diff --git a/README.md b/README.md
index 12e81b9..e992071 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ A Language Server Protocol (LSP) implementation for [Comline](https://github.com
 
 ### ✅ Fully Implemented
 
-- **Diagnostics** - Real-time syntax error detection and reporting
+- **Diagnostics** - Real-time syntax errors plus `comline-core`'s validation pass (undefined types, duplicate declarations, ...) — the same checks `comline build` runs
 - **Document Symbols** - Hierarchical outline view of structs, enums, protocols, and constants
 - **Hover Information** - Rich tooltips showing full type definitions and signatures
 - **Go to Definition** - Jump from type references to their declarations
diff --git a/src/analysis/diagnostics.rs b/src/analysis/diagnostics.rs
index f946034..275251e 100644
--- a/src/analysis/diagnostics.rs
+++ b/src/analysis/diagnostics.rs
@@ -1,8 +1,67 @@
-// Diagnostic generation from parse errors
+// Diagnostic generation — parse errors, and `comline-core`'s validation pass
 
 use crate::util::byte_range_to_lsp_range;
+use comline_core::schema::idl::grammar::Document;
+use comline_core::schema::ir::compiler::interpreter::incremental::IncrementalInterpreter;
+use comline_core::schema::ir::compiler::Compile;
+use comline_core::schema::ir::validation;
 use tower_lsp::lsp_types::{Diagnostic, DiagnosticSeverity};
 
+/// Semantic diagnostics from `comline-core`'s validation pass — undefined type
+/// references, duplicate declarations, and the like: the same checks
+/// `comline build` runs. Call only on a document that parsed cleanly.
+pub fn validation_diagnostics(source: &str, document: &Document) -> Vec<Diagnostic> {
+    let units = IncrementalInterpreter::from_declarations(document.0.clone());
+    let errors = match validation::validate(&units) {
+        Ok(()) => return vec![],
+        Err(errors) => errors,
+    };
+
+    errors
+        .into_iter()
+        .map(|error| {
+            let range = error
+                .span
+                .map(|(start, end)| byte_range_to_lsp_range(source, start, end))
+                .unwrap_or_default();
+
+            let message = if error.context.is_empty() {
+                error.message
+            } else {
+                format!("{} — {}", error.message, error.context)
+            };
+
+            Diagnostic {
+                range,
+                severity: Some(DiagnosticSeverity::ERROR),
+                code: None,
+                code_description: None,
+                source: Some("comline".to_string()),
+                message,
+                related_information: None,
+                tags: None,
+                data: None,
+            }
+        })
+        .collect()
+}
+
+/// Parse-error + validation diagnostics for `source`. Validation is skipped
+/// while the tree is malformed (parse errors present).
+pub fn all_diagnostics(
+    source: &str,
+    errors: &[rust_sitter::errors::ParseError],
+    document: Option<&Document>,
+) -> Vec<Diagnostic> {
+    let mut diagnostics = generate_diagnostics(source, errors);
+    if errors.is_empty() {
+        if let Some(doc) = document {
+            diagnostics.extend(validation_diagnostics(source, doc));
+        }
+    }
+    diagnostics
+}
+
 /// Generate LSP diagnostics from parse errors
 pub fn generate_diagnostics(source: &str, errors: &[rust_sitter::errors::ParseError]) -> Vec<Diagnostic> {
     errors
@@ -80,8 +139,50 @@ struct User {
 "#;
         let result = parser::parse(source).unwrap();
         assert!(!result.has_errors());
-        
+
         let diagnostics = generate_diagnostics(source, &result.errors);
         assert!(diagnostics.is_empty());
     }
+
+    fn diags(source: &str) -> Vec<Diagnostic> {
+        let result = parser::parse(source).unwrap();
+        all_diagnostics(source, &result.errors, result.document.as_ref())
+    }
+
+    #[test]
+    fn validation_flags_an_undefined_type_reference() {
+        let d = diags("struct Order {\n    buyer: Customer\n}\n");
+        assert!(
+            d.iter().any(|x| x.message.to_lowercase().contains("customer")),
+            "expected an undefined-type diagnostic mentioning `Customer`, got {d:?}"
+        );
+    }
+
+    #[test]
+    fn validation_flags_a_duplicate_declaration() {
+        let d = diags("struct User {\n    a: string\n}\nstruct User {\n    b: string\n}\n");
+        assert!(
+            d.iter().any(|x| x.message.to_lowercase().contains("duplicate")),
+            "expected a duplicate-definition diagnostic, got {d:?}"
+        );
+    }
+
+    #[test]
+    fn a_well_formed_schema_has_no_diagnostics() {
+        let d = diags(
+            "struct Item {\n    id: u64\n}\n\nstruct Cart {\n    items: Item[]\n}\n",
+        );
+        assert!(d.is_empty(), "expected no diagnostics, got {d:?}");
+    }
+
+    #[test]
+    fn validation_is_skipped_while_the_tree_is_malformed() {
+        // A parse error is present, so validation must not run (no panic, no
+        // spurious semantic errors) — only the parse diagnostic.
+        let source = "struct User {\n    name string\n}\n";
+        let result = parser::parse(source).unwrap();
+        assert!(result.has_errors());
+        let d = all_diagnostics(source, &result.errors, result.document.as_ref());
+        assert!(!d.is_empty());
+    }
 }
diff --git a/src/backend.rs b/src/backend.rs
index 0e8c957..127c909 100644
--- a/src/backend.rs
+++ b/src/backend.rs
@@ -1,5 +1,10 @@
-use crate::document::DocumentStore;
+//
 use std::sync::Arc;
+
+//
+use crate::document::DocumentStore;
+
+//
 use tower_lsp::jsonrpc::Result;
 use tower_lsp::lsp_types::*;
 use tower_lsp::{Client, LanguageServer};
@@ -287,9 +292,14 @@ impl Backend {
         // Parse the document
         match parser::parse(&document.text) {
             Ok(result) => {
-                // Generate LSP diagnostics from parse errors
-                let lsp_diagnostics = diagnostics::generate_diagnostics(&document.text, &result.errors);
-                
+                // Parse-error diagnostics, plus `comline-core`'s validation
+                // pass once the tree is well-formed.
+                let lsp_diagnostics = diagnostics::all_diagnostics(
+                    &document.text,
+                    &result.errors,
+                    result.document.as_ref(),
+                );
+
                 // Log parse results
                 if result.is_ok() {
                     if let Some(doc) = &result.document {