diff --git a/.github/workflows/migration-safety.yml b/.github/workflows/migration-safety.yml
new file mode 100644
index 0000000..e34bba8
--- /dev/null
+++ b/.github/workflows/migration-safety.yml
@@ -0,0 +1,86 @@
+name: Migration Safety
+
+on:
+  push:
+    branches: ["main", "master"]
+  pull_request:
+
+jobs:
+  migration-safety:
+    name: Alembic upgrade/downgrade safety
+    runs-on: ubuntu-latest
+
+    services:
+      postgres:
+        image: postgres:16
+        env:
+          POSTGRES_DB: nexapg
+          POSTGRES_USER: nexapg
+          POSTGRES_PASSWORD: nexapg
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U nexapg -d nexapg"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 30
+
+    env:
+      DB_HOST: postgres
+      DB_PORT: 5432
+      DB_NAME: nexapg
+      DB_USER: nexapg
+      DB_PASSWORD: nexapg
+      JWT_SECRET_KEY: ci-jwt-secret-key
+      ENCRYPTION_KEY: MDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDA=
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install backend dependencies
+        run: pip install -r backend/requirements.txt
+
+      - name: Install PostgreSQL client tools
+        run: sudo apt-get update && sudo apt-get install -y postgresql-client
+
+      - name: Wait for PostgreSQL
+        env:
+          PGPASSWORD: nexapg
+        run: |
+          for i in $(seq 1 60); do
+            if pg_isready -h postgres -p 5432 -U nexapg -d nexapg; then
+              exit 0
+            fi
+            sleep 2
+          done
+          echo "PostgreSQL did not become ready in time."
+          exit 1
+
+      - name: Alembic upgrade -> downgrade -1 -> upgrade
+        working-directory: backend
+        run: |
+          alembic upgrade head
+          alembic downgrade -1
+          alembic upgrade head
+
+      - name: Validate schema consistency after roundtrip
+        env:
+          PGPASSWORD: nexapg
+        run: |
+          cd backend
+          alembic upgrade head
+          pg_dump -h postgres -p 5432 -U nexapg -d nexapg --schema-only --no-owner --no-privileges \
+            | sed '/^\\restrict /d; /^\\unrestrict /d' > /tmp/schema_head_before.sql
+
+          alembic downgrade -1
+          alembic upgrade head
+          pg_dump -h postgres -p 5432 -U nexapg -d nexapg --schema-only --no-owner --no-privileges \
+            | sed '/^\\restrict /d; /^\\unrestrict /d' > /tmp/schema_head_after.sql
+
+          diff -u /tmp/schema_head_before.sql /tmp/schema_head_after.sql
diff --git a/README.md b/README.md
index 6b2d6e5..39f502c 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,7 @@ It combines FastAPI, React, and PostgreSQL in a Docker Compose stack with RBAC,
 - [Service Information](#service-information)
 - [Target Owner Notifications](#target-owner-notifications)
 - [API Overview](#api-overview)
+- [API Error Format](#api-error-format)
 - [`pg_stat_statements` Requirement](#pg_stat_statements-requirement)
 - [Reverse Proxy / SSL Guidance](#reverse-proxy--ssl-guidance)
 - [PostgreSQL Compatibility Smoke Test](#postgresql-compatibility-smoke-test)
@@ -319,6 +320,37 @@ Email alert routing is target-specific:
 - `GET /api/v1/service/info`
 - `POST /api/v1/service/info/check`
 
+## API Error Format
+
+All 4xx/5xx responses use a consistent JSON payload:
+
+```json
+{
+  "code": "validation_error",
+  "message": "Request validation failed",
+  "details": [],
+  "request_id": "c8f0f888-2365-4b86-a5de-b3f0e9df4a4b"
+}
+```
+
+Common fields:
+
+- `code`: stable machine-readable error code
+- `message`: human-readable summary
+- `details`: optional extra context (validation list, debug context, etc.)
+- `request_id`: request correlation ID (also returned in `X-Request-ID` header)
+
+Common error codes:
+
+- `bad_request` (`400`)
+- `unauthorized` (`401`)
+- `forbidden` (`403`)
+- `not_found` (`404`)
+- `conflict` (`409`)
+- `validation_error` (`422`)
+- `target_unreachable` (`503`)
+- `internal_error` (`500`)
+
 ## `pg_stat_statements` Requirement
 
