Add supports_parallel_tool_calls flag to included mcps

[josiah-openai]

Why

For more advanced MCP usage, we want the model to be able to emit parallel MCP tool calls and have Codex execute eligible ones concurrently, instead of forcing all MCP calls through the serial block.

The main design choice was where to thread the config. I made this server-level because parallel safety depends on the MCP server implementation. Codex reads the flag from mcp_servers, threads the opted-in server names into ToolRouter, and checks the parsed ToolPayload::Mcp { server, .. } at execution time. That avoids relying on model-visible tool names, which can be incomplete in deferred/search-tool paths or ambiguous for similarly named servers/tools.

What was added

Added supports_parallel_tool_calls for MCP servers.

Before:

[mcp_servers.docs]
command = "docs-server"

After:

[mcp_servers.docs]
command = "docs-server"
supports_parallel_tool_calls = true

MCP calls remain serial by default. Only tools from opted-in servers are eligible to run in parallel. Docs also now warn to enable this only when the server’s tools are safe to run concurrently, especially around shared state or read/write races.

Testing

Tested with a local stdio MCP server exposing real delay tools. The model/Responses side was mocked only to deterministically emit two MCP calls in the same turn.

Each test called query_with_delay and query_with_delay_2 with { "seconds": 25 }.

Build/config	Observed	Wall time
main with flag enabled	serial	58.79s
PR with flag enabled	parallel	31.73s
PR without flag	serial	56.70s

PR with flag enabled showed both tools start before either completed; main and PR-without-flag completed the first delay before starting the second.

Also added an integration test.

Additional checks:

• cargo test -p codex-tools passed
• cargo test -p codex-core mcp_parallel_support_uses_exact_payload_server passed
• git diff --check passed

[github-actions]

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[josiah-openai]

I have read the CLA Document and I hereby sign the CLA

[jif-oai]

Ok once my comments are processed

[jif-oai]

this looks unused here...

[josiah-openai]

Done - this one is slightly weird because it looks like js_repl uses its own tool call router/dispatcher, but I didn't want to mess around with the parameters too much to allow you to leave out the server names.