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

Context metrics shared

Unify duplicate session usage calculations

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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.

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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

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Non-goals

• Rewriting the detailed context breakdown estimator logic
• Changing translations or labels
• Moving metrics into backend API responses

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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

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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.

1. 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.

1. Migrate both components

• Replace duplicated loops in:
  • session-context-tab.tsx
  • session-context-usage.tsx
• Keep existing UI structure and i18n keys unchanged.

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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

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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.

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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.