diff --git a/packages/bb-plugin-agent-plugins/README.md b/packages/bb-plugin-agent-plugins/README.md
index 1fedf6b..650151e 100644
--- a/packages/bb-plugin-agent-plugins/README.md
+++ b/packages/bb-plugin-agent-plugins/README.md
@@ -38,6 +38,8 @@ bb plugin install git:https://github.com/patleeman/bb-plugins.git --subdirectory
 
 The MCP client delegates authentication to the official SDK, including protected-resource/authorization-server discovery, dynamic client registration, PKCE, refresh, `WWW-Authenticate`, insufficient-scope step-up, and OAuth callback completion. Tokens and PKCE state are stored in a BB secret setting; they are never placed in the database snapshot or frontend payload.
 
+For OAuth-protected MCP servers such as Fastmail, the registered redirect uses `http://localhost` on BB's port. If BB runs on another machine, sign in with your normal browser, then copy the **full** localhost URL from its address bar after the connection fails and paste it into the matching server's **Finish sign-in** field in BB. The URL contains a short-lived, one-use authorization code: paste it only into BB, never into chat or logs. BB verifies the callback URL, state and PKCE grant before exchanging the code. If the browser runs on the BB host or you have a port forward, the callback completes normally without pasting. Fastmail's MCP dynamic registration does not accept arbitrary public HTTPS redirect URIs; setting `BB_APP_URL` does not change this callback.
+
 This v0 release installs one package instance at a time. Remove an existing
 plugin before installing the same package name again; removal keeps its data
 unless you choose to purge it.
diff --git a/packages/bb-plugin-agent-plugins/app.tsx b/packages/bb-plugin-agent-plugins/app.tsx
index 4511e56..a79719c 100644
--- a/packages/bb-plugin-agent-plugins/app.tsx
+++ b/packages/bb-plugin-agent-plugins/app.tsx
@@ -277,6 +277,32 @@ function InstallBar({ onDone }: { onDone: () => void }) {
   );
 }
 
+function CallbackPasteForm({
+  pluginId, serverId, busy, onFinish,
+}: {
+  pluginId: string;
+  serverId: string;
+  busy: boolean;
+  onFinish: (url: string) => Promise<boolean>;
+}) {
+  const [callbackUrl, setCallbackUrl] = useState("");
+  return (
+    <form className="mt-3 space-y-2" onSubmit={(event) => {
+      event.preventDefault();
+      if (!callbackUrl.trim()) return;
+      void onFinish(callbackUrl.trim()).then((completed) => { if (completed) setCallbackUrl(""); });
+    }}>
+      <label className="block text-[11px] leading-snug text-muted-foreground" htmlFor={`mcp-callback-${pluginId}-${serverId}`}>
+        If your browser cannot open localhost after sign-in, copy its full address and paste it here. The URL contains a one-use code; do not paste it into chat.
+      </label>
+      <div className="flex flex-wrap gap-2">
+        <Input id={`mcp-callback-${pluginId}-${serverId}`} aria-label={`Callback URL for ${serverId}`} type="url" className="min-w-0 flex-1 text-xs" value={callbackUrl} onChange={(event) => setCallbackUrl(event.target.value)} autoComplete="off" spellCheck={false} disabled={busy} />
+        <Button type="submit" size="sm" variant="outline" disabled={busy || !callbackUrl.trim()}>{busy ? "Finishing…" : "Finish sign-in"}</Button>
+      </div>
+    </form>
+  );
+}
+
 function PluginRow({
   plugin,
   update,
@@ -288,6 +314,7 @@ function PluginRow({
   onReconnect,
   onReauthorize,
   onDisconnect,
+  onFinishAuthentication,
   onSkillEnabledChange,
   onMcpEnabledChange,
   onUpdate,
@@ -303,6 +330,7 @@ function PluginRow({
   onReconnect: (p: string, s: string) => void;
   onReauthorize: (p: string, s: string) => void;
   onDisconnect: (p: string, s: string) => void;
+  onFinishAuthentication: (p: string, s: string, url: string) => Promise<boolean>;
   onSkillEnabledChange: (pluginId: string, skillName: string, enabled: boolean) => void;
   onMcpEnabledChange: (pluginId: string, serverId: string, enabled: boolean) => void;
   onUpdate: (pluginId: string) => void;
@@ -476,6 +504,9 @@ function PluginRow({
                                 {authStatus === "authorizing" ? "Continue authentication" : "Authenticate"} {srv.serverId}
