Claude Message Threads の使い方：Messages API メッセージスレッド公開ベータ、新しいメッセージだけ送信・初回トークン約 35% 高速化（Claude Code / API 検証）

# Turn 1: full conversation + system (+ tools), create a thread
curl -sS https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "anthropic-beta: message-threads-2026-08-12" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5-5",
    "max_tokens": 1024,
    "cache_control": {"type": "ephemeral"},
    "system": "You are a support agent for Acme. Keep replies short.",
    "messages": [{"role": "user", "content": "Hi! I have a question about an order."}],
    "thread": {"type": "create"}
  }'

# Turn 2: only the new message + the id of the turn 1 response
# (no system / tools here: the thread keeps them)
curl -sS https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "anthropic-beta: message-threads-2026-08-12" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5-5",
    "max_tokens": 1024,
    "cache_control": {"type": "ephemeral"},
    "messages": [{"role": "user", "content": "How long does standard shipping usually take?"}],
    "thread": {"type": "continue", "previous_message_id": "'"$MSG_ID"'"}
  }'

import anthropic

client = anthropic.Anthropic()  # official API, reads ANTHROPIC_API_KEY

# Option A: the SDK thread helper (adds the beta header, replays on 404 for you)
thread = client.beta.messages.thread(
    model="claude-opus-5-5",
    max_tokens=1024,
    cache_control={"type": "ephemeral"},
    system="You are a support agent for Acme. Keep replies short.",
)
thread.send({"role": "user", "content": "Hi! I have a question about an order."})
second = thread.send({"role": "user", "content": "How long does standard shipping usually take?"})
print(second.content[-1].text)

# Option B: send the thread field yourself
first = client.beta.messages.create(
    model="claude-opus-5-5", max_tokens=1024,
    cache_control={"type": "ephemeral"},
    system="You are a support agent for Acme. Keep replies short.",
    messages=[{"role": "user", "content": "Hi! I have a question about an order."}],
    thread={"type": "create"},
    betas=["message-threads-2026-08-12"],
)
second = client.beta.messages.create(
    model="claude-opus-5-5", max_tokens=1024,
    cache_control={"type": "ephemeral"},
    messages=[{"role": "user", "content": "How long does standard shipping usually take?"}],
    thread={"type": "continue", "previous_message_id": first.id},
    betas=["message-threads-2026-08-12"],
)

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic(); // official API, reads ANTHROPIC_API_KEY

const thread = client.beta.messages.thread({
  model: "claude-opus-5-5",
  max_tokens: 1024,
  cache_control: { type: "ephemeral" },
  system: "You are a support agent for Acme. Keep replies short.",
});

await thread.send({ role: "user", content: "Hi! I have a question about an order." });
const second = await thread.send({
  role: "user",
  content: "How long does standard shipping usually take?",
});
for (const block of second.content) {
  if (block.type === "text") console.log(block.text);
}

export ANTHROPIC_BASE_URL="https://api.aicodewith.ai"
export ANTHROPIC_AUTH_TOKEN="YOUR_API_KEY"
claude --model claude-opus-5-5

Turn 1 (create):  "Hi! I have a question about an order."
  -> 200, "Happy to help. What's your order number, and what's the question about it?"
Turn 2 (continue, previous_message_id = turn 1 id):
  "How long does standard shipping usually take? Also, what did I say my question was about?"
  -> 200, input_tokens: 27
  -> "...As for your question, you didn't say what it was about beyond asking about standard shipping..."

import anthropic

client = anthropic.Anthropic(
    base_url="https://api.aicodewith.ai",
    api_key="YOUR_API_KEY",
)

SYSTEM = "You are a support agent for Acme. Keep replies short.\n\n" + open("policy.md").read()
history = [{"role": "user", "content": "Hi! My order number is A-1042 and it hasn't shipped yet."}]

def turn(next_user_message=None):
    r = client.messages.create(
        model="claude-opus-5-5",
        max_tokens=1024,
        cache_control={"type": "ephemeral"},  # automatic caching: moves forward each turn
        system=SYSTEM,
        messages=history,                       # full history, no thread field
    )
    history.append({"role": "assistant", "content": [b.model_dump(exclude_none=True) for b in r.content]})
    if next_user_message:
        history.append({"role": "user", "content": next_user_message})
    u = r.usage
    print(u.input_tokens, u.cache_creation_input_tokens, u.cache_read_input_tokens)
    return r

turn("When will it ship?")
turn("What was my order number again?")
turn()

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic({
  baseURL: "https://api.aicodewith.ai",
  apiKey: "YOUR_API_KEY",
});

const history: Anthropic.MessageParam[] = [
  { role: "user", content: "Hi! My order number is A-1042 and it hasn't shipped yet." },
];

const r = await client.messages.create({
  model: "claude-opus-5-5",
  max_tokens: 1024,
  cache_control: { type: "ephemeral" },
  system: "You are a support agent for Acme. Keep replies short.",
  messages: history, // full history every turn, no thread field
});
history.push({ role: "assistant", content: r.content });
console.log(r.usage);

# continue from an id that cannot exist
curl -sS https://api.aicodewith.ai/v1/messages \
  -H "x-api-key: YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "anthropic-beta: message-threads-2026-08-12" \
  -H "content-type: application/json" \
  -d '{"model":"claude-opus-5-5","max_tokens":200,"cache_control":{"type":"ephemeral"},
       "messages":[{"role":"user","content":"Hi"}],
       "thread":{"type":"continue","previous_message_id":"msg_doesnotexist123"}}'
# official API: 404, error.details.error_code = "thread_not_found"
# a gateway that ignores thread: 200 and a normal reply

{
  "type": "error",
  "error": {
    "type": "not_found_error",
    "message": "...",
    "details": { "error_code": "thread_not_found" }
  },
  "request_id": "req_..."
}