Claude Fable 5.1 vs Claude Opus 5.5 Comparison

# Python - Claude Fable 5.1: long-horizon agentic task, default high effort
import anthropic

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-fable-5-1",
    max_tokens=8000,
    output_config={"effort": "high"},
    tools=[
        {"type": "bash_20250124", "name": "bash"},
        {"type": "text_editor_20250728", "name": "str_replace_based_edit_tool"},
    ],
    messages=[
        {
            "role": "user",
            "content": (
                "Refactor the payment reconciliation module across all "
                "affected files, keep existing tests green, and explain "
                "every non-obvious tradeoff you made."
            ),
        }
    ],
)

for block in response.content:
    if block.type == "thinking":
        print("[thinking]", block.thinking[:200])
    elif block.type == "text":
        print("[answer]", block.text)

# Note: effort is set inside output_config, not as a flat effort= parameter.
# Fable 5.1 no longer supports forced tool-use via tool_choice; the model
# picks its own tools, otherwise the API returns a 400 invalid_request_error.

# Python - Claude Opus 5.5: general task, default medium effort
import anthropic

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-5-5",
    max_tokens=4096,
    # if output_config is omitted, the default "medium" is used; if a hard eval
    # class scores too low, try the higher levels:
    # output_config={"effort": "xhigh"},
    messages=[
        {
            "role": "user",
            "content": "Summarize this quarter's support tickets into 5 themes with counts.",
        }
    ],
)

for block in response.content:
    if block.type == "text":
        print(block.text)

# Note: thinking is always on in Opus 5.5; sending thinking={"type": "disabled"}
# returns a 400 invalid_request_error at every effort level. The legacy
# computer_20251124 computer-use tool is also no longer accepted on the
# Claude API or Google Cloud; move to the current tool version.