[Bugfix][Frontend] Constrain Anthropic cache_salt to non-empty

[omkar-droid]

Purpose

test_openapi_stateless[POST /v1/messages] fails intermittently with a 500. Seen on an unrelated PR in build 81852:

[500] Internal Server Error:
  `{"type":"error","error":{"type":"internal_error",
    "message":"Parameter 'cache_salt' must be a non-empty string if provided."}}`

AnthropicMessagesRequest.cache_salt (added in #49498) declares no length constraint, so its generated OpenAPI schema advertises "" as a valid value. Schemathesis runs in positive-data mode and generates requests from that schema, so it eventually emits cache_salt: "".

The handler passes the value straight through to ChatCompletionRequest:

cache_salt=anthropic_request.cache_salt,

ChatCompletionRequest.check_cache_salt_support rejects empty salts, but that validator runs during the internal conversion, so the VLLMValidationError escapes as a 500 rather than a client error.

Because Hypothesis generates inputs randomly, this passes on most runs and fails on some — it does not reproduce every build.

Fix

Add min_length=1 to the field. This does two things:

1. Publishes minLength: 1 in the OpenAPI schema, so schema-driven clients no longer generate an empty salt.
2. Rejects the request at the request-parsing layer, so an explicitly empty cache_salt returns 422 instead of 500.

The -small behaviour is unchanged: None (omitted) and non-empty salts validate exactly as before.

Not a duplicate

Checked before opening:

gh pr list --repo vllm-project/vllm --state open --search "cache_salt"
gh pr list --repo vllm-project/vllm --state open --search "anthropic cache_salt min_length"
gh pr list --repo vllm-project/vllm --state open --search "49498 in:body"
gh issue list --repo vllm-project/vllm --state open --search "cache_salt anthropic"

No open PR or issue addresses this. #46744 is a stale June draft superseded by the merged #49498.

Test

Adds TestCacheSalt::test_empty_cache_salt_rejected_at_request_validation, asserting an explicitly empty cache_salt raises ValidationError at request construction rather than surfacing later as a server error.

$ pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -k CacheSalt -v

TestCacheSalt::test_cache_salt_passed_through                        PASSED
TestCacheSalt::test_cache_salt_defaults_to_none                      PASSED
TestCacheSalt::test_empty_cache_salt_rejected_at_request_validation  PASSED

3 passed, 47 deselected in 1.29s

Full file:

$ pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -q
50 passed, 14 warnings in 14.93s

Constraint behaviour verified directly:

schema: {"anyOf": [{"minLength": 1, "type": "string"}, {"type": "null"}], "default": null}
None  -> None
valid -> tenant-abc
empty -> ValidationError (correct; FastAPI renders 422)

pre-commit run passes on the changed files (ruff, ruff-format, mypy, SPDX, forbidden-imports).

Model evaluation

Not applicable. This is a request-validation constraint on an entrypoint field; it does not touch sampling, model execution, or output.

AI assistance

AI assistance was used to diagnose the failure from the Buildkite logs and to draft this change. I reviewed every changed line.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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🚀

[noooop]

https://buildkite.com/vllm/ci/builds/81852/canvas?jid=019fc50a-bd3e-4e50-be3c-f5b48fa46af4&tab=output

It succeeded on the retry.

┓( ´∀` )┏

cc @AndreasKaratzas

[AndreasKaratzas]

@noooop on the third one through .. dont know what is this one about tbh .. i m triaging rn as well

[noooop]

I don't know why test_openai_schema has randomness...

Because both #49498 and the tests on NVIDIA devices pass, I think it should pass on AMD devices as well.

[AndreasKaratzas]

@noooop weird. In nightly it straight up passed: https://buildkite.com/vllm/ci/builds/81865/canvas?sid=019fc447-dda6-4efa-9c7a-8daf05a1e990&tab=output

Still investigating .. dont see any harm in this PR tbh though .. lmk what you think too

[AndreasKaratzas]

Ok so I think that many runs never generate cache_salt: "". And before #49498 an empty string was valid. AnthropicMessagesRequest accepted it, but ChatCompletionRequest rejected it. Because that rejection happened after request parsing, it comes out as 500 error. So min_length=1 makes sense I think. But we do still need to strengthen the test cadence. I will be adding some tests.

[DarkLight1337]

Maybe we should update this for all the other occurrences as well?

[AndreasKaratzas]

@DarkLight1337 True, but probably best as a follow up I think. CI already completed the run too..

[DarkLight1337]

Sure