3a#7 | `refactor(frontend): React Query queryOptions + skipToken` (HIGH value, largest/most semantic — split as needed) |
 
 Ranking is easiest→hardest with the unblocker (#1) first per Part B's tie-breaker. The HIGH-impact-but-large rows (error cause, React Query) sit later because they carry test/semantic surface, not because they're low value.
 
diff --git a/frontend/src/components/RequestForm.tsx b/frontend/src/components/RequestForm.tsx
index 565ba183a..d2d370828 100644
--- a/frontend/src/components/RequestForm.tsx
+++ b/frontend/src/components/RequestForm.tsx
@@ -16,7 +16,8 @@ interface RequestFormProps {
 export default function RequestForm({ campers, constraint, onSubmit, onCancel }: RequestFormProps) {
   const viewingYear = useYear()
   const [type, setType] = useState<ConstraintType>(constraint?.type ?? 'pair_together')
-  const [selectedCampers, setSelectedCampers] = useState<string[]>(constraint?.campers ?? [])
+  // Local mutable working copy; the source `constraint.campers` is readonly.
+  const [selectedCampers, setSelectedCampers] = useState<string[]>([...(constraint?.campers ?? [])])
   const [metadata, setMetadata] = useState<Record<string, unknown>>(constraint?.metadata ?? {})
 
   const handleSubmit = (e: FormEvent) => {
diff --git a/frontend/src/types/app-types.ts b/frontend/src/types/app-types.ts
index 82617a5d0..573b46cc8 100644
--- a/frontend/src/types/app-types.ts
+++ b/frontend/src/types/app-types.ts
@@ -1,6 +1,13 @@
 /**
  * Application-specific types for UI components and transformations
  * These types compose, extend, or alias the auto-generated PocketBase types
+ *
+ * Hand-written cache/API shapes carry `readonly` fields (and `readonly T[]` /
+ * `Readonly<Record>` on collections): these flow through the React Query cache
+ * and dozens of components, so the compiler forbids accidental in-place
+ * mutation of a shared instance. Defensive copies (`[...arr]`, `.map(...)`) are
+ * unaffected; the generated PB aliases (`Session`, `Bunk`, `SavedScenario`)
+ * are intentionally left mutable — their immutability belongs in codegen.
  */
 
 import type {
@@ -15,85 +22,85 @@ import type {
  * Used throughout the frontend for displaying and managing camper information
  */
 export interface Camper {
-  id: string // Composite ID: "person_id:session_id"
-  attendee_id?: string // PocketBase attendee ID if needed
-  name: string
-  age: number
-  grade: number
-  gender: 'M' | 'F' | 'NB'
-  session_cm_id: number // CampMinder session ID
-  assigned_bunk_cm_id?: number // CampMinder bunk ID
-  assigned_bunk?: string // PocketBase bunk ID relation
-  person_cm_id: number // CampMinder person ID
-  created: string
-  updated: string
+  readonly id: string // Composite ID: "person_id:session_id"
+  readonly attendee_id?: string // PocketBase attendee ID if needed
+  readonly name: string
+  readonly age: number
+  readonly grade: number
+  readonly gender: 'M' | 'F' | 'NB'
+  readonly session_cm_id: number // CampMinder session ID
+  readonly assigned_bunk_cm_id?: number // CampMinder bunk ID
+  readonly assigned_bunk?: string // PocketBase bunk ID relation
+  readonly person_cm_id: number // CampMinder person ID
+  readonly created: string
+  readonly updated: string
   // Additional fields from CampMinder
-  first_name?: string
-  last_name?: string
-  preferred_name?: string
-  birthdate?: string
-  years_at_camp?: number
-  last_year_attended?: number
-  school?: string
-  pronouns?: string
-  email?: string
-  parent_email?: string
-  phone?: string
-  tags?: string[]
-  socialize_with_best?: string
-  socialize_with_best_explain?: string
+  readonly first_name?: string
+  readonly last_name?: string
+  readonly preferred_name?: string
+  readonly birthdate?: string
+  readonly years_at_camp?: number
+  readonly last_year_attended?: number
+  readonly school?: string
+  readonly pronouns?: string
+  readonly email?: string
+  readonly parent_email?: string
+  readonly phone?: string
+  readonly tags?: readonly string[]
+  readonly socialize_with_best?: string
+  readonly socialize_with_best_explain?: string
   // Additional V2 fields
-  lead_date?: string
-  tshirt_size?: string
-  camp_grade_name?: string
-  school_grade_name?: string
+  readonly lead_date?: string
+  readonly tshirt_size?: string
+  readonly camp_grade_name?: string
+  readonly school_grade_name?: string
   // V2 Schema fields
-  gender_identity_id?: number
-  gender_identity_name?: string
-  gender_identity_write_in?: string
-  gender_pronoun_id?: number
-  gender_pronoun_name?: string
-  gender_pronoun_write_in?: string
-  household_id?: number
-  primary_household_id?: string
-  alternate_household_id?: string
-  external_id?: string
-  primary_email?: string
-  secondary_email?: string
-  bunking_requests?: Array<{
-    id?: string
-    type?: string
-    requested_person_id?: number
-    is_first_requested?: boolean
-    [key: string]: unknown
-  }> // Array of bunking requests
-  custom_fields?: Record<string, unknown>
-  attendee_status?: string
-  attendee_created?: string
-  attendee_updated?: string
-  last_updated_utc?: string
+  readonly gender_identity_id?: number
+  readonly gender_identity_name?: string
+  readonly gender_identity_write_in?: string
+  readonly gender_pronoun_id?: number
+  readonly gender_pronoun_name?: string
+  readonly gender_pronoun_write_in?: string
+  readonly household_id?: number
+  readonly primary_household_id?: string
+  readonly alternate_household_id?: string
+  readonly external_id?: string
+  readonly primary_email?: string
+  readonly secondary_email?: string
+  readonly bunking_requests?: readonly {
+    readonly id?: string
+    readonly type?: string
+    readonly requested_person_id?: number
+    readonly is_first_requested?: boolean
+    readonly [key: string]: unknown
+  }[] // Array of bunking requests
+  readonly custom_fields?: Readonly<Record<string, unknown>>
+  readonly attendee_status?: string
+  readonly attendee_created?: string
+  readonly attendee_updated?: string
+  readonly last_updated_utc?: string
   // Expanded fields
-  expand?: {
-    session?: CampSessionsResponse | null
-    assigned_bunk?: BunksResponse | null
-    tags?: Array<{
-      id: string
-      name: string
-      category: string | null
-      is_seasonal?: boolean
-    }>
-    person_tag_assignments?: Array<{
-      id: string
-      expand?: {
-        tag?: {
-          id: string
-          name: string
-          category: string | null
-          is_seasonal?: boolean
+  readonly expand?: {
+    readonly session?: CampSessionsResponse | null
+    readonly assigned_bunk?: BunksResponse | null
+    readonly tags?: readonly {
+      readonly id: string
+      readonly name: string
+      readonly category: string | null
+      readonly is_seasonal?: boolean
+    }[]
+    readonly person_tag_assignments?: readonly {
+      readonly id: string
+      readonly expand?: {
+        readonly tag?: {
+          readonly id: string
+          readonly name: string
+          readonly category: string | null
+          readonly is_seasonal?: boolean
         }
       }
-    }>
-    attendee?: AttendeesResponse
+    }[]
+    readonly attendee?: AttendeesResponse
   }
 }
 
