Crypto glossary
Solver (filler)
A solver is a specialised party that competes to fulfil users' trade intents or batch auctions, finding the best route or filling orders from its own inventory.
In intent-based protocols, users do not pick a route; solvers do. A solver, called a filler in some protocols, watches incoming orders, searches DEX liquidity, private market makers and its own balances for the best execution, and submits a bid. The winning solver settles the trade on-chain and is paid through the price spread or a fee, sometimes after posting a bond.
Solvers are the engine behind CoW Protocol, UniswapX, 1inch Fusion and cross-chain intent systems, where they front funds on the destination chain and get reimbursed later. Their competition is what gives users better prices than a single router, and some protocols redistribute part of the value they capture back to traders.
The role is capital- and skill-intensive, so a handful of professional firms tend to win most auctions, raising concerns about concentration and collusion. Solvers can also extract value within the limits a user signs, so loose slippage settings matter. Protocols rely on whitelists, bonds and slashing to keep them honest.
Example
For a large stablecoin swap, a solver matches part of the order against another user's opposite trade in the same batch and routes the rest through two DEXs, beating the best single-pool quote.