Which AI Coding Tools Read Which Instruction File? 9 Tools

A coding agent reads a Markdown file at the root of your repository before it does anything, and the filenames have multiplied: CLAUDE.md, AGENTS.md, GEMINI.md, a rules folder, a Copilot instructions file. On September 18, 2026 Claude Code version 2.1.277 began reading AGENTS.md in any project that has no CLAUDE.md. That makes the practical question sharper for every team using more than one tool: which file does each tool read, what wins when several exist, and do you need to keep two?

This matrix answers those questions for nine tools from each vendor's own documentation, read on September 22, 2026: Claude Code, OpenAI Codex, Cursor, GitHub Copilot, Gemini CLI, Google Antigravity, Cline, Continue and the AGENTS.md convention's own site. Where a vendor does not state a behaviour, the cell says "not documented" rather than guessing. This blog runs on Anthropic's models; Claude Code is one row on the same terms as the rest.

Claude Code's repository changelog carries version headings but no dates, so the date comes from the npm registry: version 2.1.277 was published at 16:22 UTC on September 18, 2026. The changelog entry is one line, quoted in full below. Anthropic's memory documentation adds the detail: reading AGENTS.md directly needs 2.1.277 or later, it does not yet work in sessions that cannot fetch feature flags (Amazon Bedrock and other third-party providers, or telemetry switched off), and in those sessions the workaround is to import AGENTS.md from a CLAUDE.md.

The change matters beyond one product because AGENTS.md has become the closest thing to a shared filename. Its own site, which is stewarded by the Agentic AI Foundation under the Linux Foundation, claims adoption in over 60,000 open-source projects; that figure is the site's own and we have not audited it. Our post on the ETH Zurich study covers what such a file costs in tokens; this post covers only who reads it.

"Project files" are the filenames the tool looks for inside a repository. "User-level" is a file outside the repository that applies to everything you open. "Nested" asks whether a second file deeper in the tree is read as well. The Gemini CLI row is kept because its documentation still describes the behaviour and the open-source project is still receiving commits; Google's May 19, 2026 announcement retired the hosted free, Pro and Ultra tiers on June 18, 2026, while Code Assist Standard and Enterprise licences kept access.

The same nine tools on the three questions that decide whether a long file is a cost or a liability: what wins when files conflict, whether a size limit is enforced, and whether the file is sent on every request or only once.

Precedence is where teams get surprised, because three different designs hide behind the same word. In Codex, precedence is position: every file from the global scope down to the working directory is concatenated, and a closer file overrides an earlier one only because the model sees it later in the prompt. In Cursor and Copilot, precedence is a merge order with every relevant set still supplied. In Claude Code, the default is an either-or: if any CLAUDE.md-family file exists at or above the working directory, AGENTS.md is not read at all.

That either-or has edges worth knowing. The user-level ~/.claude/CLAUDE.md, a managed policy file and the .claude/rules/ folder do not count for the check, so they keep loading alongside AGENTS.md. And an AGENTS.md read through the setting behaves differently from a CLAUDE.md: it is not listed by the memory and context commands, and it does not fire the hooks that a loaded CLAUDE.md fires. Claude Code exposes four settings, shown below.

Three lines cover most teams. Keep the instructions that every tool needs in AGENTS.md, because it is the file most tools read by name. Put anything specific to one tool in that tool's own file or rules folder. And if you use Claude Code alongside anything else, decide the either-or deliberately rather than by accident.

What goes inside the file is a separate question. We cut ours and measured what broke in our instruction-file audit; the plugin layer that sits beside these files is in our post on what still does not port between agents. If you are standardising coding agents across a team, our AI transformation service starts with this matrix for your toolset.

Cursor's current rules page does not mention the legacy .cursorrules file, so we make no claim that Cursor still reads it; Cline's page lists it as a format Cline reads, which is not evidence about Cursor. Continue's rules page never mentions AGENTS.md, a user-level path, a size limit or nested files; a closed GitHub issue asking for AGENTS.md support is not documentation, so all four cells say not documented. Antigravity documents its surfaces but no precedence rule between GEMINI.md, AGENTS.md and its rules folder.

Two dating caveats apply to the whole table. The npm publish time is the best public timestamp for a Claude Code release, not an Anthropic-stated date. And every documentation page was read two days after this post's editorial date and prints no last-updated date of its own, so no cell can be proven to have been on the page on September 20. The only hard-dated sources are the npm registry, Google's May 19 announcement and the GitHub repository metadata.