fix(cost): apply tier pricing for openai/azure/openrouter/helicone (#5690)

[Anuj7411]

What

Fixes #5690.

packages/cost/models/calculate-cost.ts → getThresholdValueFunction(provider) only handled vertex, google-ai-studio, anthropic and xai. Every other provider — including openai, azure, openrouter and helicone — fell to default: () => 0. Since getPricingTier selects the highest tier whose threshold <= value, a constant 0 always picked the cheapest base tier, so higher-context tiers were unreachable for those providers.

Concrete impact: gpt-5.4 (packages/cost/models/authors/openai/gpt-5.4/endpoints.ts) has a base tier and a threshold: 272000 tier (input 2×, output 1.5×). For an openai gpt-5.4 request with input=300000, output=50000:

• Expected: 300000 × 0.000005 + 50000 × 0.0000225 = $2.625
• Actual (pre-fix): base tier → 300000 × 0.0000025 + 50000 × 0.000015 = $1.50 (~43% under)

The same tiered config exists for the azure, openrouter and helicone variants of gpt-5.4, all unhandled.

Change

Adds openai / azure / openrouter / helicone cases to getThresholdValueFunction, mirroring the existing xai handler — the threshold value is the total prompt size (usage.input + (usage.cacheDetails?.cachedInput ?? 0)). For cachedInputCost, the same total prompt size is used so the cached-input rate also tier-switches at the configured threshold.

Test

Adds a regression test in packages/__tests__/cost/modelCostFromRegistry.test.ts's existing describe("threshold-based pricing") block:

• openai gpt-5.4 with input=300000, output=50000 → asserts the >272K tier rates (inputCost === 300000 * 0.000005, outputCost === 50000 * 0.0000225).

Without this PR's source change, that test computes the base-tier values ($0.75 / $0.75) and fails. With the fix it passes. (I wasn't able to spin up jest on Windows locally — running npx jest __tests__/ from /packages errored on a missing ts-jest preset in this checkout — but the change is mechanical: it adds 4 provider keys to the same switch that already handles xai identically and is exercised by the existing Anthropic 250K test at lines 299-323.)

Scope

No behaviour change for any currently-handled provider (vertex, google-ai-studio, anthropic, xai); they keep their own threshold functions. Models on openai / azure / openrouter / helicone that don't define a higher-threshold tier are unaffected — getPricingTier simply picks the base tier as before.

Related context

PR #5268 introduced the threshold mechanism and wired the four providers above; the openai-shaped providers weren't wired at the same time, which is what surfaces here.

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[Anuj7411]

Hey team, gentle nudge whenever someone has a sec 🙂 small one-file change (mirrors the existing xai handler in the same switch) and the bug currently under-charges large-context openai calls by ~40% on gpt-5.4 over 272k tokens. Happy to answer anything if useful.