## Summary
Renames the MultiAgentV2 turn-triggering tool from `assign_task` to
`followup_task` so the exposed tool name better describes sending an
additional task to an existing agent.
This updates the tool spec, handler/module names, registry wiring,
default multi-agent v2 usage hints, and tests. Rollout trace
classification keeps accepting legacy `assign_task` events so older
traces still reduce correctly, while docs show the new tool name.
## Test plan
- `just test -p codex-core followup_task`
- `just test -p codex-core -E
'test(multi_agent_feature_selects_one_agent_tool_family) |
test(multi_agent_v2_can_use_configured_tool_namespace) |
test(code_mode_only_can_expose_namespaced_multi_agent_v2_as_normal_tools)'`
- `just test -p codex-rollout-trace`
- `just fix -p codex-core`
- `just fix -p codex-rollout-trace`
Notes: `just fmt` ran `cargo fmt` but failed in the Python ruff phase
because the local environment could not resolve `hatchling>=1.27.0` from
the configured internal registry. A full `just test -p codex-core` also
hit unrelated environment-sensitive integration failures involving
missing spawned test binaries/sandbox behavior; the changed multi-agent
spec/handler tests passed in the filtered runs above.
## Summary
- rename the multi-agent v2 follow-up task tool surface to assign_task
- update core tests and spec-plan expectations
- keep rollout-trace classification backward-compatible with legacy
followup_task
## Tests
- just fmt
- just test -p codex-core
multi_agents_spec::tests::assign_task_tool_requires_message_and_has_no_output_schema
- just test -p codex-rollout-trace
- just fix -p codex-core
- just fix -p codex-rollout-trace
Note: a broad just test -p codex-core run was attempted locally, but
this sandbox produced unrelated environment failures around
sandbox-exec, missing test_stdio_server, and realtime timeouts.
## Summary
Adds the debug CLI entry point for reducing recorded rollout traces.
This gives developers a direct way to inspect whether the emitted trace
stream reduces into the expected conversation/runtime model.
## Stack
This is PR 5/5 in the rollout trace stack.
- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command
## Review Notes
This PR is intentionally last: it depends on the trace crate, core
recorder, runtime/tool events, and session/agent edge data all existing.
The command should remain a debug/developer tool and avoid adding new
runtime behavior.
The useful review question is whether the CLI exposes the reducer in the
smallest practical way for local inspection without turning the debug
command into a supported user-facing workflow.
## Summary
Adds the standalone `codex-rollout-trace` crate, which defines the raw
trace event format, replay/reduction model, writer, and reducer logic
for reconstructing model-visible conversation/runtime state from
recorded rollout data.
The crate-level design is documented in
[`codex-rs/rollout-trace/README.md`](https://github.com/openai/codex/blob/codex/rollout-trace-crate/codex-rs/rollout-trace/README.md).
## Stack
This is PR 1/5 in the rollout trace stack.
- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command
## Review Notes
This PR intentionally does not wire tracing into live Codex execution.
It establishes the data model and reducer contract first, with
crate-local tests covering conversation reconstruction, compaction
boundaries, tool/session edges, and code-cell lifecycle reduction. Later
PRs emit into this model.
The README is the best entry point for reviewing the intended trace
format and reduction semantics before diving into the reducer modules.
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