Properly isolate AGENTS.md content for all phx.new flags

This commit improves upon the previous fix by comprehensively handling
all phx.new flags to ensure AGENTS.md contains 100% accurate information
for any flag combination.

Changes:
- Split cross-cutting usage-rules into feature-specific files:
  * ecto-forms.md - Ecto + HTML/LiveView form integration
  * phoenix-html.md - HTML-specific Phoenix component guidelines
  * phoenix-live.md - LiveView-specific Phoenix layout guidelines

- Cleaned up existing files to remove cross-references:
  * ecto.md - Removed form examples, clarified changeset access
  * elixir.md - Removed LiveView socket example
  * html.md - Removed LiveView/LiveComponent references
  * liveview.md - Removed Ecto-specific form content
  * phoenix.md (template) - Removed HTML component references

- Updated generator with conditional inclusion logic for:
  * phoenix-html.md (when html flag is true)
  * phoenix-live.md (when live flag is true)
  * ecto-forms.md (when both ecto and html flags are true)

- Added comprehensive tests for flag combinations:
  * --no-ecto
  * --no-live
  * --no-html
  * --no-ecto --no-live

This ensures AI agents receive accurate, context-appropriate guidelines
without references to features not included in the project.

Author: bmalum

installer/lib/phx_new/generator.ex

@@ -123,6 +123,8 @@ defmodule Phx.New.Generator do
           # rules specific to new apps
           @new_project_rules_files["project.md"],
           @new_project_rules_files["phoenix.md"],
+          project.binding[:html] && @new_project_rules_files["phoenix-html.md"],
+          project.binding[:live] && @new_project_rules_files["phoenix-live.md"],
           # --no-assets is equivalent to --no-tailwind && --no-esbuild;
           # we check for both here
           project.binding[:javascript] && project.binding[:css] &&
@@ -157,6 +159,13 @@ defmodule Phx.New.Generator do
               @rules_files["liveview.md"],
               "\n<!-- phoenix:liveview-end -->"
             ],
+          # Include ecto-forms when both ecto and html are present
+          project.binding[:ecto] && project.binding[:html] &&
+            [
+              "<!-- phoenix:ecto-forms-start -->\n",
+              @rules_files["ecto-forms.md"],
+              "\n<!-- phoenix:ecto-forms-end -->"
+            ],
           "<!-- usage-rules-end -->"
         ]
         |> Enum.reject(fn part -> part == nil or part == false end)

installer/templates/usage-rules/phoenix-html.md

@@ -0,0 +1,6 @@
+### Phoenix Components
+
+- Out of the box, `core_components.ex` imports an `<.icon name="hero-x-mark" class="w-5 h-5"/>` component for hero icons. **Always** use the `<.icon>` component for icons, **never** use `Heroicons` modules or similar
+- **Always** use the imported `<.input>` component for form inputs from `core_components.ex` when available. `<.input>` is imported and using it will save steps and prevent errors
+- If you override the default input classes (`<.input class="myclass px-2 py-1 rounded-lg">)`) class with your own values, no default classes are inherited, so your
+custom classes must fully style the input

installer/templates/usage-rules/phoenix-live.md

@@ -0,0 +1,6 @@
+### Phoenix LiveView Layout Guidelines
+
+- **Always** begin your LiveView templates with `<Layouts.app flash={@flash} ...>` which wraps all inner content
+- Anytime you run into errors with no `current_scope` assign:
+  - You failed to follow the Authenticated Routes guidelines, or you failed to pass `current_scope` to `<Layouts.app>`
+  - **Always** fix the `current_scope` error by moving your routes to the proper `live_session` and ensure you pass `current_scope` as needed

installer/templates/usage-rules/phoenix.md

@@ -1,12 +1,4 @@
 ### Phoenix v1.8 guidelines
 
-- **Always** begin your LiveView templates with `<Layouts.app flash={@flash} ...>` which wraps all inner content
 - The `MyAppWeb.Layouts` module is aliased in the `my_app_web.ex` file, so you can use it without needing to alias it again
-- Anytime you run into errors with no `current_scope` assign:
-  - You failed to follow the Authenticated Routes guidelines, or you failed to pass `current_scope` to `<Layouts.app>`
-  - **Always** fix the `current_scope` error by moving your routes to the proper `live_session` and ensure you pass `current_scope` as needed
 - Phoenix v1.8 moved the `<.flash_group>` component to the `Layouts` module. You are **forbidden** from calling `<.flash_group>` outside of the `layouts.ex` module
-- Out of the box, `core_components.ex` imports an `<.icon name="hero-x-mark" class="w-5 h-5"/>` component for hero icons. **Always** use the `<.icon>` component for icons, **never** use `Heroicons` modules or similar
-- **Always** use the imported `<.input>` component for form inputs from `core_components.ex` when available. `<.input>` is imported and using it will save steps and prevent errors
-- If you override the default input classes (`<.input class="myclass px-2 py-1 rounded-lg">)`) class with your own values, no default classes are inherited, so your
-custom classes must fully style the input

