diff --git a/crates/pinakes-plugin-api/src/lib.rs b/crates/pinakes-plugin-api/src/lib.rs
index 34e806f..5440669 100644
--- a/crates/pinakes-plugin-api/src/lib.rs
+++ b/crates/pinakes-plugin-api/src/lib.rs
@@ -16,6 +16,7 @@ use thiserror::Error;
 pub mod manifest;
 pub mod types;
 pub mod ui_schema;
+pub mod validation;
 pub mod wasm;
 
 pub use manifest::PluginManifest;
diff --git a/crates/pinakes-plugin-api/src/validation.rs b/crates/pinakes-plugin-api/src/validation.rs
new file mode 100644
index 0000000..d232f29
--- /dev/null
+++ b/crates/pinakes-plugin-api/src/validation.rs
@@ -0,0 +1,583 @@
+//! Schema validation for plugin UI pages
+//!
+//! Provides comprehensive validation of [`UiPage`] and [`UiElement`] trees
+//! before they are rendered. Call [`SchemaValidator::validate_page`] before
+//! registering a plugin page.
+
+use thiserror::Error;
+
+use crate::{DataSource, UiElement, UiPage, UiWidget};
+
+/// Reserved routes that plugins cannot use
+const RESERVED_ROUTES: &[&str] = &[
+  "/",
+  "/search",
+  "/settings",
+  "/admin",
+  "/library",
+  "/books",
+  "/tags",
+  "/collections",
+  "/audit",
+  "/import",
+  "/duplicates",
+  "/statistics",
+  "/tasks",
+  "/database",
+  "/graph",
+];
+
+/// Errors produced by schema validation
+#[derive(Debug, Error)]
+pub enum ValidationError {
+  /// A single validation failure
+  #[error("Validation error: {0}")]
+  Single(String),
+
+  /// Multiple validation failures collected in one pass
+  #[error("Validation failed with {} errors: {}", .0.len(), .0.join("; "))]
+  Multiple(Vec<String>),
+}
+
+/// Validates plugin UI schemas before they are loaded into the registry.
+///
+/// # Example
+///
+/// ```rust,ignore
+/// let page = plugin.manifest.ui.pages[0].clone();
+/// SchemaValidator::validate_page(&page)?;
+/// ```
+pub struct SchemaValidator;
+
+impl SchemaValidator {
+  /// Validate a complete [`UiPage`] definition.
+  ///
+  /// Checks:
+  /// - Page ID format (alphanumeric + dash/underscore, starts with a letter)
+  /// - Route starts with `'/'` and is not reserved
+  /// - `DataTable` elements have at least one column
+  /// - Form elements have at least one field
+  /// - Loop and Conditional elements have valid structure
+  ///
+  /// # Errors
+  ///
+  /// Returns [`ValidationError::Multiple`] containing all collected errors
+  /// so callers can surface all problems at once.
+  pub fn validate_page(page: &UiPage) -> Result<(), ValidationError> {
+    let mut errors = Vec::new();
+
+    // ID format
+    if !Self::is_valid_id(&page.id) {
+      errors.push(format!(
+        "Invalid page ID '{}': must start with a letter and contain only \
+         alphanumeric characters, dashes, or underscores",
+        page.id
+      ));
+    }
+
+    // Route format
+    if !page.route.starts_with('/') {
+      errors.push(format!("Route must start with '/': {}", page.route));
+    }
+
+    // Reserved routes
+    if Self::is_reserved_route(&page.route) {
+      errors.push(format!("Route is reserved by the host: {}", page.route));
+    }
+
+    // Validate data sources
+    for (name, source) in &page.data_sources {
+      Self::validate_data_source(name, source, &mut errors);
+    }
+
+    // Recursively validate element tree
+    Self::validate_element(&page.root_element, &mut errors);
+
+    if errors.is_empty() {
+      Ok(())
+    } else {
+      Err(ValidationError::Multiple(errors))
+    }
+  }
+
+  /// Validate a [`UiWidget`] definition.
+  ///
+  /// # Errors
+  ///
+  /// Returns [`ValidationError::Multiple`] with all collected errors.
