llm-anthropic 0.29 Released: Adds Support for Claude Opus 5.5

Release Overview

The llm-anthropic plugin developed by Simon Willison has received a 0.29 update. This plugin is the official Anthropic model extension for the popular command-line tool LLM, enabling developers to invoke Claude models directly from the terminal.

The key change in this release is the addition of Claude Opus 5.5 support. Once users upgrade the plugin, they can immediately access Anthropic's latest flagship model with a simple command-line instruction.

LLM is an open-source command-line tool developed by Simon Willison (founder of the Datasette project), designed to give developers a unified entry point for interacting with large language models. Rather than learning separate SDKs or API conventions for each model provider, users can access dozens of models through the llm command paired with plugins. The tool also includes built-in conversation history, persistent logging (powered by SQLite), template management, and more — making it well-suited for rapid prototyping, script automation, and everyday prompt debugging. As the official Anthropic plugin, llm-anthropic is maintained by Simon Willison himself and typically adds support for new Anthropic models shortly after their release.

How to Use Claude Opus 5.5

After upgrading the plugin, calling Claude Opus 5.5 is straightforward. Simply run the following command in your terminal:

llm -m claude-opus-5.5 "prompt goes here"

The -m flag specifies the model name, followed by the prompt you want to send. This minimalist interaction style is central to LLM's design philosophy — wrapping complex LLM API calls into a single command to lower the barrier to entry.

For developers already using the LLM toolchain, this update is essentially a seamless upgrade. Running llm install -U llm-anthropic gets you the latest version without any changes to your existing workflow.

The Continued Evolution of the LLM Ecosystem

llm-anthropic is one component of the LLM command-line ecosystem Simon Willison has built. LLM itself supports a plugin architecture that connects to model providers like OpenAI, Anthropic, Google, and others through individual plugins, presenting a unified interface for all of them.

The advantage of this plugin-based design is that when a model vendor releases a new model, users don't need to wait for the main application to update — they simply upgrade the corresponding plugin to gain access right away. The rapid support for Claude Opus 5.5 is a direct reflection of this design philosophy.

For technical users who frequently switch between models, run prompt tests, or build automation scripts, the LLM toolchain offers a more efficient and integration-friendly alternative to graphical interfaces.

Claude Opus 5.5 is Anthropic's highest-tier flagship model in the Claude lineup, delivering stronger performance on reasoning, long-context tasks, and complex workflows — though at a higher API cost. In Anthropic's naming convention, "Opus" denotes flagship tier, "Sonnet" is mid-range, and "Haiku" is the lightweight, fast option; version numbers like 5.5 reflect generational iterations. For command-line users, being able to switch between different Claude model tiers directly through the llm tool makes it easier to balance performance needs against cost.

Summary

The llm-anthropic 0.29 update is a small release, but it's a practical one for command-line users who want to access Claude Opus 5.5 as soon as possible. It once again demonstrates how open-source plugin ecosystems can respond quickly and flexibly to new model releases. Interested readers can visit the GitHub releases page for the full changelog.