diff --git a/litellm/proxy/control_plane/audit/s3.py b/litellm/proxy/control_plane/audit/s3.py
new file mode 100644
index 00000000000..aab07b5f44e
--- /dev/null
+++ b/litellm/proxy/control_plane/audit/s3.py
@@ -0,0 +1,159 @@
+import base64
+import hashlib
+import re
+from collections.abc import Callable, Mapping
+from dataclasses import dataclass, field
+from datetime import UTC, datetime, timedelta
+from typing import TYPE_CHECKING, Final, Protocol, TypeAlias
+from urllib.parse import quote
+from uuid import uuid4
+
+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>")
+Clock: TypeAlias = Callable[[], datetime]
+
+
+def _utc_now() -> datetime:
+    return datetime.now(UTC)
+
+
+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, follow_redirects=False
+        )
+
+
+@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, run_id: str, batch: Batch) -> str:
+    normalized_prefix: Final = prefix.strip("/")
+    first: Final = batch[0]
+    last: Final = batch[-1]
+    suffix: Final = (
+        f"{run_id}/{first.event.at:%Y/%m/%d/%H}/{first.sequence:012d}-{last.sequence:012d}-{last.hash[-12:]}.ndjson"
+    )
+    return f"{normalized_prefix}/{suffix}" if normalized_prefix else suffix
+
+
+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}"
+    if "." in settings.bucket:
+        return f"https://{host}/{quote(settings.bucket, safe='')}/{quote(key)}"
+    return f"https://{settings.bucket}.{host}/{quote(key)}"
+
+
+def put_headers(settings: S3SinkSettings, batch: Batch, content: bytes, now: datetime) -> 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)
+    if retain_until <= now:
+        return base
+    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"
+    run_id: str = field(default_factory=lambda: uuid4().hex)
+    clock: Clock = _utc_now
+
+    async def deliver(self, batch: Batch) -> DeliveryResult:
+        key: Final = object_key(self.settings.prefix, self.run_id, batch)
+        url: Final = object_url(self.settings, key)
+        content: Final = ndjson(batch)
+        try:
+            headers: Final = put_headers(self.settings, batch, content, self.clock())
+            signed: Final = await run_aws_signing(self.sign, url, content, headers)
+            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 df85bca6496..19db1efb647 100644
--- a/litellm/proxy/control_plane/audit/sinks.py
+++ b/litellm/proxy/control_plane/audit/sinks.py
@@ -26,7 +26,7 @@ EVENT_NAME: Final = "litellm.control_plane.audit"
 LOG_PREFIX: Final = "control_plane.audit"
 
 
-def _line(record: AuditRecord) -> str:
+def ocsf_line(record: AuditRecord) -> str:
     return json.dumps(record.body, default=str, separators=(",", ":"))
 
 
@@ -41,7 +41,7 @@ class LogSink:
 
     async def deliver(self, batch: Batch) -> DeliveryResult:
         for record in batch:
-            self._logger.info("%s %s", LOG_PREFIX, _line(record))
+            self._logger.info("%s %s", LOG_PREFIX, ocsf_line(record))
         return Delivered(len(batch))
 
     async def close(self) -> None:
@@ -95,7 +95,7 @@ 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 2fed38b5b21..9cac4e2335e 100644
--- a/litellm/proxy/control_plane/container.py
+++ b/litellm/proxy/control_plane/container.py
@@ -2,17 +2,20 @@
 
 from __future__ import annotations
 
+import os
 import time
 from collections.abc import Mapping, Sequence
 from contextlib import AbstractAsyncContextManager
-from dataclasses import dataclass
+from dataclasses import dataclass, replace
 from datetime import timedelta
 from importlib.metadata import PackageNotFoundError, version
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Final
 
 import httpx
+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
@@ -20,6 +23,7 @@ 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
@@ -119,10 +123,37 @@ 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 ec706c59b66..c1cb8b97d17 100644
--- a/litellm/proxy/control_plane/settings.py
+++ b/litellm/proxy/control_plane/settings.py
@@ -4,6 +4,7 @@ import os
 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, field_validator, model_validator
@@ -15,10 +16,24 @@ from litellm.proxy.control_plane.auth.rbac import ADMIN_ROLE
 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."""
@@ -124,6 +139,51 @@ class ControlPlaneSettings(BaseSettings):
     audit_batch_size: int = Field(default=100, ge=1)
     audit_drain_seconds: float = Field(default=5.0, gt=0)
     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
+
+    @field_validator("audit_s3_bucket")
+    @classmethod
+    def _s3_bucket_is_not_blank(cls, value: str | None) -> str | None:
+        if value is not None and not value.strip():
+            raise ValueError("CONTROL_PLANE_AUDIT_S3_BUCKET must not be empty or whitespace.")
+        return value
+
+    @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,
+        )
 
