OpenAI API - Complete Guide

Version: Production Ready ✅ Package: openai@6.16.0 Last Updated: 2026-01-20

---

Status

✅ Production Ready:

• ✅ Chat Completions API (GPT-5, GPT-4o, GPT-4 Turbo)
• ✅ Embeddings API (text-embedding-3-small, text-embedding-3-large)
• ✅ Images API (DALL-E 3 generation + GPT-Image-1 editing)
• ✅ Audio API (Whisper transcription + TTS with 11 voices)
• ✅ Moderation API (11 safety categories)
• ✅ Streaming patterns (SSE)
• ✅ Function calling / Tools
• ✅ Structured outputs (JSON schemas)
• ✅ Vision (GPT-4o)
• ✅ Both Node.js SDK and fetch approaches

---

Table of Contents

1. Quick Start
2. Chat Completions API
3. GPT-5 Series Models
4. Streaming Patterns
5. Function Calling
6. Structured Outputs
7. Vision (GPT-4o)
8. Embeddings API
9. Images API
10. Audio API
11. Moderation API
12. Error Handling
13. Rate Limits
14. Common Mistakes & Gotchas
15. TypeScript Gotchas
16. Production Best Practices
17. Relationship to openai-responses

---

Quick Start

Installation

npm install openai@6.16.0

Environment Setup

export OPENAI_API_KEY="sk-..."

Or create .env file:

OPENAI_API_KEY=sk-...

First Chat Completion (Node.js SDK)

import OpenAI from 'openai';

const openai = new OpenAI({
  apiKey: process.env.OPENAI_API_KEY,
});

const completion = await openai.chat.completions.create({
  model: 'gpt-5',
  messages: [
    { role: 'user', content: 'What are the three laws of robotics?' }
  ],
});

console.log(completion.choices[0].message.content);

First Chat Completion (Fetch - Cloudflare Workers)

const response = await fetch('https://api.openai.com/v1/chat/completions', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${env.OPENAI_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    model: 'gpt-5',
    messages: [
      { role: 'user', content: 'What are the three laws of robotics?' }
    ],
  }),
});

const data = await response.json();
console.log(data.choices[0].message.content);

---

Chat Completions API

Endpoint: POST /v1/chat/completions

The Chat Completions API is the core interface for interacting with OpenAI's language models. It supports conversational AI, text generation, function calling, structured outputs, and vision capabilities.

Supported Models

GPT-5 Series (Released August 2025)

• gpt-5: Full-featured reasoning model with advanced capabilities
• gpt-5-mini: Cost-effective alternative with good performance
• gpt-5-nano: Smallest/fastest variant for simple tasks

GPT-4o Series

• gpt-4o: Multimodal model with vision capabilities
• gpt-4-turbo: Fast GPT-4 variant

GPT-4 Series (Legacy)

• gpt-4: Original GPT-4 model (deprecated - use gpt-5 or gpt-4o)

Basic Request Structure

{
  model: string,              // Model to use (e.g., "gpt-5")
  messages: Message[],        // Conversation history
  reasoning_effort?: string,  // GPT-5 only: "minimal" | "low" | "medium" | "high"
  verbosity?: string,         // GPT-5 only: "low" | "medium" | "high"
  temperature?: number,       // NOT supported by GPT-5
  max_tokens?: number,        // Max tokens to generate
  stream?: boolean,           // Enable streaming
  tools?: Tool[],             // Function calling tools
}

Response Structure

{
  id: string,                 // Unique completion ID
  object: "chat.completion",
  created: number,            // Unix timestamp
  model: string,              // Model used
  choices: [{
    index: number,
    message: {
      role: "assistant",
      content: string,        // Generated text
      tool_calls?: ToolCall[] // If function calling
    },
    finish_reason: string     // "stop" | "length" | "tool_calls"
  }],
  usage: {
    prompt_tokens: number,
    completion_tokens: number,
    total_tokens: number
  }
}

Message Roles & Multi-turn Conversations

Three roles: system (behavior), user (input), assistant (model responses).

Important: API is stateless - send full conversation history each request. For stateful conversations, use openai-responses skill.

---

GPT-5 Series Models

GPT-5 models (released August 2025) introduce reasoning and verbosity controls.

