Gemini `thinkingBudget` defaults to 0, which is incompatible with some models like Gemini 2.5 Pro

[warmboi]

Plugin Type

VSCode Extension

Cline Version

3.38.0

What happened?

Problem Description

When using Gemini models through the GeminiHandler, the thinkingBudget parameter in the API request defaults to 0. This is because the _thinkingBudget variable in src/core/api/providers/gemini.ts is initialized with this.options.thinkingBudgetTokens ?? 0.

// file: src/core/api/providers/gemini.ts

// ...
// Configure thinking budget if supported
const _thinkingBudget = this.options.thinkingBudgetTokens ?? 0;
// ...

This default value of 0 causes issues because it's not a valid setting for all models. For example, according to the official documentation, Gemini 2.5 Pro does not support disabling thinking and requires a thinkingBudget between 128 and 32768, or -1 for dynamic allocation. Sending 0 to this model is incorrect.

For other models like Gemini 2.5 Flash Lite, the default behavior is no thinking, so 0 is acceptable. However, having a single static default for all models is problematic.

Suggested Solution

The default value for thinkingBudget should be set dynamically based on the selected model to ensure compatibility and leverage the best default behavior for each.

A more robust implementation would be to check the model ID and set the default thinkingBudget accordingly:

// file: src/core/api/providers/gemini.ts (Proposed Change)

let _thinkingBudget: number;
if (options.thinkingBudgetTokens !== undefined) {
    _thinkingBudget = options.thinkingBudgetTokens;
} else {
    // Default behavior based on model, see https://ai.google.dev/gemini-api/docs/thinking#supported-models
    switch (modelId) {
        case GeminiModelId.GEMINI_2_5_PRO:
        case GeminiModelId.GEMINI_2_5_FLASH:
            _thinkingBudget = -1; // Default to dynamic thinking
            break;
        default:
            _thinkingBudget = 0; // Default to off for other models like Flash Lite
            break;
    }
}

This change ensures that:

• Gemini 2.5 Pro and Flash default to dynamic thinking (-1), which is their recommended behavior.
• Other models, like Flash Lite, default to 0, preserving their standard behavior.
• Users can still explicitly override this by setting thinkingBudgetTokens.

References

• Google AI for Developers - Supported Models & Thinking Budget: Details on model-specific constraints for thinkingBudget can be found here: https://ai.google.dev/gemini-api/docs/thinking#supported-models

Steps to reproduce

1. on extention settings
2. set API provider and apikey and model (gemini-2.5-pro)
3. uncheck enable thinking
4. click done

and chat

Provider/Model

gemini-2.5-pro

System Information

OS: mac m1
ram: 16gb

[linear]

CLINE-272

[NightTrek]

Hey Thank you for the report. I use Gemini A lot So I will follow up and make sure we address these issues as soon as possible.

[james-kinetic]

im having an issue right now cline using Gemini 2.5 pro. The "enable thinking" box is unchecked (ive tried checking and unchecking) but every prompt using 2.5 pro fails saying model doesnt support thinking.... I dont want to use gemini 3.0 as its overloaded a lot

[github-actions]

Thanks for the detailed bug report and well-researched solution, @warmboi!

Issue Confirmed

The issue you've identified is valid. Looking at the code in src/core/api/providers/gemini.ts:117:

const _thinkingBudget = this.options.thinkingBudgetTokens ?? 0

When "Enable Thinking" is unchecked, thinkingBudgetTokens is undefined, so it defaults to 0. For Gemini 2.5 Pro, which requires a thinkingBudget between 128-32768 or -1 for dynamic thinking (per Google's docs), sending 0 causes the error @james-kinetic is seeing.

The current code handles this correctly for Gemini 3 models (which use thinkingLevel instead of thinkingBudget), but Gemini 2.5 Pro doesn't support thinkingLevel and needs special handling.

The Fix Path

Your suggested fix is on the right track. The model-specific behavior should be driven by the model configuration in src/shared/api.ts. Looking at the current config for gemini-2.5-pro:

"gemini-2.5-pro": {
  thinkingConfig: {
    maxBudget: 32767,
  },
  // missing: minBudget or defaultBudget for cases where thinking can't be disabled
}

A clean implementation could add a minBudget or requiresThinking property to the model config, then update gemini.ts to default to -1 (dynamic) instead of 0 when the model doesn't allow thinkingBudget: 0.

---

SME	Reason
@abeatrix	Primary author of Gemini thinking support (#7831, #8122, #7536)
@robinnewhouse	Authored #8014 which added thinking param safeguards
@canvrno	Added Gemini 3 Pro and thinking level support (#7539, #7533)

---

Possible Duplicates

• Bug: gemini-3-pro-preview fails with "thinking_budget and thinking_level are not supported together" #7957 - Related but different: deals with thinkingLevel and thinkingBudget being sent together (which are mutually exclusive), while this issue is about the default value of thinkingBudget being invalid for Gemini 2.5 Pro
• No exact duplicates found