fix: disable reasoning by default for DeepSeek V3.1 Terminus on OpenRouter

[roomote]

Description

This PR attempts to address Issue #8270 by fixing the DeepSeek V3.1 Terminus model on OpenRouter to allow users to disable reasoning/thinking mode.

Problem

When using DeepSeek V3.1 Terminus via OpenRouter, reasoning appears to be enabled by default with no way to turn it off. This leads to:

• Worse answers for some tasks
• Increased token usage
• Users unable to opt out of reasoning mode

Solution

Added explicit reasoning exclusion logic for the model when:

• The model is being used via OpenRouter
• Reasoning is not explicitly configured
• The user has not enabled reasoning effort ( is false)

This follows the same pattern already established for Gemini 2.5 Pro models.

Changes

• Modified to add reasoning exclusion for DeepSeek V3.1 Terminus
• Added comprehensive test coverage in

Testing

• ✅ All new tests pass (4 test cases)
• ✅ Existing OpenRouter tests still pass
• ✅ Linting and type checking pass

Expected Behavior

• Default (reasoning OFF): DeepSeek V3.1 Terminus will not use reasoning tokens
• When enabled: Users can still opt-in to reasoning by setting
• Other models: No impact on other models or providers

Fixes #8270

Feedback and guidance are welcome!

---

Important

Disables reasoning by default for DeepSeek V3.1 Terminus on OpenRouter unless explicitly enabled by the user, with tests added to verify behavior.

• Behavior:
  • Disables reasoning by default for DeepSeek V3.1 Terminus in OpenRouterHandler unless enableReasoningEffort is true.
  • Applies reasoning exclusion logic in createMessage() and completePrompt() in openrouter.ts.
• Testing:
  • Adds openrouter-deepseek-terminus.spec.ts with tests for reasoning exclusion and inclusion.
  • Tests ensure no impact on other models like anthropic/claude-3-sonnet.

^{This description was created by for 389c96a. You can customize this summary. It will automatically update as commits are pushed.}

[roomote]

Self-review complete. Found some code duplication that my copy-paste skills are particularly proud of.

[roomote]

Consider extracting this reasoning exclusion logic into a helper method to avoid duplication with the same logic in completePrompt() (lines 262-268). For example:

private shouldExcludeReasoning(modelId: string, reasoning: any): boolean {
  return modelId === "deepseek/deepseek-v3.1-terminus" &&
    typeof reasoning === "undefined" &&
    !this.options.enableReasoningEffort;
}

[roomote]

Consider adding a comment explaining why DeepSeek V3.1 Terminus needs special handling, similar to the Gemini comment above. This would help future maintainers understand the reasoning behind this exclusion.

[roomote]

Consider adding a test case for when reasoning is already defined in the model configuration. The current tests cover enableReasoningEffort true/false, but not the case where typeof reasoning !== "undefined".

[roomote]

Consider integrating these tests into the main openrouter.spec.ts file or following a consistent naming pattern. Other provider-specific tests are typically included in the main provider test file.

[daniel-lxs]

#8270 (comment)