Assistant: Prompt-based tool calling polyfill for models without native function calling support

[Blackjack200]

Problem

When deepseek-reasoner (DeepSeek-V3.2-Exp) is selected in the Agent panel, Zed automatically falls back to deepseek-chat because the reasoner model does not support native tool calling.

DeepSeek API docs explicitly state:
(1) If the request to the deepseek-reasoner model includes the tools parameter, the request will actually be processed using the deepseek-chat model.
— https://api-docs.deepseek.com/quick_start/pricing

This fallback defeats the purpose for everyone who deliberately picks the reasoner model for its far superior reasoning, math, and complex refactoring capabilities.

Goal

Allow full Agent workflows while staying on the originally selected model — even when it lacks native function calling support.

Proposed Approach

Add an opt-in prompt-based tool calling polyfill (configurable via tool_call_polyfill: true) that activates when a model reports tools_supported: false.

The mechanism would:

• Inject a tailored system prompt + response parsing
• Let the model express tool use through structured text
• Have Zed execute the requested tools locally and feed results back

This pattern is already battle-tested in DeepChat, and works exceptionally well with reasoning-focused models that follow instructions strictly.

Benefits

• Users finally get real deepseek-reasoner in Agent mode
• Maximum reasoning quality on large refactors and algorithmic tasks
• Higher efficiency than the chat variant

Willing to Contribute

I'm happy to drive this entirely:

• A opt-in prompt-based tool call polyfill that activates automatically (or via toggle)
• Provider-agnostic core logic if possible, with DeepSeek as the first fully supported provider
• Probably clear in-panel indicator (Running in compatibility mode – tool use via prompt polyfill)

I’d like to implement this cleanly from the start, so before writing any code, I just want to check the team’s preference on this feature.

As soon as I get a quick go-ahead or any guidance, I’ll start the implementation and open the PR as fast as possible.

If this direction sounds good to the team, just drop a 👍 or any early feedback — I’ll start coding right away.

Thanks!

I searched the issue tracker with keywords like: deepseek-reasoner tool calling,deepseek fallback,prompt-based tool,native tool call and didn’t find any open or closed issues covering this exact problem or proposing the same polyfill approach.