diff --git a/src/chat-prefix-suggestions.ts b/src/chat-prefix-suggestions.ts
new file mode 100644
index 00000000..983af790
--- /dev/null
+++ b/src/chat-prefix-suggestions.ts
@@ -0,0 +1,30 @@
+// Copyright (c) Mehmet Bektas <mbektasgh@outlook.com>
+
+/**
+ * The slash command and participant suggestions matching what has been typed.
+ *
+ * An empty prompt offers everything; anything else is a substring match, so
+ * typing past the last match legitimately leaves nothing to suggest.
+ */
+export function prefixesMatching(prefixes: string[], prompt: string): string[] {
+  const userInput = prompt.trimStart();
+  if (userInput === '') {
+    return [...prefixes];
+  }
+  return prefixes.filter(prefix => prefix.includes(userInput));
+}
+
+/**
+ * Whether the suggestion popover should act on a keystroke.
+ *
+ * The popover is opened by typing `@` or `/` and stays open while the prompt
+ * is edited, so it can end up with nothing to offer. It renders nothing in
+ * that state, and it must not claim Enter or Tab either: a popover that
+ * accepts a suggestion it does not have leaves the message unsendable.
+ */
+export function isPrefixPopoverUsable(
+  showPopover: boolean,
+  suggestions: string[]
+): boolean {
+  return showPopover && suggestions.length > 0;
+}
diff --git a/src/chat-sidebar.tsx b/src/chat-sidebar.tsx
index 9ec29dea..8d2357a4 100644
--- a/src/chat-sidebar.tsx
+++ b/src/chat-sidebar.tsx
@@ -99,6 +99,10 @@ import { TOUR_ANCHOR } from './tour/tour-anchors';
 import { TOUR_START_EVENT, TOUR_STOP_EVENT } from './tour/tour-events';
 import { hasCompletedTour } from './tour/tour-state';
 import { IClaudeSessionInfo } from './api';
+import {
+  isPrefixPopoverUsable,
+  prefixesMatching
+} from './chat-prefix-suggestions';
 import {
   NOTEBOOK_GENERATION_PROGRESS_EVENT,
   type INotebookGenerationProgressDetail
@@ -2879,6 +2883,8 @@ function SidebarComponent(props: any) {
     }
   };
 
+  const popoverUsable = isPrefixPopoverUsable(showPopover, prefixSuggestions);
+
   const applyPrefixSuggestion = async (prefix: string) => {
     let mcpArguments = '';
     if (prefix.startsWith('/mcp:')) {
@@ -3334,14 +3340,7 @@ function SidebarComponent(props: any) {
   );
 
   const filterPrefixSuggestions = (prmpt: string) => {
-    const userInput = prmpt.trimStart();
-    if (userInput === '') {
-      setPrefixSuggestions(originalPrefixes);
-    } else {
-      setPrefixSuggestions(
-        originalPrefixes.filter(prefix => prefix.includes(userInput))
-      );
-    }
+    setPrefixSuggestions(prefixesMatching(originalPrefixes, prmpt));
   };
 
   const resetPrefixSuggestions = () => {
@@ -3408,15 +3407,16 @@ function SidebarComponent(props: any) {
       }
       event.stopPropagation();
       event.preventDefault();
-      if (showPopover) {
+      if (popoverUsable) {
         applyPrefixSuggestion(prefixSuggestions[selectedPrefixSuggestionIndex]);
         return;
       }
 
+      setShowPopover(false);
       setSelectedPrefixSuggestionIndex(0);
       handleSubmitStopChatButtonClick();
     } else if (event.key === 'Tab') {
-      if (showPopover) {
+      if (popoverUsable) {
         event.stopPropagation();
         event.preventDefault();
         applyPrefixSuggestion(prefixSuggestions[selectedPrefixSuggestionIndex]);
@@ -3433,7 +3433,7 @@ function SidebarComponent(props: any) {
       event.stopPropagation();
       event.preventDefault();
 
-      if (showPopover) {
+      if (popoverUsable) {
         setSelectedPrefixSuggestionIndex(
           (selectedPrefixSuggestionIndex - 1 + prefixSuggestions.length) %
             prefixSuggestions.length
@@ -3464,7 +3464,7 @@ function SidebarComponent(props: any) {
       event.stopPropagation();
       event.preventDefault();
 
-      if (showPopover) {
+      if (popoverUsable) {
         setSelectedPrefixSuggestionIndex(
           (selectedPrefixSuggestionIndex + 1 + prefixSuggestions.length) %
             prefixSuggestions.length
@@ -4535,7 +4535,7 @@ function SidebarComponent(props: any) {
               </button>
             </div>
           </div>
-          {showPopover && prefixSuggestions.length > 0 && (
+          {popoverUsable && (
             <div className="user-input-autocomplete" ref={autocompleteRef}>
               {prefixSuggestions.map((prefix, index) => (
                 <div
diff --git a/tests/ts/chat-prefix-suggestions.test.ts b/tests/ts/chat-prefix-suggestions.test.ts
new file mode 100644
index 00000000..688e29d1
--- /dev/null
+++ b/tests/ts/chat-prefix-suggestions.test.ts
@@ -0,0 +1,55 @@
+// Copyright (c) Mehmet Bektas <mbektasgh@outlook.com>
+
+// A prompt beginning with @ or / opens the suggestion popover, and the
+// popover used to keep claiming Enter after the prompt was typed past every
+// match, so such a message could not be sent from the keyboard at all.
+import {
+  isPrefixPopoverUsable,
+  prefixesMatching
+} from '../../src/chat-prefix-suggestions';
+
+const PREFIXES = ['@mcp', '/clear', '/newNotebook', '/newPythonFile'];
+
+describe('prefixesMatching', () => {
+  it('offers everything for an empty prompt', () => {
+    expect(prefixesMatching(PREFIXES, '')).toEqual(PREFIXES);
+    expect(prefixesMatching(PREFIXES, '   ')).toEqual(PREFIXES);
+  });
+
+  it('narrows to substring matches', () => {
+    expect(prefixesMatching(PREFIXES, '/new')).toEqual([
+      '/newNotebook',
+      '/newPythonFile'
+    ]);
+    expect(prefixesMatching(PREFIXES, '@')).toEqual(['@mcp']);
+  });
+
+  it('offers nothing once the prompt is typed past every match', () => {
+    expect(prefixesMatching(PREFIXES, '@Ada please review this')).toEqual([]);
+    expect(prefixesMatching(PREFIXES, '/usr/bin/python is missing')).toEqual(
+      []
+    );
+  });
+
+  it('does not mutate the list it was given', () => {
+    const original = [...PREFIXES];
+    prefixesMatching(PREFIXES, '').push('/mutated');
+    expect(PREFIXES).toEqual(original);
+  });
+});
+
+describe('isPrefixPopoverUsable', () => {
+  it('is false with nothing to suggest, even while open', () => {
+    // The case behind the bug: the popover renders nothing here, so it must
+    // not claim Enter or Tab either.
+    expect(isPrefixPopoverUsable(true, [])).toBe(false);
+  });
+
+  it('is true while it has something to offer', () => {
+    expect(isPrefixPopoverUsable(true, ['/clear'])).toBe(true);
+  });
+
+  it('is false when closed', () => {
+    expect(isPrefixPopoverUsable(false, ['/clear'])).toBe(false);
+  });
+});