Anthropic Messages API vs OpenAI Chat Completions API

Anthropic Messages API vs OpenAI Chat Completions API

Note: This document reflects publicly documented formats as of early 2025. It is intended for building a translation/proxy layer and focuses on shape/semantics rather than model availability or pricing.

1. Anthropic /v1/messages (new Messages API)

Request schema (high level)

POST /v1/messages
{
  "model": "claude-3-5-sonnet-20240620",
  "max_tokens": 1024,
  "system": "You are a helpful assistant.",
  "messages": [
    {
      "role": "user",
      "content": [
        {"type": "text", "text": "Summarize this:"},
        {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "..."}}
      ]
    }
  ],
  "temperature": 0.2,
  "top_p": 0.9,
  "top_k": 40,
  "stop_sequences": ["\n\nHuman:"],
  "metadata": {"user_id": "abc-123"},
  "tools": [
    {
      "name": "get_weather",
      "description": "Fetch weather for a city",
      "input_schema": {
        "type": "object",
        "properties": {"city": {"type": "string"}},
        "required": ["city"]
      }
    }
  ],
  "tool_choice": {"type": "auto"},
  "stream": false
}

Key points

• system is a top‑level field (string or array of content blocks), not a role in messages.
• messages contains only user and assistant roles.
• content is always an array of blocks, even for text‑only use cases.
• Tools are declared in tools with input_schema; tool selection uses tool_choice.

Response schema (high level)

{
  "id": "msg_01...",
  "type": "message",
  "role": "assistant",
  "model": "claude-3-5-sonnet-20240620",
  "content": [
    {"type": "text", "text": "Here's a summary..."},
    {"type": "tool_use", "id": "toolu_01...", "name": "get_weather", "input": {"city": "Boston"}}
  ],
  "stop_reason": "tool_use",
  "stop_sequence": null,
  "usage": {"input_tokens": 123, "output_tokens": 45}
}

Streaming format (SSE)

Anthropic streams structured events (each data: line is JSON, with event: names):

event: message_start

data: {"type":"message_start","message":{"id":"msg_...","role":"assistant","model":"...","content":[],"usage":{"input_tokens":123,"output_tokens":0}}}

event: content_block_start

data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}

event: content_block_delta

data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello"}}

event: content_block_stop

data: {"type":"content_block_stop","index":0}

event: message_delta

data: {"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":12}}

event: message_stop

data: {"type":"message_stop"}

Tool‑use streaming uses content_block_delta with delta.type = "input_json_delta" to incrementally stream tool input JSON.

Tool/function calling

• Tool calls are represented as content blocks of type tool_use in the assistant response.
• Tools are described in the request under tools and chosen using tool_choice.
• Tool results are sent back by the client as a user message containing a tool_result content block:

{
  "role": "user",
  "content": [
    {"type": "tool_result", "tool_use_id": "toolu_01...", "content": "72°F and sunny"}
  ]
}

System message handling

• Single top‑level system field; can be a string or an array of content blocks.
• No system role in the messages list.

2. OpenAI /v1/chat/completions (Chat Completions)

Request schema (high level)

POST /v1/chat/completions
{
  "model": "gpt-4o-mini",
  "messages": [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Summarize this:"}
  ],
  "temperature": 0.2,
  "top_p": 0.9,
  "stop": ["\n\nHuman:"],
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "get_weather",
        "description": "Fetch weather for a city",
        "parameters": {
          "type": "object",
          "properties": {"city": {"type": "string"}},
          "required": ["city"]
        }
      }
    }
  ],
  "tool_choice": "auto",
  "stream": false
}

Response schema (high level)

{
  "id": "chatcmpl_...",
  "object": "chat.completion",
  "created": 1710000000,
  "model": "gpt-4o-mini",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "Here's a summary...",
        "tool_calls": [
          {
            "id": "call_01...",
            "type": "function",
            "function": {"name": "get_weather", "arguments": "{\"city\":\"Boston\"}"}
          }
        ]
      },
      "finish_reason": "tool_calls"
    }
  ],
  "usage": {"prompt_tokens": 123, "completion_tokens": 45, "total_tokens": 168}
}

Streaming format (SSE)