                               </Button>
                             )}
+                            {canManageAuth && srv.enabled && authStatus === "authorizing" && (
+                              <CallbackPasteForm pluginId={plugin.id} serverId={srv.serverId} busy={isAuthBusy} onFinish={(url) => onFinishAuthentication(plugin.id, srv.serverId, url)} />
+                            )}
                             {canManageAuth && srv.enabled && (srv.status === "ready" || srv.status === "error" || (srv.status === "idle" && authStatus === "authenticated")) && (
                               <Button size="sm" variant="outline" className="mt-2 h-7 text-xs" disabled={isAuthBusy} onClick={() => onReconnect(plugin.id, srv.serverId)}>
                                 Reconnect {srv.serverId}
@@ -746,6 +777,25 @@ function AgentPluginsView() {
     }
   };
 
+  const finishAuthentication = async (p: string, s: string, callbackUrl: string): Promise<boolean> => {
+    const key = `mcp-auth:${p}:${s}`;
+    setPendingAction(key);
+    setLocalErr(null);
+    try {
+      await rpc.call("finishAuthentication", { id: p, serverId: s, callbackUrl });
+      toast.success(`${s} connected`, { duration: 4000 });
+      return true;
+    } catch {
+      const message = "Could not finish sign-in. Check that you pasted the full, most recent localhost URL and try again.";
+      setLocalErr(message);
+      notifyError(message, `agent-plugins:${key}:error`);
+      return false;
+    } finally {
+      setPendingAction(null);
+      await load();
+    }
+  };
+
   const authenticate = (p: string, s: string) => { void runAuthAction("authenticate", p, s); };
   const reconnect = (p: string, s: string) => { void runAuthAction("reconnect", p, s); };
   const reauthorize = (p: string, s: string) => { void runAuthAction("reauthorize", p, s); };
@@ -834,6 +884,7 @@ function AgentPluginsView() {
                   onReconnect={reconnect}
                   onReauthorize={reauthorize}
                   onDisconnect={disconnect}
+                  onFinishAuthentication={finishAuthentication}
                   onSkillEnabledChange={setSkillEnabled}
                   onMcpEnabledChange={setMcpEnabled}
                   onUpdate={updatePlugin}
diff --git a/packages/bb-plugin-agent-plugins/server.ts b/packages/bb-plugin-agent-plugins/server.ts
index 91535c5..706d462 100644
--- a/packages/bb-plugin-agent-plugins/server.ts
+++ b/packages/bb-plugin-agent-plugins/server.ts
@@ -11,6 +11,7 @@ import { McpGateway, type McpStdioCatalog, type McpStdioHost } from "./src/gatew
 import { mcpHostContract, mcpHostSignals } from "./src/host-contract.js";
 import { DEFAULT_TOOL_SEARCH_LIMIT, MAX_DESCRIBE_TOOLS, MAX_TOOL_SEARCH_LIMIT, describeTools, searchTools } from "./src/tool-search.js";
 import { DeferredOAuthCredentialStore, McpOAuthProvider, type OAuthCredentialRecord } from "./src/oauth.js";
+import { oauthCallbackUrl, validatePastedCallback } from "./src/callback-url.js";
 import { parseSource, fetchSource, probeSource } from "./src/source.js";
 import { materializeSkill, unmaterializeSkill } from "./src/skills-impl.js";
 import { ensureDir, hashDirectory, rimraf, atomicRename, LIMITS } from "./src/safe-fs.js";
@@ -257,8 +258,7 @@ export default async function plugin(bb: BbPluginApi) {
     stdioHost,
     oauth: {
       async getProvider(pluginId, serverId, serverUrl) {
-        const redirect = new URL(`/api/v1/plugins/${encodeURIComponent(bb.pluginId)}/http/oauth/callback`, bb.server.loopbackBaseUrl);
-        redirect.search = new URLSearchParams({ pluginId, serverId }).toString();
+        const redirect = oauthCallbackUrl(bb.server.loopbackBaseUrl, bb.pluginId, pluginId, serverId);
         return new McpOAuthProvider(`${pluginId}:${serverId}`, serverUrl, redirect, oauthCredentialStore);
       },
     },
@@ -362,6 +362,8 @@ export default async function plugin(bb: BbPluginApi) {
     }
   }
 
+  // Provider redirects have no BB browser session. OAuth state and PKCE, not
+  // route authentication, authorize this one-use callback.
