feat(mobile): merge model and reasoning into one agent config control (port #3821)

[Gilbert09]

Problem

The mobile app still exposed agent configuration as separate Model and Reasoning pills, and had no way to set the two new agent settings desktop shipped in #3821 (fast mode and context window). This ports that desktop change ("new agent harness selector") to apps/mobile for behavioral parity, using mobile-idiomatic UI.

Desktop PR: #3821.

Changes

Merged model + reasoning control. The two pills become one pill summarising the current model + effort (e.g. "Claude Opus 5 · Extra High"). It opens a bottom sheet with:

• A Faster → Smarter preset scale that sets model and effort together, derived from the shared capability ladder (Sonnet 5 medium/high, Opus 5 medium/xhigh, Fable 5 max for Claude; the GPT ladder for Codex). Presets that aren't offered/allowed by the live config, or whose effort is unsupported, are dropped.
• A fast mode toggle, gated on the same posthog-desktop-fast-mode flag as desktop (mobile reads it via posthog-react-native's useFeatureFlag), shown only for models that support it.
• A context window (200k / 1M) control, shown only for models that support the 1M beta.
• An Advanced section: harness (Claude/Codex), model, reasoning pickers, plus Reset to default (middle preset notch, default context window, fast mode off). Switching harness resets to that harness's middle preset.

Instead of the desktop drag-slider + dropdown-with-submenus, the sheet uses a preset list, segmented controls, and a collapsible Advanced section (no nested modals).

Payload. context_window and fast_mode now ride alongside reasoning_effort on cloud task creation and on sandbox warming, matching the desktop body shape (only sent for models that support them).

Persistence + migration. The selection persists (per-device defaults in preferencesStore, per-task in taskStore) and restores on the composers. The preferencesStore version is bumped to 1 with a migration so existing installs drop a stale last-used model/effort and land on the new defaults.

Shared policy. getCapabilityLadder, supports1MContext, and supportsFastMode were added to @posthog/shared (additive) so mobile reuses one policy rather than re-deriving lists.

Deliberately not ported

• Live per-turn toggling of context window / fast mode on a running cloud session (desktop pushes these via set_config_option). Mobile persists the choice and applies it on the next run (create / resume / retry), which keeps the change contained; reasoning/model live-push is unchanged.
• Single-sourcing the capability ladder with @posthog/agent. Desktop's ladder lives in @posthog/agent, which mobile cannot import. To respect the "don't touch desktop/agent" scope, @posthog/shared carries a mirrored copy (documented with a keep-in-sync comment) rather than refactoring @posthog/agent onto shared. That unification is a reasonable follow-up.

How did you test this?

• pnpm --filter @posthog/mobile test — 563 passing. Added/updated unit tests for the preset mapping, harness-switch reset, persistence + migration, context-window gating, and the creation/warm payload fields (options.test.ts, AgentConfigControls.test.tsx, preferencesStore.test.ts, cloudTaskRunConfig.test.ts, useWarmTask.test.tsx).
• pnpm --filter @posthog/shared --filter @posthog/api-client typecheck — clean.
• Mobile tsc --noEmit — no new errors (one pre-existing SessionActivityState error unrelated to this change).
• Biome lint clean across the changed files.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

---

Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ed4b3d9.}

[github-actions]

Contained mobile UI feature porting an already-shipped desktop capability (fast mode / context window), with additive-only backend payload fields and a well-tested, reversible local-preferences migration; no auth, billing, migration-of-server-data, or CI/dependency risk, and the diff matches the description.

• Author wrote 1% of the modified lines and has 38 merged PRs in these paths (familiarity MODERATE).

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	786L, 14F substantive, 1174L/19F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (1174L, 19F, two-areas, feat)
stamphog 2.0.0b3		.stamphog/policy.yml @ 67f0052 · reviewed head ed4b3d9

[talyn-app]

/trunk merge