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+---
+name: extension-to-functions-codebase
+description: Skill for converting a Firebase extension repository to a functions codebase
+---
+
+# Extension to Functions Codebase
+
+## Overview
+
+A user likes a Firebase Extension but it doesn't do exactly what they wanted. They
+want to convert the extension into a functions codebase that they can modify and
+deploy as their own functions. The only problem is that the extensions namespace
+doesn't express any of the Infrastructure as Code expected in extensions. This fixes that.
+
+## Triggers
+Activate this skill when a developer expresses that they wish an extension was
+a functions codebase instead of an extension.
+
+## Follow up
+If there are any tests in the extensions codebase, be sure to run them after the migration.
+
+## Rules and Constraints
+
+### Verify you are ready for the task
+If an extensions codebase has a feature that you do not know how to handle 
+yet, such as lifecycle hooks or specific IAM roles via `iamRoles` in
+`extension.yaml`, stop and tell the user that you cannot handle this task 
+yet.  Let them decide whether or not to continue.
+
+### Avoid contention
+When possible, avoid using the `firebase` cli, or firebase MCP server to avoid global contention. Try
+to inspect `extension.yaml` and code manually.
+
+## Getting started
+Make sure the git history is clean before proceeding because this skill uses commits.
+
+The user may do one of two things: ask you to convert extensions code in-place or ask you to creeate a new copy of the
+extensions code. If they do the former, and the destination is in the same directory as the source, use git cp of the
+code to the new location and commit with the message "Copying [extension] extension to [directory] in preparation for rewrite".
+
+## Steps
+
+### API Enablement
+For all API dependencies listed in `extension.yaml`, add a comment to index.js and inform the user in your final response to manually enable them.
+
+```typescript
+// APIs to enable: 
+// - vision.googleapis.com 
+```
+
+### Parameterization
+All config must be a parameter in the functions codebase. Read the list of all
+parameters in the extension's `extension.yaml` file and create a parameter for each
+one in the functions codebase. Be sure to keep all metadata such as label, description, type, and
+validation rules and error messages. If a parameter's type is `secret`, use `defineSecret('SECRET_NAME')` instead of `defineString`.
+All `process.env` calls must be instead replaced with the
+appropriate `param.value()` call. Be sure all `process.env` values referenced are defined as parameters;
+be encouraged to use built-in parameters though.
+
+Custom events should be listed as a `multiSelect` parameter with the label "Events to emit".
+The description should be "Select the events that this function should emit from the following list:"
+and then list events as options with `*[type]*: [description]\n`. The event type should be the value
+in the `multiSelect` input list.
+
+`params` must not be called with `.value()` at global scope. If a global is being initialized with a
+parameter, use the `onInit` function to initialize the global. For example:
+
+```typescript
+const myFoo = new Foo(process.env.FOO);
+```
+
+should be turned into
+
+```typescript
+const foo = defineString('FOO', { /* descriptions */ });
+let myFoo: Foo;
+onInit(() => {
+  myFoo = new Foo(foo.value());
+});
+```
+
+Never ever ever export an extension parameter's value directly or even an accessor, even through a function. This allows you to use
+the parameter without `.value()` as a functions configuration parameter in the later step. On the other hand, within a function, you may call `.value()` on the parameter if you need to actually use the value of the parameter.
+
+### Engine pinning
+In `extension.yaml` there will be a line that looks like this:
+
+```yaml
+runtime: nodejs20
+```
+
+This means that the extension is pinned to a specific runtime. If all functions
+do not have the same runtime, stop and tell the user that mixed runtimes are
+not yet supported.
+
+Learn the runtime and use that to update the customer's package.json to list the
+node engine as the runtime version.
+
+### Switching SDK versions
+For all exported functions, replace the import with "functions.extensions.foo" to just
+"functions.foo". Use the other builder functions necessary to reach the same function
+callback. Where those builder functions expect or allow a parameter, use the named functions
+parameter for the configuration.
+
+### Wrapping up
+If the destination directory looks like a firebase project (e.g. has a `firebase.json`) Offer
+to add the functions codebase to `firebase.json` for the user so that it will be included
+in subsequent deploys. If the user agrees, add the functions codebase to `firebase.json`.
+
+### Testing
+If there are any tests in the extensions codebase, be sure to run them after
+the migration. This may require modifying the test as well to point to the functions codebase.
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