Two Files, One Node Path

A node's path comes from where its FILE sits, not from what the file says. The package installer (PackageInstaller.NodePathForFile), GitSync (GitHubSyncService.ParseFile) and the file-system adapter all map a repo-relative file through NodeFileMapper.FromRelativePath: the extension is stripped, X/index.* folds onto X, and mesh paths compare case-insensitively. So each of these pairs is ONE node:

Nothing refuses the pair at import. The bulk writer treats a duplicate path as last-writer-wins (StaticRepoImporter, seenInStage), and which file is written last depends on enumeration order — so one author's node silently replaces another's: no error, no log line, nothing to grep.

The rule the guard mirrors

.github/scripts/check-node-path-collisions.py applies the importers' own rule, file for file:

• Node-file extensions are .md, .cs and .json, case-insensitive — the built-in parsers of FileFormatParserRegistry. Every parser MeshWeaver.Plugins contributes (agent, skill, slide) claims .md only; a module that contributes a NEW extension must add it to the guard too.
• A .json is a node only when it looks like one — an object carrying $type, id or nodeType (JsonFileParser.LooksLikeMeshNode). A package manifest beside a node never collides. A .json that is not strict JSON is not compared — but it is NAMED on a warning line, because the hub's serializer options may accept a comment-bearing file this parser refuses, and a same-stem sibling of one would then be exactly the silent collision this guard exists for.
• Not a node by design (PackageInstaller.IsNotANodeFile): the tree-root README.md, any manifest.lock, every file under a content/ segment (an asset, ContentAssetMapper), and a node repo's top-level src/ (module sources).

Every colliding group is reported with ALL its files, and an empty scan is red — a gate over nothing passes vacuously. The self-test proves each half of the rule can fail: every case asserts its own expected outcome (its banner counts the cases it actually ran), and each of eight mutants of the rule (no extension strip, no case fold, no index fold, no content filter, every .json a node, src/ scanned, the file-system pair admitted everywhere, the vacuity line removed) reds it.

Where it runs

• Every node repo, through the shared validate lane (node-repo-validate.yml), fetched at the lane's scripts ref like every other central guard: --root ., the whole Git-visible tree except top-level dot-directories and src/. That is deliberately WIDER than the package enumeration — the canonical gen-manifests.py --list-packages refuses a repo without a gen-manifests.config.json, and two lane callers have none. Over-inclusion can only add files to compare; measured over every caller's main it found no group outside a package.
• Core, in CI's own shell: the self-test, then the doc tree and the samples tree.

The one pair that is merged on purpose

FileSystemStorageAdapter.MergeIndexMarkdownAsync reads X.json and lays X/index.md in as its content — the "JSON registry + index.md split". Only that adapter merges; the installer and GitSync do not. So the pair is admitted only with --filesystem-layout, only for a --tree, and only as exactly those two files — the same pair in a node repo is an ordinary collision.

Core's samples tree used it for one pair, ACME.json + ACME/index.md, and CI passed --filesystem-layout for that tree. ACME left core with the other product demos (see Carving Projects Out Of Core). Its new home in MeshWeaver.Plugins keeps ACME/index.md alone, because that repo's --root check refuses the pair. So core's samples tree is now checked WITHOUT the flag, and no tree in the fleet relies on the exemption. The flag stays in the script for a FileSystem-served tree that genuinely needs it.

Measured when the guard landed

Every caller's default branch plus core: 19,569 files, 16,353 node candidates. One group, core's samples/Graph/Data/ACME.json + ACME/index.md — the adapter's merged pair above. No collision in MeshWeaver.Plugins, .Education, .Reinsurance, .SocialMedia, .Manufacturing, .Crm or Memex.