GPT-5.2 (Released December 11, 2025)

Latest flagship model:

• gpt-5.2: 400k context window, 128k output tokens
• xhigh reasoning_effort: New level beyond "high" for complex problems
• Compaction: Extends context for long workflows (via API endpoint)
• Pricing: $1.75/$14 per million tokens (1.4x of GPT-5.1)

// GPT-5.2 with maximum reasoning
const completion = await openai.chat.completions.create({
  model: 'gpt-5.2',
  messages: [{ role: 'user', content: 'Solve this extremely complex problem...' }],
  reasoning_effort: 'xhigh', // NEW: Beyond "high"
});

GPT-5.1 (Released November 13, 2025)

Warmer, more intelligent model:

• gpt-5.1: Adaptive reasoning that varies thinking time dynamically
• 24-hour extended prompt caching: Faster follow-up queries at lower cost
• New developer tools: apply_patch (code editing), shell (command execution)

BREAKING CHANGE: GPT-5.1/5.2 default to reasoning_effort: 'none' (vs GPT-5 defaulting to 'medium').

O-Series Reasoning Models

Dedicated reasoning models (separate from GPT-5):

Model	Released	Purpose
o3	Apr 16, 2025	Successor to o1, advanced reasoning
o3-pro	Jun 10, 2025	Extended compute version of o3
o3-mini	Jan 31, 2025	Smaller, faster o3 variant
o4-mini	Apr 16, 2025	Fast, cost-efficient reasoning

// O-series models
const completion = await openai.chat.completions.create({
  model: 'o3',  // or 'o3-mini', 'o4-mini'
  messages: [{ role: 'user', content: 'Complex reasoning task...' }],
});

Note: O-series may be deprecated in favor of GPT-5 with reasoning_effort parameter.

reasoning_effort Parameter

Controls thinking depth (GPT-5/5.1/5.2):

• "none": No reasoning (fastest) - GPT-5.1/5.2 default
• "minimal": Quick responses (Note: May not be available - Issue #1690)
• "low": Basic reasoning
• "medium": Balanced - GPT-5 default
• "high": Deep reasoning
• "xhigh": Maximum reasoning (GPT-5.2 only)

verbosity Parameter

Controls output detail (GPT-5 series):

• "low": Concise
• "medium": Balanced (default)
• "high": Verbose

GPT-5 Limitations

NOT Supported:

• ❌ temperature, top_p, logprobs parameters
• ❌ Stateful Chain of Thought between turns

Alternatives: Use GPT-4o for temperature/top_p, or openai-responses skill for stateful reasoning

---

Streaming Patterns

Enable with stream: true for token-by-token delivery.

Node.js SDK

const stream = await openai.chat.completions.create({
  model: 'gpt-5.1',
  messages: [{ role: 'user', content: 'Write a poem' }],
  stream: true,
});

for await (const chunk of stream) {
  const content = chunk.choices[0]?.delta?.content || '';
  process.stdout.write(content);
}

Fetch (Cloudflare Workers)

const response = await fetch('https://api.openai.com/v1/chat/completions', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${env.OPENAI_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    model: 'gpt-5.1',
    messages: [{ role: 'user', content: 'Write a poem' }],
    stream: true,
  }),
});

const reader = response.body?.getReader();
const decoder = new TextDecoder();

while (true) {
  const { done, value } = await reader!.read();
  if (done) break;

  const chunk = decoder.decode(value);
  const lines = chunk.split('\n').filter(line => line.trim() !== '');

  for (const line of lines) {
    if (line.startsWith('data: ')) {
      const data = line.slice(6);
      if (data === '[DONE]') break;

      try {
        const json = JSON.parse(data);
        const content = json.choices[0]?.delta?.content || '';
        console.log(content);
      } catch (e) {
        // Skip invalid JSON
      }
    }
  }
}

Server-Sent Events (SSE) format:

data: {"id":"chatcmpl-xyz","choices":[{"delta":{"content":"Hello"}}]}
data: [DONE]

Key Points: Handle incomplete chunks, [DONE] signal, and invalid JSON gracefully.

---

Function Calling

Define tools with JSON schema, model invokes them based on context.

Tool Definition & Request

const tools = [{
  type: 'function',
  function: {
    name: 'get_weather',
    description: 'Get current weather for a location',
    parameters: {
      type: 'object',
      properties: {
        location: { type: 'string', description: 'City name' },
        unit: { type: 'string', enum: ['celsius', 'fahrenheit'] }
      },
      required: ['location']
    }
  }
}];

const completion = await openai.chat.completions.create({
  model: 'gpt-5.1',
  messages: [{ role: 'user', content: 'What is the weather in SF?' }],
  tools: tools,
});

Handle Tool Calls

const message = completion.choices[0].message;

if (message.tool_calls) {
  for (const toolCall of message.tool_calls) {
    const args = JSON.parse(toolCall.function.arguments);
    const result = await executeFunction(toolCall.function.name, args);

    // Send result back to model
    await openai.chat.completions.create({
      model: 'gpt-5.1',
      messages: [
        ...messages,
        message,
        {
          role: 'tool',
          tool_call_id: toolCall.id,
          content: JSON.stringify(result)
        }
      ],
      tools: tools,
    });
  }
}

Loop pattern: Continue calling API until no tool_calls in response.

