diff --git a/src/data_access.rs b/src/data_access.rs
index f05b6d8..1ebac52 100644
--- a/src/data_access.rs
+++ b/src/data_access.rs
@@ -712,6 +712,43 @@ mod tests {
         assert!(matches!(err, AlkTypeError::Access { .. }));
     }
 
+    #[test]
+    fn m4_write_bytes_indirect_pair_at_nonzero_offset_data_ok() {
+        // The indirect write's pair offset and data offset are
+        // independent; pin the pair landing mid-buffer with the data
+        // elsewhere (review #006 M4 item 3 — the write_bytes_indirect
+        // guard family is the canonical overflow-guard pattern, review
+        // #002 M2, and its nonzero-offset paths were uncovered).
+        let mut buf = vec![0u8; 24];
+        let written = write_bytes_indirect(&mut buf, 4, 16, b"xyz", "blob", BE).unwrap();
+        assert_eq!(written, 8);
+        assert_eq!(&buf[4..8], &16u32.to_be_bytes());
+        assert_eq!(&buf[8..12], &3u32.to_be_bytes());
+        assert_eq!(&buf[16..19], b"xyz");
+        let bytes = read_bytes_indirect(&buf, 4, "blob", BE).unwrap();
+        assert_eq!(bytes, b"xyz");
+    }
+
+    #[test]
+    fn m4_write_bytes_indirect_data_length_exceeds_bounds() {
+        // The data-region bounds check in write_bytes_indirect: the pair
+        // fits, but the {data_offset, length} target runs past the buffer
+        // end — the write must refuse without corrupting the pair.
+        let mut buf = vec![0u8; 20];
+        let err = write_bytes_indirect(&mut buf, 0, 16, b"hello", "blob", LE).unwrap_err();
+        match err {
+            AlkTypeError::Access { field_path, reason } => {
+                assert_eq!(field_path, "blob");
+                assert!(reason.contains("bounds"), "reason: {reason}");
+            }
+            other => panic!("expected Access, got {other:?}"),
+        }
+        // The pair was already written (offset+length at 0..8) — that's
+        // fine; the error refuses the data copy only.
+        assert_eq!(&buf[0..4], &16u32.to_le_bytes());
+        assert_eq!(&buf[4..8], &5u32.to_le_bytes());
+    }
+
     #[test]
     fn read_at_nonzero_offset() {
         let mut buf = vec![0u8; 16];
diff --git a/src/materialize.rs b/src/materialize.rs
index f435c01..b757c0f 100644
--- a/src/materialize.rs
+++ b/src/materialize.rs
@@ -727,18 +727,42 @@ fn materialize_union_packed(
         }
         BastDiscriminator::Field { name } => {
             let fields = union_node.fields();
-            let disc_field = fields
-                .iter()
-                .find(|f| f.name() == *name)
-                .ok_or_else(|| {
-                    AlkTypeError::Schema(format!(
-                        "materialize: union at {field_path} has no discriminator field '{name}'"
-                    ))
-                })?;
-            let disc_path = format!("{field_path}.{name}");
-            let disc_value =
-                materialize_field_packed(doc, disc_field, &disc_path, endian, buffer, offset)?;
-            let key = union_discriminator_key(&disc_value, field_path)?;
+            if fields.is_empty() {
+                return Err(AlkTypeError::Schema(format!(
+                    "materialize: union at {field_path} has no shared fields (the \
+                     discriminator field must be declared first)"
+                )));
+            }
+            // The shared-then-variant wire convention (H3): walk ALL
+            // declared `fields` in order — capturing the disc value at its
+            // real position — and start the variant after the whole shared
+            // walk. (Review #006 M6: the pre-fix arm materialized only the
+            // disc field and started the variant immediately after it,
+            // silently reading the remaining shared fields' bytes as
+            // variant data whenever the union had any.)
