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| let Some(content_type) = headers | ||
| .get(axum::http::header::CONTENT_TYPE) | ||
| .and_then(|value| value.to_str().ok()) | ||
| else { | ||
| return Err(unsupported_media_type_error()); | ||
| return Ok(()); | ||
| }; |
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🟡 Requests declaring an unreadable content format are now accepted instead of rejected
A request whose declared format cannot be read as text is now let through as if no format had been declared (to_str().ok() combined with the new return Ok(()) at lib/llm/src/http/service/openai.rs:2017-2022), so a caller declaring a format the server cannot parse is silently accepted instead of being told the format is unsupported.
Impact: Requests that assert a non-JSON, unreadable format no longer get the clear "unsupported format" reply and instead fail later with a confusing parse error.
How the fallthrough merges "absent header" and "unreadable header" cases
headers.get(CONTENT_TYPE).and_then(|v| v.to_str().ok()) yields None in two distinct situations: the header is absent, and the header is present but contains bytes that are not valid UTF-8 (e.g. Content-Type: appl\xffication/xml). Before this change both produced 415, which was acceptable. Now both produce Ok(()), which contradicts the stated intent in the new comment at lib/llm/src/http/service/openai.rs:2015-2016 ("A header that is present but not JSON is still rejected"). The fix is to check presence of the header separately from decoding it: only treat a truly absent header as JSON, and reject a present-but-undecodable header with unsupported_media_type_error(). Relatedly, a present-but-empty value (Content-Type: ) is decodable and falls into is_json_content_type("") → 415, whereas FastAPI treats an empty value the same as absent, so the compatibility goal of the PR is not fully met for that case either.
| let Some(content_type) = headers | |
| .get(axum::http::header::CONTENT_TYPE) | |
| .and_then(|value| value.to_str().ok()) | |
| else { | |
| return Err(unsupported_media_type_error()); | |
| return Ok(()); | |
| }; | |
| let Some(content_type) = headers.get(axum::http::header::CONTENT_TYPE) else { | |
| return Ok(()); | |
| }; | |
| let Ok(content_type) = content_type.to_str() else { | |
| return Err(unsupported_media_type_error()); | |
| }; | |
| if content_type.trim().is_empty() { | |
| // An empty value is equivalent to declining to assert a format, like FastAPI. | |
| return Ok(()); | |
| } |
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Good catch — fixed in e5d6c74.
You're right on both points. to_str().ok() collapsed absent with present-but-undecodable, so an invalid header was being accepted as JSON rather than returning 415. And an empty value should be treated as absent: FastAPI's if not content_type_value is falsy for "", so it parses those as JSON too.
Now:
- absent →
Ok - present, empty/whitespace →
Ok(matches FastAPI) - present, undecodable → 415
- present, non-JSON → 415
Added regression tests for the empty and undecodable cases (HeaderValue::from_bytes(&[0xff, 0xfe])). cargo test -p dynamo-llm --lib: 1880 passed, 0 failed.
WalkthroughChangesJSON Content-Type handling
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to Requests with malformed present Content-Type headers may be accepted as JSON instead of rejected with 415. This is a localized compatibility-validation risk that should be addressed or explicitly accepted before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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Inline comments:
In `@lib/llm/src/http/service/openai.rs`:
- Line 2021: Update the Content-Type validation in the surrounding
request-handling function to distinguish an absent header from a present header
whose to_str conversion fails: allow Ok(()) only when the header is absent, and
return the existing 415 response for undecodable values. Add a regression test
using HeaderValue::from_bytes.
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ensure_json_content_type() rejects a request with no Content-Type header with 415.
FastAPI - which backs the vLLM and SGLang HTTP frontends Dynamo stands in for - parses
the body as JSON when the header is absent:
content_type_value = request.headers.get("content-type")
if not content_type_value:
if not actual_strict_content_type:
json_body = await request.json()
(fastapi/routing.py; the strict variant is opt-in and defaults off). So a client that
omits the header succeeds against `vllm serve` and gets 415 from Dynamo. That is a
silent compatibility break on migration - the request shape is unchanged and the client
is unmodified, but it starts failing 100%% of the time.
Observed in production after moving an endpoint from vLLM to Dynamo: a measurable rise
in 415s from clients that had worked for months.
A header that is present but not JSON is still rejected. That is a caller asserting a
format we cannot parse, rather than declining to assert one.
NOTE FOR REVIEWERS: this inverts an existing assertion in
test_ensure_json_content_type_rejects_missing_or_non_json, which explicitly required a
missing header to fail. I could not find the rationale in the history available to me,
so if that strictness was deliberate please say so - an opt-in flag preserving the
current default would also solve our case, and I am happy to rework it that way.
cargo test -p dynamo-llm --lib: 1878 passed, 0 failed.
Signed-off-by: Chan Nguyen <channguyen@nvidia.com>
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to_str().ok() collapsed "header absent" with "header present but not valid UTF-8", so an undecodable value was accepted as JSON instead of returning 415. Separate the two: absent or empty is treated as JSON (FastAPI reads an empty value as absent), undecodable is a 415. Signed-off-by: Chan Nguyen <channguyen@nvidia.com>
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/ok to test e5d6c74 |
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vLLM does not explicitly set this strictness check to True, but in recent versions of vLLM 0.26.0+ -- It uses a newer FastAPI version and inherits FastAPI’s default which is newly strict by default :
- vLLM 0.26.0 constructs FastAPI() without strict_content_type.
