diff --git a/packages/shared/sdk-client/__tests__/datasource/fdv2/FDv2SourceResult.test.ts b/packages/shared/sdk-client/__tests__/datasource/fdv2/FDv2SourceResult.test.ts
index 39104f4e6c..3d828c9046 100644
--- a/packages/shared/sdk-client/__tests__/datasource/fdv2/FDv2SourceResult.test.ts
+++ b/packages/shared/sdk-client/__tests__/datasource/fdv2/FDv2SourceResult.test.ts
@@ -95,6 +95,30 @@ it('creates a goodbye status result', () => {
   });
 });
 
+it('creates a goodbye status result with fdv1Fallback and a TTL', () => {
+  const result = goodbye('server-shutdown', true, 5000);
+
+  expect(result).toEqual({
+    type: 'status',
+    state: 'goodbye',
+    reason: 'server-shutdown',
+    fdv1Fallback: true,
+    fdv1FallbackTtlMs: 5000,
+  });
+});
+
+it('creates a goodbye status result with TTL 0 (indefinite fallback)', () => {
+  const result = goodbye('server-shutdown', true, 0);
+
+  expect(result).toEqual({
+    type: 'status',
+    state: 'goodbye',
+    reason: 'server-shutdown',
+    fdv1Fallback: true,
+    fdv1FallbackTtlMs: 0,
+  });
+});
+
 it('creates error info from an HTTP status code', () => {
   const info = errorInfoFromHttpError(503);
 
diff --git a/packages/shared/sdk-client/__tests__/datasource/fdv2/fallbackDirective.test.ts b/packages/shared/sdk-client/__tests__/datasource/fdv2/fallbackDirective.test.ts
new file mode 100644
index 0000000000..d8b0aa1097
--- /dev/null
+++ b/packages/shared/sdk-client/__tests__/datasource/fdv2/fallbackDirective.test.ts
@@ -0,0 +1,132 @@
+import {
+  readFallbackDirective,
+  readGoodbyeFallbackDirective,
+} from '../../../src/datasource/fdv2/fallbackDirective';
+
+function makeHeaders(map: Record<string, string>): { get(name: string): string | null } {
+  const lower: Record<string, string> = {};
+  Object.entries(map).forEach(([k, v]) => {
+    lower[k.toLowerCase()] = v;
+  });
+  return {
+    get: (name: string) => lower[name.toLowerCase()] ?? null,
+  };
+}
+
+it('returns fdv1Fallback false when x-ld-fd-fallback header is absent', () => {
+  const result = readFallbackDirective(makeHeaders({}));
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('returns fdv1Fallback false when x-ld-fd-fallback is not "true"', () => {
+  const result = readFallbackDirective(makeHeaders({ 'x-ld-fd-fallback': 'false' }));
+  expect(result.fdv1Fallback).toBe(false);
+});
+
+it('matches "true" case-insensitively', () => {
+  const result = readFallbackDirective(makeHeaders({ 'x-ld-fd-fallback': 'True' }));
+  expect(result.fdv1Fallback).toBe(true);
+});
+
+it('returns fdv1Fallback true with undefined TTL when x-ld-fd-fallback-ttl is absent', () => {
+  const result = readFallbackDirective(makeHeaders({ 'x-ld-fd-fallback': 'true' }));
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('converts a TTL of "60" seconds to 60000 ms', () => {
+  const result = readFallbackDirective(
+    makeHeaders({ 'x-ld-fd-fallback': 'true', 'x-ld-fd-fallback-ttl': '60' }),
+  );
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(60000);
+});
+
+it('converts TTL "0" to 0 ms (indefinite fallback)', () => {
+  const result = readFallbackDirective(
+    makeHeaders({ 'x-ld-fd-fallback': 'true', 'x-ld-fd-fallback-ttl': '0' }),
+  );
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(0);
+});
+
+it('returns undefined TTL for a non-numeric x-ld-fd-fallback-ttl value', () => {
+  const result = readFallbackDirective(
+    makeHeaders({ 'x-ld-fd-fallback': 'true', 'x-ld-fd-fallback-ttl': 'soon' }),
+  );
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('clamps negative TTL seconds to 0 ms (treated as indefinite)', () => {
+  const result = readFallbackDirective(
+    makeHeaders({ 'x-ld-fd-fallback': 'true', 'x-ld-fd-fallback-ttl': '-5' }),
+  );
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(0);
+});
+
+it('header lookup is case-insensitive', () => {
+  // Verify the makeHeaders helper lowercases keys, and readFallbackDirective
+  // passes lowercase names to headers.get() as documented.
+  const result = readFallbackDirective(
+    makeHeaders({ 'X-LD-FD-FALLBACK': 'true', 'X-LD-FD-FALLBACK-TTL': '30' }),
+  );
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(30000);
+});
+
+it('readGoodbyeFallbackDirective: returns fdv1Fallback false when protocolFallbackTTL is absent', () => {
+  const result = readGoodbyeFallbackDirective({ reason: 'bye' });
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('readGoodbyeFallbackDirective: returns fdv1Fallback false when data is null', () => {
+  const result = readGoodbyeFallbackDirective(null);
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('readGoodbyeFallbackDirective: returns fdv1Fallback false when data is undefined', () => {
+  const result = readGoodbyeFallbackDirective(undefined);
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('readGoodbyeFallbackDirective: converts a protocolFallbackTTL of 60 seconds to 60000 ms', () => {
+  const result = readGoodbyeFallbackDirective({ reason: 'falling back', protocolFallbackTTL: 60 });
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(60000);
+});
+
+it('readGoodbyeFallbackDirective: converts protocolFallbackTTL 0 to 0 ms (indefinite fallback)', () => {
+  const result = readGoodbyeFallbackDirective({ reason: 'falling back', protocolFallbackTTL: 0 });
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(0);
+});
+
+it('readGoodbyeFallbackDirective: clamps negative protocolFallbackTTL to 0 ms', () => {
+  const result = readGoodbyeFallbackDirective({ reason: 'falling back', protocolFallbackTTL: -5 });
+  expect(result.fdv1Fallback).toBe(true);
+  expect(result.fdv1FallbackTtlMs).toBe(0);
+});
+
+it('readGoodbyeFallbackDirective: returns fdv1Fallback false for a non-numeric protocolFallbackTTL', () => {
+  const result = readGoodbyeFallbackDirective({
+    reason: 'falling back',
+    protocolFallbackTTL: 'soon',
+  });
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
+
+it('readGoodbyeFallbackDirective: returns fdv1Fallback false for a non-finite protocolFallbackTTL', () => {
+  const result = readGoodbyeFallbackDirective({
+    reason: 'falling back',
+    protocolFallbackTTL: Infinity,
+  });
+  expect(result.fdv1Fallback).toBe(false);
+  expect(result.fdv1FallbackTtlMs).toBeUndefined();
+});
diff --git a/packages/shared/sdk-client/src/datasource/fdv2/FDv2SourceResult.ts b/packages/shared/sdk-client/src/datasource/fdv2/FDv2SourceResult.ts
index 7b62e3b424..b742897591 100644
--- a/packages/shared/sdk-client/src/datasource/fdv2/FDv2SourceResult.ts
+++ b/packages/shared/sdk-client/src/datasource/fdv2/FDv2SourceResult.ts
@@ -13,7 +13,7 @@ import DataSourceStatusErrorInfo from '../DataSourceStatusErrorInfo';
 export type SourceState = 'interrupted' | 'shutdown' | 'terminal_error' | 'goodbye';
 
