diff --git a/crates/alknet-typedef/src/tunion.rs b/crates/alknet-typedef/src/tunion.rs
index fbeeb0b..1dbccc9 100644
--- a/crates/alknet-typedef/src/tunion.rs
+++ b/crates/alknet-typedef/src/tunion.rs
@@ -2,6 +2,645 @@
 //!
 //! TUnion supports two discriminator kinds: byte-offset (protocol
 //! dispatch, e.g., SFTP type bytes) and field-name (typedef.ts string
-//! pattern).
+//! pattern). This module reads the discriminator value from a byte
+//! buffer, looks up the variant schema in the union's `mapping`, and
+//! reports the offset where the variant struct begins.
+//!
+//! All reads go through [`crate::data_access`] so bounds checks and
+//! endianness handling are uniform with the rest of the engine.
 
-// TODO: implement
\ No newline at end of file
+use crate::data_access::{read_enum, read_string, read_u16, read_u32, read_u8};
+use crate::error::TypedefError;
+use crate::schema::{get_typedef_kind, parse_discriminator, DiscriminatorKind, Endian};
+use serde_json::Value;
+
+const U32_SIZE: usize = 4;
+const STRING_PREFIX_SIZE: usize = 4;
+const DISCRIMINATOR_PATH: &str = "__discriminator";
+
+/// The result of reading a TUnion discriminator.
+#[derive(Debug, Clone)]
+pub struct UnionDispatch {
+    /// The mapping key (stringified discriminator value for byte-offset,
+    /// string value for field-name).
+    pub key: String,
+    /// The byte offset where the variant struct starts.
+    pub variant_offset: usize,
+    /// The size of the discriminator in bytes.
+    pub discriminator_size: usize,
+}
+
+/// Read the discriminator value from a byte-offset TUnion.
+///
+/// The discriminator is a fixed-size integer at a known byte offset.
+/// Returns the mapping key (as a string) and the variant struct offset.
+///
+/// This is the SFTP `Packet` enum pattern — byte 0 is the type byte,
+/// bytes 1..N are the variant struct. The call protocol's event type
+/// dispatch uses the same pattern.
+///
+/// # Errors
+///
+/// - [`TypedefError::Schema`] if the discriminator annotation is missing
+///   or malformed, or if the discriminator `type` is not one of
+///   `TypeDef:Uint8` / `TypeDef:Uint16` / `TypeDef:Uint32`.
+/// - [`TypedefError::Access`] if the buffer is too short to contain the
+///   discriminator, or if the read value is not present in the union's
+///   `mapping`.
+pub fn read_byte_discriminator(
+    buffer: &[u8],
+    union_schema: &Value,
+    endian: Endian,
+) -> Result<UnionDispatch, TypedefError> {
+    let disc = parse_discriminator(union_schema)?;
+    let (offset, disc_type) = match disc {
+        DiscriminatorKind::Byte { offset, disc_type } => (offset, disc_type),
+        DiscriminatorKind::Field { .. } => {
+            return Err(TypedefError::Schema(
+                "read_byte_discriminator requires a byte-offset discriminator".to_string(),
+            ));
+        }
+    };
+
+    let (disc_value, discriminator_size) = match disc_type.as_str() {
+        "TypeDef:Uint8" => (u32::from(read_u8(buffer, offset, DISCRIMINATOR_PATH)?), 1),
+        "TypeDef:Uint16" => (
+            u32::from(read_u16(buffer, offset, DISCRIMINATOR_PATH, endian)?),
+            2,
+        ),
+        "TypeDef:Uint32" => (read_u32(buffer, offset, DISCRIMINATOR_PATH, endian)?, 4),
+        other => {
+            return Err(TypedefError::Schema(format!(
+                "unsupported byte discriminator type: {other}"
+            )));
+        }
+    };
+
+    let key = disc_value.to_string();
+    verify_mapping_key(union_schema, &key, DISCRIMINATOR_PATH, &key)?;
+
+    let variant_offset = offset
+        .checked_add(discriminator_size)
+        .ok_or_else(|| TypedefError::Access {
+            field_path: DISCRIMINATOR_PATH.to_string(),
+            reason: format!(
+                "offset {offset} + discriminator_size {discriminator_size} overflows usize"
+            ),
+        })?;
+
+    Ok(UnionDispatch {
+        key,
+        variant_offset,
+        discriminator_size,
+    })
+}
+
+/// Read the discriminator value from a field-name TUnion.
