diff --git a/backend/dependencies.py b/backend/dependencies.py
index 6891d67..45956b9 100644
--- a/backend/dependencies.py
+++ b/backend/dependencies.py
@@ -2,6 +2,7 @@
 Shared dependencies and service instances.
 All route modules import from here to avoid circular imports.
 """
+from typing import Optional
 from fastapi import HTTPException, Depends
 
 from services.indexer_optimized import OptimizedCodeIndexer
@@ -43,21 +44,25 @@
 redis_client = cache.redis if cache.redis else None
 
 
-def get_repo_or_404(repo_id: str, user_id: str) -> dict:
+def get_repo_or_404(repo_id: str, user_id: Optional[str]) -> dict:
     """
     Get repository with ownership verification.
-    Returns 404 if not found or user doesn't own it.
+    Raises 401 if user_id is None, 404 if not found or user doesn't own it.
     """
+    if user_id is None:
+        raise HTTPException(status_code=401, detail="User ID required for this operation")
     repo = repo_manager.get_repo_for_user(repo_id, user_id)
     if not repo:
         raise HTTPException(status_code=404, detail="Repository not found")
     return repo
 
 
-def verify_repo_access(repo_id: str, user_id: str) -> None:
+def verify_repo_access(repo_id: str, user_id: Optional[str]) -> None:
     """
     Verify user has access to repository.
-    Raises 404 if no access.
+    Raises 401 if user_id is None, 404 if no access.
     """
+    if user_id is None:
+        raise HTTPException(status_code=401, detail="User ID required for this operation")
     if not repo_manager.verify_ownership(repo_id, user_id):
         raise HTTPException(status_code=404, detail="Repository not found")
diff --git a/backend/middleware/auth.py b/backend/middleware/auth.py
index 511fa40..68c8964 100644
--- a/backend/middleware/auth.py
+++ b/backend/middleware/auth.py
@@ -24,6 +24,13 @@ async def demo_search(auth: AuthContext = Depends(public_auth)):
 import os
 import hashlib
 
+from services.exceptions import (
+    AuthenticationError,
+    TokenExpiredError,
+    InvalidTokenError,
+    TokenMissingClaimError,
+)
+
 from fastapi import Depends, HTTPException, status
 from fastapi.security import HTTPBearer
 from fastapi.security.http import HTTPAuthorizationCredentials
@@ -59,7 +66,11 @@ def identifier(self) -> str:
 # Core validation functions
 
 def _validate_jwt(token: str) -> Optional[AuthContext]:
-    """Validate Supabase JWT token"""
+    """Validate Supabase JWT token.
+    
+    Returns AuthContext on success, None if token isn't a JWT (allows API key fallback).
+    Raises HTTPException for specific JWT failures (expired, bad audience, missing claim).
+    """
     try:
         from services.auth import get_auth_service
         auth_service = get_auth_service()
@@ -70,6 +81,15 @@ def _validate_jwt(token: str) -> Optional[AuthContext]:
             email=user.get("email"),
             tier=user.get("metadata", {}).get("tier", "free")
         )
+    except TokenExpiredError:
+        raise HTTPException(status_code=401, detail="Token expired")
+    except TokenMissingClaimError as e:
+        raise HTTPException(status_code=401, detail=str(e))
+    except InvalidTokenError:
+        # Could be a non-JWT token (API key) -- allow fallback
+        return None
+    except AuthenticationError as e:
+        raise HTTPException(status_code=401, detail=str(e))
     except Exception:
         return None
 
@@ -181,7 +201,12 @@ async def get_current_user(
     """
     from services.auth import get_auth_service
     auth_service = get_auth_service()
-    return auth_service.verify_jwt(credentials.credentials)
+    try:
+        return auth_service.verify_jwt(credentials.credentials)
+    except (TokenExpiredError, TokenMissingClaimError) as e:
+        raise HTTPException(status_code=401, detail=str(e))
+    except AuthenticationError:
+        raise HTTPException(status_code=401, detail="Invalid or expired token")
 
 
 async def get_optional_user(
diff --git a/backend/services/auth.py b/backend/services/auth.py
index a6bd688..b0af2de 100644
--- a/backend/services/auth.py
+++ b/backend/services/auth.py
@@ -5,11 +5,17 @@
 from fastapi import HTTPException, status
 from typing import Optional, Dict, Any
 import os
-import jwt
+import jwt as pyjwt
 from datetime import datetime
 from supabase import create_client, Client
 
 from services.observability import logger
+from services.exceptions import (
+    AuthenticationError,
+    TokenExpiredError,
+    InvalidTokenError,
+    TokenMissingClaimError,
+)
 
 
 class SupabaseAuthService:
@@ -48,7 +54,7 @@ def verify_jwt(self, token: str) -> Dict[str, Any]:
     def _verify_local(self, token: str) -> Dict[str, Any]:
         """Decode and verify JWT locally with HS256 secret."""
         try:
-            payload = jwt.decode(
+            payload = pyjwt.decode(
                 token,
                 self.jwt_secret,
                 algorithms=["HS256"],
@@ -58,10 +64,7 @@ def _verify_local(self, token: str) -> Dict[str, Any]:
             
             user_id = payload.get("sub")
             if not user_id:
-                raise HTTPException(
-                    status_code=status.HTTP_401_UNAUTHORIZED,
-                    detail="Token missing subject claim",
-                )
+                raise TokenMissingClaimError("sub")
             
             return {
                 "user_id": user_id,
@@ -69,22 +72,13 @@ def _verify_local(self, token: str) -> Dict[str, Any]:
                 "metadata": payload.get("user_metadata") or {},
             }
         
