Anthropic-Compatible API

Overview

CallMissed provides an Anthropic Messages API-compatible endpoint alongside the OpenAI-compatible API. If you're already using the Anthropic SDK, you can switch to CallMissed by changing only the base_url.

Endpoints:

• POST /v1/messages — chat completions (streaming + non-streaming)
• POST /v1/messages/count_tokens — token count estimation (real BPE, not char-based)
• GET /anthropic/v1/models — list models in Anthropic shape with capability metadata
• GET /anthropic/v1/models/{model_id} — single model detail
• POST /anthropic/v1/messages — alternate path for the chat endpoint

Authentication: Use either header style:

• x-api-key: cm_your_key (Anthropic SDK default)
• Authorization: Bearer cm_your_key (OpenAI style)

Basic Usage

import anthropic

client = anthropic.Anthropic(
    api_key="cm_your_key",
    base_url="https://api.callmissed.com"
)

message = client.messages.create(
    model="gpt-5.6-sol",
    max_tokens=1024,
    system="You are a helpful assistant.",
    messages=[
        {"role": "user", "content": "What is the capital of India?"}
    ]
)

print(message.content[0].text)

Response:

{
  "id": "msg-abc123def456",
  "type": "message",
  "role": "assistant",
  "content": [
    {"type": "text", "text": "The capital of India is New Delhi."}
  ],
  "model": "gpt-5.6-sol",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 25,
    "output_tokens": 12
  }
}

Streaming

Set stream: true to receive Server-Sent Events with the full Anthropic streaming lifecycle:

import anthropic

client = anthropic.Anthropic(
    api_key="cm_your_key",
    base_url="https://api.callmissed.com"
)

with client.messages.stream(
    model="gpt-5.6-sol",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Tell me a short story."}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

SSE event lifecycle:

event: message_start        → message metadata + input token count
event: content_block_start  → new content block begins
event: content_block_delta  → text chunks (repeats)
event: content_block_stop   → content block complete
event: message_delta        → stop_reason + output token count
event: message_stop          → stream complete

Parameters

Note: Unlike the OpenAI API, max_tokens is required and system is a top-level parameter (not a message with role: "system").

Model Selection

Send any model ID from the Models catalog — not just Anthropic-shaped names. The model field takes the same values as /v1/chat/completions.

{ "model": "gpt-5.6-sol", "max_tokens": 1024, "messages": [...] }

Token Counting

Estimate input token count before sending a request. The endpoint uses a BPE tokenizer (tiktoken cl100k_base) — close to Claude's real tokenizer on typical English prompts, and noticeably more accurate than char-length heuristics.

curl -X POST https://api.callmissed.com/v1/messages/count_tokens \
  -H "x-api-key: cm_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5.6-sol",
    "messages": [{"role": "user", "content": "Hello, how are you?"}],
    "system": "You are a helpful assistant."
  }'

Response:

{"input_tokens": 15}

Image content blocks contribute a fixed estimate (~258 tokens per image) rather than a fetch-and-resize pass. Tool definitions are counted against the total by serializing each to JSON and tokenizing the schema.

Listing Models

List all available models via the Anthropic-shape endpoint:

curl https://api.callmissed.com/anthropic/v1/models \
  -H "x-api-key: cm_your_key"

Response:

{
  "data": [
    {
      "type": "model",
      "id": "gpt-5.6-sol",
      "display_name": "GPT-5.6 Sol",
      "created_at": "2023-11-14T22:13:20+00:00",
      "description": "Frontier model for complex professional work. Multimodal, reasoning + tools.",
      "category": "llm",
      "context_window": 1050000,
      "context_length": 1050000,
      "pricing": {"input": 5.00, "output": 30.00, "unit": "per_million_tokens", "currency": "USD"},
      "supports_streaming": true,
      "supports_tools": true,
      "supports_reasoning": true,
      "supports_vision": true
    }
  ],
  "has_more": false,
  "first_id": "...",
  "last_id": "..."
}

Fetch a single model at GET /anthropic/v1/models/{model_id}.

Vision (Image Input)

Send images on any model whose supports_vision flag is true in the model listing. That is the authoritative source; see the vision list for the current set. Models without vision reject image content with 400 invalid_request_error before the upstream call — you are not charged.

curl -X POST https://api.callmissed.com/v1/messages \
  -H "x-api-key: cm_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5.6-sol",
    "max_tokens": 1024,
    "messages": [{
      "role": "user",
      "content": [
        {"type": "text", "text": "What is in this image?"},
        {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "<base64>"}}
      ]
    }]
  }'

Error Format

Errors return the Anthropic format (different from the OpenAI endpoints):

{
  "type": "error",
  "error": {
    "type": "authentication_error",
    "message": "Invalid API key"
  }
}

Rate limit headers are returned in Anthropic format:

anthropic-ratelimit-requests-limit: 60
anthropic-ratelimit-requests-remaining: 45
anthropic-ratelimit-requests-reset: 2026-05-01T00:00:00+00:00

Differences from Anthropic

This endpoint is designed to work with the Anthropic SDK out of the box. Key differences from the official Anthropic API:

• anthropic-version header is accepted but not required
• Model routing — requests can target any model in the CallMissed catalogue, not just Anthropic-shaped names.
• Token counting uses a BPE tokenizer approximation (tiktoken cl100k_base). Expect ~5-10% variance from Anthropic's native counts on English prompts; larger on CJK and heavy-punctuation text.
• Tools are supported — tools and tool_choice work as documented, and tool_use/tool_result content blocks are preserved.
• Vision is supported on models whose supports_vision flag is true. Image content sent to text-only models is rejected with a 400 invalid_request_error before the upstream call, so your credits are safe.
• Message Batches API (/v1/messages/batches) is not implemented — use the regular /v1/messages endpoint.
• Billing uses CallMissed credits, not Anthropic billing.