v1.17.13: Session title generation fails on OpenRouter with "Reasoning is mandatory for this endpoint"

[pathbouncer]

Description

I am a human!
In v1.17.13, auto-generated session titles silently fail for users whose auto-picked small_model is an OpenRouter reasoning-mandatory model (e.g. openrouter/google/gemini-3.5-flash).

The auto-pick changed in v1.17.12 ("Pick better default small models across providers") and 1.17.13 added "Force reasoning mode for OpenAI-compatible reasoning models". The combination forces reasoning on the title call, which the upstream endpoint rejects with:

AI_APICallError: Reasoning is mandatory for this endpoint and cannot be disabled.

The title agent runs, errors, and the session is left with its default "New session - " title. No user-visible feedback.

• opencode v1.17.13
• Main model: openrouter/minimax/minimax-m3
• Auto-picked small: openrouter/google/gemini-3.5-flash
• Ubuntu 24.04.4 LTS

Plugins

No response

OpenCode version

1.17.13

Steps to reproduce

1. opencode.json has only model: "openrouter/<some-model>", no small_model set.
2. Start a session in v1.17.13.
3. Send a prompt.
4. Observe: title stays default; in ~/.local/share/opencode/log/opencode.log:
   stream providerID=openrouter modelID=google/gemini-3.5-flash
   small=true agent=title mode=primary
   ERROR "stream error" error.error="AI_APICallError: Reasoning is mandatory
   for this endpoint and cannot be disabled."

Screenshot and/or share link

No response

Operating System

Ubuntu 24.04.4 LTS

Terminal

Gnome Terminal

[github-actions]

This issue might be related to existing issues about session title generation failures. Please check if any of these are the same underlying problem:

• [BUG] Session title generation fails silently since v1.3.3 — effort parameter leaks into small model call #20269: Session title generation fails silently — effort parameter leaks into small model call (similar pattern: unsupported parameters being passed to the title agent's model)
• Session title generation silently fails with no fallback or logging #29734: Session title generation silently fails with no fallback or logging (same symptom, different root cause)
• Auto session title generation fails for opencode provider models (smallOptions missing provider config) #30662: Auto session title generation fails for opencode provider models (smallOptions missing provider config)

Your issue appears to be a distinct root cause (reasoning mode being forced on a model that rejects it at the endpoint level), but the maintainers may want to track these together.