diff --git a/litellm/proxy/control_plane/audit/s3.py b/litellm/proxy/control_plane/audit/s3.py
new file mode 100644
index 00000000000..3d4fcd97ed1
--- /dev/null
+++ b/litellm/proxy/control_plane/audit/s3.py
@@ -0,0 +1,144 @@
+"""Append-only audit objects on S3: one NDJSON object per delivered batch, written once with ``If-None-Match: *`` and,
+when configured, held under Object Lock so nobody can shorten or delete it inside the retention period."""
+
+import base64
+import hashlib
+import re
+from collections.abc import Mapping
+from dataclasses import dataclass
+from datetime import timedelta
+from typing import TYPE_CHECKING, Final, Protocol
+from urllib.parse import quote
+
+import httpx
+
+from litellm.litellm_core_utils.aws_partition import get_aws_dns_suffix
+from litellm.llms.bedrock.base_aws_llm import BaseAWSLLM, run_aws_signing
+from litellm.llms.custom_httpx.http_handler import AsyncHTTPHandler
+from litellm.proxy.control_plane.audit.pipeline import Batch, Delivered, DeliveryResult, Undelivered
+from litellm.proxy.control_plane.audit.sinks import ocsf_line
+from litellm.proxy.control_plane.settings import S3SinkSettings
+
+if TYPE_CHECKING:
+    from botocore.credentials import Credentials
+
+CONTENT_TYPE: Final = "application/x-ndjson"
+RETAIN_UNTIL_FORMAT: Final = "%Y-%m-%dT%H:%M:%SZ"
+_ERROR_CODE: Final = re.compile(r"<Code>([^<]+)</Code>")
+
+
+class ObjectStore(Protocol):
+    """The one call the sink makes. Raises only httpx errors and never retries on its own."""
+
+    async def put(self, url: str, content: bytes, headers: Mapping[str, str], timeout: float) -> httpx.Response: ...
+
+
+class Signer(Protocol):
+    def __call__(self, url: str, content: bytes, headers: Mapping[str, str]) -> Mapping[str, str]: ...
+
+
+@dataclass(frozen=True, slots=True)
+class HttpxObjectStore:
+    """Holds the handler, not its client: the shared cache may close and replace the client between batches."""
+
+    handler: AsyncHTTPHandler
+
+    async def put(self, url: str, content: bytes, headers: Mapping[str, str], timeout: float) -> httpx.Response:
+        return await self.handler.client.put(url, content=content, headers=dict(headers), timeout=timeout)
+
+
+@dataclass(frozen=True, slots=True)
+class SigV4Signer:
+    """SigV4 over the usual credential chain (env, profile, IRSA, IMDS), the same way the proxy's s3 logger signs.
+    Runs in the signing executor because the credential chain may block on STS or IMDS."""
+
+    region: str
+    aws: BaseAWSLLM
+
+    def __call__(self, url: str, content: bytes, headers: Mapping[str, str]) -> Mapping[str, str]:
+        from botocore.auth import S3SigV4Auth
+        from botocore.awsrequest import AWSRequest
+        from botocore.credentials import RefreshableCredentials
+
+        credentials: Final[Credentials] = self.aws.get_credentials(aws_region_name=self.region)
+        frozen: Final = (
+            credentials.get_frozen_credentials() if isinstance(credentials, RefreshableCredentials) else credentials
+        )
+        request: Final = AWSRequest(method="PUT", url=url, data=content, headers=dict(headers))
+        S3SigV4Auth(frozen, "s3", self.region).add_auth(request)
+        return dict(request.headers.items())
+
+
+def ndjson(batch: Batch) -> bytes:
+    return "".join(f"{ocsf_line(record)}\n" for record in batch).encode()
+
+
+def object_key(prefix: str, batch: Batch) -> str:
+    """Dated, ordered by sequence, suffixed with the last hash so an identical retry hits the same key."""
