Eclipse Theia 1.66 Release: News and Noteworthy

We are happy to announce the Eclipse Theia 1.66 release! The release contains in total 78 merged pull requests. In this article, we will highlight some selected improvements and provide an overview of the latest news around Theia.

Before we dive into the release details, we want to highlight that TheiaCon 2025 just concluded and it was a fantastic event! The conference brought together the Theia community to share insights, showcase innovations, and discuss the future of cloud and desktop IDE development. If you missed the event or want to revisit any of the sessions, all the videos are now available on our YouTube playlist. We encourage everyone to check out the recordings!

For those new to Eclipse Theia, it is the next-generation platform for building IDEs and tools for the web or desktop, based on modern state-of-the-art web technologies. With Theia AI, part of the Theia platform, you can also build AI-powered tools and IDEs with ease. For more details about Eclipse Theia, please refer to this article and visit the Theia website.

The Theia project also releases a product, the Theia IDE. The Eclipse Theia IDE is a modern, AI-powered, and open IDE for cloud and desktop environments, aimed at end users. The Theia IDE is based on the Theia platform and also includes advanced AI powered features. For more details, see the Theia IDE website.

If you are looking for a simple way to check out the new release, please download and install the Theia IDE, which is based on Theia 1.66.

Eclipse Theia 1.66: Selected features and improvements

In the following, we will highlight some selected improvements in the new release. As usual, we cannot mention all 78 improvements, however we will focus on the most notable changes as well as changes visible to end users. The corresponding pull requests are linked under the respective heading when applicable.

Chat Session Persistence

One of the most requested features for Theia AI has been the ability to persist chat sessions across application restarts. With Theia 1.66, chat sessions are automatically stored to disk, allowing you to seamlessly continue your AI conversations even after restarting the Theia IDE or any custom tool based on Theia AI. Learn more about using the chat interface in the Theia IDE.

The implementation stores chat sessions in the user ~/.theia/chatSessions/ directory with a limit of 25 sessions. When this limit is exceeded, the oldest sessions are automatically deleted. The “Show Chats…” dialog now displays both active and archived sessions, making it easy to restore previous conversations. The persistence system maintains the full chat tree including message alternatives and branches.

Beyond just text content, the system also persists changesets with fully functional apply, revert, and open operations. This means that code (or other artifact) changes suggested by AI agents in previous sessions remain actionable even after a restart. This feature is also available for custom content types implemented by adopters of Theia AI.

Slash Commands for AI Chat

Theia 1.66 introduces slash commands in the AI chat interface, providing a more intuitive way to invoke pre-defined prompt templates. Instead of the previously supported and technical #prompt:fragmentid syntax, you can now simply type /commandname arguments to execute commands with a familiar, user-friendly interface. This will insert a pre-defined prompt fragment into your current chat.

Commands can be pre-defined by the tool provider. In the Theia IDE, custom commands can be additionally added by the end user. They are registered as prompt fragments with isCommand: true metadata and support argument substitution using placeholders like $ARGUMENTS, $1, $2, and so on, see the documentation on commands in Theia AI for more details.

The system includes autocomplete support directly in the chat input, making it easy to discover and use available commands. Commands can be scoped to specific agents, ensuring that only relevant commands appear based on your current context.

Under the hood, slash commands are converted to the existing prompt variable reference system, maintaining compatibility with the broader Theia AI architecture.

We’ll soon be publishing a more detailed piece on slash commands covering advanced use cases and best practices. Make sure to subscribe to our YouTube channel to be notified when it’s available!

Agent Mode Support

Theia AI 1.66 adds optional mode support for chat agents, allowing them to define different operational modes that users can select from. This feature enables agents to adjust their behavior based on user preferences – for example, switching between “edit” and “agent” code agent behevaior, or “plan” versus “act” approaches to problem-solving. Learn more about agent modes in the Theia AI documentation.

A mode selector appears in the chat input UI when the current agent supports multiple modes, and users can select modes via dropdown or cycle through them using the Shift+Tab keyboard shortcut. The selected mode is passed with the chat request, allowing agents to tailor their responses accordingly. This mechanism also supports multi-agent workflows, where different agents can be configured with specialized modes for specific tasks.

The Video below shows a sample ModeChatAgent demonstrating how to implement mode-based behavior with concise versus detailed response modes.

We will soon integrated this feature into the existing agents available in the Theia IDE, e.g. to allows to switch between the available modes of Theia Coder. Please note, that this switch is already possible by selecting the propmpt template in the agent settings, but with the new mode support, it will be much more accessible.

GitHub Agent

Theia 1.66 introduces a new GitHub Agent that enables natural language interaction with GitHub repositories. This agent can help with a wide range of GitHub-related tasks, from querying repository information to creating issues and managing pull requests. Learn more about using the GitHub agent.

Users can simply address the GitHub agent in the AI chat to perform actions like checking repository status, listing issues, creating pull requests, or reviewing code. The agent leverages the Model Context Protocol (MCP) to communicate with GitHub’s API and includes built-in support for setting up the required MCP server connection, making it easy to get started.

