Show HN: Gambit, an open-source agent harness for building reliable AI agents

[randall]

Hey HN!

Wanted to show our open source agent harness called Gambit.

If you’re not familiar, agent harnesses are sort of like an operating system for an agent... they handle tool calling, planning, context window management, and don’t require as much developer orchestration.

Normally you might see an agent orchestration framework pipeline like:

compute -> compute -> compute -> LLM -> compute -> compute -> LLM

we invert this so with an agent harness, it’s more like:

LLM -> LLM -> LLM -> compute -> LLM -> LLM -> compute -> LLM

Essentially you describe each agent in either a self contained markdown file, or as a typescript program. Your root agent can bring in other agents as needed, and we create a typesafe way for you to define the interfaces between those agents. We call these decks.

Agents can call agents, and each agent can be designed with whatever model params make sense for your task.

Additionally, each step of the chain gets automatic evals, we call graders. A grader is another deck type… but it’s designed to evaluate and score conversations (or individual conversation turns).

We also have test agents you can define on a deck-by-deck basis, that are designed to mimic scenarios your agent would face and generate synthetic data for either humans or graders to grade.

Prior to Gambit, we had built an LLM based video editor, and we weren’t happy with the results, which is what brought us down this path of improving inference time LLM quality.

We know it’s missing some obvious parts, but we wanted to get this out there to see how it could help people or start conversations. We’re really happy with how it’s working with some of our early design partners, and we think it’s a way to implement a lot of interesting applications:

- Truly open source agents and assistants, where logic, code, and prompts can be easily shared with the community.

- Rubric based grading to guarantee you (for instance) don’t leak PII accidentally

- Spin up a usable bot in minutes and have Codex or Claude Code use our command line runner / graders to build a first version that is pretty good w/ very little human intervention.

We’ll be around if ya’ll have any questions or thoughts. Thanks for checking us out!

Walkthrough video: https://youtu.be/J_hQ2L_yy60

[elgrantomate]

I've been playing with this for the past 24 hours or so. I like the atomic containment of the LLM, and the clear separation of logic, code, and prompts.

You have some great working examples, but, for example: translate_text specifies the default language in three places: the card, the input schema, and the deck. This can't be necessary; I'll experiment, but shouldn't it just be defined in one place?

The descriptive language of the project is a bit dense for me too. I'm having a hard time figuring out how to do basic things like parameters -- let's say that I want to constrain summarize_text to a certain length... I've tried to write language in the cards/decks, but the model doesn't seem to be paying attention.

I also want to be able to load a file, e.g. not just "translate 'hello my friend' to Italian" but "translate '/test/hello_my_friend.txt' to Italian" and have it load the contents of the file as input text. How do I do that?

Super cool project!

[randall]

yeah the way to do that stuff is through zod schemas… input and output schemas.

you can set up really complex validation.

thanks for checking it out!!

[niyikiza]

Nice architecture. The typed deck composition pattern is exactly right for making agent workflows testable.

One thing I've been thinking about is that schema validation catches "is this data shaped correctly?" but not "is this action permitted given who initiated the request?" When you have deck → child deck → grandchild deck chains, a prompt injection at any level could trigger actions the root caller never intended.

I've been working on offline capability verification for this using cryptographically signed warrants that attenuate as they propagate down the call chain. Curious if you've thought about that layer, or if you're relying on the model to self-police tool selection?

[randall]

So two things.

1/ crypto signing is totally the right way to think about this. 2/ I'm limiting prompt injection by using chain of command: https://model-spec.openai.com/2025-12-18.html#chain_of_comma...

we have a "gambit_init" tool call that is synthetically injected into every call which has the context. Because it's the result of a tool call, it gets injected into layer 6 of the chain of command, so it's less likely to be subject to prompt injections.

Also, relatedly, yes i have thought EXTREMELY deeply about cryptographic primitives to replace HTTP with peer-to-peer webs of trust as the primary units of compute and information.

Imagine being able to authenticate the source of an image using "private blockchains" ala holepunch's hypercore.

[iainctduncan]

You might want to know that Gambit is an open source Scheme implementation that has been around a very long time.

[Trufa]

Is this an alternative to https://mastra.ai/docs

How would it compare?

[randall]

So I look at something like Mastra (or LangChain) as agent orchestration, where you do computing tasks to line up things for an LLM to execute against.

I look at Gambit as more of an "agent harness", meaning you're building agents that can decide what to do more than you're orchestrating pipelines.

Basically, if we're successful, you should be able to chain agents together to accomplish things extremely simply (using markdown). Mastra, as far as I'm aware, is focused on helping people use programming languages (typescript) to build pipelines and workflows.

So yes it's an alternative, but more like an alternative approach rather than a direct competitor if that makes sense.

[yencabulator]

> - Rubric based grading to guarantee you (for instance) don’t leak PII accidentally

That does not sound like a "guarantee", at all.

[randall]

once your team comes to a consensus on what PII is, you can roughly guarantee it... especially as models improve.

[benban]

nice work. the idea of breaking agents into short-lived executors with explicit inputs/outputs makes a lot of sense - most failures i've seen come from agents staying alive too long and leaking assumptions across steps.

curious how you're handling context lifetimes when agents call other agents. do you drop context between calls or is there a way to bound it? that's been the trickiest part for us.

[randall]

right now yeah we’re just dropping context… sub agents are short lived.

thinking about ways to deal with that but we haven’t yet done it.

[elgrantomate]

also, it seems like this works with openrouter, and perhaps OpenAI -- what about Gemini API?

[randall]

thanks for the PR! :)

[tomhow]

[under-the-rug stub]

[see https://news.ycombinator.com/item?id=45988611 for explanation]

[franciscomello]

This looks quite interesting in terms of the architecture. Seems like a fresh take on stuff like Langchain, which at least last time I checked sucks.

[sofdao]

this is awesome

are things like file system baked in?

fan of the design of the system. looks great architecturally

[alberson]

I’m excited to give this a spin at Agentive! Really interesting approach.

[pych]

wow this looks cool - been meaning to dig into harness stuff this looks like a good starting point

[randall]

thx, i appreciate it, believe it or not. :)

[randall]

i have a huge theory here that idk when we’ll implement but it has to do with “quorums” and other stuff.

hard to explain… we’ll keep going.

[brap]

This seems to be where it’s at right now, we can’t seem to make the models significantly more intelligent, so we “inject” our own intelligence into the system, in the form of good old fashioned code.

My philosophy is make the LLMs do as little work as possible. Only small, simple steps. Anything that can be reasonably done in code (orchestration, tool calls, etc) should be done in code. Basically any time you find yourself instructing an LLM to follow a certain recipe, just break it down to multiple agents and do what you can with code.