diff --git a/packages/core/src/mcp/oauth.ts b/packages/core/src/mcp/oauth.ts
index 2e72bd2d256a..72450bb6cfff 100644
--- a/packages/core/src/mcp/oauth.ts
+++ b/packages/core/src/mcp/oauth.ts
@@ -373,8 +373,17 @@ export const authorize = (input: {
       }
       const error = url.searchParams.get("error_description") ?? url.searchParams.get("error")
       if (error) return fail(error, "authorization_error")
-      if (url.searchParams.get("state") !== state) return fail("OAuth state mismatch", "state_mismatch")
+      const returned = url.searchParams.get("state")
       const value = url.searchParams.get("code")
+      // Browsers can reach the callback without any OAuth parameters (preflights, prefetches, reloads). Only a
+      // redirect from the authorization server may end the login, so keep waiting for it.
+      if (returned === null && !value) {
+        response
+          .writeHead(400, { "Content-Type": "text/html" })
+          .end(OauthCallbackPage.error("Missing authorization code", { provider: input.name }))
+        return
+      }
+      if (returned !== state) return fail("OAuth state mismatch", "state_mismatch")
       if (!value) return fail("Missing authorization code", "missing_code")
       // The page waits for the token exchange so the browser never reports success for a rejected code.
       if (
diff --git a/packages/core/test/mcp-oauth.test.ts b/packages/core/test/mcp-oauth.test.ts
index d65ce15f336b..4deeccb8d777 100644
--- a/packages/core/test/mcp-oauth.test.ts
+++ b/packages/core/test/mcp-oauth.test.ts
@@ -160,6 +160,37 @@ describe("MCP OAuth", () => {
     expect(tokenRequests[0]?.get("resource")).toBe(server.url.href)
   })
 
+  test("keeps waiting for the redirect after callback requests without OAuth parameters", async () => {
+    const server = Bun.serve({
+      port: 0,
+      async fetch(request) {
+        if (request.method !== "POST" || new URL(request.url).pathname !== "/token")
+          return new Response(null, { status: 404 })
+        return Response.json({ access_token: "access", token_type: "Bearer", refresh_token: "refresh", expires_in: 3600 })
+      },
+    })
+
+    const credential = await Effect.runPromise(
+      Effect.scoped(
+        Effect.gen(function* () {
+          const { authorization, url: authorizationUrl } = yield* start(server, { client_id: "client" })
+          const redirectValue = authorizationUrl.searchParams.get("redirect_uri")
+          const state = authorizationUrl.searchParams.get("state")
+          if (!redirectValue || !state) throw new Error("Missing OAuth redirect parameters")
+          const callback = new URL(redirectValue)
+          expect((yield* Effect.promise(() => fetch(callback, { method: "OPTIONS" }))).status).toBe(400)
+          expect((yield* Effect.promise(() => fetch(callback))).status).toBe(400)
+          callback.searchParams.set("code", "accepted")
+          callback.searchParams.set("state", state)
+          expect((yield* Effect.promise(() => fetch(callback))).status).toBe(200)
+          return yield* authorization.callback
+        }),
+      ),
+    ).finally(() => server.stop(true))
+
+    expect(credential.access).toBe("access")
+  })
+
   test("reports a rejected token exchange on the callback page", async () => {
     const server = Bun.serve({
       port: 0,