# Keywords ### block-height {% hint style="danger" %} Deprecated in Clarity 3. {% endhint %} Introduced in: Clarity 1 output: `uint` description: Returns the current block height of the Stacks blockchain in Clarity 1 and 2. Upon activation of epoch 3.0, `block-height` will return the same value as `tenure-height`. In Clarity 3, `block-height` is removed and has been replaced with `stacks-block-height`. example: ```clarity (> block-height u1000) ;; returns true if the current block-height has passed 1000 blocks. ``` *** ### burn-block-height {% hint style="danger" %} There is a bug in Clarity 3 when `burn-block-height` is used within an `at-block` expression. Normally, keywords executed within an `at-block` expression will return the data for that specified block. This bug causes `burn-block-height` always to return the burn block at the current chain tip, even within an `at-block` expression. This behavior affects any Clarity 3 contracts and will be fixed in the upcoming Clarity 5 version. {% endhint %} Introduced in: Clarity 1 output: `uint` description: Returns the current block height of the underlying burn blockchain as a uint example: ```clarity (> burn-block-height u1000) ;; returns true if the current height of the underlying burn blockchain has passed 1000 blocks. ``` *** ### chain-id Introduced in: Clarity 2 output: `uint` description: Returns the 32-bit chain ID of the blockchain running this transaction example: ```clarity (print chain-id) ;; Will print 'u1' if the code is running on mainnet, and 'u2147483648' on testnet, and other values on different chains. ``` *** ### contract-caller Introduced in: Clarity 1 output: `principal` description: Returns the caller of the current contract context. If this contract is the first one called by a signed transaction, the caller will be equal to the signing principal. If `contract-call?` was used to invoke a function from a new contract, `contract-caller` changes to the *calling* contract's principal. If `as-contract` is used to change the `tx-sender` context, `contract-caller` *also* changes to the same contract principal. example: ```clarity (print contract-caller) ;; Will print out a Stacks address of the transaction sender ``` {% hint style="warning" %} Use caution when leveraging all contract calls, particularly tx-sender and contract-caller as based on the design, you can unintentionally introduce attack surface area. [Read more](https://www.setzeus.com/community-blog-posts/clarity-carefully-tx-sender). {% endhint %} *** ### current-contract Introduced in: Clarity 4 output: `principal` description: Returns the principal of the current contract. example: ``` (stx-transfer? u1000000 tx-sender current-contract) ``` *** ### false Introduced in: Clarity 1 output: `bool` description: Boolean false constant. example: ```clarity (and true false) ;; Evaluates to false (or false true) ;; Evaluates to true ``` *** ### is-in-mainnet Introduced in: Clarity 2 output: `bool` description: Returns a boolean indicating whether or not the code is running on the mainnet example: ```clarity (print is-in-mainnet) ;; Will print 'true' if the code is running on the mainnet ``` *** ### is-in-regtest Introduced in: Clarity 1 output: `bool` description: Returns whether or not the code is running in a regression test example: ```clarity (print is-in-regtest) ;; Will print 'true' if the code is running in a regression test ``` *** ### none Introduced in: Clarity 1 output: `(optional ?)` description: Represents the *none* option indicating no value for a given optional (analogous to a null value). example: ```clarity (define-public (only-if-positive (a int)) (if (> a 0) (some a) none)) (only-if-positive 4) ;; Returns (some 4) (only-if-positive (- 3)) ;; Returns none ``` ```clarity (print stx-liquid-supply) ;; Will print out the total number of liqui ``` *** ### stacks-block-height Introduced in: Clarity 3 output: `uint` description: Returns the current Stacks block height. example: ```clarity (print stacks-block-height) ;; Will print out the current Stacks block height ``` *** ### stacks-block-time Introduced in: Clarity 4 output: `uint` description: Returns the timestamp of the current block in seconds since the Unix epoch {% hint style="info" %} This same timestamp can also be retrieved for previous blocks using `(get-stacks-block-info? time height)`, which exists since Clarity 3, but cannot be used for the current block. Note that `stacks-block-time` will properly account for the context of an `at-block` expression. If the `at-block` sets the context to a block that is from before Clarity 4 has activated, attempting to use `stacks-block-time` in that context will result in a runtime error. {% endhint %} ``` (if (> stacks-block-time 1755820800) (print "after 2025-07-22") (print "before 2025-07-22")) ``` *** ### stx-liquid-supply Introduced in: Clarity 1 output: `uint` description: Returns the total number of micro-STX (uSTX) that are liquid in the system as of this block. example: ```clarity (print stx-liquid-supply) ;; Will print out the total number of liquid uSTX ``` *** ### tenure-height Introduced in: Clarity 3 output: `uint` description: Returns the number of tenures that have passed. When the Nakamoto block-processing starts, this will be equal to the chain length. example: ```clarity (print tenure-height) ;; Will print out the current tenure height ``` *** ### true Introduced in: Clarity 1 output: `bool` description: Boolean true constant. example: ```clarity (and true false) ;; Evaluates to false (or false true) ;; Evaluates to true ``` *** ### tx-sender Introduced in: Clarity 1 output: `principal` description: Returns the original sender of the current transaction, or if `as-contract` was called to modify the sending context, it returns that contract principal. example: ```clarity (print tx-sender) ;; Will print out a Stacks address of the transaction sender ``` {% hint style="warning" %} Use caution when leveraging all contract calls, particularly tx-sender and contract-caller as based on the design, you can unintentionally introduce attack surface area. [Read more](https://www.setzeus.com/community-blog-posts/clarity-carefully-tx-sender). {% endhint %} *** ### tx-sponsor? Introduced in: Clarity 2 output: `optional principal` description: Returns the sponsor of the current transaction if there is a sponsor, otherwise returns None. example: ```clarity (print tx-sponsor?) ;; Will print out an optional value containing the Stacks address of the transaction sponsor ```