[NX-101 Issue] Refactor error handling to use consistent API error format

Replaced all inline error messages with the standardized `api_error` helper for consistent error response formatting. This improves clarity, maintainability, and ensures uniform error structures across the application. Updated logging for collector failures to include error class and switched to warning level for target unreachable scenarios.

README.md

@@ -348,6 +348,7 @@ Common error codes:
 - `not_found` (`404`)
 - `conflict` (`409`)
 - `validation_error` (`422`)
+- `target_unreachable` (`503`)
 - `internal_error` (`500`)
 
 ## `pg_stat_statements` Requirement

backend/app/api/routes/admin_settings.py

@@ -9,6 +9,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.db import get_db
 from app.core.deps import require_roles
+from app.core.errors import api_error
 from app.models.models import EmailNotificationSettings, User
 from app.schemas.admin_settings import EmailSettingsOut, EmailSettingsTestRequest, EmailSettingsUpdate
 from app.services.audit import write_audit_log
@@ -96,9 +97,9 @@ async def test_email_settings(
 ) -> dict:
     settings = await _get_or_create_settings(db)
     if not settings.smtp_host:
-        raise HTTPException(status_code=400, detail="SMTP host is not configured")
+        raise HTTPException(status_code=400, detail=api_error("smtp_host_missing", "SMTP host is not configured"))
     if not settings.from_email:
-        raise HTTPException(status_code=400, detail="From email is not configured")
+        raise HTTPException(status_code=400, detail=api_error("smtp_from_email_missing", "From email is not configured"))
 
     password = decrypt_secret(settings.encrypted_smtp_password) if settings.encrypted_smtp_password else None
     message = EmailMessage()
@@ -126,7 +127,10 @@ async def test_email_settings(
                     smtp.login(settings.smtp_username, password or "")
                 smtp.send_message(message)
     except Exception as exc:
-        raise HTTPException(status_code=400, detail=f"SMTP test failed: {exc}")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("smtp_test_failed", "SMTP test failed", {"error": str(exc)}),
+        ) from exc
 
     await write_audit_log(db, "admin.email_settings.test", admin.id, {"recipient": str(payload.recipient)})
     return {"status": "sent", "recipient": str(payload.recipient)}

backend/app/api/routes/admin_users.py

@@ -3,6 +3,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.db import get_db
 from app.core.deps import require_roles
+from app.core.errors import api_error
 from app.core.security import hash_password
 from app.models.models import User
 from app.schemas.user import UserCreate, UserOut, UserUpdate
@@ -22,7 +23,7 @@ async def list_users(admin: User = Depends(require_roles("admin")), db: AsyncSes
 async def create_user(payload: UserCreate, admin: User = Depends(require_roles("admin")), db: AsyncSession = Depends(get_db)) -> UserOut:
     exists = await db.scalar(select(User).where(User.email == payload.email))
     if exists:
-        raise HTTPException(status_code=409, detail="Email already exists")
+        raise HTTPException(status_code=409, detail=api_error("email_exists", "Email already exists"))
     user = User(
         email=payload.email,
         first_name=payload.first_name,
@@ -46,13 +47,13 @@ async def update_user(
 ) -> UserOut:
     user = await db.scalar(select(User).where(User.id == user_id))
     if not user:
-        raise HTTPException(status_code=404, detail="User not found")
+        raise HTTPException(status_code=404, detail=api_error("user_not_found", "User not found"))
     update_data = payload.model_dump(exclude_unset=True)
     next_email = update_data.get("email")
     if next_email and next_email != user.email:
         existing = await db.scalar(select(User).where(User.email == next_email))
         if existing and existing.id != user.id:
-            raise HTTPException(status_code=409, detail="Email already exists")
+            raise HTTPException(status_code=409, detail=api_error("email_exists", "Email already exists"))
     if "password" in update_data:
         raw_password = update_data.pop("password")
         if raw_password:
@@ -68,10 +69,10 @@ async def update_user(
 @router.delete("/{user_id}")
 async def delete_user(user_id: int, admin: User = Depends(require_roles("admin")), db: AsyncSession = Depends(get_db)) -> dict:
     if user_id == admin.id:
-        raise HTTPException(status_code=400, detail="Cannot delete yourself")
+        raise HTTPException(status_code=400, detail=api_error("cannot_delete_self", "Cannot delete yourself"))
     user = await db.scalar(select(User).where(User.id == user_id))
     if not user:
-        raise HTTPException(status_code=404, detail="User not found")
+        raise HTTPException(status_code=404, detail=api_error("user_not_found", "User not found"))
     await db.delete(user)
     await db.commit()
     await write_audit_log(db, "admin.user.delete", admin.id, {"deleted_user_id": user_id})