+            let mut key = None;
+            let mut disc_value: Option<Value> = None;
+            let mut shared_values: Vec<(String, Value)> = Vec::new();
+            for field in fields {
+                let path = format!("{field_path}.{}", field.name());
+                let value = materialize_field_packed(doc, field, &path, endian, buffer, offset)?;
+                if field.name() == name.as_str() {
+                    key = Some(union_discriminator_key(&value, field_path)?);
+                    disc_value = Some(value);
+                } else {
+                    shared_values.push((field.name().to_string(), value));
+                }
+            }
+            let key = key.ok_or_else(|| {
+                AlkTypeError::Schema(format!(
+                    "materialize: union at {field_path} has no discriminator field '{name}'"
+                ))
+            })?;
+            let disc_value = disc_value.ok_or_else(|| {
+                AlkTypeError::Schema(format!(
+                    "internal: union {field_path} discriminator field {name:?} not walked"
+                ))
+            })?;
             let mapping = union_node.mapping();
             let variant_ty = mapping
                 .iter()
@@ -760,6 +784,9 @@ fn materialize_union_packed(
             let mut obj = Map::new();
             obj.insert(DISCRIMINATOR_KEY.to_string(), Value::String(key.clone()));
             obj.insert((*name).to_string(), disc_value);
+            for (k, v) in shared_values {
+                obj.insert(k, v);
+            }
             flatten_variant_into(&mut obj, variant_value);
             Ok(Value::Object(obj))
         }
@@ -1029,6 +1056,7 @@ fn materialize_leaf_at(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::offset_map::ByteRange;
     use serde_json::json;
 
     fn doc_from(root: &Value, name: &str) -> BastDoc {
@@ -1648,6 +1676,410 @@ mod tests {
         assert_eq!(v["crc"], json!(0x01020304u32));
     }
 
+    // ----- M4 item 1: aligned nested structs + error arms ----------------
+
+    #[test]
+    fn m4_aligned_nested_struct_recursion_roundtrips() {
+        // The nested-struct recursion (materialize_struct_aligned entered
+        // with a non-empty path_prefix) had 0 executions in the review's
+        // coverage run — the heart of aligned validate_bytes was never
+        // exercised beyond a flat 2-field struct. This drives a
+        // three-level nesting through OffsetMap + materialize_aligned.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        {
+                            "name": "header",
+                            "kind": {
+                                "kind": "struct",
+                                "fields": [
+                                    { "name": "magic", "kind": "uint32" },
+                                    {
+                                        "name": "meta",
+                                        "kind": {
+                                            "kind": "struct",
+                                            "fields": [
+                                                { "name": "ver",  "kind": "uint16" },
+                                                { "name": "flag", "kind": "uint8" }
+                                            ]
+                                        }
+                                    }
+                                ]
+                            }
+                        },
+                        { "name": "body", "kind": "uint32" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        // Layout: header.magic @0..4, header.meta.ver @4..6, flag @6..7,
+        // (pad 1) body @8..12 — total 12.
+        assert_eq!(map.get("header.magic").map(|e| e.range), Some(ByteRange { start: 0, end: 4 }));
+        assert_eq!(map.get("header.meta.ver").map(|e| e.range), Some(ByteRange { start: 4, end: 6 }));
+        assert_eq!(map.get("header.meta.flag").map(|e| e.range), Some(ByteRange { start: 6, end: 7 }));
+        assert_eq!(map.get("body").map(|e| e.range), Some(ByteRange { start: 8, end: 12 }));
+        assert_eq!(map.total_size(), 12);
+        let mut buf = vec![0u8; 12];
+        buf[0..4].copy_from_slice(&0xDEADBEEFu32.to_le_bytes());
+        buf[4..6].copy_from_slice(&7u16.to_le_bytes());
+        buf[6] = 1;
+        buf[8..12].copy_from_slice(&99u32.to_le_bytes());
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["header"]["magic"], json!(0xDEADBEEFu32));
+        assert_eq!(v["header"]["meta"]["ver"], json!(7));
+        assert_eq!(v["header"]["meta"]["flag"], json!(1));
+        assert_eq!(v["body"], json!(99));
+    }
+
+    #[test]
+    fn m4_aligned_nested_struct_maxlength_string_deep() {
+        // maxLength trim inside nested aligned structs — the trim arm is
+        // shared with the flat case, but the nested entry lookup is not.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        {
+                            "name": "inner",
+                            "kind": {
+                                "kind": "struct",
+                                "fields": [
+                                    { "name": "name", "kind": "string", "maxLength": 8 }
+                                ]
+                            }
+                        },
+                        { "name": "id", "kind": "uint32" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let mut buf = vec![0u8; 12];
