Install, Pin, and Verify the SDK

Pin the SDK like you pin your model

The anthropic package on PyPI moves quickly. New SDK releases add helpers, refine types, and occasionally tighten validation. Pin the version in pyproject.toml or requirements.txt the same way you pin your model id — then upgrade deliberately, not accidentally.

Verify what is actually installed

Before you debug an SDK quirk, prove which version is running in the venv. pip show anthropic, python -c "import anthropic; print(anthropic.__version__)", and a quick look at anthropic.Anthropic.__doc__ are the three commands that catch 80% of 'why does this work locally and not in CI' bugs.

The two clients

The SDK ships two clients in one package: Anthropic (sync) and AsyncAnthropic (async). Same surface, different I/O model. Pick by your runtime — sync for scripts, async for FastAPI routes and any code that already runs an event loop.

Principle: Treat the SDK as a versioned dependency with a real upgrade story. Drift between local and prod is a planning bug, not a deploy surprise.

Code

Pinned install (pyproject.toml fragment)·text

[project]
dependencies = [
  "anthropic>=0.49,<0.60",  # range, not loose; bump intentionally
  # Optional extras when you need them:
  # "anthropic[bedrock]",
  # "anthropic[vertex]",
]

Verify locally and in CI·bash

python -c "import anthropic; print(anthropic.__version__)"
pip show anthropic | head -n 5
python - <<'PY'
from anthropic import Anthropic
client = Anthropic()
print(type(client).__name__, [m for m in dir(client) if not m.startswith('_')][:8])
PY

Sync and async clients side by side·python

from anthropic import Anthropic, AsyncAnthropic
import asyncio

# Sync — fine for scripts and CLI tools
sync_client = Anthropic()
resp = sync_client.messages.create(
    model="claude-haiku-4-5-20251001",
    max_tokens=64,
    messages=[{"role": "user", "content": "sync hello"}],
)
print("sync:", resp.content[0].text)

# Async — required for FastAPI / event-loop code
async_client = AsyncAnthropic()
async def go():
    r = await async_client.messages.create(
        model="claude-haiku-4-5-20251001",
        max_tokens=64,
        messages=[{"role": "user", "content": "async hello"}],
    )
    print("async:", r.content[0].text)
asyncio.run(go())

Install, Pin, and Verify the SDK

Pin the SDK like you pin your model

Verify what is actually installed

The two clients

Code

External links

Exercise

Progress

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