ACP `usage_update` mislabels non-USD costs as USD

[DinahK-2SO]

Description

OpenCode's custom provider model configuration accepts numeric token cost rates but does not accept
their currency. The internal assistant-message cost is also stored as a bare number. When OpenCode
projects that number into ACP, it unconditionally emits currency: "USD".

As a result, a custom provider priced in EUR can have the correct numeric amount reported with the
wrong currency. In the deterministic reproduction below, a provider cost of 30 EUR is emitted
as 30 USD.

opencode/packages/opencode/src/acp/usage.ts

Line 212 in aaa42fe

cost: { amount: totalSessionCost(messages), currency: "USD" },

Plugins

No response

OpenCode version

OpenCode 1.18.3

Steps to reproduce

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Screenshot and/or share link

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Operating System

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Terminal

No response

[kagura-agent]

I'd like to work on this. The root cause is clear: both usage.ts (L212) and service.ts (L653) hardcode currency: "USD" in the ACP usage_update. The ProviderCost schema has no currency field, so there's no way for custom providers to specify their pricing currency.

Proposed fix:

1. Add optional currency field to ProviderCost in provider.ts (defaults to "USD" for backward compat)
2. Add findCostCurrency() utility to usage.ts (mirrors existing findContextLimit())
3. Thread the currency through sendUpdate in both usage.ts and service.ts
4. Add test coverage for custom currency propagation

[DinahK-2SO]

I'd like to work on this. The root cause is clear: both usage.ts (L212) and service.ts (L653) hardcode currency: "USD" in the ACP usage_update. The ProviderCost schema has no currency field, so there's no way for custom providers to specify their pricing currency.

Proposed fix:

1. Add optional currency field to ProviderCost in provider.ts (defaults to "USD" for backward compat)
2. Add findCostCurrency() utility to usage.ts (mirrors existing findContextLimit())
3. Thread the currency through sendUpdate in both usage.ts and service.ts
4. Add test coverage for custom currency propagation

@kagura-agent Why not

[kagura-agent]

Thanks @DinahK-2SO — I'll get a PR up for this. Will follow the approach outlined above (optional currency field with USD default for backward compat).