Diagnostic records can contain sensitive values several levels below the top of the object. Implement redaction so matching fields are masked throughout the structure without mutating the input.
Build the implementation in Ruby under /app, with /app/run.sh as its entry point. Read one JSON request from standard input and write one JSON result to standard output. Input is any JSON value. Recursively replace values of object keys whose lowercase name is exactly password, secret, token or api_key with the string [REDACTED]. Preserve original keys, arrays, scalar values and nonmatching fields. Match the key regardless of the value type; do not inspect or rewrite ordinary string contents.
Reject inputs that violate an explicit validation rule with exit status 2, no standard output, and {"error":"invalid_input"} on standard error. For valid input, exit with status 0. Keep the implementation reusable for new inputs; it must work offline and must not depend on the example data being the only data it receives.
Repair or replace the unfinished /app/run.sh and add source files under /app. Malformed JSON must use the same invalid-input response. Do not read /tests or /oracle from the application.