diff --git a/packages/amazonq/.changes/next-release/Bug Fix-6f1c2d3e-4b5a-4c6d-9e8f-0a1b2c3d4e5f.json b/packages/amazonq/.changes/next-release/Bug Fix-6f1c2d3e-4b5a-4c6d-9e8f-0a1b2c3d4e5f.json
new file mode 100644
index 0000000000..6ebdf547e1
--- /dev/null
+++ b/packages/amazonq/.changes/next-release/Bug Fix-6f1c2d3e-4b5a-4c6d-9e8f-0a1b2c3d4e5f.json	
@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "Amazon Q: In browser-only VS Code (vscode.dev, github.dev), a clear message now guides you to desktop VS Code or a remote environment where Amazon Q is fully supported."
+}
diff --git a/packages/amazonq/src/extension.ts b/packages/amazonq/src/extension.ts
index 9b83695205..496d5dc421 100644
--- a/packages/amazonq/src/extension.ts
+++ b/packages/amazonq/src/extension.ts
@@ -127,7 +127,11 @@ export async function activateAmazonQCommon(context: vscode.ExtensionContext, is
     // This contains every lsp agnostic things (auth, security scan, code scan)
     await activateCodeWhisperer(extContext as ExtContext)
 
-    if (!isAmazonLinux2() || hasGlibcPatch()) {
+    if (isWeb) {
+        // The language server runs as a child process, which the browser-only extension host cannot spawn.
+        // Skip it instead of surfacing a confusing "Failed to launch Amazon Q language server" error.
+        getLogger('amazonqLsp').info('skipping language server activation: not supported in the web extension host')
+    } else if (!isAmazonLinux2() || hasGlibcPatch()) {
         // Activate Amazon Q LSP for everyone unless they're using AL2 without the glibc patch
         await activateAmazonqLsp(context)
     }
@@ -169,7 +173,9 @@ export async function activateAmazonQCommon(context: vscode.ExtensionContext, is
     // reload webviews
     await vscode.commands.executeCommand('workbench.action.webview.reloadWebviewAction')
 
-    if (AuthUtils.ExtensionUse.instance.isFirstUse()) {
+    // The chat panel is not available in the web extension host (see `!aws.isWebExtHost` in package.json),
+    // so there is nothing to focus there.
+    if (!isWeb && AuthUtils.ExtensionUse.instance.isFirstUse()) {
         // Give time for the extension to finish initializing.
         globals.clock.setTimeout(async () => {
             CommonAuthWebview.authSource = ExtStartUpSources.firstStartUp
diff --git a/packages/amazonq/src/extensionWeb.ts b/packages/amazonq/src/extensionWeb.ts
index 3091cda2da..e55b089b43 100644
--- a/packages/amazonq/src/extensionWeb.ts
+++ b/packages/amazonq/src/extensionWeb.ts
@@ -5,11 +5,16 @@
 
 import type { ExtensionContext } from 'vscode'
 import { activateWebShared } from 'aws-core-vscode/webShared'
+import { showWebModeUnsupportedMessageOnce } from 'aws-core-vscode/amazonq'
 import { activateAmazonQCommon, deactivateCommon } from './extension'
 
 export async function activate(context: ExtensionContext) {
     await activateWebShared(context)
     await activateAmazonQCommon(context, true)
+    // Amazon Q needs a Node.js extension host (desktop VS Code, or a browser editor connected to a
+    // remote environment). In the browser-only extension host the language server cannot start, so
+    // tell the user up front instead of letting them click "Sign in" and see nothing happen.
+    showWebModeUnsupportedMessageOnce()
 }
 