+    first: Final = batch[0]
+    last: Final = batch[-1]
+    return f"{prefix}/{first.event.at:%Y/%m/%d/%H}/{first.sequence:012d}-{last.sequence:012d}-{last.hash[-12:]}.ndjson"
+
+
+def object_url(settings: S3SinkSettings, key: str) -> str:
+    if settings.endpoint_url is not None:
+        return f"{settings.endpoint_url.rstrip('/')}/{settings.bucket}/{quote(key)}"
+    suffix: Final = get_aws_dns_suffix(settings.region)
+    host: Final = f"s3.{settings.region}.{suffix}" if settings.region else f"s3.{suffix}"
+    return f"https://{settings.bucket}.{host}/{quote(key)}"
+
+
+def put_headers(settings: S3SinkSettings, batch: Batch, content: bytes) -> Mapping[str, str]:
+    """Integrity headers, the never-overwrite condition and, when a lock mode is set, retention counted from the
+    newest event in the batch so the clock in the record is the clock of the lock."""
+    base: Final = {
+        "Content-Type": CONTENT_TYPE,
+        "Content-MD5": base64.b64encode(hashlib.md5(content, usedforsecurity=False).digest()).decode(),
+        "x-amz-content-sha256": hashlib.sha256(content).hexdigest(),
+        "If-None-Match": "*",
+    }
+    if settings.object_lock_mode is None:
+        return base
+    retain_until: Final = max(record.event.at for record in batch) + timedelta(days=settings.retention_days)
+    return {
+        **base,
+        "x-amz-object-lock-mode": settings.object_lock_mode,
+        "x-amz-object-lock-retain-until-date": retain_until.strftime(RETAIN_UNTIL_FORMAT),
+    }
+
+
+def _error_code(response: httpx.Response) -> str:
+    found: Final = _ERROR_CODE.search(response.text)
+    return found.group(1) if found else "no error code"
+
+
+@dataclass(frozen=True, slots=True)
+class S3ObjectLockSink:
+    """A 412 means the key already exists from an earlier attempt whose acknowledgement was lost. The key carries the
+    last record's hash, which covers every record in the batch, so the stored object is this batch and it counts
+    as delivered."""
+
+    settings: S3SinkSettings
+    store: ObjectStore
+    sign: Signer
+    name: str = "s3"
+
+    async def deliver(self, batch: Batch) -> DeliveryResult:
+        key: Final = object_key(self.settings.prefix, batch)
+        url: Final = object_url(self.settings, key)
+        content: Final = ndjson(batch)
+        try:
+            signed: Final = await run_aws_signing(self.sign, url, content, put_headers(self.settings, batch, content))
+            response: Final = await self.store.put(url, content, signed, self.settings.timeout_seconds)
+        except httpx.HTTPError as error:
+            return Undelivered(f"{type(error).__name__} putting {key}")
+        except Exception as error:  # noqa: BLE001  # a failing credential chain is a failed delivery, not a dead worker
+            return Undelivered(f"{type(error).__name__} signing {key}")
+        match response.status_code:
+            case 200 | 412:
+                return Delivered(len(batch))
+            case status:
+                return Undelivered(f"HTTP {status} {_error_code(response)} putting {key}")
+
+    async def close(self) -> None:
+        return None
diff --git a/litellm/proxy/control_plane/audit/sinks.py b/litellm/proxy/control_plane/audit/sinks.py
index 53ea83bddce..4f5d9fcb021 100644
--- a/litellm/proxy/control_plane/audit/sinks.py
+++ b/litellm/proxy/control_plane/audit/sinks.py
@@ -28,7 +28,7 @@ AUDIT_LOGGER: Final = verbose_proxy_logger.getChild(LOG_PREFIX)
 AUDIT_LOGGER.setLevel(logging.INFO)
 
