diff --git a/AGENTS.md b/AGENTS.md
index 950e309..56a43bc 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -81,10 +81,10 @@ cd web
 flask --app app.py run
 ```
 
-The web dependencies (Flask) are not in `pyproject.toml`; install separately:
+Install the package with the `web` extra to include Flask:
 
 ```
-pip install flask
+pip install -e ".[web]"
 ```
 
 ## Type checking
diff --git a/README.md b/README.md
index 9ee0234..c25e469 100644
--- a/README.md
+++ b/README.md
@@ -88,8 +88,8 @@ routes as GeoJSON.
 #### Running the dev server
 
 ```
-cd web
-flask --app app.py run
+pip install -e ".[web]"
+flask --app web/app.py run
 ```
 
 Open `http://127.0.0.1:5000`.
diff --git a/pyproject.toml b/pyproject.toml
index 45ed064..cb66129 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,11 +15,15 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+web = [
+    "flask>=3.0",
+]
 dev = [
     "mypy",
     "pytest",
     "responses",
     "types-requests",
+    "flask>=3.0",
 ]
 
 [project.scripts]