+        buf[..5].copy_from_slice(b"hello");
+        buf[8..12].copy_from_slice(&5u32.to_le_bytes());
+        let v = materialize_aligned_strict(&doc, &buf).expect("materialize");
+        assert_eq!(v["inner"]["name"], json!("hello"));
+        assert_eq!(v["id"], json!(5));
+    }
+
+    #[test]
+    fn m4_aligned_max_length_string_invalid_utf8_is_access_error() {
+        // The UTF-8 error arm of the maxLength trim (0-execution in the
+        // review's coverage run): garbage bytes in the reservation must
+        // be a clean Access error, not a panic or a silent String.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "name", "kind": "string", "maxLength": 8 }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let mut buf = vec![0u8; 8];
+        buf[..4].copy_from_slice(&[0xFF, 0xFE, 0x00, 0x80]);
+        let err = materialize_aligned_strict(&doc, &buf).unwrap_err();
+        match err {
+            AlkTypeError::Access { field_path, reason } => {
+                assert_eq!(field_path, "name");
+                assert!(reason.contains("UTF-8"), "reason: {reason}");
+            }
+            other => panic!("expected Access, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn m4_aligned_offset_indirect_out_of_bounds_is_access_error() {
+        // The offset-indirect arms' error path: the {offset, length} pair
+        // points outside the buffer — data_access's bounds check rejects.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "name", "kind": "string", "encoding": "offset-indirect" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let mut buf = vec![0u8; 8];
+        buf[0..4].copy_from_slice(&1000u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&4u32.to_le_bytes());
+        let err = materialize_aligned_strict(&doc, &buf).unwrap_err();
+        assert!(matches!(err, AlkTypeError::Access { .. }), "got {err:?}");
+    }
+
+    #[test]
+    fn m4_aligned_offset_indirect_string_roundtrips_after_sibling() {
+        // Offset-indirect with a following field — the indirect pair's
+        // data can point anywhere in the buffer (that is the point of
+        // the encoding); pin the data landing after the sibling field.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "id",   "kind": "uint32" },
+                        { "name": "name", "kind": "string", "encoding": "offset-indirect" },
+                        { "name": "tail", "kind": "uint32" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        // Layout: id @0..4, pair @4..12, tail @12..16 (total 16);
+        // string data at 16..21.
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        assert_eq!(map.get("name").map(|e| e.range), Some(ByteRange { start: 4, end: 12 }));
+        assert_eq!(map.get("tail").map(|e| e.range), Some(ByteRange { start: 12, end: 16 }));
+        let mut buf = vec![0u8; 21];
+        buf[0..4].copy_from_slice(&3u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&16u32.to_le_bytes());
+        buf[8..12].copy_from_slice(&5u32.to_le_bytes());
+        buf[12..16].copy_from_slice(&7u32.to_le_bytes());
+        buf[16..21].copy_from_slice(b"hello");
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["id"], json!(3));
+        assert_eq!(v["name"], json!("hello"));
+        assert_eq!(v["tail"], json!(7));
+    }
+
+    #[test]
+    fn m4_aligned_array_of_fixed_struct_elements_materializes() {
+        // Struct elements are rejected in aligned mode (their kind is
+        // `Struct`, which is_fixed_size() refuses — OQ-001's conservative
+        // gate; the packed reader supports fixed-struct arrays via the
+        // plan's computed stride but the aligned map does not). Pin the
+        // aligned array walk over scalar fixed elements + the map's
+        // per-element entries, which is the aligned-mode shape.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        {
+                            "name": "points",
+                            "kind": {
+                                "kind": "array",
+                                "element": "uint32",
+                                "count": 2
+                            }
+                        }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        assert_eq!(map.get("points[0]").map(|e| e.range), Some(ByteRange { start: 0, end: 4 }));
+        assert_eq!(map.get("points[1]").map(|e| e.range), Some(ByteRange { start: 4, end: 8 }));
+        let buf = [1u8, 0, 0, 0, 2, 0, 0, 0];
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["points"], json!([1, 2]));
+    }
+
+    #[test]
+    fn m4_aligned_record_with_struct_values_walks_nested_fields() {
+        // A record whose values are a struct: the aligned record arm
+        // drops into the packed BAST walker (materialize_typeref_packed →
+        // materialize_struct_packed), which was 0-execution in the review's
+        // coverage run. Pin the recursion + wire arithmetic.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "meta", "kind": { "kind": "record", "values": { "$ref": "#/$defs/V" } } }
+                    ]
+                },
+                "V": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "tag",  "kind": "uint8" },
+                        { "name": "val",  "kind": "uint32" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        // The record's internal walk is the packed walker (no padding):
+        // count@0..4 + entry0: keylen@4..8=1, key@8='a', tag@9, val@10..14
+        // = 14 bytes total.
