Ask HN: How are thinking efforts implemented?

[simianwords]

Claude and ChatGPT have thinking efforts where you can tune the amount of thinking allowed.

Like low, medium, high, xhigh and so on.

But are they different models underneath? Or same model with different parameter?

The reason I ask is because, if I change the effort param mid conversation in Claude code, I get a warning suggesting I’m breaking the cache.

I don’t think this happens in Codex because when I change the effort, the responses are still quick.

[masfuerte]

I suspect it's not possible (as an end user) to get a thinking trace from one of the models. But what happens with "thinking" is that the model has a conversation with itself in an attempt to home in on a better answer to the original prompt.

The "amount of thinking" is how long this internal conversation is allowed to progress. The longer it goes on the more it costs. It's all part of the token budget but, because this internal dialogue is hidden, it's not obvious to the end user.

[pyentropy]

Take a look at the harmony repo which specifies the internal OpenAI format - the effort level is specified in the context after the <|start|> tag - https://github.com/openai/harmony

Note that inference libs also have parsers that put hard limits on reasoning tokens with separate counters (similar to how you can put a limit on token generation per completion versus waiting for an <eos>). For that, take a look at vllm reasoning docs.

[pyentropy]

Examples with inference of different reasoning effort levels is in the OpenAI docs as well - https://developers.openai.com/cookbook/articles/openai-harmo...

https://docs.vllm.ai/en/latest/features/reasoning_outputs/#a...

https://developers.openai.com/api/docs/guides/reasoning

[simianwords]

I think you have the right answer but I'm struggling to understand: does changing the effort change the prompt at the start of the conversation? I wonder why come up with this way at all? Why not just add a parameter at the end or something? At least it won't break cache.

Maybe like: add a secret suffix to your chat in the conversation to think more like

   conversation....

   Hey please help
   [think more]

[pyentropy]

I'm considering the possibility that it's good to break the prefix and cache because the LLM itself was rewarded (during post-training) with different prefixes/system prompts, each containing reasoning traces of the correct size.

I might be very very wrong though and LLMs disagree with me, insisting that cache is preserved and the system message doesn't have to change (even though it often contains effort level in context) if effort level changes across turns, and that all you have to do is tell the inference lib that parses think tags to early-close think tags that are too long.

[simianwords]

This seems correct but again I would like to think post training could have been also done by checking only the string in the last message sent.

[aabdi]

Different models do slight variants.

Usually it’s done in post training to enforce behavior based on prompt. Ie. System prompt with thinking:max or low or wtv.

Enforcement then goes via constrained decoding, checking for think token start and end with max lengths, or other variations

[bjourne]

LLMs work by generating the most likely continuation to a prompt. But they can also generate multiple likely continuations. This create multiple branches which in turn can generate even more branches. The LLM can then evaluate the branches, prune the unpromising ones, and merge the best ones. More branches means more tokens, means more effort.

[simianwords]

this has nothing to do with the thinking effort however

[bjourne]

Yes, it does. Breadth of search is exactly what the effort setting controls.

[pyentropy]

LLM-judge/parallel branching ≠ multi-token prediction ≠ reasoning effort.

See https://developers.openai.com/cookbook/articles/openai-harmo... and src/openai/types/shared/reasoning_effort.py

[bjourne]

[flagged]

[simianwords]

No it doesn't and lets not call people names. You can verify this using ChatGPT or anything else. You are mistaken and there are no "branches" happening.

[FergusArgyll]

I think you may be confusing the openai "pro" series models with thinking. Thos are rumored to be multi "branched"

[__patchbit__]

At a guess. May be associated with token length context window. Down selecting is consistent with warning message, forcing cutoff to context window. The technical term cache being a synonym. Increasing the headroom for more "thinking" should allow the implementation to access more resources without warning about the cache breaking.

[sometimelurker]

they use multitoken prediction behind the scenes, that might interact with the CoT in a strange way. maybe for different thinking modes they have different MTP models? if so thats interesting

[pyentropy]

The number of tokens you predict at time (multi or not) has nothing to do with whether the model wants to emit any, some or a lot of reasoning tokens in reasoning tag -- similar to how branch prediction will not really change the for loop iteration count.

[sometimelurker]

no it might. a high reasoning task is probably harder than a low reasoning task, so the same MTP LLM will predict more correct tokens on the low reasoning task. to compensate for this, big labs likely have different MTP LLMs for different cases. it would make sense for them to do this

[Yahyaaa]

Usually it’s not a different model, it’s the same model with different inference-time settings. “Thinking effort” typically changes the compute budget and decoding behavior (how many steps, how much exploration, sometimes internal planning loops).

Some stacks also tie it to orchestration layers or system/prompt signals, which is why it can look inconsistent across products

[shanewei]

My understanding is that it’s mostly an inference-time knob, not different weights.

OpenAI describes reasoning.effort as controlling how many reasoning tokens get used before the answer. Anthropic’s docs are even more explicit that effort trades off thoroughness vs token efficiency “with a single model”.

So I wouldn’t read the Claude Code cache warning as proof that a different model is being used. It may just mean the thinking/effort setting is part of the cache key.