diff --git a/backend/src/media_library_viewer_api/integrations/alertmanager.py b/backend/src/media_library_viewer_api/integrations/alertmanager.py
index d4d86ba..64d8016 100644
--- a/backend/src/media_library_viewer_api/integrations/alertmanager.py
+++ b/backend/src/media_library_viewer_api/integrations/alertmanager.py
@@ -6,6 +6,7 @@ from typing import Any
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
     WidgetConfigBase,
@@ -16,7 +17,7 @@ from media_library_viewer_api.integrations.base import (
 class AlertmanagerConfig(ServiceConfigBase):
     """Non-secret Alertmanager connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     timeout_seconds: int = 5
 
 
diff --git a/backend/src/media_library_viewer_api/integrations/base.py b/backend/src/media_library_viewer_api/integrations/base.py
index 09da520..d116f7f 100644
--- a/backend/src/media_library_viewer_api/integrations/base.py
+++ b/backend/src/media_library_viewer_api/integrations/base.py
@@ -16,9 +16,37 @@ map. There is no runtime plugin loading.
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from typing import Any
+from typing import Annotated, Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, BeforeValidator, Field
+
+
+def _validate_service_base_url(value: Any) -> str:
+    """Require an absolute http(s) URL for service ``base_url`` fields.
+
+    Relative hosts (e.g. ``grafana.example.com``) break downstream HTTP clients
+    because ``requests`` treats them as relative paths, so we fail fast with a
+    clear error instead of letting the call silently malfunction.
+    """
+    if not isinstance(value, str):
+        raise ValueError("base_url must be a string starting with http:// or https://")
+    text = value.strip()
+    if not text:
+        raise ValueError("base_url must not be empty")
+    lowered = text.lower()
+    if not (lowered.startswith("http://") or lowered.startswith("https://")):
+        raise ValueError("base_url must start with http:// or https:// (include the schema)")
+    return text
+
+
+#: Shared annotated type for service ``base_url`` fields. applying the validator
+#: uniformly across every integration so missing schemas are rejected at the
+#: config boundary with a helpful message.
+ServiceBaseUrl = Annotated[
+    str,
+    Field(description="Absolute URL including the http:// or https:// schema."),
+    BeforeValidator(_validate_service_base_url),
+]
 
 
 class ServiceConfigBase(BaseModel):
@@ -26,6 +54,9 @@ class ServiceConfigBase(BaseModel):
 
     Subclass this in each integration module and declare the connection fields.
     The JSON schema is derived via ``model_json_schema()`` and exposed to the UI.
+
+    Connection URLs should use the :data:`ServiceBaseUrl` type so the
+    ``http(s)://`` schema is enforced consistently across integrations.
     """
 
 
diff --git a/backend/src/media_library_viewer_api/integrations/grafana.py b/backend/src/media_library_viewer_api/integrations/grafana.py
index cf95f55..842dea0 100644
--- a/backend/src/media_library_viewer_api/integrations/grafana.py
+++ b/backend/src/media_library_viewer_api/integrations/grafana.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
     WidgetConfigBase,
@@ -14,7 +15,7 @@ from media_library_viewer_api.integrations.base import (
 class GrafanaConfig(ServiceConfigBase):
     """Non-secret Grafana connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     timeout_seconds: int = 5
 
 
diff --git a/backend/src/media_library_viewer_api/integrations/jellyfin.py b/backend/src/media_library_viewer_api/integrations/jellyfin.py
index 78897fd..efbab6b 100644
--- a/backend/src/media_library_viewer_api/integrations/jellyfin.py
+++ b/backend/src/media_library_viewer_api/integrations/jellyfin.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
     WidgetConfigBase,
@@ -14,7 +15,7 @@ from media_library_viewer_api.integrations.base import (
 class JellyfinConfig(ServiceConfigBase):
     """Non-secret Jellyfin connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     user_id: str = ""
     timeout_seconds: int = 10
 
diff --git a/backend/src/media_library_viewer_api/integrations/jellyseerr.py b/backend/src/media_library_viewer_api/integrations/jellyseerr.py
index cea0b8b..1cf5d76 100644
--- a/backend/src/media_library_viewer_api/integrations/jellyseerr.py
+++ b/backend/src/media_library_viewer_api/integrations/jellyseerr.py
@@ -9,6 +9,7 @@ from __future__ import annotations
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
 )
@@ -17,7 +18,7 @@ from media_library_viewer_api.integrations.base import (
 class JellyseerrConfig(ServiceConfigBase):
     """Non-secret Jellyseerr connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
 