   bb.http.route("GET", "/oauth/callback", async (context) => {
     const url = new URL(context.req.url);
     const pluginId = url.searchParams.get("pluginId");
@@ -375,7 +377,7 @@ export default async function plugin(bb: BbPluginApi) {
       bb.log.warn(`[agent-plugins] OAuth callback failed for ${serverId}: ${errorText(e)}`);
       return new Response("<p>Authentication failed. Return to BB and try again.</p>", { status: 400, headers: { "content-type": "text/html; charset=utf-8" } });
     }
-  });
+  }, { auth: "none" });
 
   async function approveServer(id: string, serverId: string): Promise<void> {
     const p = store.getPlugin(id) ?? store.getPluginByName(id);
@@ -1129,8 +1131,9 @@ export default async function plugin(bb: BbPluginApi) {
     async finishAuthentication({ id, serverId, callbackUrl }) {
       const p = store.getPlugin(id) ?? store.getPluginByName(id);
       if (!p) throw new Error(`not found: ${id}`);
-      const callback = new URL(callbackUrl);
-      await withDeferredOAuthPersistence(() => gateway.finishAuth(p.id, serverId, callback.searchParams));
+      const expected = oauthCallbackUrl(bb.server.loopbackBaseUrl, bb.pluginId, p.id, serverId);
+      const params = validatePastedCallback(callbackUrl, expected);
+      await withDeferredOAuthPersistence(() => gateway.finishAuth(p.id, serverId, params));
       await publishChanged({ kind: "oauth", id: p.id, serverId });
       return { authenticated: true };
     },
diff --git a/packages/bb-plugin-agent-plugins/src/app.test.tsx b/packages/bb-plugin-agent-plugins/src/app.test.tsx
index 723b9f2..cc842e5 100644
--- a/packages/bb-plugin-agent-plugins/src/app.test.tsx
+++ b/packages/bb-plugin-agent-plugins/src/app.test.tsx
@@ -400,4 +400,42 @@ describe("Agent Plugins settings", () => {
     }));
     await slot.findByRole("button", { name: "Authenticate fastmail" });
   });
+
+  it("finishes remote sign-in with a pasted localhost callback without showing the code in a message", async () => {
+    const callbackUrl = "http://localhost:38886/api/v1/plugins/agent-plugins/http/oauth/callback?pluginId=plugin-1&serverId=fastmail&code=one-use-secret&state=nonce";
+    let snapshot = {
+      plugins: [{
+        id: "plugin-1", name: "Fastmail", version: "1.0.0", specVersion: "1.0.0",
+        sourceType: "path", sourceIntent: "path:/tmp/fastmail", sourceResolved: null,
+        status: "active", approval: "approved", lastError: null,
+      }],
+      skills: [],
+      mcpServers: [{
+        pluginId: "plugin-1", serverId: "fastmail", type: "streamable-http", status: "needs-auth",
+        authStatus: "authorizing", lastError: null as string | null, approved: 1, enabled: true,
+        configJson: JSON.stringify({ type: "streamable-http", url: "https://api.fastmail.com/mcp" }),
+      }],
+      dataDir: "/tmp/bb",
+    };
+    const calls: unknown[] = [];
+    const slot = renderSlot(panel, { subPath: "" }, {
+      rpc: {
+        snapshot: () => snapshot,
+        checkUpdates: () => ({ updates: [] }),
+        finishAuthentication: (input: unknown) => {
+          calls.push(input);
+          snapshot = { ...snapshot, mcpServers: [{ ...snapshot.mcpServers[0]!, status: "ready", authStatus: "authenticated" }] };
+          return { authenticated: true };
+        },
+      } as never,
+    });
+
+    fireEvent.click(await slot.findByRole("button", { name: /Fastmail/ }));
+    const field = await slot.findByRole("textbox", { name: "Callback URL for fastmail" });
+    fireEvent.change(field, { target: { value: callbackUrl } });
+    fireEvent.click(slot.getByRole("button", { name: "Finish sign-in" }));
