diff --git a/crates/alknet-typedef/src/error.rs b/crates/alknet-typedef/src/error.rs
index bfddeeb..41501cf 100644
--- a/crates/alknet-typedef/src/error.rs
+++ b/crates/alknet-typedef/src/error.rs
@@ -4,4 +4,48 @@
 //! conditions across the engine's three phases (schema parsing, offset
 //! computation, read/write) plus validation.
 
-// TODO: implement
\ No newline at end of file
+use std::fmt;
+
+/// Errors produced by the typedef engine across all phases.
+#[derive(Debug)]
+pub enum TypedefError {
+    /// Schema parsing errors — invalid JSON, missing required keywords,
+    /// unknown `TypeDef:*` kinds, malformed annotations.
+    Schema(String),
+
+    /// Offset computation errors — field not found, type not supported
+    /// for offset computation, recursive depth exceeded.
+    Offset {
+        field_path: String,
+        reason: String,
+    },
+
+    /// Read/write errors — buffer too short, invalid UTF-8, value out
+    /// of range for the target type.
+    Access {
+        field_path: String,
+        reason: String,
+    },
+
+    /// Validation errors — delegated to the `jsonschema` crate.
+    /// The `'static` lifetime is correct: the validator owns its schema
+    /// reference and lives for the lifetime of the `TypedefEngine`.
+    Validation(jsonschema::ValidationError<'static>),
+}
+
+impl fmt::Display for TypedefError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            TypedefError::Schema(msg) => write!(f, "schema error: {msg}"),
+            TypedefError::Offset { field_path, reason } => {
+                write!(f, "offset error at {field_path}: {reason}")
+            }
+            TypedefError::Access { field_path, reason } => {
+                write!(f, "access error at {field_path}: {reason}")
+            }
+            TypedefError::Validation(inner) => write!(f, "validation error: {inner}"),
+        }
+    }
+}
+
+impl std::error::Error for TypedefError {}
\ No newline at end of file
diff --git a/crates/alknet-typedef/src/lib.rs b/crates/alknet-typedef/src/lib.rs
index 333f5df..f5afdef 100644
--- a/crates/alknet-typedef/src/lib.rs
+++ b/crates/alknet-typedef/src/lib.rs
@@ -29,4 +29,6 @@ pub mod offset_map;
 pub mod schema;
 pub mod sequential_reader;
 pub mod tunion;
-pub mod validation;
\ No newline at end of file
+pub mod validation;
+
+pub use error::TypedefError;
\ No newline at end of file
diff --git a/tasks/typedef/error-type.md b/tasks/typedef/error-type.md
index 9dc0495..dad5eab 100644
--- a/tasks/typedef/error-type.md
+++ b/tasks/typedef/error-type.md
@@ -1,7 +1,7 @@
 ---
 id: typedef/error-type
 name: Implement TypedefError enum with Schema, Offset, Access, and Validation variants
-status: pending
+status: completed
 depends_on: [typedef/crate-init]
 scope: narrow
 risk: low
@@ -107,4 +107,4 @@ Since the `jsonschema` crate is a dependency, this is straightforward.
 
 ## Summary
 
-> To be filled on completion
+Implemented the `TypedefError` enum in `crates/alknet-typedef/src/error.rs` with four variants (`Schema`, `Offset`, `Access`, `Validation`) per ADR-098, plus `Display` (with field-path-aware messages for `Offset` and `Access`) and `std::error::Error` impls and a derived `Debug`. Re-exported `TypedefError` from `crates/alknet-typedef/src/lib.rs` so it is accessible as `alknet_typedef::TypedefError`. `cargo check`, `cargo clippy -D warnings`, and `cargo build --workspace` all pass with no warnings.