## ADDED Requirements

### Requirement: Cost accounting uses model and service-tier pricing
When computing API key `cost_usd` usage, the system MUST price requests using the resolved model pricing and the authoritative `service_tier` reported by the upstream response when available, falling back to the forwarded request `service_tier` only when the response omits it. Requests sent with non-standard service tiers MUST use the published pricing for the tier actually used instead of falling back to standard-tier pricing.

#### Scenario: Priority-tier request increments cost limit
- **WHEN** an authenticated request for a priced model is finalized with `service_tier: "priority"`
- **THEN** the system computes `cost_usd` using the priority-tier rate for that model

#### Scenario: Flex-tier request increments cost limit
- **WHEN** an authenticated request for a priced model is finalized with `service_tier: "flex"`
- **THEN** the system computes `cost_usd` using the flex-tier rate for that model

#### Scenario: Standard-tier request keeps standard pricing
- **WHEN** an authenticated request for the same model is finalized without `service_tier`
- **THEN** the system computes `cost_usd` using the standard-tier rate

### Requirement: gpt-5.4 pricing is recognized
The system MUST recognize `gpt-5.4` pricing when computing request costs. For standard-tier requests with more than 272K input tokens, the system MUST apply the published higher long-context rates.

#### Scenario: gpt-5.4 request priced at standard tier
- **WHEN** a request for `gpt-5.4` completes with standard service tier
- **THEN** the system computes non-zero cost using the configured `gpt-5.4` standard rates

#### Scenario: gpt-5.4 long-context request priced at long-context rates
- **WHEN** a standard-tier `gpt-5.4` request completes with more than 272K input tokens
- **THEN** the system computes cost using the configured long-context `gpt-5.4` rates