+        let mut buf = vec![0u8; 14];
+        buf[0..4].copy_from_slice(&1u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&1u32.to_le_bytes());
+        buf[8] = b'a';
+        buf[9] = 7;
+        buf[10..14].copy_from_slice(&4242u32.to_le_bytes());
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["meta"]["a"], json!({ "tag": 7, "val": 4242 }));
+    }
+
+    #[test]
+    fn m4_aligned_record_with_array_values_walks_elements() {
+        // Record values that are an array of fixed primitives — the
+        // packed BAST walker's array arm through the aligned record path.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "bins", "kind": { "kind": "record", "values": { "kind": "array", "element": "uint16", "count": 2 } } }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        // count@0..4=1, keylen@4..8=1, key@8='a', elems@9..13 (2×u16).
+        let mut buf = vec![0u8; 13];
+        buf[0..4].copy_from_slice(&1u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&1u32.to_le_bytes());
+        buf[8] = b'a';
+        buf[9..11].copy_from_slice(&10u16.to_le_bytes());
+        buf[11..13].copy_from_slice(&20u16.to_le_bytes());
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["bins"], json!({ "a": [10, 20] }));
+    }
+
+    #[test]
+    fn m4_aligned_record_with_byte_disc_union_values_dispatches() {
+        // Record values that are a byte-disc union — the packed BAST
+        // walker's union arm through the aligned record path.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "events", "kind": { "kind": "record", "values": { "$ref": "#/$defs/Ev" } } }
+                    ]
+                },
+                "Ev": {
+                    "kind": "union",
+                    "discriminator": { "kind": "byte", "offset": 0, "type": "uint8" },
+                    "mapping": { "3": { "$ref": "#/$defs/Read" } }
+                },
+                "Read": { "kind": "struct", "fields": [ { "name": "n", "kind": "uint32" } ] }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        // count@0..4=1, keylen@4..8=1, key@8='a', disc@9=3, n@10..14.
+        let mut buf = vec![0u8; 14];
+        buf[0..4].copy_from_slice(&1u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&1u32.to_le_bytes());
+        buf[8] = b'a';
+        buf[9] = 3;
+        buf[10..14].copy_from_slice(&77u32.to_le_bytes());
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        assert_eq!(v["events"]["a"]["__discriminator"], json!(3));
+        assert_eq!(v["events"]["a"]["n"], json!(77));
+    }
+
+    #[test]
+    fn m4_aligned_record_with_field_disc_union_values_dispatches() {
+        // A field-disc union as record values — the union walker's field
+        // arm (shared walk + variant flatten) through the aligned record
+        // path.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "events", "kind": { "kind": "record", "values": { "$ref": "#/$defs/Ev" } } }
+                    ]
+                },
+                "Ev": {
+                    "kind": "union",
+                    "discriminator": { "kind": "field", "name": "type" },
+                    "fields": [ { "name": "type", "kind": "uint8" }, { "name": "seq", "kind": "uint32" } ],
+                    "mapping": { "1": { "$ref": "#/$defs/Read" } }
+                },
+                "Read": { "kind": "struct", "fields": [ { "name": "handle", "kind": "uint32" } ] }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        // count@0..4=1, keylen@4..8=1, key@8='a', shared: type@9, seq@10..14,
+        // variant: handle@14..18 → 18 bytes.
