diff --git a/crates/oa-core/src/language.rs b/crates/oa-core/src/language.rs
index 1342c092..1b48474e 100644
--- a/crates/oa-core/src/language.rs
+++ b/crates/oa-core/src/language.rs
@@ -1,6 +1,7 @@
 //! Typed OA/SA language, tolerant source analysis, and explicit V1/V2/V3/V4 migration.
 #[path = "language/semantic_source.rs"]
 mod semantic_source;
+mod source3_bridge;
 // Staged syntax foundation only. Public source8 admission waits for complete
 // declaration lowering, source updates and the root-owned resource gate.
 #[cfg(test)]
@@ -1834,10 +1835,7 @@ pub(crate) fn resolve_declarations(
         target.nodes.insert(node.id, node);
     }
     if legacy {
-        let mut value = serde_json::to_value(&project).expect("project serialization");
-        value["elements"] = Value::Object(legacy_elements);
-        let migrated = serde_json::from_value(value)
-            .map_err(crate::persistence::FileError::Json)
+        let migrated = source3_bridge::from_scratch(project, legacy_elements)
             .and_then(|old| crate::persistence::v3::migrate(old, limits))
             .map_err(|error| {
                 let issue = if let crate::persistence::FileError::Model { issues, .. } = &error {
diff --git a/crates/oa-core/src/language/source3_bridge.rs b/crates/oa-core/src/language/source3_bridge.rs
new file mode 100644
index 00000000..65127f42
--- /dev/null
+++ b/crates/oa-core/src/language/source3_bridge.rs
@@ -0,0 +1,52 @@
+//! Convert only source3 lowering scratch state into its frozen migration input.
+//! This is not a current-to-legacy exporter: fields outside that scratch state
+//! must reject instead of being silently discarded.
+use crate::Project;
+use crate::persistence::{FileError, v3};
+use serde_json::{Map, Value};
+
+pub(super) fn from_scratch(
+    project: Project,
+    legacy_elements: Map<String, Value>,
+) -> Result<v3::Project3, FileError> {
+    // No `..`: a future Project field requires an explicit compatibility decision.
+    let Project {
+        schema_version,
+        id,
+        alias,
+        name,
+        description,
+        definitions,
+        elements,
+        relations,
+        diagrams,
+        artifact_links,
+        traces,
+    } = project;
+    if schema_version != 3 {
+        return Err(FileError::Invalid(
+            "Expected source3 lowering state.".into(),
+        ));
+    }
+    if !definitions.is_empty() || !elements.is_empty() {
+        return Err(FileError::Invalid(
+            "Source3 lowering state cannot contain current definitions or elements.".into(),
+        ));
+    }
+    Ok(v3::Project3 {
+        schema_version,
+        id,
+        alias,
+        name,
+        description,
+        elements: serde_json::from_value(Value::Object(legacy_elements))?,
+        // Keep the frozen nested contracts, including strict trace baselines.
+        relations: serde_json::from_value(serde_json::to_value(relations)?)?,
+        diagrams,
+        artifact_links,
+        traces: serde_json::from_value(serde_json::to_value(traces)?)?,
+    })
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/crates/oa-core/src/language/source3_bridge/tests.rs b/crates/oa-core/src/language/source3_bridge/tests.rs
new file mode 100644
index 00000000..cb2c6542
--- /dev/null
+++ b/crates/oa-core/src/language/source3_bridge/tests.rs
@@ -0,0 +1,188 @@
+use super::*;
+use crate::{Id, language, persistence};
+use serde_json::json;
+
+const SOURCE3: &str = include_str!("../../../tests/fixtures/v3-data-contract.arcadia");
+const JSON3: &[u8] = include_bytes!("../../../tests/fixtures/v3-data-contract.json");
+const PROSE: &str = r#"arcadia 3;
+project review "Review" id "00000000-0000-0000-0000-000000000001" {
+    oa_activity reviewed_activity "Live OA" id "00000000-0000-0000-0000-000000000002" {}
+    sa_function function "Live SA" id "00000000-0000-0000-0000-000000000003" {}
+    trace review_link "Review link" id "00000000-0000-0000-0000-000000000004" {
+        oa @reviewed_activity; sa @function; kind Refinement;
+        baseline {
+            oa { name "Approved OA"; description "First\nΩ rationale"; };
+            sa { name "Approved SA"; description "Prior SA prose"; };
+        };
+    }
+}"#;
+
+fn id(value: u128) -> Id {
+    Id::from_u128(value)
+}
+
+fn prose_json() -> Value {
+    json!({
+        "schema_version":3,"id":id(1),"alias":"review","name":"Review","description":"",
+        "elements": {
+            id(2).to_string(): {"id":id(2),"alias":"reviewed_activity","name":"Live OA","description":"","parent":null,"body":{"Oa":"Activity"}},
+            id(3).to_string(): {"id":id(3),"alias":"function","name":"Live SA","description":"","parent":null,"body":{"Sa":"Function"}}
+        },
+        "relations":{},"diagrams":{},"artifact_links":{},
+        "traces": {id(4).to_string():{
+            "id":id(4),"alias":"review_link","name":"Review link","description":"",
+            "oa":id(2),"sa":id(3),"kind":"Refinement",
+            "baseline":{"oa":{"name":"Approved OA","description":"First\nΩ rationale"},"sa":{"name":"Approved SA","description":"Prior SA prose"}}
+        }}
+    })
+}
+
+#[test]
+fn frozen_source3_and_json3_have_complete_model_and_migration_parity() {
+    let source = language::parse_with_report(SOURCE3).unwrap();
+    let json = persistence::decode_with_report(JSON3).unwrap();
+    assert_eq!(source.project, json.project);
+    assert_eq!(source.migration, json.migration);
+    assert!(source.project.definitions.is_empty());
+    let report = source.migration.unwrap();
+    assert_eq!(
+        (report.from_schema, report.to_schema),
+        (3, crate::SCHEMA_VERSION)
+    );
+    // This tests direct legacy lowering. Canonical source8 reopening remains a
+    // separate, unchanged public integration gate.