backend/app/api/routes/alerts.py

@@ -4,6 +4,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.db import get_db
 from app.core.deps import get_current_user, require_roles
+from app.core.errors import api_error
 from app.models.models import AlertDefinition, Target, User
 from app.schemas.alert import (
     AlertDefinitionCreate,
@@ -33,7 +34,7 @@ async def _validate_target_exists(db: AsyncSession, target_id: int | None) -> No
         return
     target_exists = await db.scalar(select(Target.id).where(Target.id == target_id))
     if target_exists is None:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
 
 
 @router.get("/status", response_model=AlertStatusResponse)
@@ -101,7 +102,7 @@ async def update_alert_definition(
 ) -> AlertDefinitionOut:
     definition = await db.scalar(select(AlertDefinition).where(AlertDefinition.id == definition_id))
     if definition is None:
-        raise HTTPException(status_code=404, detail="Alert definition not found")
+        raise HTTPException(status_code=404, detail=api_error("alert_definition_not_found", "Alert definition not found"))
 
     updates = payload.model_dump(exclude_unset=True)
     if "target_id" in updates:
@@ -131,7 +132,7 @@ async def delete_alert_definition(
 ) -> dict:
     definition = await db.scalar(select(AlertDefinition).where(AlertDefinition.id == definition_id))
     if definition is None:
-        raise HTTPException(status_code=404, detail="Alert definition not found")
+        raise HTTPException(status_code=404, detail=api_error("alert_definition_not_found", "Alert definition not found"))
     await db.delete(definition)
     await db.commit()
     invalidate_alert_cache()
@@ -148,7 +149,7 @@ async def test_alert_definition(
     _ = user
     target = await db.scalar(select(Target).where(Target.id == payload.target_id))
     if target is None:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     try:
         value = await run_scalar_sql_for_target(target, payload.sql_text)
         return AlertDefinitionTestResponse(ok=True, value=value)

backend/app/api/routes/auth.py

@@ -5,6 +5,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.config import get_settings
 from app.core.db import get_db
 from app.core.deps import get_current_user
+from app.core.errors import api_error
 from app.core.security import create_access_token, create_refresh_token, verify_password
 from app.models.models import User
 from app.schemas.auth import LoginRequest, RefreshRequest, TokenResponse
@@ -19,7 +20,10 @@ settings = get_settings()
 async def login(payload: LoginRequest, db: AsyncSession = Depends(get_db)) -> TokenResponse:
     user = await db.scalar(select(User).where(User.email == payload.email))
     if not user or not verify_password(payload.password, user.password_hash):
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_credentials", "Invalid credentials"),
+        )
 
     await write_audit_log(db, action="auth.login", user_id=user.id, payload={"email": user.email})
     return TokenResponse(access_token=create_access_token(str(user.id)), refresh_token=create_refresh_token(str(user.id)))
@@ -30,14 +34,23 @@ async def refresh(payload: RefreshRequest, db: AsyncSession = Depends(get_db)) -
     try:
         token_payload = jwt.decode(payload.refresh_token, settings.jwt_secret_key, algorithms=[settings.jwt_algorithm])
     except jwt.InvalidTokenError as exc:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid refresh token") from exc
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_refresh_token", "Invalid refresh token"),
+        ) from exc
 
     if token_payload.get("type") != "refresh":
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid refresh token type")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_refresh_token_type", "Invalid refresh token type"),
+        )
     user_id = token_payload.get("sub")
     user = await db.scalar(select(User).where(User.id == int(user_id)))
     if not user:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not found")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("user_not_found", "User not found"),
+        )
 
     await write_audit_log(db, action="auth.refresh", user_id=user.id, payload={})
     return TokenResponse(access_token=create_access_token(str(user.id)), refresh_token=create_refresh_token(str(user.id)))

backend/app/api/routes/me.py

@@ -2,6 +2,7 @@ from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.db import get_db
 from app.core.deps import get_current_user
+from app.core.errors import api_error
 from app.core.security import hash_password, verify_password
 from app.models.models import User
 from app.schemas.user import UserOut, UserPasswordChange
@@ -22,10 +23,16 @@ async def change_password(
     db: AsyncSession = Depends(get_db),
 ) -> dict:
     if not verify_password(payload.current_password, user.password_hash):
-        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="Current password is incorrect")
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=api_error("invalid_current_password", "Current password is incorrect"),
+        )
 
