diff --git a/Cargo.lock b/Cargo.lock
index ddd2d6b..4806204 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -74,6 +74,12 @@ dependencies = [
  "windows-sys 0.48.0",
 ]
 
+[[package]]
+name = "anyhow"
+version = "1.0.75"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4668cab20f66d8d020e1fbc0ebe47217433c1b6c8f2040faf858554e394ace6"
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -698,6 +704,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "stef",
+ "stef-compiler",
  "stef-parser",
  "syn",
  "thiserror",
@@ -719,6 +726,7 @@ name = "stef-compiler"
 version = "0.1.0"
 dependencies = [
  "stef-parser",
+ "thiserror",
 ]
 
 [[package]]
@@ -746,6 +754,7 @@ dependencies = [
 name = "stef-playground"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
  "stef",
  "stef-build",
 ]
@@ -838,18 +847,18 @@ dependencies = [
 
 [[package]]
 name = "thiserror"
-version = "1.0.49"
+version = "1.0.50"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1177e8c6d7ede7afde3585fd2513e611227efd6481bd78d2e82ba1ce16557ed4"
+checksum = "f9a7210f5c9a7156bb50aa36aed4c95afb51df0df00713949448cf9e97d382d2"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "1.0.49"
+version = "1.0.50"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "10712f02019e9288794769fba95cd6847df9874d49d871d062172f9dd41bc4cc"
+checksum = "266b2e40bc00e5a6c09c3584011e08b06f123c00362c92b975ba9843aaaa14b8"
 dependencies = [
  "proc-macro2",
  "quote",
diff --git a/Cargo.toml b/Cargo.toml
index 8f6c80e..97d87f0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,7 +21,7 @@ mimalloc = "0.1.39"
 proc-macro2 = { version = "1.0.69", default-features = false }
 quote = { version = "1.0.33", default-features = false }
 syn = "2.0.38"
-thiserror = "1.0.49"
+thiserror = "1.0.50"
 
 [profile.release]
 lto = true
diff --git a/crates/stef-build/Cargo.toml b/crates/stef-build/Cargo.toml
index d804589..8baa2ef 100644
--- a/crates/stef-build/Cargo.toml
+++ b/crates/stef-build/Cargo.toml
@@ -15,6 +15,7 @@ miette.workspace = true
 prettyplease = "0.2.15"
 proc-macro2.workspace = true
 quote.workspace = true
+stef-compiler = { path = "../stef-compiler" }
 stef-parser = { path = "../stef-parser" }
 syn.workspace = true
 thiserror.workspace = true
diff --git a/crates/stef-build/src/lib.rs b/crates/stef-build/src/lib.rs
index 034b244..01a01ee 100644
--- a/crates/stef-build/src/lib.rs
+++ b/crates/stef-build/src/lib.rs
@@ -38,27 +38,39 @@ pub enum Error {
     },
     #[error("failed parsing schema from {file:?}: {message}")]
     Parse { message: String, file: PathBuf },
+    #[error("failed compiling schema from {file:?}")]
+    Compile {
+        #[source]
+        source: stef_compiler::Error,
+        file: PathBuf,
+    },
 }
 
 pub fn compile(schemas: &[impl AsRef<str>], _includes: &[impl AsRef<Path>]) -> Result<()> {
     let out_dir = PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
 
     for schema in schemas.iter().map(AsRef::as_ref) {
-        for schema in glob::glob(schema).map_err(|e| Error::Pattern {
-            source: e,
+        for schema in glob::glob(schema).map_err(|source| Error::Pattern {
+            source,
             glob: schema.to_owned(),
         })? {
             let path = schema.map_err(|e| Error::Glob { source: e })?;
 
-            let input = std::fs::read_to_string(&path).map_err(|e| Error::Read {
-                source: e,
+            let input = std::fs::read_to_string(&path).map_err(|source| Error::Read {
+                source,
                 file: path.clone(),
             })?;
 
             let schema = Schema::parse(&input).map_err(|e| Error::Parse {
-                message: e.to_string(),
+                message: format!("{e:?}"),
                 file: path.clone(),
             })?;
+
+            stef_compiler::validate_schema(&schema).map_err(|source| Error::Compile {
+                source,
+                file: path.clone(),
+            })?;
+
             let code = definition::compile_schema(&schema);
             let code = prettyplease::unparse(&syn::parse2(code).unwrap());
 
diff --git a/crates/stef-compiler/Cargo.toml b/crates/stef-compiler/Cargo.toml
index ba3843f..25bbde6 100644
--- a/crates/stef-compiler/Cargo.toml
+++ b/crates/stef-compiler/Cargo.toml
@@ -11,3 +11,4 @@ license.workspace = true
 