 
-def _line(record: AuditRecord) -> str:
+def ocsf_line(record: AuditRecord) -> str:
     return json.dumps(record.body, default=str, separators=(",", ":"))
 
 
@@ -38,7 +38,7 @@ class LogSink:
 
     async def deliver(self, batch: Batch) -> DeliveryResult:
         for record in batch:
-            AUDIT_LOGGER.info("%s %s", LOG_PREFIX, _line(record))
+            AUDIT_LOGGER.info("%s %s", LOG_PREFIX, ocsf_line(record))
         return Delivered(len(batch))
 
     async def close(self) -> None:
@@ -89,7 +89,7 @@ def _log_data(record: AuditRecord, resource: Resource) -> ReadableLogRecord:
             ),
             severity_text=severity_text,
             severity_number=severity_number,
-            body=_line(record),
+            body=ocsf_line(record),
             attributes=dict(_attributes(record)),
         ),
         resource,
diff --git a/litellm/proxy/control_plane/container.py b/litellm/proxy/control_plane/container.py
index e2d6c2a7697..9807fb9ae0b 100644
--- a/litellm/proxy/control_plane/container.py
+++ b/litellm/proxy/control_plane/container.py
@@ -1,8 +1,9 @@
 """Long-lived objects built once per process and handed to the app. Tests build their own."""
 
+import os
 import time
 from collections.abc import Mapping, Sequence
-from dataclasses import dataclass
+from dataclasses import dataclass, replace
 from datetime import timedelta
 from importlib.metadata import PackageNotFoundError, version
 from types import MappingProxyType
@@ -10,7 +11,9 @@ from typing import Final
 
 import httpx
 from opentelemetry.sdk.trace import TracerProvider
+from pydantic import TypeAdapter
 
+from litellm.llms.bedrock.base_aws_llm import BaseAWSLLM
 from litellm.llms.custom_httpx.http_handler import (
     AsyncHTTPHandler,
     get_async_httpx_client,  # pyright: ignore[reportUnknownVariableType]  # params is an untyped dict upstream
@@ -18,6 +21,7 @@ from litellm.llms.custom_httpx.http_handler import (
 from litellm.proxy.control_plane.audit.log import AuditLog, build_audit_log
 from litellm.proxy.control_plane.audit.ocsf import Product
 from litellm.proxy.control_plane.audit.pipeline import AuditPipeline, AuditSink, build_pipeline
+from litellm.proxy.control_plane.audit.s3 import HttpxObjectStore, S3ObjectLockSink, SigV4Signer
 from litellm.proxy.control_plane.audit.sinks import LogSink, OtlpLogSink
 from litellm.proxy.control_plane.auth.jwt import JwksKeyLookup, SessionTokenCodec, TokenVerifier, build_token_verifier
 from litellm.proxy.control_plane.auth.login import LoginService, LoginThrottle
@@ -113,10 +117,37 @@ def _shared_http_client(settings: ControlPlaneSettings) -> HttpTransport:
     return HandlerTransport(_shared_http_handler(settings))
 
 
+_REGION: Final = TypeAdapter(str)
+_OPTIONAL_REGION: Final[TypeAdapter[str | None]] = TypeAdapter(str | None)
+
+
+def _s3_sink(settings: ControlPlaneSettings) -> AuditSink:
+    s3: Final = settings.audit_s3
+    if s3 is None:
+        raise ValueError("CONTROL_PLANE_AUDIT_SINKS names s3 but CONTROL_PLANE_AUDIT_S3_BUCKET is not set.")
+    aws: Final = BaseAWSLLM()
+    region: Final = s3.region or _environment_region(aws)
+    return S3ObjectLockSink(
+        replace(s3, region=region), HttpxObjectStore(_shared_http_handler(settings)), SigV4Signer(region, aws)
+    )
+
+
+def _environment_region(aws: BaseAWSLLM) -> str:
+    """The AWS SDK order: ``AWS_REGION``, then botocore's chain (``AWS_DEFAULT_REGION``, profile, config), then
+    litellm's resolver (``AWS_REGION_NAME`` and its default)."""
+    from botocore.session import Session
+
+    explicit: Final = os.environ.get("AWS_REGION")
+    configured: Final = explicit or _OPTIONAL_REGION.validate_python(Session().get_config_variable("region"))
+    return configured or _REGION.validate_python(aws.get_aws_region_name_for_non_llm_api_calls())
+
+
 def _sink(kind: str, settings: ControlPlaneSettings) -> AuditSink:
     match kind:
         case "otlp":
             return OtlpLogSink(build_log_exporter(settings), build_resource(settings))
+        case "s3":
+            return _s3_sink(settings)
         case _:
             return LogSink()
 