+  pub fn validate_widget(widget: &UiWidget) -> Result<(), ValidationError> {
+    let mut errors = Vec::new();
+
+    if !Self::is_valid_id(&widget.id) {
+      errors.push(format!(
+        "Invalid widget ID '{}': must start with a letter and contain only \
+         alphanumeric characters, dashes, or underscores",
+        widget.id
+      ));
+    }
+
+    if widget.target.is_empty() {
+      errors.push("Widget target must not be empty".to_string());
+    }
+
+    Self::validate_element(&widget.content, &mut errors);
+
+    if errors.is_empty() {
+      Ok(())
+    } else {
+      Err(ValidationError::Multiple(errors))
+    }
+  }
+
+  /// Recursively validate a [`UiElement`] subtree.
+  pub fn validate_element(element: &UiElement, errors: &mut Vec<String>) {
+    match element {
+      UiElement::Container { children, .. } => {
+        for child in children {
+          Self::validate_element(child, errors);
+        }
+      },
+
+      UiElement::Grid { children, .. } => {
+        for child in children {
+          Self::validate_element(child, errors);
+        }
+      },
+
+      UiElement::Flex { children, .. } => {
+        for child in children {
+          Self::validate_element(child, errors);
+        }
+      },
+
+      UiElement::Split { sidebar, main, .. } => {
+        Self::validate_element(sidebar, errors);
+        Self::validate_element(main, errors);
+      },
+
+      UiElement::Tabs { tabs, .. } => {
+        if tabs.is_empty() {
+          errors.push("Tabs element must have at least one tab".to_string());
+        }
+        for tab in tabs {
+          Self::validate_element(&tab.content, errors);
+        }
+      },
+
+      UiElement::DataTable { data, columns, .. } => {
+        if data.is_empty() {
+          errors
+            .push("DataTable 'data' source key must not be empty".to_string());
+        }
+        if columns.is_empty() {
+          errors.push("DataTable must have at least one column".to_string());
+        }
+      },
+
+      UiElement::Form { fields, .. } => {
+        if fields.is_empty() {
+          errors.push("Form must have at least one field".to_string());
+        }
+        for field in fields {
+          if field.id.is_empty() {
+            errors.push("Form field id must not be empty".to_string());
+          }
+        }
+      },
+
+      UiElement::Conditional {
+        then, else_element, ..
+      } => {
+        Self::validate_element(then, errors);
+        if let Some(else_branch) = else_element {
+          Self::validate_element(else_branch, errors);
+        }
+      },
+
+      UiElement::Loop { template, .. } => {
+        Self::validate_element(template, errors);
+      },
+
+      UiElement::Card {
+        content, footer, ..
+      } => {
+        for child in content.iter().chain(footer.iter()) {
+          Self::validate_element(child, errors);
+        }
+      },
+
+      UiElement::List { data, .. } => {
+        if data.is_empty() {
+          errors.push("List 'data' source key must not be empty".to_string());
+        }
+      },
+
+      // Leaf elements with no children to recurse into
+      UiElement::Heading { .. }
+      | UiElement::Text { .. }
+      | UiElement::Code { .. }
+      | UiElement::MediaGrid { .. }
+      | UiElement::DescriptionList { .. }
+      | UiElement::Button { .. }
+      | UiElement::Link { .. }
+      | UiElement::Progress { .. }
+      | UiElement::Badge { .. }
+      | UiElement::Chart { .. } => {},
+    }
+  }
+
+  fn validate_data_source(
+    name: &str,
+    source: &DataSource,
+    errors: &mut Vec<String>,
+  ) {
+    match source {
+      DataSource::Endpoint { path, .. } => {
+        if path.is_empty() {
+          errors.push(format!(
+            "Data source '{name}': endpoint path must not be empty"
+          ));
+        }
+        if !path.starts_with('/') {
+          errors.push(format!(
+            "Data source '{name}': endpoint path must start with '/': {path}"
+          ));
+        }
+      },
+      DataSource::Transform { source_name, .. } => {
+        if source_name.is_empty() {
+          errors.push(format!(
+            "Data source '{name}': transform source_name must not be empty"
+          ));
+        }
+      },
+      DataSource::Static { .. } => {},
+    }
+  }
+
+  fn is_valid_id(id: &str) -> bool {
+    if id.is_empty() || id.len() > 64 {
+      return false;
+    }
+    let mut chars = id.chars();
+    chars.next().is_some_and(|c| c.is_ascii_alphabetic())
+      && chars.all(|c| c.is_ascii_alphanumeric() || c == '_' || c == '-')
