fix(server): include Origin in CORS preflight Vary header (#26445)

packages/opencode/src/server/routes/instance/httpapi/middleware/cors-vary.ts

@@ -0,0 +1,29 @@
+import { Effect } from "effect"
+import { HttpRouter, HttpServerResponse } from "effect/unstable/http"
+
+// effect-smol's HttpMiddleware.cors builds OPTIONS preflight responses by
+// spreading allowOrigin() and allowHeaders() into the same record. Both set
+// the `vary` key, so allowHeaders' `Vary: Access-Control-Request-Headers`
+// overwrites allowOrigin's `Vary: Origin`. With dynamic origin echoing, the
+// missing `Vary: Origin` lets shared caches reuse a preflight cached for one
+// origin against a different origin.
+//
+// TODO: upstream a fix that merges Vary values in headersFromRequestOptions
+// (packages/effect/src/unstable/http/HttpMiddleware.ts ~line 332).
+export const corsVaryFix = HttpRouter.middleware(
+  (effect) =>
+    Effect.gen(function* () {
+      const response = yield* effect
+      const allowOrigin = response.headers["access-control-allow-origin"]
+      if (!allowOrigin || allowOrigin === "*") return response
+
+      const vary = response.headers["vary"]
+      if (!vary) return HttpServerResponse.setHeader(response, "vary", "Origin")
+
+      const tokens = vary.split(",").map((s) => s.trim().toLowerCase())
+      if (tokens.includes("origin") || tokens.includes("*")) return response
+
+      return HttpServerResponse.setHeader(response, "vary", `${vary}, Origin`)
+    }),
+  { global: true },
+)

packages/opencode/src/server/routes/instance/httpapi/server.ts

@@ -82,6 +82,7 @@ import { disposeMiddleware } from "./lifecycle"
 import { memoMap } from "@opencode-ai/core/effect/memo-map"
 import * as ServerBackend from "@/server/backend"
 import { compressionLayer } from "./middleware/compression"
+import { corsVaryFix } from "./middleware/cors-vary"
 import { errorLayer } from "./middleware/error"
 import { fenceLayer } from "./middleware/fence"
 
@@ -176,6 +177,7 @@ export function createRoutes(corsOptions?: CorsOptions) {
     Layer.provide([
       errorLayer,
       compressionLayer,
+      corsVaryFix,
       fenceLayer,
       cors(corsOptions),
       runtime,