stack/apps/gateway/src/agent/routing.service.ts

import { Inject, Injectable, Logger } from '@nestjs/common';
import type { ModelInfo } from '@mosaic/types';
import type { RoutingCriteria, RoutingResult, CostTier } from '@mosaic/types';
import { ProviderService } from './provider.service.js';

/** Per-million-token cost thresholds for tier classification */
const COST_TIER_THRESHOLDS: Record<CostTier, { maxInput: number }> = {
  cheap: { maxInput: 1 },
  standard: { maxInput: 10 },
  premium: { maxInput: Infinity },
  // local = self-hosted; treat as cheapest tier for cost scoring purposes
  local: { maxInput: 0 },
};

@Injectable()
export class RoutingService {
  private readonly logger = new Logger(RoutingService.name);

  constructor(@Inject(ProviderService) private readonly providerService: ProviderService) {}

  /**
   * Select the best available model for the given criteria.
   * Returns null if no model matches the requirements.
   */
  route(criteria: RoutingCriteria = {}): RoutingResult | null {
    const available = this.providerService.listAvailableModels();
    if (available.length === 0) {
      this.logger.warn('No available models for routing');
      return null;
    }

    // If a specific model is preferred, try it first
    if (criteria.preferredProvider && criteria.preferredModel) {
      const match = available.find(
        (m) => m.provider === criteria.preferredProvider && m.id === criteria.preferredModel,
      );
      if (match) {
        return {
          provider: match.provider,
          modelId: match.id,
          modelName: match.name,
          score: 100,
          reasoning: 'Preferred model selected',
        };
      }
    }

    // Score and rank candidates
    const scored = available
      .map((model) => this.scoreModel(model, criteria))
      .filter((s) => s.score > 0)
      .sort((a, b) => b.score - a.score);

    if (scored.length === 0) {
      this.logger.warn('No models matched routing criteria', criteria);
      return null;
    }

    const best = scored[0] as RoutingResult;
    this.logger.debug(
      `Routed to ${best.provider}/${best.modelId} (score=${best.score}): ${best.reasoning}`,
    );
    return best;
  }

  /**
   * List all available models ranked by suitability for the given criteria.
   */
  rank(criteria: RoutingCriteria = {}): RoutingResult[] {
    const available = this.providerService.listAvailableModels();
    return available
      .map((model) => this.scoreModel(model, criteria))
      .filter((s) => s.score > 0)
      .sort((a, b) => b.score - a.score);
  }

  private scoreModel(model: ModelInfo, criteria: RoutingCriteria): RoutingResult {
    let score = 50; // Base score
    const reasons: string[] = [];

    // Hard requirements — disqualify if not met
    if (criteria.requireReasoning && !model.reasoning) {
      return this.disqualified(model, 'reasoning required but not supported');
    }

    if (criteria.requireImageInput && !model.inputTypes.includes('image')) {
      return this.disqualified(model, 'image input required but not supported');
    }

    if (criteria.minContextWindow && model.contextWindow < criteria.minContextWindow) {
      return this.disqualified(
        model,
        `context window ${model.contextWindow} < required ${criteria.minContextWindow}`,
      );
    }

    // Cost tier matching
    if (criteria.costTier) {
      const tier = this.classifyTier(model);
      if (tier === criteria.costTier) {
        score += 20;
        reasons.push(`cost tier match (${tier})`);
      } else if (
        (criteria.costTier === 'cheap' && tier === 'standard') ||
        (criteria.costTier === 'standard' && tier === 'premium')
      ) {
        score += 5;
        reasons.push(`adjacent cost tier (wanted ${criteria.costTier}, got ${tier})`);
      }
    }

    // Prefer cheaper models when no cost tier specified
    if (!criteria.costTier) {
      const costPerMillion = model.cost.input;
      if (costPerMillion <= 1) score += 10;
      else if (costPerMillion <= 5) score += 5;
    }

    // Provider preference
    if (criteria.preferredProvider && model.provider === criteria.preferredProvider) {
      score += 15;
      reasons.push('preferred provider');
    }

    // Reasoning bonus for complex tasks
    if (model.reasoning) {
      if (criteria.taskType === 'coding' || criteria.taskType === 'analysis') {
        score += 10;
        reasons.push('reasoning model for complex task');
      }
    }

    // Large context bonus for analysis tasks
    if (criteria.taskType === 'analysis' && model.contextWindow >= 128_000) {
      score += 5;
      reasons.push('large context window');
    }

    return {
      provider: model.provider,
      modelId: model.id,
      modelName: model.name,
      score,
      reasoning: reasons.length > 0 ? reasons.join('; ') : 'base score',
    };
  }

  private classifyTier(model: ModelInfo): CostTier {
    const cost = model.cost.input;
    const cheapThreshold = COST_TIER_THRESHOLDS['cheap'];
    const standardThreshold = COST_TIER_THRESHOLDS['standard'];

    if (cost <= cheapThreshold.maxInput) return 'cheap';
    if (cost <= standardThreshold.maxInput) return 'standard';
    return 'premium';
  }

  private disqualified(model: ModelInfo, reason: string): RoutingResult {
    return {
      provider: model.provider,
      modelId: model.id,
      modelName: model.name,
      score: 0,
      reasoning: `disqualified: ${reason}`,
    };
  }
}