diff --git a/packages/common/container-definitions/api-report/container-definitions.legacy.beta.api.md b/packages/common/container-definitions/api-report/container-definitions.legacy.beta.api.md
index 9cd30d8bcdb9..8b9a872fd978 100644
--- a/packages/common/container-definitions/api-report/container-definitions.legacy.beta.api.md
+++ b/packages/common/container-definitions/api-report/container-definitions.legacy.beta.api.md
@@ -147,7 +147,7 @@ export interface IContainer extends IEventProvider<IContainerEvents> {
     attach(request: IRequest, attachProps?: {
         deltaConnection?: "none" | "delayed";
     }): Promise<void>;
-    readonly attachState: AttachState;
+    readonly attachState: AttachState | undefined;
     readonly audience: IAudience;
     readonly clientId?: string | undefined;
     close(error?: ICriticalContainerError): void;
diff --git a/packages/common/container-definitions/package.json b/packages/common/container-definitions/package.json
index e3c838939e78..7dfa47ba23da 100644
--- a/packages/common/container-definitions/package.json
+++ b/packages/common/container-definitions/package.json
@@ -110,7 +110,11 @@
 		"typescript": "~5.4.5"
 	},
 	"typeValidation": {
-		"broken": {},
+		"broken": {
+			"Interface_IContainer": {
+				"backCompat": false
+			}
+		},
 		"entrypoint": "legacy"
 	}
 }
diff --git a/packages/common/container-definitions/src/loader.ts b/packages/common/container-definitions/src/loader.ts
index 703c80a9dad6..a16424d68afc 100644
--- a/packages/common/container-definitions/src/loader.ts
+++ b/packages/common/container-definitions/src/loader.ts
@@ -352,8 +352,9 @@ export interface IContainer extends IEventProvider<IContainerEvents> {
 
 	/**
 	 * Indicates the attachment state of the container to a host service.
+	 * Will be `undefined` when the container is being loaded and the state is not yet known.
 	 */
-	readonly attachState: AttachState;
+	readonly attachState: AttachState | undefined;
 
 	/**
 	 * Get the code details that are currently specified for the container.
diff --git a/packages/common/container-definitions/src/test/types/validateContainerDefinitionsPrevious.generated.ts b/packages/common/container-definitions/src/test/types/validateContainerDefinitionsPrevious.generated.ts
index 7f01d94ec8b5..538a3c0e058a 100644
--- a/packages/common/container-definitions/src/test/types/validateContainerDefinitionsPrevious.generated.ts
+++ b/packages/common/container-definitions/src/test/types/validateContainerDefinitionsPrevious.generated.ts
@@ -240,6 +240,7 @@ declare type old_as_current_for_Interface_IContainer = requireAssignableTo<TypeO
  * typeValidation.broken:
  * "Interface_IContainer": {"backCompat": false}
  */
+// @ts-expect-error compatibility expected to be broken
 declare type current_as_old_for_Interface_IContainer = requireAssignableTo<TypeOnly<current.IContainer>, TypeOnly<old.IContainer>>
 
 /*
diff --git a/packages/framework/fluid-static/src/fluidContainer.ts b/packages/framework/fluid-static/src/fluidContainer.ts
index 0f57df727a36..a981f5dc4db4 100644
--- a/packages/framework/fluid-static/src/fluidContainer.ts
+++ b/packages/framework/fluid-static/src/fluidContainer.ts
@@ -341,7 +341,10 @@ class FluidContainer<TContainerSchema extends ContainerSchema = ContainerSchema>
 	}
 
 	public get attachState(): AttachState {
-		return this.container.attachState;
+		// attachState can be undefined on IContainer while loading, but FluidContainer is only
+		// created after loading completes, so attachState is always defined here.
+		// eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+		return this.container.attachState!;
 	}
 
 	public get disposed(): boolean {
diff --git a/packages/loader/container-loader/src/container.ts b/packages/loader/container-loader/src/container.ts
index f6035607f005..a6ae5886a03d 100644
--- a/packages/loader/container-loader/src/container.ts
+++ b/packages/loader/container-loader/src/container.ts
@@ -558,7 +558,7 @@ export class Container
 	private readonly connectionTransitionTimes: number[] = [];
 	private _loadedFromVersion: IVersion | undefined;
 	private _dirtyContainer = false;
-	private attachmentData: AttachmentData = { state: AttachState.Detached };
+	private attachmentData: AttachmentData | undefined;
 	private readonly serializedStateManager: SerializedStateManager;
 	private readonly _containerId: string;
 
@@ -757,6 +757,12 @@ export class Container
 		this.connectionTransitionTimes[ConnectionState.Disconnected] = performanceNow();
 		const pendingLocalState = loadProps?.pendingLocalState;
 
+		// When loading an existing container, the attachment state is unknown until the load
+		// completes. Only initialize to Detached when creating a new container.
+		if (loadProps === undefined) {
+			this.attachmentData = { state: AttachState.Detached };
+		}
+
 		this._canReconnect = canReconnect ?? true;
 		this.clientDetailsOverride = clientDetailsOverride;
 		this.urlResolver = urlResolver;
@@ -1155,7 +1161,7 @@ export class Container
 			);
 		}
 		assert(
-			this.attachmentData.state === AttachState.Attached,
+			this.attachmentData?.state === AttachState.Attached,
 			0x0d1 /* "Container should be attached before close" */,
 		);
 		assert(
@@ -1170,8 +1176,8 @@ export class Container
 		return pendingState;
 	}
 
-	public get attachState(): AttachState {
-		return this.attachmentData.state;
+	public get attachState(): AttachState | undefined {
+		return this.attachmentData?.state;
 	}
 
 	/**
@@ -1181,7 +1187,11 @@ export class Container
 	 * @returns stringified {@link IPendingDetachedContainerState} for the container
 	 */
 	public serialize(): string {
-		if (this.attachmentData.state === AttachState.Attached || this.closed) {
+		if (
+			this.attachmentData === undefined ||
+			this.attachmentData.state === AttachState.Attached ||
+			this.closed
+		) {
 			throw new UsageError("Container must not be attached or closed.");
 		}
 
@@ -1222,6 +1232,7 @@ export class Container
 				async () => {
 					if (
 						this._lifecycleState !== "loaded" ||
+						this.attachmentData === undefined ||
 						this.attachmentData.state === AttachState.Attached
 					) {
 						// pre-0.58 error message: containerNotValidForAttach
@@ -1243,7 +1254,7 @@ export class Container
 					const setAttachmentData: AttachProcessProps["setAttachmentData"] = (
 						attachmentData,
 					) => {
-						const previousState = this.attachmentData.state;
+						const previousState = this.attachmentData?.state;
 						this.attachmentData = attachmentData;
 						const state = this.attachmentData.state;
 						if (state !== previousState && state !== AttachState.Detached) {
@@ -2422,7 +2433,7 @@ export class Container
 				getAbsoluteUrl: this.getAbsoluteUrl,
 				getContainerDiagnosticId: () => this.resolvedUrl?.id,
 				getClientId: () => this.clientId,
-				getAttachState: () => this.attachState,
+				getAttachState: () => this.attachState as AttachState,
 				getConnected: () => this.connected,
 				getConnectionState: () => this.connectionState,
 				clientDetails: this._deltaManager.clientDetails,