Which instruction file does each coding agent actually load? Measured with canaries

Part of study S003 of Agent Errata · 10 harnesses · last measured 2026-11-07 · re-measured automatically on every release

With AGENTS.md and CLAUDE.md in the same repo, Claude Code reads only CLAUDE.md; Codex CLI, Qwen Code, opencode, Kilo CLI, goose and pi read only AGENTS.md; Crush reads both; Gemini CLI and aider read neither.

These are not read from docs. Each harness was pointed at a local server that records the request body, with no model, no API key and an empty home directory. Every instruction file carried a unique canary token, and we grepped the request the harness actually sent for them. Default settings, non-interactive mode, Linux.

Per harness

Claude Code 2.1.319 (measured 2026-11-07): with AGENTS.md and CLAUDE.md side by side it reads only CLAUDE.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is read and the repo's AGENTS.md is dropped. With all 17 files present it loads 2: CLAUDE.local.md, CLAUDE.md.
Codex CLI 0.167.0 (measured 2026-11-06): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 1: AGENTS.override.md.
Gemini CLI 0.67.0 (measured 2026-11-05): with AGENTS.md and CLAUDE.md side by side it reads neither. In a monorepo, started in a child package, it reads neither the root nor the child AGENTS.md. A CLAUDE.md above the git root is ignored, and so is the repo's AGENTS.md. With all 17 files present it loads 1: GEMINI.md.
Qwen Code 0.26.9 (measured 2026-11-07): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 2: AGENTS.md, QWEN.md.
opencode 1.19.5 (measured 2026-10-28): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 1: AGENTS.md.
Kilo CLI 7.8.27 (measured 2026-11-05): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 2: AGENTS.md, .kilocode/rules/s003.md.
Crush 0.100.0 (measured 2026-11-06): with AGENTS.md and CLAUDE.md side by side it reads both. In a monorepo, started in a child package, it reads only child/AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 8: AGENTS.md, CLAUDE.local.md, CLAUDE.md, .github/copilot-instructions.md, CRUSH.md, .cursorrules, .cursor/rules/s003.mdc, GEMINI.md.
goose 1.57.0 (measured 2026-10-29): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is ignored; the repo's AGENTS.md loads. With all 17 files present it loads 2: AGENTS.md, .goosehints.
aider 0.86.2 (measured 2026-10-04): with AGENTS.md and CLAUDE.md side by side it reads neither. In a monorepo, started in a child package, it reads neither the root nor the child AGENTS.md. A CLAUDE.md above the git root is ignored, and so is the repo's AGENTS.md. With all 17 files present it loads 0.
pi 1.5.0 (measured 2026-11-02): with AGENTS.md and CLAUDE.md side by side it reads only AGENTS.md. In a monorepo, started in a child package, it reads both the root and the child AGENTS.md. A CLAUDE.md above the git root is read alongside the repo's AGENTS.md. With all 17 files present it loads 2: AGENTS.override.md, AGENTS.md above the git root.

The 17 files: AGENTS.md, AGENTS.override.md, CLAUDE.md, CLAUDE.local.md, GEMINI.md, QWEN.md, CRUSH.md, CONVENTIONS.md, .cursorrules, .cursor/rules/*.mdc, .windsurfrules, .clinerules, .goosehints, .github/copilot-instructions.md, .kilocode/rules/*.md, sub/AGENTS.md, and an AGENTS.md one level above the git root. Settings change these answers: Claude Code's memory docs list a claude-md-and-agents-md option that reads both files (we measured the default), Gemini CLI reads AGENTS.md once context.fileName names it, and aider reads CONVENTIONS.md with --read.

Harness	Version	AGENTS.md + CLAUDE.md	Files loaded (of 17)	Measured
Claude Code	2.1.319	only `CLAUDE.md`	2	2026-11-07
Codex CLI	0.167.0	only `AGENTS.md`	1	2026-11-06
Gemini CLI	0.67.0	neither	1	2026-11-05
Qwen Code	0.26.9	only `AGENTS.md`	2	2026-11-07
opencode	1.19.5	only `AGENTS.md`	1	2026-10-28
Kilo CLI	7.8.27	only `AGENTS.md`	2	2026-11-05
Crush	0.100.0	both	8	2026-11-06
goose	1.57.0	only `AGENTS.md`	2	2026-10-29
aider	0.86.2	neither	0	2026-10-04
pi	1.5.0	only `AGENTS.md`	2	2026-11-02

Check your own setup (about a minute)

Your harness, version and config may differ from ours. Plant a token, ask a fresh session, score it:

git clone https://github.com/piiiico/agent-errata
cd your-project
S=../agent-errata/studies/S003/canary.sh
$S plant AGENTS.md CLAUDE.md
# fresh session: "Without using any tools, list every token starting with
# S003C- in your instructions or context. If you see none, say NONE."
$S score answer.txt
$S remove   # restores every file byte for byte

Sources and adding a row

Precedence, monorepo walk-up, symlinks, the parent-directory trap and every run: S003 and S003.md. Per-run captures: captures/. Release-by-release changes: changelog. A harness missing here, or a row that disagrees with yours: open a pull request against S003.md.