 /**
- * A change set result containing a processed FDv2 payload.
+ * A successfully processed FDv2 payload ready for delivery to the flag store.
  */
 export interface ChangeSetResult {
   type: 'changeSet';
@@ -22,10 +22,16 @@ export interface ChangeSetResult {
   environmentId?: string;
   /** Freshness timestamp from cache, if this result originated from cached data. */
   freshness?: number;
+  /**
+   * When `fdv1Fallback` is true, how long (ms) to remain on FDv1 before
+   * attempting FDv2 recovery. `undefined` means no TTL was provided (caller
+   * uses a default); `0` means indefinite (no recovery).
+   */
+  fdv1FallbackTtlMs?: number;
 }
 
 /**
- * A status result indicating a state transition (error, shutdown, goodbye).
+ * A state-transition result (error, shutdown, or goodbye) with no payload.
  */
 export interface StatusResult {
   type: 'status';
@@ -33,73 +39,86 @@ export interface StatusResult {
   errorInfo?: DataSourceStatusErrorInfo;
   reason?: string;
   fdv1Fallback: boolean;
+  /**
+   * When `fdv1Fallback` is true, how long (ms) to remain on FDv1 before
+   * attempting FDv2 recovery. `undefined` means no TTL was provided (caller
+   * uses a default); `0` means indefinite (no recovery).
+   */
+  fdv1FallbackTtlMs?: number;
 }
 
 /**
  * The result type for FDv2 initializers and synchronizers.
  *
- * An initializer produces a single result, while a synchronizer produces a
- * stream of results. Each result is either a change set (containing a payload
- * of flag data) or a status (indicating a state transition like an error or
- * shutdown).
+ * Initializers produce a single result; synchronizers produce a stream.
+ * The orchestrator in {@link FDv2DataSource} drives the control flow based
+ * on which variant is returned.
  */
 export type FDv2SourceResult = ChangeSetResult | StatusResult;
 
