Bug Description

When configuring a custom OpenAI-compatible API provider (e.g., spanagent.xyz), if the baseUrl is set to https://spanagent.xyz (without /v1 suffix), OpenClaw constructs the request URL as https://spanagent.xyz/chat/completions. This endpoint returns an HTML page (200 OK, Content-Type: text/html), which the streaming handler cannot parse. The error surfaced to the user is cryptic:

FailoverError: spanagent/deepseek-v4-flash ended with an incomplete terminal response

Meanwhile, the log shows contentType=text/html; charset=utf-8 but no clear indication that the URL path is wrong.

Steps to Reproduce

1. Add a custom provider via openclaw onboard or manual config:

   {
     "baseUrl": "https://spanagent.xyz",
     "api": "openai-completions",
     "models": [{ "id": "deepseek-v4-flash", "reasoning": true, ... }]
   }

2. Run openclaw agent --agent <agent> --message "hi" --model <provider>/deepseek-v4-flash --local
3. Observe: request goes to https://spanagent.xyz/chat/completions (no /v1 prefix)
4. API returns HTML page with 200 OK
5. Error: incomplete terminal response — no mention of wrong URL

Expected Behavior

1. The onboard wizard or docs should guide users to use https://spanagent.xyz/v1 as the base URL (OpenAI SDK appends /chat/completions to the base URL)
2. When the provider returns non-JSON/non-SSE content (e.g., HTML), the error should be descriptive: "API endpoint returned HTML instead of a valid response — check baseUrl (should typically end with /v1 for OpenAI-compatible APIs)"

Actual Behavior

Silent failure with misleading incomplete terminal response message. Requires log inspection to find contentType=text/html and discover the wrong URL path.

Environment

• OpenClaw version: 2026.5.19-beta.1 (167e73c)
• Deployment: Docker container (npm global install)
• Provider: Custom OpenAI-compatible API (spanagent.xyz)

Additional Context

The OpenAI JavaScript SDK (used internally by pi-ai/pi-agent-core) constructs the endpoint URL as ${baseUrl}/chat/completions. This means for any provider serving under a path prefix (like /v1/), the baseUrl MUST include that prefix.

For example:

• ❌ https://spanagent.xyz → request goes to https://spanagent.xyz/chat/completions (wrong)
• ✅ https://spanagent.xyz/v1 → request goes to https://spanagent.xyz/v1/chat/completions (correct)

A secondary issue was also encountered: the default maxTokens of 384000 for deepseek models exceeded the providers limit (65536), causing HTTP 400. The error message for this was clear (400 field MaxTokens invalid, should be in [1, 65536]), so that part is fine — just noting the config guidance gap.