     @model_validator(mode="after")
     def _strict_audit_requires_a_sink(self) -> "ControlPlaneSettings":
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..5dab6a75c94
--- /dev/null
+++ b/tests/unit/proxy/control_plane/audit/test_s3.py
@@ -0,0 +1,193 @@
+import base64
+import hashlib
+import json
+from collections.abc import Mapping
+from dataclasses import dataclass, field, replace
+from datetime import UTC, datetime, timedelta
+from functools import reduce
+from typing import Final
+from uuid import UUID
+
+import httpx
+import pytest
+
+from litellm.llms.custom_httpx.http_handler import AsyncHTTPHandler
+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 HttpxObjectStore, S3ObjectLockSink, object_key
+from litellm.proxy.control_plane.settings import S3SinkSettings
+
+T0 = datetime(2026, 10, 13, 12, 0, tzinfo=UTC)
+
+
+def test_empty_s3_prefix_does_not_add_a_leading_slash() -> None:
+    assert not object_key("", "run", _batch(1)).startswith("/")
+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: Final = self.outcomes.pop(0)
+        if isinstance(outcome, Exception):
+            raise outcome
+        return outcome
+
+
+@pytest.mark.asyncio
+async def test_s3_put_does_not_follow_redirects_from_a_following_http_client() -> None:
+    transport: Final = httpx.MockTransport(
+        lambda request: httpx.Response(
+            307 if request.url.host == "bucket.example" else 200,
+            headers={"Location": "https://redirected.example/object"} if request.url.host == "bucket.example" else {},
+        )
+    )
+    handler: Final = AsyncHTTPHandler(transport=transport, follow_redirects=True)
+
+    response: Final = await HttpxObjectStore(handler).put("https://bucket.example/object", b"body", {}, 1.0)
+    await handler.close()
+
+    assert (response.status_code, str(response.request.url)) == (307, "https://bucket.example/object")
+
+
+def _sign(url: str, content: bytes, headers: Mapping[str, str]) -> Mapping[str, str]:
+    return {**headers, "Authorization": f"AWS4-HMAC-SHA256 test {len(content)}"}
+
+
+def _append_record(state: tuple[ChainState, Batch], index: int) -> tuple[ChainState, Batch]:
+    previous, records = state
+    event: Final = AuditEvent(T0, "alice", None, "eu", "POST", f"/key/{index}", "allowed", 200, None, None)
+    next_state, record = chain(event, Product("t"), previous)
+    return next_state, (*records, record)
+
+
+def _batch(count: int) -> Batch:
+    return reduce(_append_record, range(count), (ChainState(), ()))[1]
+
+
+@pytest.mark.asyncio
+async def test_a_batch_is_one_locked_ndjson_object_under_a_dated_key_that_is_never_overwritten() -> None:
+    store: Final = FakeStore([httpx.Response(200)])
+    batch: Final = _batch(3)
+    sink: Final = S3ObjectLockSink(SETTINGS, store, _sign, clock=lambda: T0)
+    assert await sink.deliver(batch) == Delivered(3)
+    ((url, content, headers, timeout),) = store.puts
+    suffix: Final = batch[-1].hash[-12:]
+    assert (
+        url == f"https://audit.s3.eu-west-1.amazonaws.com/cp/{sink.run_id}/2026/10/13/12/"
+        f"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: Final = FakeStore([httpx.Response(200)])
+    settings: Final = replace(SETTINGS, object_lock_mode=None, endpoint_url="https://minio.internal:9000/")
+    sink: Final = S3ObjectLockSink(settings, store, _sign)
+    assert await sink.deliver(_batch(1)) == Delivered(1)
+    ((url, _, headers, _),) = store.puts
+    assert url.startswith(f"https://minio.internal:9000/audit/cp/{sink.run_id}/2026/10/13/12/000000000001-")
+    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: Final = FakeStore([httpx.Response(200)])
+    sink: Final = S3ObjectLockSink(replace(SETTINGS, region="cn-north-1"), store, _sign)
+    await sink.deliver(_batch(1))
+    ((url, _, _, _),) = store.puts
+    assert url.startswith(f"https://audit.s3.cn-north-1.amazonaws.com.cn/cp/{sink.run_id}/")
+
+
+@pytest.mark.asyncio
+async def test_sink_keys_are_unique_per_sink_and_stable_across_retries() -> None:
+    batch: Final = _batch(1)
+    first_store: Final = FakeStore([httpx.Response(200), httpx.Response(200)])
+    second_store: Final = FakeStore([httpx.Response(200)])
+    first: Final = S3ObjectLockSink(SETTINGS, first_store, _sign)
+    second: Final = S3ObjectLockSink(SETTINGS, second_store, _sign)
+
+    await first.deliver(batch)
+    await first.deliver(batch)
+    await second.deliver(batch)
+
+    first_urls: Final = tuple(url for url, _, _, _ in first_store.puts)
+    ((second_url, _, _, _),) = second_store.puts
+    assert UUID(first.run_id).version == UUID(second.run_id).version == 4
+    assert first_urls[0] == first_urls[1] and first_urls[0] != second_url
+
+
+@pytest.mark.asyncio
+async def test_expired_retention_omits_lock_headers() -> None:
+    store: Final = FakeStore([httpx.Response(200)])
+    sink: Final = S3ObjectLockSink(SETTINGS, store, _sign, clock=lambda: T0 + timedelta(days=30))
+
+    await sink.deliver(_batch(1))
+
+    ((_, _, headers, _),) = store.puts
+    assert not [key for key in headers if key.startswith("x-amz-object-lock")]
+
+
+@pytest.mark.asyncio
+async def test_dotted_bucket_uses_regional_path_style_url() -> None:
+    store: Final = FakeStore([httpx.Response(200)])
+    sink: Final = S3ObjectLockSink(replace(SETTINGS, bucket="audit.logs"), store, _sign)
+
+    await sink.deliver(_batch(1))
+
+    ((url, _, _, _),) = store.puts
+    assert url.startswith(f"https://s3.eu-west-1.amazonaws.com/audit.logs/cp/{sink.run_id}/")
+
+
+@pytest.mark.asyncio
+async def test_an_existing_key_is_delivered_while_s3_errors_and_network_errors_are_retried() -> None:
+    store: Final = FakeStore(
+        [
+            httpx.Response(412, text="<Error><Code>PreconditionFailed</Code></Error>"),
+            httpx.Response(503, text="<Error><Code>SlowDown</Code></Error>"),
+            httpx.ConnectError("boom"),
+        ]
+    )
+    sink: Final = S3ObjectLockSink(SETTINGS, store, _sign)
+    batch: Final = _batch(2)
+    assert await sink.deliver(batch) == Delivered(2), "the object is already there from an unacknowledged attempt"
+    slow: Final = await sink.deliver(batch)
+    assert isinstance(slow, Undelivered) and slow.reason.startswith(
+        f"HTTP 503 SlowDown putting cp/{sink.run_id}/2026/10/13/12/"
+    )
+    down: Final = await sink.deliver(batch)
+    assert isinstance(down, Undelivered) and down.reason.startswith(f"ConnectError putting cp/{sink.run_id}/")
+
+
+@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: Final = FakeStore([])
+    sink: Final = S3ObjectLockSink(SETTINGS, store, no_credentials)
+    result: Final = await sink.deliver(_batch(1))
+    assert isinstance(result, Undelivered) and result.reason.startswith(f"RuntimeError signing cp/{sink.run_id}/")
+    assert store.puts == []
diff --git a/tests/unit/proxy/control_plane/test_container.py b/tests/unit/proxy/control_plane/test_container.py
index 4735b6af098..8c6eee4f0e9 100644
--- a/tests/unit/proxy/control_plane/test_container.py
+++ b/tests/unit/proxy/control_plane/test_container.py
@@ -5,8 +5,9 @@ import httpx
 import pytest
 from pydantic import TypeAdapter
 