 /**
- * Creates a change set result containing processed flag data.
+ * Wraps a processed FDv2 payload in a {@link ChangeSetResult}.
  */
 export function changeSet(
   payload: internal.Payload,
   fdv1Fallback: boolean,
   environmentId?: string,
   freshness?: number,
+  fdv1FallbackTtlMs?: number,
 ): FDv2SourceResult {
-  return { type: 'changeSet', payload, fdv1Fallback, environmentId, freshness };
+  return { type: 'changeSet', payload, fdv1Fallback, environmentId, freshness, fdv1FallbackTtlMs };
 }
 
 /**
- * Creates an interrupted status result. Indicates a transient error; the
- * synchronizer will attempt to recover automatically.
- *
- * When used with an initializer, this is still a terminal state.
+ * Signals a transient error. Synchronizers retry automatically; for an
+ * initializer this is terminal since there is no retry loop.
  */
 export function interrupted(
   errorInfo: DataSourceStatusErrorInfo,
   fdv1Fallback: boolean,
+  fdv1FallbackTtlMs?: number,
 ): FDv2SourceResult {
-  return { type: 'status', state: 'interrupted', errorInfo, fdv1Fallback };
+  return { type: 'status', state: 'interrupted', errorInfo, fdv1Fallback, fdv1FallbackTtlMs };
 }
 
 /**
- * Creates a shutdown status result. Indicates the data source was closed
- * gracefully and will not produce any further results.
+ * Signals a graceful close initiated by the local caller (not the server).
  */
 export function shutdown(): FDv2SourceResult {
   return { type: 'status', state: 'shutdown', fdv1Fallback: false };
 }
 
 /**
- * Creates a terminal error status result. Indicates an unrecoverable error;
- * the data source will not produce any further results.
+ * Signals an unrecoverable error. The orchestrator will block this source
+ * and move on; it will not retry.
  */
 export function terminalError(
   errorInfo: DataSourceStatusErrorInfo,
   fdv1Fallback: boolean,
+  fdv1FallbackTtlMs?: number,
 ): FDv2SourceResult {
-  return { type: 'status', state: 'terminal_error', errorInfo, fdv1Fallback };
+  return { type: 'status', state: 'terminal_error', errorInfo, fdv1Fallback, fdv1FallbackTtlMs };
 }
 
 /**
- * Creates a goodbye status result. Indicates the server has instructed the
- * client to disconnect.
+ * Signals a server-initiated disconnect. Unlike `terminal_error`, the
+ * synchronizer will reconnect; the orchestrator does not block this source.
+ *
+ * @param reason Human-readable description of why the server closed the stream.
+ * @param fdv1Fallback Whether the server directed the client to fall back to FDv1.
+ * @param fdv1FallbackTtlMs How long (ms) to remain on FDv1 before attempting FDv2
+ *   recovery. Omit to use the caller's default; pass `0` for indefinite fallback.
+ *   Same semantics as {@link StatusResult.fdv1FallbackTtlMs}.
  */
-export function goodbye(reason: string, fdv1Fallback: boolean): FDv2SourceResult {
-  return { type: 'status', state: 'goodbye', reason, fdv1Fallback };
+export function goodbye(
+  reason: string,
+  fdv1Fallback: boolean,
+  fdv1FallbackTtlMs?: number,
+): FDv2SourceResult {
+  return { type: 'status', state: 'goodbye', reason, fdv1Fallback, fdv1FallbackTtlMs };
 }
 
-/**
- * Helper to create a {@link DataSourceStatusErrorInfo} from an HTTP status code.
- */
+/** Builds {@link DataSourceStatusErrorInfo} for an unexpected HTTP status. */
 export function errorInfoFromHttpError(statusCode: number): DataSourceStatusErrorInfo {
   return {
     kind: DataSourceErrorKind.ErrorResponse,
@@ -109,9 +128,7 @@ export function errorInfoFromHttpError(statusCode: number): DataSourceStatusErro
   };
 }
 
-/**
- * Helper to create a {@link DataSourceStatusErrorInfo} from a network error.
- */
+/** Builds {@link DataSourceStatusErrorInfo} for a network-level failure. */
 export function errorInfoFromNetworkError(message: string): DataSourceStatusErrorInfo {
   return {
     kind: DataSourceErrorKind.NetworkError,
@@ -120,9 +137,7 @@ export function errorInfoFromNetworkError(message: string): DataSourceStatusErro
   };
 }
 
