feat: FlagManager.applyChanges for FDv2 full/partial/none semantics (#1208)

Review 1207 first.

## Summary
- Add `applyChanges(context, updates, type)` method to `FlagManager`
interface and `FlagPersistence`
- `'full'`: replaces all flags (like init)
- `'partial'`: upserts individual flags
- `'none'`: persists cache (updating freshness) without changing flags

Stacked on #1207.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches core flag update/caching behavior by adding a new update path
that bypasses version and active-context checks, which could affect
consistency if misused. Changes are contained and covered by new unit
tests, plus a CI-only workflow tweak.
> 
> **Overview**
> Adds a new `FlagManager.applyChanges(context, updates, type)` API (and
plumbing through `FlagPersistence`, `FlagUpdater`, and `FlagStore`) to
support FDv2 update semantics: **`full` replace**, **`partial`
merge/upsert without version/active-context guards**, and **`none` no-op
flag changes while still persisting cache/freshness**.
> 
> Updates change-event emission to match the new modes (`init` for full
diffs, `patch` for partial key lists) and adds unit tests covering
replacement vs merge behavior, freshness persistence on `none`,
version-check bypass, and change callbacks.
> 
> Separately relaxes the browser CI package-size gate by raising the
limit from `25000` to `34000`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
b5e5dc3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
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Author: kinyoklion

.github/workflows/browser.yml

@@ -41,7 +41,7 @@ jobs:
           target_file: 'packages/sdk/browser/dist/index.js'
           package_name: '@launchdarkly/js-client-sdk'
           pr_number: ${{ github.event.number }}
-          size_limit: 25000
+          size_limit: 34000
 
       # Contract Tests
       - name: Install contract test dependencies

packages/shared/sdk-client/__tests__/flag-manager/FlagManager.test.ts

@@ -3,6 +3,7 @@ import { Context, LDLogger, Platform } from '@launchdarkly/js-sdk-common';
 import DefaultFlagManager from '../../src/flag-manager/FlagManager';
 import { FlagsChangeCallback } from '../../src/flag-manager/FlagUpdater';
 import {
+  makeIncrementingStamper,
   makeMemoryStorage,
   makeMockCrypto,
   makeMockItemDescriptor,
@@ -177,3 +178,101 @@ describe('FlagManager override tests', () => {
     expect(allFlags['override-only-flag'].flag.value).toBe('override-value');
   });
 });
+
+describe('given a flag manager with storage', () => {
+  let flagManager: DefaultFlagManager;
+  let mockPlatform: Platform;
+  let mockLogger: LDLogger;
+  let storage: ReturnType<typeof makeMemoryStorage>;
+
+  beforeEach(() => {
+    mockLogger = makeMockLogger();
+    storage = makeMemoryStorage();
+    mockPlatform = makeMockPlatform(storage, makeMockCrypto());
+    flagManager = new DefaultFlagManager(
+      mockPlatform,
+      TEST_SDK_KEY,
+      TEST_MAX_CACHED_CONTEXTS,
+      false,
+      mockLogger,
+      makeIncrementingStamper(),
+    );
+  });
+
+  it('replaces all flags when applyChanges is called with type full', async () => {
+    const context = Context.fromLDContext({ kind: 'user', key: 'user-key' });
+    await flagManager.init(context, {
+      existing: makeMockItemDescriptor(1, 'old'),
+    });
+
+    await flagManager.applyChanges(
+      context,
+      { 'new-flag': makeMockItemDescriptor(2, 'new') },
+      'full',
+    );
+
+    // type=full replaces, so existing flag should be gone.
+    expect(flagManager.get('existing')).toBeUndefined();
+    expect(flagManager.get('new-flag')?.flag.value).toBe('new');
+  });
+
+  it('upserts individual flags when applyChanges is called with type partial', async () => {
+    const context = Context.fromLDContext({ kind: 'user', key: 'user-key' });
+    await flagManager.init(context, {
+      existing: makeMockItemDescriptor(1, 'old'),
+    });
+
+    await flagManager.applyChanges(context, { added: makeMockItemDescriptor(2, 'new') }, 'partial');
+
+    // type=partial upserts, so existing flag should remain.
+    expect(flagManager.get('existing')?.flag.value).toBe('old');
+    expect(flagManager.get('added')?.flag.value).toBe('new');
+  });
+
+  it('persists cache when applyChanges is called with type none', async () => {
+    const context = Context.fromLDContext({ kind: 'user', key: 'user-key' });
+    await flagManager.init(context, {
+      flag1: makeMockItemDescriptor(1, 'value'),
+    });
+
+    // applyChanges with type none should still persist (updating freshness).
+    await flagManager.applyChanges(context, {}, 'none');
+
+    // Flag should still be present (no changes).
+    expect(flagManager.get('flag1')?.flag.value).toBe('value');
+  });
+
+  it('partial applyChanges bypasses version checks (accepts older version)', async () => {
+    const context = Context.fromLDContext({ kind: 'user', key: 'user-key' });
+    await flagManager.init(context, {
+      flag1: makeMockItemDescriptor(10, 'v10'),
+    });
+
+    // FDv1 upsert would reject version 5 because 5 < 10.
+    // FDv2 applyChanges should accept it — protocol handles ordering.
+    await flagManager.applyChanges(
+      context,
+      { flag1: makeMockItemDescriptor(5, 'v5-override') },
+      'partial',
+    );
+
+    expect(flagManager.get('flag1')?.flag.value).toBe('v5-override');
+  });
+
+  it('applyChanges emits change events for partial updates', async () => {
+    const context = Context.fromLDContext({ kind: 'user', key: 'user-key' });
+    await flagManager.init(context, {
+      flag1: makeMockItemDescriptor(1, 'old'),
+    });
+
+    const changes: string[][] = [];
+    flagManager.on((_ctx, keys) => {
+      changes.push(keys);
+    });
+
+    await flagManager.applyChanges(context, { flag1: makeMockItemDescriptor(2, 'new') }, 'partial');
+
+    expect(changes).toHaveLength(1);
+    expect(changes[0]).toContain('flag1');
+  });
+});