 Query Insights requires `pg_stat_statements` on the monitored target:
diff --git a/backend/app/api/routes/admin_settings.py b/backend/app/api/routes/admin_settings.py
index 5c227c5..6c8a4bc 100644
--- a/backend/app/api/routes/admin_settings.py
+++ b/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)}
diff --git a/backend/app/api/routes/admin_users.py b/backend/app/api/routes/admin_users.py
index 22ff253..362b65e 100644
--- a/backend/app/api/routes/admin_users.py
+++ b/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})
diff --git a/backend/app/api/routes/alerts.py b/backend/app/api/routes/alerts.py
index bba8624..2faa592 100644
--- a/backend/app/api/routes/alerts.py
+++ b/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)
diff --git a/backend/app/api/routes/auth.py b/backend/app/api/routes/auth.py
index 32d3d98..ed97cea 100644
--- a/backend/app/api/routes/auth.py
+++ b/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)))
diff --git a/backend/app/api/routes/me.py b/backend/app/api/routes/me.py
index 0b43821..14cd815 100644
--- a/backend/app/api/routes/me.py
+++ b/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()
diff --git a/backend/app/api/routes/targets.py b/backend/app/api/routes/targets.py
index 6ab0322..ea437a7 100644
--- a/backend/app/api/routes/targets.py
+++ b/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
diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index e54f34a..899dba8 100644
--- a/backend/app/core/config.py
+++ b/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):
diff --git a/backend/app/core/deps.py b/backend/app/core/deps.py
index 0260c1b..b91a92a 100644
--- a/backend/app/core/deps.py
+++ b/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
diff --git a/backend/app/core/errors.py b/backend/app/core/errors.py
new file mode 100644
index 0000000..60c2bbc
--- /dev/null
+++ b/backend/app/core/errors.py
@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+from typing import Any
+
+
+def error_payload(code: str, message: str, details: Any, request_id: str) -> dict[str, Any]:
+    return {
+        "code": code,
+        "message": message,
+        "details": details,
+        "request_id": request_id,
+    }
+
+
+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",
+        401: "unauthorized",
+        403: "forbidden",
+        404: "not_found",
+        405: "method_not_allowed",
+        409: "conflict",
+        422: "validation_error",
+        429: "rate_limited",
+        500: "internal_error",
+        502: "bad_gateway",
+        503: "service_unavailable",
+        504: "gateway_timeout",
+    }
+    return mapping.get(status_code, f"http_{status_code}")
diff --git a/backend/app/main.py b/backend/app/main.py
index d1420ed..a70b02c 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -1,12 +1,17 @@
 import asyncio
 import logging
+from uuid import uuid4
 from contextlib import asynccontextmanager
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException, Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
+from starlette.exceptions import HTTPException as StarletteHTTPException
 from sqlalchemy import select
 from app.api.router import api_router
 from app.core.config import get_settings
 from app.core.db import SessionLocal
+from app.core.errors import error_payload, http_status_to_code
 from app.core.logging import configure_logging
 from app.core.security import hash_password
 from app.models.models import User
@@ -57,4 +62,67 @@ app.add_middleware(
     allow_methods=["*"],
     allow_headers=["*"],
 )
+
+
+@app.middleware("http")
+async def request_id_middleware(request: Request, call_next):
+    request_id = request.headers.get("x-request-id") or str(uuid4())
+    request.state.request_id = request_id
+    response = await call_next(request)
+    response.headers["X-Request-ID"] = request_id
+    return response
+
+
+@app.exception_handler(HTTPException)
+@app.exception_handler(StarletteHTTPException)
+async def http_exception_handler(request: Request, exc: HTTPException | StarletteHTTPException):
+    request_id = getattr(request.state, "request_id", str(uuid4()))
+    code = http_status_to_code(exc.status_code)
+    message = "Request failed"
+    details = None
+
+    if isinstance(exc.detail, str):
+        message = exc.detail
+    elif isinstance(exc.detail, dict):
+        code = str(exc.detail.get("code", code))
+        message = str(exc.detail.get("message", message))
+        details = exc.detail.get("details")
+    elif isinstance(exc.detail, list):
+        message = "Request validation failed"
+        details = exc.detail
+
+    return JSONResponse(
+        status_code=exc.status_code,
+        content=error_payload(code=code, message=message, details=details, request_id=request_id),
+    )
+
+
+@app.exception_handler(RequestValidationError)
+async def request_validation_exception_handler(request: Request, exc: RequestValidationError):
+    request_id = getattr(request.state, "request_id", str(uuid4()))
+    return JSONResponse(
+        status_code=422,
+        content=error_payload(
+            code="validation_error",
+            message="Request validation failed",
+            details=exc.errors(),
+            request_id=request_id,
+        ),
+    )
+
+
+@app.exception_handler(Exception)
+async def unhandled_exception_handler(request: Request, exc: Exception):
+    request_id = getattr(request.state, "request_id", str(uuid4()))
+    logger.exception("unhandled_exception request_id=%s", request_id, exc_info=exc)
+    return JSONResponse(
+        status_code=500,
+        content=error_payload(
+            code="internal_error",
+            message="Internal server error",
+            details=None,
+            request_id=request_id,
+        ),
+    )
+
 app.include_router(api_router, prefix=settings.api_v1_prefix)
diff --git a/backend/app/services/alerts.py b/backend/app/services/alerts.py
index 199205d..0180961 100644
--- a/backend/app/services/alerts.py
+++ b/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
 
