diff --git a/src/core/release/statusCommand.test.ts b/src/core/release/statusCommand.test.ts
index 360a18b..120b8f9 100644
--- a/src/core/release/statusCommand.test.ts
+++ b/src/core/release/statusCommand.test.ts
@@ -4,6 +4,7 @@ import { NodeHttpClient } from '@effect/platform-node';
 import { createLogger, makeLaunchLoggerTest } from '../services/logger.js';
 import type { AppDescriptor } from '../types/app.js';
 import { classifyVerdict, type ReleaseStatus } from './appStoreRelease.js';
+import type { MutableDeep } from '../types/mutable.js';
 import {
   formatStatusLine,
   selectIosApps,
@@ -32,7 +33,7 @@ const releaseStatus = (overrides: Partial<ReleaseStatus> = {}): ReleaseStatus =>
 };
 
 const discoveredApp = (appName: string, bundleId?: string): AppDescriptor => {
-  const appDescriptor: AppDescriptor = {
+  const appDescriptor: MutableDeep<AppDescriptor> = {
     name: appName,
     dir: `/repo/${appName}`,
     configPath: `/repo/${appName}/app.json`,
diff --git a/src/core/release/testflightFeedback.ts b/src/core/release/testflightFeedback.ts
index a2d5fa2..25b9d52 100644
--- a/src/core/release/testflightFeedback.ts
+++ b/src/core/release/testflightFeedback.ts
@@ -143,7 +143,7 @@ const safeFeedbackIdentifier = (feedbackId: string): string => {
 /** Download screenshot attachments serially into one directory. */
 export const downloadFeedbackAttachments = (
   appleStore: AscFeedbackApi,
-  feedbackEntries: BetaFeedback[],
+  feedbackEntries: readonly BetaFeedback[],
   outputDirectory: string,
 ): Effect.Effect<DownloadedAttachment[], unknown, FileSystem.FileSystem | Path.Path> =>
   Effect.gen(function* () {
diff --git a/src/core/release/version.ts b/src/core/release/version.ts
index 6b917bb..b79dc99 100644
--- a/src/core/release/version.ts
+++ b/src/core/release/version.ts
@@ -72,7 +72,7 @@ export const compareVersions = (a: string, b: string): number => {
  * Used to fold App Store + TestFlight versions into a single "latest on record" without trusting the
  * store's own (lexical) sort, which would order `1.10.0` below `1.9.0`.
  */
-export const highestVersion = (versions: string[]): string | null => {
+export const highestVersion = (versions: readonly string[]): string | null => {
   const parseable = versions.filter((version) => parseVersion(version) !== null);
   if (parseable.length === 0) return null;
   return parseable.reduce((highest, version) => {
diff --git a/src/core/releaseTrain/orchestrator.ts b/src/core/releaseTrain/orchestrator.ts
index 0b41aae..fcaf3b4 100644
--- a/src/core/releaseTrain/orchestrator.ts
+++ b/src/core/releaseTrain/orchestrator.ts
@@ -10,6 +10,7 @@ import type {
   TrainState,
 } from '../types/releaseTrain.js';
 import { isCarTerminal, isNativeCar, isOtaCar } from './guards.js';
+import type { MutableDeep } from '../types/mutable.js';
 
 /** Store and OTA operations driven by the release-train state machine. */
 export type TrainEngine<Requirements = never> = Readonly<{
@@ -100,7 +101,7 @@ export const startTrain = <Requirements>(
   Effect.gen(function* () {
     const trainCars: Car[] = [];
     for (const platform of trainInput.platforms) {
-      const nativeCar: NativeCar = {
+      const nativeCar: MutableDeep<NativeCar> = {
         kind: platform,
         state: 'building',
         updatedAt: trainInput.now,
@@ -148,7 +149,7 @@ export const advanceTrain = <Requirements>(
     if (trainRecord.state === 'done') return trainRecord;
     if (trainRecord.state === 'aborted') return trainRecord;
     const forced = advanceOptions.force === true;
-    const trainCars = trainRecord.cars.map((trainCar): Car => ({ ...trainCar }));
+    const trainCars = trainRecord.cars.map((trainCar): MutableDeep<Car> => ({ ...trainCar }));
 
     for (const trainCar of trainCars) {
       if (!isNativeCar(trainCar)) continue;
@@ -160,7 +161,11 @@ export const advanceTrain = <Requirements>(
       if (!isNativeFailure(trainCar)) delete trainCar.error;
     }
 
