feat: support context_tiers for arbitrary tiered pricing

[Pxys-io]

Problem

OpenCode currently uses a hardcoded 200_000 token threshold to switch between base and long-context pricing (experimentalOver200K). This doesn't match how providers like OpenRouter handle tiered pricing, which supports arbitrary context length thresholds.

Solution

This PR adds support for the new context_tiers schema from models.dev#1324, replacing the hardcoded threshold with dynamic tier selection.

Changes

File	Change
src/provider/models.ts	Add context_tiers array to ModelsDev cost schema
src/provider/provider.ts	Add contextTiers to internal Model cost type; parse tiers from models.dev
src/session/index.ts	Replace hardcoded 200_000 check with dynamic tier selection — picks the highest tier where input + cache_read >= min_context

How It Works

const costInfo = (() => {
  const totalInputTokens = tokens.input + tokens.cache.read;

  // Prefer new context_tiers with arbitrary thresholds
  if (input.model.cost?.contextTiers?.length > 0) {
    let selected = input.model.cost; // base cost
    for (const tier of input.model.cost.contextTiers) {
      if (totalInputTokens >= tier.min_context) {
        selected = tier;  // highest matching tier wins
      }
    }
    return selected;
  }

  // Fall back to legacy experimentalOver200K (hardcoded 200K)
  if (input.model.cost?.experimentalOver200K && totalInputTokens > 200_000) {
    return input.model.cost.experimentalOver200K;
  }

  return input.model.cost;
})();

Backward Compatibility

• Falls back to experimentalOver200K for models using the legacy context_over_200k field
• Falls back to base cost if neither is defined
• No breaking changes to existing models or cost calculations

Companion PR

• models.dev#1324 — adds context_tiers schema and example usage

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