## Summary
`TriggerChatTransport`, `AgentChat`, and `chat.createStartSessionAction`
now accept a string-or-function `baseURL` so callers can route per
endpoint — e.g. `.in/append` through a trusted edge proxy while keeping
`.out` SSE direct. The same surfaces add a `fetch` override for header
injection, custom retries, or proxy rewrites that go beyond URL routing.
SSE GETs are covered too via a new `fetchClient` option on
`SSEStreamSubscription`.
```ts
// TriggerChatTransport / AgentChat — endpoints: "in" | "out"
baseURL: ({ endpoint }) =>
endpoint === "out" ? DIRECT : PROXY,
fetch: (url, init, ctx) => {
init.headers = new Headers(init.headers);
init.headers.set("traceparent", currentTraceparent());
return globalThis.fetch(url, init);
},
// chat.createStartSessionAction — endpoints: "sessions" | "auth"
chat.createStartSessionAction("my-agent", {
baseURL: ({ endpoint }) => (endpoint === "sessions" ? PROXY : DIRECT),
});
```
`streamBaseURL` on `TriggerChatTransport` is kept as a backwards-compat
alias and continues to win for the `"out"` endpoint when set.
Plain-string `baseURL` still applies to every endpoint, matching prior
behavior.
Eric Allam
427d9e078a