diff --git a/backend/app/modules/design/domain/services.py b/backend/app/modules/design/domain/services.py
index e69de29..38da6a7 100644
--- a/backend/app/modules/design/domain/services.py
+++ b/backend/app/modules/design/domain/services.py
@@ -0,0 +1,128 @@
+from dataclasses import dataclass
+
+from app.modules.design.domain.entities import (
+    Capability,
+    Entity,
+    Module,
+    TraceabilityLink,
+)
+from app.modules.design.domain.value_objects import FileStatus
+
+TEMPLATE_MARKERS = ["TODO", "EXAMPLE", "<replace", "{{", "Lorem ipsum"]
+
+
+@dataclass
+class ConstraintViolation:
+    rule: str
+    entity_id: str
+    message: str
+
+
+class DesignValidationService:
+    @staticmethod
+    def determine_file_status(content: str, file_path: str) -> FileStatus:
+        if not content or not content.strip():
+            return FileStatus.MISSING
+
+        lines = [l for l in content.strip().splitlines() if l.strip()]
+        is_csv = file_path.endswith(".csv")
+
+        # Sparse check
+        if is_csv and len(lines) < 2:
+            return FileStatus.SPARSE
+        if not is_csv and len(lines) < 5:
+            return FileStatus.SPARSE
+
+        # Count placeholder tokens
+        total_cells = 0
+        placeholder_cells = 0
+        for line in lines:
+            cells = line.split(",") if is_csv else [line]
+            for cell in cells:
+                total_cells += 1
+                if any(m.lower() in cell.lower() for m in TEMPLATE_MARKERS):
+                    placeholder_cells += 1
+
+        # Placeholder heavy: >50%
+        if total_cells > 0 and placeholder_cells / total_cells > 0.5:
+            return FileStatus.PLACEHOLDER_HEAVY
+
+        # Template residue: any marker present but <=50%
+        if placeholder_cells > 0:
+            return FileStatus.TEMPLATE_RESIDUE
+
+        return FileStatus.OK
+
+    @staticmethod
+    def check_capability_module_linkage(
+        capabilities: list[Capability],
+        traceability_links: list[TraceabilityLink],
+    ) -> list[ConstraintViolation]:
+        linked_caps = {link.capability_id for link in traceability_links}
+        return [
+            ConstraintViolation(
+                rule="capability_module_linkage",
+                entity_id=cap.capability_id,
+                message=f"Capability {cap.capability_id} has no TraceabilityLink to any module",
+            )
+            for cap in capabilities
+            if cap.capability_id not in linked_caps
+        ]
+
+    @staticmethod
+    def check_entity_owner(entities: list[Entity]) -> list[ConstraintViolation]:
+        return [
+            ConstraintViolation(
+                rule="entity_owner",
+                entity_id=ent.entity_id,
+                message=f"Entity {ent.entity_id} has no owner module",
+            )
+            for ent in entities
+            if not ent.owner_module
+        ]
+
+    @staticmethod
+    def check_traceability_references(
+        links: list[TraceabilityLink],
+        capabilities: list[Capability],
+        modules: list[Module],
+        entities: list[Entity],
+    ) -> list[ConstraintViolation]:
+        cap_ids = {c.capability_id for c in capabilities}
+        mod_ids = {m.module_id for m in modules}
+        ent_ids = {e.entity_id for e in entities}
+        violations: list[ConstraintViolation] = []
+        for link in links:
+            if link.capability_id not in cap_ids:
+                violations.append(ConstraintViolation(
+                    "traceability_ref", link.trace_id,
+                    f"References unknown capability {link.capability_id}",
+                ))
+            if link.module_id not in mod_ids:
+                violations.append(ConstraintViolation(
+                    "traceability_ref", link.trace_id,
+                    f"References unknown module {link.module_id}",
+                ))
+            for eid in link.entity_ids:
+                if eid not in ent_ids:
+                    violations.append(ConstraintViolation(
+                        "traceability_ref", link.trace_id,
+                        f"References unknown entity {eid}",
+                    ))
+        return violations
+
+    @classmethod
+    def validate_all(
+        cls,
+        capabilities: list[Capability],
