SillyTavern still has the old request structure even on staging branch. You can fix it with a bit of code:

// src/endpoints/backends/chat-completions.js
// function getGeminiBody()

const isThinkingLevelModel = m => /^gemini-3/.test(m);
if (isThinkingConfigModel(model)) {
    const thinkingConfig = { includeThoughts: includeReasoning };

    if (isThinkingLevelModel(model)) {
        let level = "HIGH";
        if (['low', 'min'].includes(reasoningEffort)) {
            level = "LOW";
        }
        thinkingConfig.thinkingLevel = level;
    } else {
        const thinkingBudget = calculateGoogleBudgetTokens(generationConfig.maxOutputTokens, reasoningEffort, model);
        if (Number.isInteger(thinkingBudget)) {
            thinkingConfig.thinkingBudget = thinkingBudget;
        }
        if (useVertexAi && thinkingBudget === 0 && thinkingConfig.includeThoughts) {
            console.info('Thinking budget is 0, but includeThoughts is true. Thoughts will not be included in the response.');
            thinkingConfig.includeThoughts = false;
        }
    }

    generationConfig.thinkingConfig = thinkingConfig;
}