Mobile Chat 9b — Agentic Reasoning Timeline

Status: active — feature-flow complete (all 5 gated artifacts + plan-challenge). Ready to execute PR-by-PR.

Sub-phase 9b of the deferred rich-chat work in ../05-pr-roadmap.md (the parent mobile-chat roadmap). A faithful 1:1 port of web's agent timeline shell (web/src/app/app/message/messageComponents/**) to the React Native app, wiring only the reasoning renderer. Approach C — Faithful Shell First (owner: "match web, no refactor later, want everything"). Every tool renderer (search/fetch/python/custom-tool/deep-research/memory) and 9c–9e is an immediate follow-up PR on the zero-refactor seam this phase builds. Builds on the 9a foundation (../9a-citations/, shipped #13025). No backend/DB changes.

Artifacts

1. 01-research.md — requirement, locked scope, codebase + backend + web + industry findings, the three approaches, and the chosen one (C).
2. 02-high-level-design.md — end-to-end flow, component interaction diagram, key decisions.
3. 03-detailed-design.md — exact contracts, new files, file tree, per-file responsibilities, integration points, the two ref-restructures, the documented divergences.
4. 04-implementation-plan.md — CLAUDE.md-format plan + appended plan-challenge results (all six pass; web-verified).
5. 05-pr-roadmap.md — the 7-PR delivery sequence (9b.1 engine → … → 9b.7 composition/device gate).

Supporting reference (gitignored): .context/pr9b-deepread/*.md — verbatim web timeline contracts extracted for the faithful port.

One-line summary

Port web's agent reasoning timeline shell to mobile 1:1 (grouping engine + section_end synthesis + pacing + 7-state machine + render-prop renderer contract + StepContainer/collapse), wiring only the reasoning renderer; delivered as 7 layered PRs so the tool renderers drop in later with zero refactor. One accepted, platform-forced divergence: the two ref-during-render hooks are restructured (behavior-preserving) to satisfy react-hooks/refs.