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opencode/specs/12-session-context-metrics-shared.md

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+## Context metrics shared
+
+Unify duplicate session usage calculations
+
+---
+
+### Summary
+
+`session-context-tab.tsx` and `session-context-usage.tsx` both compute overlapping session metrics (cost, last assistant token totals, provider/model context usage). This creates duplicate loops and raises drift risk. We will centralize shared calculations in one helper module and have both components consume it.
+
+---
+
+### Goals
+
+- Compute shared session usage metrics in one place
+- Remove duplicate loops for cost and latest-token context usage
+- Keep UI output unchanged in both components
+
+---
+
+### Non-goals
+
+- Rewriting the detailed context breakdown estimator logic
+- Changing translations or labels
+- Moving metrics into backend API responses
+
+---
+
+### Parallel execution contract
+
+This spec owns:
+
+- `packages/app/src/components/session/session-context-tab.tsx`
+- `packages/app/src/components/session-context-usage.tsx`
+- New helper in `packages/app/src/components/session/*` or `packages/app/src/utils/*`
+
+This spec should not modify:
+
+- `packages/app/src/pages/session.tsx`
+- `packages/app/src/context/sync.tsx`
+
+---
+
+### Implementation plan
+
+1. Add shared metrics helper
+
+- Create helper for raw metrics from message list + provider map, e.g.:
+  - `totalCost`
+  - `lastAssistantWithTokens`
+  - `tokenTotal`
+  - `tokenUsagePercent`
+  - provider/model labels
+- Return raw numeric values; keep locale formatting in consumers.
+
+2. Add memoization guard
+
+- Use reference-based memoization (e.g. by message-array identity) inside helper or component-level memo to avoid duplicate recalculation on unchanged arrays.
+
+3. Migrate both components
+
+- Replace duplicated loops in:
+  - `session-context-tab.tsx`
+  - `session-context-usage.tsx`
+- Keep existing UI structure and i18n keys unchanged.
+
+---
+
+### Acceptance criteria
+
+- Shared cost + token calculations are defined in one module.
+- Both components read from the shared helper.
+- Rendered values remain identical for:
+  - total cost
+  - token totals
+  - usage percentage
+  - provider/model fallback labels
+
+---
+
+### Validation plan
+
+- Manual:
+  - Open session context tab and compare values with header/context indicator tooltip.
+  - Verify values update correctly while new assistant messages stream in.
+- Regression:
+  - locale change still formats numbers/currency correctly.
+
+---
+
+### Risks and mitigations
+
+- Risk: helper changes semantic edge cases (no provider, no model, missing token fields).
+  - Mitigation: preserve existing fallback behavior (`"—"`, null percent).
+- Risk: memoization over-caches stale values.
+  - Mitigation: key cache by message-array reference and dependent IDs only.