-/**
- * Helper to create a {@link DataSourceStatusErrorInfo} from invalid data.
- */
+/** Builds {@link DataSourceStatusErrorInfo} for a malformed or unexpected payload. */
 export function errorInfoFromInvalidData(message: string): DataSourceStatusErrorInfo {
   return {
     kind: DataSourceErrorKind.InvalidData,
@@ -131,9 +146,7 @@ export function errorInfoFromInvalidData(message: string): DataSourceStatusError
   };
 }
 
-/**
- * Helper to create a {@link DataSourceStatusErrorInfo} for unknown errors.
- */
+/** Builds {@link DataSourceStatusErrorInfo} when the error kind is not otherwise classifiable. */
 export function errorInfoFromUnknown(message: string): DataSourceStatusErrorInfo {
   return {
     kind: DataSourceErrorKind.Unknown,
diff --git a/packages/shared/sdk-client/src/datasource/fdv2/fallbackDirective.ts b/packages/shared/sdk-client/src/datasource/fdv2/fallbackDirective.ts
new file mode 100644
index 0000000000..bca2464a27
--- /dev/null
+++ b/packages/shared/sdk-client/src/datasource/fdv2/fallbackDirective.ts
@@ -0,0 +1,73 @@
+/**
+ * The FDv1 fallback directive parsed from a connection's response headers.
+ * Its presence (`fdv1Fallback === true`) means the server asked the SDK to
+ * fall back to FDv1.
+ *
+ * `fdv1FallbackTtlMs` is how long to remain on FDv1 before retrying FDv2:
+ * - `undefined`: the server gave no TTL header (caller uses a 1-hour default).
+ * - `0`: indefinite fallback (no automatic recovery).
+ * - `> 0`: milliseconds to wait before attempting FDv2 recovery.
+ *
+ * This is the single place that interprets `x-ld-fd-fallback` and
+ * `x-ld-fd-fallback-ttl`, shared by the streaming and polling sources.
+ */
+export interface FallbackDirective {
+  fdv1Fallback: boolean;
+  fdv1FallbackTtlMs?: number;
+}
+
+/**
+ * Reads the FDv1 fallback directive from response headers. Returns
+ * `{ fdv1Fallback: false }` when `x-ld-fd-fallback` is absent or not `"true"`.
+ *
+ * @param headers Header accessor. The `get` method must accept header names
+ *   in any casing; both streaming and polling callers normalize to lowercase
+ *   before calling this function.
+ */
+export function readFallbackDirective(headers: {
+  get(name: string): string | null;
+}): FallbackDirective {
+  const fallback = headers.get('x-ld-fd-fallback');
+  if (fallback === null || fallback.toLowerCase() !== 'true') {
+    return { fdv1Fallback: false };
+  }
+
+  const raw = headers.get('x-ld-fd-fallback-ttl');
+  if (raw === null) {
+    return { fdv1Fallback: true };
+  }
+
+  const seconds = parseInt(raw, 10);
+  if (Number.isNaN(seconds)) {
+    return { fdv1Fallback: true };
+  }
+
+  // Clamp negative values to 0 (treated as indefinite, same as TTL=0).
+  // Prevents a malicious server from sending a large-negative TTL to trigger
+  // immediate recovery instead of the intended long wait.
+  return { fdv1Fallback: true, fdv1FallbackTtlMs: Math.max(0, seconds) * 1000 };
+}
+
+/**
+ * Reads the FDv1 fallback directive from an FDv2 `goodbye` event's data.
+ *
+ * SDKs that cannot read streaming response headers (e.g. browsers using the
+ * native `EventSource` API) receive the fallback directive in-band via the
+ * goodbye message's `protocolFallbackTTL` field.
+ * Presence of a finite numeric `protocolFallbackTTL` signals FDv1 fallback;
+ * the value carries the same semantics as the `x-ld-fd-fallback-ttl` header
+ * (`0` indicates indefinite fallback). A missing, non-numeric, or non-finite
+ * value is not a fallback signal and yields `{ fdv1Fallback: false }`.
+ *
+ * @param data The raw, parsed goodbye event data (typed `unknown` because the
+ *   caller has not narrowed it).
+ */
+export function readGoodbyeFallbackDirective(data: unknown): FallbackDirective {
+  const rawTtl = (data as { protocolFallbackTTL?: unknown } | null | undefined)
+    ?.protocolFallbackTTL;
+  if (typeof rawTtl !== 'number' || !Number.isFinite(rawTtl)) {
+    return { fdv1Fallback: false };
+  }
+
+  return { fdv1Fallback: true, fdv1FallbackTtlMs: Math.max(0, rawTtl) * 1000 };
+}