diff --git a/.github/workflows/_build.yml b/.github/workflows/_build.yml
index 34e8d0f29ed..f6965e026f8 100644
--- a/.github/workflows/_build.yml
+++ b/.github/workflows/_build.yml
@@ -40,6 +40,14 @@ on:
         type: number
         default: 3
 
+      # Custom environment variables to inject into the jobs.
+      # Expected to be a JSON-formatted string.
+      # Example: '{"FIREBASE_APP_CHECK_BRANCH": "nc/target-split"}'
+      env_vars:
+        type: string
+        required: false
+        default: "{}"
+
     # IMPORTANT: When adding new secrets to this workflow, update the
     # 'has_secrets' logic in the 'check_secrets' job to include the new secret.
     secrets:
@@ -96,6 +104,24 @@ jobs:
     - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
       with:
         persist-credentials: false
+    - name: Set Custom Environment Variables
+      run: |
+        python3 -c '
+        import os, json
+        try:
+            env_vars = json.loads(os.environ.get("CUSTOM_ENV_VARS", "{}"))
+            if not isinstance(env_vars, dict):
+                raise ValueError("env_vars must be a JSON object")
+            with open(os.environ["GITHUB_ENV"], "a") as file:
+                for key, value in env_vars.items():
+                    file.write(f"{key}={value}\n")
+        except json.JSONDecodeError:
+            print("Warning: env_vars is not valid JSON. Skipping.")
+        except Exception as error:
+            print(f"Error setting env vars: {error}")
+        '
+      env:
+        CUSTOM_ENV_VARS: ${{ inputs.env_vars }}
     - uses: ruby/setup-ruby@354a1ad156761f5ee2b7b13fa8e09943a5e8d252 # v1
       if: inputs.method != 'spm' && inputs.method != 'spmbuildonly' && inputs.method != 'cmake'
     - name: Setup Bundler