diff --git a/.changeset/bright-data-bearer-prefix.md b/.changeset/bright-data-bearer-prefix.md
new file mode 100644
index 000000000..cac7dad77
--- /dev/null
+++ b/.changeset/bright-data-bearer-prefix.md
@@ -0,0 +1,5 @@
+---
+"@truefoundry/trueforge-ui": patch
+---
+
+Fix header-auth MCP connectors (e.g. Bright Data) rejecting pasted API keys with 401 because the required `Bearer ` scheme prefix wasn't applied. The Connect / Replace Key dialog now tests the pasted key live against the upstream server — as typed, then with a `Bearer ` prefix, then with a `Basic ` prefix — reporting each attempt in the dialog, and stores whichever one actually connects.
diff --git a/packages/trueforge-ui/src/containers/SettingsBuilder/ConnectorSettings.tsx b/packages/trueforge-ui/src/containers/SettingsBuilder/ConnectorSettings.tsx
index 9bd2529c4..1fbba1687 100644
--- a/packages/trueforge-ui/src/containers/SettingsBuilder/ConnectorSettings.tsx
+++ b/packages/trueforge-ui/src/containers/SettingsBuilder/ConnectorSettings.tsx
@@ -44,6 +44,7 @@ const ConnectorSettings = () => {
   const [connectorAwaitingKey, setConnectorAwaitingKey] = useState<ConnectorCatalogEntry | null>(null);
   const [selectedConnector, setSelectedConnector] = useState<ConnectorBase | null>(null);
   const [apiKey, setApiKey] = useState('');
+  const [probeLog, setProbeLog] = useState<string[]>([]);
 
   const connectorIconMap = useMemo(() => {
     return (catalog ?? []).reduce(
@@ -141,6 +142,7 @@ const ConnectorSettings = () => {
   const closeApiKeyModal = () => {
     setConnectorAwaitingKey(null);
     setApiKey('');
+    setProbeLog([]);
     setFormError(null);
   };
 
@@ -166,6 +168,7 @@ const ConnectorSettings = () => {
   const handleConnect = (entry: ConnectorCatalogEntry) => {
     if (entry.auth.type === 'header') {
       setApiKey('');
+      setProbeLog([]);
       setFormError(null);
       setConnectorAwaitingKey(entry);
       return;
@@ -176,38 +179,78 @@ const ConnectorSettings = () => {
     }).catch(() => {});
   };
 
+  /**
+   * A pasted key's required scheme (none, `Bearer `, `Basic `, …) isn't knowable up front, so try
+   * the key as typed first, then the common schemes, in order — deduping a candidate that's
+   * byte-identical to one already tried (e.g. the user already typed the `Bearer ` prefix).
+   */
+  const buildAuthCandidates = (rawKey: string): { label: string; value: string }[] => {
+    const trimmed = rawKey.trim();
+    const candidates: { label: string; value: string }[] = [{ label: 'Testing key…', value: trimmed }];
+    if (!/^bearer\s/i.test(trimmed)) {
+      candidates.push({ label: 'Trying with prefix "Bearer"…', value: `Bearer ${trimmed}` });
+    }
+    if (!/^basic\s/i.test(trimmed)) {
+      candidates.push({ label: 'Trying with prefix "Basic"…', value: `Basic ${trimmed}` });
+    }
+    return candidates;
+  };
+
   const handleApiKeySubmit = (event: FormEvent<HTMLFormElement>) => {
     event.preventDefault();
     if (!connectorAwaitingKey || !apiKey.trim()) return;
 