- vLLM requires FastAPI 0.133–0.136.
- FastAPI defines strict_content_type=True by default.
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@chanh just want to double check if we really want to allow empty content-type, rather than continue to reject it. It seems like latest vllm serve versions also reject it.
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You're right, and the PR's premise is wrong. I checked the FastAPI source rather than argue from memory, and it does not say what my description claims.
strict_content_type landed in FastAPI 0.132.0 and defaults to Default(True):
0.131.0 fastapi/routing.py: 0 occurrences
0.132.0 fastapi/routing.py: 17 occurrences <- introduced here
0.136.1 fastapi/routing.py: 17
0.141.1 fastapi/routing.py: 26
vLLM 0.26.0 builds FastAPI(lifespan=lifespan) with no override, so it inherits the strict default, exactly as you said. My PR description asserts "the strict variant is opt-in and defaults off" — that was true before 0.132.0 and is false for every FastAPI version vLLM 0.26.0 accepts. The production behaviour I measured was against an older vLLM, and I generalised from it without checking the current default. My mistake.
One correction to both of us, though: under strict, FastAPI does not return 415. The enforcement is a fallthrough, not a rejection (routing.py 0.136.1, L409-426):
body_bytes = await request.body()
if body_bytes:
json_body: Any = Undefined
content_type_value = request.headers.get("content-type")
if not content_type_value:
if not actual_strict_content_type:
json_body = await request.json()
else:
... # application/json or */*+json -> parse
if json_body != Undefined:
body = json_body
else:
body = body_bytes # <- raw bytes, not an errorWith strict on and no Content-Type, json_body stays Undefined, the body is handed to the model as raw bytes, and Pydantic validation fails — so vllm serve answers 422, not 415. Two further details that matter here:
if not content_type_valueis falsy for the empty string as well, so FastAPI groups absent and empty identically. My PR added a separate "empty → accept" branch; the grouping is right, the outcome is not.- The check only fires when the body is non-empty, so an empty body with no
Content-Typenever reaches it.
So the honest picture is that neither current Dynamo nor this PR matches vLLM: Dynamo returns 415, vLLM returns 422, and this PR returns 200.
Given that, plus the CSRF rationale FastAPI documents for the default — a browser can send a no-Content-Type cross-origin POST with no CORS preflight — I don't think "accept it" is defensible. I'd rather not ship it.
Two options, and I'll take your call:
- Close this PR. Current behaviour is safe and stricter than vLLM's; the only divergence is 415 vs 422, which no client should be branching on.
- Narrow it to the status code: keep rejecting, but group empty with absent the way FastAPI does, and return 422 instead of 415 so a client migrating off
vllm servesees the same status. No change in what is accepted, only in how the rejection is reported.
I lean toward (1) — the divergence is cosmetic, and 415 is arguably the more correct status for this condition regardless of what FastAPI happens to do. Say the word and I'll close it, or push (2).
Either way I'll drop the "FastAPI treats an absent Content-Type as JSON" claim from the description so it doesn't mislead anyone reading the history.
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Want to address this thread first: https://github.com/ai-dynamo/dynamo/pull/13268/changes#r3787772870
|
Closing after the review corrected the premise: current FastAPI/vLLM strict content-type handling rejects a missing |
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2 participants
Problem
ensure_json_content_type()returns 415 for a request with noContent-Typeheader.FastAPI — which backs the vLLM and SGLang HTTP frontends Dynamo stands in for — parses the body as JSON when the header is absent:
(
fastapi/routing.py; the strict variant is opt-in and defaults off.)So a client that omits the header succeeds against
vllm serveand gets 415 from Dynamo. It's a silent compatibility break on migration: same request shape, unmodified client, and it starts failing 100% of the time rather than intermittently.We hit this in production after moving an endpoint from vLLM to Dynamo — a measurable rise in 415s from clients that had worked for months. Reproduced against the live endpoint:
Content-Typeapplication/jsonapplication/json; charset=utf-8Application/JSONapplication/vnd.x+jsontext/plainChange
Treat an absent
Content-Typeas JSON. A header that is present but not JSON is still rejected — that's a caller asserting a format we can't parse, rather than declining to assert one.Reviewer note — this inverts an existing assertion
test_ensure_json_content_type_rejects_missing_or_non_jsonexplicitly required a missing header to fail, so the current behaviour looks deliberate rather than accidental. I could not find the rationale in the history available to me.If that strictness was intentional, please say so and I'll rework this. An opt-in flag preserving the current default would also solve our case — I went with changing the default because Dynamo's value here is a drop-in OpenAI-compatible contract, and silently rejecting requests the replaced server accepted works against that. Happy to defer to maintainer preference.
Testing
cargo test -p dynamo-llm --lib --no-default-features→ 1878 passed, 0 failed.Test coverage: absent header accepted; JSON variants (charset, mixed case,
+jsonsuffix) accepted;text/plainstill 415.Summary by CodeRabbit
Content-Typeheader are now accepted as JSON.