---

Structured Outputs

Structured outputs allow you to enforce JSON schema validation on model responses.

Using JSON Schema

const completion = await openai.chat.completions.create({
  model: 'gpt-4o', // Note: Structured outputs best supported on GPT-4o
  messages: [
    { role: 'user', content: 'Generate a person profile' }
  ],
  response_format: {
    type: 'json_schema',
    json_schema: {
      name: 'person_profile',
      strict: true,
      schema: {
        type: 'object',
        properties: {
          name: { type: 'string' },
          age: { type: 'number' },
          skills: {
            type: 'array',
            items: { type: 'string' }
          }
        },
        required: ['name', 'age', 'skills'],
        additionalProperties: false
      }
    }
  }
});

const person = JSON.parse(completion.choices[0].message.content);
// { name: "Alice", age: 28, skills: ["TypeScript", "React"] }

JSON Mode (Simple)

For simpler use cases without strict schema validation:

const completion = await openai.chat.completions.create({
  model: 'gpt-5',
  messages: [
    { role: 'user', content: 'List 3 programming languages as JSON' }
  ],
  response_format: { type: 'json_object' }
});

const data = JSON.parse(completion.choices[0].message.content);

Important: When using response_format, include "JSON" in your prompt to guide the model.

---

Vision (GPT-4o)

GPT-4o supports image understanding alongside text.

Image via URL

const completion = await openai.chat.completions.create({
  model: 'gpt-4o',
  messages: [
    {
      role: 'user',
      content: [
        { type: 'text', text: 'What is in this image?' },
        {
          type: 'image_url',
          image_url: {
            url: 'https://example.com/image.jpg'
          }
        }
      ]
    }
  ]
});

Image via Base64

import fs from 'fs';

const imageBuffer = fs.readFileSync('./image.jpg');
const base64Image = imageBuffer.toString('base64');

const completion = await openai.chat.completions.create({
  model: 'gpt-4o',
  messages: [
    {
      role: 'user',
      content: [
        { type: 'text', text: 'Describe this image in detail' },
        {
          type: 'image_url',
          image_url: {
            url: `data:image/jpeg;base64,${base64Image}`
          }
        }
      ]
    }
  ]
});

Multiple Images

const completion = await openai.chat.completions.create({
  model: 'gpt-4o',
  messages: [
    {
      role: 'user',
      content: [
        { type: 'text', text: 'Compare these two images' },
        { type: 'image_url', image_url: { url: 'https://example.com/image1.jpg' } },
        { type: 'image_url', image_url: { url: 'https://example.com/image2.jpg' } }
      ]
    }
  ]
});

---

Embeddings API

Endpoint: POST /v1/embeddings

Convert text to vectors for semantic search and RAG.

Models

• text-embedding-3-large: 3072 dims (custom: 256-3072), highest quality
• text-embedding-3-small: 1536 dims (custom: 256-1536), cost-effective, recommended

Basic Request

const embedding = await openai.embeddings.create({
  model: 'text-embedding-3-small',
  input: 'The food was delicious.',
});
// Returns: { data: [{ embedding: [0.002, -0.009, ...] }] }

Custom Dimensions (OpenAI-Specific)

const embedding = await openai.embeddings.create({
  model: 'text-embedding-3-small',
  input: 'Sample text',
  dimensions: 256, // Reduced from 1536 default
});

Benefits: 4x-12x storage reduction, faster search, minimal quality loss.

Batch Processing

const embeddings = await openai.embeddings.create({
  model: 'text-embedding-3-small',
  input: ['First doc', 'Second doc', 'Third doc'],
});

Limits: 8192 tokens/input, 300k tokens total across batch, 2048 max array size.

Key Points: Use custom dimensions for efficiency, batch up to 2048 docs, cache embeddings (deterministic).