+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, _shared_http_handler
+from litellm.proxy.control_plane.container import HandlerTransport, _environment_region, _shared_http_handler
 from litellm.proxy.control_plane.settings import ControlPlaneSettings
 
 _JSON_COUNT: Final = TypeAdapter(dict[str, int])
@@ -67,3 +68,14 @@ async def test_transport_never_follows_a_dataplane_redirect_even_on_a_following_
     async with transport.stream("GET", "https://dp.test/v1/mcp/server/oauth/x/authorize", timeout=1.0) as response:
         status: Final = response.status_code
     assert status == 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 ee043acb2b0..5acab7b3179 100644
--- a/tests/unit/proxy/control_plane/test_settings.py
+++ b/tests/unit/proxy/control_plane/test_settings.py
@@ -4,7 +4,7 @@ import pytest
 from pydantic import ValidationError
 
 from litellm.proxy.control_plane.audit.event import CONTROL_PLANE_ID
-from litellm.proxy.control_plane.settings import ControlPlaneSettings, load_settings
+from litellm.proxy.control_plane.settings import ControlPlaneSettings, S3SinkSettings, load_settings
 from litellm.types.proxy.control_plane.auth import PASSWORD_MAX_LENGTH, USERNAME_MAX_LENGTH
 
 MASTER_KEY: Final = "01234567890123456789012345678901"
@@ -43,6 +43,45 @@ def test_password_login_without_default_roles_requires_a_custom_admin_role() ->
     assert not settings.include_default_roles and "admin" in settings.roles
 
 
+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": MASTER_KEY, "audit_sinks": ["log", "s3"]})
+    settings: Final = ControlPlaneSettings.model_validate(
+        {
+            "LITELLM_MASTER_KEY": MASTER_KEY,
+            "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": MASTER_KEY}).audit_s3 is None
+
+
+@pytest.mark.parametrize("bucket", ["", " \t "])
+def test_the_s3_sink_rejects_a_blank_bucket(bucket: str) -> None:
+    with pytest.raises(ValidationError, match="must not be empty or whitespace"):
+        ControlPlaneSettings.model_validate(
+            {"LITELLM_MASTER_KEY": MASTER_KEY, "audit_sinks": ["s3"], "audit_s3_bucket": bucket}
+        )
+
+
+def test_a_plain_http_s3_endpoint_is_refused_unless_explicitly_allowed() -> None:
+    base: Final = {
+        "LITELLM_MASTER_KEY": MASTER_KEY,
+        "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: Final = 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: Final = ControlPlaneSettings.model_validate(
         {