If I put a rule in the wrong place initially and later want to adjust it, is that a hassle?
Technically not too much of a hassle—moving content from one place to another is a simple operation. The real cost lies in "discovering it was misplaced" in the first place—if you don't realize the classification logic is off, you might keep accumulating more misplaced rules, and by the time you actually want to adjust things, you find the scope of content needing review has grown quite large.
A good practical habit is applying the "would this rule still apply in a different context" judgment rule right at the first setup, rather than waiting until rules have accumulated to a certain point and behavior inconsistency shows up before going back to troubleshoot—this avoids the extra cost of dealing with a problem only after it's accumulated over time.
If a rule fits both "fixed role behavior" and "project-specific context" at the same time, how do I judge which one it belongs to?
This situation usually means the rule can actually be split into two parts, each placed at its corresponding layer. For example, "answer in a professional but friendly tone" is a fixed role behavior pattern that belongs in System Prompt; "this project's readers are internal employees, so company-internal jargon can be used" is project-specific context that belongs in Project instructions. What looks like one rule on the surface is actually two things operating simultaneously once split apart.
If, after splitting, you find the two parts are genuinely tightly bound and can't be described separately, that situation is rarer, but if you do run into it, you can place it in whichever layer is more commonly used first (usually Project instructions, since it tends to change more frequently), then later consider extracting the common part into System Prompt if you find this role configuration is being reused across projects with increasing frequency.
Do everyday users on the web app or Desktop App also have these two layers, System Prompt and Project instructions?
Yes, just presented differently in the interface. The Custom Instructions everyday users set up through the Claude Projects feature correspond to the Project instructions layer covered in this article; as for System Prompt, everyday users usually don't directly see the technical term "System Prompt," but behind every product context, there's actually an underlying configuration determining Claude's basic behavior pattern running in the background—it's just that this layer is pre-configured for everyday users and isn't something they need to (or usually can) adjust themselves.
This means that for everyday users using Claude via the web app or Desktop App, the layer that actually needs hands-on setup, and where classification problems most commonly come up in practice, is mainly the Project instructions layer—the most practically useful part of the judgment logic covered in this article, for everyday users, is helping you decide "which Project's instructions should this information go into," rather than getting mixed up with a different Project.
When a team shares the same Project across multiple people, what should be watched for in maintaining Project instructions?
Since Project instructions are shared background information for this batch of conversations, the most important thing to watch for when multiple people share it is content timeliness—if the project's background information gets updated (like client requirements changing, or project scope being adjusted), Project instructions need to be updated in sync. Otherwise, different team members conversing within the same Project get outdated background context, potentially leading to different members receiving inconsistent answers, which might get mistaken for a problem with Claude itself rather than background information simply not being updated.
A better practice in practice is designating a fixed person on the team responsible for maintaining Project instruction updates, rather than letting each member individually judge whether to update it—which can lead to a situation where an update gets overwritten by someone else. This division of maintenance responsibility is just as important as the content classification logic covered earlier, but another aspect that often gets overlooked.
If you use Claude via the API or through Claude Projects, you'll quickly run into two places that both let you "pre-configure behavior rules": System Prompt and Custom Instructions inside a Project. The two sound similar—both set rules before a conversation begins—but in actual use they often get mixed together, leading to scattered configuration and difficulty later finding where a given rule actually lives when you want to adjust it. This article covers how to cleanly divide the two layers of configuration, rather than repeating each one's definition.
System Prompt is the role configuration sent along with every user message on every single API call—it determines "what role Claude should play in this conversation, and what fixed rules it should follow." This configuration is typically hardcoded logic for a specific application scenario—like "you are a customer support assistant, only answer product-related questions, politely decline anything out of scope." This kind of rule doesn't change based on who the user is or what the specific topic is; it's the underlying logic that always holds true for this application scenario.
Custom instructions inside a Project serve a completely different need: placing a set of related conversations inside the same container, so those conversations can all access shared background information. What goes here is typically project-specific context—like "this project's client is a company called X, their product line includes A, B, and C"—information that only makes sense within this specific Project, and doesn't apply at all if moved to a different one.
Confusion typically arises when a user puts the specific rule for "how this task should be done" into System Prompt by mistake (resulting in old rules still being applied even after switching application scenarios), or puts "the fixed behavior standard this role should always follow" into Project instructions by mistake (resulting in inconsistent role behavior across different Projects). The judgment rule is simple: would this rule still apply if moved to a completely different context or Project? If the answer is "this is the role's fixed behavior pattern, which should apply regardless of which conversation it's in," it belongs in System Prompt; if the answer is "this is background specific to this batch of conversations, and wouldn't apply to a different batch," it belongs in Project instructions.
In practice, the two layers are often used together: System Prompt defines the role's fixed behavior pattern (like "you are a professional technical writing assistant, answer using concise, clear language"), while Project instructions supplement background information specific to this particular project (like "this project's readers are a non-technical marketing team, avoid excessive jargon"). Stacked together, they form the complete behavior rules for this conversation—there's no need to force everything into a single layer.
If you're developing an application that needs to reuse the same role configuration repeatedly, correctly classifying fixed rules into System Prompt ensures role behavior stays consistent across different projects, without needing to re-configure it for every single Project. Correctly classifying project-specific background information into Project instructions avoids background material from different projects contaminating each other. Misclassification won't make the application completely stop working, but it accumulates over time into a lot of duplicated configuration maintenance cost and hard-to-diagnose behavioral inconsistencies.