## Summary
- add tracing-based diagnostics for nested `codex.tool(...)` calls made
from `js_repl`
- emit a bounded, sanitized summary at `info!`
- emit the exact raw serialized response object or error string seen by
JavaScript at `trace!`
- document how to enable these logs and where to find them, especially
for `codex app-server`
## Why
Nested `codex.tool(...)` calls inside `js_repl` are a debugging
boundary: JavaScript sees the tool result, but that result is otherwise
hard to inspect from outside the kernel.
This change adds explicit tracing for that path using the repo’s normal
observability pattern:
- `info` for compact summaries
- `trace` for exact raw payloads when deep debugging is needed
## What changed
- `js_repl` now summarizes nested tool-call results across the response
shapes it can receive:
- message content
- function-call outputs
- custom tool outputs
- MCP tool results and MCP error results
- direct error strings
- each nested `codex.tool(...)` completion logs:
- `exec_id`
- `tool_call_id`
- `tool_name`
- `ok`
- a bounded summary struct describing the payload shape
- at `trace`, the same path also logs the exact serialized response
object or error string that JavaScript received
- docs now include concrete logging examples for `codex app-server`
- unit coverage was added for multimodal function output summaries and
error summaries
## How to use it
### Summary-only logging
Set:
```sh
RUST_LOG=codex_core::tools::js_repl=info
```
For `codex app-server`, tracing output is written to the server process
`stderr`.
Example:
```sh
RUST_LOG=codex_core::tools::js_repl=info \
LOG_FORMAT=json \
codex app-server \
2> /tmp/codex-app-server.log
```
This emits bounded summary lines for nested `codex.tool(...)` calls.
### Full raw debugging
Set:
```sh
RUST_LOG=codex_core::tools::js_repl=trace
```
Example:
```sh
RUST_LOG=codex_core::tools::js_repl=trace \
LOG_FORMAT=json \
codex app-server \
2> /tmp/codex-app-server.log
```
At `trace`, you get:
- the same `info` summary line
- a `trace` line with the exact serialized response object seen by
JavaScript
- or the exact error string if the nested tool call failed
### Where the logs go
For `codex app-server`, these logs go to process `stderr`, so redirect
or capture `stderr` to inspect them.
Example:
```sh
RUST_LOG=codex_core::tools::js_repl=trace \
LOG_FORMAT=json \
/Users/fjord/code/codex/codex-rs/target/debug/codex app-server \
2> /tmp/codex-app-server.log
```
Then inspect:
```sh
rg "js_repl nested tool call" /tmp/codex-app-server.log
```
Without an explicit `RUST_LOG` override, these `js_repl` nested
tool-call logs are typically not visible.
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