my-cohere-app/
├── src/
│   ├── cohere/
│   │   ├── client.ts           # CohereClientV2 singleton
│   │   ├── models.ts           # Model selection logic
│   │   ├── types.ts            # Cohere-specific types
│   │   └── errors.ts           # Error classification
│   ├── services/
│   │   ├── chat.ts             # Chat completions + streaming
│   │   ├── rag.ts              # RAG pipeline (embed → rerank → chat)
│   │   ├── agents.ts           # Tool-use agent loops
│   │   ├── embed.ts            # Batch embedding + caching
│   │   ├── rerank.ts           # Document reranking
│   │   └── classify.ts         # Few-shot classification
│   ├── tools/                  # Tool definitions for agents
│   │   ├── registry.ts         # Tool name → executor mapping
│   │   ├── search.ts
│   │   └── calculator.ts
│   ├── api/
│   │   ├── chat.ts             # POST /api/chat (streaming)
│   │   ├── embed.ts            # POST /api/embed
│   │   └── health.ts           # GET /api/health
│   └── cache/
│       └── embeddings.ts       # LRU cache for embeddings
├── tests/
│   ├── unit/
│   │   ├── chat.test.ts
│   │   ├── rag.test.ts
│   │   └── fixtures/           # Mocked API responses
│   └── integration/
│       └── cohere.test.ts      # Real API tests (gated)
├── config/
│   ├── models.json             # Model selection per environment
│   └── tools.json              # Tool definitions
└── package.json

┌─────────────────────────────────────────┐
│             API Layer                    │
│   (Express/Next.js routes, SSE stream)  │
├─────────────────────────────────────────┤
│           Service Layer                  │
│  (RAG pipeline, agent loop, classify)   │
├─────────────────────────────────────────┤
│          Cohere Layer                    │
│   (CohereClientV2, retry, model select) │
├─────────────────────────────────────────┤
│         Infrastructure Layer             │
│    (Embed cache, tool registry, queue)   │
└─────────────────────────────────────────┘

// src/cohere/client.ts
import { CohereClientV2, CohereError, CohereTimeoutError } from 'cohere-ai';

let instance: CohereClientV2 | null = null;

export function getCohere(): CohereClientV2 {
  if (!instance) {
    instance = new CohereClientV2({
      token: process.env.CO_API_KEY,
    });
  }
  return instance;
}

// src/cohere/models.ts
export const MODELS = {
  chat: {
    premium: 'command-a-03-2025',
    standard: 'command-r-08-2024',
    fast: 'command-r7b-12-2024',
  },
  embed: {
    latest: 'embed-v4.0',
    english: 'embed-english-v3.0',
    multilingual: 'embed-multilingual-v3.0',
  },
  rerank: {
    latest: 'rerank-v3.5',
  },
} as const;

// src/services/rag.ts
import { getCohere } from '../cohere/client';
import { MODELS } from '../cohere/models';

interface RAGResult {
  answer: string;
  citations: Array<{ start: number; end: number; text: string; sources: string[] }>;
  model: string;
}

export async function ragQuery(
  query: string,
  documents: Array<{ id: string; text: string }>,
  options?: { model?: string; topN?: number }
): Promise<RAGResult> {
  const cohere = getCohere();
  const model = options?.model ?? MODELS.chat.standard;

  // Step 1: Rerank documents
  const reranked = await cohere.rerank({
    model: MODELS.rerank.latest,
    query,
    documents: documents.map(d => d.text),
    topN: options?.topN ?? 5,
  });

  // Step 2: Chat with top documents
  const topDocs = reranked.results.map(r => ({
    id: documents[r.index].id,
    data: { text: documents[r.index].text },
  }));

  const response = await cohere.chat({
    model,
    messages: [{ role: 'user', content: query }],
    documents: topDocs,
  });

  return {
    answer: response.message?.content?.[0]?.text ?? '',
    citations: (response.message?.citations ?? []).map(c => ({
      start: c.start,
      end: c.end,
      text: c.text,
      sources: c.sources?.map((s: any) => s.id) ?? [],
    })),
    model,
  };
}

