feat(save-mode): add Save Mode cost controls — FE + local dev wiring

[ricardo-leiva]

Problem

Heavy AI coding sessions cost real money. A user running Opus at max effort for 10+ turns is spending ~$0.80 per session — and the agent is doing routine file reads and edits where Opus quality is overkill. There's no way to dial this back without leaving the app.

How it works

                         COST PER TURN
 ┌──────────────────────────────────────────────────────────────┐
 │  [  model price  ] × [  input tokens  +  output tokens  ]    │
 │        ▲                    ▲                   ▲            │
 │  Lever 1: downshift   Lever 3: cache      Lever 2: effort    │
 │  Opus → Sonnet (~3×)  TTL 1h (gateway)   cap + terse prompt  │
 └──────────────────────────────────────────────────────────────┘

  OFF  ────────────────────────────────────▶  Full power, no changes.

  BALANCED  ──────────────────────────────▶  Keep model. Cap effort → high
                                             (removes xhigh/max think budgets).
                                             Add terse system reminder.
                                             Est. savings: 20–40%.

  MAX SAVINGS  ───────────────────────────▶  Downshift Opus → Sonnet.
                                             Cap effort → medium.
                                             Add terse system reminder.
                                             Est. savings: 50–70%.

The toggle lives next to the reasoning-level selector in both the task-input bar and the live session bar. Setting is persisted in settingsStore and applied immediately to the active session when changed.

Request flow

Changes

New files:

• packages/core/src/save-mode/saveMode.ts — pure resolveSaveMode() resolver + tests
• packages/core/src/save-mode/budget.ts — evaluateBudget() helper + tests
• packages/ui/src/features/sessions/components/SaveModeToggle.tsx — toggle UI component
• packages/ui/src/features/sessions/hooks/useApplySaveMode.ts — wraps one service call (AGENTS.md rule 5)
• scripts/dev-cost-controls.sh — local dev helper

Core wiring (task start):

• shared/task-creation-domain.ts → systemPromptOverride field in TaskCreationInput
• core/task-detail/taskInput.ts → forwards it from PrepareTaskInputOptions
• core/task-detail/taskCreationSaga.ts → passes to ConnectParams
• core/sessions/sessionService.ts → ConnectParams.systemPromptOverride, createNewLocalSession, _saveModeBaselines, applySaveMode()

Agent wiring:

• agent/adapters/claude/session/options.ts → saveModeHeaders in Options
• agent/adapters/claude/claude-agent.ts → passes saveModeHeaders through
• agent/adapters/claude/types.ts → NewSessionMeta.saveModeHeaders
• agent/utils/gateway.ts → LLM_GATEWAY_BASE_URL env override
• workspace-server/services/agent/agent.ts → SessionConfig.saveModeHeaders, wired into newSession/resumeSession _meta
• workspace-server/services/agent/schemas.ts → Zod schema for saveModeHeaders

UI:

• ui/features/sessions/components/SessionView.tsx → SaveModeToggle in reasoningSelector
• ui/features/task-detail/components/TaskInput.tsx → SaveModeToggle in reasoningSelector
• ui/features/task-detail/hooks/useTaskCreation.ts → computes saveModeResult, passes systemPromptOverride, stamps save_mode on TASK_CREATED event
• ui/features/settings/settingsStore.ts → saveMode: SaveMode + setSaveMode + persistence
• ui/features/settings/sections/GeneralSettings.tsx → save mode in Settings panel
• ui/features/home/hooks/useRunWorkstreamAction.ts → save mode for workstream tasks

Also fixes: missing BROWSER_TAB_OPENED / LINK_CLICKED_IN_CHAT analytics event types in @posthog/shared (were used in the browser feature but never registered in the schema).

How did you test this?

Automated tests run and passing:

• packages/core/src/save-mode/saveMode.test.ts — 5 cases: off pass-through, balanced caps effort, max_save downshifts model, never raises low effort, perTaskFullPower bypass
• packages/core/src/save-mode/budget.test.ts — 5 cases: disabled/no cap, ok, warn@70%, engage@85%, blocked@100%
• Full pnpm typecheck passes across all packages (including apps/web and apps/code)
• pnpm --filter @posthog/core test run — 1610 tests, all pass

Manual testing by human:

• Got app running (gateway from feat(llm-gateway): add Save Mode cost controls — cache upgrade + budget guard + telemetry posthog#65690)
• Tested the visual aspects of it.
• Validated prompts and model switching.
• Real testing for this would be by analyzing a broader data set to see how much it actually saves. As iteration goes on, savings could increase.

Automatic notifications

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

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Built with Claude Code (claude-sonnet-4-6) across two sessions. The implementation covers:

• Pure resolveSaveMode function (testable, host-neutral) computing effective model/effort/reminder
• Gateway wiring via saveModeHeaders (x-posthog-property-* headers) fed to the LLM Gateway in a companion BE PR
• Architecture review finding fixes: moved multi-store hook logic to SessionService.applySaveMode() (AGENTS.md rule 5), replaced dead SETTING_CHANGED event with typed SAVE_MODE_CHANGED, fixed claude-fable downshift target to claude-sonnet-4-6
• Branch was rebased off main (not feat/skills-improvements) to avoid contamination

[greptile-apps]

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