     if verify_password(payload.new_password, user.password_hash):
-        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="New password must be different")
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=api_error("password_reuse_not_allowed", "New password must be different"),
+        )
 
     user.password_hash = hash_password(payload.new_password)
     await db.commit()

backend/app/api/routes/targets.py

@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.db import get_db
 from app.core.deps import get_current_user, require_roles
+from app.core.errors import api_error
 from app.models.models import Metric, QueryStat, Target, TargetOwner, User
 from app.schemas.metric import MetricOut, QueryStatOut
 from app.schemas.overview import DatabaseOverviewOut
@@ -85,7 +86,10 @@ async def _discover_databases(payload: TargetCreate) -> list[str]:
         )
         return [row["datname"] for row in rows if row["datname"]]
     except Exception as exc:
-        raise HTTPException(status_code=400, detail=f"Database discovery failed: {exc}")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("database_discovery_failed", "Database discovery failed", {"error": str(exc)}),
+        )
     finally:
         if conn:
             await conn.close()
@@ -131,7 +135,10 @@ async def test_target_connection(
         version = await conn.fetchval("SHOW server_version")
         return {"ok": True, "message": "Connection successful", "server_version": version}
     except Exception as exc:
-        raise HTTPException(status_code=400, detail=f"Connection failed: {exc}")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("connection_test_failed", "Connection test failed", {"error": str(exc)}),
+        )
     finally:
         if conn:
             await conn.close()
@@ -147,7 +154,10 @@ async def create_target(
     if owner_ids:
         owners_exist = (await db.scalars(select(User.id).where(User.id.in_(owner_ids)))).all()
         if len(set(owners_exist)) != len(owner_ids):
-            raise HTTPException(status_code=400, detail="One or more owner users were not found")
+            raise HTTPException(
+                status_code=400,
+                detail=api_error("owner_users_not_found", "One or more owner users were not found"),
+            )
 
     encrypted_password = encrypt_secret(payload.password)
     created_targets: list[Target] = []
@@ -155,7 +165,10 @@ async def create_target(
     if payload.discover_all_databases:
         databases = await _discover_databases(payload)
         if not databases:
-            raise HTTPException(status_code=400, detail="No databases discovered on target")
+            raise HTTPException(
+                status_code=400,
+                detail=api_error("no_databases_discovered", "No databases discovered on target"),
+            )
         group_id = str(uuid4())
         base_tags = payload.tags or {}
         for dbname in databases:
@@ -194,7 +207,10 @@ async def create_target(
                 await _set_target_owners(db, target.id, owner_ids, user.id)
 
         if not created_targets:
-            raise HTTPException(status_code=400, detail="All discovered databases already exist as targets")
+            raise HTTPException(
+                status_code=400,
+                detail=api_error("all_discovered_databases_exist", "All discovered databases already exist as targets"),
+            )
         await db.commit()
         for item in created_targets:
             await db.refresh(item)
@@ -247,7 +263,7 @@ async def get_target(target_id: int, user: User = Depends(get_current_user), db:
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     owner_map = await _owners_by_target_ids(db, [target.id])
     return _target_out_with_owners(target, owner_map.get(target.id, []))
 
@@ -261,7 +277,7 @@ async def update_target(
 ) -> TargetOut:
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
 
     updates = payload.model_dump(exclude_unset=True)
     owner_user_ids = updates.pop("owner_user_ids", None)
@@ -273,7 +289,10 @@ async def update_target(
     if owner_user_ids is not None:
         owners_exist = (await db.scalars(select(User.id).where(User.id.in_(owner_user_ids)))).all()
         if len(set(owners_exist)) != len(set(owner_user_ids)):
-            raise HTTPException(status_code=400, detail="One or more owner users were not found")
+            raise HTTPException(
+                status_code=400,
+                detail=api_error("owner_users_not_found", "One or more owner users were not found"),
+            )
         await _set_target_owners(db, target.id, owner_user_ids, user.id)
 
