fix: show sub-cent costs in context panel instead of rounding to $0.00

[VJ-yadav]

Summary

• The cost display used Intl.NumberFormat USD currency formatting which rounds to 2 decimal places, causing any cost below $0.005 to show as "$0.00" — making it appear cost never increases even as tokens accumulate
• Added Locale.cost() formatter with tiered precision: 4 decimal places for sub-10-cent costs (trailing zeros stripped), standard 2 decimal places for larger amounts
• Applied the formatter consistently across all 5 cost display points: TUI sidebar, TUI header, stats command, trace list, and trajectory table

Root Cause Analysis

A single LLM message with 1K input tokens on Claude Sonnet costs ~$0.003. The standard Intl.NumberFormat("en-US", { style: "currency", currency: "USD" }) rounds this to $0.00. The cost IS being calculated correctly by Session.getUsage() (verified via Decimal.js multiplication of token counts × model pricing from models.dev), but the 2-decimal formatting hid it.

The trace viewer (viewer.ts line 321) already had a local fc() function with the correct 4-decimal behavior — this PR extracts the pattern into a shared Locale.cost() utility and applies it everywhere.

For free models (opencode provider with apiKey: "public"), cost is genuinely $0.00 since those models have cost: { input: 0, output: 0 } in models.dev. The formatter correctly shows "$0.00" for truly zero costs.

Test Plan

• Locale.cost() unit tests: zero, sub-cent, under-10-cents, standard amounts, typical session costs
• Session.getUsage() tests: non-zero cost for small token counts (5K input + 1K output on Claude Sonnet pricing), zero cost for free models
• All 44 tests pass across locale.test.ts and compaction.test.ts

Checklist

• Formatting examples: $0.003 (1K input on Sonnet), $0.03 (5K input + 1K output), $0.25 (multi-message), $1.23 (standard)
• No breaking changes — only formatting output changes
• Consistent with existing fc() in viewer.ts

Fixes #585

Summary by CodeRabbit

• Improvements
  • Enhanced cost formatting with intelligent decimal precision across the application. Costs below $0.10 now display with four decimal places for improved visibility of small amounts, while costs $0.10 and above use standard two-decimal currency formatting for consistent and clear cost presentation throughout the interface.

[claude]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 4ca199f5-679b-49b4-a59d-f03c3e145699

📥 Commits

Reviewing files that changed from the base of the PR and between a14c9f8 and 3d9d767.

📒 Files selected for processing (2)

• packages/opencode/src/util/locale.ts
• packages/opencode/test/util/locale.test.ts

📝 Walkthrough

Walkthrough

A new Locale.cost() utility function is introduced to standardize USD cost formatting with tiered precision across the application. All hardcoded cost formatting throughout CLI commands and TUI components is replaced with calls to this centralized utility, including comprehensive test coverage.

Changes

Cohort / File(s)	Summary
CLI Command Cost Formatting packages/opencode/src/cli/cmd/stats.ts, packages/opencode/src/cli/cmd/trace.ts, packages/opencode/src/cli/cmd/trajectory.ts	Replaced manual cost formatting (toFixed(), $ prefix concatenation) with Locale.cost() for "Total Cost", "Avg Cost/Day", and per-model cost rendering.
TUI Component Cost Formatting packages/opencode/src/cli/cmd/tui/routes/session/header.tsx, packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx	Replaced hardcoded Intl.NumberFormat("en-US", { style: "currency", currency: "USD" }) with Locale.cost() for cost display in session header and sidebar components.
Locale Utility Implementation packages/opencode/src/util/locale.ts	Added new Locale.cost(amount: number): string function implementing tiered precision: 4 decimal places for amounts < $0.10 (with trailing zero stripping), standard 2 decimal places for amounts ≥ $0.10.
Test Coverage packages/opencode/test/util/locale.test.ts, packages/opencode/test/session/compaction.test.ts	Added comprehensive test suite for Locale.cost() covering zero values, sub-cent precision, boundary cases, and standard currency formatting; added cost calculation assertions for session usage.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• AltimateAI/altimate-code#369: Modifies the same Locale utility module with overlapping changes to cost formatting and related test coverage.

Suggested labels

contributor

Poem

🐰 Costs were scattered, now consolidated tight,
From stats to traces, they all format right,
Locale.cost() hops in with precision so keen,
Four decimals for pennies, two for the green! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: introducing a formatter that displays sub-cent costs instead of rounding them to $0.00.
Description check	✅ Passed	The description covers all required sections: summary of changes and root cause, test plan with specific test cases, and checklist with all items completed.
Linked Issues check	✅ Passed	The PR directly addresses issue #585 by implementing a cost formatter that displays sub-cent values, fixing the issue where costs appeared to never increase due to 2-decimal rounding.
Out of Scope Changes check	✅ Passed	All changes are scoped to implementing and applying the Locale.cost() formatter across cost display locations, with supporting test coverage—no unrelated modifications detected.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/opencode/src/util/locale.ts`:
- Around line 101-111: The special sub-$0.10 formatting branch currently
triggers for any amount < 0.10, incorrectly handling negative values; update the
conditional in the block that checks amount (the "if (amount < 0.10)" branch) to
only apply to positive sub-cent amounts (e.g., amount > 0 && amount < 0.10) so
negative amounts fall through to the standard currency/Intl formatting path;
verify behavior for zero if desired and adjust condition to include/exclude 0
accordingly.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 13fd3048-acff-4f9c-828e-8c49eb0ad3f3

📥 Commits

Reviewing files that changed from the base of the PR and between 5d0ada3 and a14c9f8.

📒 Files selected for processing (8)

• packages/opencode/src/cli/cmd/stats.ts
• packages/opencode/src/cli/cmd/trace.ts
• packages/opencode/src/cli/cmd/trajectory.ts
• packages/opencode/src/cli/cmd/tui/routes/session/header.tsx
• packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx
• packages/opencode/src/util/locale.ts
• packages/opencode/test/session/compaction.test.ts
• packages/opencode/test/util/locale.test.ts

[anandgupta42]

Working as expected. Check the issue details.