 
diff --git a/backend/app/services/collector.py b/backend/app/services/collector.py
index 51bbe3c..60cd4d2 100644
--- a/backend/app/services/collector.py
+++ b/backend/app/services/collector.py
@@ -1,6 +1,7 @@
 import asyncio
 import logging
 from datetime import datetime, timezone
+from random import uniform
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.exc import SQLAlchemyError
@@ -15,6 +16,9 @@ logger = logging.getLogger(__name__)
 settings = get_settings()
 _failure_state: dict[int, dict[str, object]] = {}
 _failure_log_interval_seconds = 300
+_backoff_base_seconds = max(3, int(settings.poll_interval_seconds))
+_backoff_max_seconds = 300
+_backoff_jitter_factor = 0.15
 
 
 def build_target_dsn(target: Target) -> str:
@@ -181,31 +185,66 @@ async def collect_once() -> None:
     async with SessionLocal() as db:
         targets = (await db.scalars(select(Target))).all()
 
+    active_target_ids = {target.id for target in targets}
+    stale_target_ids = [target_id for target_id in _failure_state.keys() if target_id not in active_target_ids]
+    for stale_target_id in stale_target_ids:
+        _failure_state.pop(stale_target_id, None)
+
     for target in targets:
+        now = datetime.now(timezone.utc)
+        state = _failure_state.get(target.id)
+        if state:
+            next_attempt_at = state.get("next_attempt_at")
+            if isinstance(next_attempt_at, datetime) and now < next_attempt_at:
+                continue
+
         try:
             await collect_target(target)
             prev = _failure_state.pop(target.id, None)
             if prev:
+                first_failure_at = prev.get("first_failure_at")
+                downtime_seconds = None
+                if isinstance(first_failure_at, datetime):
+                    downtime_seconds = max(0, int((now - first_failure_at).total_seconds()))
                 logger.info(
-                    "collector_target_recovered target=%s after_failures=%s last_error=%s",
+                    "collector_target_recovered target=%s after_failures=%s downtime_seconds=%s last_error=%s",
                     target.id,
                     prev.get("count", 0),
+                    downtime_seconds,
                     prev.get("error"),
                 )
         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:
+                next_delay = min(_backoff_max_seconds, _backoff_base_seconds)
+                jitter = next_delay * _backoff_jitter_factor
+                next_delay = max(1, int(next_delay + uniform(-jitter, jitter)))
+                next_attempt_at = now.timestamp() + next_delay
                 _failure_state[target.id] = {
                     "count": 1,
+                    "first_failure_at": now,
                     "last_log_at": now,
                     "error": current_error,
+                    "next_attempt_at": datetime.fromtimestamp(next_attempt_at, tz=timezone.utc),
                 }
-                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 retry_in_seconds=%s",
+                    target.id,
+                    error_class,
+                    current_error,
+                    1,
+                    next_delay,
+                )
                 continue
 