+  }
+
+  fn is_reserved_route(route: &str) -> bool {
+    RESERVED_ROUTES.iter().any(|reserved| {
+      route == *reserved || route.starts_with(&format!("{reserved}/"))
+    })
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use std::collections::HashMap;
+
+  use super::*;
+  use crate::UiElement;
+
+  fn make_page(id: &str, route: &str) -> UiPage {
+    UiPage {
+      id:           id.to_string(),
+      title:        "Test Page".to_string(),
+      route:        route.to_string(),
+      icon:         None,
+      root_element: UiElement::Container {
+        children: vec![],
+        gap:      0,
+        padding:  None,
+      },
+      data_sources: HashMap::new(),
+    }
+  }
+
+  #[test]
+  fn test_valid_page() {
+    let page = make_page("my-plugin-page", "/plugins/test/page");
+    assert!(SchemaValidator::validate_page(&page).is_ok());
+  }
+
+  #[test]
+  fn test_invalid_id_starts_with_digit() {
+    let page = make_page("1invalid", "/plugins/test/page");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_invalid_id_empty() {
+    let page = make_page("", "/plugins/test/page");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_reserved_route() {
+    let page = make_page("my-page", "/settings");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_route_missing_slash() {
+    let page = make_page("my-page", "plugins/test");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_datatable_no_columns() {
+    let mut page = make_page("my-page", "/plugins/test/page");
+    page.root_element = UiElement::DataTable {
+      data:        "items".to_string(),
+      columns:     vec![],
+      sortable:    false,
+      filterable:  false,
+      page_size:   0,
+      row_actions: vec![],
+    };
+    let result = SchemaValidator::validate_page(&page);
+    assert!(result.is_err());
+    let err = result.unwrap_err().to_string();
+    assert!(err.contains("at least one column"));
+  }
+
+  #[test]
+  fn test_form_no_fields() {
+    let mut page = make_page("my-page", "/plugins/test/page");
+    page.root_element = UiElement::Form {
+      fields:        vec![],
+      submit_action: crate::ActionRef::Name("submit".to_string()),
+      submit_label:  "Submit".to_string(),
+      cancel_label:  None,
+    };
+    let result = SchemaValidator::validate_page(&page);
+    assert!(result.is_err());
+    let err = result.unwrap_err().to_string();
+    assert!(err.contains("at least one field"));
+  }
+
+  #[test]
+  fn test_multiple_errors_collected() {
+    let page = make_page("1bad-id", "/settings");
+    let result = SchemaValidator::validate_page(&page);
+    assert!(result.is_err());
+    match result.unwrap_err() {
+      ValidationError::Multiple(errs) => assert!(errs.len() >= 2),
+      ValidationError::Single(_) => panic!("expected Multiple"),
+    }
+  }
+
+  #[test]
+  fn test_reserved_route_subpath_rejected() {
+    // A sub-path of a reserved route must also be rejected
+    for route in &[
+      "/settings/theme",
+      "/admin/users",
+      "/library/foo",
+      "/search/advanced",
+      "/tasks/pending",
+    ] {
+      let page = make_page("my-page", route);
+      let result = SchemaValidator::validate_page(&page);
+      assert!(
+        result.is_err(),
+        "expected error for sub-path of reserved route: {route}"
+      );
+    }
+  }
+
+  #[test]
+  fn test_plugin_route_not_reserved() {
+    // Routes under /plugins/ are allowed (not in RESERVED_ROUTES)
+    let page = make_page("my-page", "/plugins/my-plugin/page");
+    assert!(SchemaValidator::validate_page(&page).is_ok());
+  }
+
+  #[test]
+  fn test_id_max_length_accepted() {
+    let id = "a".repeat(64);
+    let page = make_page(&id, "/plugins/test");
+    assert!(SchemaValidator::validate_page(&page).is_ok());
+  }
+
+  #[test]
+  fn test_id_too_long_rejected() {
+    let id = "a".repeat(65);
+    let page = make_page(&id, "/plugins/test");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_id_with_dash_and_underscore() {
+    let page = make_page("my-plugin_page", "/plugins/test");
+    assert!(SchemaValidator::validate_page(&page).is_ok());
+  }
+
+  #[test]
+  fn test_id_with_special_chars_rejected() {
+    let page = make_page("my page!", "/plugins/test");
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_datatable_empty_data_key() {
+    let col: crate::ColumnDef =
+      serde_json::from_value(serde_json::json!({"key": "id", "header": "ID"}))
+        .unwrap();
+    let mut page = make_page("my-page", "/plugins/test/page");
+    page.root_element = UiElement::DataTable {
