# Why
Codex currently negotiates MCP `2025-06-18`, where the client
elicitation capability is represented as an empty object. We were still
serializing `capabilities.elicitation.form`, which belongs to the later
capability shape and can cause strict `2025-06-18` servers to reject
`initialize` with an unrecognized-field error.
This keeps the handshake aligned with the protocol version Codex
actually negotiates and fixes the compatibility regression tracked in
#17492.
# What
- Serialize the client elicitation capability as `elicitation: {}` for
`2025-06-18`.
- Keep elicitation advertised for both Codex Apps and custom MCP
servers.
- Tighten regression coverage so the unit test asserts both the Rust
value and the serialized wire shape.
- Add an app-server integration test that round-trips a form elicitation
from a custom MCP server; the existing connector round-trip continues to
cover the connector path.
# Verification
- `cargo test -p codex-mcp`
- `cargo test -p codex-app-server mcp_server_elicitation_round_trip`
- `cargo test -p codex-app-server
mcp_server_tool_call_round_trips_elicitation`
# Next steps
- Decide whether `tool_call_mcp_elicitation=false` should also suppress
capability advertisement during `initialize`.
- Revisit `form` / `url` capability advertisement when Codex is ready to
negotiate MCP `2025-11-25`, which defines that newer shape.
## Why
Large MCP tool call outputs can make rollout JSONL files enormous. In
the session that motivated this change, the biggest JSONL records were:
- `event_msg/mcp_tool_call_end`
- `response_item/function_call_output`
both containing the same unbounded MCP payloads - just 3 MCP tool calls
that each were multi-hundred MBs 😱
This PR truncates both of those JSONL records.
## How
#### For `response_item/function_call_output`
Unified exec already bounds tool output before it is injected into
model-facing history, which also keeps the corresponding rollout
`response_item/function_call_output` records small.
MCP should follow the same pattern: truncate the model-facing tool
output at the tool-output boundary, while leaving code-mode/raw hook
consumers alone.
#### For `event_msg/mcp_tool_call_end`
`McpToolCallEnd` also needs its own bounded event copy because it is the
app-server/replay/UI event shape that backs `ThreadItem::McpToolCall`.
Unfortunately this is _not_ downstream of the `ToolOutput` trait.
## Model behavior
Model behavior is actually unchanged as a result of this PR.
Before this PR, MCP output was:
1. Converted to `FunctionCallOutput`.
2. Recorded into in-memory history.
3. Truncated by `ContextManager::record_items()` before later model
turns saw it.
After this branch, MCP output is truncated earlier, in
`McpToolOutput::response_payload()`, using the same helper. Then
`ContextManager::record_items()` sees an already-truncated output and
effectively has little/no additional work to do.
So the model should still see the same kind of truncated function-call
output. The practical difference is where truncation happens: earlier,
before rollout persistence/app-server emission can see the giant
payload.
## Verification
- `cargo test -p codex-core mcp_tool_output`
- `cargo test -p codex-core
mcp_tool_call::tests::truncate_mcp_tool_result_for_event`
- `cargo test -p codex-core
mcp_post_tool_use_payload_uses_model_tool_name_args_and_result`
- `just fmt`
- `just fix -p codex-core`
- `git diff --check`
## Summary
- attach the authoritative Codex thread id to MCP tool request
`_meta.threadId` for model-initiated tool calls
- attach the same thread id for manual `mcpServer/tool/call` requests
before invoking the MCP server
- cover both metadata helper behavior and the manual app-server MCP path
in tests
needed because the Rust app-server is the last place that still has
authoritative knowledge of “this model-generated MCP tool call belongs
to conversation/thread X” before the request leaves Codex and reaches
Hoopa. It adds threadId to MCP request metadata in the model-generated
tool-call path, using sess.conversation_id, and also does the same for
the manual mcpServer/tool/call path.
## Test plan
- `cargo test -p codex-core
mcp_tool_call_thread_id_meta_is_added_to_request_meta --lib`
- `cargo test -p codex-app-server
mcp_server_tool_call_returns_tool_result`
Paired Hoopa consumer PR: https://github.com/openai/openai/pull/833263
Matthew Zeng