fix(sglang): return sidecar request refusals to the client

[KrishnanPrash]

Overview:

The SGLang sidecar refuses some requests before it calls the engine, for example an image request or n=2. The client then gets 400 {"message":"Invalid request"}, and the real reason is only in the worker log. Since #14396, the HTTP body shows DynamoError::public_message() and falls back to "Invalid request" when the error has no public message. The sidecar sets only the private message. This PR gives the fixed request-validation texts of the sidecar gRPC request builder a public message.

Repro (stock SGLang, one SGLang sidecar worker):

curl -s localhost:8000/v1/chat/completions -H 'Content-Type: application/json' -d '{
  "model": "Qwen/Qwen3-VL-2B-Instruct", "max_tokens": 16,
  "messages": [{"role": "user", "content": [
    {"type": "image_url", "image_url": {"url": "data:image/jpeg;base64,<JPEG>"}},
    {"type": "text", "text": "Describe this image."}]}]}'

Before:

{"message":"Invalid request","type":"Bad Request","code":400}

After:

{"message":"multimodal payloads are not supported by SGLang's native Generate RPC","type":"Bad Request","code":400}

The same change applies to n=2 ("n must be 1 for the SGLang sidecar") and to the other request checks.

Details:

lib/sidecar/sglang/src/client.rs adds invalid_request(&'static str). The helper sets the same fixed text as the message and as the public message. The parameter type is &'static str, so client input and runtime values cannot become public through this helper.

lib/sidecar/sglang/src/protocol.rs uses the helper for its fixed request-validation texts. These are the checks in validate_request and the range checks for max_tokens, min_tokens, logprobs, the data-parallel rank, token ids, and stop-token ids. The token-id texts no longer include the client value. Errors that carry endpoints, bootstrap ports and rooms, engine state, or engine text stay private.

The prompt_embeds check now runs before the empty-token check. A prompt_embeds request arrives with empty token_ids, so the old order gave the wrong reason. The thinking-budget text now names the client field thinking_token_budget. This PR does not add or remove a refusal.

The tests check the public text for three refusals. They also check that a disaggregation handoff error has no public message.

Out of scope: the vLLM and TRT-LLM sidecars have the same gap through invalid_argument in lib/sidecar/common/src/error.rs. The refusals on the native HTTP path of the SGLang sidecar (native_http.rs) also keep a private message.

Related Issues

• Part of DIS-2975

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Request validation errors now provide consistent, clearer messages, including for unsupported prompt embeddings and out-of-range stop tokens.
  • Requests that include both prompt embeddings and empty token IDs now report the unsupported-embeddings error.
  • The thinking token budget error now identifies the alternate name for the setting.
  • Missing decode rendezvous parameters no longer expose a public error message.

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📥 Commits

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📒 Files selected for processing (2)

• lib/sidecar/sglang/src/client.rs
• lib/sidecar/sglang/src/protocol.rs

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Walkthrough

SGLang request-conversion and validation errors now use a shared InvalidArgument helper. Prompt embeddings are checked before empty token IDs. Tests verify selected public error messages and that a missing decode rendezvous parameter error has no public message.

Changes

SGLang request errors

Layer / File(s)	Summary
Invalid request error helper lib/sidecar/sglang/src/client.rs	Added a crate-visible helper that sets the InvalidArgument error type and uses the provided text for both internal and public messages.
Request conversion and validation errors lib/sidecar/sglang/src/protocol.rs	Updated conversion and validation failures to use the helper. Prompt embeddings are checked before empty token IDs, the thinking-token-budget error names both parameter forms, and tests check selected public error messages.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to afec1

The SGLang request-error change appears mergeable after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: returning SGLang sidecar request-refusal messages to clients.
Description check	✅ Passed	The description explains the problem, behavior before and after the change, implementation details, tests, and scope. The Related Issues section identifies DIS-2975, although it does not use the templ…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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