Add API docs page and update README/AGENTS

- Add /docs route serving web/templates/api.html: full Bootstrap 5
  documentation page covering all three API endpoints with parameter
  tables, example requests, and example responses
- Add 'API docs' link to the navbar on the main map page
- Update README.md: add web frontend section with feature list, dev
  server instructions, and API endpoint summary table
- Update AGENTS.md: add web/ layout, API endpoint table, Flask run
  instructions, and route_master example relation IDs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -4,9 +4,8 @@ Guidelines for AI coding agents working in this repository.
 
 ## Project overview
 
-A collection of Python CLI tools for working with OpenStreetMap data. Each
+A collection of Python tools for working with OpenStreetMap public transport
-tool is a standalone Python script using Click for the CLI interface and
+data. Includes a CLI tool and a Flask web frontend with a JSON API.
-Requests for HTTP calls.
 
 ## Repository layout
 
@@ -15,10 +14,20 @@ pyproject.toml              - Package metadata and build configuration
 src/
   osm_geojson/
     __init__.py             - Top-level package marker
+    py.typed                - Marker for mypy typed package
     pt/
       __init__.py           - Public transport subpackage
       core.py               - Data fetching and processing functions
       cli.py                - Click CLI commands
+web/
+  app.py                    - Flask application (routes + API endpoints)
+  templates/
+    index.html              - Main map UI
+    api.html                - API documentation page (served at /docs)
+  static/
+    app.js                  - Frontend JavaScript
+    style.css               - CSS for the map UI
+    favicon.svg             - SVG favicon
 tests/
   fixtures/                 - Saved OSM API responses for offline testing
   test_osm_pt_geojson.py    - Test suite
@@ -35,10 +44,13 @@ point registered in `pyproject.toml` under `[project.scripts]`.
 - Python 3.11+
 - CLI via [Click](https://click.palletsprojects.com/)
 - HTTP via [Requests](https://requests.readthedocs.io/)
+- Web frontend via [Flask](https://flask.palletsprojects.com/) 3.x
 - Parse XML with lxml if needed; prefer the OSM JSON API where possible
-- Errors go to stderr; data output goes to stdout
+- Errors go to stderr; data output goes to stdout (CLI)
 - GeoJSON output uses `ensure_ascii=False`
 - All modules, functions, and test functions must have docstrings
+- Library code (`core.py`) raises `OsmError` instead of calling `sys.exit()`;
+  the CLI and Flask views catch `OsmError` and handle it appropriately
 
 ## OSM API
 
@@ -51,6 +63,30 @@ https://www.openstreetmap.org/api/0.6/
 No authentication is required for read-only access. Include a descriptive
 `User-Agent` header in all requests.
 
+## Web frontend API endpoints
+
+| Endpoint | Description |
+|---|---|
+| `GET /api/route/<id>` | Full route GeoJSON, stop list, and sibling routes |
+| `GET /api/segment/<id>?from=NAME&to=NAME[&stops=0]` | Segment between two named stops |
+| `GET /api/route_master/<id>` | All member routes of a route_master |
+| `GET /docs` | API documentation page |
+
+All API responses are JSON. Errors return `{"error": "<code>", "message": "<text>"}`.
+
+## Running the web frontend
+
+```
+cd web
+flask --app app.py run
+```
+
+The web dependencies (Flask) are not in `pyproject.toml`; install separately:
+
+```
+pip install flask
+```
+
 ## Type checking
 
 All code uses type hints. Run mypy in strict mode to check:
@@ -74,7 +110,9 @@ API. Fixture data is stored in `tests/fixtures/` as saved API responses.
 
 | ID       | Description                             |
 |----------|------------------------------------------|
-| 15083963 | M11 Istanbul Metro (subway)              |
+| 15083963 | M11 Istanbul Metro (subway, direction 1) |
+| 15083964 | M11 Istanbul Metro (subway, direction 2) |
+| 15083966 | M11 Istanbul Metro (route_master)        |
 | 18892969 | Bus A1: Bristol Airport → Bus Station    |
 
 ## Dependencies

README.md

@@ -1,6 +1,6 @@
 # openstreetmap-tools
 
-A collection of command-line tools for working with OpenStreetMap data.
+A collection of tools for working with OpenStreetMap public transport data.
 
 ## Tools
 
@@ -9,7 +9,7 @@ A collection of command-line tools for working with OpenStreetMap data.
 Fetch an OSM public transport route relation and list its stops, or export the
 route as GeoJSON.
 
