diff --git a/crates/stef-compiler/src/validate/generics.rs b/crates/stef-compiler/src/validate/generics.rs
index 5696145..598187c 100644
--- a/crates/stef-compiler/src/validate/generics.rs
+++ b/crates/stef-compiler/src/validate/generics.rs
@@ -24,10 +24,12 @@ pub enum InvalidGenericType {
 pub struct DuplicateGenericName {
     /// Name of the parameter.
     pub name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Defined but unused type parameter.
@@ -37,8 +39,9 @@ pub struct DuplicateGenericName {
 pub struct UnusedGeneric {
     /// Name of the parameter.
     pub name: String,
+    /// Source location of the declaration.
     #[label("declared here")]
-    definition: Range<usize>,
+    pub declared: Range<usize>,
 }
 
 /// Ensure all generics in a struct are unique and used.
@@ -57,7 +60,7 @@ pub fn validate_struct_generics(value: &Struct<'_>) -> Result<(), InvalidGeneric
     unvisited.into_iter().next().map_or(Ok(()), |(name, span)| {
         Err(UnusedGeneric {
             name: name.to_owned(),
-            definition: span.into(),
+            declared: span.into(),
         }
         .into())
     })
@@ -81,7 +84,7 @@ pub fn validate_enum_generics(value: &Enum<'_>) -> Result<(), InvalidGenericType
     unvisited.into_iter().next().map_or(Ok(()), |(name, span)| {
         Err(UnusedGeneric {
             name: name.to_owned(),
-            definition: span.into(),
+            declared: span.into(),
         }
         .into())
     })
diff --git a/crates/stef-compiler/src/validate/ids.rs b/crates/stef-compiler/src/validate/ids.rs
index f33116c..9bb6ca8 100644
--- a/crates/stef-compiler/src/validate/ids.rs
+++ b/crates/stef-compiler/src/validate/ids.rs
@@ -28,10 +28,12 @@ pub struct DuplicateVariantId {
     pub name: String,
     /// Name of the variant that used the ID for the first time.
     pub other_name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Duplicate ID for fields of a struct or enum variant.
@@ -58,10 +60,12 @@ pub struct DuplicateNamedFieldId {
     pub name: String,
     /// Name of the field that used the ID for the first time.
     pub other_name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Duplicate ID for unnamed fields.
@@ -75,10 +79,12 @@ pub struct DuplicateUnnamedFieldId {
     pub position: usize,
     /// 1-base position of the field that used the ID for the first time.
     pub other_position: usize,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Ensure all IDs inside a struct are unique (which are the field IDs).
diff --git a/crates/stef-compiler/src/validate/names.rs b/crates/stef-compiler/src/validate/names.rs
index 6a27cc4..5d5c378 100644
--- a/crates/stef-compiler/src/validate/names.rs
+++ b/crates/stef-compiler/src/validate/names.rs
@@ -28,10 +28,12 @@ pub enum DuplicateName {
 pub struct DuplicateVariantName {
     /// Name of the variant.
     pub name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Duplicate name for fields of a struct or enum variant.
@@ -41,10 +43,12 @@ pub struct DuplicateVariantName {
 pub struct DuplicateFieldName {
     /// Name of the field.
     pub name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Duplicate name for definitions inside a module.
@@ -56,10 +60,12 @@ pub struct DuplicateFieldName {
 pub struct DuplicateNameInModule {
     /// Name of the declaration.
     pub name: String,
+    /// Source location of the first occurrence.
     #[label("first declared here")]
-    first: Range<usize>,
+    pub first: Range<usize>,
+    /// Source location of the duplicate.
     #[label("used here again")]
-    second: Range<usize>,
+    pub second: Range<usize>,
 }
 
 /// Ensure all field names inside a struct are unique.
diff --git a/crates/stef-compiler/src/validate/tuples.rs b/crates/stef-compiler/src/validate/tuples.rs
index 9843258..ab78112 100644
--- a/crates/stef-compiler/src/validate/tuples.rs
+++ b/crates/stef-compiler/src/validate/tuples.rs
@@ -20,8 +20,9 @@ use crate::highlight;
 pub struct TupleSize {
     /// The amount that's not allowed.
     pub amount: InvalidTupleAmount,
+    /// Source location of the declaration.
     #[label("declared here")]
-    declared: Range<usize>,
+    pub declared: Range<usize>,
 }
 
 /// Possible amount of tuple elements that are invalid.
diff --git a/crates/stef-lsp/src/compile.rs b/crates/stef-lsp/src/compile.rs
index 0a45562..9563bd9 100644
--- a/crates/stef-lsp/src/compile.rs
+++ b/crates/stef-lsp/src/compile.rs
@@ -1,25 +1,36 @@
 use std::ops::Range;
 