     const entry = connectorAwaitingKey;
+    const headerName = entry.auth.type === 'header' ? entry.auth.headerName : undefined;
+    const existing = connectors.ordered.find(({ connector }) => connector.id === entry.id);
+    const existingConnector = existing?.isConfigured ? existing.connector : undefined;
+
     setFormError(null);
-    void runMutation(async () => {
-      const auth: ConnectorAuth = {
-        type: 'header',
-        apiKey: apiKey.trim(),
-        ...(entry.auth.type === 'header' && entry.auth.headerName ? { headerName: entry.auth.headerName } : {}),
-      };
-      const existing = connectors.ordered.find(({ connector }) => connector.id === entry.id);
-      const existingConnector = existing?.isConfigured ? existing.connector : undefined;
-      if (existingConnector) {
-        await connectorCatalog.updateConnector({
-          id: existingConnector.id,
-          name: existingConnector.name,
-          description: existingConnector.description,
-          url: existingConnector.url,
-          auth,
-        });
-      } else {
-        await createFromCatalog(entry, auth);
+    setError(null);
+    setProbeLog([]);
+    setBusy(true);
+    void (async () => {
+      let lastError = 'Connection failed';
+      for (const candidate of buildAuthCandidates(apiKey)) {
+        setProbeLog(log => [...log, candidate.label]);
+        const auth: ConnectorAuth = {
+          type: 'header',
+          apiKey: candidate.value,
+          ...(headerName ? { headerName } : {}),
+        };
+        try {
+          if (existingConnector) {
+            await connectorCatalog.updateConnector({
+              id: existingConnector.id,
+              name: existingConnector.name,
+              description: existingConnector.description,
+              url: existingConnector.url,
+              auth,
+            });
+          } else {
+            await createFromCatalog(entry, auth);
+          }
+          await connectorCatalog.getToolsByConnectorId({ id: entry.id });
+          setProbeLog(log => [...log.slice(0, -1), `${candidate.label} succeeded`]);
+          setBusy(false);
+          closeApiKeyModal();
+          await refresh();
+          setTimeout(() => {
+            toaster?.showSuccess({
+              title: `${entry.name} ${existingConnector ? 'updated' : 'connected'}`,
+            });
+          }, 100);
+          return;
+        } catch (err) {
+          lastError = getErrorMessage(err, 'Connection failed');
+          setProbeLog(log => [...log.slice(0, -1), `${candidate.label} failed`]);
+        }
       }
-      closeApiKeyModal();
-      setTimeout(() => {
-        toaster?.showSuccess({
-          title: `${entry.name} ${existingConnector ? 'updated' : 'connected'}`,
-        });
-      }, 100);
-    }, setFormError).catch(() => {});
+      setBusy(false);
+      setFormError(
+        `Could not connect with the provided key (tried as-is, with "Bearer", and with "Basic"). Last error: ${lastError}`,
+      );
+    })();
   };
 
   const handleAddMcpServer = async (draft: AddMcpServerDraft) => {
@@ -305,6 +348,7 @@ const ConnectorSettings = () => {
                 onClick={event => {
                   event.stopPropagation();
                   setApiKey('');
+                  setProbeLog([]);
                   setConnectorAwaitingKey(connector);
                 }}
               >
@@ -507,9 +551,15 @@ const ConnectorSettings = () => {
                     placeholder={`Paste the token from ${connectorAwaitingKey?.name ?? 'the provider'}`}
                     autoFocus
                     required
-                    className={auiInputClass('h-11')}
+                    disabled={busy}
+                    className={auiInputClass('h-11 disabled:opacity-60')}
                   />
                 </div>
+                {probeLog.length > 0 ? (
+                  <pre className="whitespace-pre-wrap rounded-md border border-border bg-secondary-bg/40 px-3 py-2 font-mono text-xs text-text-secondary">
+                    {probeLog.join('\n')}
+                  </pre>
+                ) : null}
                 {formError ? <p className="text-failure-bg text-sm">{formError}</p> : null}
               </div>
 
