What is Blue? — language.blue docs

Blue is a language for describing shared, executable interactions — documents with participants, rules, state, and a verifiable record of what happened.

30-second answer

A Blue document defines who participates, what they can do, what state exists, and how that state changes when timeline entries arrive. Any compliant processor, given the same document and the same entries, derives the same next state.

Document + Timelines + Rules = Deterministic State

The core idea

A Blue document is not just data. It is a shared interaction with rules.

Inside a Blue document:

participants have named roles
each participant contributes actions through a timeline
rules define what can happen
state changes when allowed actions occur
any compliant processor derives the same result from the same document and the same events

An order, a payment, a mandate, a negotiation, a service engagement, a delivery flow — all expressed in the same shape.

Why that matters

Today, every interaction works differently.

People use emails, contracts, PDFs, signatures
Services use APIs
Users use apps and websites
Payments move through separate rails and systems

Each has a different structure, a different trust model, a different source of truth. That is friction for humans. It becomes a real bottleneck when agents enter the picture.

Blue solves that by giving interactions a common structure.

A concrete example: an order between Alice and Bob

Alice runs a flower shop. Bob wants to buy a tulip bundle. The order between them is a Blue document with:

Participants: Alice (seller), Bob (buyer).
State: item, quantity, status, amount.
Rules: Bob can place or update the order. Alice can accept or reject it. Alice can only mark it shipped after she has accepted it. Both can see the current state.
References: when Alice accepts, the order can trigger a linked payment block.

That document is the same kind of object as a payment, a mandate, or a delivery confirmation. Different content, same shape. You can watch a live document process step by step in the playground.

Blue is not just a format

Blue gives you:

a language for expressing interactions
a way to refer to shared types via repo.blue
a way to process documents deterministically
a timeline-based trust model
composability between interactions

That is why we describe Blue as an interaction language, not just a schema format.

What Blue is not

Not a blockchain. No global ledger, no consensus protocol, no token.
Not a database. Documents are governed interactions, not rows or buckets.
Not one platform. Multiple runtimes and timeline providers can host and process Blue documents.
Not just YAML. YAML is a convenient authoring form. The protocol is the typed structure underneath, identified by BlueId.
Not an API gateway. A Blue document carries its own state, rules, and history — not just a request/response.

It is a common language for interactions, a trust-aware execution model, and an open way to build structured, agent-usable processes.