Trail URLs — the shared reading-context format

A trail URL serializes a reading context: an ordered path of panes, root → focus, exactly as the Reading Room lays it out on screen. It is the context object humans and agents exchange (ADR 0005 decision 14): an agent mints a trail to hand a human its reasoning chain, reconstructed pane by pane; an agent parses a human's trail to load what they were reading. The format is stable and constructible — nothing in it requires the room.

The canonical copy of this document is published in-universe; the repository copy at docs/trail-format.md (repo latebit-io/demarkus-library) is its source. The companion style document is the authoring contract.

Format

/t/<pane>/~/<pane>/~/<pane>?focus=<i>

• Pane chunk — the tail of the pane's standalone /w/ route:
  • document: <world>/d/<path> — e.g. soul.demarkus.io/d/adr/0005.md
  • listing: <world>/d/<dir>/ (trailing slash) — e.g. root/d/plans/
  • tag page: <world>/tags/<tag> — e.g. root/tags/architecture
  • the floor: the single segment u — the universe view (pane zero), no world part because the floor IS the whole universe. / redirects to /t/u.
• ~ — the reserved separator: a path segment that is exactly ~ between consecutive pane chunks.
• focus — 0-based index of the focused pane (full render + margin + live fetch). Omitted ⇒ the last pane. Out of range ⇒ clamped.
• Worlds are knowledge-system names (root) or demarkus hosts (soul.demarkus.io, wiki.example.org:6310). Paths keep raw slashes — no percent-encoded /.
• Depth cap: 10 panes. URLs with more are rejected (400). When a click would push past the cap, the room drops the oldest pane.

Example — the floor, a tag page, and a document, attention on the tag page:

/t/u/~/root/tags/adr/~/root/d/adr/0005.md?focus=1

Semantics (what the room guarantees)

• The trail is a single reasoning path, not a browsing history. A link clicked in pane N discards panes right of N and appends the target, focusing it. A target already on the path is focused, never duplicated.
• The whole canvas renders from the URL alone — server state, no client state. Sharing the URL shares the exact reconstruction.
• Every link the room renders already carries its post-click trail URL, so fetching a trail page and following hrefs is itself a valid way for an agent to walk the space of next states.
• The floor pane renders the same data an agent reads via mark_worlds plus per-world mark_lookup with the match-all query * (importance order) — the projection adds layout, never information.

Minting (agent → human)

Concatenate route tails with /~/, set focus if not last. Rules:

• ≤ 10 panes; each pane's world+path must read (an unreadable unfocused pane renders as a tombstone, the trail survives; an unreadable focused pane is the page's error).
• Order IS the argument: pane 0 is where the reasoning starts, focus is where you want the reader's attention to land. Starting at u hands the reader the map before the path.
• Don't percent-encode slashes; do percent-encode characters that are not valid in a URL path segment.

Parsing (human → agent)

Split the path after /t/ on /~/; each chunk is <world>/<kind>/<value> (kind d ⇒ document path /<value>, kind tags ⇒ tag page) or the bare u floor chunk. Read the documents over mark:// (mark://<world>/<path>) in order — that ordered list, plus the focus index, is the reader's current context.