-        except jwt.ExpiredSignatureError:
-            raise HTTPException(
-                status_code=status.HTTP_401_UNAUTHORIZED,
-                detail="Token expired",
-            )
-        except jwt.InvalidAudienceError:
-            raise HTTPException(
-                status_code=status.HTTP_401_UNAUTHORIZED,
-                detail="Invalid token audience",
-            )
-        except jwt.InvalidTokenError as e:
+        except pyjwt.ExpiredSignatureError as e:
+            raise TokenExpiredError("Token expired") from e
+        except pyjwt.InvalidAudienceError as e:
+            raise InvalidTokenError("Invalid token audience") from e
+        except pyjwt.InvalidTokenError as e:
             logger.debug("JWT decode failed", error=str(e))
-            raise HTTPException(
-                status_code=status.HTTP_401_UNAUTHORIZED,
-                detail="Invalid token",
-            )
+            raise InvalidTokenError("Invalid token") from e
     
     def _verify_via_api(self, token: str) -> Dict[str, Any]:
         """Fallback: verify via Supabase API call (requires network)."""
@@ -92,24 +86,18 @@ def _verify_via_api(self, token: str) -> Dict[str, Any]:
             response = self.client.auth.get_user(token)
             
             if not response.user:
-                raise HTTPException(
-                    status_code=status.HTTP_401_UNAUTHORIZED,
-                    detail="Invalid or expired token",
-                )
+                raise InvalidTokenError("Invalid or expired token")
             
             return {
                 "user_id": response.user.id,
                 "email": response.user.email,
                 "metadata": response.user.user_metadata or {},
             }
-        except HTTPException:
+        except AuthenticationError:
             raise
         except Exception as e:
             logger.debug("API-based JWT verification failed", error=str(e))
-            raise HTTPException(
-                status_code=status.HTTP_401_UNAUTHORIZED,
-                detail="Token verification failed",
-            )
+            raise AuthenticationError("Token verification failed") from e
     
