OP_DEPTH - Counting Stack Items

13. OP_DEPTH — Counting Stack Items

Overview

OP_DEPTH pushes the current number of items on the stack onto the top of the stack as a numeric value. It is an introspective opcode — it lets the script itself examine how many items are currently on the stack, which can then be used in comparisons or loop constructs.

Opcode Reference

Opcode:     OP_DEPTH
Hex:        0x74
Word:       DEPTH
Input:      (nothing consumed)
Output:     <stack_depth>
Stack Before: [ x1, x2, ..., xN ]
Stack After:  [ x1, x2, ..., xN, N ]

Execution Trace

Script: OP_1 OP_2 OP_3 OP_DEPTH

After pushes: Stack: [ 1, 2, 3 ]
After OP_DEPTH: Stack: [ 1, 2, 3, 3 ]   ← depth (3) pushed to top

Counting After OP_DEPTH

Note that OP_DEPTH does not count itself — it pushes the depth before its own execution:

Empty stack:
OP_DEPTH → Stack: [ 0 ]

Stack with 5 items:
OP_DEPTH → Stack: [ x1, x2, x3, x4, x5, 5 ]

Practical Use Cases

1. Input Validation:

Scripts can use OP_DEPTH to verify that exactly the right number of items were provided:

OP_DEPTH OP_3 OP_EQUALVERIFY

This asserts that there are exactly 3 items on the stack before proceeding.

2. Variable-Argument Scripts:

Some advanced scripts — particularly those built for Simplicity or Script extensions — use OP_DEPTH to handle variable numbers of arguments:

OP_DEPTH OP_0 OP_GREATERTHAN
OP_IF
    <process first item>
OP_ENDIF

3. Debugging and Testing:

During development and testing of Bitcoin scripts, OP_DEPTH is used to assert stack state at specific execution points, catching incorrect operand counts early.

Security Implications

OP_DEPTH can be used to craft scripts that behave differently based on how many arguments were provided. Script authors must ensure that OP_DEPTH-based branching does not inadvertently enable bypass conditions:

OP_DEPTH OP_2 OP_EQUAL
OP_IF
    <secure path>   ← executes when exactly 2 items on stack
OP_ELSE
    OP_1            ← DANGER: always returns TRUE with 0 or 1 items!
OP_ENDIF

The else branch must be carefully written to avoid unconditional success.

Failure Conditions

OP_DEPTH itself never fails — even on an empty stack, it pushes 0. However, scripts that subsequently use the depth value in comparisons may behave unexpectedly if assumptions about stack depth are wrong.

Summary

OP_DEPTH is a powerful introspection tool that exposes the current stack size to the script. It enables dynamic, depth-aware script logic but must be used carefully to avoid security vulnerabilities in conditional branches.