diff --git a/packages/react-dom/src/server/ReactDOMLegacyServerBrowser.js b/packages/react-dom/src/server/ReactDOMLegacyServerBrowser.js
index 5435c3b6449c7..5a01fa9327955 100644
--- a/packages/react-dom/src/server/ReactDOMLegacyServerBrowser.js
+++ b/packages/react-dom/src/server/ReactDOMLegacyServerBrowser.js
@@ -22,7 +22,7 @@ import {
 import {
   createResponseState,
   createRootFormatContext,
-} from './ReactDOMServerFormatConfig';
+} from './ReactDOMServerLegacyFormatConfig';
 
 type ServerOptions = {
   identifierPrefix?: string,
@@ -59,7 +59,7 @@ function renderToString(
   const request = createRequest(
     children,
     destination,
-    createResponseState(options ? options.identifierPrefix : undefined),
+    createResponseState(false, options ? options.identifierPrefix : undefined),
     createRootFormatContext(undefined),
     Infinity,
     onError,
diff --git a/packages/react-dom/src/server/ReactDOMLegacyServerNode.js b/packages/react-dom/src/server/ReactDOMLegacyServerNode.js
index 29ba74f8ffaf7..55b05d90c4753 100644
--- a/packages/react-dom/src/server/ReactDOMLegacyServerNode.js
+++ b/packages/react-dom/src/server/ReactDOMLegacyServerNode.js
@@ -21,7 +21,7 @@ import {
 import {
   createResponseState,
   createRootFormatContext,
-} from './ReactDOMServerFormatConfig';
+} from './ReactDOMServerLegacyFormatConfig';
 
 import {
   version,
@@ -77,7 +77,7 @@ function renderToNodeStream(
   const request = createRequest(
     children,
     destination,
-    createResponseState(options ? options.identifierPrefix : undefined),
+    createResponseState(false, options ? options.identifierPrefix : undefined),
     createRootFormatContext(undefined),
     Infinity,
     onError,
diff --git a/packages/react-dom/src/server/ReactDOMLegacyServerStreamConfig.js b/packages/react-dom/src/server/ReactDOMLegacyServerStreamConfig.js
index c1d6c1df07ef9..4cfe16091bc58 100644
--- a/packages/react-dom/src/server/ReactDOMLegacyServerStreamConfig.js
+++ b/packages/react-dom/src/server/ReactDOMLegacyServerStreamConfig.js
@@ -16,8 +16,6 @@ export type Destination = {
 export type PrecomputedChunk = string;
 export type Chunk = string;
 
-export const isPrimaryStreamConfig = false;
-
 export function scheduleWork(callback: () => void) {
   callback();
 }
diff --git a/packages/react-dom/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
index 7d705a6abe1d3..1e77b80184d22 100644
--- a/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
@@ -23,7 +23,6 @@ import {
   writeChunk,
   stringToChunk,
   stringToPrecomputedChunk,
-  isPrimaryStreamConfig,
 } from 'react-server/src/ReactServerStreamConfig';
 
 import {
@@ -51,7 +50,7 @@ import isArray from 'shared/isArray';
 
 // Used to distinguish these contexts from ones used in other renderers.
 // E.g. this can be used to distinguish legacy renderers from this modern one.
-export const isPrimaryRenderer = isPrimaryStreamConfig;
+export const isPrimaryRenderer = true;
 
 // Per response, global state that is not contextual to the rendering subtree.
 export type ResponseState = {
@@ -63,18 +62,20 @@ export type ResponseState = {
   nextOpaqueID: number,
   sentCompleteSegmentFunction: boolean,
   sentCompleteBoundaryFunction: boolean,
-  sentClientRenderFunction: boolean,
+  sentClientRenderFunction: boolean, // We allow the legacy renderer to extend this object.
+  ...
 };
 