     await db.commit()
@@ -292,12 +311,15 @@ async def set_target_owners(
 ) -> list[TargetOwnerOut]:
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     owner_user_ids = sorted(set(payload.user_ids))
     if owner_user_ids:
         owners_exist = (await db.scalars(select(User.id).where(User.id.in_(owner_user_ids)))).all()
         if len(set(owners_exist)) != len(set(owner_user_ids)):
-            raise HTTPException(status_code=400, detail="One or more owner users were not found")
+            raise HTTPException(
+                status_code=400,
+                detail=api_error("owner_users_not_found", "One or more owner users were not found"),
+            )
     await _set_target_owners(db, target_id, owner_user_ids, user.id)
     await db.commit()
     await write_audit_log(db, "target.owners.update", user.id, {"target_id": target_id, "owner_user_ids": owner_user_ids})
@@ -321,7 +343,7 @@ async def get_target_owners(
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     rows = (
         await db.execute(
             select(User.id, User.email, User.role)
@@ -341,7 +363,7 @@ async def delete_target(
 ) -> dict:
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     await db.delete(target)
     await db.commit()
     await write_audit_log(db, "target.delete", user.id, {"target_id": target_id})
@@ -369,7 +391,22 @@ async def get_metrics(
 
 
 async def _live_conn(target: Target) -> asyncpg.Connection:
-    return await asyncpg.connect(dsn=build_target_dsn(target))
+    try:
+        return await asyncpg.connect(dsn=build_target_dsn(target))
+    except (OSError, asyncpg.PostgresError) as exc:
+        raise HTTPException(
+            status_code=503,
+            detail=api_error(
+                "target_unreachable",
+                "Target database is not reachable",
+                {
+                    "target_id": target.id,
+                    "host": target.host,
+                    "port": target.port,
+                    "error": str(exc),
+                },
+            ),
+        ) from exc
 
 
 @router.get("/{target_id}/locks")
@@ -377,7 +414,7 @@ async def get_locks(target_id: int, user: User = Depends(get_current_user), db:
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     conn = await _live_conn(target)
     try:
         rows = await conn.fetch(
@@ -398,7 +435,7 @@ async def get_activity(target_id: int, user: User = Depends(get_current_user), d
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     conn = await _live_conn(target)
     try:
         rows = await conn.fetch(
@@ -420,7 +457,7 @@ async def get_top_queries(target_id: int, user: User = Depends(get_current_user)
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
     if not target.use_pg_stat_statements:
         return []
     rows = (
@@ -450,5 +487,20 @@ async def get_overview(target_id: int, user: User = Depends(get_current_user), d
     _ = user
     target = await db.scalar(select(Target).where(Target.id == target_id))
     if not target:
-        raise HTTPException(status_code=404, detail="Target not found")
-    return await get_target_overview(target)
+        raise HTTPException(status_code=404, detail=api_error("target_not_found", "Target not found"))
+    try:
+        return await get_target_overview(target)
+    except (OSError, asyncpg.PostgresError) as exc:
+        raise HTTPException(
+            status_code=503,
+            detail=api_error(
+                "target_unreachable",
+                "Target database is not reachable",
+                {
+                    "target_id": target.id,
+                    "host": target.host,
+                    "port": target.port,
+                    "error": str(exc),
+                },
+            ),
+        ) from exc

backend/app/core/config.py

@@ -2,7 +2,7 @@ from functools import lru_cache
 from pydantic import field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
-NEXAPG_VERSION = "0.1.8"
+NEXAPG_VERSION = "0.2.0"
 
 
 class Settings(BaseSettings):

backend/app/core/deps.py

@@ -5,6 +5,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.config import get_settings
 from app.core.db import get_db
+from app.core.errors import api_error
 from app.models.models import User
 
 settings = get_settings()
@@ -16,27 +17,42 @@ async def get_current_user(
     db: AsyncSession = Depends(get_db),
 ) -> User:
     if not credentials:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Missing token")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("missing_token", "Missing token"),
+        )
     token = credentials.credentials
     try:
         payload = jwt.decode(token, settings.jwt_secret_key, algorithms=[settings.jwt_algorithm])
     except jwt.InvalidTokenError as exc:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token") from exc
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_token", "Invalid token"),
+        ) from exc
 
     if payload.get("type") != "access":
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token type")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_token_type", "Invalid token type"),
+        )
 
     user_id = payload.get("sub")
     user = await db.scalar(select(User).where(User.id == int(user_id)))
     if not user:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not found")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("user_not_found", "User not found"),
+        )
     return user
 
 
 def require_roles(*roles: str):
     async def role_dependency(user: User = Depends(get_current_user)) -> User:
         if user.role not in roles:
-            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Forbidden")
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail=api_error("forbidden", "Forbidden"),
+            )
         return user
 
     return role_dependency

backend/app/core/errors.py

@@ -12,6 +12,14 @@ def error_payload(code: str, message: str, details: Any, request_id: str) -> dic
     }
 