-#### Usage
+#### CLI usage
 
 ```
 osm-pt-geojson list-stations <relation_id>
@@ -67,6 +67,46 @@ cd openstreetmap-tools
 pip install -e .
 ```
 
+---
+
+### Web frontend
+
+An interactive map interface for browsing and downloading public transport
+routes as GeoJSON.
+
+#### Features
+
+- Enter a relation ID or OSM URL to load a route onto the map.
+- Click stops in the sidebar list or on the map to set start and end points.
+- Preview the selected segment highlighted on the map.
+- Toggle stop points in/out of the GeoJSON before downloading.
+- Download a segment or the full route as a `.geojson` file.
+- Entering a `route_master` ID shows all directions on the map with links to
+  each individual route.
+- Bookmarkable URLs: `/<relation_id>` loads that route directly.
+
+#### Running the dev server
+
+```
+cd web
+flask --app app.py run
+```
+
+Open `http://127.0.0.1:5000`.
+
+#### API
+
+The web frontend exposes a JSON API. Full documentation is available at `/docs`
+when the server is running.
+
+| Endpoint | Description |
+|---|---|
+| `GET /api/route/<id>` | Full route GeoJSON, stop list, and sibling routes |
+| `GET /api/segment/<id>?from=NAME&to=NAME` | Segment between two named stops |
+| `GET /api/route_master/<id>` | All member routes of a route_master |
+
+---
+
 ## Licence
 
 MIT License. Copyright (c) 2026 Edward Betts.

web/app.py

@@ -80,6 +80,12 @@ def check_route_tags(
     return None
 
 
+@app.route("/docs")
+def docs() -> ResponseReturnValue:
+    """Render the API documentation page."""
+    return render_template("api.html")
+
+
 @app.route("/")
 def index() -> ResponseReturnValue:
     """Render the landing page with no relation loaded."""