             count = int(state.get("count", 0)) + 1
+            raw_backoff = min(_backoff_max_seconds, _backoff_base_seconds * (2 ** min(count - 1, 10)))
+            jitter = raw_backoff * _backoff_jitter_factor
+            next_delay = max(1, int(raw_backoff + uniform(-jitter, jitter)))
+            state["next_attempt_at"] = datetime.fromtimestamp(now.timestamp() + next_delay, tz=timezone.utc)
+
             last_log_at = state.get("last_log_at")
             last_logged_error = str(state.get("error", ""))
             should_log = False
@@ -220,18 +259,23 @@ 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 retry_in_seconds=%s",
                     target.id,
+                    error_class,
                     current_error,
                     count,
+                    next_delay,
                 )
 
 
 async def collector_loop(stop_event: asyncio.Event) -> None:
     while not stop_event.is_set():
+        cycle_started = asyncio.get_running_loop().time()
         await collect_once()
+        elapsed = asyncio.get_running_loop().time() - cycle_started
+        sleep_for = max(0.0, settings.poll_interval_seconds - elapsed)
         try:
-            await asyncio.wait_for(stop_event.wait(), timeout=settings.poll_interval_seconds)
+            await asyncio.wait_for(stop_event.wait(), timeout=sleep_for)
         except asyncio.TimeoutError:
             pass
diff --git a/frontend/src/api.js b/frontend/src/api.js
index 5ee9bfb..6a1ca4d 100644
--- a/frontend/src/api.js
+++ b/frontend/src/api.js
@@ -35,8 +35,21 @@ export async function apiFetch(path, options = {}, tokens, onUnauthorized) {
     }
   }
   if (!res.ok) {
-    const txt = await res.text();
-    throw new Error(txt || `HTTP ${res.status}`);
+    const raw = await res.text();
+    let parsed = null;
+    try {
+      parsed = raw ? JSON.parse(raw) : null;
+    } catch {
+      parsed = null;
+    }
+
+    const message = parsed?.message || raw || `HTTP ${res.status}`;
+    const err = new Error(message);
+    err.status = res.status;
+    err.code = parsed?.code || null;
+    err.details = parsed?.details || null;
+    err.requestId = parsed?.request_id || res.headers.get("x-request-id") || null;
+    throw err;
   }
   if (res.status === 204) return null;
   return res.json();
diff --git a/frontend/src/pages/TargetDetailPage.jsx b/frontend/src/pages/TargetDetailPage.jsx
index 21c26e1..8337190 100644
--- a/frontend/src/pages/TargetDetailPage.jsx
+++ b/frontend/src/pages/TargetDetailPage.jsx
@@ -76,6 +76,10 @@ function didMetricSeriesChange(prev = [], next = []) {
   return prevLast?.ts !== nextLast?.ts || Number(prevLast?.value) !== Number(nextLast?.value);
 }
 