// src/services/agents.ts
import { getCohere } from '../cohere/client';
import { MODELS } from '../cohere/models';
import { toolRegistry } from '../tools/registry';

export async function runAgent(
  userMessage: string,
  maxSteps = 5
): Promise<string> {
  const cohere = getCohere();
  const messages: any[] = [{ role: 'user', content: userMessage }];
  const tools = toolRegistry.getToolDefinitions();

  for (let step = 0; step < maxSteps; step++) {
    const response = await cohere.chat({
      model: MODELS.chat.premium,
      messages,
      tools,
    });

    if (response.finishReason !== 'TOOL_CALL') {
      return response.message?.content?.[0]?.text ?? '';
    }

    const toolCalls = response.message?.toolCalls ?? [];
    messages.push({ role: 'assistant', toolCalls });

    for (const tc of toolCalls) {
      const result = await toolRegistry.execute(
        tc.function.name,
        JSON.parse(tc.function.arguments)
      );
      messages.push({ role: 'tool', toolCallId: tc.id, content: result });
    }
  }

  return 'Agent reached max steps.';
}

// src/tools/registry.ts
interface ToolDefinition {
  type: 'function';
  function: {
    name: string;
    description: string;
    parameters: Record<string, unknown>;
  };
}

class ToolRegistry {
  private tools: Map<string, {
    definition: ToolDefinition;
    executor: (args: any) => Promise<string>;
  }> = new Map();

  register(
    name: string,
    description: string,
    parameters: Record<string, unknown>,
    executor: (args: any) => Promise<string>
  ) {
    this.tools.set(name, {
      definition: {
        type: 'function',
        function: { name, description, parameters },
      },
      executor,
    });
  }

  getToolDefinitions(): ToolDefinition[] {
    return Array.from(this.tools.values()).map(t => t.definition);
  }

  async execute(name: string, args: any): Promise<string> {
    const tool = this.tools.get(name);
    if (!tool) return JSON.stringify({ error: `Unknown tool: ${name}` });
    try {
      return await tool.executor(args);
    } catch (err) {
      return JSON.stringify({ error: String(err) });
    }
  }
}

export const toolRegistry = new ToolRegistry();

// src/cohere/errors.ts
import { CohereError, CohereTimeoutError } from 'cohere-ai';

export type ErrorCategory = 'auth' | 'rate_limit' | 'bad_request' | 'server' | 'timeout' | 'unknown';

export function classifyError(err: unknown): {
  category: ErrorCategory;
  retryable: boolean;
  message: string;
} {
  if (err instanceof CohereTimeoutError) {
    return { category: 'timeout', retryable: true, message: 'Request timed out' };
  }
  if (err instanceof CohereError) {
    switch (err.statusCode) {
      case 401: return { category: 'auth', retryable: false, message: 'Invalid API key' };
      case 429: return { category: 'rate_limit', retryable: true, message: 'Rate limited' };
      case 400: return { category: 'bad_request', retryable: false, message: err.message };
      default:
        if (err.statusCode && err.statusCode >= 500) {
          return { category: 'server', retryable: true, message: err.message };
        }
    }
  }
  return { category: 'unknown', retryable: false, message: String(err) };
}

User Query
     │
     ▼
┌─────────────┐
│   API Route  │  POST /api/chat
└──────┬──────┘
       │
       ▼
┌─────────────┐    ┌─────────────┐
│  RAG Service │───▶│  Rerank     │  rerank-v3.5
│  or Agent   │    │  Service    │
└──────┬──────┘    └─────────────┘
       │
       ▼
┌─────────────┐    ┌─────────────┐
│ Chat/Stream  │───▶│  Embed      │  embed-v4.0
│  Service    │    │  Cache      │  (cached)
└──────┬──────┘    └─────────────┘
       │
       ▼
┌─────────────┐
│ CohereClient │  command-a-03-2025
│     V2      │
└─────────────┘