     async def signup(self, email: str, password: str, github_username: Optional[str] = None) -> Dict[str, Any]:
         """
diff --git a/backend/services/exceptions.py b/backend/services/exceptions.py
new file mode 100644
index 0000000..cd20b6d
--- /dev/null
+++ b/backend/services/exceptions.py
@@ -0,0 +1,49 @@
+"""
+Domain exceptions for authentication.
+
+Services raise these; the middleware/route layer translates them to HTTP responses.
+This decouples business logic from the HTTP framework.
+"""
+
+
+class AuthenticationError(Exception):
+    """Base auth error. All auth exceptions inherit from this."""
+    pass
+
+
+class TokenExpiredError(AuthenticationError):
+    """JWT token has expired."""
+    pass
+
+
+class InvalidTokenError(AuthenticationError):
+    """JWT token is malformed, wrong signature, or invalid audience."""
+    pass
+
+
+class TokenMissingClaimError(AuthenticationError):
+    """JWT token is missing a required claim (e.g., sub)."""
+
+    def __init__(self, claim: str = "sub"):
+        super().__init__(f"Token missing required claim: {claim}")
+        self.claim = claim
+
+
+class InvalidCredentialsError(AuthenticationError):
+    """Login failed due to wrong email/password."""
+    pass
+
+
+class SignupError(AuthenticationError):
+    """User registration failed."""
+    pass
+
+
+class SessionError(AuthenticationError):
+    """Token refresh or logout failed."""
+    pass
+
+
+class UserIdRequiredError(AuthenticationError):
+    """Operation requires a user_id but auth context has None (e.g., API key without user)."""
+    pass
diff --git a/backend/tests/test_auth_hardening.py b/backend/tests/test_auth_hardening.py
new file mode 100644
index 0000000..f4b14c3
--- /dev/null
+++ b/backend/tests/test_auth_hardening.py
@@ -0,0 +1,77 @@
+"""Tests for auth hardening -- domain exceptions + null safety (OPE-76, OPE-77)."""
+import pytest
+from unittest.mock import patch, MagicMock
+from fastapi import HTTPException
+
+
+class TestNullUserIdSafety:
+    """API key users (no user_id) get 401, not confusing 404."""
+
+    def test_search_with_null_user_id_returns_401(self, client, valid_headers):
+        """Search should reject None user_id via the real verify_repo_access null guard."""
+        from fastapi.testclient import TestClient
+        from main import app
+        from middleware.auth import require_auth, AuthContext
+
+        # Mock auth to return a context with no user_id (API key user)
+        async def mock_auth_no_user():
+            return AuthContext(api_key_name="test-key", user_id=None)
+
+        app.dependency_overrides[require_auth] = mock_auth_no_user
+        try:
+            resp = client.post(
+                "/api/v1/search",
+                json={"query": "auth", "repo_id": "test"},
+                headers=valid_headers,
+            )
+            # verify_repo_access in dependencies.py should catch None user_id
+            assert resp.status_code == 401
+            assert "User ID required" in resp.json()["detail"]
+        finally:
+            app.dependency_overrides.pop(require_auth, None)
+
+    def test_get_repo_or_404_rejects_none_user_id(self):
+        """get_repo_or_404 should raise 401 when user_id is None."""
+        from dependencies import get_repo_or_404
+        from fastapi import HTTPException
+
+        with pytest.raises(HTTPException) as exc:
+            get_repo_or_404("some-repo", None)
+        assert exc.value.status_code == 401
+
+    def test_verify_repo_access_rejects_none_user_id(self):
+        """verify_repo_access should raise 401 when user_id is None."""
+        from dependencies import verify_repo_access
+        from fastapi import HTTPException
+
+        with pytest.raises(HTTPException) as exc:
+            verify_repo_access("some-repo", None)
+        assert exc.value.status_code == 401
+
+
+class TestDomainExceptions:
+    """Auth service raises domain exceptions, not HTTPException."""
+
+    def test_expired_token_raises_domain_exception(self):
+        """Auth service should raise TokenExpiredError, not HTTPException."""
+        from services.exceptions import TokenExpiredError
+        assert issubclass(TokenExpiredError, Exception)
+        assert not issubclass(TokenExpiredError, HTTPException)
+
+    def test_exception_hierarchy(self):
+        """All auth exceptions inherit from AuthenticationError."""
+        from services.exceptions import (
+            AuthenticationError,
+            TokenExpiredError,
+            InvalidTokenError,
+            TokenMissingClaimError,
+            InvalidCredentialsError,
+            SignupError,
+            SessionError,
+            UserIdRequiredError,
+        )
+        for exc_class in [
+            TokenExpiredError, InvalidTokenError, TokenMissingClaimError,
+            InvalidCredentialsError, SignupError, SessionError, UserIdRequiredError,
+        ]:
+            assert issubclass(exc_class, AuthenticationError)
diff --git a/backend/tests/test_jwt_local_decode.py b/backend/tests/test_jwt_local_decode.py
index 07e08f0..c525e5f 100644
--- a/backend/tests/test_jwt_local_decode.py
+++ b/backend/tests/test_jwt_local_decode.py
@@ -50,40 +50,34 @@ def test_bearer_prefix_stripped(self, auth_service):
 
         assert result["user_id"] == "user-456"
 
-    def test_expired_token_raises_401(self, auth_service):
+    def test_expired_token_raises_error(self, auth_service):
         token = _make_token({"sub": "user-789", "exp": int(time.time()) - 60})
 
-        from fastapi import HTTPException
-        with pytest.raises(HTTPException) as exc:
+        from services.exceptions import TokenExpiredError
+        with pytest.raises(TokenExpiredError):
             auth_service.verify_jwt(token)
-        assert exc.value.status_code == 401
-        assert "expired" in exc.value.detail.lower()
 
-    def test_wrong_secret_raises_401(self, auth_service):
+    def test_wrong_secret_raises_error(self, auth_service):
         token = _make_token({"sub": "user-000"}, secret="wrong-secret")
 
-        from fastapi import HTTPException
-        with pytest.raises(HTTPException) as exc:
+        from services.exceptions import InvalidTokenError
+        with pytest.raises(InvalidTokenError):
             auth_service.verify_jwt(token)
-        assert exc.value.status_code == 401
 
-    def test_missing_sub_claim_raises_401(self, auth_service):
+    def test_missing_sub_claim_raises_error(self, auth_service):
         token = _make_token({"email": "no-sub@test.com"})
 
-        from fastapi import HTTPException
-        with pytest.raises(HTTPException) as exc:
+        from services.exceptions import TokenMissingClaimError
+        with pytest.raises(TokenMissingClaimError):
             auth_service.verify_jwt(token)
-        assert exc.value.status_code == 401
-        assert "subject" in exc.value.detail.lower()
 
-    def test_wrong_audience_raises_401(self, auth_service):
+    def test_wrong_audience_raises_error(self, auth_service):
         payload = {"sub": "user-aud", "aud": "wrong-audience", "exp": int(time.time()) + 3600}
         token = pyjwt.encode(payload, JWT_SECRET, algorithm="HS256")
 
-        from fastapi import HTTPException
-        with pytest.raises(HTTPException) as exc:
+        from services.exceptions import InvalidTokenError
+        with pytest.raises(InvalidTokenError):
             auth_service.verify_jwt(token)
-        assert exc.value.status_code == 401
 
     def test_no_network_call_made(self, auth_service):
         """The whole point of OPE-75: verify_jwt should NOT hit the network."""