 
 DEFINITION = ServiceDefinition(
diff --git a/backend/src/media_library_viewer_api/integrations/nextcloud.py b/backend/src/media_library_viewer_api/integrations/nextcloud.py
index dd44558..fe62d97 100644
--- a/backend/src/media_library_viewer_api/integrations/nextcloud.py
+++ b/backend/src/media_library_viewer_api/integrations/nextcloud.py
@@ -8,6 +8,7 @@ from __future__ import annotations
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
 )
@@ -16,7 +17,7 @@ from media_library_viewer_api.integrations.base import (
 class NextcloudConfig(ServiceConfigBase):
     """Non-secret Nextcloud connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     username: str = ""
 
 
diff --git a/backend/src/media_library_viewer_api/integrations/prometheus.py b/backend/src/media_library_viewer_api/integrations/prometheus.py
index 458304f..a862997 100644
--- a/backend/src/media_library_viewer_api/integrations/prometheus.py
+++ b/backend/src/media_library_viewer_api/integrations/prometheus.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
     WidgetConfigBase,
@@ -14,7 +15,7 @@ from media_library_viewer_api.integrations.base import (
 class PrometheusConfig(ServiceConfigBase):
     """Non-secret Prometheus connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     timeout_seconds: int = 10
 
 
diff --git a/backend/tests/test_services.py b/backend/tests/test_services.py
index a7676ec..b041762 100644
--- a/backend/tests/test_services.py
+++ b/backend/tests/test_services.py
@@ -8,6 +8,7 @@ from unittest.mock import patch
 import pytest
 from cryptography.fernet import Fernet
 from fastapi.testclient import TestClient
+from pydantic import ValidationError
 
 from media_library_viewer_api.dependencies import get_settings_store
 from media_library_viewer_api.integrations.registry import (
@@ -244,7 +245,8 @@ def test_invalid_config_rejected(client):
         "/api/services/instances",
         json={"service_type": "grafana", "name": "x", "config": {"base_url": ""}},
     )
-    # Pydantic accepts empty string; force a real validation error via bad type.
+    assert response.status_code == 422
+    # Force a real validation error via bad type.
     response = client.post(
         "/api/services/instances",
         json={"service_type": "grafana", "name": "x", "config": {"timeout_seconds": "fast"}},
@@ -252,6 +254,25 @@ def test_invalid_config_rejected(client):
     assert response.status_code == 422
 
 
+@pytest.mark.parametrize(
+    "bad_url", ["grafana.example.com", "localhost:3000", "//grafana.example.com", "ftp://grafana.example.com"]
+)
+def test_service_base_url_requires_http_schema(bad_url):
+    """Every service base_url must include an http:// or https:// schema."""
+    model = get_service_definition("grafana").config_model
+    with pytest.raises(ValidationError):
+        model.model_validate({"base_url": bad_url, "timeout_seconds": 5})
+
+
+@pytest.mark.parametrize(
+    "service_type", ["grafana", "prometheus", "alertmanager", "jellyfin", "jellyseerr", "nextcloud"]
+)
+def test_service_base_url_accepts_absolute_urls(service_type):
+    model = get_service_definition(service_type).config_model
+    instance = model.model_validate({"base_url": "https://example.com"})
+    assert instance.base_url == "https://example.com"
+
+
 def test_unknown_secret_field_rejected(client):
     response = client.post(
         "/api/services/instances",
diff --git a/docs/REQUIREMENTS.md b/docs/REQUIREMENTS.md
index 47ce7bd..e425a18 100644
--- a/docs/REQUIREMENTS.md
+++ b/docs/REQUIREMENTS.md
@@ -271,6 +271,10 @@ Service definitions live as Pydantic modules in the backend
 (`integrations/`); they declare the service config schema, secret fields, and
 the widget kinds the service provides. There is no runtime plugin loading.
 
+Every service `base_url` uses the shared `ServiceBaseUrl` type, which rejects
+values missing an `http://` or `https://` schema with a clear validation error
+(relative hosts break downstream HTTP clients).
+
 ### Services
 
 - **Grafana** — base URL + optional API key; provides a dashboard-link widget.