+        modules: list[Module],
+        entities: list[Entity],
+        traceability_links: list[TraceabilityLink],
+    ) -> list[ConstraintViolation]:
+        violations: list[ConstraintViolation] = []
+        violations.extend(cls.check_capability_module_linkage(capabilities, traceability_links))
+        violations.extend(cls.check_entity_owner(entities))
+        violations.extend(cls.check_traceability_references(
+            traceability_links, capabilities, modules, entities,
+        ))
+        return violations
diff --git a/backend/tests/test_design_services.py b/backend/tests/test_design_services.py
new file mode 100644
index 0000000..f163134
--- /dev/null
+++ b/backend/tests/test_design_services.py
@@ -0,0 +1,77 @@
+import pytest
+from app.modules.design.domain.entities import (
+    Capability,
+    Entity,
+    Module,
+    TraceabilityLink,
+)
+from app.modules.design.domain.services import DesignValidationService
+from app.modules.design.domain.value_objects import FileStatus
+
+
+class TestFileStatusDetermination:
+    def test_empty_content_is_missing(self):
+        assert DesignValidationService.determine_file_status("", "test.csv") == FileStatus.MISSING
+
+    def test_csv_header_only_is_sparse(self):
+        assert DesignValidationService.determine_file_status("id,name\n", "test.csv") == FileStatus.SPARSE
+
+    def test_csv_with_data_is_ok(self):
+        content = "id,name\n1,foo\n2,bar\n"
+        assert DesignValidationService.determine_file_status(content, "test.csv") == FileStatus.OK
+
+    def test_md_too_short_is_sparse(self):
+        assert DesignValidationService.determine_file_status("# Title\n\nShort.\n", "test.md") == FileStatus.SPARSE
+
+    def test_template_residue_detected(self):
+        content = "id,name\nTODO,<replace this>\nreal,data\n"
+        assert DesignValidationService.determine_file_status(content, "test.csv") == FileStatus.TEMPLATE_RESIDUE
+
+    def test_placeholder_heavy(self):
+        content = "id,name,desc\nTODO,TODO,TODO\nTODO,TODO,TODO\n"
+        assert DesignValidationService.determine_file_status(content, "test.csv") == FileStatus.PLACEHOLDER_HEAVY
+
+    def test_ok_md_file(self):
+        content = "---\ndoc_id: DOC-01\ntitle: Test\n---\n\n# Title\n\nThis is a real document with enough content.\nLine 4.\nLine 5.\nLine 6.\n"
+        assert DesignValidationService.determine_file_status(content, "test.md") == FileStatus.OK
+
+
+class TestConstraintValidation:
+    def _make_cap(self, cap_id: str) -> Capability:
+        return Capability(cap_id, "n", "d", "must", "MVP", [])
+
+    def _make_mod(self, mod_id: str) -> Module:
+        return Module(mod_id, "n", "backend", "d", "MVP", [], [])
+
+    def _make_ent(self, ent_id: str, owner: str) -> Entity:
+        return Entity(ent_id, "n", "d", owner, "d", "MVP", "f.csv")
+
+    def _make_link(self, trace_id: str, cap: str, mod: str, ents: list[str]) -> TraceabilityLink:
+        return TraceabilityLink(trace_id, cap, mod, ents, [], "")
+
+    def test_capability_without_module_link_is_violation(self):
+        caps = [self._make_cap("CAP-01")]
+        links = []
+        violations = DesignValidationService.check_capability_module_linkage(caps, links)
+        assert len(violations) == 1
+
+    def test_entity_without_owner_is_violation(self):
+        entities = [self._make_ent("ENT-01", "")]
+        violations = DesignValidationService.check_entity_owner(entities)
+        assert len(violations) == 1
+
+    def test_valid_traceability_passes(self):
+        caps = [self._make_cap("CAP-01")]
+        mods = [self._make_mod("MOD-01")]
+        ents = [self._make_ent("ENT-01", "MOD-01")]
+        links = [self._make_link("TR-01", "CAP-01", "MOD-01", ["ENT-01"])]
+        violations = DesignValidationService.check_traceability_references(links, caps, mods, ents)
+        assert len(violations) == 0
+
+    def test_broken_traceability_reference_is_violation(self):
+        caps = [self._make_cap("CAP-01")]
+        mods = [self._make_mod("MOD-01")]
+        ents = [self._make_ent("ENT-01", "MOD-01")]
+        links = [self._make_link("TR-01", "CAP-MISSING", "MOD-01", ["ENT-01"])]
+        violations = DesignValidationService.check_traceability_references(links, caps, mods, ents)
+        assert len(violations) >= 1