+    await waitFor(() => expect(calls).toEqual([{ id: "plugin-1", serverId: "fastmail", callbackUrl }]));
+    await waitFor(() => expect(slot.queryByRole("textbox", { name: "Callback URL for fastmail" })).toBeNull());
+    expect(slot.queryByText(/one-use-secret/)).toBeNull();
+  });
 });
diff --git a/packages/bb-plugin-agent-plugins/src/callback-url.test.ts b/packages/bb-plugin-agent-plugins/src/callback-url.test.ts
new file mode 100644
index 0000000..81ee55a
--- /dev/null
+++ b/packages/bb-plugin-agent-plugins/src/callback-url.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from "vitest";
+import { oauthCallbackUrl, validatePastedCallback } from "./callback-url.js";
+
+describe("MCP OAuth loopback callback", () => {
+  const expected = "http://localhost:38886/api/v1/plugins/agent-plugins/http/oauth/callback?pluginId=pa-fastmail-spike&serverId=fastmail";
+
+  it("uses localhost even when BB has a remote public URL", () => {
+    expect(oauthCallbackUrl("http://127.0.0.1:38886", "agent-plugins", "pa-fastmail-spike", "fastmail").toString()).toBe(expected);
+  });
+
+  it("preserves an already-localhost loopback address", () => {
+    expect(oauthCallbackUrl("http://localhost:38886", "agent-plugins", "pa-fastmail-spike", "fastmail").toString()).toBe(expected);
+  });
+
+  it("only accepts loopback as the callback base", () => {
+    expect(() => oauthCallbackUrl("http://remote.example:38886", "agent-plugins", "pa-fastmail-spike", "fastmail")).toThrow();
+  });
+
+  it("accepts the complete pasted callback URL after browser sign-in", () => {
+    const params = validatePastedCallback(`${expected}&code=one-time-code&state=nonce`, new URL(expected));
+    expect(params.get("code")).toBe("one-time-code");
+    expect(params.get("state")).toBe("nonce");
+  });
+
+  it.each([
+    "https://attacker.example/oauth/callback?code=secret&state=nonce",
+    "http://localhost:38886/other?code=secret&state=nonce",
+    "http://localhost:38886/api/v1/plugins/agent-plugins/http/oauth/callback?pluginId=other&serverId=fastmail&code=secret&state=nonce",
+    "http://localhost:38886/api/v1/plugins/agent-plugins/http/oauth/callback?pluginId=pa-fastmail-spike&serverId=other&code=secret&state=nonce",
+  ])("rejects a callback from a different URL or server without returning it (%s)", (pasted) => {
+    expect(() => validatePastedCallback(pasted, new URL(expected))).toThrow("Callback URL does not match this MCP connection");
+  });
+});
diff --git a/packages/bb-plugin-agent-plugins/src/callback-url.ts b/packages/bb-plugin-agent-plugins/src/callback-url.ts
new file mode 100644
index 0000000..60815c5
--- /dev/null
+++ b/packages/bb-plugin-agent-plugins/src/callback-url.ts
@@ -0,0 +1,31 @@
+// Fastmail's dynamic registration accepts localhost but not an arbitrary
+// HTTPS callback. A remote BB user can paste the failed browser redirect into
+// BB; the OAuth state/PKCE grant still authorizes the code exchange.
+export function oauthCallbackUrl(loopbackBaseUrl: string, bridgeId: string, pluginId: string, serverId: string): URL {
+  const base = new URL(loopbackBaseUrl);
+  if (base.protocol !== "http:" || !["localhost", "127.0.0.1", "[::1]"].includes(base.hostname) || base.username || base.password) {
+    throw new Error("MCP OAuth requires a local BB loopback URL");
+  }
+  base.hostname = "localhost";
+  const redirect = new URL(`/api/v1/plugins/${encodeURIComponent(bridgeId)}/http/oauth/callback`, base);
+  redirect.search = new URLSearchParams({ pluginId, serverId }).toString();
+  return redirect;
+}
+
+export function validatePastedCallback(pasted: string, expected: URL): URLSearchParams {
+  // Never include the supplied URL in an error: it carries a one-use code.
