## What changed
- Upgrade `rmcp` from 3.0.0 to 3.1.2.
- Use `rmcp`'s native JSON-RPC decoding and remove the local compatibility layer for multi-round-trip tool results.
- Preserve response metadata on `input_required` SSE results.
- Support OAuth protected-resource metadata discovery and include the discovered resource audience when refreshing tokens.
## Testing
- Add coverage for metadata preservation on modern SSE `input_required` responses.
- Add an OAuth startup scenario that discovers protected-resource metadata and verifies the refresh-token resource audience.
GitOrigin-RevId: 689f009b4a33552b640304111b74585b4009eeb8
## What changed
- Drive multi-round `tools/call` and `resources/read` requests through
`input_required` responses, preserving opaque request state and elicitation
metadata across JSON, SSE, and stdio transports.
- Decode modern discovery and elicitation result shapes while retaining legacy
initialization, elicitation defaults, and discovery fallback behavior.
- Apply the 8 MiB modern-protocol response limit to JSON, SSE events, and stdio
messages, and reject executor stream recovery when output sequence gaps would
corrupt the protocol stream.
## Testing
Added integration coverage for discovery, multi-round requests, legacy
fallbacks, message limits, stdio launchers, SSE responses, session recovery,
and executor output recovery.
GitOrigin-RevId: d26a08883a2825e8ce2408ac498114b33859225e
## 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
## What changed
- Guard executor-backed MCP stdio sends with a single-permit semaphore so a
second JSON-RPC message cannot start writing while the first write is pending.
- Add a regression test that drives two concurrent sends and verifies the
process receives each newline-delimited message in order without overlap.
GitOrigin-RevId: 30c2e51827ecf57b5eb23d9ad1e876734d6e784e
## Why
Remote MCP stdio buffers executor-provided stdout until a JSON-RPC line
completes and stderr until a diagnostic line completes. A malicious or
broken executor can keep sending chunks without a newline, making
orchestrator memory grow without bound.
Executor event boundaries are arbitrary, so a valid message before an
oversized line must not be lost merely because both arrived in one
event.
## What changed
- Limit one stdout JSON-RPC line to 8 MiB.
- Limit one stderr diagnostic line to 1 MiB.
- Check each logical line before copying its bytes into the retained
buffer.
- Preserve complete stdout messages that precede an oversized line and
deliver them before closing the stream.
- Close the MCP transport on overflow; existing transport cleanup
terminates the executor-managed process.
- Preserve multiple lines per chunk, CRLF framing, and final
unterminated messages at EOF.
- Box the pending stdio transport so the added framing state does not
inflate every `ClientState` value.
- Add regression coverage for cross-chunk overflow, bounded lines in a
larger chunk, and a valid prefix before an oversized line.
## Scope
This bounds pre-newline buffering for executor-backed MCP stdio only. It
does not change local stdio framing or address memory amplification
while parsing a complete JSON message.
## Summary
This PR adds `memchr` for some low-hanging performance improvements
(namely, in MCP stdio, Ollama streaming, and full message-history
newline counts).
Codex produced the following release benchmarks:
| Operation | Before | After | Speedup |
| --- | ---: | ---: | ---: |
| MCP 1 MiB chunked line | 2.172 s | 3.984 ms | 545x |
| Ollama 1 MiB chunked line | 1.673 s | 2.790 ms | 600x |
| Count newlines in 10 MiB history | 132.83 ms | 20.05 ms | 6.6x |
With a "real" MCP setup (`ExecutorStdioServerLauncher` started a Python
MCP server, completed `initialize`, requested `tools/list`, and
deserialized a 1 MiB tool description over newline-delimited stdio),
it's about 16x faster end-to-end:
| Branch | 50 calls | Per call |
| --- | ---: | ---: |
| `main` | 862.53 ms | 17.25 ms |
| this branch | 53.89 ms | 1.08 ms |
`memchr` is already in our dependency tree and extremely widely used for
this kind of optimized scanning.
## Summary
- Add the executor-backed RMCP stdio transport.
- Wire MCP stdio placement through the executor environment config.
- Cover local and executor-backed stdio paths with the existing MCP test
helpers.
## Stack
```text
o #18027 [6/6] Fail exec client operations after disconnect
│
@ #18212 [5/6] Wire executor-backed MCP stdio
│
o #18087 [4/6] Abstract MCP stdio server launching
│
o #18020 [3/6] Add pushed exec process events
│
o #18086 [2/6] Support piped stdin in exec process API
│
o #18085 [1/6] Add MCP server environment config
│
o main
```
---------
Co-authored-by: Codex <noreply@openai.com>
thomas