[Bug] Prometheus agent ignores fallback chain when no explicit model is configured

[LaceLetho]

Problem Description

When using the Prometheus agent without an explicit model configuration, it incorrectly uses the UI-selected model directly instead of following the fallback chain defined in AGENT_MODEL_REQUIREMENTS. This causes Prometheus to bypass the intended fallback logic (claude-opus-4-6 → gpt-5.4 → glm-5 → gemini-3.1-pro) and use whatever model is currently selected in the UI.

Expected Behavior

When agents.prometheus.model is not explicitly configured, Prometheus should:

1. Use the UI-selected model only if it matches the primary model (claude-opus-4-6)
2. Otherwise, follow the fallback chain to find an available model (gpt-5.4, etc.)

This behavior should be consistent with other primary agents like Sisyphus and Atlas.

Actual Behavior

Prometheus always uses params.currentModel (UI-selected model) when no explicit configuration exists, completely bypassing the fallback chain resolution.

Root Cause Analysis

The issue is in src/plugin-handlers/prometheus-agent-config-builder.ts at line 47:

Current code (problematic):

const configuredPrometheusModel =
  params.pluginPrometheusOverride?.model ?? categoryConfig?.model;

const modelResolution = resolveModelPipeline({
  intent: {
    uiSelectedModel: configuredPrometheusModel ? undefined : params.currentModel,  // ← BUG
    userModel: params.pluginPrometheusOverride?.model,
    categoryDefaultModel: categoryConfig?.model,
  },
  // ...
});

The condition configuredPrometheusModel ? undefined : params.currentModel passes uiSelectedModel to the resolution pipeline when either pluginPrometheusOverride?.model or categoryConfig?.model is undefined.

However, when neither is configured (common case), this causes:

1. configuredPrometheusModel = undefined
2. uiSelectedModel = params.currentModel (UI-selected model)
3. resolveModelPipeline returns the UI model immediately (line 49-53 in model-resolution-pipeline.ts), skipping the fallback chain

Comparison with Other Agents

Other primary agents (Sisyphus, Atlas) use the correct pattern:

Sisyphus (src/agents/builtin-agents/sisyphus-agent.ts line 55):

uiSelectedModel: sisyphusOverride?.model !== undefined ? undefined : uiSelectedModel,

Atlas (src/agents/builtin-agents/atlas-agent.ts line 42):

uiSelectedModel: orchestratorOverride?.model !== undefined ? undefined : uiSelectedModel,

These only skip uiSelectedModel when the user explicitly configures a model, not when category config is missing.

Suggested Fix

Change line 47 in prometheus-agent-config-builder.ts to match the pattern used by other agents:

// Before (incorrect):
uiSelectedModel: configuredPrometheusModel ? undefined : params.currentModel,

// After (correct):
uiSelectedModel: params.pluginPrometheusOverride?.model !== undefined ? undefined : params.currentModel,

This ensures:

1. If user explicitly sets agents.prometheus.model, use that model
2. Otherwise, use UI-selected model AND allow fallback chain resolution to work

Environment

• oh-my-opencode version: 3.14.0
• OpenCode version: v1.3.5

Additional Context

This issue makes it impossible to rely on the fallback chain for Prometheus unless the user explicitly configures a model, which defeats the purpose of having sensible defaults for primary agents.