Owen Lin 3516cb9751 fix(core): truncate large mcp tool outputs in rollouts (#20260) ...

## 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`

2026-04-30 16:30:43 +00:00

..

history_tests.rs

Remove ghost snapshots (#19481)

2026-04-27 18:48:57 -07:00

history.rs

fix(core): truncate large mcp tool outputs in rollouts (#20260)

2026-04-30 16:30:43 +00:00

mod.rs

fix(core): truncate large mcp tool outputs in rollouts (#20260)

2026-04-30 16:30:43 +00:00

normalize.rs

Add notify to code-mode (#14842)

2026-03-18 09:37:13 -07:00

updates.rs

Delete unused ResponseItem::Message.end_turn (#19605)

2026-04-26 17:18:09 -07:00