@@ -518,7 +568,7 @@ const ConnectorSettings = () => {
                   Cancel
                 </Button>
                 <Button type="submit" disabled={!apiKey.trim() || busy}>
-                  {isReplacingKey ? 'Replace Key' : 'Connect'}
+                  {busy ? 'Testing…' : isReplacingKey ? 'Replace Key' : 'Connect'}
                 </Button>
               </footer>
             </form>
diff --git a/packages/trueforge-ui/test/containers/SettingsBuilder/ConnectorSettings.test.tsx b/packages/trueforge-ui/test/containers/SettingsBuilder/ConnectorSettings.test.tsx
new file mode 100644
index 000000000..d61e73955
--- /dev/null
+++ b/packages/trueforge-ui/test/containers/SettingsBuilder/ConnectorSettings.test.tsx
@@ -0,0 +1,143 @@
+// @vitest-environment jsdom
+import { fireEvent, render, screen, waitFor, within } from '@testing-library/react';
+import { beforeAll, describe, expect, it } from 'vitest';
+
+import ConnectorSettings from '@/containers/SettingsBuilder/ConnectorSettings.js';
+import { ServerProvider } from '@/server/ServerContext.js';
+import type { ConnectorBase, ConnectorCatalogEntry } from '@/server/types.js';
+import { createMockAgentUIServer, createMockCatalog } from '../../server/mockServer.js';
+
+beforeAll(() => {
+  HTMLDialogElement.prototype.showModal = function showModal() {
+    this.setAttribute('open', '');
+  };
+  HTMLDialogElement.prototype.close = function close() {
+    this.removeAttribute('open');
+    this.dispatchEvent(new Event('close'));
+  };
+});
+
+const brightData: ConnectorCatalogEntry = {
+  id: 'bright-data',
+  name: 'bright-data',
+  description: 'Search the web and scrape pages, including sites behind bot protection.',
+  url: 'https://mcp.brightdata.com/mcp',
+  auth: { type: 'header', headerName: 'Authorization' },
+};
+
+const connectedBrightData: ConnectorBase = {
+  id: 'bright-data',
+  name: 'bright-data',
+  description: brightData.description ?? '',
+  url: brightData.url,
+  auth: { type: 'header', headerName: 'Authorization' },
+  requiresAuth: false,
+  authenticated: true,
+};
+
+/** `succeedsWhen` decides, per attempt, whether the just-saved header value should test as reachable. */
+function renderConnectorSettings(succeedsWhen: (headerValue: string) => boolean) {
+  let lastAuthValue: string | undefined;
+  const attempts: string[] = [];
+  const server = createMockAgentUIServer({
+    catalog: createMockCatalog({
+      connectorCatalog: {
+        getConnectorCatalog: async () => [brightData],
+        listConnectors: async () => [],
+        getConnector: async () => connectedBrightData,
+        getToolsByConnectorId: async () => {
+          if (lastAuthValue === undefined || !succeedsWhen(lastAuthValue)) {
+            throw new Error(`upstream returned 401 Unauthorized for "${lastAuthValue}"`);
+          }
+          return [];
+        },
+        createConnector: async req => {
+          const value = req.auth.type === 'header' ? (req.auth.apiKey ?? '') : '';
+          lastAuthValue = value;
+          attempts.push(value);
+          return connectedBrightData;
+        },
+        updateConnector: async req => {
+          const value = req.auth.type === 'header' ? (req.auth.apiKey ?? '') : '';
+          lastAuthValue = value;
+          attempts.push(value);
+          return connectedBrightData;
+        },
+        authenticateConnector: async () => ({ authorization_endpoint: '' }),
+        disconnectConnector: async () => connectedBrightData,
+      },
+    }),
+  });
+
+  render(
+    <ServerProvider server={server}>
+      <ConnectorSettings />
+    </ServerProvider>,
+  );
+
+  return { attempts };
+}
+
+async function openConnectModal() {
+  const row = await screen.findByText('bright-data');
+  const connectButton = within(row.closest('article') as HTMLElement).getByRole('button', { name: 'Connect' });
+  fireEvent.click(connectButton);
+  return within(await screen.findByRole('dialog'));
+}
+
+function submit(dialog: ReturnType<typeof within>, apiKey: string) {
+  fireEvent.change(dialog.getByLabelText('API key / token'), { target: { value: apiKey } });
+  fireEvent.click(dialog.getByRole('button', { name: /Connect|Testing/ }));
+}
+
+describe('ConnectorSettings auto-probing header auth (fixes #490)', () => {
+  it('stores the raw key when it works on the first try', async () => {
+    const { attempts } = renderConnectorSettings(value => value === 'abc123');
+    const dialog = await openConnectModal();
+    submit(dialog, 'abc123');
+
+    await waitFor(() => expect(screen.queryByRole('dialog')).not.toBeInTheDocument());
+    expect(attempts).toEqual(['abc123']);
+  });
+
+  it('falls back to a Bearer prefix when the raw key is rejected, logging each attempt', async () => {
+    const { attempts } = renderConnectorSettings(value => value === 'Bearer abc123');
+    const dialog = await openConnectModal();
+    submit(dialog, 'abc123');
+
+    await waitFor(() => expect(screen.queryByRole('dialog')).not.toBeInTheDocument());
+    expect(attempts).toEqual(['abc123', 'Bearer abc123']);
+  });
+
+  it('falls back to Basic when both the raw key and Bearer are rejected', async () => {
+    const { attempts } = renderConnectorSettings(value => value === 'Basic abc123');
+    const dialog = await openConnectModal();
+    submit(dialog, 'abc123');
+
+    await waitFor(() => expect(screen.queryByRole('dialog')).not.toBeInTheDocument());
+    expect(attempts).toEqual(['abc123', 'Bearer abc123', 'Basic abc123']);
+  });
+
+  it('shows a final error, keeps the dialog open, and logs every failed attempt when all candidates fail', async () => {
+    const { attempts } = renderConnectorSettings(() => false);
+    const dialog = await openConnectModal();
+    submit(dialog, 'abc123');
+
+    expect(await dialog.findByText(/Could not connect with the provided key/)).toBeInTheDocument();
+    expect(attempts).toEqual(['abc123', 'Bearer abc123', 'Basic abc123']);
+    expect(screen.getByRole('dialog')).toBeInTheDocument();
+    expect(dialog.getByText(/Testing key… failed/)).toBeInTheDocument();
+    expect(dialog.getByText(/Trying with prefix "Bearer"… failed/)).toBeInTheDocument();
+    expect(dialog.getByText(/Trying with prefix "Basic"… failed/)).toBeInTheDocument();
+  });
+
+  it('does not double-prefix a key already typed with "Bearer "', async () => {
+    const { attempts } = renderConnectorSettings(value => value === 'Basic Bearer abc123');
+    const dialog = await openConnectModal();
+    submit(dialog, 'Bearer abc123');
+
+    await waitFor(() => expect(screen.queryByRole('dialog')).not.toBeInTheDocument());
+    // No separate "Bearer Bearer abc123" attempt — the raw try already carried the prefix.
+    expect(attempts).toEqual(['Bearer abc123', 'Basic Bearer abc123']);
+  });
+});