+  if (pasted.length > 8192) throw new Error("Callback URL is too long");
+  let actual: URL;
+  try { actual = new URL(pasted); }
+  catch { throw new Error("Paste the full callback URL from your browser's address bar"); }
+  const expectedQuery = expected.searchParams;
+  const actualQuery = actual.searchParams;
+  if (actual.protocol !== expected.protocol ||
+      ![expected.hostname, "127.0.0.1", "[::1]"].includes(actual.hostname) ||
+      actual.port !== expected.port || actual.pathname !== expected.pathname ||
+      actual.username || actual.password || actual.hash ||
+      [...expectedQuery.keys()].some((key) => actualQuery.getAll(key).length !== 1 || actualQuery.get(key) !== expectedQuery.get(key))) {
+    throw new Error("Callback URL does not match this MCP connection");
+  }
+  return actualQuery;
+}
diff --git a/packages/bb-plugin-agent-plugins/src/oauth.integration.test.ts b/packages/bb-plugin-agent-plugins/src/oauth.integration.test.ts
index 9d5eba1..9a831fe 100644
--- a/packages/bb-plugin-agent-plugins/src/oauth.integration.test.ts
+++ b/packages/bb-plugin-agent-plugins/src/oauth.integration.test.ts
@@ -173,7 +173,11 @@ describe("MCP SDK OAuth integration", () => {
     const state = new URL(url!).searchParams.get("state");
     expect(state).toBeTruthy();
 
+    await expect(gateway.finishAuth(plugin.id, "fastmail", new URLSearchParams({ code: "fixture-code" }))).rejects.toThrow("state mismatch");
+    await expect(gateway.finishAuth(plugin.id, "fastmail", new URLSearchParams({ code: "fixture-code", state: "wrong" }))).rejects.toThrow("state mismatch");
+    expect(tokenRequests).toBe(0);
     await gateway.finishAuth(plugin.id, "fastmail", new URLSearchParams({ code: "fixture-code", state: state! }));
+    await expect(gateway.finishAuth(plugin.id, "fastmail", new URLSearchParams({ code: "fixture-code", state: state! }))).rejects.toThrow("state mismatch");
     const tools = await gateway.listTools();
     expect(tools.map((tool) => tool.name)).toEqual(["oauth-tool"]);
     expect(tokenRequests).toBe(1);
diff --git a/packages/bb-plugin-agent-plugins/src/server.test.ts b/packages/bb-plugin-agent-plugins/src/server.test.ts
index 5aaa764..c1216ea 100644
--- a/packages/bb-plugin-agent-plugins/src/server.test.ts
+++ b/packages/bb-plugin-agent-plugins/src/server.test.ts
@@ -14,6 +14,20 @@ afterEach(async () => {
   while (tempDirs.length > 0) await rimraf(tempDirs.pop()!);
 });
 
+describe("Agent Plugins OAuth route", () => {
+  it("accepts provider redirects without BB browser authentication and rejects missing context", async () => {
+    const host = createFakePluginHost({ pluginId: "agent-plugins" });
+    hosts.push(host);
+    await plugin(host.bb);
+    const callback = host.harness.registrations.httpRoutes.find((route) => route.path === "/oauth/callback");
+    expect(callback?.auth).toBe("none");
+    const route = callback!;
+    const response = await route.handler({ req: { url: "https://bb.example.test/api/v1/plugins/agent-plugins/http/oauth/callback" } } as Parameters<typeof route.handler>[0]);
+    expect(response.status).toBe(400);
+    expect(await response.text()).toContain("Missing Agent Plugins OAuth callback context");
+  });
+});
+
 describe("Agent Plugins update flow", () => {
   it("keeps the installed identity, data path, and disabled skill state across an update", async () => {
     const dataDir = await fsp.mkdtemp(path.join(os.tmpdir(), "agent-plugins-data-"));