web/templates/api.html

@@ -0,0 +1,269 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>API Documentation – OSM Public Transport → GeoJSON</title>
+  <link rel="icon" type="image/svg+xml" href="{{ url_for('static', filename='favicon.svg') }}">
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css">
+  <style>
+    pre { background: #f8f9fa; border: 1px solid #dee2e6; border-radius: 6px; padding: 1rem; font-size: 0.85rem; }
+    .endpoint-url { font-family: monospace; font-size: 1rem; }
+    .badge-get { background: #0d6efd; }
+    h2 { margin-top: 2.5rem; }
+    h3 { margin-top: 2rem; font-size: 1.1rem; }
+  </style>
+</head>
+<body>
+
+<nav class="navbar navbar-dark bg-dark px-3" style="height:56px">
+  <a class="navbar-brand" href="/">OSM Public Transport → GeoJSON</a>
+  <a class="nav-link text-white" href="/docs">API docs</a>
+</nav>
+
+<div class="container py-5" style="max-width:860px">
+
+  <h1 class="mb-1">API Documentation</h1>
+  <p class="text-muted mb-4">JSON API for fetching OSM public transport routes as GeoJSON.</p>
+
+  <p>All endpoints are read-only and require no authentication. Data is fetched
+  live from the <a href="https://wiki.openstreetmap.org/wiki/API_v0.6">OSM API v0.6</a>.
+  Relation IDs can be found on
+  <a href="https://www.openstreetmap.org">openstreetmap.org</a> — search for a
+  route and click through to its relation page.</p>
+
+  <p>All error responses return JSON with at least <code>error</code> (machine-readable
+  code) and <code>message</code> (human-readable description) fields.</p>
+
+  <!-- ── GET /api/route/<relation_id> ──────────────────────────────────── -->
+  <h2>
+    <span class="badge badge-get me-2" style="background:#0d6efd;font-size:0.75rem;vertical-align:middle">GET</span>
+    <span class="endpoint-url">/api/route/<em>&lt;relation_id&gt;</em></span>
+  </h2>
+  <p>Returns the full route as a GeoJSON <code>FeatureCollection</code>, the
+  ordered list of stops, and links to sibling routes in the same
+  <code>route_master</code> (if any).</p>
+
+  <h3>Path parameters</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Parameter</th><th>Type</th><th>Description</th></tr></thead>
+    <tbody>
+      <tr><td><code>relation_id</code></td><td>integer</td><td>OSM relation ID of a public transport route.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Success response <span class="badge bg-success">200 OK</span></h3>
+<pre>{
+  "name": "M11: Arnavutköy Hastane → Gayrettepe",
+  "ref": "M11",
+  "stops": [
+    { "name": "Arnavutköy Hastane", "lat": 41.1234, "lon": 28.7654 },
+    { "name": "İstanbul Havalimanı", "lat": 41.2701, "lon": 28.7519 },
+    ...
+  ],
+  "geojson": {
+    "type": "FeatureCollection",
+    "features": [
+      {
+        "type": "Feature",
+        "geometry": { "type": "LineString", "coordinates": [[28.765, 41.123], ...] },
+        "properties": { "name": "M11: Arnavutköy Hastane → Gayrettepe",
+                        "ref": "M11", "from": "Arnavutköy Hastane",
+                        "to": "Gayrettepe", "route": "subway" }
+      },
+      {
+        "type": "Feature",
+        "geometry": { "type": "Point", "coordinates": [28.765, 41.123] },
+        "properties": { "name": "Arnavutköy Hastane" }
+      },
+      ...
+    ]
+  },
+  "other_directions": [
+    { "id": 15083964, "name": "M11: Gayrettepe → Arnavutköy Hastane", "ref": "M11" }
+  ]
+}</pre>
+
+  <p>The <code>geojson</code> field is a <code>FeatureCollection</code> containing
+  one <code>LineString</code> for the route geometry followed by one
+  <code>Point</code> per stop. The <code>LineString</code> carries the route tags
+  as properties (<code>name</code>, <code>ref</code>, <code>from</code>,
+  <code>to</code>, <code>route</code>). Each <code>Point</code> has a
+  <code>name</code> property.</p>
+
+  <p><code>other_directions</code> lists sibling routes from the same
+  <code>route_master</code> relation. It is an empty array if the route has no
+  parent <code>route_master</code>.</p>
+
+  <h3>Error responses</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Status</th><th><code>error</code></th><th>When</th></tr></thead>
+    <tbody>
+      <tr><td>404</td><td><code>osm_error</code></td><td>Relation not found on OpenStreetMap.</td></tr>
+      <tr><td>422</td><td><code>is_route_master</code></td><td>Relation is a <code>route_master</code>. Use <code>/api/route_master/&lt;id&gt;</code> instead.</td></tr>
+      <tr><td>422</td><td><code>not_public_transport</code></td><td>Relation exists but is not a supported public transport route.</td></tr>
+      <tr><td>502</td><td><code>osm_error</code></td><td>OSM API returned an unexpected error.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Example</h3>
+  <pre>GET /api/route/15083963</pre>
+
+  <!-- ── GET /api/segment/<relation_id> ────────────────────────────────── -->
+  <h2>
+    <span class="badge me-2" style="background:#0d6efd;font-size:0.75rem;vertical-align:middle">GET</span>
+    <span class="endpoint-url">/api/segment/<em>&lt;relation_id&gt;</em></span>
+  </h2>
+  <p>Returns a GeoJSON <code>FeatureCollection</code> for the portion of the
+  route between two named stops. Stops are matched case-insensitively.</p>
+
+  <h3>Path parameters</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Parameter</th><th>Type</th><th>Description</th></tr></thead>
+    <tbody>
+      <tr><td><code>relation_id</code></td><td>integer</td><td>OSM relation ID of a public transport route.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Query parameters</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Parameter</th><th>Required</th><th>Description</th></tr></thead>
+    <tbody>
+      <tr><td><code>from</code></td><td>Yes</td><td>Name of the start stop (case-insensitive).</td></tr>