@@ -115,66 +122,66 @@ export type Session = CampSessionsResponse
 export type Bunk = BunksResponse
 
 export interface BunkRequest {
-  id: string
-  requester_id: number // CampMinder person ID
-  requestee_id?: number | null // CampMinder person ID
-  request_type: 'bunk_with' | 'not_bunk_with' | 'age_preference'
-  is_first_requested?: boolean
-  year: number
-  session_id: number // CampMinder session ID
-  status: 'resolved' | 'pending' | 'declined'
-  original_text?: string
-  confidence_score?: number
-  parse_notes?: string
-  socialize_explain?: string
-  source_field?: string // CSV field this came from (bunk_with, not_bunk_with, manual, etc.)
-  is_reciprocal?: boolean
-  manual_notes?: string
+  readonly id: string
+  readonly requester_id: number // CampMinder person ID
+  readonly requestee_id?: number | null // CampMinder person ID
+  readonly request_type: 'bunk_with' | 'not_bunk_with' | 'age_preference'
+  readonly is_first_requested?: boolean
+  readonly year: number
+  readonly session_id: number // CampMinder session ID
+  readonly status: 'resolved' | 'pending' | 'declined'
+  readonly original_text?: string
+  readonly confidence_score?: number
+  readonly parse_notes?: string
+  readonly socialize_explain?: string
+  readonly source_field?: string // CSV field this came from (bunk_with, not_bunk_with, manual, etc.)
+  readonly is_reciprocal?: boolean
+  readonly manual_notes?: string
   // Age preference specific
-  age_preference_target?: string // 'older' or 'younger'
-  metadata?: Record<string, unknown> // JSON metadata field
-  ai_reasoning?: {
-    csv_source_field?: string
-    [key: string]: unknown
+  readonly age_preference_target?: string // 'older' or 'younger'
+  readonly metadata?: Readonly<Record<string, unknown>> // JSON metadata field
+  readonly ai_reasoning?: {
+    readonly csv_source_field?: string
+    readonly [key: string]: unknown
   }
   // Additional fields from DB
-  confidence_level?: string
-  keywords_found?: Record<string, unknown>
-  can_be_dropped?: boolean
-  is_placeholder?: boolean
-  requires_manual_review?: boolean
-  manual_review_reason?: string
-  was_dropped_for_spread?: boolean
+  readonly confidence_level?: string
+  readonly keywords_found?: Readonly<Record<string, unknown>>
+  readonly can_be_dropped?: boolean
+  readonly is_placeholder?: boolean
+  readonly requires_manual_review?: boolean
+  readonly manual_review_reason?: string
+  readonly was_dropped_for_spread?: boolean
   // Merge tracking — empty string or undefined means not merged; non-empty is the PB record ID it was merged into
-  merged_into?: string
-  created: string
-  updated: string
+  readonly merged_into?: string
+  readonly created: string
+  readonly updated: string
 }
 
