🧠 First: What is an immutable HTTP method?

In HTTP, a method is considered immutable (or safe) if:

• It does NOT change anything on the server.
• It’s supposed to be read-only (just retrieves data).

✅ Examples of immutable (safe) methods:

Method	Safe?	What it does
GET	✅ Yes	Retrieves a resource (no changes)
HEAD	✅ Yes	Same as GET but no response body
OPTIONS	✅ Yes	Asks what methods are supported
TRACE	✅ Yes	Echoes the request for diagnostics

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🎯 Now, answering your question:

Which HTTP method is not immutable?

👉 POST is not immutable!

✅ POST is intended to:

• Create new resources
• Modify server-side data
• Submit forms
• Trigger side-effects (e.g., send email, update a database)

Thus, POST changes server state —
→ so it’s not immutable (not safe).

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🚀 Other HTTP methods that are also not immutable:

Method	Mutability	Purpose
POST	Mutable	Create or process a resource
PUT	Mutable	Replace a resource
DELETE	Mutable	Remove a resource
PATCH	Mutable	Partially update a resource

All of these modify the server somehow.

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🛠️ Quick Summary:

HTTP Method	Immutable (Safe)?
GET	✅ Yes
HEAD	✅ Yes
OPTIONS	✅ Yes
TRACE	✅ Yes
POST	❌ No
PUT	❌ No
DELETE	❌ No
PATCH	❌ No

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⚡ Very simple memory trick:

If the method	Then
Only reads / asks	✅ Immutable (safe)
Creates / updates / deletes	❌ Not immutable (unsafe)