---

Images API

Image Generation (DALL-E 3)

Endpoint: POST /v1/images/generations

const image = await openai.images.generate({
  model: 'dall-e-3',
  prompt: 'A white siamese cat with striking blue eyes',
  size: '1024x1024', // Also: 1024x1536, 1536x1024, 1024x1792, 1792x1024
  quality: 'standard', // or 'hd'
  style: 'vivid', // or 'natural'
});

console.log(image.data[0].url);
console.log(image.data[0].revised_prompt); // DALL-E 3 may revise for safety

DALL-E 3 Specifics:

• Only supports n: 1 (one image per request)
• May revise prompts for safety/quality (check revised_prompt)
• URLs expire in 1 hour (use response_format: 'b64_json' for persistence)

Image Editing (GPT-Image-1)

Endpoint: POST /v1/images/edits

Important: Uses multipart/form-data, not JSON.

import FormData from 'form-data';

const formData = new FormData();
formData.append('model', 'gpt-image-1');
formData.append('image', fs.createReadStream('./woman.jpg'));
formData.append('image_2', fs.createReadStream('./logo.png')); // Optional composite
formData.append('prompt', 'Add the logo to the fabric.');
formData.append('input_fidelity', 'high'); // low|medium|high
formData.append('format', 'png'); // Supports transparency
formData.append('background', 'transparent'); // transparent|white|black

const response = await fetch('https://api.openai.com/v1/images/edits', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.OPENAI_API_KEY}`,
    ...formData.getHeaders(),
  },
  body: formData,
});

GPT-Image-1 Features: Supports transparency (PNG/WebP), compositing with image_2, output compression control.

---

Audio API

Whisper Transcription

Endpoint: POST /v1/audio/transcriptions

const transcription = await openai.audio.transcriptions.create({
  file: fs.createReadStream('./audio.mp3'),
  model: 'whisper-1',
});
// Returns: { text: "Transcribed text..." }

Formats: mp3, mp4, mpeg, mpga, m4a, wav, webm

Text-to-Speech (TTS)

Endpoint: POST /v1/audio/speech

Models:

• tts-1: Standard quality, lowest latency
• tts-1-hd: High definition audio
• gpt-4o-mini-tts: Supports voice instructions (November 2024), streaming

11 Voices: alloy, ash, ballad, coral, echo, fable, onyx, nova, sage, shimmer, verse

const mp3 = await openai.audio.speech.create({
  model: 'tts-1',
  voice: 'alloy',
  input: 'Text to speak (max 4096 chars)',
  speed: 1.0, // 0.25-4.0
  response_format: 'mp3', // mp3|opus|aac|flac|wav|pcm
});

Voice Instructions (gpt-4o-mini-tts Only)

const speech = await openai.audio.speech.create({
  model: 'gpt-4o-mini-tts',
  voice: 'nova',
  input: 'Welcome to support.',
  instructions: 'Speak in a calm, professional tone.', // Custom voice control
});

Streaming TTS (gpt-4o-mini-tts Only)

const response = await fetch('https://api.openai.com/v1/audio/speech', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.OPENAI_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    model: 'gpt-4o-mini-tts',
    voice: 'nova',
    input: 'Long text...',
    stream_format: 'sse', // Server-Sent Events
  }),
});

Note: instructions and stream_format: "sse" only work with gpt-4o-mini-tts.

---

Moderation API

Endpoint: POST /v1/moderations

Check content across 11 safety categories.

const moderation = await openai.moderations.create({
  model: 'omni-moderation-latest',
  input: 'Text to moderate',
});

console.log(moderation.results[0].flagged);
console.log(moderation.results[0].categories);
console.log(moderation.results[0].category_scores); // 0.0-1.0

11 Safety Categories

1. sexual: Sexual content
2. hate: Hateful content based on identity
3. harassment: Bullying, intimidation
4. self-harm: Promoting self-harm
5. sexual/minors: Child sexualization (CSAM)
6. hate/threatening: Violent threats based on identity
7. violence/graphic: Extreme gore
8. self-harm/intent: Suicidal ideation
9. self-harm/instructions: Self-harm how-to guides
10. harassment/threatening: Violent personal threats
11. violence: Violence threats/glorification

Scores: 0.0 (low confidence) to 1.0 (high confidence)

Batch Moderation

const moderation = await openai.moderations.create({
  model: 'omni-moderation-latest',
  input: ['Text 1', 'Text 2', 'Text 3'],
});

Best Practices: Use lower thresholds for severe categories (sexual/minors: 0.1, self-harm/intent: 0.2), batch requests, fail closed on errors.

---

Realtime API (Voice/Audio)

Low-latency voice and audio interactions via WebSocket/WebRTC. GA August 28, 2025.

Update (Feb 2025): Concurrent session limit removed - unlimited simultaneous connections now supported.

