diff --git a/backend/app/modules/design/domain/entities.py b/backend/app/modules/design/domain/entities.py
index e69de29..9c3bd72 100644
--- a/backend/app/modules/design/domain/entities.py
+++ b/backend/app/modules/design/domain/entities.py
@@ -0,0 +1,304 @@
+from dataclasses import dataclass
+
+
+# ── Business Layer ──
+
+@dataclass
+class Capability:
+    capability_id: str
+    name: str
+    description: str
+    priority: str          # must / should / could
+    phase: str
+    related_value_flows: list[str]
+
+
+@dataclass
+class ValueFlow:
+    value_flow_id: str
+    name: str
+    trigger: str
+    actor: str
+    steps: str
+    outcome: str
+    phase: str
+    related_capabilities: list[str]
+
+
+@dataclass
+class UserJourney:
+    journey_id: str
+    name: str
+    actor: str
+    precondition: str
+    steps: str
+    postcondition: str
+    phase: str
+    related_value_flows: list[str]
+
+
+@dataclass
+class ScopeAndGoals:
+    doc_id: str
+    title: str
+    core_problem: str
+    users: str
+    constraints: str
+
+
+# ── Application Layer ──
+
+@dataclass
+class Module:
+    module_id: str
+    name: str
+    layer: str             # backend / frontend
+    description: str
+    phase: str
+    depends_on: list[str]
+    capabilities: list[str]
+
+
+@dataclass
+class Integration:
+    integration_id: str
+    source_id: str
+    target_id: str
+    target_type: str
+    direction: str
+    protocol: str
+    trigger: str
+    phase: str
+    description: str
+
+
+@dataclass
+class ExternalSystem:
+    system_id: str
+    name: str
+    type: str
+    protocol: str
+    direction: str
+    phase: str
+    description: str
+
+
+@dataclass
+class ApiContract:
+    doc_id: str
+    path: str
+    method: str
+    operation_id: str
+    summary: str
+
+
+@dataclass
+class CodebaseAlignment:
+    module_id: str
+    repo_root: str
+    code_root: str
+    package_name: str
+
+
+@dataclass
+class ModuleBoundaryRule:
+    doc_id: str
+    title: str
+    content: str
+
+
+@dataclass
+class SystemContext:
+    doc_id: str
+    title: str
+    content: str
+
+
+@dataclass
+class SolutionLayer:
+    doc_id: str
+    title: str
+    content: str
+
+
+# ── Data Layer ──
+
+@dataclass
+class Entity:
+    entity_id: str
+    name: str
+    domain: str
+    owner_module: str
+    description: str
+    phase: str
+    source_file: str
+
+
+@dataclass
+class DataFlow:
+    data_flow_id: str
+    source: str
+    target: str
+    data_content: str
+    trigger: str
+    protocol: str
+    phase: str
+    description: str
+
+
+@dataclass
+class DataSecurity:
+    security_id: str
+    sensitivity: str
+    entities: str
+    protection: str
+
+
+# ── Technology Layer ──
+
+@dataclass
+class TechSelection:
+    category: str
+    technology: str
+    version: str
+    purpose: str
+    rationale: str
+    alternatives_considered: str
+    phase: str
+
+
+@dataclass
+class RuntimeComponent:
+    component_id: str
+    name: str
+    type: str
+    technology: str
+    port: str
+
+
+@dataclass
+class RuntimeTopology:
+    doc_id: str
+    title: str
+    content: str
+
+
+@dataclass
+class Environment:
+    env_id: str
+    name: str
+    purpose: str
+    infra: str
+
+
+@dataclass
+class OperationalBaseline:
+    doc_id: str
+    title: str
+    content: str
+
+
+@dataclass
+class ReleasePlan:
+    doc_id: str
+    title: str
+    content: str
+
+
+# ── Cross-Layer ──
+
+@dataclass
+class TraceabilityLink:
+    trace_id: str
+    capability_id: str
+    module_id: str
+    entity_ids: list[str]
+    value_flow_ids: list[str]
+    notes: str
+
+
+@dataclass
+class ChangeLogEntry:
+    change_id: str
+    date: str
+    scope: str
+    description: str
+
+
+@dataclass
+class ADR:
+    adr_id: str
+    title: str
+    status: str
+    context: str
+    decision: str
+
+
+@dataclass
+class DesignDocument:
+    doc_id: str
+    title: str
+    version: str
+    status: str
+    owners: list[str]
