**This document is canonical.** The ECHO vault holds data only — there are no `CLAUDE.md` / `STRUCTURE.md` / `BOOTSTRAP.md` / `index.md` control docs in it. Layout, taxonomy, and frontmatter conventions live here in the plugin; the bootstrap procedure (`references/bootstrap.md`) builds the tree below into any empty vault.
**Entity index:**`_agent/index/entities.json` is a machine-maintained registry mapping each entity slug to its `{path, kind, title, aliases, last_seen}`. It makes routing/resolve an O(1), alias-aware lookup and supplies the name→path map for cross-linking and recall. `resolve` matches slug/title/alias exactly; when nothing matches exactly it falls back to **fuzzy candidates** — entities sharing a distinctive token with the mention — so a shortened name ("echo memory" → the project `echo`) surfaces the existing note instead of spawning a duplicate. Aliases are auto-derived from titles, learned from mentions on update, and re-folded from note frontmatter on `sweep`. It is rebuilt automatically by `echo.py capture` and by `sweep.py`; do not hand-edit. The only JSON file in the vault.
**Heartbeat:**`_agent/heartbeat/last-session.md` is a one-line pointer (`<session-log-path> @ <ISO-timestamp>`) the **session-logging procedure writes (PUT, overwrite) at session end** and the **loading procedure reads first (Step 4)** as an O(1) shortcut to the latest session log. It is a hint, not a source of truth — fall back to the `sessions/` directory listing if it's missing or stale. Because it's PUT-overwritten, it never grows or duplicates.
`aliases:` is the **durable, Obsidian-native home for alternate names** of an entity (e.g. the project `echo` is also "echo-memory" / "echo plugin"). `capture` auto-derives the obvious case/punctuation variants of the title and writes them here; `sweep` folds frontmatter aliases back into the entity index, so they survive an index rebuild. List plain strings, never `[[wikilinks]]`.
`agent_written: true` + a populated `source_notes` is the key signal separating
agent-managed content from human-authored content. When appending with POST, do
not rewrite frontmatter — the append goes after existing content. To change
`updated:` or `status:`, use PATCH with `Target-Type: frontmatter`.
**Frontmatter field semantics:**
-`created:` is the **earliest known date** the entity was tracked in the vault — *not* "today". When merging notes (e.g. promoting an `on-hold/` project into `active/`), preserve the earliest `created:` from any merged source and only update `updated:`.
-`source_notes` is a **backward link** — the note(s) that triggered or supplied content for this one (e.g. the session log a project update came from). Forward links go in the `## Related` body section, never here.
-`status:` for a project MUST match its folder under `projects/` (`active`, `incubating`, `on-hold`, `archived`). Moving the file and updating `status:` are the same operation.
> **No `[[wikilinks]]` in frontmatter.** YAML parses `[[...]]` as nested lists, so wiki
> links there break and never render as clickable links in reading view. Put all
> cross-references in a **`## Related`** section in the note **body** (bulleted `[[links]]`).
Append observed facts under `## Observations` by default. Promote to `## Fact / Pattern` (dropping the date) once a pattern stabilizes. "The operator" is the vault owner — refer to them generically and in the third person (the display name is the runtime `owner` config value).
See `session-log-template.md`. ECHO uses an **HHMM time component** in the filename — this is **canonical, not optional**. The four-digit local-time component makes filenames lex-sort in true chronological order, which the loading procedure relies on. Older session logs without HHMM exist; leave them alone, but every new one must use the full `YYYY-MM-DD-HHMM-<slug>.md` form.
ADR-style: Context → Decision → Consequences. If the decision belongs to an existing project, PATCH-append the wikilink into that project's `## Key Decisions` heading. Don't use `decisions/by-project/` — it's legacy scaffolding that's intentionally left empty.
`type: company`. An organization the vault owner works with — client, vendor, partner, or employer (e.g. `acme.md`). Distinct from `people/` (individuals, who *belong to* companies) and `references/` (external sources). Lowercase kebab-case slug.
