diff --git a/backend/src/kf_oj/api/app.py b/backend/src/kf_oj/api/app.py
index c1da944..3578162 100644
--- a/backend/src/kf_oj/api/app.py
+++ b/backend/src/kf_oj/api/app.py
@@ -8,6 +8,7 @@ from starlette.responses import JSONResponse
 from kf_oj import __version__
 from kf_oj.api.availability import AvailabilityMiddleware
 from kf_oj.api.body_limits import BodyLimitMiddleware
+from kf_oj.api.dependency_errors import install_handlers
 from kf_oj.api.v1.admin_imports import router as import_router
 from kf_oj.api.v1.admin_problems import router as maintenance_router
 from kf_oj.api.v1.identity import router as identity_router
@@ -148,6 +149,8 @@ def create_app(settings: Settings | None = None) -> FastAPI:
     async def invalid_request(_, __):
         return JSONResponse({"detail": "invalid request"}, status_code=422)
 
+    install_handlers(app)
+
     app.include_router(router, prefix="/api/v1")
     app.include_router(identity_router, prefix="/api/v1")
     app.include_router(problems_router, prefix="/api/v1")
diff --git a/backend/src/kf_oj/api/dependency_errors.py b/backend/src/kf_oj/api/dependency_errors.py
new file mode 100644
index 0000000..96abd09
--- /dev/null
+++ b/backend/src/kf_oj/api/dependency_errors.py
@@ -0,0 +1,42 @@
+"""Bounded, non-secret API responses for transient dependency/capacity failures."""
+
+import errno
+
+from asyncpg import CannotConnectNowError, PostgresConnectionError
+from fastapi import FastAPI
+from sqlalchemy.exc import OperationalError
+from sqlalchemy.exc import TimeoutError as PoolTimeoutError
+from starlette.responses import JSONResponse
+
+
+async def temporarily_unavailable(_, __):
+    # Write outcomes may be uncertain. Existing operation/submission UUIDs, not
+    # automatic server retries, own recovery from a lost response.
+    return JSONResponse(
+        {"detail": "service temporarily unavailable"},
+        status_code=503,
+        headers={"Cache-Control": "no-store", "Retry-After": "5"},
+    )
+
+
+async def filesystem_failure(_, error: OSError):
+    if error.errno in {errno.ENOSPC, errno.EDQUOT, errno.EMFILE, errno.ENFILE}:
+        return await temporarily_unavailable(_, error)
+    # Unrelated filesystem/programming failures retain server-error semantics.
+    # Never serialize path/source/query/credential text from an exception.
+    return JSONResponse(
+        {"detail": "internal server error"}, status_code=500, headers={"Cache-Control": "no-store"}
+    )
+
+
+def install_handlers(app: FastAPI) -> None:
+    for error in (
+        ConnectionError,
+        TimeoutError,
+        OperationalError,
+        PoolTimeoutError,
+        PostgresConnectionError,
+        CannotConnectNowError,
+    ):
+        app.add_exception_handler(error, temporarily_unavailable)
+    app.add_exception_handler(OSError, filesystem_failure)
diff --git a/backend/src/kf_oj/infra/db/session.py b/backend/src/kf_oj/infra/db/session.py
index ade3488..d037ffb 100644
--- a/backend/src/kf_oj/infra/db/session.py
+++ b/backend/src/kf_oj/infra/db/session.py
@@ -13,6 +13,7 @@ def build_engine(settings: Settings) -> AsyncEngine:
         max_overflow=0,
         pool_timeout=5,
         connect_args={
+            "timeout": 5,
             "command_timeout": 10,
             "server_settings": {
                 "statement_timeout": "10000",
diff --git a/backend/tests/test_dependency_errors.py b/backend/tests/test_dependency_errors.py
new file mode 100644
index 0000000..c896a1e
--- /dev/null
+++ b/backend/tests/test_dependency_errors.py
@@ -0,0 +1,58 @@
+"""Ordinary exception-boundary checks; real dependency failure is VM-only."""
+
+import asyncio
+import errno
+
+import httpx
+import pytest
+from asyncpg import CannotConnectNowError
+from sqlalchemy.exc import OperationalError, ProgrammingError
+from sqlalchemy.exc import TimeoutError as PoolTimeoutError
+
+from kf_oj.api.app import create_app
+from kf_oj.core.settings import Settings
+
+
+@pytest.mark.parametrize(
+    "error,expected",
+    [
+        (ConnectionRefusedError("private_connection_text"), 503),
+        (TimeoutError("private_timeout_text"), 503),
+        (CannotConnectNowError("private_database_text"), 503),
+        (PoolTimeoutError("private_pool_text"), 503),
+        (
+            OperationalError(
+                "private_query", {"secret": "private_value"}, RuntimeError("private_driver")
+            ),
+            503,
+        ),
+        (OSError(errno.ENOSPC, "private_source_path"), 503),
+        (OSError(errno.EDQUOT, "private_quota_path"), 503),
+        (FileNotFoundError(errno.ENOENT, "private_missing_path"), 500),
+    ],
+)
+def test_dependency_boundary_preserves_status_without_exception_text(error, expected):
+    app = create_app(Settings(_env_file=None, environment="test"))
+
+    @app.get("/api/v1/auth/boundary-test")
+    async def fail():
+        raise error
+
+    async def request():
+        async with httpx.AsyncClient(
+            transport=httpx.ASGITransport(app=app), base_url="http://test"
+        ) as client:
+            response = await client.get("/api/v1/auth/boundary-test")
+            assert response.status_code == expected
+            assert "private_" not in response.text
+            assert response.headers["cache-control"] == "no-store"
+            if expected == 503:
+                assert response.headers["retry-after"] == "5"
+
+    asyncio.run(request())
+
+
+def test_programming_errors_are_not_classified_as_transient_dependency_failures():
+    app = create_app(Settings(_env_file=None, environment="test"))
+    assert ProgrammingError not in app.exception_handlers
+    assert ValueError not in app.exception_handlers