Summary

Resolves #517 — openshell inference set fails for OpenAI GPT-5 models because the validation probe sends the deprecated max_tokens parameter, which GPT-5+ rejects with HTTP 400.

• Send max_completion_tokens as the primary parameter in the OpenAI chat completions validation probe
• Automatically fall back to max_tokens when the backend returns HTTP 400 (for legacy or self-hosted backends)
• Extract try_validation_request() helper to avoid duplicating the request/response classification logic

Root Cause

OpenAI introduced max_completion_tokens as a replacement for max_tokens starting with the o1 series. GPT-5 and later models reject max_tokens entirely, returning HTTP 400. The validation probe only sent max_tokens, so inference setup would fail for any GPT-5+ model even though the endpoint was perfectly healthy.

graph TD
    subgraph "Before (broken)"
        A["validation_probe()"] -->|"max_tokens: 32"| B[OpenAI API]
        B -->|"HTTP 400: unsupported parameter"| C["ValidationFailure ❌"]
    end

    subgraph "After (fixed)"
        D["validation_probe()"] -->|"max_completion_tokens: 32"| E[OpenAI API]
        E -->|"HTTP 200"| F["ValidatedEndpoint ✅"]
        E -->|"HTTP 400"| G{fallback_body?}
        G -->|"yes"| H["retry with max_tokens: 32"]
        H -->|"HTTP 200"| I["ValidatedEndpoint ✅"]
        G -->|"no"| J["ValidationFailure ❌"]
    end

Changes

Test Plan

• cargo test -p openshell-router — 11 passed, 0 failed
• New test: verify_openai_chat_uses_max_completion_tokens — primary probe succeeds with max_completion_tokens
• New test: verify_openai_chat_falls_back_to_max_tokens — HTTP 400 on primary triggers retry with max_tokens
• New test: verify_non_chat_completions_no_fallback — non-chat protocols (e.g. anthropic_messages) do not retry on 400

sequenceDiagram
    participant CLI as openshell inference set
    participant Router as Privacy Router
    participant Backend as OpenAI API

    CLI->>Router: verify_backend_endpoint()
    Router->>Backend: POST /v1/chat/completions<br/>{"max_completion_tokens": 32}

    alt GPT-5+ model
        Backend->>Router: HTTP 200
        Router->>CLI: ValidatedEndpoint ✅
    else Legacy backend
        Backend->>Router: HTTP 400 (unknown param)
        Router->>Backend: POST /v1/chat/completions<br/>{"max_tokens": 32}
        Backend->>Router: HTTP 200
        Router->>CLI: ValidatedEndpoint ✅
    end