diff --git a/packages/2-sql/2-authoring/contract-psl/test/interpreter.diagnostics.test.ts b/packages/2-sql/2-authoring/contract-psl/test/interpreter.diagnostics.test.ts
index c9ae75c773..2b4273e546 100644
--- a/packages/2-sql/2-authoring/contract-psl/test/interpreter.diagnostics.test.ts
+++ b/packages/2-sql/2-authoring/contract-psl/test/interpreter.diagnostics.test.ts
@@ -22,6 +22,24 @@ const testSpan: PslSpan = {
   end: { line: 1, column: 7, offset: 6 },
 };
 
+function expectDiagnosticForSchema(
+  schema: string,
+  diagnostic: { readonly code: string; readonly message: string },
+): void {
+  const document = parsePslDocument({ schema, sourceId: 'schema.prisma' });
+  const result = interpretPslDocumentToSqlContract({
+    ...baseInput,
+    document,
+    controlMutationDefaults: builtinControlMutationDefaults,
+  });
+
+  expect(result.ok).toBe(false);
+  if (result.ok) return;
+  expect(result.failure.diagnostics).toEqual(
+    expect.arrayContaining([expect.objectContaining(diagnostic)]),
+  );
+}
+
 describe('interpretPslDocumentToSqlContract diagnostics', () => {
   it('returns diagnostics when target context is missing', () => {
     const document = parsePslDocument({
@@ -193,6 +211,56 @@ model User {
     );
   });
 
+  it('returns diagnostics for duplicate field and model primary keys', () => {
+    expectDiagnosticForSchema(
+      `model Membership {
+  id Int @id
+  orgId String
+  userId String
+
+  @@id([orgId, userId])
+}
+`,
+      {
+        code: 'PSL_INVALID_ATTRIBUTE_ARGUMENT',
+        message: 'Model "Membership" cannot declare both field-level @id and model-level @@id',
+      },
+    );
+  });
+
+  it('returns diagnostics for nullable composite primary key fields', () => {
+    expectDiagnosticForSchema(
+      `model Membership {
+  orgId String
+  userId String?
+
+  @@id([orgId, userId])
+}
+`,
+      {
+        code: 'PSL_INVALID_ATTRIBUTE_ARGUMENT',
+        message:
+          'Model "Membership" @@id cannot include optional field "userId"; primary key columns must be NOT NULL',
+      },
+    );
+  });
+
+  it('returns diagnostics for unknown composite primary key fields', () => {
+    expectDiagnosticForSchema(
+      `model Membership {
+  orgId String
+  userId String
+
+  @@id([orgId, missingId])
+}
+`,
+      {
+        code: 'PSL_INVALID_ATTRIBUTE_ARGUMENT',
+        message: 'Model "Membership" @@id references unknown field "Membership.missingId"',
+      },
+    );
+  });
+
   it('returns diagnostics for model attributes with unrecognized extension namespace', () => {
     const document = parsePslDocument({
       schema: `model Team {
diff --git a/packages/2-sql/2-authoring/contract-psl/test/interpreter.test.ts b/packages/2-sql/2-authoring/contract-psl/test/interpreter.test.ts
index 0f3397dc4d..45d9369aeb 100644
--- a/packages/2-sql/2-authoring/contract-psl/test/interpreter.test.ts
+++ b/packages/2-sql/2-authoring/contract-psl/test/interpreter.test.ts
@@ -526,4 +526,34 @@ model OrderItem {
       });
     });
   });
+
+  it('maps model-level composite primary keys to storage columns', () => {
+    const document = parsePslDocument({
+      schema: `model Membership {
+  orgId String @map("org_id")
+  userId String @map("user_id")
+
+  @@id([orgId, userId], map: "membership_pkey")
+  @@map("membership")
+}
+`,
+      sourceId: 'schema.prisma',
+    });
+
+    const result = interpretPslDocumentToSqlContract({
+      document,
+      controlMutationDefaults: builtinControlMutationDefaults,
+    });
+
+    expect(result.ok).toBe(true);
+    if (!result.ok) return;
+
+    expect(result.value.storage).toMatchObject({
+      tables: {
+        membership: {
+          primaryKey: { columns: ['org_id', 'user_id'], name: 'membership_pkey' },
+        },
+      },
+    });
+  });
 });