 export type ConstraintType = 'pair_together' | 'keep_apart' | 'age_preference' | 'bunk_preference'
 
 export interface Constraint {
-  id: string
-  description: string // Required in DB
-  session_id: number // CampMinder session ID
-  scope?: 'global' | 'single' | 'pair'
-  severity?: 'hard' | 'soft'
-  single_camper_id?: number // CampMinder person ID for single constraints
-  pair_camper1_id?: number // CampMinder person ID for pair constraints
-  pair_camper2_id?: number // CampMinder person ID for pair constraints
-  year: number
-  created: string
-  updated: string
+  readonly id: string
+  readonly description: string // Required in DB
+  readonly session_id: number // CampMinder session ID
+  readonly scope?: 'global' | 'single' | 'pair'
+  readonly severity?: 'hard' | 'soft'
+  readonly single_camper_id?: number // CampMinder person ID for single constraints
+  readonly pair_camper1_id?: number // CampMinder person ID for pair constraints
+  readonly pair_camper2_id?: number // CampMinder person ID for pair constraints
+  readonly year: number
+  readonly created: string
+  readonly updated: string
   // Legacy fields for backward compatibility
-  constraint_type?: string
-  type?: ConstraintType
-  session?: string
-  campers?: string[]
-  metadata?: Record<string, unknown>
+  readonly constraint_type?: string
+  readonly type?: ConstraintType
+  readonly session?: string
+  readonly campers?: readonly string[]
+  readonly metadata?: Readonly<Record<string, unknown>>
   // Expanded fields
-  expand?: {
-    session?: CampSessionsResponse
-    campers?: Camper[]
+  readonly expand?: {
+    readonly session?: CampSessionsResponse
+    readonly campers?: readonly Camper[]
   }
 }
 
@@ -190,48 +197,48 @@ export type SavedScenario = SavedScenariosResponse<unknown, { session?: CampSess
  * Kept in app-types because it represents a solver API response shape.
  */
 export interface SolverRun {
-  id: string
-  session: string
-  status: 'pending' | 'running' | 'completed' | 'failed'
-  constraints_snapshot?: Record<string, unknown>
-  locked_bunks?: string[]
-  results?: {
-    assignments: Array<{
-      camper_id: string
-      bunk_id: string
-    }>
-    stats: {
-      total_campers: number
-      assigned_campers: number
-      satisfied_constraints: number
-      total_constraints: number
-      solve_time_ms: number
+  readonly id: string
+  readonly session: string
+  readonly status: 'pending' | 'running' | 'completed' | 'failed'
+  readonly constraints_snapshot?: Readonly<Record<string, unknown>>
+  readonly locked_bunks?: readonly string[]
+  readonly results?: {
+    readonly assignments: readonly {
+      readonly camper_id: string
+      readonly bunk_id: string
+    }[]
+    readonly stats: {
+      readonly total_campers: number
+      readonly assigned_campers: number
+      readonly satisfied_constraints: number
+      readonly total_constraints: number
+      readonly solve_time_ms: number
       // Request-based stats (newer API)
-      satisfied_request_count?: number
-      total_requests?: number
-      request_validation?: {
-        impossible_requests: number
-        affected_campers: number
+      readonly satisfied_request_count?: number
+      readonly total_requests?: number
+      readonly request_validation?: {
+        readonly impossible_requests: number
+        readonly affected_campers: number
       }
     }
   }
-  error_message?: string
-  started_at?: string
-  completed_at?: string
-  created: string
-  updated: string
+  readonly error_message?: string
+  readonly started_at?: string
+  readonly completed_at?: string
+  readonly created: string
+  readonly updated: string
 }
 
 // UI-specific types
 export interface DragItem {
-  id: string
-  type: 'camper'
-  camper: Camper
-  sourceBunkId?: string
+  readonly id: string
+  readonly type: 'camper'
+  readonly camper: Camper
+  readonly sourceBunkId?: string
 }
 
 export interface BunkWithCampers extends Bunk {
-  campers: Camper[]
-  occupancy: number
-  utilization: number
+  readonly campers: readonly Camper[]
+  readonly occupancy: number
+  readonly utilization: number
 }
diff --git a/frontend/src/utils/csvExportHelpers.ts b/frontend/src/utils/csvExportHelpers.ts
index a2a3e5797..49aa6b6fc 100644
--- a/frontend/src/utils/csvExportHelpers.ts
+++ b/frontend/src/utils/csvExportHelpers.ts
@@ -70,7 +70,7 @@ export interface BuildCamperRowsOptions {
  * @returns        - Array of string[] where each inner array is one CSV data row
  */
 export function buildCamperRows(
-  campers: Camper[],
+  campers: readonly Camper[],
   sessions: Session[],
   options: BuildCamperRowsOptions = {}
 ): string[][] {