# BlockChalk

> BlockChalk was a location-based neighborhood messaging service (2009-2012). Users with
> GPS-enabled phones left short public messages called "chalks" pinned to a geographic point,
> and read the chalks other people had left nearby. The company was founded by Stephen Hood
> (former Delicious product lead) and Dave Baggeroer (Stanford Institute of Design), and
> raised $1M in seed funding in May 2010 from Joshua Schachter, Battery Ventures, Founder
> Collective, Harrison Metal, Mitch Kapor and others.

IMPORTANT: BlockChalk is RETIRED. The product is gone and blockchalk.com does not resolve —
the domain registration is active through 2028 but its delegated nameservers return no
records. There is no live API. Everything in this repository is historical documentation
reconstructed from the provider's own archived developer page and from captured live
responses. Do not attempt to integrate with BlockChalk.

## Historical API surface

BlockChalk published three read-only, anonymous interfaces on its /developers page, all
parameterized by a longitude/latitude pair:

- XML: http://blockchalk.com/api/xml/chalk?long=[longitude]&lat=[latitude]
- JSON: http://blockchalk.com/api/json/chalk?long=[longitude]&lat=[latitude]
- RSS: http://blockchalk.com/rss/chalk?long=[longitude]&lat=[latitude]

A later versioned path is evidenced by captured 200 responses:

- GeoRSS: http://blockchalk.com/api/v0.6/chalks/[latitude],[longitude]

BlockChalk's own caveat on that page: "these interfaces are early prototypes and are likely
to change in the future."

There was no authentication, no API key, no registration, no pagination, no rate-limit
signaling, no documented error contract, no HTTPS endpoint, and no write API. Chalks were
posted through the mobile clients only.

## Artifacts in this repository

- [OpenAPI (reconstruction)](openapi/blockchalk-chalks-openapi.yml): the four documented and
  observed read endpoints, derived from the archived developer page and captured responses.
- [Overlay](overlays/blockchalk-chalks-overlay.yaml): API Evangelist annotations marking the
  retirement posture and evidence links.
- [Lifecycle](lifecycle/blockchalk-lifecycle.yml): retirement status, version history, live
  probe results and a dated timeline.
- [Authentication](authentication/blockchalk-authentication.yml): the anonymous access model.
- [Conventions](conventions/blockchalk-conventions.yml): cross-cutting runtime semantics,
  mostly a record of what was never published.
- [Data model](data-model/blockchalk-data-model.yml): the Chalk entity and the observed
  proximity and neighborhood vocabulary.
- [Conformance](conformance/blockchalk-conformance.yml): RSS 2.0 and GeoRSS conformance;
  no OAuth, no compliance program.
- [Packages](packages/blockchalk-packages.yml): no first-party SDK ever existed; one
  community webOS client.
- [Well-known](well-known/blockchalk-well-known.yml): probe results, all unreachable.
- [Domain security](security/blockchalk-domain-security.yml): TLS/DNS/email posture probe.
- [Examples](examples/): verbatim captured GeoRSS responses for San Francisco and New York.

## Data shape

A chalk is a feed item carrying a title (the message text), a permalink and matching guid
using a 12-character hex id, an RFC 1123 pubDate, a georss:point with latitude and longitude,
and a description holding a human-readable proximity label such as "on this block", "within
a few blocks" or a named neighborhood like "in South of Market".

## Provenance

generated: 2026-07-20
method: generated
source: apis.yml plus the artifacts listed above; upstream evidence is the archived
BlockChalk /developers page (2009-08-10) and captured /api/v0.6/chalks responses (2010).