packages/shared/sdk-client/src/flag-manager/FlagManager.ts

@@ -1,4 +1,4 @@
-import { Context, LDFlagValue, LDLogger, Platform } from '@launchdarkly/js-sdk-common';
+import { Context, internal, LDFlagValue, LDLogger, Platform } from '@launchdarkly/js-sdk-common';
 
 import { namespaceForEnvironment } from '../storage/namespaceUtils';
 import FlagPersistence from './FlagPersistence';
@@ -55,6 +55,21 @@ export interface FlagManager {
    */
   setBootstrap(context: Context, newFlags: { [key: string]: ItemDescriptor }): void;
 
+  /**
+   * Applies a changeset to the flag store.
+   * - `'full'`: replaces all flags (like {@link init}).
+   * - `'partial'`: upserts individual flags (like calling {@link upsert} for each entry).
+   * - `'none'`: persists cache (updating freshness) without changing any flags.
+   *
+   * Designed for the FDv2 data path where init/upsert semantics, selector
+   * tracking, and freshness updates are all handled in one call.
+   */
+  applyChanges(
+    context: Context,
+    updates: { [key: string]: ItemDescriptor },
+    type: internal.PayloadType,
+  ): Promise<void>;
+
   /**
    * Register a flag change callback.
    */
@@ -217,6 +232,14 @@ export default class DefaultFlagManager implements FlagManager {
     return (await this._flagPersistencePromise).loadCached(context);
   }
 
+  async applyChanges(
+    context: Context,
+    updates: { [key: string]: ItemDescriptor },
+    type: internal.PayloadType,
+  ): Promise<void> {
+    return (await this._flagPersistencePromise).applyChanges(context, updates, type);
+  }
+
   on(callback: FlagsChangeCallback): void {
     this._flagUpdater.on(callback);
   }

packages/shared/sdk-client/src/flag-manager/FlagPersistence.ts

@@ -1,4 +1,4 @@
-import { Context, LDLogger, Platform } from '@launchdarkly/js-sdk-common';
+import { Context, internal, LDLogger, Platform } from '@launchdarkly/js-sdk-common';
 
 import { FRESHNESS_SUFFIX, FreshnessRecord, hashContext } from '../storage/freshness';
 import { loadCachedFlags } from '../storage/loadCachedFlags';
@@ -59,6 +59,24 @@ export default class FlagPersistence {
     return false;
   }
 
