2 files changed, 18 insertions, 4 deletions
diff --git a/crates/shirabe/src/plugin/capability/capability.rs b/crates/shirabe/src/plugin/capability/capability.rs
index 1788d9a0..aa70ffa0 100644
--- a/crates/shirabe/src/plugin/capability/capability.rs
+++ b/crates/shirabe/src/plugin/capability/capability.rs
@@ -1,4 +1,14 @@
 //! ref: composer/src/Composer/Plugin/Capability/Capability.php
 
-// TODO(plugin): Marker interface for Plugin capabilities. Every new Capability which is added to the Plugin API must implement this interface.
-pub trait Capability {}
+use crate::plugin::capability::CommandProvider;
+
+/// Marker interface for Plugin capabilities. Every new Capability which is added to the
+/// Plugin API must implement this interface.
+///
+/// The accessor replaces PHP's `instanceof` downcast on a capability instance of unknown
+/// concrete type (the way `PluginInterface::as_capable` does for plugins).
+pub trait Capability {
+    fn as_command_provider(&self) -> Option<&dyn CommandProvider> {
+        None
+    }
+}
diff --git a/crates/shirabe/src/plugin/capability/command_provider.rs b/crates/shirabe/src/plugin/capability/command_provider.rs
index 6d6f6cf1..2453860a 100644
--- a/crates/shirabe/src/plugin/capability/command_provider.rs
+++ b/crates/shirabe/src/plugin/capability/command_provider.rs
@@ -1,9 +1,13 @@
 //! ref: composer/src/Composer/Plugin/Capability/CommandProvider.php
 
-// TODO(plugin): Commands Provider Interface. Plugins implementing this capability provide a list of commands.
 use crate::command::BaseCommand;
 use crate::plugin::capability::Capability;
 
+/// Commands Provider Interface. Plugins implementing this capability provide a list of
+/// commands.
+///
+/// The sole implementor is the PHP capability proxy (Composer itself never implements a
+/// capability), so the method is fallible: the answer crosses the RPC boundary.
 pub trait CommandProvider: Capability {
-    fn get_commands(&self) -> Vec<Box<dyn BaseCommand>>;
+    fn get_commands(&self) -> anyhow::Result<Vec<Box<dyn BaseCommand>>>;
 }