Thanks! I just found the issue: it works after i removed the BatchLinkPlugin in the client and BatchHandlerPlugin in the server. Nothing else is changed. May i ask if this is expected?

The error message goes like this:

Error: Internal Server Error
    at StandardRPCLinkCodec.decode (http://localhost:3000/node_modules/.vite/deps/chunk-YVHPG2GL.js?v=047c3448:328:13)
    at async http://localhost:3000/node_modules/.vite/deps/chunk-YVHPG2GL.js?v=047c3448:86:26
    at async http://localhost:3000/node_modules/.vite/deps/chunk-FANAFXAN.js?v=047c3448:434:14
    at async Proxy.procedureClient (http://localhost:3000/node_modules/.vite/deps/chunk-TG47255X.js?v=047c3448:70:12)
    at async Object.reconnectToStream (http://localhost:3000/src/components/panels/explanation-panel/index.tsx?t=1762606993761:119:28)
    at async Chat.makeRequest (http://localhost:3000/node_modules/.vite/deps/chunk-OFYKFGD6.js?v=047c3448:11380:27)
    at async AbstractChat.resumeStream (http://localhost:3000/node_modules/.vite/deps/chunk-OFYKFGD6.js?v=047c3448:11240:7)

edit: this is what gpt told me:

The whole problem was the batching layer:

• src/lib/orpc/client.ts:50-83 wires up BatchLinkPlugin with condition: () => true, so every procedure call—including
  long‑lived stream resumptions—gets queued for /api/orpc/batch whenever multiple calls fire in the same tick (plan
  usage query, explanation lookup, reconnect, etc.).
• On the server, src/routes/api/orpc/$.ts:8-22 installs BatchHandlerPlugin. That plugin’s own code (node_modules/@orpc/
  server/dist/plugins/index.mjs:33-79) explicitly rejects any batch entry whose response body is a Blob/FormData/event
  iterator; it returns a 500 with the message “Batch responses do not support file/blob, or event-iterator. Please call
  this procedure separately outside of the batch request.”
• reconnectToExplainStream returns an event iterator. When the reconnect call is wrapped inside a batch,
  the handler never even executes—the batch plugin short‑circuits with that 500. The client surfaces it as
  StandardRPCLinkCodec.decode … Internal Server Error, which matches what you saw. When you removed both batch plugins,
  the reconnect request went out as a normal POST, the server streamed from Redis, and everything worked with no other
  changes.