WebSocket Connection

const ws = new WebSocket('wss://api.openai.com/v1/realtime', {
  headers: {
    Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
    'OpenAI-Beta': 'realtime=v1',
  },
});

ws.onopen = () => {
  ws.send(JSON.stringify({
    type: 'session.update',
    session: {
      voice: 'alloy',  // or: echo, fable, onyx, nova, shimmer, marin, cedar
      instructions: 'You are a helpful assistant',
      input_audio_transcription: { model: 'whisper-1' },
    },
  }));
};

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  switch (data.type) {
    case 'response.audio.delta':
      // Handle audio chunk (base64 encoded)
      playAudioChunk(data.delta);
      break;
    case 'response.text.delta':
      // Handle text transcript
      console.log(data.delta);
      break;
  }
};

// Send user audio
ws.send(JSON.stringify({
  type: 'input_audio_buffer.append',
  audio: base64AudioData,
}));

Model

• gpt-realtime: Production model ($32/1M input, $64/1M output)
• gpt-realtime-mini: Smaller, faster variant

Features

• Voice activity detection
• Interruption handling
• Function calling while speaking
• 13 voices (including new: marin, cedar)
• WebRTC, WebSocket, SIP connections

---

Batch API (50% Cost Savings)

Process large volumes with 24-hour maximum turnaround at 50% lower cost.

Note: While the completion window is 24 hours maximum, jobs often complete much faster (reports show completion in under 1 hour for tasks estimated at 10+ hours).

Create Batch

// 1. Create JSONL file with requests
const requests = [
  { custom_id: 'req-1', method: 'POST', url: '/v1/chat/completions',
    body: { model: 'gpt-5.1', messages: [{ role: 'user', content: 'Hello 1' }] } },
  { custom_id: 'req-2', method: 'POST', url: '/v1/chat/completions',
    body: { model: 'gpt-5.1', messages: [{ role: 'user', content: 'Hello 2' }] } },
];

// 2. Upload file
const file = await openai.files.create({
  file: new File([requests.map(r => JSON.stringify(r)).join('\n')], 'batch.jsonl'),
  purpose: 'batch',
});

// 3. Create batch
const batch = await openai.batches.create({
  input_file_id: file.id,
  endpoint: '/v1/chat/completions',
  completion_window: '24h',
});

console.log(batch.id); // batch_abc123

Check Status

const batch = await openai.batches.retrieve('batch_abc123');

console.log(batch.status);  // validating, in_progress, completed, failed
console.log(batch.request_counts); // { total, completed, failed }

if (batch.status === 'completed') {
  const results = await openai.files.content(batch.output_file_id);
  // Parse JSONL results
}

When to Use Batch API

Use Case	Batch API?
Content moderation at scale	✅
Document processing (embeddings)	✅
Bulk summarization	✅
Real-time chat	❌ Use Chat API
Streaming responses	❌ Use Chat API

---

Error Handling & Rate Limits

Common Errors

• 401: Invalid API key
• 429: Rate limit exceeded (implement exponential backoff)
• 500/503: Server errors (retry with backoff)

async function completionWithRetry(params, maxRetries = 3) {
  for (let i = 0; i < maxRetries; i++) {
    try {
      return await openai.chat.completions.create(params);
    } catch (error) {
      if (error.status === 429 && i < maxRetries - 1) {
        await new Promise(resolve => setTimeout(resolve, Math.pow(2, i) * 1000));
        continue;
      }
      throw error;
    }
  }
}

Rate Limit Headers (OpenAI-Specific)

response.headers.get('x-ratelimit-limit-requests');
response.headers.get('x-ratelimit-remaining-requests');
response.headers.get('x-ratelimit-reset-requests');

Limits: Based on RPM (Requests/Min), TPM (Tokens/Min), IPM (Images/Min). Varies by tier and model.

---

Common Mistakes & Gotchas

Mistake #1: Using Wrong Model Name "gpt-5.1-mini"

Error: 400 The requested model 'gpt-5.1-mini' does not exist Source: GitHub Issue #1706

Wrong:

model: 'gpt-5.1-mini' // Does not exist

Correct:

model: 'gpt-5-mini' // Correct (no .1 suffix)

Available GPT-5 series models:

• gpt-5, gpt-5-mini, gpt-5-nano
• gpt-5.1, gpt-5.2
• Note: No gpt-5.1-mini or gpt-5.2-mini - mini variant doesn't have .1/.2 versions

Mistake #2: Embeddings Dimension Mismatch

Error: ValueError: shapes (0,256) and (1536,) not aligned

Ensure vector database dimensions match embeddings API dimensions parameter:

// ❌ Wrong - missing dimensions, returns 1536 default
const embedding = await openai.embeddings.create({
  model: 'text-embedding-3-small',
  input: 'text',
});