+        let mut buf = vec![0u8; 18];
+        buf[0..4].copy_from_slice(&1u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&1u32.to_le_bytes());
+        buf[8] = b'a';
+        buf[9] = 1;
+        buf[10..14].copy_from_slice(&5u32.to_le_bytes());
+        buf[14..18].copy_from_slice(&77u32.to_le_bytes());
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        let ev = &v["events"]["a"];
+        assert_eq!(ev["type"], json!(1));
+        assert_eq!(ev["seq"], json!(5));
+        assert_eq!(ev["handle"], json!(77));
+        assert_eq!(ev["__discriminator"], json!("1"));
+    }
+
+    #[test]
+    fn m4_aligned_record_with_wide_primitive_values_walks_all_arms() {
+        // The packed BAST walker's remaining primitive arms (i8..bool,
+        // u64, floats, string, bytes) as record values — 0-execution in
+        // the review's coverage run.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "vals", "kind": { "kind": "record", "values": { "$ref": "#/$defs/V" } } }
+                    ]
+                },
+                "V": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "i8",  "kind": "int8" },
+                        { "name": "u64", "kind": "uint64" },
+                        { "name": "f32", "kind": "float32" },
+                        { "name": "f64", "kind": "float64" },
+                        { "name": "b",   "kind": "bool" },
+                        { "name": "s",   "kind": "string" },
+                        { "name": "raw", "kind": "bytes" }
+                    ]
+                }
+            }
+        });
+        let doc = doc_from(&root, "S");
+        let map = crate::offset_map::OffsetMap::compute(&doc).expect("map");
+        let mut buf = vec![0u8; 64];
+        buf[0..4].copy_from_slice(&1u32.to_le_bytes());
+        buf[4..8].copy_from_slice(&1u32.to_le_bytes());
+        buf[8] = b'a';
+        let mut off = 9;
+        buf[off] = 0xF3; off += 1; // i8 = -13
+        buf[off..off+8].copy_from_slice(&0x0102030405060708u64.to_le_bytes()); off += 8;
+        buf[off..off+4].copy_from_slice(&1.5f32.to_le_bytes()); off += 4;
+        buf[off..off+8].copy_from_slice(&2.5f64.to_le_bytes()); off += 8;
+        buf[off] = 0x01; off += 1; // bool = true
+        buf[off..off+4].copy_from_slice(&1u32.to_le_bytes()); off += 4; // string len
+        buf[off] = b'z'; off += 1;
+        buf[off..off+4].copy_from_slice(&2u32.to_le_bytes()); off += 4; // bytes len
+        buf[off] = 0xAA;
+        buf[off+1] = 0xBB;
+        let v = materialize_aligned(&doc, &buf, &map).expect("materialize");
+        let obj = &v["vals"]["a"];
+        assert_eq!(obj["i8"], json!(-13));
+        assert_eq!(obj["u64"], json!(0x0102030405060708u64));
+        assert_eq!(obj["f32"], json!(1.5));
+        assert_eq!(obj["f64"], json!(2.5));
+        assert_eq!(obj["s"], json!("z"));
+        assert_eq!(obj["raw"], json!([0xAA, 0xBB]));
+    }
+
     #[test]
     fn materialize_packed_rejects_non_finite_float() {
         let root = json!({
diff --git a/src/sequential_reader.rs b/src/sequential_reader.rs
index 5ebe2a3..428f4e9 100644
--- a/src/sequential_reader.rs
+++ b/src/sequential_reader.rs
@@ -1464,6 +1464,259 @@ mod tests {
         assert!(matches!(err, AlkTypeError::Access { .. }), "got {err:?}");
     }
 
+    // ----- M4 item 2: disc-kind arms, nested-union walk, accessors -------
+
+    #[test]
+    fn m4_field_disc_uint16_dispatches_on_wide_value() {
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "event", "kind": { "$ref": "#/$defs/Event" } }
+                    ]
+                },
+                "Event": {
+                    "kind": "union",
