A large repository gives the agent too many reasons to be distracted: generated artifacts, data snapshots, test coverage - all of it inflates the index and mixes into context without helping the task. The tool to remove this is an ignore file, preferably .devinignore; on the corresponding surfaces the legacy files .codeiumignore and .windsurfignore are also recognized, and by default gitignored paths, node_modules and hidden directories are excluded from the index anyway.
The naive move is born from the word "ignore" itself: since the agent ignores this, you can hide secrets and sensitive data there - .env, keys, prod dumps - and they are safe. The logic is seductive in its simplicity: one line in a file seemingly replaces a permission setting. It is exactly this substitution that is dangerous.
It breaks on the fact that ignore is about noise, not about security. It reduces the index and lowers the chance of a file accidentally landing in context, but it is not a universal security boundary: the actual read and write behavior depends on the agent and its settings, not on the presence of a path in .devinignore. An ignore file governs attention, not permissions.
The difference shows already in documented details. For instance, the agent cannot edit files from .gitignore - this is a functional indexing constraint, not a guarantee that the path is unreachable in every scenario. Ignore tells the engine "do not keep this in the index"; it does not tell the operating system "do not let the process read this". Mixing the two levels is exactly what it means to pretend a safe where there is only a sign.
The professional technique is to split two tasks across different mechanisms. For reducing noise and speeding up the index - .devinignore with explicit patterns. For real protection - true boundaries: for Devin Local these are permission rules with an explicit deny and a system sandbox that limits what the process can read at all and where it can connect. For secrets - not an ignore file and not file trees but a secret manager, so that the values do not lie in the repository in the first place.
The file itself, meanwhile, stays useful and is worth keeping carefully. A small .devinignore that removes environments, secret directories, coverage, the build and fixtures with production data from the index lowers both noise and accidental context at once. Such a file reads in seconds and travels between projects together with the repository.
Enterprise can set ignore globally: a .codeiumignore file in ~/.codeium/ applies to all Devin Desktop workspaces on the machine. This is convenient for a single policy of "never index this", but here too it is important to remember its nature: a global ignore is wider in coverage but just as much not a security boundary - it merely spreads the rule about attention to all projects at once.
Why the tool is deliberately not made a security boundary. Ignore is a hint to the indexer, cheap and portable, living in the repository next to the code. Real isolation costs more and lives on a different level - in the agent's permissions and in the OS sandbox. Reducing protection to a line in a text file would give a false sense of safety that is more dangerous than its honest absence: a visible sign makes people stop fitting a real lock.
The cost of the substitution is concrete. A secret "hidden" in .devinignore still lies in the working tree in plain text; another access scenario, another surface or another setting is enough - and it is read. Worse, an entry in the ignore file dulls vigilance: since it is "hidden", no one recalls the real deny and the secret manager anymore. The radius of that mistake is a leak, not an extra line in the index.
The vendor's estimate of the index's cost should be kept as a guide, not a guarantee. At the snapshot the documentation names about 5-10 minutes of indexing and around 300 MB of RAM per 5000 files and recommends staying within 10,000 files on a machine with roughly 10 GB of RAM. The numbers depend on the machine and the project, so the .devinignore helps performance here too: the less excess in the index, the closer you are to the comfortable range.
The result must be checked along two different lines, and they must not be confused. For noise and speed - that the index shrank, the excess does not mix into context, and indexing fits the guide. For security - that secrets are protected not by ignore but by deny rules, a sandbox and a secret manager; the check here is to try to make sure that even without an entry in the ignore file the agent has no permission to read the secret. The typical failure is one: a line in .devinignore is taken for a lock. The sign is the phrase "it's in the ignore file" instead of a reference to a permission rule or a sandbox. Name the real boundary first - and if there is none, no ignore file will replace it.
# .devinignore
.env*
secrets/**
coverage/**
dist/**
fixtures/production-data/**
*.pem
*.p12