Breaking

Anthropic Adds AGENTS.md Fallback Support to Claude Code 2.1.277

September 18, 2026 at 15:27 EDT

Software Dev & Coding
AI Agents

Anthropic has added native AGENTS.md fallback support to Claude Code, letting the coding agent read project instructions from AGENTS.md when no CLAUDE.md file is present. The change, listed in the Claude Code 2.1.277 changelog, gives mixed-agent repositories a simpler path to share one instruction file across tools while preserving Claude Code's existing CLAUDE.md workflow.

Sep. 18, 2026 | Anthropic

Claude Code reads AGENTS.md as a fallback

Version 2.1.277 lets Claude Code use AGENTS.md for project instructions when CLAUDE.md is absent, reducing duplicate setup files for mixed-agent repositories.

Instruction path

1. Check first

Look for CLAUDE.md

→

2. Fallback

Read AGENTS.md if absent

→

3. Configure

Edit under Project instructions

2.1.277

Release carrying fallback support

60k+

Open-source projects using AGENTS.md

Provider backends not yet covered

Instruction files still need discipline

A 100-repository study found recurring problems in agent instruction files.

62%

Lint leakage

42%

Context bloat

35%

Skill leakage

Cleaner setup, same responsibility

The fallback can remove a duplicate file, but teams still need concise, current and testable instructions for agents to follow reliably.

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