Claude Code 2.1.278: @AGENTS.md import works, stand-alone AGENTS.md fails

The test exists because the author wanted to separate the official specification from what actually happens on one machine. The current documentation places native support at 2.1.277 and later, with conditions tied to the presence of other instruction files, plus feature enablement and first-session constraints. Rather than assume an updated build behaves the same everywhere, he fixed the versions and compared unique marker strings in responses.

The 12 runs span six conditions each on 2.1.170 and 2.1.278, and every run logged an empty tool, MCP and skills set with no actual tool calls. That matters because it closes off the ordinary route where the model simply opens a file and later repeats what it read. Under those conditions the @AGENTS.md import produced both markers on both builds, while the explicit both-files mode produced a marker only on the new build.

The unresolved point is the lone-placement case on 2.1.278, which returned NONE even though the same build surfaced the AGENTS.md marker when both files were read. The author is careful not to call this a verdict on native support; he simply did not capture the internal load path, and interactive mode, normal tool-enabled runs, user settings and the effect of flags such as --disable-slash-commands were left untested. For teams relying on shared instruction files, the practical reading is that behavior may hinge on the exact launch configuration rather than the version number alone, and a small marker-string check before changing a live project looks like the safer path.