parameterized config mismatch in Firebase Functions v4 between emulator and production deployment

Parameter presence enforcement

Confirmed: firebase deploy runs a pre-deploy validation that checks every non-optional parameter declared with defineString, defineInt or defineSecret against functions:config:get and Cloud Secret Manager bindings. If a required parameter is missing, the deploy exits before upload. The emulator skips this check; it only loads values from .env, .env.local or .env.secret.local and starts the runtime. Likely explanation: the emulator is designed for speed, not parity, so remote config and IAM checks are omitted.

TypeScript return types

Confirmed: params.MY_VAR.value() returns string | undefined in the emulator when the key is absent from .env. In production, a required parameter that is unset causes the SDK to throw on first access. To keep a consistent signature, declare optional parameters with defineString('MY_VAR', {optional:true}) so both environments return string | undefined.

Migrate functions.config() without branching

1. Create a single params.ts exported at repo root and import it in each function.
2. Replace functions.config().my_var with params.MY_VAR.value() and add the matching define* declaration at top level.
3. Do not add process.env.FUNCTIONS_EMULATOR branching; the params object resolves from local .env in emulator and from remote config/secrets in production once declared.

Monorepo synchronization pattern

Confirmed: there is no built-in CLI diff. Recommended discipline: check in .env.example with keys only, keep all define* calls in the shared params.ts, and add a CI step that runs firebase functions:config:export --project prod and compares keys against params declarations. For secrets, create them in Secret Manager and grant the Cloud Functions service account roles/secretmanager.secretAccessor; the emulator reads .env.secret.local but does not validate IAM.

Verification

1. firebase deploy --dry-run to trigger validation without upload.
2. firebase functions:config:get --project prod to list set parameters.

One diagnostic detail that changes recommendation: which firebase-tools version your CI uses. Pre-deploy validation was strengthened in v13.x and later; older versions may allow deploys that later fail at runtime.