FrozenError: can't modify frozen String Ruby / Rails Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Ruby: FrozenError: can't modify frozen String: "abc"

You tried to change an object that's frozen — string literals under # frozen_string_literal: true, constants, symbols or frozen hashes.

Exact message you may see FrozenError: can't modify frozen String: "abc"FrozenError (can't modify frozen String)can't modify frozen Hashcan't modify frozen ArrayFrozenError: can't modify frozen object
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Seen on: Ruby

Meaning

Use non-mutating methods (+, dup, String.new, gsub instead of gsub!), or unfreeze with +"text"/dup. Rails freezes some options/params; copy with dup before changing. Ruby 3.4 warns about mutating string literals.

Common causes

  • # frozen_string_literal: true with << or gsub!
  • Modifying a constant (CONST << x)
  • Mutating a frozen hash (params.freeze)
  • Ruby 3.4 chilled strings
  • Gems returning frozen objects

⚡ Quick fix

  1. s = +"text" or s = "text".dup then mutate
  2. Prefer s += "x"
  3. Use String.new for buffers

Detailed fix by platform

Ruby

  1. buffer = String.new; buffer << "a"

How to diagnose

  1. Object — Frozen why?
  2. Method — Mutating bang?
  3. Fix — dup?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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