+///
+/// The discriminator is a named field within the struct — its offset
+/// is computed like any other field. The consumer provides the
+/// discriminator field's offset (from the OffsetMap or LayoutBuilder).
+///
+/// This is the typedef.ts `TUnion` pattern — the discriminator is a
+/// field like any other, and the mapping keys are string values.
+///
+/// # Errors
+///
+/// - [`TypedefError::Schema`] if the discriminator annotation is missing
+///   or malformed, the discriminator field is not declared in
+///   `properties`, the field has no `TypeDef:*` kind, or the field's
+///   kind is not one of `TypeDef:String` / `TypeDef:Uint8` /
+///   `TypeDef:Enum`.
+/// - [`TypedefError::Access`] if the buffer is too short to contain the
+///   discriminator field, or if the read value is not present in the
+///   union's `mapping`.
+pub fn read_field_discriminator(
+    buffer: &[u8],
+    union_schema: &Value,
+    disc_field_offset: usize,
+    endian: Endian,
+) -> Result<UnionDispatch, TypedefError> {
+    let disc = parse_discriminator(union_schema)?;
+    let name = match disc {
+        DiscriminatorKind::Field { name } => name,
+        DiscriminatorKind::Byte { .. } => {
+            return Err(TypedefError::Schema(
+                "read_field_discriminator requires a field-name discriminator".to_string(),
+            ));
+        }
+    };
+
+    let field_schema = union_schema
+        .get("properties")
+        .and_then(Value::as_object)
+        .and_then(|props| props.get(&name))
+        .ok_or_else(|| {
+            TypedefError::Schema(format!(
+                "discriminator field '{name}' not found in union properties"
+            ))
+        })?;
+
+    let kind = get_typedef_kind(field_schema).ok_or_else(|| {
+        TypedefError::Schema(format!(
+            "discriminator field '{name}' has no TypeDef:* kind"
+        ))
+    })?;
+
+    let (key, discriminator_field_size) = match kind {
+        "TypeDef:String" => {
+            let s = read_string(buffer, disc_field_offset, &name, endian)?;
+            let size = STRING_PREFIX_SIZE
+                .checked_add(s.len())
+                .ok_or_else(|| TypedefError::Access {
+                    field_path: name.clone(),
+                    reason: format!(
+                        "string prefix {STRING_PREFIX_SIZE} + data length {} overflows usize",
+                        s.len()
+                    ),
+                })?;
+            (s.to_string(), size)
+        }
+        "TypeDef:Uint8" => {
+            let v = read_u8(buffer, disc_field_offset, &name)?;
+            (v.to_string(), 1)
+        }
+        "TypeDef:Enum" => {
+            let v = read_enum(buffer, disc_field_offset, &name, endian)?;
+            (v.to_string(), U32_SIZE)
+        }
+        other => {
+            return Err(TypedefError::Schema(format!(
+                "unsupported discriminator field type: {other}"
+            )));
+        }
+    };
+
+    verify_mapping_key(union_schema, &key, &name, key.as_str())?;
+
+    let variant_offset = disc_field_offset
+        .checked_add(discriminator_field_size)
+        .ok_or_else(|| TypedefError::Access {
+            field_path: name.clone(),
+            reason: format!(
+                "disc_field_offset {disc_field_offset} + discriminator_field_size {discriminator_field_size} overflows usize"
+            ),
+        })?;
+
+    Ok(UnionDispatch {
+        key,
+        variant_offset,
+        discriminator_size: discriminator_field_size,
+    })
+}
+
+/// Look up a variant schema from the union's mapping.
+///
+/// Returns the variant schema. Inline schemas are returned directly.
+/// `$ref` pointers of the form `"#/$defs/<name>"` are resolved against
+/// the `union_schema`'s own `$defs` block (when the union schema is the
+/// schema root). For nested unions whose `$defs` live on an ancestor,
+/// the caller (typically `TypedefEngine::compile`) is expected to
+/// resolve refs before reaching this function, or to inline the
+/// variant schemas into the mapping at load time.