 // Allows us to keep track of what we've already written so we can refer back to it.
 export function createResponseState(
-  identifierPrefix: string = '',
+  identifierPrefix: string | void,
 ): ResponseState {
+  const idPrefix = identifierPrefix === undefined ? '' : identifierPrefix;
   return {
-    placeholderPrefix: stringToPrecomputedChunk(identifierPrefix + 'P:'),
-    segmentPrefix: stringToPrecomputedChunk(identifierPrefix + 'S:'),
-    boundaryPrefix: identifierPrefix + 'B:',
-    opaqueIdentifierPrefix: identifierPrefix + 'R:',
+    placeholderPrefix: stringToPrecomputedChunk(idPrefix + 'P:'),
+    segmentPrefix: stringToPrecomputedChunk(idPrefix + 'S:'),
+    boundaryPrefix: idPrefix + 'B:',
+    opaqueIdentifierPrefix: idPrefix + 'R:',
     nextSuspenseID: 0,
     nextOpaqueID: 0,
     sentCompleteSegmentFunction: false,
diff --git a/packages/react-dom/src/server/ReactDOMServerLegacyFormatConfig.js b/packages/react-dom/src/server/ReactDOMServerLegacyFormatConfig.js
new file mode 100644
index 0000000000000..09e4f84a4a153
--- /dev/null
+++ b/packages/react-dom/src/server/ReactDOMServerLegacyFormatConfig.js
@@ -0,0 +1,77 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {createResponseState as createResponseStateImpl} from './ReactDOMServerFormatConfig';
+
+import type {PrecomputedChunk} from 'react-server/src/ReactServerStreamConfig';
+
+export const isPrimaryRenderer = false;
+
+export type ResponseState = {
+  // Keep this in sync with ReactDOMServerFormatConfig
+  placeholderPrefix: PrecomputedChunk,
+  segmentPrefix: PrecomputedChunk,
+  boundaryPrefix: string,
+  opaqueIdentifierPrefix: string,
+  nextSuspenseID: number,
+  nextOpaqueID: number,
+  sentCompleteSegmentFunction: boolean,
+  sentCompleteBoundaryFunction: boolean,
+  sentClientRenderFunction: boolean,
+  // This is an extra field for the legacy renderer
+  generateStaticMarkup: boolean,
+};
+
+export function createResponseState(
+  generateStaticMarkup: boolean,
+  identifierPrefix: string | void,
+): ResponseState {
+  const responseState = createResponseStateImpl(identifierPrefix);
+  return {
+    // Keep this in sync with ReactDOMServerFormatConfig
+    placeholderPrefix: responseState.placeholderPrefix,
+    segmentPrefix: responseState.segmentPrefix,
+    boundaryPrefix: responseState.boundaryPrefix,
+    opaqueIdentifierPrefix: responseState.opaqueIdentifierPrefix,
+    nextSuspenseID: responseState.nextSuspenseID,
+    nextOpaqueID: responseState.nextOpaqueID,
+    sentCompleteSegmentFunction: responseState.sentCompleteSegmentFunction,
+    sentCompleteBoundaryFunction: responseState.sentCompleteBoundaryFunction,
+    sentClientRenderFunction: responseState.sentClientRenderFunction,
+    // This is an extra field for the legacy renderer
+    generateStaticMarkup,
+  };
+}
+
+export type {
+  FormatContext,
+  SuspenseBoundaryID,
+  OpaqueIDType,
+} from './ReactDOMServerFormatConfig';
+
+export {
+  createRootFormatContext,
+  getChildFormatContext,
+  createSuspenseBoundaryID,
+  makeServerID,
+  pushEmpty,
+  pushTextInstance,
+  pushStartInstance,
+  pushEndInstance,
+  writePlaceholder,
+  writeStartCompletedSuspenseBoundary,
+  writeStartPendingSuspenseBoundary,
+  writeStartClientRenderedSuspenseBoundary,
+  writeEndSuspenseBoundary,
+  writeStartSegment,
+  writeEndSegment,
+  writeCompletedSegmentInstruction,
+  writeCompletedBoundaryInstruction,
+  writeClientRenderBoundaryInstruction,
+} from './ReactDOMServerFormatConfig';
diff --git a/packages/react-server-dom-relay/src/ReactServerStreamConfigFB.js b/packages/react-server-dom-relay/src/ReactServerStreamConfigFB.js
index b13e5f3792601..e0477fd6197e7 100644
--- a/packages/react-server-dom-relay/src/ReactServerStreamConfigFB.js
+++ b/packages/react-server-dom-relay/src/ReactServerStreamConfigFB.js
@@ -17,8 +17,6 @@ export type Destination = {
 export type PrecomputedChunk = string;
 export type Chunk = string;
 
-export const isPrimaryStreamConfig = true;
-
 export function scheduleWork(callback: () => void) {
   // We don't schedule work in this model, and instead expect performWork to always be called repeatedly.
 }
diff --git a/packages/react-server/src/ReactServerStreamConfigBrowser.js b/packages/react-server/src/ReactServerStreamConfigBrowser.js
index b669002aff771..3714be4302765 100644
--- a/packages/react-server/src/ReactServerStreamConfigBrowser.js
+++ b/packages/react-server/src/ReactServerStreamConfigBrowser.js
@@ -12,8 +12,6 @@ export type Destination = ReadableStreamController;
 export type PrecomputedChunk = Uint8Array;
 export type Chunk = Uint8Array;
 
-export const isPrimaryStreamConfig = true;
-
 export function scheduleWork(callback: () => void) {
   callback();
 }
diff --git a/packages/react-server/src/ReactServerStreamConfigNode.js b/packages/react-server/src/ReactServerStreamConfigNode.js
index 58ce127ba9659..348deb7605039 100644
--- a/packages/react-server/src/ReactServerStreamConfigNode.js
+++ b/packages/react-server/src/ReactServerStreamConfigNode.js
@@ -19,8 +19,6 @@ export type Destination = Writable & MightBeFlushable;
 export type PrecomputedChunk = Uint8Array;
 export type Chunk = string;
 
-export const isPrimaryStreamConfig = true;
-
 export function scheduleWork(callback: () => void) {
   setImmediate(callback);
 }
diff --git a/packages/react-server/src/forks/ReactServerFormatConfig.dom-legacy.js b/packages/react-server/src/forks/ReactServerFormatConfig.dom-legacy.js
index c6e482efeb60c..acbac5042b62a 100644
--- a/packages/react-server/src/forks/ReactServerFormatConfig.dom-legacy.js
+++ b/packages/react-server/src/forks/ReactServerFormatConfig.dom-legacy.js
@@ -7,4 +7,4 @@
  * @flow
  */
 
-export * from 'react-dom/src/server/ReactDOMServerFormatConfig';
+export * from 'react-dom/src/server/ReactDOMServerLegacyFormatConfig';