Problem: you run more than one coding agent on the same repo, and every tool reads its project rules from a different file. Keeping those files in sync by hand is tedious, and the moment you forget one, the agents drift out of sync.

Solution: put everything in AGENTS.md and point the others at it.

AGENTS.md is an open Markdown format (agents.md) supported by Codex, Cursor, Junie, and Gemini.

• Codex reads it directly.
• Cursor also reads AGENTS.md at the repo root, in addition to its own .cursor/rules/*.mdc files.
• Junie looks for .junie/AGENTS.md first, then a root AGENTS.md, with .junie/guidelines.md kept as a legacy fallback.
• Gemini CLI defaults to GEMINI.md, but at the time of writing you can optionally add AGENTS.md to context.fileName in .gemini/settings.json:

{
  "context": {
    "fileName": [
      "AGENTS.md",
      "GEMINI.md"
    ]
  }
}

That leaves Claude Code, which still reads CLAUDE.md and, as of mid-2026, does not read AGENTS.md (feature request #6235, one of the most-upvoted open issues in the tracker, no official response yet). There are two ways to fix it.

Option 1: symlink

Claude Code follows symlinks, so make CLAUDE.md point at AGENTS.md:

#!/usr/bin/env bash

ln -s AGENTS.md CLAUDE.md

• Now both names refer to the same file.
• Commit the symlink; git stores it as a link, not a copy.
• The same trick works for any other tool that wants its own filename:

#!/usr/bin/env bash

ln -s AGENTS.md GEMINI.md
ln -s AGENTS.md .junie/guidelines.md

Option 2: import

If you'd rather not deal with symlinks, make CLAUDE.md a one-liner that imports AGENTS.md:

@AGENTS.md

A real second file, but it never changes; Claude pulls in AGENTS.md at runtime.

Proof

The trick: put a codeword in AGENTS.md, then ask each agent for it. If an agent repeats the codeword, it read the file. This script builds a throwaway repo, wires CLAUDE.md both ways (symlink and import), and checks Codex and Claude Code against each. Paste it into a terminal that has the codex and claude CLIs installed:

#!/usr/bin/env bash

set -euo pipefail

QUESTION="What is the project codeword? Reply with only the codeword and nothing else."

# Ask both agents, confirm each answer contains the codeword from AGENTS.md.
verify() {
  local label=$1 expected=$2

  codex exec -s read-only --skip-git-repo-check -o codex-answer.txt "$QUESTION" >/dev/null 2>&1
  local from_codex from_claude
  from_codex=$(cat codex-answer.txt)
  from_claude=$(claude -p "$QUESTION")

  echo "$label:"
  [[ "$from_codex"  == *"$expected"* ]] && echo "  codex  PASS" || { echo "  codex  FAIL (got: $from_codex)";  exit 1; }
  [[ "$from_claude" == *"$expected"* ]] && echo "  claude PASS" || { echo "  claude FAIL (got: $from_claude)"; exit 1; }
}

workdir=$(mktemp -d)

# Option 1: CLAUDE.md is a symlink to AGENTS.md
mkdir "$workdir/symlink" && cd "$workdir/symlink"
echo "If asked for the project codeword, reply with exactly: SYMLINK-OK" > AGENTS.md
ln -s AGENTS.md CLAUDE.md
verify "symlink" "SYMLINK-OK"

# Option 2: CLAUDE.md imports AGENTS.md
mkdir "$workdir/import" && cd "$workdir/import"
echo "If asked for the project codeword, reply with exactly: IMPORT-OK" > AGENTS.md
echo "@AGENTS.md" > CLAUDE.md
verify "import" "IMPORT-OK"

rm -rf "$workdir"
echo "All good."

Output, run in 2026 with Codex (codex-cli 0.138.0) and Claude Code (2.1.169):

symlink:
  codex  PASS
  claude PASS
import:
  codex  PASS
  claude PASS
All good.

References

• AGENTS.md, the open standard
• Claude Code memory and CLAUDE.md
• Custom instructions with AGENTS.md (Codex)
• Provide context with GEMINI.md files (Gemini CLI)
• Rules and AGENTS.md (Cursor)
• Guidelines and memory (Junie)
• Symlink AGENTS.md to tool-specific files (SSW.Rules)
• Feature Request: Support AGENTS.md (#6235)