+///
+/// # Errors
+///
+/// - [`TypedefError::Schema`] if the union has no `mapping` object, the
+///   `key` is not present, a `$ref` is malformed, or a `$ref` cannot be
+///   resolved against the union schema's own `$defs`.
+pub fn resolve_variant<'a>(
+    union_schema: &'a Value,
+    key: &str,
+) -> Result<&'a Value, TypedefError> {
+    let mapping = union_schema
+        .get("mapping")
+        .and_then(Value::as_object)
+        .ok_or_else(|| TypedefError::Schema("union is missing 'mapping' object".to_string()))?;
+
+    let variant = mapping
+        .get(key)
+        .ok_or_else(|| TypedefError::Schema(format!("unknown mapping key: {key}")))?;
+
+    let ref_str = match variant.get("$ref").and_then(Value::as_str) {
+        Some(r) => r,
+        None => return Ok(variant),
+    };
+
+    let pointer = ref_str
+        .strip_prefix('#')
+        .ok_or_else(|| TypedefError::Schema(format!("unsupported $ref form: {ref_str}")))?;
+
+    let resolved = resolve_json_pointer(union_schema, pointer).ok_or_else(|| {
+        TypedefError::Schema(format!(
+            "cannot resolve $ref {ref_str} against union schema; ensure refs are inlined or the union schema contains $defs"
+        ))
+    })?;
+    Ok(resolved)
+}
+
+/// Get the discriminator size in bytes for a byte-offset discriminator.
+///
+/// Returns 1 for `TypeDef:Uint8`, 2 for `TypeDef:Uint16`, and 4 for
+/// `TypeDef:Uint32`. Field-name discriminators have no fixed size and
+/// produce a [`TypedefError::Schema`].
+///
+/// # Errors
+///
+/// - [`TypedefError::Schema`] if the discriminator annotation is
+///   missing/malformed, the discriminator `type` is unsupported, or the
+///   discriminator is a field-name discriminator.
+pub fn discriminator_size(union_schema: &Value) -> Result<usize, TypedefError> {
+    let disc = parse_discriminator(union_schema)?;
+    match disc {
+        DiscriminatorKind::Byte { disc_type, .. } => match disc_type.as_str() {
+            "TypeDef:Uint8" => Ok(1),
+            "TypeDef:Uint16" => Ok(2),
+            "TypeDef:Uint32" => Ok(4),
+            other => Err(TypedefError::Schema(format!(
+                "unsupported byte discriminator type: {other}"
+            ))),
+        },
+        DiscriminatorKind::Field { .. } => Err(TypedefError::Schema(
+            "field-name discriminator has no fixed size".to_string(),
+        )),
+    }
+}
+
+fn verify_mapping_key(
+    union_schema: &Value,
+    key: &str,
+    field_path: &str,
+    raw_value: &str,
+) -> Result<(), TypedefError> {
+    let in_mapping = union_schema
+        .get("mapping")
+        .and_then(Value::as_object)
+        .map(|m| m.contains_key(key))
+        .unwrap_or(false);
+    if in_mapping {
+        Ok(())
+    } else {
+        Err(TypedefError::Access {
+            field_path: field_path.to_string(),
+            reason: format!("unknown discriminator value: {raw_value}"),
+        })
+    }
+}
+
+fn resolve_json_pointer<'a>(root: &'a Value, pointer: &str) -> Option<&'a Value> {
+    if pointer.is_empty() {
+        return Some(root);
+    }
+    let trimmed = pointer.strip_prefix('/')?;
+    let mut current = root;
+    for unescaped in trimmed.split('/') {
+        let segment = unescape_json_pointer_token(unescaped)?;
+        current = current.get(&segment)?;
+    }
+    Some(current)
+}
+
+fn unescape_json_pointer_token(token: &str) -> Option<String> {
+    let mut out = String::with_capacity(token.len());
+    let mut chars = token.chars();
+    while let Some(c) = chars.next() {
+        match c {
+            '~' => match chars.next() {
+                Some('0') => out.push('~'),
+                Some('1') => out.push('/'),
+                _ => return None,
+            },
+            other => out.push(other),
+        }
+    }
+    Some(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    const LE: Endian = Endian::Little;
+    const BE: Endian = Endian::Big;
+
+    fn byte_union_schema(offset: usize, disc_type: &str) -> Value {
+        json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "byte", "offset": offset, "type": disc_type},
+            "mapping": {
+                "5": {"TypeDef:Struct": true, "properties": {"id": {"TypeDef:Uint32": true}}},