installer/test/phx_new_test.exs

@@ -518,6 +518,40 @@ defmodule Mix.Tasks.Phx.NewTest do
       assert_file("phx_blog/config/test.exs", fn file ->
         refute file =~ ~s|config :phoenix_live_view|
       end)
+
+      assert_file("phx_blog/AGENTS.md", fn file ->
+        refute file =~ "<.form"
+        refute file =~ "<.input"
+        refute file =~ "LiveView"
+      end)
+    end)
+  end
+
+  test "new with --no-live" do
+    in_tmp("new with no_live", fn ->
+      Mix.Tasks.Phx.New.run([@app_name, "--no-live"])
+
+      assert_file("phx_blog/AGENTS.md", fn file ->
+        refute file =~ "## Phoenix LiveView guidelines"
+        refute file =~ "LiveView streams"
+        refute file =~ "push_event"
+        refute file =~ "handle_event"
+        refute file =~ "phx-hook"
+      end)
+    end)
+  end
+
+  test "new with --no-ecto --no-live" do
+    in_tmp("new with no_ecto and no_live", fn ->
+      Mix.Tasks.Phx.New.run([@app_name, "--no-ecto", "--no-live"])
+
+      assert_file("phx_blog/AGENTS.md", fn file ->
+        refute file =~ "Ecto"
+        refute file =~ "changeset"
+        refute file =~ "## Phoenix LiveView guidelines"
+        refute file =~ "LiveView streams"
+        refute file =~ "phx-hook"
+      end)
     end)
   end
 
@@ -560,6 +594,7 @@ defmodule Mix.Tasks.Phx.NewTest do
         refute file =~ "Ecto"
         refute file =~ "changeset"
         refute file =~ "Ecto.Schema"
+        refute file =~ "Ecto.Changeset"
       end)
     end)
   end

usage-rules/ecto-forms.md

@@ -0,0 +1,45 @@
+## Ecto Forms
+
+### Creating forms from changesets
+
+When using changesets, the underlying data, form params, and errors are retrieved from it. The `:as` option is automatically computed too. E.g. if you have a user schema:
+
+    defmodule MyApp.Users.User do
+      use Ecto.Schema
+      ...
+    end
+
+And then you create a changeset that you pass to `to_form`:
+
+    %MyApp.Users.User{}
+    |> Ecto.Changeset.change()
+    |> to_form()
+
+Once the form is submitted, the params will be available under `%{"user" => user_params}`.
+
+In the template, the form assign can be passed to the `<.form>` function component:
+
+    <.form for={@form} id="todo-form" phx-submit="save">
+      <.input field={@form[:field]} type="text" />
+    </.form>
+
+Always give the form an explicit, unique DOM ID, like `id="todo-form"`.
+
+### Avoiding form errors with changesets
+
+**Always** use a form assigned via `to_form/1` or `to_form/2`, and the `<.input>` component in the template. In the template **always access forms this way**:
+
+    <%!-- ALWAYS do this (valid) --%>
+    <.form for={@form} id="my-form">
+      <.input field={@form[:field]} type="text" />
+    </.form>
+
+And **never** do this:
+
+    <%!-- NEVER do this (invalid) --%>
+    <.form for={@changeset} id="my-form">
+      <.input field={@changeset[:field]} type="text" />
+    </.form>
+
+- You are FORBIDDEN from accessing the changeset in the template as it will cause errors
+- **Never** use `<.form let={f} ...>` in the template, instead **always use `<.form for={@form} ...>`**, then drive all form references from the form assign as in `@form[:field]`. The UI should **always** be driven by a `to_form/1` or `to_form/2` that is derived from a changeset

usage-rules/ecto.md

@@ -4,7 +4,7 @@
 - Remember `import Ecto.Query` and other supporting modules when you write `seeds.exs`
 - `Ecto.Schema` fields always use the `:string` type, even for `:text`, columns, ie: `field :name, :string`
 - `Ecto.Changeset.validate_number/2` **DOES NOT SUPPORT the `:allow_nil` option**. By default, Ecto validations only run if a change for the given field exists and the change value is not nil, so such as option is never needed
-- You **must** use `Ecto.Changeset.get_field(changeset, :field)` to access changeset fields
+- **Never** use map access syntax (`changeset[:field]`) on changesets. You **must** use `Ecto.Changeset.get_field(changeset, :field)` to access changeset fields
 - Fields which are set programatically, such as `user_id`, must not be listed in `cast` calls or similar for security purposes. Instead they must be explicitly set when creating the struct
 - **Always** invoke `mix ecto.gen.migration migration_name_using_underscores` when generating migration files, so the correct timestamp and conventions are applied
 

usage-rules/elixir.md

@@ -18,18 +18,18 @@
   you *must* bind the result of the expression to a variable if you want to use it and you CANNOT rebind the result inside the expression, ie:
 