diff --git a/litellm/proxy/control_plane/settings.py b/litellm/proxy/control_plane/settings.py
index 740b8c8a49b..ecc7f0931ab 100644
--- a/litellm/proxy/control_plane/settings.py
+++ b/litellm/proxy/control_plane/settings.py
@@ -3,6 +3,7 @@
 from collections.abc import Mapping
 from dataclasses import dataclass
 from typing import Final, Literal, TypeAlias
+from urllib.parse import urlsplit
 
 import yaml
 from pydantic import BaseModel, ConfigDict, Field, SecretStr, TypeAdapter, model_validator
@@ -12,10 +13,24 @@ from litellm.litellm_core_utils.oauth_token_exchange import TokenExchangeProfile
 from litellm.types.proxy.control_plane.auth import PASSWORD_MAX_LENGTH, USERNAME_MAX_LENGTH
 
 DEFAULT_SESSION_TTL_SECONDS: Final = 12 * 60 * 60
-AuditSinkKind: TypeAlias = Literal["log", "otlp"]
+AuditSinkKind: TypeAlias = Literal["log", "otlp", "s3"]
+ObjectLockMode: TypeAlias = Literal["COMPLIANCE", "GOVERNANCE"]
 DEFAULT_DATAPLANE_TIMEOUT_SECONDS: Final = 10.0
 
 
+@dataclass(frozen=True, slots=True)
+class S3SinkSettings:
+    """Where the s3 audit sink writes. ``region`` is None until the container resolves it from the AWS environment."""
+
+    bucket: str
+    prefix: str
+    region: str | None
+    endpoint_url: str | None
+    object_lock_mode: ObjectLockMode | None
+    retention_days: int
+    timeout_seconds: float
+
+
 @dataclass(frozen=True, slots=True)
 class OidcSettings:
     """External IdP used to verify caller JWTs and, later, to exchange them for dataplane tokens."""
@@ -117,6 +132,44 @@ class ControlPlaneSettings(BaseSettings):
     audit_queue_size: int = 10_000
     audit_batch_size: int = 100
     audit_strict: bool = False
+    audit_s3_bucket: str | None = None
+    audit_s3_prefix: str = "control-plane-audit"
+    audit_s3_region: str | None = None
+    audit_s3_endpoint_url: str | None = None
+    audit_s3_allow_insecure_http: bool = False
+    audit_s3_object_lock_mode: ObjectLockMode | None = None
+    audit_s3_retention_days: int = Field(default=365, ge=1)
+    audit_s3_timeout_seconds: float = 30.0
+
+    @model_validator(mode="after")
+    def _s3_sink_needs_a_bucket(self) -> "ControlPlaneSettings":
+        if "s3" in self.audit_sinks and self.audit_s3_bucket is None:
+            raise ValueError("CONTROL_PLANE_AUDIT_SINKS names s3 but CONTROL_PLANE_AUDIT_S3_BUCKET is not set.")
+        return self
+
+    @model_validator(mode="after")
+    def _s3_endpoint_is_https_unless_opted_out(self) -> "ControlPlaneSettings":
+        endpoint: Final = self.audit_s3_endpoint_url
+        if endpoint is None or self.audit_s3_allow_insecure_http or urlsplit(endpoint).scheme.lower() == "https":
+            return self
+        raise ValueError(
+            f"audit s3 endpoint {endpoint!r} is not https, audit records and the SigV4 authorization would travel "
+            "unencrypted. Use https or set CONTROL_PLANE_AUDIT_S3_ALLOW_INSECURE_HTTP=true for an in-cluster store."
+        )
+
+    @property
+    def audit_s3(self) -> S3SinkSettings | None:
+        if self.audit_s3_bucket is None:
+            return None
+        return S3SinkSettings(
+            bucket=self.audit_s3_bucket,
+            prefix=self.audit_s3_prefix.strip("/"),
+            region=self.audit_s3_region,
+            endpoint_url=self.audit_s3_endpoint_url,
+            object_lock_mode=self.audit_s3_object_lock_mode,
+            retention_days=self.audit_s3_retention_days,
+            timeout_seconds=self.audit_s3_timeout_seconds,
+        )
 
