diff --git a/apps/mcp/src/server/client/index.ts b/apps/mcp/src/server/client/index.ts
index 0beb9f8d..220eea92 100644
--- a/apps/mcp/src/server/client/index.ts
+++ b/apps/mcp/src/server/client/index.ts
@@ -377,7 +377,9 @@ export class SupermemoryClient {
 				case 402:
 					throw new Error("Memory limit reached. Upgrade at supermemory.ai")
 				case 403:
-					throw new Error("Access forbidden.")
+					throw new Error(
+						`Access forbidden: you don't have access to container tag '${this.containerTag}', or it doesn't exist. Use listSpaces to see the available container tags.`,
+					)
 				case 404:
 					throw new Error("Not found.")
 				case 429:
diff --git a/apps/mcp/src/server/tools/search-memory.ts b/apps/mcp/src/server/tools/search-memory.ts
index 0c7ca491..a4b7431b 100644
--- a/apps/mcp/src/server/tools/search-memory.ts
+++ b/apps/mcp/src/server/tools/search-memory.ts
@@ -1,6 +1,10 @@
 import { z } from "zod"
 import { getMemoryText } from "../client"
 import type { ToolDeps } from "./types"
+import {
+	containerTagExists,
+	unknownContainerTagError,
+} from "./validate-container-tag"
 
 export function register(deps: ToolDeps) {
 	const containerTagField: Record<string, z.ZodTypeAny> = deps.rbac
@@ -43,6 +47,12 @@ export function register(deps: ToolDeps) {
 						),
 					)
 				}
+				if (
+					args.containerTag &&
+					!(await containerTagExists(deps, args.containerTag))
+				) {
+					return deps.errorResult(unknownContainerTagError(args.containerTag))
+				}
 				const effectiveTag = await deps.resolveContainerTag(args.containerTag)
 				const client = deps.getClient(effectiveTag)
 
diff --git a/apps/mcp/src/server/tools/set-active-tag.ts b/apps/mcp/src/server/tools/set-active-tag.ts
index bc43fe6a..31e65441 100644
--- a/apps/mcp/src/server/tools/set-active-tag.ts
+++ b/apps/mcp/src/server/tools/set-active-tag.ts
@@ -2,6 +2,10 @@ import { registerAppTool } from "@modelcontextprotocol/ext-apps/server"
 import { z } from "zod"
 import { SUPERMEMORY_RESOURCE_URI, type ViewMessage } from "../../shared/types"
 import type { ToolDeps } from "./types"
+import {
+	containerTagExists,
+	unknownContainerTagError,
+} from "./validate-container-tag"
 
 export function register(deps: ToolDeps) {
 	registerAppTool(
@@ -26,6 +30,13 @@ export function register(deps: ToolDeps) {
 					new Error(`No access to container tag '${containerTag}'.`),
 				)
 			}
+			try {
+				if (!(await containerTagExists(deps, containerTag))) {
+					return deps.errorResult(unknownContainerTagError(containerTag))
+				}
+			} catch (error) {
+				return deps.errorResult(error)
+			}
 			await deps.storage.put("activeContainerTag", containerTag)
 			const sc: ViewMessage = {
 				view: "confirmation",
diff --git a/apps/mcp/src/server/tools/validate-container-tag.ts b/apps/mcp/src/server/tools/validate-container-tag.ts
new file mode 100644
index 00000000..5ba0c860
--- /dev/null
+++ b/apps/mcp/src/server/tools/validate-container-tag.ts
@@ -0,0 +1,21 @@
+import type { ToolDeps } from "./types"
+
+/**
+ * Checks whether a container tag actually exists for this user.
+ * Fast path is the cached tag list from init; on a miss we re-fetch once so
+ * tags created after the session started are not falsely rejected.
+ */
+export async function containerTagExists(
+	deps: ToolDeps,
+	containerTag: string,
+): Promise<boolean> {
+	if (deps.cachedContainerTags().includes(containerTag)) return true
+	await deps.refreshContainerTags()
+	return deps.cachedContainerTags().includes(containerTag)
+}
+
+export function unknownContainerTagError(containerTag: string): Error {
+	return new Error(
+		`Container tag '${containerTag}' does not exist. Use listSpaces to see the available container tags.`,
+	)
+}