+                "6": {"TypeDef:Struct": true, "properties": {"len": {"TypeDef:Uint16": true}}}
+            }
+        })
+    }
+
+    fn field_union_schema(field_name: &str, field_kind: &str) -> Value {
+        let field_schema = match field_kind {
+            "TypeDef:Enum" => json!({
+                "TypeDef:Enum": true,
+                "enum": ["read", "write"]
+            }),
+            _ => json!({field_kind: true}),
+        };
+        let (key_a, key_b) = match field_kind {
+            "TypeDef:String" => ("read", "write"),
+            _ => ("0", "1"),
+        };
+        json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "field", "name": field_name},
+            "properties": {
+                field_name: field_schema
+            },
+            "mapping": {
+                key_a: {"TypeDef:Struct": true, "properties": {"n": {"TypeDef:Uint32": true}}},
+                key_b: {"TypeDef:Struct": true, "properties": {"m": {"TypeDef:Uint16": true}}}
+            }
+        })
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint8_default_offset() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let buf = [5u8, 0xAA, 0xBB, 0xCC];
+        let d = read_byte_discriminator(&buf, &schema, LE).expect("read");
+        assert_eq!(d.key, "5");
+        assert_eq!(d.variant_offset, 1);
+        assert_eq!(d.discriminator_size, 1);
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint8_big_endian() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let buf = [6u8];
+        let d = read_byte_discriminator(&buf, &schema, BE).expect("read");
+        assert_eq!(d.key, "6");
+        assert_eq!(d.variant_offset, 1);
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint16_little_endian() {
+        let schema = byte_union_schema(2, "TypeDef:Uint16");
+        let mut buf = vec![0u8; 4];
+        buf[2..4].copy_from_slice(&5u16.to_le_bytes());
+        let d = read_byte_discriminator(&buf, &schema, LE).expect("read");
+        assert_eq!(d.key, "5");
+        assert_eq!(d.variant_offset, 4);
+        assert_eq!(d.discriminator_size, 2);
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint16_big_endian() {
+        let schema = byte_union_schema(0, "TypeDef:Uint16");
+        let buf = [0x00, 0x06, 0xAA, 0xBB];
+        let d = read_byte_discriminator(&buf, &schema, BE).expect("read");
+        assert_eq!(d.key, "6");
+        assert_eq!(d.variant_offset, 2);
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint32_little_endian() {
+        let schema = byte_union_schema(0, "TypeDef:Uint32");
+        let mut buf = vec![0u8; 8];
+        buf[0..4].copy_from_slice(&5u32.to_le_bytes());
+        let d = read_byte_discriminator(&buf, &schema, LE).expect("read");
+        assert_eq!(d.key, "5");
+        assert_eq!(d.variant_offset, 4);
+        assert_eq!(d.discriminator_size, 4);
+    }
+
+    #[test]
+    fn read_byte_discriminator_uint32_big_endian() {
+        let schema = byte_union_schema(0, "TypeDef:Uint32");
+        let mut buf = vec![0u8; 8];
+        buf[0..4].copy_from_slice(&6u32.to_be_bytes());
+        let d = read_byte_discriminator(&buf, &schema, BE).expect("read");
+        assert_eq!(d.key, "6");
+        assert_eq!(d.variant_offset, 4);
+    }
+
+    #[test]
+    fn read_byte_discriminator_unknown_value_is_access_error() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let buf = [99u8];
+        let err = read_byte_discriminator(&buf, &schema, LE).unwrap_err();
+        match err {
+            TypedefError::Access { field_path, reason } => {
+                assert_eq!(field_path, DISCRIMINATOR_PATH);
+                assert!(reason.contains("99"), "reason: {reason}");
+            }
+            other => panic!("expected Access, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn read_byte_discriminator_buffer_too_short_is_access_error() {
+        let schema = byte_union_schema(4, "TypeDef:Uint32");
+        let buf = [0u8; 2];
+        let err = read_byte_discriminator(&buf, &schema, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Access { .. }));
+    }
+
+    #[test]
+    fn read_byte_discriminator_field_kind_is_schema_error() {
+        let schema = field_union_schema("type", "TypeDef:String");
+        let buf = [0u8; 16];