 
+def api_error(code: str, message: str, details: Any = None) -> dict[str, Any]:
+    return {
+        "code": code,
+        "message": message,
+        "details": details,
+    }
+
+
 def http_status_to_code(status_code: int) -> str:
     mapping = {
         400: "bad_request",
@@ -28,4 +36,3 @@ def http_status_to_code(status_code: int) -> str:
         504: "gateway_timeout",
     }
     return mapping.get(status_code, f"http_{status_code}")
-

backend/app/services/alerts.py

@@ -11,6 +11,7 @@ from sqlalchemy import desc, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.config import get_settings
+from app.core.errors import api_error
 from app.models.models import AlertDefinition, Metric, QueryStat, Target
 from app.schemas.alert import AlertStatusItem, AlertStatusResponse
 from app.services.collector import build_target_dsn
@@ -144,25 +145,40 @@ def get_standard_alert_reference() -> list[dict[str, str]]:
 
 def validate_alert_thresholds(comparison: str, warning_threshold: float | None, alert_threshold: float) -> None:
     if comparison not in _ALLOWED_COMPARISONS:
-        raise HTTPException(status_code=400, detail=f"Invalid comparison. Use one of {sorted(_ALLOWED_COMPARISONS)}")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error(
+                "invalid_comparison",
+                f"Invalid comparison. Use one of {sorted(_ALLOWED_COMPARISONS)}",
+            ),
+        )
     if warning_threshold is None:
         return
 
     if comparison in {"gte", "gt"} and warning_threshold > alert_threshold:
-        raise HTTPException(status_code=400, detail="For gte/gt, warning_threshold must be <= alert_threshold")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("invalid_thresholds", "For gte/gt, warning_threshold must be <= alert_threshold"),
+        )
     if comparison in {"lte", "lt"} and warning_threshold < alert_threshold:
-        raise HTTPException(status_code=400, detail="For lte/lt, warning_threshold must be >= alert_threshold")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("invalid_thresholds", "For lte/lt, warning_threshold must be >= alert_threshold"),
+        )
 
 
 def validate_alert_sql(sql_text: str) -> str:
     sql = sql_text.strip().rstrip(";")
     lowered = sql.lower().strip()
     if not lowered.startswith("select"):
-        raise HTTPException(status_code=400, detail="Alert SQL must start with SELECT")
+        raise HTTPException(status_code=400, detail=api_error("invalid_alert_sql", "Alert SQL must start with SELECT"))
     if _FORBIDDEN_SQL_WORDS.search(lowered):
-        raise HTTPException(status_code=400, detail="Only read-only SELECT statements are allowed")
+        raise HTTPException(
+            status_code=400,
+            detail=api_error("invalid_alert_sql", "Only read-only SELECT statements are allowed"),
+        )
     if ";" in sql:
-        raise HTTPException(status_code=400, detail="Only a single SQL statement is allowed")
+        raise HTTPException(status_code=400, detail=api_error("invalid_alert_sql", "Only a single SQL statement is allowed"))
     return sql
 
 

backend/app/services/collector.py

@@ -195,6 +195,7 @@ async def collect_once() -> None:
         except (OSError, SQLAlchemyError, asyncpg.PostgresError, Exception) as exc:
             now = datetime.now(timezone.utc)
             current_error = str(exc)
+            error_class = exc.__class__.__name__
             state = _failure_state.get(target.id)
             if state is None:
                 _failure_state[target.id] = {
@@ -202,7 +203,13 @@ async def collect_once() -> None:
                     "last_log_at": now,
                     "error": current_error,
                 }
-                logger.exception("collector_error target=%s err=%s", target.id, exc)
+                logger.warning(
+                    "collector_target_unreachable target=%s error_class=%s err=%s consecutive_failures=%s",
+                    target.id,
+                    error_class,
+                    current_error,
+                    1,
+                )
                 continue
 
             count = int(state.get("count", 0)) + 1
@@ -220,9 +227,10 @@ async def collect_once() -> None:
             if should_log:
                 state["last_log_at"] = now
                 state["error"] = current_error
-                logger.error(
-                    "collector_error_throttled target=%s err=%s consecutive_failures=%s",
+                logger.warning(
+                    "collector_target_unreachable target=%s error_class=%s err=%s consecutive_failures=%s",
                     target.id,
+                    error_class,
                     current_error,
                     count,
                 )