 export async function deactivate() {
diff --git a/packages/core/src/amazonq/index.ts b/packages/core/src/amazonq/index.ts
index 2d4b830b5e..327bacd2c1 100644
--- a/packages/core/src/amazonq/index.ts
+++ b/packages/core/src/amazonq/index.ts
@@ -9,6 +9,7 @@ export { MessagePublisher } from './messages/messagePublisher'
 export { MessageListener } from './messages/messageListener'
 export { AuthController } from './auth/controller'
 export { showAmazonQWalkthroughOnce } from './onboardingPage/walkthrough'
+export { showWebModeUnsupportedMessage, showWebModeUnsupportedMessageOnce, webModeUnsupportedMessage } from './webMode'
 export {
     focusAmazonQChatWalkthrough,
     openAmazonQWalkthrough,
diff --git a/packages/core/src/amazonq/webMode.ts b/packages/core/src/amazonq/webMode.ts
new file mode 100644
index 0000000000..064c9c26de
--- /dev/null
+++ b/packages/core/src/amazonq/webMode.ts
@@ -0,0 +1,34 @@
+/*!
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { showMessage } from '../shared/utilities/messages'
+import { once } from '../shared/utilities/functionUtils'
+
+/**
+ * Amazon Q depends on the Amazon Q language server, which runs as a Node.js child process.
+ * The browser-only VS Code extension host (for example vscode.dev or github.dev opened without
+ * a remote/backend environment) cannot spawn processes, so Amazon Q features cannot work there.
+ *
+ * Users end up with an extension that appears installed but does nothing when they click "Sign in".
+ * This message tells them why, and what to do instead.
+ */
+export const webModeUnsupportedMessage =
+    'Amazon Q is not available in the browser-only version of VS Code (for example vscode.dev or github.dev without a remote environment). ' +
+    'Use VS Code on your desktop, or connect the browser editor to a remote environment such as a codespace or a remote host, to use Amazon Q.'
+
+/**
+ * Shows a non-modal notification explaining that Amazon Q does not work in the browser-only extension host.
+ */
+export function showWebModeUnsupportedMessage(): Thenable<string | undefined> {
+    return showMessage('info', webModeUnsupportedMessage, [], {}, { id: 'amazonqWebModeUnsupported' })
+}
+
+/**
+ * Same as {@link showWebModeUnsupportedMessage}, but only shows the notification once per session.
+ * Intended for extension activation so the notification is not repeated.
+ */
+export const showWebModeUnsupportedMessageOnce = once(() => {
+    void showWebModeUnsupportedMessage()
+})
diff --git a/packages/core/src/codewhisperer/ui/codeWhispererNodes.ts b/packages/core/src/codewhisperer/ui/codeWhispererNodes.ts
index c1934ec6a7..92618e1040 100644
--- a/packages/core/src/codewhisperer/ui/codeWhispererNodes.ts
+++ b/packages/core/src/codewhisperer/ui/codeWhispererNodes.ts
@@ -29,6 +29,7 @@ import { submitFeedback } from '../../feedback/vue/submitFeedback'
 import { focusAmazonQPanel } from '../../codewhispererChat/commands/registerCommands'
 import { isWeb } from '../../shared/extensionGlobals'
 import { getLogger } from '../../shared/logger/logger'
+import { showWebModeUnsupportedMessage } from '../../amazonq/webMode'
 
 export function createAutoSuggestions(running: boolean): DataQuickPickItem<'autoSuggestions'> {
     const labelResume = localize('AWS.codewhisperer.resumeCodeWhispererNode.label', 'Resume Auto-Suggestions')
@@ -249,9 +250,10 @@ export function createSignIn(): DataQuickPickItem<'signIn'> {
         })
     }
     if (isWeb()) {
-        // TODO: nkomonen, call a Command instead
+        // Amazon Q cannot run in the browser-only extension host (no language server), so signing in
+        // would not lead anywhere. Explain why instead of starting an auth flow that appears to do nothing.
         onClick = () => {
-            void AuthUtil.instance.connectToAwsBuilderId()
+            void showWebModeUnsupportedMessage()
         }
     }
 
diff --git a/packages/core/src/test/amazonq/webMode.test.ts b/packages/core/src/test/amazonq/webMode.test.ts
new file mode 100644
index 0000000000..9838595c0b
--- /dev/null
+++ b/packages/core/src/test/amazonq/webMode.test.ts
@@ -0,0 +1,56 @@
+/*!
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import assert from 'assert'
+import sinon from 'sinon'
+import globals from '../../shared/extensionGlobals'
+import { showWebModeUnsupportedMessage, webModeUnsupportedMessage } from '../../amazonq/webMode'
+import { createSignIn } from '../../codewhisperer/ui/codeWhispererNodes'
+import { getTestWindow } from '../shared/vscode/window'
+
+/** `waitForMessage` treats strings as regex patterns, so match on a prefix without special characters. */
+const webModeMessagePattern = /^Amazon Q is not available in the browser-only version of VS Code/
+
+describe('amazonq webMode', function () {
+    let sandbox: sinon.SinonSandbox
+
+    beforeEach(function () {
+        sandbox = sinon.createSandbox()
+    })
+
+    afterEach(function () {
+        sandbox.restore()
+    })
+
+    describe('showWebModeUnsupportedMessage', function () {
+        it('shows an informational notification', async function () {
+            const shown = getTestWindow().waitForMessage(webModeMessagePattern)
+            void showWebModeUnsupportedMessage()
+            const message = await shown
+            message.assertInfo(webModeUnsupportedMessage)
+        })
+    })
+
+    describe('createSignIn', function () {
+        it('explains that Amazon Q is unavailable when running in the web extension host', async function () {
+            sandbox.stub(globals, 'isWeb').value(true)
+
+            const shown = getTestWindow().waitForMessage(webModeMessagePattern)
+            createSignIn().onClick?.()
+            const message = await shown
+            message.assertInfo(webModeUnsupportedMessage)
+        })
+
+        it('does not show the web notice outside of the web extension host', function () {
+            sandbox.stub(globals, 'isWeb').value(false)
+
+            createSignIn().onClick?.()
+            assert.strictEqual(
+                getTestWindow().shownMessages.some((m) => m.message === webModeUnsupportedMessage),
+                false
+            )
+        })
+    })
+})