OpenAI streams choice deltas:

data: {"id":"chatcmpl_...","object":"chat.completion.chunk","choices":[{"delta":{"role":"assistant","content":"Hel"},"index":0,"finish_reason":null}]}

data: {"id":"chatcmpl_...","object":"chat.completion.chunk","choices":[{"delta":{"content":"lo"},"index":0,"finish_reason":null}]}

data: {"id":"chatcmpl_...","object":"chat.completion.chunk","choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_01...","type":"function","function":{"name":"get_weather","arguments":"{\"city\":\"Bos"}}]}}]}

data: {"id":"chatcmpl_...","object":"chat.completion.chunk","choices":[{"delta":{"tool_calls":[{"index":0,"function":{"arguments":"ton\"}"}}]}}]}

data: {"id":"chatcmpl_...","object":"chat.completion.chunk","choices":[{"delta":{},"index":0,"finish_reason":"tool_calls"}]}

data: [DONE]

Tool/function calling

• Tools are declared in tools with type: "function" and a JSON Schema‑like parameters object.
• Tool calls are returned in message.tool_calls.
• Tool results are sent back as a role: "tool" message with tool_call_id and content.

System message handling

• System instructions are message(s) with role: "system" in the messages list.

3. Comparison Matrix

4. Translation Layer Design

4.1 OpenAI → Anthropic

Mapping rules

• System messages → concatenate into Anthropic system (string or content blocks).
• User/assistant messages → map to Anthropic messages with block array content.
• Tool schema → map tools[].function.parameters → tools[].input_schema.
• Tool choice → map tool_choice:
  • "auto" → { "type": "auto" }
  • "none" → { "type": "auto" } with no tools (or omit tools)
  • {"type":"function","function":{"name":"X"}} → { "type": "tool", "name": "X" }

Pseudocode

function openai_to_anthropic(request):
  anthropic = {}
  anthropic.model = request.model
  anthropic.max_tokens = request.max_tokens
  anthropic.temperature = request.temperature
  anthropic.top_p = request.top_p
  anthropic.stop_sequences = request.stop

  system_msgs = filter(request.messages, role == "system")
  anthropic.system = join_system(system_msgs)

  anthropic.messages = []
  for msg in request.messages where role in ["user", "assistant", "tool"]:
    if msg.role == "tool":
      # convert tool result to user message with tool_result block
      anthropic.messages.append({
        role: "user",
        content: [{type: "tool_result", tool_use_id: msg.tool_call_id, content: msg.content}]
      })
    else:
      anthropic.messages.append({
        role: msg.role,
        content: to_content_blocks(msg.content)
      })

  anthropic.tools = map_tools_openai_to_anthropic(request.tools)
  anthropic.tool_choice = map_tool_choice_openai_to_anthropic(request.tool_choice)
  return anthropic

Edge cases / gotchas

• OpenAI allows multiple system messages; Anthropic expects a single system field. Decide whether to concatenate with separators or preserve as an array of blocks.
• OpenAI tool call arguments are stringified JSON; Anthropic expects a native JSON object in tool_use.input.
• OpenAI role: "tool" messages may contain non‑string content (e.g., structured JSON in content) — must be stringified or embedded as tool_result text.

4.2 Anthropic → OpenAI

Mapping rules

• Top‑level system → first messages[] entry with role: "system".
• Messages array → map content blocks to OpenAI content string (or multimodal parts) where possible.
• Tool schema → map tools[].input_schema → tools[].function.parameters.
• Tool calls → map tool_use blocks to message.tool_calls[].
• Tool results → map tool_result blocks to role: "tool" messages.

