diff --git a/crates/djls-server/src/diagnostics.rs b/crates/djls-server/src/diagnostics.rs
index 05b0322..09ce53a 100644
--- a/crates/djls-server/src/diagnostics.rs
+++ b/crates/djls-server/src/diagnostics.rs
@@ -1,5 +1,8 @@
 use tower_lsp::lsp_types::*;
 use crate::documents::TextDocument;
+use djls_template_ast::{Lexer, Parser};
+use djls_template_ast::ast::{Ast, Node, LineOffsets, Span};
+use djls_template_ast::tokens::TokenStream;
 
 pub struct Diagnostics;
 
@@ -7,29 +10,82 @@ impl Diagnostics {
     pub fn generate_for_document(document: &TextDocument) -> Vec<Diagnostic> {
         let mut diagnostics = Vec::new();
         
-        // Simple example: Check for TODO comments
-        for (line_num, line) in document.get_text().lines().enumerate() {
-            if let Some(col) = line.find("TODO") {
+        let text = document.get_text();
+        let lexer = Lexer::new(text);
+        let token_stream = match lexer.tokenize() {
+            Ok(tokens) => tokens,
+            Err(e) => {
                 diagnostics.push(Diagnostic {
-                    range: Range {
-                        start: Position { 
-                            line: line_num as u32, 
-                            character: col as u32 
-                        },
-                        end: Position { 
-                            line: line_num as u32, 
-                            character: (col + 4) as u32 
-                        },
-                    },
-                    severity: Some(DiagnosticSeverity::INFORMATION),
-                    code: Some(NumberOrString::String("django.todo".to_string())),
+                    range: Range::new(Position::new(0, 0), Position::new(0, 0)),
+                    severity: Some(DiagnosticSeverity::ERROR),
+                    code: Some(NumberOrString::String("lexing_error".to_string())),
                     source: Some("Django LSP".to_string()),
-                    message: "Found TODO comment".to_string(),
+                    message: format!("Lexing error: {}", e),
                     ..Default::default()
                 });
+                return diagnostics;
+            }
+        };
+
+        let mut parser = Parser::new(token_stream);
+        let (ast, parse_errors) = match parser.parse() {
+            Ok(result) => result,
+            Err(e) => {
+                diagnostics.push(Diagnostic {
+                    range: Range::new(Position::new(0, 0), Position::new(0, 0)),
+                    severity: Some(DiagnosticSeverity::ERROR),
+                    code: Some(NumberOrString::String("parsing_error".to_string())),
+                    source: Some("Django LSP".to_string()),
+                    message: format!("Parsing error: {}", e),
+                    ..Default::default()
+                });
+                return diagnostics;
+            }
+        };
+
+        for error in parse_errors {
+            diagnostics.push(Diagnostic {
+                range: Range::new(Position::new(0, 0), Position::new(0, 0)), // Adjust range based on error
+                severity: Some(DiagnosticSeverity::ERROR),
+                code: Some(NumberOrString::String("parse_error".to_string())),
+                source: Some("Django LSP".to_string()),
+                message: format!("Parse error: {}", error),
+                ..Default::default()
+            });
+        }
+
+        for node in ast.nodes() {
+            match node {
+                Node::Block(block) => {
+                    if let Some(closing) = block.closing() {
+                    } else {
+                        let span = block.tag().span;
+                        let range = get_range_from_span(&ast.line_offsets(), &span);
+                        diagnostics.push(Diagnostic {
+                            range,
+                            severity: Some(DiagnosticSeverity::ERROR),
+                            code: Some(NumberOrString::String("unclosed_block".to_string())),
+                            source: Some("Django LSP".to_string()),
+                            message: format!("Unclosed block tag '{}'", block.tag().name),
+                            ..Default::default()
+                        });
+                    }
+                },
+                Node::Variable { .. } => {},
+                _ => {},
             }
         }
 
         diagnostics
     }
 }
+
+fn get_range_from_span(line_offsets: &LineOffsets, span: &Span) -> Range {
+    let (start_line, start_col) = line_offsets.position_to_line_col(span.start as usize);
+    let (end_line, end_col) = line_offsets.position_to_line_col((span.start + span.length) as usize);
+
+    Range {
+        start: Position::new(start_line as u32 - 1, start_col as u32),
+        end: Position::new(end_line as u32 - 1, end_col as u32),
+    }
+}