diff --git a/crates/komorebi-config/src/lib.rs b/crates/komorebi-config/src/lib.rs
index ba126d86..a40e456b 100644
--- a/crates/komorebi-config/src/lib.rs
+++ b/crates/komorebi-config/src/lib.rs
@@ -19,4 +19,6 @@ pub mod changes;
 pub mod defaults;
 pub mod matching;
 pub mod placement;
+#[cfg(feature = "schemars")]
+pub mod platforms;
 pub mod static_config;
diff --git a/crates/komorebi-config/src/platforms.rs b/crates/komorebi-config/src/platforms.rs
new file mode 100644
index 00000000..f56e0acb
--- /dev/null
+++ b/crates/komorebi-config/src/platforms.rs
@@ -0,0 +1,48 @@
+//! Which platform reads which key of `komorebi.json`.
+//!
+//! The table is not written a second time: it is read from the `x-komorebi-platforms` markers
+//! of the one `StaticConfig`, so a key cannot be added without saying who reads it.
+
+use komorebi_protocol::platforms::Platform;
+use serde_json::Value;
+
+use crate::schema::PLATFORMS;
+use crate::schema::platform_name;
+use crate::schema::union;
+use crate::static_config::StaticConfig;
+
+/// Whether `platform` reads the top-level `key` of a configuration. A key the type does not
+/// know is read by nobody (it is ignored everywhere).
+#[must_use]
+pub fn reads(platform: Platform, key: &str) -> bool {
+    let schema = union::<StaticConfig>();
+    match schema["properties"].get(key) {
+        None => false,
+        Some(property) => match property.get(PLATFORMS) {
+            Some(Value::Array(platforms)) => platforms
+                .iter()
+                .any(|p| p.as_str() == Some(platform_name(platform))),
+            _ => true,
+        },
+    }
+}
+
+/// The top-level keys of the configuration text `raw` that are keys of `komorebi.json` but that
+/// `platform` does not read, in file order: what a daemon says at load time instead of silently
+/// dropping them. Keys that are not known at all are not reported.
+///
+/// # Errors
+///
+/// The text is not JSON.
+pub fn unsupported_keys(raw: &str, platform: Platform) -> color_eyre::Result<Vec<String>> {
+    let value: Value = serde_json::from_str(raw)?;
+    let schema = union::<StaticConfig>();
+    let known = schema["properties"].as_object();
+    Ok(value
+        .as_object()
+        .into_iter()
+        .flat_map(|object| object.keys())
+        .filter(|key| known.is_some_and(|k| k.contains_key(*key)) && !reads(platform, key))
+        .cloned()
+        .collect())
+}
diff --git a/crates/komorebi-config/tests/schemas.rs b/crates/komorebi-config/tests/schemas.rs
index 87c48471..1d4ac3a6 100644
--- a/crates/komorebi-config/tests/schemas.rs
+++ b/crates/komorebi-config/tests/schemas.rs
@@ -251,3 +251,55 @@ fn the_schema_of_each_platform_is_a_view_of_the_one_static_config() {
         );
     }
 }
+
+#[test]
+fn the_platform_table_comes_from_the_markers() {
+    use komorebi_config::platforms::reads;
+    use komorebi_config::platforms::unsupported_keys;
+    use komorebi_protocol::platforms::Platform;
+
+    let windows_only = ["stackbar", "border_style", "slow_application_identifiers"];
+    let kde_macos_only = [
+        "border_radius",
+        "titleless_applications",
+        "tabbed_applications",
+    ];
+    let windows_kde = ["transparency", "focus_follows_mouse"];
+
+    for key in windows_only {
+        assert!(reads(Platform::Windows, key));
+        assert!(
+            !reads(Platform::Kde, key) && !reads(Platform::MacOs, key),
+            "{key}"
+        );
+    }
+    for key in kde_macos_only {
+        assert!(!reads(Platform::Windows, key), "{key}");
+        assert!(reads(Platform::Kde, key) && reads(Platform::MacOs, key));
+    }
+    for key in windows_kde {
+        assert!(reads(Platform::Windows, key) && reads(Platform::Kde, key));
+        assert!(!reads(Platform::MacOs, key), "{key}");
+    }
+    assert!(reads(Platform::MacOs, "monitors") && !reads(Platform::MacOs, "no_such_key"));
+
+    // the keys each tree's schema.json holds are the keys that platform reads
+    for (tree, platform) in [
+        (&TREES[0], Platform::Windows),
+        (&TREES[1], Platform::Kde),
+        (&TREES[2], Platform::MacOs),
+    ] {
+        let schema: Value = serde_json::from_str(&checked_in(tree, "schema.json")).unwrap();
+        for key in schema["properties"].as_object().unwrap().keys() {
+            assert!(reads(platform, key), "{} reads {key}", tree.name);
+        }
+    }
+
+    let raw = r#"{ "stackbar": {}, "border": true, "a_key_nobody_knows": 1, "border_radius": 5 }"#;
+    assert_eq!(unsupported_keys(raw, Platform::Kde).unwrap(), ["stackbar"]);
+    assert_eq!(
+        unsupported_keys(raw, Platform::Windows).unwrap(),
+        ["border_radius"]
+    );
+    assert!(unsupported_keys("{ ", Platform::Kde).is_err());
+}