How Claude Project Instructions Are Injected — And Why Changing Them Mid-Conversation Breaks History

A Reddit user (OHOLshoukanjuu) dug into how Claude's Project Instructions actually work by asking Claude to print its full system prompt verbatim in both a project and non-project conversation, then diffing the two dumps. Here's what they found.

Key Finding: Single Injection at Start

Project Instructions (and User Preferences) are not re-injected every turn. They get loaded into the system prompt at conversation start and stay in context from there. This means Claude only sees them once, at the beginning.

The Mid-Conversation Change Bug

If you change the project instructions mid-conversation, Claude does not know you changed them. It reads the updated version as if that was the original instruction from the very first message. This leads to two weird behaviors:

• Immediate obedience: If your instructions say "start every response with HELP I'M A BUG" and you get one response following that, then you change to "start every response with HELLO WORLD," the next response says HELLO WORLD.
• False memory: If you ask Claude what the project instructions were for the first turn, it says HELLO WORLD. It will actually conclude it made an error on the first response by not following the instructions it now sees.

No Explicit Label

Project instructions aren't labeled as "project instructions" anywhere in the prompt. Claude follows them, but if you ask "what are the project instructions?" it may tell you there aren't any — because nothing in its context is tagged that way.

How It Was Discovered

The user (on iOS, Max subscriber since 2023, self-described non-developer) asked Claude to print its full system prompt verbatim in both a project conversation and a non-project conversation. By diffing the two dumps and watching Claude's thought process while testing changes, they confirmed the single-injection behavior.

This means: if you rely on project instructions evolving over the course of a long conversation, Claude will rewrite its past understanding to match the latest version of the instructions. The original context is lost.