+use stef_compiler::validate;
 use stef_parser::{
     error::{
         ParseAliasCause, ParseAttributeCause, ParseCommentError, ParseConstCause,
         ParseDefinitionError, ParseEnumCause, ParseFieldsCause, ParseFieldsError,
         ParseGenericsError, ParseIdError, ParseImportCause, ParseLiteralCause, ParseModuleCause,
-        ParseSchemaCause, ParseStructCause, ParseTypeCause, ParseTypeError,
+        ParseSchemaCause, ParseSchemaError, ParseStructCause, ParseTypeCause, ParseTypeError,
     },
     Schema,
 };
-use tower_lsp::lsp_types::{self as lsp, Diagnostic};
+use tower_lsp::lsp_types::{self as lsp, Diagnostic, Url};
 
 use crate::utf16;
 
-pub fn compile(schema: &str) -> std::result::Result<Schema<'_>, Diagnostic> {
-    stef_parser::Schema::parse(schema, None).map_err(|e| match &e.cause {
+pub fn compile(file: Url, schema: &str) -> std::result::Result<Schema<'_>, Diagnostic> {
+    let parsed = stef_parser::Schema::parse(schema, None)
+        .map_err(|e| parse_schema_diagnostic(schema, &e))?;
+
+    stef_compiler::validate_schema(&parsed)
+        .map_err(|e| validate_schema_diagnostic(file, schema, e))?;
+
+    Ok(parsed)
+}
+
+fn parse_schema_diagnostic(schema: &str, e: &ParseSchemaError) -> Diagnostic {
+    match &e.cause {
         ParseSchemaCause::Parser(_) => {
             Diagnostic::new_simple(get_range(schema, 0..schema.len()), e.to_string())
         }
         ParseSchemaCause::Definition(e) => parse_definition_diagnostic(schema, e),
-    })
+    }
 }
 
 fn parse_definition_diagnostic(schema: &str, e: &ParseDefinitionError) -> Diagnostic {
@@ -183,6 +194,53 @@ fn parse_id_diagnostic(schema: &str, e: &ParseIdError) -> Diagnostic {
     Diagnostic::new_simple(get_range(schema, e.at.clone()), e.to_string())
 }
 
+fn validate_schema_diagnostic(file: Url, schema: &str, e: validate::Error) -> Diagnostic {
+    use validate::{DuplicateFieldId, DuplicateId, DuplicateName, Error, InvalidGenericType};
+
+    let (message, first, second) = match e {
+        Error::DuplicateId(e) => match e {
+            DuplicateId::EnumVariant(e) => (e.to_string(), e.first, e.second),
+            DuplicateId::Field(e) => match e {
+                DuplicateFieldId::Named(e) => (e.to_string(), e.first, e.second),
+                DuplicateFieldId::Unnamed(e) => (e.to_string(), e.first, e.second),
+            },
+        },
+        Error::DuplicateName(e) => match e {
+            DuplicateName::EnumVariant(e) => (e.to_string(), e.first, e.second),
+            DuplicateName::Field(e) => (e.to_string(), e.first, e.second),
+            DuplicateName::InModule(e) => (e.to_string(), e.first, e.second),
+        },
+        Error::InvalidGeneric(e) => match e {
+            InvalidGenericType::Duplicate(e) => (e.to_string(), e.first, e.second),
+            InvalidGenericType::Unused(e) => {
+                let message = e.to_string();
+                return Diagnostic::new_simple(get_range(schema, e.declared), message);
+            }
+        },
+        Error::TupleSize(e) => {
+            let message = e.to_string();
+            return Diagnostic::new_simple(get_range(schema, e.declared), message);
+        }
+    };
+
+    diagnostic_with_related(
+        get_range(schema, second),
+        message,
+        vec![lsp::DiagnosticRelatedInformation {
+            location: lsp::Location::new(file, get_range(schema, first)),
+            message: "first used here".to_owned(),
+        }],
+    )
+}
+
+fn diagnostic_with_related(
+    range: lsp::Range,
+    message: String,
+    related: Vec<lsp::DiagnosticRelatedInformation>,
+) -> Diagnostic {
+    Diagnostic::new(range, None, None, None, message, Some(related), None)
+}
+
 #[allow(clippy::cast_possible_truncation)]
 fn get_range(schema: &str, location: Range<usize>) -> lsp::Range {
     let start_line = schema[..location.start].lines().count().saturating_sub(1);
diff --git a/crates/stef-lsp/src/main.rs b/crates/stef-lsp/src/main.rs
index fc5c8c0..a3dff47 100644
--- a/crates/stef-lsp/src/main.rs
+++ b/crates/stef-lsp/src/main.rs
@@ -172,7 +172,7 @@ impl LanguageServer for Backend {
 
         let file = FileBuilder {
             content: params.text_document.text,
-            schema_builder: |schema| compile::compile(schema),
+            schema_builder: |schema| compile::compile(params.text_document.uri.clone(), schema),
         }
         .build();
 
@@ -199,7 +199,7 @@ impl LanguageServer for Backend {
 
         let file = FileBuilder {
             content: params.content_changes.remove(0).text,
-            schema_builder: |schema| compile::compile(schema),
+            schema_builder: |schema| compile::compile(params.text_document.uri.clone(), schema),
         }
         .build();