Multi-Model Routing

This tutorial assigns different models to different tasks in a single workflow — a cheap, fast model for the spec, a capable model for coding, and a different model for review. The routing is controlled by a CSS-like stylesheet.

The workflow

Multi-Model workflow: Start → Write Spec (Haiku) → Implement (Sonnet) → Write Tests (Sonnet) → Code Review (Sonnet) → Exit

multi-model.fabro

digraph MultiModel {
    graph [
        goal="Build and review a utility function using multiple models",
        model_stylesheet="
            * { model: claude-haiku-4-5;reasoning_effort: low; }
            .coding { model: claude-sonnet-4-5;reasoning_effort: high; }
            #review { model: claude-sonnet-4-5;reasoning_effort: high; }
        "
    ]
    rankdir=LR

    start [shape=Mdiamond, label="Start"]
    exit  [shape=Msquare, label="Exit"]

    spec      [label="Write Spec", prompt="Write a brief spec for a TypeScript string utility module with 3 functions: slugify, truncate, and capitalize. Output the spec only.", shape=tab]
    implement [label="Implement", prompt="Implement the TypeScript string utility module from the spec. Write it to string-utils.ts.", class="coding"]
    test      [label="Write Tests", prompt="Write tests for the string utility module using Bun's test runner. Write to string-utils.test.ts.", class="coding"]
    review    [label="Code Review", prompt="Review the implementation and tests. Check for edge cases, type safety, and correctness. Provide a brief verdict.", shape=tab]

    start -> spec -> implement -> test -> review -> exit
}

fabro run docs/internal/demo/08-multi-model.fabro

Model stylesheets

The model_stylesheet graph attribute contains CSS-like rules that assign models to nodes:

* { model: claude-haiku-4-5;reasoning_effort: low; }
.coding { model: claude-sonnet-4-5;reasoning_effort: high; }
#review { model: claude-sonnet-4-5;reasoning_effort: high; }

Selectors

Selector	Syntax	Matches	Specificity
Universal	`*`	All nodes	0
Shape	`box`, `tab`, etc.	Nodes with that shape	1
Class	`.classname`	Nodes with `class="classname"`	2
ID	`#nodeid`	A specific node by ID	3

Higher specificity wins. If two rules have the same specificity, the last one in the stylesheet wins.

How this workflow routes

Node	Matches	Model	Why
`spec`	`*` (universal)	Haiku	Simple generation task — fast and cheap
`implement`	`.coding` (class)	Sonnet	Coding requires a capable model
`test`	`.coding` (class)	Sonnet	Test writing also needs coding capability
`review`	`#review` (ID)	Sonnet	Review needs careful analysis

Assigning classes

Set the class attribute on a node to target it with class selectors:

implement [label="Implement", class="coding"]

Multiple classes are space-separated: class="coding critical".

Properties

Stylesheets support four properties:

Property	Description
`model`	Model ID or alias (e.g. `claude-sonnet-4-5`, `opus`, `gemini-pro`)
`provider`	Provider name (optional — auto-inferred from the model catalog when omitted)
`reasoning_effort`	`low`, `medium`, or `high`
`backend`	`api` (default), `cli`, or `acp`

Why route models?

Not every task needs a frontier model:

Spec writing, classification, summarization — use a fast, cheap model (Haiku, Flash Lite)
Code implementation, complex reasoning — use a capable model (Sonnet, Opus, GPT-5.4)
Cross-critique — use a different provider so the reviewer brings fresh eyes

Model routing lets you optimize cost and latency without changing the workflow structure. Swap claude-haiku-4-5 to gemini-3-flash-preview in the stylesheet and the workflow behaves the same — just with a different model underneath.

Explicit overrides

A model set directly on a node attribute always beats the stylesheet:

implement [label="Implement", class="coding", model="claude-opus-4-6"]

This node uses Opus regardless of what .coding says. See Model Stylesheets for the full reference and Models for available model IDs.

What you’ve learned

Model stylesheets use CSS-like rules to assign models to nodes
Selectors match by universal (*), shape, class (.name), or ID (#name)
Specificity determines which rule wins when multiple match
Route cheap models to simple tasks and capable models to hard ones

Ensemble

Fan out to multiple providers and synthesize their independent opinions.

Workflow Tutorials

Example Workflows

Deployment

Multi-Model Routing

The workflow

Model stylesheets

Selectors

How this workflow routes

Assigning classes

Properties

Why route models?

Explicit overrides

What you’ve learned

Next

Ensemble

​The workflow

​Model stylesheets

​Selectors

​How this workflow routes

​Assigning classes

​Properties

​Why route models?

​Explicit overrides

​What you’ve learned

​Next

Ensemble

The workflow

Model stylesheets

Selectors

How this workflow routes

Assigning classes

Properties

Why route models?

Explicit overrides

What you’ve learned

Next