fix: cryptic error message for empty messages list in /chat/completions #2067

[heisenberglit]

Overview:

This PR addresses issue #2064 where the /chat/completions endpoint returns a cryptic internal error when receiving an empty messages array. The fix adds proper validation and returns a clear, user-friendly error message instead.

Details:

Added validation: Check for empty messages array before processing the request
Improved error handling: Return HTTP 400 with descriptive error message instead of internal server error

Where should the reviewer start?

Added a function to validate chat completion required field in llm->http->service->openai.rs

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Fixes [BUG]: /chat/completions requests with empty messages list has cryptic error message #2064

Summary by CodeRabbit

• Bug Fixes
  • Improved validation for chat completion requests to ensure that required fields are present, returning an error if the messages field is missing or empty.

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[github-actions]

👋 Hi heisenberglit! Thank you for contributing to ai-dynamo/dynamo.

Just a reminder: The NVIDIA Test Github Validation CI runs an essential subset of the testing framework to quickly catch errors.Your PR reviewers may elect to test the changes comprehensively before approving your changes.

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Walkthrough

A new validation function was introduced to ensure that the messages field in chat completion requests is not empty. This check is performed early in the request handler, and if the field is empty, a 400 Bad Request error is returned. The function is publicly exported for use in the handler.

Changes

File(s)	Change Summary
lib/llm/src/http/service/openai.rs	Added validate_chat_completion_required_fields function; integrated it into request handling.

Sequence Diagram(s)

sequenceDiagram
    participant Client
    participant Handler
    participant Validator

    Client->>Handler: POST /chat/completions (with request)
    Handler->>Validator: validate_chat_completion_required_fields(request)
    alt messages empty
        Validator-->>Handler: Err (400 Bad Request)
        Handler-->>Client: Respond with error
    else messages present
        Validator-->>Handler: Ok
        Handler->>Handler: Continue processing (template, engine, etc.)
        Handler-->>Client: Respond with completion
    end

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Estimated code review effort

1 (~4 minutes)

Poem

In the warren of code, a new check appears,
No more cryptic errors, just clarity cheers!
If your messages are empty, a Bad Request you’ll see—
The handler now guards, as strict as can be.
🐇✨
Hop along, dear user, with requests that are right,
For clear error messages make debugging light!

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📜 Recent review details

Configuration used: .coderabbit.yaml
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 2c642fd and b54fb16.

📒 Files selected for processing (1)

• lib/llm/src/http/service/openai.rs (2 hunks)

🔇 Additional comments (2)

lib/llm/src/http/service/openai.rs (2)

416-417: LGTM! Well-integrated validation call.

The validation is correctly placed early in the request processing flow, after unsupported field validation but before template application. The error handling follows the established pattern in the codebase.

---

564-578: Excellent implementation! Addresses the core issue effectively.

The validation function is well-implemented:

• Clear error message: Replaces the cryptic internal server error with a descriptive HTTP 400 response that clearly explains the problem and requirement
• Appropriate status code: Uses HTTP 400 Bad Request which is correct for client input validation errors
• Consistent pattern: Follows the same signature and error handling approach as existing validation functions
• Focused logic: Simple and targeted check that directly addresses the issue described in the PR objectives

This perfectly solves the problem of unclear error messages when the messages array is empty.

---

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[rmccorm4]

Thanks for the contribution @heisenberglit !