The video below demonstrates the GitHub agent in action, embedded into a full workflow:

Project Info Agent

A new Project Info Agent has been introduced in Theia 1.66 to help developers create and maintain project context artefacts. This agent can initially generate structured “project information” files that capture essential details about your codebase, making it easier for AI agents to understand the context and purpose of your project. Learn more about using the Project Info agent.

The Project Info Agent auto-discovers relevant project artefacts. It also considers existing project documentation from popular sources like .copilot-instructions.md or Claude.md files, consolidating them into a unified project information format. This centralized documentation helps other AI agents such as Theia Coder to provide more accurate and contextually relevant assistance by having a clear understanding of your project’s structure, conventions, architecture and goals.

We will soon publish a comprehensive tutorial on creating, maintaining and using project context artefacts in several coding agents. Make sure to subscribe to our YouTube channel to be notified when it’s available!

Debugging and UI Enhancements

Debugging capabilities in Theia 1.66 have been enhanced with better handling of session-bound metadata for disabled breakpoints. When a breakpoint is disabled during a debug session, its relocated position is now properly retained until the session ends, rather than immediately reverting to its original location. This provides a more consistent debugging experience, especially when working with transpiled code or source maps.

The Add to Watch command has been fixed and now correctly appears in the Debug Variables widget context menu, allowing users to easily add variables to the watch expressions list. This functionality has been aligned with VS Code’s implementation for consistency.

Context menu performance has been improved by evaluating enablement and toggle state only once before showing the menu, rather than on every mouse hover event. This reduces unnecessary computation and provides a more responsive user interface.

The reveal position behavior during debugging has been made smoother by using the ‘auto’ option instead of always centering the current line. This prevents the visible part of the document from jumping around unnecessarily during each debug step, providing a more comfortable debugging experience.

OpenAI Response API

OpenAI integration has been enhanced with support for the new Response API. This modern API endpoint provides improved reliability and performance compared to the legacy completion endpoint. In 1.66, the feature can be enabled for built-in models via the ai-features.openAiOfficial.useResponseApi preference, and custom model definitions can also leverage this capability through the useResponseApi field. While currently disabled by default to maintain compatibility, this option allows users to take advantage of OpenAI’s latest API improvements. We encourage Theia AI users and adopters to test the new API for their use cases, as this will help stabilize it and it will become the default in an upcoming release. Learn more about configuring OpenAI models.

Terminal Command Suggestion

A new tool function has been added that enables AI agents to suggest terminal commands without immediately executing them. When an agent recommends a command, it is inserted into the terminal buffer where users can review and potentially modify it before execution. This provides an important safety mechanism while still streamlining command-line workflows. If no terminal is currently active, the function automatically creates a new terminal with the appropriate workspace context. Learn more about using terminal assistance.

Additional Improvements

As always, the 1.66 release contains much more than described in this article. The compatibility with VS Code Extensions has been upgraded to API version 1.105.0, Electron has been updated to version 38.4.0, and proxy configuration is now supported for accessing LLMs when working behind corporate firewalls (PR #16453).

Several authentication improvements have been made, including better handling of session changes (PR #16252). The Claude Code integration has been migrated to the official Agent SDK and now includes a dedicated API key preference for easier configuration (PR #16508).

The Orchestrator agent now has a configurable exclusion list to prevent delegation to specific agents (PR #16510), and symbol icons in the breadcrumbs and outline view now include default colors that adapt to your selected theme (PR #15860).

Memory leak issues have been addressed in the Output Widget and Monaco Editor (PR #16487), and several localization enhancements have been made across various AI-related packages to improve the internationalization experience.

Improved Code Editing with Coder Agent: A new, more flexible content replacer strategy has been introduced for the Coder agent’s Edit Mode (PR #16322). The new suggestFileChangesNext function provides enhanced accuracy when applying code changes. You can try this experimental feature by switching the Coder agent to use the “edit-next” prompt in the AI Configuration View. Early testing shows promising results, and we are considering making this the default approach in future releases. We encourage users to test this new strategy and share their feedback! Learn more about using Theia Coder.

For a complete overview of all changes in this release, please refer to the 1.66 milestone. All these features and improvements (in total 78) were the result of one month of intensive development. Eclipse Theia follows a monthly release schedule. We are looking forward to the next release due next month, stay tuned! To be notified about future releases, follow us on LinkedIn or follow Theia on Twitter and subscribe to our mailing list.

If you are interested in building custom tools or IDEs based on Eclipse Theia, EclipseSource provides consulting and implementation services for Eclipse Theia, for AI-powered tools, as well as for web-based tools in general.

Furthermore, if you want to extend Theia, Theia AI or the Theia IDE with new features or strategically invest into the project, EclipseSource provides sponsored development for Theia, too. Finally, we provide consulting and support for hosting web-based tools in the cloud.

👉 Get in contact with us, to discuss your use case!

👉 Subscribe to our YouTube channel