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How did the action of assumpsit make informal agreements enforceable?

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King Henry II and St. Thomas Becket — Common law Henry II (left) disputing with Thomas Becket (center), miniature from a 14th-century manuscript; in the British Library (Cotton MS. Claudius D.ii).

In the development of common law, the action of assumpsit was essential for making informal agreements enforceable because existing legal actions, such as debt and covenant, were limited to specific circumstances like fixed sums of money or promises made under seal.1

Assumpsit evolved from a 14th-century remedy for the negligent performance of an undertaking—such as damaging goods entrusted to another—into a broader contractual remedy through two key modifications:21

Shift in Focus: The legal emphasis moved from the defendant's negligent act to their failure to fulfill a specific promise.2
Expansion of Scope: The action became available as a remedy whenever a defendant either performed an act improperly or neglected to perform an act they had promised to do.2

By the mid-16th century, this allowed courts to address breaches of informal agreements. To prevent the scope of this remedy from becoming unlimited, courts introduced the doctrine of consideration, which requires that a promise generally must be supported by something of value that is bargained for to be legally binding.1