+function isTargetUnreachableError(err) {
+  return err?.code === "target_unreachable" || err?.status === 503;
+}
+
 async function loadMetric(targetId, metric, range, tokens, refresh) {
   const { from, to } = toQueryRange(range);
   return apiFetch(
@@ -99,6 +103,7 @@ export function TargetDetailPage() {
   const [targetMeta, setTargetMeta] = useState(null);
   const [owners, setOwners] = useState([]);
   const [groupTargets, setGroupTargets] = useState([]);
+  const [offlineState, setOfflineState] = useState(null);
   const [error, setError] = useState("");
   const [loading, setLoading] = useState(true);
   const refreshRef = useRef(refresh);
@@ -114,22 +119,16 @@ export function TargetDetailPage() {
         setLoading(true);
       }
       try {
-        const [connections, xacts, cache, locksTable, activityTable, overviewData, targetInfo, ownerRows, allTargets] = await Promise.all([
+        const [connections, xacts, cache, targetInfo, ownerRows, allTargets] = await Promise.all([
           loadMetric(id, "connections_total", range, tokens, refreshRef.current),
           loadMetric(id, "xacts_total", range, tokens, refreshRef.current),
           loadMetric(id, "cache_hit_ratio", range, tokens, refreshRef.current),
-          apiFetch(`/targets/${id}/locks`, {}, tokens, refreshRef.current),
-          apiFetch(`/targets/${id}/activity`, {}, tokens, refreshRef.current),
-          apiFetch(`/targets/${id}/overview`, {}, tokens, refreshRef.current),
           apiFetch(`/targets/${id}`, {}, tokens, refreshRef.current),
           apiFetch(`/targets/${id}/owners`, {}, tokens, refreshRef.current),
           apiFetch("/targets", {}, tokens, refreshRef.current),
         ]);
         if (!active) return;
         setSeries({ connections, xacts, cache });
-        setLocks(locksTable);
-        setActivity(activityTable);
-        setOverview(overviewData);
         setTargetMeta(targetInfo);
         setOwners(ownerRows);
         const groupId = targetInfo?.tags?.monitor_group_id;
@@ -141,6 +140,34 @@ export function TargetDetailPage() {
         } else {
           setGroupTargets([]);
         }
+        try {
+          const [locksTable, activityTable, overviewData] = await Promise.all([
+            apiFetch(`/targets/${id}/locks`, {}, tokens, refreshRef.current),
+            apiFetch(`/targets/${id}/activity`, {}, tokens, refreshRef.current),
+            apiFetch(`/targets/${id}/overview`, {}, tokens, refreshRef.current),
+          ]);
+          if (!active) return;
+          setLocks(locksTable);
+          setActivity(activityTable);
+          setOverview(overviewData);
+          setOfflineState(null);
+        } catch (liveErr) {
+          if (!active) return;
+          if (isTargetUnreachableError(liveErr)) {
+            setLocks([]);
+            setActivity([]);
+            setOverview(null);
+            setOfflineState({
+              message:
+                "Target is currently unreachable. Check host/port, network route, SSL mode, and database availability.",
+              host: liveErr?.details?.host || targetInfo?.host || "-",
+              port: liveErr?.details?.port || targetInfo?.port || "-",
+              requestId: liveErr?.requestId || null,
+            });
+          } else {
+            throw liveErr;
+          }
+        }
         setError("");
       } catch (e) {
         if (active) setError(String(e.message || e));
@@ -281,6 +308,17 @@ export function TargetDetailPage() {
         <span className="muted">Responsible users:</span>
         {owners.length > 0 ? owners.map((item) => <span key={item.user_id} className="owner-pill">{item.email}</span>) : <span className="muted">none assigned</span>}
       </div>
+      {offlineState && (
+        <div className="card target-offline-card">
+          <h3>Target Offline</h3>
+          <p>{offlineState.message}</p>
+          <div className="target-offline-meta">
+            <span><strong>Host:</strong> {offlineState.host}</span>
+            <span><strong>Port:</strong> {offlineState.port}</span>
+            {offlineState.requestId ? <span><strong>Request ID:</strong> {offlineState.requestId}</span> : null}
+          </div>
+        </div>
+      )}
       {uiMode === "easy" && overview && easySummary && (
         <>
           <div className={`card easy-status ${easySummary.health}`}>
diff --git a/frontend/src/styles.css b/frontend/src/styles.css
index 25d10d0..9c1b75f 100644
--- a/frontend/src/styles.css
+++ b/frontend/src/styles.css
@@ -2022,6 +2022,29 @@ select:-webkit-autofill {
   font-size: 12px;
 }
 
+.target-offline-card {
+  border-color: #a85757;
+  background: linear-gradient(130deg, #2c1724 0%, #1f1f38 100%);
+}
+
+.target-offline-card h3 {
+  margin: 0 0 8px;
+  color: #fecaca;
+}
+
+.target-offline-card p {
+  margin: 0 0 10px;
+  color: #fde2e2;
+}
+
+.target-offline-meta {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 16px;
+  font-size: 12px;
+  color: #d4d4f5;
+}
+
 .chart-tooltip {
   background: #0f1934ee;
   border: 1px solid #2f4a8b;