diff --git a/litellm/constants.py b/litellm/constants.py
index 137eea15174..4342c7d7ebe 100644
--- a/litellm/constants.py
+++ b/litellm/constants.py
@@ -1532,6 +1532,7 @@ PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS: Final = 4096
 PASSTHROUGH_UPSTREAM_ERROR_REPORT_TASK_NAME: Final = "passthrough-upstream-error-report"
 PASSTHROUGH_UPSTREAM_ERROR_REPORT_SHUTDOWN_WAIT_SECONDS: Final = 10.0
 PASSTHROUGH_UPSTREAM_ERROR_REDACT_MARGIN_CHARS: Final = 256
+PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_SCAN_BYTES: Final = 65536
 SSE_LATE_RELAY_BACKGROUND_WAIT_SECONDS: Final = 10.0
 
 # Headers to control callbacks
diff --git a/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py b/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
index b015f5afe71..aae04fa96e2 100644
--- a/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
+++ b/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
@@ -44,6 +44,7 @@ from litellm._uuid import uuid
 from litellm.constants import (
     MAXIMUM_TRACEBACK_LINES_TO_LOG,
     PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS,
+    PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_SCAN_BYTES,
     PASSTHROUGH_UPSTREAM_ERROR_REDACT_MARGIN_CHARS,
     PASSTHROUGH_UPSTREAM_ERROR_REPORT_TASK_NAME,
     REDACTED_BY_LITELLM,
@@ -863,13 +864,10 @@ def _resolve_team_callback_wiring(
     )
 
 
-def _truncate_upstream_error_body(body: str) -> str:
-    if len(body) <= PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS:
+def _truncate_upstream_error_body(body: str, *, truncated: bool) -> str:
+    if not truncated:
         return body
-    return (
-        f"{body[:PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS]}... "
-        f"(truncated at {PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS} chars)"
-    )
+    return f"{body}... (truncated at {PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS} chars)"
 
 
 def _sanitize_upstream_error_body(body: str) -> str:
@@ -949,7 +947,7 @@ def _headers_without_body_framing(headers: httpx.Headers) -> httpx.Headers:
     )
 
 
-_DANGLING_PRIVATE_KEY: Final = re.compile(r"-----BEGIN[A-Z \-]*PRIVATE KEY-----")
+_DANGLING_PRIVATE_KEY: Final = re.compile(r"-----BEGIN[A-Z \-]*PRIVATE KEY----- ?[A-Za-z0-9+/= ]{32,}")
 
 
 def _mask_dangling_private_key(text: str) -> str:
@@ -957,6 +955,29 @@ def _mask_dangling_private_key(text: str) -> str:
     return text if match is None else text[: match.start()] + REDACTED
 
 
+def _redact_preview_head(text: str, redact: Callable[[str], str]) -> str:
+    head: Final = text[:PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS]
+    margin: Final = text[
+        PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS : PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS
+        + PASSTHROUGH_UPSTREAM_ERROR_REDACT_MARGIN_CHARS
+    ]
+    redacted_head: Final = redact(head)
+    if not margin or redact(head + margin) == redacted_head + redact(margin):
+        return redacted_head
+    return redacted_head[: redacted_head.rfind(" ") + 1] + REDACTED
+
+
+def _upstream_error_body_for_log(preview: bytes, encoding: str, redact: Callable[[str], str]) -> str:
+    sanitized: Final = _sanitize_upstream_error_body(
+        preview[:PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_SCAN_BYTES].decode(encoding, errors="replace")
+    )
+    return _truncate_upstream_error_body(
+        _mask_dangling_private_key(_redact_preview_head(sanitized, redact)),
+        truncated=len(preview) > PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_SCAN_BYTES
+        or len(sanitized) > PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS,
+    )
+
+
 def _passthrough_upstream_failure_reporter(
     response: httpx.Response,
     user_api_key_dict: UserAPIKeyAuth,
@@ -967,17 +988,10 @@ def _passthrough_upstream_failure_reporter(
     redact: Callable[[str], str] = redact_secrets,
 ) -> _ReportPreview:
     async def report(preview: bytes) -> None:
-        preview_text: Final = preview[
-            : (PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS + PASSTHROUGH_UPSTREAM_ERROR_REDACT_MARGIN_CHARS) * 4
-        ].decode(response.encoding or "utf-8", errors="replace")[
-            : PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS + PASSTHROUGH_UPSTREAM_ERROR_REDACT_MARGIN_CHARS
-        ]
         upstream_error_body: Final = (
             REDACTED_BY_LITELLM
             if should_redact_message_logging(logging_obj.model_call_details)
-            else _truncate_upstream_error_body(
-                _sanitize_upstream_error_body(_mask_dangling_private_key(redact(preview_text)))
-            )
+            else _upstream_error_body_for_log(preview, response.encoding or "utf-8", redact)
         )
         log_warning(
             "pass_through_endpoint: upstream %s %s returned %s: %s",
diff --git a/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py b/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
index 6f444c7db04..7ab281e7192 100644
--- a/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
+++ b/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
@@ -23,8 +23,8 @@ from starlette.datastructures import FormData, Headers, QueryParams
 from starlette.datastructures import UploadFile as StarletteUploadFile
 