+        let err = read_byte_discriminator(&buf, &schema, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn read_field_discriminator_string() {
+        let schema = field_union_schema("type", "TypeDef:String");
+        let mut buf = vec![0u8; 32];
+        let value = "read";
+        let len_bytes = (value.len() as u32).to_le_bytes();
+        buf[0..4].copy_from_slice(&len_bytes);
+        buf[4..4 + value.len()].copy_from_slice(value.as_bytes());
+        let d = read_field_discriminator(&buf, &schema, 0, LE).expect("read");
+        assert_eq!(d.key, "read");
+        assert_eq!(d.variant_offset, 4 + value.len());
+        assert_eq!(d.discriminator_size, 4 + value.len());
+    }
+
+    #[test]
+    fn read_field_discriminator_uint8() {
+        let schema = field_union_schema("type", "TypeDef:Uint8");
+        let mut buf = vec![0u8; 8];
+        buf[0] = 0;
+        let d = read_field_discriminator(&buf, &schema, 0, LE).expect("read");
+        assert_eq!(d.key, "0");
+        assert_eq!(d.variant_offset, 1);
+        assert_eq!(d.discriminator_size, 1);
+    }
+
+    #[test]
+    fn read_field_discriminator_enum() {
+        let schema = field_union_schema("type", "TypeDef:Enum");
+        let mut buf = vec![0u8; 8];
+        buf[0..4].copy_from_slice(&0u32.to_le_bytes());
+        let d = read_field_discriminator(&buf, &schema, 0, LE).expect("read");
+        assert_eq!(d.key, "0");
+        assert_eq!(d.variant_offset, 4);
+        assert_eq!(d.discriminator_size, 4);
+    }
+
+    #[test]
+    fn read_field_discriminator_string_big_endian() {
+        let schema = field_union_schema("type", "TypeDef:String");
+        let mut buf = vec![0u8; 32];
+        let value = "write";
+        let len_bytes = (value.len() as u32).to_be_bytes();
+        buf[0..4].copy_from_slice(&len_bytes);
+        buf[4..4 + value.len()].copy_from_slice(value.as_bytes());
+        let d = read_field_discriminator(&buf, &schema, 0, BE).expect("read");
+        assert_eq!(d.key, "write");
+        assert_eq!(d.variant_offset, 4 + value.len());
+    }
+
+    #[test]
+    fn read_field_discriminator_unknown_value_is_access_error() {
+        let schema = field_union_schema("type", "TypeDef:Uint8");
+        let mut buf = vec![0u8; 8];
+        buf[0] = 99;
+        let err = read_field_discriminator(&buf, &schema, 0, LE).unwrap_err();
+        match err {
+            TypedefError::Access { field_path, reason } => {
+                assert_eq!(field_path, "type");
+                assert!(reason.contains("99"), "reason: {reason}");
+            }
+            other => panic!("expected Access, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn read_field_discriminator_field_not_found_is_schema_error() {
+        let schema = json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "field", "name": "missing"},
+            "properties": {"other": {"TypeDef:Uint8": true}},
+            "mapping": {"5": {"TypeDef:Struct": true}}
+        });
+        let buf = [0u8; 4];
+        let err = read_field_discriminator(&buf, &schema, 0, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn read_field_discriminator_no_typedef_kind_is_schema_error() {
+        let schema = json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "field", "name": "type"},
+            "properties": {"type": {"type": "string"}},
+            "mapping": {"read": {"TypeDef:Struct": true}}
+        });
+        let buf = [0u8; 4];
+        let err = read_field_discriminator(&buf, &schema, 0, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn read_field_discriminator_unsupported_kind_is_schema_error() {
+        let schema = field_union_schema("type", "TypeDef:Float32");
+        let buf = [0u8; 8];
+        let err = read_field_discriminator(&buf, &schema, 0, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn read_field_discriminator_byte_kind_is_schema_error() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let buf = [5u8];
+        let err = read_field_discriminator(&buf, &schema, 0, LE).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn resolve_variant_inline_schema() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let variant = resolve_variant(&schema, "5").expect("resolve");
+        assert_eq!(
+            variant.get("TypeDef:Struct").and_then(Value::as_bool),