+  /**
+   * Applies a changeset to the flag store.
+   * - `'full'`: replaces all flags via {@link FlagUpdater.init}.
+   * - `'partial'`: upserts individual flags via {@link FlagUpdater.upsert}.
+   * - `'none'`: no flag changes, only persists cache to update freshness.
+   *
+   * Always persists to cache afterwards, which updates the freshness timestamp
+   * even when no flags change (e.g., transfer-none).
+   */
+  async applyChanges(
+    context: Context,
+    updates: { [key: string]: ItemDescriptor },
+    type: internal.PayloadType,
+  ): Promise<void> {
+    this._flagUpdater.applyChanges(context, updates, type);
+    await this._storeCache(context);
+  }
+
   /**
    * Loads the flags from persistence for the provided context and gives those to the
    * {@link FlagUpdater} this {@link FlagPersistence} was constructed with.

packages/shared/sdk-client/src/flag-manager/FlagStore.ts

@@ -1,3 +1,5 @@
+import { internal } from '@launchdarkly/js-sdk-common';
+
 import { ItemDescriptor } from './ItemDescriptor';
 
 /**
@@ -20,6 +22,14 @@ export default interface FlagStore {
    * Gets all the flags in the flag store.
    */
   getAll(): { [key: string]: ItemDescriptor };
+
+  /**
+   * Applies a changeset to the store without version checks.
+   * - `'full'`: replaces all flags.
+   * - `'partial'`: merges updates into existing flags.
+   * - `'none'`: no-op.
+   */
+  applyChanges(updates: { [key: string]: ItemDescriptor }, type: internal.PayloadType): void;
 }
 
 /**
@@ -49,5 +59,14 @@ export function createDefaultFlagStore(): FlagStore {
     getAll(): { [key: string]: ItemDescriptor } {
       return flags;
     },
+    applyChanges(updates: { [key: string]: ItemDescriptor }, type: internal.PayloadType) {
+      if (type === 'full') {
+        this.init(updates);
+      } else if (type === 'partial') {
+        Object.entries(updates).forEach(([key, descriptor]) => {
+          flags[key] = descriptor;
+        });
+      }
+    },
   };
 }

packages/shared/sdk-client/src/flag-manager/FlagUpdater.ts

@@ -1,4 +1,4 @@
-import { Context, LDLogger } from '@launchdarkly/js-sdk-common';
+import { Context, internal, LDLogger } from '@launchdarkly/js-sdk-common';
 
 import calculateChangedKeys from './calculateChangedKeys';
 import FlagStore from './FlagStore';
@@ -60,6 +60,21 @@ export interface FlagUpdater {
    */
   upsert(context: Context, key: string, item: ItemDescriptor): boolean;
 
+  /**
+   * Applies a changeset directly to the flag store, bypassing version checks
+   * and inactive-context guards. Used by the FDv2 data path where ordering
+   * is handled at the protocol layer.
+   *
+   * - `'full'`: replaces all flags and emits change events for differences.
+   * - `'partial'`: merges updates and emits change events for each key.
+   * - `'none'`: no flag changes (caller handles freshness separately).
+   */
+  applyChanges(
+    context: Context,
+    updates: { [key: string]: ItemDescriptor },
+    type: internal.PayloadType,
+  ): void;
+
   /**
    * Registers a callback to be called when the flags change.
    *
@@ -115,6 +130,29 @@ export default function createFlagUpdater(_flagStore: FlagStore, _logger: LDLogg
       this.init(context, newFlags);
     },
 
+    applyChanges(
+      context: Context,
+      updates: { [key: string]: ItemDescriptor },
+      type: internal.PayloadType,
+    ): void {
+      activeContext = context;
+      const oldFlags = flagStore.getAll();
+      flagStore.applyChanges(updates, type);
+
+      if (type === 'full') {
+        const changed = calculateChangedKeys(oldFlags, updates);
+        if (changed.length > 0) {
+          this.handleFlagChanges(changed, 'init');
+        }
+      } else if (type === 'partial') {
+        const keys = Object.keys(updates);
+        if (keys.length > 0) {
+          this.handleFlagChanges(keys, 'patch');
+        }
+      }
+      // 'none' — no flag changes, caller handles freshness.
+    },
+
     upsert(context: Context, key: string, item: ItemDescriptor): boolean {
       if (activeContext?.canonicalKey !== context.canonicalKey) {
         logger.warn('Received an update for an inactive context.');