+}
+
+#[test]
+fn frozen_wire_still_rejects_new_unknown_and_duplicate_fields() {
+    let original: Value = serde_json::from_slice(JSON3).unwrap();
+    for definitions in [
+        json!({}),
+        Value::Null,
+        json!({id(90).to_string(): {"name":"new"}}),
+    ] {
+        let mut bad = original.clone();
+        bad["definitions"] = definitions;
+        let error = persistence::decode(&serde_json::to_vec(&bad).unwrap())
+            .err()
+            .unwrap()
+            .to_string();
+        assert!(error.contains("unknown field `definitions`"), "{error}");
+    }
+    let mut bad = original;
+    bad["unrecognized"] = json!(true);
+    assert!(
+        persistence::decode(&serde_json::to_vec(&bad).unwrap())
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("unknown field `unrecognized`")
+    );
+    let raw = std::str::from_utf8(JSON3).unwrap();
+    let duplicate = raw.replacen(
+        "\"schema_version\": 3",
+        "\"schema_version\": 3, \"schema_version\": 3",
+        1,
+    );
+    assert_ne!(duplicate, raw);
+    assert!(
+        persistence::decode(duplicate.as_bytes())
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("duplicate")
+    );
+}
+
+#[test]
+fn old_review_prose_survives_without_admitting_semantic_fields() {
+    let source = language::parse_with_report(PROSE).unwrap();
+    let json =
+        persistence::decode_with_report(&serde_json::to_vec(&prose_json()).unwrap()).unwrap();
+    assert_eq!(source.project, json.project);
+    assert_eq!(source.migration, json.migration);
+    let baseline = source.project.traces[&id(4)].baseline.as_ref().unwrap();
+    assert_eq!(baseline.oa.name, "Approved OA");
+    assert_eq!(baseline.oa.description, "First\nΩ rationale");
+    assert_eq!(baseline.sa.description, "Prior SA prose");
+    assert!(baseline.semantic.is_none());
+    for semantic in [Value::Null, json!({"facets":[]})] {
+        let mut bad = prose_json();
+        bad["traces"][id(4).to_string()]["baseline"]["semantic"] = semantic;
+        let error = persistence::decode(&serde_json::to_vec(&bad).unwrap())
+            .err()
+            .unwrap()
+            .to_string();
+        assert!(error.contains("semantic"), "{error}");
+    }
+    for field in ["semantic none;", "semantic { facets []; };"] {
+        let bad = PROSE.replace("baseline {", &format!("baseline {{ {field}"));
+        let errors = language::parse(&bad).err().unwrap();
+        assert!(
+            errors
+                .iter()
+                .any(|e| e.message.contains("require schema/source5")),
+            "{errors:?}"
+        );
+    }
+}
+
+#[test]
+fn scratch_bridge_refuses_to_discard_current_only_data() {
+    let mut empty = Project::empty();
+    empty.schema_version = 3;
+    assert!(from_scratch(empty.clone(), Map::new()).is_ok());
+    let mut wrong_version = empty.clone();
+    wrong_version.schema_version = crate::SCHEMA_VERSION;
+    assert!(from_scratch(wrong_version, Map::new()).is_err());
+    let mut definition = empty;
+    definition.definitions.insert(
+        id(90),
+        crate::analysis::Definition {
+            id: id(90),
+            alias: "dimensionless".into(),
+            name: "Dimensionless".into(),
+            description: String::new(),
+            body: crate::analysis::DefinitionBody::QuantityKind(
+                crate::analysis::QuantityKindDefinition {
+                    dimension: crate::analysis_numeric::Dimension::ZERO,
+                    algebra: crate::analysis::QuantityAlgebra::Linear,
+                },
+            ),
+        },
+    );
+    assert!(
+        from_scratch(definition, Map::new())
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("definitions or elements")
+    );
+    let mut current = persistence::decode_with_report(&serde_json::to_vec(&prose_json()).unwrap())
+        .unwrap()
+        .project;
+    current.schema_version = 3;
+    assert!(
+        from_scratch(current.clone(), Map::new())
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("definitions or elements")
+    );
+    current.elements.clear();
+    current
+        .traces
+        .get_mut(&id(4))
+        .unwrap()
+        .baseline
+        .as_mut()
+        .unwrap()
+        .semantic = Some(crate::semantic::SemanticBaseline { facets: Vec::new() });
+    assert!(
+        from_scratch(current, Map::new())
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("semantic")
+    );
+}