+                    "discriminator": { "kind": "field", "name": "type" },
+                    "fields": [ { "name": "type", "kind": "uint16" } ],
+                    "mapping": {
+                        "514": { "$ref": "#/$defs/Read" }
+                    }
+                },
+                "Read": {
+                    "kind": "struct",
+                    "fields": [ { "name": "x", "kind": "uint8" } ]
+                }
+            }
+        });
+        let mut buf = vec![0u8; 4];
+        buf[0..2].copy_from_slice(&514u16.to_le_bytes());
+        buf[2] = 42;
+        let mut reader = reader(&root, "S");
+        let (name, value) = reader.read_next(&buf).unwrap().unwrap();
+        assert_eq!(name, "event");
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "514");
+                assert_eq!(variant_start, 2);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader.position(), 3);
+    }
+
+    #[test]
+    fn m4_field_disc_uint32_dispatches_on_wide_value() {
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "event", "kind": { "$ref": "#/$defs/Event" } }
+                    ]
+                },
+                "Event": {
+                    "kind": "union",
+                    "discriminator": { "kind": "field", "name": "type" },
+                    "fields": [ { "name": "type", "kind": "uint32" } ],
+                    "mapping": {
+                        "258": { "$ref": "#/$defs/Read" }
+                    }
+                },
+                "Read": {
+                    "kind": "struct",
+                    "fields": [ { "name": "x", "kind": "uint8" } ]
+                }
+            }
+        });
+        let mut buf = vec![0u8; 6];
+        buf[0..4].copy_from_slice(&258u32.to_le_bytes());
+        buf[4] = 42;
+        let mut reader = reader(&root, "S");
+        let (name, value) = reader.read_next(&buf).unwrap().unwrap();
+        assert_eq!(name, "event");
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "258");
+                assert_eq!(variant_start, 4);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader.position(), 5);
+    }
+
+    #[test]
+    fn m4_field_disc_enum_dispatches_on_index() {
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "event", "kind": { "$ref": "#/$defs/Event" } }
+                    ]
+                },
+                "Event": {
+                    "kind": "union",
+                    "discriminator": { "kind": "field", "name": "kind" },
+                    "fields": [ { "name": "kind", "kind": { "$ref": "#/$defs/Kind" } } ],
+                    "mapping": {
+                        "3": { "$ref": "#/$defs/Read" }
+                    }
+                },
+                "Kind": { "kind": "enum", "values": ["A", "B", "C", "D"] },
+                "Read": {
+                    "kind": "struct",
+                    "fields": [ { "name": "x", "kind": "uint8" } ]
+                }
+            }
+        });
+        let mut buf = vec![0u8; 8];
+        buf[0..4].copy_from_slice(&3u32.to_le_bytes());
+        buf[4] = 42;
+        let mut reader = reader(&root, "S");
+        let (name, value) = reader.read_next(&buf).unwrap().unwrap();
+        assert_eq!(name, "event");
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "3");
+                assert_eq!(variant_start, 4);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader.position(), 5);
+    }
+
+    #[test]
+    fn m4_byte_disc_uint16_and_uint32_arms() {
+        // The plan reader's byte-disc uint16/uint32 arms (0-execution in
+        // the review's coverage run — wire-visible dispatch widths).