     @property
     def oidc(self) -> OidcSettings | None:
diff --git a/tests/unit/proxy/control_plane/audit/test_s3.py b/tests/unit/proxy/control_plane/audit/test_s3.py
new file mode 100644
index 00000000000..2fd2593c740
--- /dev/null
+++ b/tests/unit/proxy/control_plane/audit/test_s3.py
@@ -0,0 +1,120 @@
+import base64
+import hashlib
+import json
+from collections.abc import Mapping
+from dataclasses import dataclass, field, replace
+from datetime import UTC, datetime
+from typing import Final
+
+import httpx
+import pytest
+
+from litellm.proxy.control_plane.audit.event import AuditEvent
+from litellm.proxy.control_plane.audit.ocsf import AuditRecord, ChainState, Product, chain
+from litellm.proxy.control_plane.audit.pipeline import Batch, Delivered, Undelivered
+from litellm.proxy.control_plane.audit.s3 import S3ObjectLockSink
+from litellm.proxy.control_plane.settings import S3SinkSettings
+
+T0 = datetime(2026, 10, 13, 12, 0, tzinfo=UTC)
+SETTINGS: Final = S3SinkSettings(
+    bucket="audit",
+    prefix="cp",
+    region="eu-west-1",
+    endpoint_url=None,
+    object_lock_mode="COMPLIANCE",
+    retention_days=30,
+    timeout_seconds=5.0,
+)
+
+
+@dataclass
+class FakeStore:  # mutable-ok: scripts responses and records puts for assertions
+    outcomes: list[httpx.Response | Exception]
+    puts: list[tuple[str, bytes, Mapping[str, str], float]] = field(default_factory=list)
+
+    async def put(self, url: str, content: bytes, headers: Mapping[str, str], timeout: float) -> httpx.Response:
+        self.puts.append((url, content, headers, timeout))
+        outcome = self.outcomes.pop(0)
+        if isinstance(outcome, Exception):
+            raise outcome
+        return outcome
+
+
+def _sign(url: str, content: bytes, headers: Mapping[str, str]) -> Mapping[str, str]:
+    return {**headers, "Authorization": f"AWS4-HMAC-SHA256 test {len(content)}"}
+
+
+def _batch(count: int) -> Batch:
+    state: Final[list[ChainState]] = [ChainState()]
+    out: Final[list[AuditRecord]] = []
+    for index in range(count):
+        event = AuditEvent(T0, "alice", None, "eu", "POST", f"/key/{index}", "allowed", 200, None, None)
+        state[0], record = chain(event, Product("t"), state[0])
+        out.append(record)
+    return tuple(out)
+
+
+@pytest.mark.asyncio
+async def test_a_batch_is_one_locked_ndjson_object_under_a_dated_key_that_is_never_overwritten() -> None:
+    store = FakeStore([httpx.Response(200)])
+    batch = _batch(3)
+    assert await S3ObjectLockSink(SETTINGS, store, _sign).deliver(batch) == Delivered(3)
+    ((url, content, headers, timeout),) = store.puts
+    suffix = batch[-1].hash[-12:]
+    assert url == f"https://audit.s3.eu-west-1.amazonaws.com/cp/2026/10/13/12/000000000001-000000000003-{suffix}.ndjson"
+    assert [json.loads(line) for line in content.decode().splitlines()] == [record.body for record in batch]
+    assert headers["Authorization"] == f"AWS4-HMAC-SHA256 test {len(content)}" and timeout == 5.0
+    assert headers["If-None-Match"] == "*" and headers["Content-Type"] == "application/x-ndjson"