 [dependencies]
 stef-parser = { path = "../stef-parser" }
+thiserror.workspace = true
diff --git a/crates/stef-compiler/src/ids.rs b/crates/stef-compiler/src/ids.rs
new file mode 100644
index 0000000..fafde4d
--- /dev/null
+++ b/crates/stef-compiler/src/ids.rs
@@ -0,0 +1,108 @@
+use std::collections::HashMap;
+
+use stef_parser::{Enum, Fields, Id, Struct};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum DuplicateId {
+    #[error("duplicate ID in an enum variant")]
+    EnumVariant(#[from] DuplicateVariantId),
+    #[error("duplicate ID in a field")]
+    Field(#[from] DuplicateFieldId),
+}
+
+#[derive(Debug, Error)]
+#[error("duplicate ID {} in enum variant `{name}`, already used in `{other_name}`", id.0)]
+pub struct DuplicateVariantId {
+    pub id: Id,
+    pub name: String,
+    pub other_name: String,
+}
+
+#[derive(Debug, Error)]
+pub enum DuplicateFieldId {
+    #[error("duplicate ID {} in field `{name}`, already used in `{other_name}`", id.0)]
+    Named {
+        id: Id,
+        name: String,
+        other_name: String,
+    },
+    #[error("duplicate ID {} in field {position}, already used in {other_position}", id.0)]
+    Unnamed {
+        id: Id,
+        position: usize,
+        other_position: usize,
+    },
+}
+
+/// Ensure all IDs inside a struct are unique (which are the field IDs).
+pub(crate) fn validate_struct_ids(value: &Struct<'_>) -> Result<(), DuplicateFieldId> {
+    validate_field_ids(&value.fields)
+}
+
+/// Ensure all IDs inside an enum are unique, which means all variants have a unique ID, plus all
+/// potential fields in a variant are unique (within that variant).
+pub(crate) fn validate_enum_ids(value: &Enum<'_>) -> Result<(), DuplicateId> {
+    let mut visited = HashMap::with_capacity(value.variants.len());
+    value
+        .variants
+        .iter()
+        .find_map(|variant| {
+            visited
+                .insert(variant.id, variant.name)
+                .map(|other_name| {
+                    DuplicateVariantId {
+                        id: variant.id,
+                        name: variant.name.to_owned(),
+                        other_name: other_name.to_owned(),
+                    }
+                    .into()
+                })
+                .or_else(|| {
+                    validate_field_ids(&variant.fields)
+                        .err()
+                        .map(DuplicateId::from)
+                })
+        })
+        .map_or(Ok(()), Err)
+}
+
+/// Ensure all field IDs of a struct or enum are unique.
+fn validate_field_ids(value: &Fields) -> Result<(), DuplicateFieldId> {
+    match value {
+        Fields::Named(named) => {
+            let mut visited = HashMap::with_capacity(named.len());
+            named
+                .iter()
+                .find_map(|field| {
+                    visited.insert(field.id, field.name).map(|other_field| {
+                        DuplicateFieldId::Named {
+                            id: field.id,
+                            name: field.name.to_owned(),
+                            other_name: other_field.to_owned(),
+                        }
+                    })
+                })
+                .map_or(Ok(()), Err)?;
+        }
+        Fields::Unnamed(unnamed) => {
+            let mut visited = HashMap::with_capacity(unnamed.len());
+            unnamed
+                .iter()
+                .enumerate()
+                .find_map(|(pos, field)| {
+                    visited
+                        .insert(field.id, pos)
+                        .map(|other_position| DuplicateFieldId::Unnamed {
+                            id: field.id,
+                            position: pos,
+                            other_position,
+                        })
+                })
+                .map_or(Ok(()), Err)?;
+        }
+        Fields::Unit => {}
+    }
+
+    Ok(())
+}
diff --git a/crates/stef-compiler/src/lib.rs b/crates/stef-compiler/src/lib.rs
index 8b13789..4c6c23c 100644
--- a/crates/stef-compiler/src/lib.rs
+++ b/crates/stef-compiler/src/lib.rs
@@ -1 +1,30 @@
+pub use ids::{DuplicateFieldId, DuplicateId, DuplicateVariantId};
+use stef_parser::{Definition, Schema};
+use thiserror::Error;
 