+            Some(true)
+        );
+    }
+
+    #[test]
+    fn resolve_variant_ref_against_own_defs() {
+        let schema = json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "byte"},
+            "mapping": {
+                "5": {"$ref": "#/$defs/Read"}
+            },
+            "$defs": {
+                "Read": {"TypeDef:Struct": true, "properties": {"id": {"TypeDef:Uint32": true}}}
+            }
+        });
+        let variant = resolve_variant(&schema, "5").expect("resolve");
+        assert_eq!(
+            variant.get("TypeDef:Struct").and_then(Value::as_bool),
+            Some(true)
+        );
+    }
+
+    #[test]
+    fn resolve_variant_unknown_key_is_schema_error() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        let err = resolve_variant(&schema, "999").unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn resolve_variant_missing_mapping_is_schema_error() {
+        let schema = json!({"TypeDef:Union": true, "discriminator": {"kind": "byte"}});
+        let err = resolve_variant(&schema, "5").unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn resolve_variant_unresolvable_ref_is_schema_error() {
+        let schema = json!({
+            "TypeDef:Union": true,
+            "discriminator": {"kind": "byte"},
+            "mapping": {
+                "5": {"$ref": "#/$defs/Read"}
+            }
+        });
+        let err = resolve_variant(&schema, "5").unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn discriminator_size_uint8() {
+        let schema = byte_union_schema(0, "TypeDef:Uint8");
+        assert_eq!(discriminator_size(&schema).unwrap(), 1);
+    }
+
+    #[test]
+    fn discriminator_size_uint16() {
+        let schema = byte_union_schema(0, "TypeDef:Uint16");
+        assert_eq!(discriminator_size(&schema).unwrap(), 2);
+    }
+
+    #[test]
+    fn discriminator_size_uint32() {
+        let schema = byte_union_schema(0, "TypeDef:Uint32");
+        assert_eq!(discriminator_size(&schema).unwrap(), 4);
+    }
+
+    #[test]
+    fn discriminator_size_field_kind_is_schema_error() {
+        let schema = field_union_schema("type", "TypeDef:String");
+        let err = discriminator_size(&schema).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+
+    #[test]
+    fn discriminator_size_missing_discriminator_is_schema_error() {
+        let schema = json!({"TypeDef:Union": true});
+        let err = discriminator_size(&schema).unwrap_err();
+        assert!(matches!(err, TypedefError::Schema(_)));
+    }
+}
\ No newline at end of file
diff --git a/tasks/typedef/tunion.md b/tasks/typedef/tunion.md
index ceb3be4..4af13d6 100644
--- a/tasks/typedef/tunion.md
+++ b/tasks/typedef/tunion.md
@@ -1,7 +1,7 @@
 ---
 id: typedef/tunion
 name: Implement TUnion discriminator dispatch for byte-offset and field-name discriminators
-status: pending
+status: completed
 depends_on: [typedef/schema-types, typedef/error-type, typedef/data-access]
 scope: narrow
 risk: medium
@@ -142,4 +142,17 @@ are full JSON Pointer paths).
 
 ## Summary
 
-> To be filled on completion
+Implemented TUnion discriminator dispatch in `crates/alknet-typedef/src/tunion.rs`
+with the `UnionDispatch` result type and four public functions: `read_byte_discriminator`,
+`read_field_discriminator`, `resolve_variant`, and `discriminator_size`. Byte-offset
+discriminators support `TypeDef:Uint8`/`Uint16`/`Uint32` via the endian-aware
+`data_access` readers, stringifying the integer value as the mapping key and returning
+`offset + discriminator_size` as the variant start. Field-name discriminators support
+`TypeDef:String`/`Uint8`/`Enum` fields, reading the value at the caller-provided offset
+and computing the variant start as `disc_field_offset + discriminator_field_size`
+(`4 + str.len()` for strings, 1 for u8, 4 for enum). `resolve_variant` returns inline
+schemas directly and resolves `"#/$defs/<name>"` refs against the union schema's own
+`$defs` block, returning `TypedefError::Schema` for unresolvable refs (the engine
+layer is expected to inline or pass the root for nested unions). All 28 unit tests
+pass, covering both discriminator kinds, all supported sub-types, both endiannesses,
+buffer-too-short, unknown mapping keys, and ref resolution.