diff --git a/src-tauri/src/github.rs b/src-tauri/src/github.rs
index a4f9172e..cda8fe1d 100644
--- a/src-tauri/src/github.rs
+++ b/src-tauri/src/github.rs
@@ -2,8 +2,10 @@ use serde::{Deserialize, Serialize};
 use std::path::Path;
 use std::process::Command;
 
-/// GitHub OAuth App client ID. Replace with your registered GitHub App's client_id.
-const GITHUB_CLIENT_ID: &str = "Ov23liCuBz7Z5hKk6T8c";
+/// GitHub App client ID for OAuth device flow.
+/// To set up: GitHub Settings → Developer settings → GitHub Apps → New GitHub App.
+/// Enable "Device authorization flow" under Optional features. Webhook can be disabled.
+const GITHUB_CLIENT_ID: &str = "Ov23liwee215tDMs9u4L";
 
 #[derive(Debug, Serialize, Deserialize, Clone)]
 pub struct GithubRepo {
@@ -175,14 +177,23 @@ async fn github_device_flow_start_with_base(base_url: &str) -> Result<DeviceFlow
     let response = client
         .post(format!("{}/login/device/code", base_url))
         .header("Accept", "application/json")
+        .header("User-Agent", "Laputa-App")
         .form(&[("client_id", GITHUB_CLIENT_ID), ("scope", "repo")])
         .send()
         .await
         .map_err(|e| format!("Device flow request failed: {}", e))?;
 
     if !response.status().is_success() {
+        let status = response.status();
         let body = response.text().await.unwrap_or_default();
-        return Err(format!("Device flow start failed: {}", body));
+        if status.as_u16() == 404 {
+            return Err(
+                "GitHub device flow not available. Ensure a GitHub App is registered with \
+                 'Device authorization flow' enabled (Settings → Developer settings → GitHub Apps)."
+                    .to_string(),
+            );
+        }
+        return Err(format!("Device flow start failed ({}): {}", status, body));
     }
 
     response
@@ -204,6 +215,7 @@ async fn github_device_flow_poll_with_base(
     let response = client
         .post(format!("{}/login/oauth/access_token", base_url))
         .header("Accept", "application/json")
+        .header("User-Agent", "Laputa-App")
         .form(&[
             ("client_id", GITHUB_CLIENT_ID),
             ("device_code", device_code),
@@ -794,6 +806,24 @@ mod tests {
         assert!(result.unwrap_err().contains("Device flow start failed"));
     }
 
+    #[tokio::test]
+    async fn test_github_device_flow_start_404_gives_clear_message() {
+        let mut server = mockito::Server::new_async().await;
+        let mock = server
+            .mock("POST", "/login/device/code")
+            .with_status(404)
+            .with_body(r#"{"error":"Not Found"}"#)
+            .create_async()
+            .await;
+
+        let result = github_device_flow_start_with_base(&server.url()).await;
+        mock.assert_async().await;
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+        assert!(err.contains("device flow not available"));
+        assert!(err.contains("Device authorization flow"));
+    }
+
     #[tokio::test]
     async fn test_github_device_flow_poll_complete() {
         let mut server = mockito::Server::new_async().await;
diff --git a/src/components/SettingsPanel.test.tsx b/src/components/SettingsPanel.test.tsx
index 39cd7789..71aac1a7 100644
--- a/src/components/SettingsPanel.test.tsx
+++ b/src/components/SettingsPanel.test.tsx
@@ -291,5 +291,56 @@ describe('SettingsPanel', () => {
       )
       expect(screen.getByText(/Connect your GitHub account/)).toBeInTheDocument()
     })
+
+    it('displays the actual backend error string when device flow start fails', async () => {
+      mockInvokeFn.mockImplementation(async (cmd: string) => {
+        if (cmd === 'github_device_flow_start') {
+          // Tauri invoke rejects with a plain string, not an Error instance
+          throw 'GitHub device flow not available. Ensure a GitHub App is registered.'
+        }
+        return null
+      })
+
+      render(
+        <SettingsPanel open={true} settings={emptySettings} onSave={onSave} onClose={onClose} />
+      )
+
+      fireEvent.click(screen.getByTestId('github-login'))
+
+      await waitFor(() => {
+        expect(screen.getByTestId('github-error')).toHaveTextContent(
+          'GitHub device flow not available. Ensure a GitHub App is registered.'
+        )
+      })
+    })
+
+    it('prevents double-click by disabling button during login flow', async () => {
+      let resolveStart: ((v: unknown) => void) | null = null
+      mockInvokeFn.mockImplementation(async (cmd: string) => {
+        if (cmd === 'github_device_flow_start') {
+          return new Promise(r => { resolveStart = r })
+        }
+        return null
+      })
+
+      render(
+        <SettingsPanel open={true} settings={emptySettings} onSave={onSave} onClose={onClose} />
+      )
+
+      const loginBtn = screen.getByTestId('github-login') as HTMLButtonElement
+      fireEvent.click(loginBtn)
+
+      // Button should be disabled while waiting
+      await waitFor(() => {
+        expect(loginBtn.disabled).toBe(true)
+      })
+
+      // Clean up
+      resolveStart?.({
+        device_code: 'dc', user_code: 'UC-1234',
+        verification_uri: 'https://github.com/login/device',
+        expires_in: 900, interval: 5,
+      })
+    })
   })
 })
diff --git a/src/components/SettingsPanel.tsx b/src/components/SettingsPanel.tsx
index fe448585..70409701 100644
--- a/src/components/SettingsPanel.tsx
+++ b/src/components/SettingsPanel.tsx
@@ -172,11 +172,11 @@ function GitHubSection({ githubUsername, githubToken, onConnected, onDisconnect
       pollLoop().catch(err => {
         stopPolling()
         setOauthStatus('error')
-        setErrorMessage(err instanceof Error ? err.message : 'Polling failed.')
+        setErrorMessage(typeof err === 'string' ? err : err instanceof Error ? err.message : 'Polling failed.')
       })
     } catch (err) {
       setOauthStatus('error')
-      setErrorMessage(err instanceof Error ? err.message : 'Failed to start login.')
+      setErrorMessage(typeof err === 'string' ? err : err instanceof Error ? err.message : 'Failed to start login.')
     }
   }, [onConnected, stopPolling])