Pseudocode

function anthropic_to_openai(request):
  openai = {}
  openai.model = request.model
  openai.max_tokens = request.max_tokens
  openai.temperature = request.temperature
  openai.top_p = request.top_p
  openai.stop = request.stop_sequences

  openai.messages = []
  if request.system:
    openai.messages.append({role: "system", content: system_to_string(request.system)})

  for msg in request.messages:
    if has_tool_result(msg):
      for block in msg.content where block.type == "tool_result":
        openai.messages.append({
          role: "tool",
          tool_call_id: block.tool_use_id,
          content: block.content
        })
    else:
      openai.messages.append({
        role: msg.role,
        content: blocks_to_openai_content(msg.content)
      })

  openai.tools = map_tools_anthropic_to_openai(request.tools)
  openai.tool_choice = map_tool_choice_anthropic_to_openai(request.tool_choice)
  return openai

Edge cases / gotchas

• Anthropic content blocks can include non‑text (images, citations, tool_use) that may not map 1:1 into OpenAI chat content unless using multimodal message parts.
• Anthropic streams content_block events which must be reassembled into OpenAI’s delta chunk format.
• Anthropic can return multiple content blocks (text + tool_use); OpenAI expects tool calls in a separate tool_calls array, not in content.

4.3 What cannot be translated 1:1

• Streaming semantics: Anthropic’s typed SSE events do not map cleanly to OpenAI’s delta‑only streaming model without re‑chunking and state management.
• Tool input streaming: Anthropic streams input_json_delta for tool calls, whereas OpenAI streams string fragments in function.arguments.
• Content block richness: Anthropic’s block types (citations, images, tool_use) may not have direct equivalents in plain OpenAI chat content.
• Usage accounting: token counters are named differently and may include different categories in edge cases.

5. Side‑by‑Side Request Examples

Basic text request

### Tool call request

| Anthropic | OpenAI |
|---|---|
| ```json
{ "model":"claude-3-5-sonnet-20240620", "max_tokens":256, "messages":[{"role":"user","content":[{"type":"text","text":"Weather in Boston"}]}], "tools":[{"name":"get_weather","description":"Fetch weather","input_schema":{"type":"object","properties":{"city":{"type":"string"}},"required":["city"]}}], "tool_choice":{"type":"auto"} }
``` | ```json
{ "model":"gpt-4o-mini", "messages":[{"role":"user","content":"Weather in Boston"}], "tools":[{"type":"function","function":{"name":"get_weather","description":"Fetch weather","parameters":{"type":"object","properties":{"city":{"type":"string"}},"required":["city"]}}}], "tool_choice":"auto" }
``` |

---

## 6. Side‑by‑Side Response Examples

### Tool call response

| Anthropic | OpenAI |
|---|---|
| ```json
{ "id":"msg_...", "type":"message", "role":"assistant", "content":[{"type":"tool_use","id":"toolu_01...","name":"get_weather","input":{"city":"Boston"}}], "stop_reason":"tool_use" }
``` | ```json
{ "id":"chatcmpl_...", "object":"chat.completion", "choices":[{"message":{"role":"assistant","tool_calls":[{"id":"call_01...","type":"function","function":{"name":"get_weather","arguments":"{\"city\":\"Boston\"}"}}]},"finish_reason":"tool_calls"}] }
``` |

---

## 7. Streaming Format Comparison

| Aspect | Anthropic | OpenAI |
|---|---|---|
| Framing | `event:` + `data:` SSE with typed events | `data:` SSE chunks + `[DONE]` sentinel |
| Granularity | Content blocks + deltas (`text_delta`, `input_json_delta`) | Choice deltas (`delta.content`, `delta.tool_calls`) |
| Tool input streaming | `input_json_delta` | string fragments in `function.arguments` |

---

## 8. Tool Calling Comparison

| Topic | Anthropic | OpenAI |
|---|---|---|
| Declaration | `tools[].input_schema` | `tools[].function.parameters` |
| Call location | `content` block `tool_use` | `message.tool_calls[]` |
| Call arguments type | native JSON object | JSON **string** |
| Tool result | `tool_result` block in `user` message | `role: "tool"` message |

---

## 9. Compatibility Notes for Proxy Implementation

- **Normalize system instructions** to a single source of truth: assemble/disassemble system messages consistently.
- **Preserve tool call IDs**: OpenAI uses `tool_call_id`, Anthropic uses `tool_use_id` — map carefully.
- **Round‑trip tool arguments**: stringify/parse JSON deterministically (stable key ordering helps debugging).
- **Streaming bridge**: maintain a small state machine to convert between event types and deltas.
- **Message ordering**: ensure tool results always follow tool calls to avoid invalid API errors.
- **Token accounting**: expose both counts; do not rely on equality when returning metrics to clients.