diff --git a/src/client/GameModeSelector.ts b/src/client/GameModeSelector.ts
index 660dd222a..b1dc74cb0 100644
--- a/src/client/GameModeSelector.ts
+++ b/src/client/GameModeSelector.ts
@@ -34,6 +34,7 @@ const CARD_BG = "bg-surface";
 export class GameModeSelector extends LitElement {
   @state() private lobbies: PublicGames | null = null;
   @state() private mapAspectRatios: Map<GameMapType, number> = new Map();
+  @state() private inputValid: boolean = true;
   private serverTimeOffset: number = 0;
   private defaultLobbyTime: number = 0;
 
@@ -45,28 +46,44 @@ export class GameModeSelector extends LitElement {
     return this;
   }
 
-  /**
-   * Validates username input and shows error message if invalid.
-   * Returns true if valid, false otherwise.
-   */
+  // Silent backstop; the buttons are already disabled while input is invalid.
   private validateUsername(): boolean {
     const usernameInput = document.querySelector(
       "username-input",
     ) as UsernameInput | null;
-    return usernameInput ? usernameInput.validateOrShowError() : true;
+    return usernameInput ? usernameInput.canPlay() : true;
   }
 
   connectedCallback() {
     super.connectedCallback();
     this.lobbySocket.start();
     this.defaultLobbyTime = ClientEnv.gameCreationRate() / 1000;
+    window.addEventListener(
+      "username-validity-change",
+      this.handleValidityChange,
+    );
+    // Pick up the current value in case username-input validated before us.
+    const usernameInput = document.querySelector(
+      "username-input",
+    ) as UsernameInput | null;
+    if (usernameInput) {
+      this.inputValid = usernameInput.canPlay();
+    }
   }
 
   disconnectedCallback() {
     this.stop();
+    window.removeEventListener(
+      "username-validity-change",
+      this.handleValidityChange,
+    );
     super.disconnectedCallback();
   }
 
+  private handleValidityChange = (e: Event) => {
+    this.inputValid = (e as CustomEvent).detail?.isValid ?? true;
+  };
+
   public stop() {
     this.lobbySocket.stop();
   }
@@ -259,7 +276,11 @@ export class GameModeSelector extends LitElement {
     return html`
       <button
         @click=${onClick}
-        class="flex items-center justify-center w-full h-full rounded-lg ${bgClass} transition-all duration-200 text-sm lg:text-base font-medium text-white uppercase tracking-wider text-center"
+        ?disabled=${!this.inputValid}
+        class="flex items-center justify-center w-full h-full rounded-lg ${bgClass} transition-all duration-200 text-sm lg:text-base font-medium text-white uppercase tracking-wider text-center ${!this
+          .inputValid
+          ? "opacity-50 cursor-not-allowed pointer-events-none"
+          : ""}"
       >
         ${title}
       </button>
@@ -305,7 +326,11 @@ export class GameModeSelector extends LitElement {
     return html`
       <button
         @click=${() => this.validateAndJoin(lobby)}
-        class="group relative w-full h-44 sm:h-full text-white uppercase rounded-2xl transition-all duration-200 hover:scale-[1.02] active:scale-[0.98] bg-surface hover:shadow-[var(--shadow-lobby-card-hover)]"
+        ?disabled=${!this.inputValid}
+        class="group relative w-full h-44 sm:h-full text-white uppercase rounded-2xl transition-all duration-200 hover:scale-[1.02] active:scale-[0.98] bg-surface hover:shadow-[var(--shadow-lobby-card-hover)] ${!this
+          .inputValid
+          ? "opacity-50 cursor-not-allowed pointer-events-none"
+          : ""}"
       >
         <!-- Image clipped separately so overflow-hidden doesn't block absolute children -->
         <div
diff --git a/src/client/Main.ts b/src/client/Main.ts
index 82c9c38dd..d63f691c6 100644
--- a/src/client/Main.ts
+++ b/src/client/Main.ts
@@ -814,7 +814,7 @@ class Client {
   private async handleJoinLobby(event: CustomEvent<JoinLobbyEvent>) {
     const lobby = event.detail;
     this.mostRecentJoinEvent = event.timeStamp;
-    if (this.usernameInput && !this.usernameInput.validateOrShowError()) {
+    if (this.usernameInput && !this.usernameInput.canPlay()) {
       return;
     }
 
diff --git a/src/client/UsernameInput.ts b/src/client/UsernameInput.ts
index 50c6dba74..52d2a44ea 100644
--- a/src/client/UsernameInput.ts
+++ b/src/client/UsernameInput.ts
@@ -28,9 +28,8 @@ export class UsernameInput extends LitElement {
   @state() private clanTag: string = "";
 
   @property({ type: String }) validationError: string = "";
-  // Ownership-check feedback (i18n key) shown inline beneath the tag input. This
-  // is advisory only — it does not gate play; the tag is stripped on submit and
-  // the server re-checks authoritatively.
+  // Ownership-check feedback (i18n key) shown inline beneath the tag input. Only
+  // "not a member" gates the buttons (see emitValidity); the rest is advisory.
   @state() private clanTagOwnershipError: string = "";
   @state() private clanCheckPending: boolean = false;
   private _isValid: boolean = true;
@@ -71,6 +70,7 @@ export class UsernameInput extends LitElement {
     const gen = ++this.clanCheckGen;
     const tag = this.clanTag;
     this.clanTagOwnershipError = "";
+    this.emitValidity();
     if (tag.length === 0 || !validateClanTag(tag).isValid) {
       this.clanCheckPending = false;
       this.clanCheck = Promise.resolve(null);
@@ -81,6 +81,7 @@ export class UsernameInput extends LitElement {
       if (gen === this.clanCheckGen) {
         this.clanTagOwnershipError = res.error ?? "";
         this.clanCheckPending = false;
+        this.emitValidity();
       }
       return res.tag;
     });
@@ -239,6 +240,7 @@ export class UsernameInput extends LitElement {
     if (!clanTagResult.isValid) {
       this._isValid = false;
       this.validationError = clanTagResult.error ?? "";
+      this.emitValidity();
       return;
     }
 
@@ -251,32 +253,23 @@ export class UsernameInput extends LitElement {
     } else {
       this.validationError = result.error ?? "";
     }
+    this.emitValidity();
   }
 
-  public isValid(): boolean {
-    return this._isValid;
-  }
-
-  public showValidationFeedback(): void {
-    const message =
-      this.validationError || translateText("username.invalid_chars");
+  // Broadcast play-eligibility so action buttons can disable themselves.
+  private emitValidity() {
     window.dispatchEvent(
-      new CustomEvent("show-message", {
-        detail: {
-          message,
-          color: "red",
-          duration: 2500,
-        },
+      new CustomEvent("username-validity-change", {
+        detail: { isValid: this.canPlay() },
       }),
     );
   }
 
-  public validateOrShowError(): boolean {
-    if (this.isValid()) {
-      return true;
-    }
-    this.showValidationFeedback();
-    return false;
+  // Play-eligibility: syntax-valid and not blocked by clan membership.
+  public canPlay(): boolean {
+    return (
+      this._isValid && this.clanTagOwnershipError !== "username.tag_not_member"
+    );
   }
 }