+    assert headers["Content-MD5"] == base64.b64encode(hashlib.md5(content, usedforsecurity=False).digest()).decode()
+    assert headers["x-amz-content-sha256"] == hashlib.sha256(content).hexdigest()
+    assert (headers["x-amz-object-lock-mode"], headers["x-amz-object-lock-retain-until-date"]) == (
+        "COMPLIANCE",
+        "2026-11-12T12:00:00Z",
+    )
+
+
+@pytest.mark.asyncio
+async def test_without_a_lock_mode_there_are_no_lock_headers_and_a_custom_endpoint_is_path_style() -> None:
+    store = FakeStore([httpx.Response(200)])
+    settings = replace(SETTINGS, object_lock_mode=None, endpoint_url="https://minio.internal:9000/")
+    assert await S3ObjectLockSink(settings, store, _sign).deliver(_batch(1)) == Delivered(1)
+    ((url, _, headers, _),) = store.puts
+    assert url.startswith("https://minio.internal:9000/audit/cp/2026/10/13/12/000000000001-000000000001-")
+    assert not [key for key in headers if key.startswith("x-amz-object-lock")]
+
+
+@pytest.mark.asyncio
+async def test_a_china_partition_region_gets_its_own_dns_suffix() -> None:
+    store = FakeStore([httpx.Response(200)])
+    await S3ObjectLockSink(replace(SETTINGS, region="cn-north-1"), store, _sign).deliver(_batch(1))
+    ((url, _, _, _),) = store.puts
+    assert url.startswith("https://audit.s3.cn-north-1.amazonaws.com.cn/cp/")
+
+
+@pytest.mark.asyncio
+async def test_an_existing_key_is_delivered_while_s3_errors_and_network_errors_are_retried() -> None:
+    store = FakeStore(
+        [
+            httpx.Response(412, text="<Error><Code>PreconditionFailed</Code></Error>"),
+            httpx.Response(503, text="<Error><Code>SlowDown</Code></Error>"),
+            httpx.ConnectError("boom"),
+        ]
+    )
+    sink = S3ObjectLockSink(SETTINGS, store, _sign)
+    batch = _batch(2)
+    assert await sink.deliver(batch) == Delivered(2), "the object is already there from an unacknowledged attempt"
+    slow = await sink.deliver(batch)
+    assert isinstance(slow, Undelivered) and slow.reason.startswith("HTTP 503 SlowDown putting cp/2026/10/13/12/")
+    down = await sink.deliver(batch)
+    assert isinstance(down, Undelivered) and down.reason.startswith("ConnectError putting cp/")
+
+
+@pytest.mark.asyncio
+async def test_a_signer_that_cannot_get_credentials_is_a_failed_delivery_not_a_dead_worker() -> None:
+    def no_credentials(url: str, content: bytes, headers: Mapping[str, str]) -> Mapping[str, str]:
+        raise RuntimeError("no credential chain")
+
+    store = FakeStore([])
+    result = await S3ObjectLockSink(SETTINGS, store, no_credentials).deliver(_batch(1))
+    assert isinstance(result, Undelivered) and result.reason.startswith("RuntimeError signing cp/")
+    assert store.puts == []
diff --git a/tests/unit/proxy/control_plane/test_container.py b/tests/unit/proxy/control_plane/test_container.py
index 485e509ef4e..16d031b0629 100644
--- a/tests/unit/proxy/control_plane/test_container.py
+++ b/tests/unit/proxy/control_plane/test_container.py
@@ -4,8 +4,9 @@ from typing import cast
 import httpx
 import pytest
 
