feat(sessions): surface per-task cost estimate#3545
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The Claude Agent SDK already reports a per-turn `total_cost_usd`, which flowed into `ContextUsage.cost` but was never rendered. Sum it across turns (each result reports only its own spend, matching how per-turn token usage is already accumulated) and show the running total in the context usage indicator chip and breakdown popover. Codex sessions emit tokens without a cost, so `cost` stays null and the UI hides it. Generated-By: PostHog Code Task-Id: f6d673c3-965e-4598-a4bd-663f2651a5a5
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Wrap the cost readout (indicator chip suffix and breakdown popover row) in a feature flag so it can roll out gradually. On in local dev via the import.meta.env.DEV fallback, matching the other UI flags. Generated-By: PostHog Code Task-Id: f6d673c3-965e-4598-a4bd-663f2651a5a5
tatoalo
approved these changes
Jul 17, 2026
Gating the cost readout behind useFeatureFlag made the component call useService(FEATURE_FLAGS), which throws in the test's provider-less render. Mock the hook (matching the pattern used across the UI test suite) and add a case covering the cost suffix when the flag is on. Generated-By: PostHog Code Task-Id: f6d673c3-965e-4598-a4bd-663f2651a5a5
Generated-By: PostHog Code Task-Id: f6d673c3-965e-4598-a4bd-663f2651a5a5
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Summary
Surfaces a per-task cost estimate in the session UI, gated behind a feature flag. The Claude Agent SDK already reports a per-turn
total_cost_usd, which flowed intoContextUsage.costbut was never rendered anywhere. This wires it through to the UI.resultmessage reports only that turn's spend, so the running session total is the sum — mirroring how the codebase already resets and re-accumulates per-turn token usage. Split cost out of the point-in-time context aggregate (used/sizestay a snapshot of the newest turn;costaccrues), in both the batch extractor and the streaming append-only tracker (kept equivalent).ContextUsageIndicatorchip shows· $X.XX, andContextBreakdownPopovergets an "Estimated cost" row.posthog-code-task-cost(TASK_COST_FLAG). On in local dev via theimport.meta.env.DEVfallback, matching the other UI flags. Until the flag is created/enabled in PostHog,isFeatureEnabledreturnsfalse, so the cost stays hidden in production.coststaysnulland the UI hides it regardless of the flag.Labeled "Estimated cost" since it's the SDK's own estimate. Background task-notification turns are excluded (they don't feed the ACP
usage_updatethat carries cost).Rollout
Create the
posthog-code-task-costflag in the PostHog project the desktop app reads flags from, then enable for the target cohort.Test plan
contextUsage.test.ts: added cases for single-turn cost, cross-turn summation, null-when-absent, append-only accumulation, and batch/tracker equivalence — 15 tests pass.@posthog/core+@posthog/uitypecheck clean; Biome lint clean on all changed files (zeronoRestrictedImports).Follow-up (not in this PR)
Phase 2 — a local per-model pricing table for a per-model cost breakdown and Codex coverage.
Created with PostHog Code