 import litellm
-from litellm._logging import verbose_proxy_logger
-from litellm.constants import DEFAULT_REQUEST_TIMEOUT_SECONDS
+from litellm._logging import redact_secrets, verbose_proxy_logger
+from litellm.constants import DEFAULT_REQUEST_TIMEOUT_SECONDS, PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS
 from litellm.integrations.custom_logger import CustomLogger
 from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObj
 from litellm.proxy._types import ProxyException, UserAPIKeyAuth
@@ -35,6 +35,8 @@ from litellm.proxy.pass_through_endpoints.pass_through_endpoints import (
     InitPassThroughEndpointHelpers,
     _PreviewReportingStream,
     _passthrough_upstream_failure_reporter,
+    _sanitize_upstream_error_body,
+    _upstream_error_body_for_log,
     _registered_pass_through_routes,
     _truncate_upstream_error_body,
     _with_trace_context,
@@ -4297,11 +4299,12 @@ async def test_pass_through_request_streaming_upstream_error_body_reaches_client
 @pytest.mark.asyncio
 async def test_truncate_upstream_error_body_caps_at_log_limit():
     short_body: Final = "x" * 4096
-    assert _truncate_upstream_error_body(short_body) == short_body
+    assert _truncate_upstream_error_body(short_body, truncated=False) == short_body
+
+    truncated: Final = _truncate_upstream_error_body("a" * 4096, truncated=True)
+    assert truncated == f"{'a' * 4096}... (truncated at 4096 chars)"
 
     long_body: Final = "a" * 5000
-    truncated: Final = _truncate_upstream_error_body(long_body)
-    assert truncated == f"{'a' * 4096}... (truncated at 4096 chars)"
 