+      <tr><td><code>to</code></td><td>Yes</td><td>Name of the end stop (case-insensitive).</td></tr>
+      <tr><td><code>stops</code></td><td>No</td><td>Include stop <code>Point</code> features. <code>1</code> (default) to include, <code>0</code> to omit.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Success response <span class="badge bg-success">200 OK</span></h3>
+<pre>{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "geometry": { "type": "LineString", "coordinates": [[28.800, 41.259], ...] },
+      "properties": { "name": "M11: Arnavutköy Hastane → Gayrettepe",
+                      "ref": "M11", "from": "Arnavutköy Hastane",
+                      "to": "Gayrettepe", "route": "subway" }
+    },
+    {
+      "type": "Feature",
+      "geometry": { "type": "Point", "coordinates": [28.800, 41.259] },
+      "properties": { "name": "İstanbul Havalimanı" }
+    },
+    {
+      "type": "Feature",
+      "geometry": { "type": "Point", "coordinates": [28.820, 41.247] },
+      "properties": { "name": "Hasdal" }
+    }
+  ]
+}</pre>
+
+  <p>The <code>LineString</code> is trimmed to the shortest path between the
+  two stops along the route geometry. The <code>from</code> stop is treated as
+  the start regardless of the order the names are supplied — the segment always
+  follows the route direction.</p>
+
+  <h3>Error responses</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Status</th><th><code>error</code></th><th>When</th></tr></thead>
+    <tbody>
+      <tr><td>400</td><td><code>missing_params</code></td><td><code>from</code> or <code>to</code> parameter is absent.</td></tr>
+      <tr><td>404</td><td><code>osm_error</code></td><td>Relation not found on OpenStreetMap.</td></tr>
+      <tr><td>404</td><td><code>station_not_found</code></td><td>One or both stop names were not found. The response includes an <code>available</code> array listing valid stop names.</td></tr>
+      <tr><td>422</td><td><code>not_public_transport</code></td><td>Relation is not a supported public transport route.</td></tr>
+      <tr><td>502</td><td><code>osm_error</code></td><td>OSM API returned an unexpected error.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Example</h3>
+  <pre>GET /api/segment/15083963?from=İstanbul+Havalimanı&to=Hasdal</pre>
+  <pre>GET /api/segment/15083963?from=İstanbul+Havalimanı&to=Hasdal&stops=0</pre>
+
+  <!-- ── GET /api/route_master/<relation_id> ────────────────────────────── -->
+  <h2>
+    <span class="badge me-2" style="background:#0d6efd;font-size:0.75rem;vertical-align:middle">GET</span>
+    <span class="endpoint-url">/api/route_master/<em>&lt;relation_id&gt;</em></span>
+  </h2>
+  <p>Returns all member routes of an OSM <code>route_master</code> relation,
+  each with its GeoJSON geometry (stops omitted). Use this to display all
+  directions of a line on a map.</p>
+
+  <h3>Path parameters</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Parameter</th><th>Type</th><th>Description</th></tr></thead>
+    <tbody>
+      <tr><td><code>relation_id</code></td><td>integer</td><td>OSM relation ID of a <code>route_master</code> relation.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Success response <span class="badge bg-success">200 OK</span></h3>
+<pre>{
+  "name": "M11",
+  "ref": "M11",
+  "routes": [
+    {
+      "id": 15083963,
+      "name": "M11: Arnavutköy Hastane → Gayrettepe",
+      "ref": "M11",
+      "from": "Arnavutköy Hastane",
+      "to": "Gayrettepe",
+      "geojson": {
+        "type": "FeatureCollection",
+        "features": [
+          {
+            "type": "Feature",
+            "geometry": { "type": "LineString", "coordinates": [[28.765, 41.123], ...] },
+            "properties": { "name": "M11: Arnavutköy Hastane → Gayrettepe", ... }
+          }
+        ]
+      }
+    },
+    {
+      "id": 15083964,
+      "name": "M11: Gayrettepe → Arnavutköy Hastane",
+      "ref": "M11",
+      "from": "Gayrettepe",
+      "to": "Arnavutköy Hastane",
+      "geojson": { ... }
+    }
+  ]
+}</pre>
+
+  <p>Each entry in <code>routes</code> contains the individual route's
+  <code>id</code>, <code>name</code>, <code>ref</code>, <code>from</code>,
+  <code>to</code> tags, and a <code>geojson</code> <code>FeatureCollection</code>
+  with only the <code>LineString</code> geometry (no stop points). If a member
+  route cannot be fetched, it is included with <code>"geojson": null</code>.</p>
+
+  <h3>Error responses</h3>
+  <table class="table table-sm table-bordered">
+    <thead class="table-light"><tr><th>Status</th><th><code>error</code></th><th>When</th></tr></thead>
+    <tbody>
+      <tr><td>404</td><td><code>osm_error</code></td><td>Relation not found on OpenStreetMap.</td></tr>
+      <tr><td>422</td><td><code>osm_error</code></td><td>Relation exists but is not a <code>route_master</code>.</td></tr>
+      <tr><td>502</td><td><code>osm_error</code></td><td>OSM API returned an unexpected error.</td></tr>
+    </tbody>
+  </table>
+
+  <h3>Example</h3>
+  <pre>GET /api/route_master/15083966</pre>
+
+  <!-- ── Supported route types ─────────────────────────────────────────── -->
+  <h2>Supported route types</h2>
+  <p>The following OSM <code>route</code> tag values are accepted:</p>
+  <p>
+    <code>bus</code> &nbsp;
+    <code>ferry</code> &nbsp;
+    <code>funicular</code> &nbsp;
+    <code>light_rail</code> &nbsp;
+    <code>monorail</code> &nbsp;
+    <code>subway</code> &nbsp;
+    <code>train</code> &nbsp;
+    <code>tram</code> &nbsp;
+    <code>trolleybus</code>
+  </p>
+
+</div>
+</body>
+</html>

web/templates/index.html

@@ -13,6 +13,7 @@
 
 <nav class="navbar navbar-dark bg-dark px-3" style="height:56px">
   <a class="navbar-brand" href="/">OSM Public Transport → GeoJSON</a>
+  <a class="nav-link text-white" href="/docs">API docs</a>
 </nav>
 
 <div class="container-fluid h-100 p-0">