+        let u16_root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "packet", "kind": { "$ref": "#/$defs/P16" } }
+                    ]
+                },
+                "P16": {
+                    "kind": "union",
+                    "discriminator": { "kind": "byte", "offset": 0, "type": "uint16" },
+                    "mapping": { "515": { "$ref": "#/$defs/Read" } }
+                },
+                "Read": { "kind": "struct", "fields": [ { "name": "x", "kind": "uint8" } ] }
+            }
+        });
+        let mut buf = vec![0u8; 4];
+        buf[0..2].copy_from_slice(&515u16.to_le_bytes());
+        buf[2] = 42;
+        let mut reader16 = reader(&u16_root, "S");
+        let (_, value) = reader16.read_next(&buf).unwrap().unwrap();
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "515");
+                assert_eq!(variant_start, 2);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader16.position(), 3);
+
+        let u32_root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "packet", "kind": { "$ref": "#/$defs/P32" } }
+                    ]
+                },
+                "P32": {
+                    "kind": "union",
+                    "discriminator": { "kind": "byte", "offset": 0, "type": "uint32" },
+                    "mapping": { "258": { "$ref": "#/$defs/Read" } }
+                },
+                "Read": { "kind": "struct", "fields": [ { "name": "x", "kind": "uint8" } ] }
+            }
+        });
+        let mut buf = vec![0u8; 6];
+        buf[0..4].copy_from_slice(&258u32.to_le_bytes());
+        buf[4] = 42;
+        let mut reader32 = reader(&u32_root, "S");
+        let (_, value) = reader32.read_next(&buf).unwrap().unwrap();
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "258");
+                assert_eq!(variant_start, 4);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader32.position(), 5);
+    }
+
+    #[test]
+    fn m4_nested_union_variant_size_walk() {
+        // A byte-disc union whose variant is itself a union — the
+        // plan_walk_variant_size union arm (0-execution in the review's
+        // coverage run; the capability phase 1 restored). The inner
+        // union dispatches on its own discriminator after the outer's.
+        let root = json!({
+            "$defs": {
+                "S": {
+                    "kind": "struct",
+                    "fields": [
+                        { "name": "outer", "kind": { "$ref": "#/$defs/Outer" } }
+                    ]
+                },
+                "Outer": {
+                    "kind": "union",
+                    "discriminator": { "kind": "byte", "offset": 0, "type": "uint8" },
+                    "mapping": { "1": { "$ref": "#/$defs/Inner" } }
+                },
+                "Inner": {
+                    "kind": "union",
+                    "discriminator": { "kind": "byte", "offset": 0, "type": "uint8" },
+                    "mapping": { "7": { "$ref": "#/$defs/Leaf" } }
+                },
+                "Leaf": { "kind": "struct", "fields": [ { "name": "v", "kind": "uint32" } ] }
+            }
+        });
+        // Wire: outer disc (1) + inner disc (7) + u32 — 6 bytes.
+        let mut buf = vec![0u8; 6];
+        buf[0] = 1;
+        buf[1] = 7;
+        buf[2..6].copy_from_slice(&42u32.to_le_bytes());
+        let mut reader = reader(&root, "S");
+        let (name, value) = reader.read_next(&buf).unwrap().unwrap();
+        assert_eq!(name, "outer");
+        match value {
+            FieldValue::Union { discriminator, variant_start } => {
+                assert_eq!(discriminator, "1");
+                assert_eq!(variant_start, 1);
+            }
+            other => panic!("expected Union, got {other:?}"),
+        }
+        assert_eq!(reader.position(), 6, "nested union's full size walked");
+    }
+
+    #[test]
+    fn m4_public_schema_and_plan_accessors_return_compiled_doc() {
+        // The public accessors (phase-2 deliverables) had 0 test calls:
+        // `schema()` returns the compiled document, `plan()` the compiled
+        // ReadPlan handle.
+        let root = json!({
+            "$defs": {
+                "S": { "kind": "struct", "fields": [ { "name": "a", "kind": "uint8" } ] }
+            }
+        });
+        let plan = ReadPlan::compile(&root, "S").expect("plan");
+        let reader = SequentialReader::new(Arc::new(plan));
+        let schema = reader.schema();
+        assert_eq!(schema["$defs"]["S"]["fields"][0]["name"], json!("a"));
+        let p = reader.plan();
+        assert_eq!(p.fields().len(), 1);
+        assert_eq!(p.fields()[0].name(), "a");
+        assert_eq!(p.schema()["$defs"]["S"]["fields"][0]["name"], json!("a"));
+    }
+
     #[test]
     fn rejects_schema_with_missing_root() {
         let root = json!({ "$defs": { "Other": { "kind": "struct", "fields": [] } } });