+from litellm.llms.bedrock.base_aws_llm import BaseAWSLLM
 from litellm.llms.custom_httpx.http_handler import AsyncHTTPHandler
-from litellm.proxy.control_plane.container import HandlerTransport
+from litellm.proxy.control_plane.container import HandlerTransport, _environment_region
 
 
 @dataclass
@@ -53,3 +54,14 @@ async def test_transport_never_follows_a_dataplane_redirect_even_on_a_following_
     transport = HandlerTransport(cast(AsyncHTTPHandler, handler))  # cast-ok: only the client property is used
     reply = await transport.request("GET", "https://dp.test/v1/mcp/server/oauth/x/authorize", timeout=1.0)
     assert reply.status_code == 302 and handler.hops == ["https://dp.test/v1/mcp/server/oauth/x/authorize"]
+
+
+def test_the_s3_region_follows_the_aws_sdk_order_aws_region_then_default_region(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setenv("AWS_REGION", "us-east-1")
+    monkeypatch.setenv("AWS_DEFAULT_REGION", "eu-west-1")
+    monkeypatch.delenv("AWS_REGION_NAME", raising=False)
+    assert _environment_region(BaseAWSLLM()) == "us-east-1"
+    monkeypatch.delenv("AWS_REGION")
+    assert _environment_region(BaseAWSLLM()) == "eu-west-1"
diff --git a/tests/unit/proxy/control_plane/test_settings.py b/tests/unit/proxy/control_plane/test_settings.py
index aaabadc6477..393041a6dd8 100644
--- a/tests/unit/proxy/control_plane/test_settings.py
+++ b/tests/unit/proxy/control_plane/test_settings.py
@@ -1,7 +1,7 @@
 import pytest
 from pydantic import ValidationError
 
-from litellm.proxy.control_plane.settings import ControlPlaneSettings
+from litellm.proxy.control_plane.settings import ControlPlaneSettings, S3SinkSettings
 from litellm.types.proxy.control_plane.auth import PASSWORD_MAX_LENGTH, USERNAME_MAX_LENGTH
 
 
@@ -16,6 +16,33 @@ def test_configured_admin_credentials_share_the_login_request_bounds(username: s
         )
 
 
+def test_the_s3_sink_refuses_to_start_without_a_bucket_and_reads_its_block_with_one() -> None:
+    with pytest.raises(ValidationError, match="CONTROL_PLANE_AUDIT_S3_BUCKET"):
+        ControlPlaneSettings.model_validate({"LITELLM_MASTER_KEY": "sk-m", "audit_sinks": ["log", "s3"]})
+    settings = ControlPlaneSettings.model_validate(
+        {
+            "LITELLM_MASTER_KEY": "sk-m",
+            "audit_sinks": ["s3"],
+            "audit_s3_bucket": "audit",
+            "audit_s3_prefix": "/cp/",
+            "audit_s3_object_lock_mode": "COMPLIANCE",
+            "audit_s3_retention_days": 7,
+        }
+    )
+    assert settings.audit_s3 == S3SinkSettings("audit", "cp", None, None, "COMPLIANCE", 7, 30.0)
+    assert ControlPlaneSettings.model_validate({"LITELLM_MASTER_KEY": "sk-m"}).audit_s3 is None
+
+
+def test_a_plain_http_s3_endpoint_is_refused_unless_explicitly_allowed() -> None:
+    base = {"LITELLM_MASTER_KEY": "sk-m", "audit_s3_bucket": "audit", "audit_s3_endpoint_url": "http://minio:9000"}
+    with pytest.raises(ValidationError, match="not https"):
+        ControlPlaneSettings.model_validate(base)
+    with pytest.raises(ValidationError, match="not https"):
+        ControlPlaneSettings.model_validate({**base, "audit_s3_endpoint_url": "HTTP://minio:9000"})
+    allowed = ControlPlaneSettings.model_validate({**base, "audit_s3_allow_insecure_http": True}).audit_s3
+    assert allowed is not None and allowed.endpoint_url == "http://minio:9000"
+
+
 def test_credentials_at_the_bound_are_accepted() -> None:
     settings = ControlPlaneSettings.model_validate(
         {