       # INVALID: we are rebinding inside the `if` and the result never gets assigned
-      if connected?(socket) do
-        socket = assign(socket, :val, val)
+      if some_condition?(data) do
+        data = transform(data)
       end
 
       # VALID: we rebind the result of the `if` to a new variable
-      socket =
-        if connected?(socket) do
-          assign(socket, :val, val)
+      data =
+        if some_condition?(data) do
+          transform(data)
         end
 
 - **Never** nest multiple modules in the same file as it can cause cyclic dependencies and compilation errors
-- **Never** use map access syntax on structs as they do not implement the Access behaviour by default. For structs, you **must** access the fields directly, such as `my_struct.field` or use higher level APIs that are available on the struct if they exist
+- **Never** use map access syntax (`my_struct[:field]`) on structs as they do not implement the Access behaviour by default. For structs, you **must** access the fields directly, such as `my_struct.field` or use higher level APIs that are available on the struct if they exist
 - Elixir's standard library has everything necessary for date and time manipulation. Familiarize yourself with the common `Time`, `Date`, `DateTime`, and `Calendar` interfaces by accessing their documentation as necessary. **Never** install additional dependencies unless asked or for date/time parsing (which you can use the `date_time_parser` package)
 - Don't use `String.to_atom/1` on user input (memory leak risk)
 - Predicate function names should not start with `is_` and should end in a question mark. Names like `is_thing` should be reserved for guards

usage-rules/html.md

@@ -2,9 +2,9 @@
 
 - Phoenix templates **always** use `~H` or .html.heex files (known as HEEx), **never** use `~E`
 - **Always** use the imported `Phoenix.Component.form/1` and `Phoenix.Component.inputs_for/1` function to build forms. **Never** use `Phoenix.HTML.form_for` or `Phoenix.HTML.inputs_for` as they are outdated
-- When building forms **always** use the already imported `Phoenix.Component.to_form/2` (`assign(socket, form: to_form(...))` and `<.form for={@form} id="msg-form">`), then access those forms in the template via `@form[:field]`
+- When building forms **always** use the already imported `Phoenix.Component.to_form/2` (`assign(:form, to_form(...))` and `<.form for={@form} id="msg-form">`), then access those forms in the template via `@form[:field]`
 - **Always** add unique DOM IDs to key elements (like forms, buttons, etc) when writing templates, these IDs can later be used in tests (`<.form for={@form} id="product-form">`)
-- For "app wide" template imports, you can import/alias into the `my_app_web.ex`'s `html_helpers` block, so they will be available to all LiveViews, LiveComponent's, and all modules that do `use MyAppWeb, :html` (replace "my_app" by the actual app name)
+- For "app wide" template imports, you can import/alias into the `my_app_web.ex`'s `html_helpers` block, so they will be available to all templates and all modules that do `use MyAppWeb, :html` (replace "my_app" by the actual app name)
 
 - Elixir supports `if/else` but **does NOT support `if/else if` or `if/elsif`**. **Never use `else if` or `elseif` in Elixir**, **always** use `cond` or `case` for multiple conditionals.
 

usage-rules/liveview.md

@@ -186,22 +186,10 @@ You can also specify a name to nest the params:
       {:noreply, assign(socket, form: to_form(user_params, as: :user))}
     end
 
-Always give the form an explicit, unique DOM ID, like `id="todo-form"`.
-
-#### Avoiding form errors
+In the template, the form assign can be passed to the `<.form>` function component:
 
-**Always** use a form assigned via `to_form/2` in the LiveView, and the `<.input>` component in the template. In the template **always access forms this**:
-
-    <%!-- ALWAYS do this (valid) --%>
-    <.form for={@form} id="my-form">
+    <.form for={@form} id="todo-form" phx-change="validate" phx-submit="save">
       <.input field={@form[:field]} type="text" />
     </.form>
 
-And **never** do this:
-
-    <%!-- NEVER do this (invalid) --%>
-    <.form for={@data} id="my-form">
-      <.input field={@data[:field]} type="text" />
-    </.form>
-
-- **Never** use `<.form let={f} ...>` in the template, instead **always use `<.form for={@form} ...>`**, then drive all form references from the form assign as in `@form[:field]`. The UI should **always** be driven by a `to_form/1` or `to_form/2` assigned in the LiveView module
+Always give the form an explicit, unique DOM ID, like `id="todo-form"`.