/v1/chat/completions vs /v1/responses vs /v1/messages: Which AI API Endpoint Should You Use?

A common support issue in AI API gateways is not the API key, not the model, and not the SDK. It is the endpoint.

A user picks a model that exists, but sends the request to the wrong endpoint. The result looks like:

• model unavailable;
• unsupported endpoint;
• invalid request body;
• tool calling not working;
• streaming format mismatch;
• Claude model works in one tool but fails in another.

This guide explains the difference between three endpoint families:

/v1/chat/completions
/v1/responses
/v1/messages

Short version:

Endpoint	Native ecosystem	Best for	Request style
/v1/chat/completions	OpenAI-compatible legacy/current chat	Most apps, SDKs, LiteLLM, Cursor-style tools, simple chat	messages: [{role, content}]
/v1/responses	Newer OpenAI Responses API	Tool use, multimodal, reasoning items, newer OpenAI-style agents	input, tools, structured response items
/v1/messages	Anthropic Claude API	Claude-native SDKs and Claude-style apps	messages plus top-level system, Anthropic schema

If your client says “OpenAI-compatible”, start with:

https://crazyrouter.com/v1/chat/completions

or set base URL to:

https://crazyrouter.com/v1

and let the SDK append /chat/completions.

If your tool specifically requires the OpenAI Responses API, use /v1/responses.

If your tool is Anthropic-native and expects Claude’s Messages API, use /v1/messages.

The most common mistake

The most common mistake is mixing the model family and the endpoint family.

For example, a Claude model can be exposed through an OpenAI-compatible gateway, but that does not automatically mean your request body should use Anthropic’s native /v1/messages schema.

Likewise, a tool that sends Anthropic-native requests to /v1/messages cannot be fixed by only changing the model name. The endpoint and request body must match.

Think of it this way:

model name + endpoint + request schema must match

If one of the three is wrong, the model may look unavailable even when the model itself is healthy.

What /v1/chat/completions is

/v1/chat/completions is the classic OpenAI-compatible chat endpoint.

A typical request looks like this:

curl https://crazyrouter.com/v1/chat/completions \
  -H "Authorization: Bearer $CRAZYROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5.5",
    "messages": [
      {"role": "system", "content": "You are a helpful assistant."},
      {"role": "user", "content": "Explain API endpoints in one paragraph."}
    ]
  }'

Use /v1/chat/completions when:

• the app says it supports an OpenAI-compatible API;
• the config asks for OPENAI_BASE_URL or base_url;
• the request body uses messages with role and content;
• you are using common OpenAI SDK compatibility mode;
• you are configuring tools like Cursor-style clients, LiteLLM-style routers, FastGPT-style apps, or many chat UIs.

For many users, this is the safest default endpoint.

What /v1/responses is

/v1/responses is the newer OpenAI Responses API endpoint.

It is designed around a more general response object, and it can represent things like:

• text output;
• tool calls;
• multimodal input;
• reasoning items;
• structured output;
• agent-like workflows.

A simplified request looks like this:

curl https://crazyrouter.com/v1/responses \
  -H "Authorization: Bearer $CRAZYROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5.5",
    "input": "Explain the difference between chat completions and responses."
  }'

Use /v1/responses when:

• the tool explicitly says it uses the OpenAI Responses API;
• the config mentions responses rather than chat.completions;
• the request body uses input instead of messages;
• the model/provider route supports Responses API behavior;
• you need newer OpenAI-style tool or reasoning output formats.

Do not send a Chat Completions body to /v1/responses unless the gateway explicitly documents that conversion.

What /v1/messages is

/v1/messages is the Anthropic Messages API style endpoint.

A typical Claude-native request looks like this:

curl https://crazyrouter.com/v1/messages \
  -H "Authorization: Bearer $CRAZYROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-6",
    "max_tokens": 1024,
    "system": "You are a helpful assistant.",
    "messages": [
      {"role": "user", "content": "Explain Claude Messages API."}
    ]
  }'

Use /v1/messages when:

• the client is Anthropic-native;
• the SDK expects Claude Messages API format;
• the request has top-level system instead of a system message inside the array;
• the request requires Anthropic-specific content blocks or tool schema;
• the documentation explicitly says to call /v1/messages.

Do not assume every Claude model should be called with /v1/messages. If you are using an OpenAI-compatible gateway or SDK, Claude models may be called through /v1/chat/completions instead.

Why endpoint mistakes make models look unavailable

When a model fails, users often assume the model is down. But in many cases, the model is fine and the request format is wrong.

Common mismatch examples:

Mistake	What happens	Fix
Sending messages chat body to /v1/responses	Invalid request body or ignored fields	Use /v1/chat/completions or change body to input
Sending Anthropic system + messages body to /v1/chat/completions	Schema mismatch	Use /v1/messages or convert to OpenAI message format
Using /v1/messages in an OpenAI SDK	SDK may append the wrong path or parse the wrong response	Use OpenAI-compatible base URL with chat completions
Missing /v1 in base URL	404 or unknown route	Set base URL to https://crazyrouter.com/v1
Adding /chat/completions to base URL and SDK appends it again	Double path like /chat/completions/chat/completions	Base URL should usually end at /v1
Adding UTM parameters to API endpoint	Auth/routing errors	UTM only belongs on website links

Base URL vs full endpoint

Many tools ask for a Base URL, not a full endpoint.

Correct Base URL:

https://crazyrouter.com/v1

Then the SDK calls:

https://crazyrouter.com/v1/chat/completions

Wrong Base URL for most SDKs:

https://crazyrouter.com/v1/chat/completions

Why? Because the SDK may append /chat/completions again.

Rule:

• If the field is called base_url, use /v1.
• If the field is called endpoint and asks for a full URL, use the full path the tool requires.

Which endpoint should I choose?

Use this decision tree:

1. Does your tool say “OpenAI-compatible API”?
   
   Use /v1/chat/completions or Base URL https://crazyrouter.com/v1.

2. Does your tool specifically say “Responses API”?
   
   Use /v1/responses.

3. Does your tool use the Anthropic SDK or Claude Messages API?
   
   Use /v1/messages.

4. Are you unsure?
   
   Start with /v1/chat/completions, because most third-party clients support it.

Recommended Crazyrouter configuration

For most OpenAI-compatible tools:

API Key: your Crazyrouter API key
Base URL: https://crazyrouter.com/v1
Model: choose a model from the Crazyrouter model list

For users in regions where the global route is unstable:

Base URL: https://cn.crazyrouter.com/v1

Human-facing links can use UTM tracking:

https://crazyrouter.com/models?utm_source=blog&utm_medium=article&utm_campaign=api_endpoints

API endpoints should not:

https://api-endpoint.example.invalid/v1?utm_source=blog

Quick debugging checklist

Before reporting “model unavailable,” check these five things:

1. Is the model name spelled exactly as shown in the model list?
2. Is the endpoint family correct: chat completions, responses, or messages?
3. Does the request body match that endpoint’s schema?
4. Is the Base URL exactly /v1, not missing it and not duplicating endpoint paths?
5. Are you using the right SDK mode: OpenAI-compatible or Anthropic-native?

Final recommendation

If you are building general app integrations, start with /v1/chat/completions. It is the broadest compatibility path.

Use /v1/responses when your client or workflow explicitly requires the Responses API.

Use /v1/messages when you are using Anthropic-native tooling.

Most endpoint problems disappear when you remember this rule:

OpenAI-compatible client → /v1/chat/completions
OpenAI Responses client → /v1/responses
Anthropic-native client → /v1/messages

If the model exists but appears unavailable, do not only change the model name. Check the endpoint and request schema first.