     upstream_response: Final = httpx.Response(
         status_code=500,
@@ -8326,8 +8329,8 @@ async def test_passthrough_upstream_failure_reporter_redacts_only_the_bounded_pr
     )
     await report(preview)
     max_plus_margin: Final = 4096 + 256
-    assert len(redacted_inputs) == 1, redacted_inputs
-    assert len(redacted_inputs[0]) <= max_plus_margin, len(redacted_inputs[0])
+    assert 1 <= len(redacted_inputs) <= 3, redacted_inputs
+    assert max(len(text) for text in redacted_inputs) <= max_plus_margin, redacted_inputs
     assert len(logged_details) == 1, logged_details
     assert "REDACTED-KEY" in logged_details[0], logged_details[0]
     assert marker_key not in logged_details[0], logged_details[0]
@@ -8416,3 +8419,76 @@ async def test_passthrough_upstream_failure_reporter_keeps_text_after_a_complete
     assert len(logged_details) == 1, logged_details
     assert "MIIEvwIBADANBgkqhkiG9w0BAQEFAASCBKkwggSlAgEAAoIBAQshortkey==" not in logged_details[0], logged_details[0]
     assert "INTERNAL" in logged_details[0], logged_details[0]
+
+
+_TRUNCATION_MARKER: Final = f"... (truncated at {PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS} chars)"
+
+
+@pytest.mark.asyncio
+async def test_upstream_error_body_for_log_keeps_a_bare_pem_header_unmasked():
+    """Vertex-style error text mentions the PEM header literally; only a header
+    followed by real key material may be masked."""
+    preview: Final = (
+        '{"error":"private_key field must start with -----BEGIN PRIVATE KEY----- header, got garbage; request_id=abc"}'
+    ).encode()
+    output: Final = _upstream_error_body_for_log(preview, "utf-8", redact_secrets)
+    assert "-----BEGIN PRIVATE KEY----- header" in output, output
+    assert "request_id=abc" in output, output
+    assert "REDACTED" not in output, output
+
+
+@pytest.mark.asyncio
+async def test_upstream_error_body_for_log_drops_a_secret_fragment_cut_at_the_preview_edge():
+    """A secret whose match spans the head/margin cut must not leak its head into
+    the log: the straddle branch drops the trailing partial and writes REDACTED."""
+    aws_key: Final = "AKIAIOSFODNN7EXAMPLE"
+    pem: Final = "-----BEGIN PRIVATE KEY----- " + "b64chars" * 75 + " -----END PRIVATE KEY-----"
+    pad_len: Final = 4096 + 256 - 19 - len(pem) - 2
+    assert pad_len > 0
+    sanitized: Final = _sanitize_upstream_error_body(pem + " " + "x" * pad_len + " " + aws_key)
+    assert sanitized.index(aws_key) == 4096 + 256 - 19
+    output: Final = _upstream_error_body_for_log(sanitized.encode(), "utf-8", redact_secrets)
+    assert "AKIA" not in output, output
+    assert "AKIAIOSFODNN7EXAMPL" not in output, output
+    assert "REDACTED" in output, output
+    assert output.endswith(_TRUNCATION_MARKER), output
+
+
+@pytest.mark.asyncio
+async def test_upstream_error_body_for_log_keeps_non_secret_straddle_text():
+    """Plain text that simply runs past the head must survive unchanged: the
+    straddle branch only fires when a redaction match spans the cut."""
+    sanitized: Final = "word " * 840
+    preview: Final = sanitized.encode()
+    output: Final = _upstream_error_body_for_log(preview, "utf-8", redact_secrets)
+    expected: Final = sanitized[:PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS] + _TRUNCATION_MARKER
+    assert output == expected, output[:200]
+
+
+@pytest.mark.asyncio
+async def test_upstream_error_body_for_log_sanitizes_before_slicing():
+    """Whitespace must collapse before the head is taken, or the useful part of a
+    pretty-printed error body would be mostly indentation."""
+    body: Final = json.dumps(
+        {"error": {"message": "x" * 100, "details": [{"k": f"v{i}", "pad": " " * 40} for i in range(400)]}},
+        indent=4,
+    )
+    sanitized: Final = _sanitize_upstream_error_body(body)
+    assert len(sanitized) > PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS, len(sanitized)
+    output: Final = _upstream_error_body_for_log(body.encode(), "utf-8", redact_secrets)
+    useful: Final = output.removesuffix(_TRUNCATION_MARKER)
+    assert len(useful) == PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS, len(useful)
+    assert output.endswith(_TRUNCATION_MARKER), output
+    assert useful == sanitized[:PASSTHROUGH_UPSTREAM_ERROR_BODY_MAX_LOG_CHARS], useful[:200]
+
+
+@pytest.mark.asyncio
+async def test_upstream_error_body_for_log_marks_bodies_past_the_scan_cap():
+    """A body beyond the 64 KiB raw scan cap must carry the truncation marker
+    even when its scanned head collapses below the log cap: the tail we never
+    decoded is unaccounted for."""
+    preview: Final = b"a" * 3000 + b" " * 5_000_000
+    output: Final = _upstream_error_body_for_log(preview, "utf-8", redact_secrets)
+    useful: Final = output.removesuffix(_TRUNCATION_MARKER)
+    assert useful == "a" * 3000, useful[:200]
+    assert output.endswith(_TRUNCATION_MARKER), output