GitHub Copilot Adds Claude Fable 5.1, Gemini 3.8 Flash, and Agent Merge

GitHub's August 31 Copilot release brings two new model options to paying subscribers: Claude Fable 5.1 lands for Pro+, Max, Business, and Enterprise tiers, while Gemini 3.8 Flash rolls out across all paid plans including Pro. On the tooling side, VS Code 1.136 ships Agent Merge in public preview, a feature that takes a pull request from "reviewed but messy" to merge-ready by handling feedback, failed checks, and conflicts without manual intervention.

What happened

Feature	Detail
Claude Fable 5.1	Generally available for Copilot Pro+, Max, Business, Enterprise
Gemini 3.8 Flash	Rolling out to Copilot Pro, Pro+, Max, Business, Enterprise
Content exclusions	Now honored in Copilot app and CLI across agentic workflows
Agent Merge	Public preview in VS Code 1.136
Multi-root workspaces	Experimental in VS Code 1.136
JetBrains harness	Generally available

GitHub published this batch of changes on September 4, 2026, covering the week ending August 31. The release covers the Copilot web app, CLI, VS Code, and JetBrains integrations simultaneously.

New models: what each tier gets

Claude Fable 5.1 is restricted to the higher-cost tiers: Pro+, Max, Business, and Enterprise. Gemini 3.8 Flash has a broader rollout and is available to every paid plan, starting at the entry-level Pro tier. GitHub notes the rollout for Gemini is still in progress, so not every user will see it immediately.

More model options inside a single tool means less context-switching between separate AI services. For teams already on Business or Enterprise plans, having both Claude Fable 5.1 and Gemini 3.8 Flash available in the same interface is a practical consolidation. Our coverage of AI dev jargon like “harnesses” and agent loops explains the terminology appearing more frequently in these changelogs.

Why content exclusions matter for agentic workflows

Until this release, content exclusion rules (settings that prevent specific files or directories from being sent to Copilot’s models) only applied in certain editors. Agentic workflows, where Copilot autonomously runs multi-step tasks, could bypass those boundaries. Now the Copilot app and CLI both respect the same exclusions, so secrets files, proprietary configs, or legally sensitive code stay out of model context regardless of how Copilot is invoked.

This is a meaningful compliance fix. If your team has content exclusions configured, verify they are set up correctly before letting agents run unsupervised. Our AI integration work regularly surfaces exactly this gap: exclusion policies configured for one surface but not enforced end-to-end.

VS Code 1.136: what Agent Merge actually does

Agent Merge is the headline VS Code feature. Once a pull request is open and reviewed, the agent reads the review comments, applies the requested changes, reruns any failing CI checks, and resolves merge conflicts. The goal is to compress the gap between “approved with comments” and “merged.” It is in public preview, so expect rough edges.

Other VS Code additions

• Multi-root workspaces (experimental): Copilot and Claude agent sessions now span every folder in a multi-root workspace, not just the active one.
• Chat backgrounds (experimental): The Agents window can display built-in patterns or a custom image. Cosmetic, but useful for distinguishing sessions at a glance.
• Chat session hierarchy: Related chats are grouped and flagged when they need attention, making it easier to track parallel agent threads.

JetBrains harness reaches general availability

The Copilot harness for JetBrains IDEs (the internal abstraction layer that standardises how Copilot features are delivered) is now generally available. GitHub says this enables faster feature delivery and better code quality for JetBrains users, who have historically received updates later than VS Code users.

Why it matters

Two trends are accelerating here. First, model choice inside developer tools is becoming a real differentiator. Teams can now match model to task: a faster, cheaper model like Gemini 3.8 Flash for autocomplete and a more capable one for complex refactors. Second, agentic features are moving from demos to production fixtures. Agent Merge is a direct attempt to automate one of the most tedious parts of shipping code.

For businesses evaluating whether to standardise on Copilot versus maintaining separate AI subscriptions, this week’s release adds weight to the consolidation argument.

Our take

The content exclusion fix is the quietest item in this release and possibly the most important. Any team running Copilot agents on codebases with sensitive files should treat this as a prompt to audit their exclusion policies, not just accept that the fix “makes things safe.” Configurations vary by IDE, and a gap in one surface can expose data you thought was protected.

Agent Merge is promising but public preview means it will break on edge cases, especially complex merge conflicts with interleaved changes. Test it on low-stakes branches before relying on it in a busy main branch. If you need help thinking through how to wire Copilot or other AI tools into your team’s actual workflow, get in touch and we can talk through what’s production-ready today.

What to do about it

1. Check your Copilot plan tier and confirm whether Claude Fable 5.1 or Gemini 3.8 Flash is now available in your model selector.
2. Review your content exclusion settings in each surface (editor, CLI, app) to confirm sensitive paths are consistently excluded.
3. Enable Agent Merge in VS Code 1.136 on a low-stakes feature branch and test it against a PR with minor review comments before using it on main.
4. If you use JetBrains, update the Copilot plugin to pick up the now-stable harness and expect faster feature parity with VS Code going forward.

The content exclusion fix alone is worth a quick audit of your Copilot policy settings this week.

Source: GitHub Changelog