Tannakian Reconstruction

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July 14, 2026

Tannakian reconstruction

Posted by Bartosz Milewski under Category Theory | Tags: Category Theory, Fiber Functor, Tannakian Reconstruction |

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Two friends, Alice and Bob, live in the same city, but on the opposite sides of a wide river. Every night, Bob looks at the lights on the other side and tries to guess, which one belongs to Alice. They come up with a clever arrangement: Alice will turn on her lights for 10 minutes every night at 10 p.m. Every night Bob will take a long-exposure photo at the pre-arranged time. At the end of the year, Bob will superimpose all the photos, and hopefully the only bright spot will be Alice’s window. This is Tannakian reconstruction in a nutshell.

A functor produces a picture of one category inside another. It’s a potentially lossy encoding, but it always preserves the structure of the source. If there is a connection (morphism) between two objects in the source category, there will always be a connection between their images in the target category.

In general it’s impossible to recover the structure of the source category by looking at only one such picture. But if the target category has enough resolution, and we superimpose all available pictures, we can recover the morphisms of the source category.

Fiber functors

A category with just the right resolution is the category of sets. Therefore we’ll be looking at functors in (for historical reasons, these are called co-presheaves). Such a functor maps objects to sets, and morphisms to functions.

When dealing with functors, we usually imagine varying objects and morphisms while keeping the functor constant. Here, we are interested in using the totality of all functors while keeping the objects constant. To every object we will associate a mapping from functors to sets, simply by applying each functor to this object :

This mapping is functorial. Indeed, a natural transformation between and is a family of functions . We define the action of on a natural transformation by taking its component .

is called a fiber functor. You may think of it as probing an object and, through morphisms, its immediate neighborhood.

Tannakian reconstruction

To probe a hom-set we’ll be looking at the set of functions under all possible functors .

This set happens to be the set of natural transformations between two fiber functors and , a hom-set in the functor category:

A set of natural transformations can be written as an end:

An end is like a gigantic product. In our analogy, it correspond to the superposition of all photos. Like with any product, if any of its components is empty, the whole end is empty. Since the end goes over every possible functor, what prevents us from hand-picking a functor that’s non-empty at and empty at ? Such a single bad apple would spoil the whole batch (there is no function from a non-empty set to an empty set).

What makes this end non-trivial is functoriality. Any time there is a morphism , we automatically have a function , for any functor . In fact, because of the Yoneda embedding being full and faithful, we have as many such functions as there are morphisms in . We have the isomorphism:

By superimposing the images of all functors, we recover the original hom-set. This is the categorical version of Tannakian reconstruction.

We’ll use the double-Yoneda trick to prove it. First, we use the Yoneda lemma:

to explode the functors under the end:

We can now apply the Yoneda reduction to "integrate" over . The result is:

which, again by Yoneda, is equivalent to:

As an example, let’s apply Tannakian reconstruction to a simple one-object category. Such a category has a single hom set, which forms a monoid under composition. A set-valued functor maps the single object of to a set — a representation of this monoid. Natural transformations between such functors are called equivariant maps. Tannakian reconstruction lets us recover the monoid from the totality of its representations. Notice that naturality/equivariance is baked into the definition of an end.

Tannakian Reconstruction

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