+    upstream: list[str]
+    downstream: list[str]
+    file_path: str
+
+
+# ── Domain Layer ──
+
+@dataclass
+class Domain:
+    domain_name: str
+    overview: str
+    modules: list[str]
+    entities: list[str]
+
+
+@dataclass
+class UbiquitousTerm:
+    term_id: str
+    term: str
+    english_term: str
+    code_symbol: str
+    domain: str
+    definition: str
+
+
+@dataclass
+class SharedTerm:
+    term_id: str
+    term: str
+    english_term: str
+    definition: str
+    used_by_domains: list[str]
+
+
+@dataclass
+class Scenario:
+    scenario_id: str
+    name: str
+    trigger: str
+    actors: str
+    steps: str
+    outcome: str
+    related_capabilities: list[str]
+
+
+@dataclass
+class DomainModule:
+    module_id: str
+    module_name: str
+    domain: str
+    description: str
+    layer_in_code: str
+
+
+@dataclass
+class DomainEntity:
+    entity_id: str
+    entity_name: str
+    type: str
+    description: str
+    key_attributes: str
diff --git a/backend/tests/test_design_entities.py b/backend/tests/test_design_entities.py
index 139c405..d49ecda 100644
--- a/backend/tests/test_design_entities.py
+++ b/backend/tests/test_design_entities.py
@@ -25,3 +25,105 @@ def test_module_layer_values():
     assert ModuleLayer.APPLICATION == "application"
     assert ModuleLayer.INFRASTRUCTURE == "infrastructure"
     assert ModuleLayer.INTERFACES == "interfaces"
+
+
+from app.modules.design.domain.entities import (
+    ADR,
+    ApiContract,
+    Capability,
+    ChangeLogEntry,
+    CodebaseAlignment,
+    DataFlow,
+    DataSecurity,
+    DesignDocument,
+    Domain,
+    DomainEntity,
+    DomainModule,
+    Entity,
+    Environment,
+    ExternalSystem,
+    Integration,
+    Module,
+    ModuleBoundaryRule,
+    OperationalBaseline,
+    ReleasePlan,
+    RuntimeComponent,
+    RuntimeTopology,
+    Scenario,
+    ScopeAndGoals,
+    SharedTerm,
+    SolutionLayer,
+    SystemContext,
+    TechSelection,
+    TraceabilityLink,
+    UbiquitousTerm,
+    UserJourney,
+    ValueFlow,
+)
+
+
+def test_capability_creation():
+    cap = Capability(
+        capability_id="CAP-01",
+        name="test",
+        description="desc",
+        priority="must",
+        phase="MVP",
+        related_value_flows=["VF-01"],
+    )
+    assert cap.capability_id == "CAP-01"
+    assert cap.related_value_flows == ["VF-01"]
+
+
+def test_module_creation():
+    mod = Module(
+        module_id="MOD-01",
+        name="test",
+        layer="backend",
+        description="desc",
+        phase="MVP",
+        depends_on=["MOD-02"],
+        capabilities=["CAP-01"],
+    )
+    assert mod.depends_on == ["MOD-02"]
+
+
+def test_traceability_link_list_fields():
+    tl = TraceabilityLink(
+        trace_id="TR-01",
+        capability_id="CAP-01",
+        module_id="MOD-01",
+        entity_ids=["ENT-01", "ENT-02"],
+        value_flow_ids=["VF-01"],
+        notes="test",
+    )
+    assert len(tl.entity_ids) == 2
+
+
+def test_design_document_list_fields():
+    dd = DesignDocument(
+        doc_id="DOC-01",
+        title="test",
+        version="0.1",
+        status="draft",
+        owners=["owner1"],
+        upstream=["a.md"],
+        downstream=["b.md"],
+        file_path="test.md",
+    )
+    assert dd.owners == ["owner1"]
+
+
+def test_all_31_entities_importable():
+    """Verify all 31 entity classes can be imported."""
+    entities = [
+        Capability, ValueFlow, UserJourney, ScopeAndGoals,
+        Module, Integration, ExternalSystem, ApiContract,
+        CodebaseAlignment, ModuleBoundaryRule, SystemContext, SolutionLayer,
+        Entity, DataFlow, DataSecurity,
+        TechSelection, RuntimeComponent, RuntimeTopology, Environment,
+        OperationalBaseline, ReleasePlan,
+        TraceabilityLink, ChangeLogEntry, ADR, DesignDocument,
+        Domain, UbiquitousTerm, SharedTerm, Scenario, DomainModule, DomainEntity,
+    ]
+    assert len(entities) == 31