-    const nativeCars = trainCars.filter(isNativeCar);
+    const nativeCars: MutableDeep<NativeCar>[] = [];
+    for (const trainCar of trainCars) {
+      if (!isNativeCar(trainCar)) continue;
+      nativeCars.push(trainCar);
+    }
     const hasNativeFailure = nativeCars.some(isNativeFailure);
     const allApproved = nativeCars.every(isNativeApprovedOrReleased);
     const gateOpen = isReleaseGateOpen(trainRecord.hold, forced, hasNativeFailure, allApproved);
diff --git a/src/core/releaseTrain/record.ts b/src/core/releaseTrain/record.ts
index 4cd099a..80d2f41 100644
--- a/src/core/releaseTrain/record.ts
+++ b/src/core/releaseTrain/record.ts
@@ -8,47 +8,41 @@ import {
 } from '../services/paths.js';
 import type { TrainRecord } from '../types/releaseTrain.js';
 
-const NativeCarSchema = Schema.mutable(
-  Schema.Struct({
-    kind: Schema.Literal('ios', 'android'),
-    state: Schema.Literal(
-      'building',
-      'submitted',
-      'in-review',
-      'approved',
-      'released',
-      'rejected',
-      'failed',
-    ),
-    buildId: Schema.optionalWith(Schema.String, { exact: true }),
-    error: Schema.optionalWith(Schema.String, { exact: true }),
-    updatedAt: Schema.String,
-  }),
-);
-
-const OtaCarSchema = Schema.mutable(
-  Schema.Struct({
-    kind: Schema.Literal('ota'),
-    platform: Schema.Literal('ios', 'android'),
-    channel: Schema.String,
-    runtimeVersion: Schema.String,
-    state: Schema.Literal('pending', 'published'),
-    manifestId: Schema.optionalWith(Schema.String, { exact: true }),
-    updatedAt: Schema.String,
-  }),
-);
-
-const TrainRecordSchema: Schema.Schema<TrainRecord> = Schema.mutable(
-  Schema.Struct({
-    id: Schema.String,
-    app: Schema.String,
-    hold: Schema.Boolean,
-    state: Schema.Literal('running', 'blocked', 'done', 'aborted'),
-    createdAt: Schema.String,
-    updatedAt: Schema.String,
-    cars: Schema.mutable(Schema.Array(Schema.Union(NativeCarSchema, OtaCarSchema))),
-  }),
-);
+const NativeCarSchema = Schema.Struct({
+  kind: Schema.Literal('ios', 'android'),
+  state: Schema.Literal(
+    'building',
+    'submitted',
+    'in-review',
+    'approved',
+    'released',
+    'rejected',
+    'failed',
+  ),
+  buildId: Schema.optionalWith(Schema.String, { exact: true }),
+  error: Schema.optionalWith(Schema.String, { exact: true }),
+  updatedAt: Schema.String,
+});
+
+const OtaCarSchema = Schema.Struct({
+  kind: Schema.Literal('ota'),
+  platform: Schema.Literal('ios', 'android'),
+  channel: Schema.String,
+  runtimeVersion: Schema.String,
+  state: Schema.Literal('pending', 'published'),
+  manifestId: Schema.optionalWith(Schema.String, { exact: true }),
+  updatedAt: Schema.String,
+});
+
+const TrainRecordSchema: Schema.Schema<TrainRecord> = Schema.Struct({
+  id: Schema.String,
+  app: Schema.String,
+  hold: Schema.Boolean,
+  state: Schema.Literal('running', 'blocked', 'done', 'aborted'),
+  createdAt: Schema.String,
+  updatedAt: Schema.String,
+  cars: Schema.Array(Schema.Union(NativeCarSchema, OtaCarSchema)),
+});
 
 /** A persisted release-train record could not be read or written. */
 export type TrainRecordFailure = Readonly<{