## What changed
- Add an opt-in `mcp_2026_07_28` protocol mode while preserving the legacy
lifecycle by default.
- Negotiate the new protocol over streamable HTTP with `server/discover`,
including bounded responses, redirect protection, and fallback only when a
response establishes that the endpoint is legacy-only.
- Require stdio servers to opt in with
`CODEX_MCP_PROTOCOL_VERSION=2026-07-28`, and add a bounded local stdio
transport for the modern lifecycle.
- Consume paginated tool, resource, and resource-template catalogs in modern
mode, reject repeated cursors, and retain discovered server identity.
- Reconnect reusable MCP clients when their selected protocol mode changes.
## Testing
- Cover HTTP JSON and SSE discovery, legacy fallback and rejection cases,
redirects, retries, response limits, and pagination.
- Cover local and executor stdio discovery, protocol markers, message limits,
and legacy compatibility.
GitOrigin-RevId: f6a78816e127d2a482292d63b91c8384f1595903
be2e4afcd7