+mod ids;
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("duplicate ID found")]
+    DuplicateId(#[from] DuplicateId),
+}
+
+impl From<DuplicateFieldId> for Error {
+    fn from(v: DuplicateFieldId) -> Self {
+        Self::DuplicateId(v.into())
+    }
+}
+
+pub fn validate_schema(value: &Schema<'_>) -> Result<(), Error> {
+    value.definitions.iter().try_for_each(validate_definition)
+}
+
+fn validate_definition(value: &Definition<'_>) -> Result<(), Error> {
+    match value {
+        Definition::Module(m) => m.definitions.iter().try_for_each(validate_definition),
+        Definition::Struct(s) => ids::validate_struct_ids(s).map_err(Into::into),
+        Definition::Enum(e) => ids::validate_enum_ids(e).map_err(Into::into),
+        Definition::TypeAlias(_) | Definition::Const(_) | Definition::Import(_) => Ok(()),
+    }
+}
diff --git a/crates/stef-parser/Cargo.toml b/crates/stef-parser/Cargo.toml
index 03d264d..f03ef29 100644
--- a/crates/stef-parser/Cargo.toml
+++ b/crates/stef-parser/Cargo.toml
@@ -16,7 +16,7 @@ rustc-args = ["--cfg", "docsrs"]
 miette.workspace = true
 owo-colors = { version = "3.5.0", features = ["supports-colors"] }
 stef-derive = { path = "../stef-derive" }
-winnow = "0.5.16"
+winnow = "0.5.17"
 
 [dev-dependencies]
 indoc.workspace = true
diff --git a/crates/stef-parser/src/lib.rs b/crates/stef-parser/src/lib.rs
index 86abfc5..3ebde34 100644
--- a/crates/stef-parser/src/lib.rs
+++ b/crates/stef-parser/src/lib.rs
@@ -763,7 +763,7 @@ impl<'a> Display for Generics<'a> {
 /// ```txt
 /// @1
 /// ```
-#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct Id(pub u32);
 
 impl Display for Id {
diff --git a/crates/stef-playground/Cargo.toml b/crates/stef-playground/Cargo.toml
index 0dbc0f6..2bfb7e5 100644
--- a/crates/stef-playground/Cargo.toml
+++ b/crates/stef-playground/Cargo.toml
@@ -14,4 +14,5 @@ publish = false
 stef = { path = "../stef" }
 
 [build-dependencies]
+anyhow = "1.0.75"
 stef-build = { path = "../stef-build" }
diff --git a/crates/stef-playground/build.rs b/crates/stef-playground/build.rs
index e8e4c26..d6c27f9 100644
--- a/crates/stef-playground/build.rs
+++ b/crates/stef-playground/build.rs
@@ -1,4 +1,5 @@
-fn main() {
-    stef_build::compile(&["src/sample.stef"], &["src/"]).unwrap();
-    stef_build::compile(&["schemas/*.stef"], &["schemas/"]).unwrap();
+fn main() -> anyhow::Result<()> {
+    stef_build::compile(&["src/sample.stef"], &["src/"])?;
+    stef_build::compile(&["schemas/*.stef"], &["schemas/"])?;
+    Ok(())
 }
diff --git a/crates/stef/src/buf/decode.rs b/crates/stef/src/buf/decode.rs
index dc921e4..bc5096f 100644
--- a/crates/stef/src/buf/decode.rs
+++ b/crates/stef/src/buf/decode.rs
@@ -14,7 +14,7 @@ pub type Result<T, E = Error> = std::result::Result<T, E>;
 
 #[derive(Debug)]
 pub enum Error {
-    InsufficentData,
+    InsufficientData,
     DecodeInt(varint::DecodeIntError),
     NonUtf8(std::string::FromUtf8Error),
     MissingField { id: u32, name: Option<&'static str> },
@@ -39,7 +39,7 @@ pub const END_MARKER: u32 = 0;
 macro_rules! ensure_size {
     ($r:ident, $size:expr) => {
         if $r.remaining() < $size {
-            return Err(Error::InsufficentData);
+            return Err(Error::InsufficientData);
         }
     };
 }
@@ -155,7 +155,7 @@ where
 {
     let len = decode_u64(r)?;
     if (len as usize) < N {
-        return Err(Error::InsufficentData);
+        return Err(Error::InsufficientData);
     }
 
     let buf = (0..N).map(|_| decode(r)).collect::<Result<Vec<_>>>()?;