+      data:        String::new(),
+      columns:     vec![col],
+      sortable:    false,
+      filterable:  false,
+      page_size:   0,
+      row_actions: vec![],
+    };
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_form_field_empty_id_rejected() {
+    let field: crate::FormField = serde_json::from_value(
+      serde_json::json!({"id": "", "label": "Name", "type": {"type": "text"}}),
+    )
+    .unwrap();
+    let mut page = make_page("my-page", "/plugins/test/page");
+    page.root_element = UiElement::Form {
+      fields:        vec![field],
+      submit_action: crate::ActionRef::Name("submit".to_string()),
+      submit_label:  "Submit".to_string(),
+      cancel_label:  None,
+    };
+    let result = SchemaValidator::validate_page(&page);
+    assert!(result.is_err());
+    assert!(result.unwrap_err().to_string().contains("field id"));
+  }
+
+  #[test]
+  fn test_valid_widget() {
+    let widget = crate::UiWidget {
+      id:      "my-widget".to_string(),
+      target:  "library_header".to_string(),
+      content: UiElement::Container {
+        children: vec![],
+        gap:      0,
+        padding:  None,
+      },
+    };
+    assert!(SchemaValidator::validate_widget(&widget).is_ok());
+  }
+
+  #[test]
+  fn test_widget_invalid_id() {
+    let widget = crate::UiWidget {
+      id:      "1bad".to_string(),
+      target:  "library_header".to_string(),
+      content: UiElement::Container {
+        children: vec![],
+        gap:      0,
+        padding:  None,
+      },
+    };
+    assert!(SchemaValidator::validate_widget(&widget).is_err());
+  }
+
+  #[test]
+  fn test_widget_empty_target() {
+    let widget = crate::UiWidget {
+      id:      "my-widget".to_string(),
+      target:  String::new(),
+      content: UiElement::Container {
+        children: vec![],
+        gap:      0,
+        padding:  None,
+      },
+    };
+    assert!(SchemaValidator::validate_widget(&widget).is_err());
+  }
+
+  #[test]
+  fn test_data_source_empty_endpoint_path() {
+    use crate::{DataSource, HttpMethod};
+    let mut page = make_page("my-page", "/plugins/test");
+    page
+      .data_sources
+      .insert("items".to_string(), DataSource::Endpoint {
+        path:          String::new(),
+        method:        HttpMethod::Get,
+        params:        Default::default(),
+        poll_interval: 0,
+        transform:     None,
+      });
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_data_source_endpoint_path_no_leading_slash() {
+    use crate::{DataSource, HttpMethod};
+    let mut page = make_page("my-page", "/plugins/test");
+    page
+      .data_sources
+      .insert("items".to_string(), DataSource::Endpoint {
+        path:          "api/v1/items".to_string(),
+        method:        HttpMethod::Get,
+        params:        Default::default(),
+        poll_interval: 0,
+        transform:     None,
+      });
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_data_source_endpoint_valid() {
+    use crate::{DataSource, HttpMethod};
+    let mut page = make_page("my-page", "/plugins/test");
+    page
+      .data_sources
+      .insert("items".to_string(), DataSource::Endpoint {
+        path:          "/api/v1/items".to_string(),
+        method:        HttpMethod::Get,
+        params:        Default::default(),
+        poll_interval: 0,
+        transform:     None,
+      });
+    assert!(SchemaValidator::validate_page(&page).is_ok());
+  }
+
+  #[test]
+  fn test_data_source_transform_empty_source_name() {
+    use crate::DataSource;
+    let mut page = make_page("my-page", "/plugins/test");
+    page
+      .data_sources
+      .insert("derived".to_string(), DataSource::Transform {
+        source_name: String::new(),
+        expression:  crate::Expression::Literal(serde_json::Value::Null),
+      });
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_tabs_empty_rejected() {
+    let mut page = make_page("my-page", "/plugins/test");
+    page.root_element = UiElement::Tabs {
+      tabs:        vec![],
+      default_tab: 0,
+    };
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+
+  #[test]
+  fn test_list_empty_data_key() {
+    let mut page = make_page("my-page", "/plugins/test");
+    page.root_element = UiElement::List {
+      data:          String::new(),
+      item_template: Box::new(UiElement::Container {
+        children: vec![],
+        gap:      0,
+        padding:  None,
+      }),
+      dividers:      false,
+    };
+    assert!(SchemaValidator::validate_page(&page).is_err());
+  }
+}