Compare two files with Ansible

Why not using stat to compare the two files? Just a simple example:

- name: Get cksum of my First file
  stat:
    path : "/poc/files/{{ inventory_hostname }}.result"
  register: myfirstfile

- name: Current SHA1
  set_fact:
    mf1sha1: "{{ myfirstfile.stat.checksum }}"

- name: Get cksum of my Second File (If needed you can jump this)
  stat:
    path : "/poc/files/transport.results"
  register: mysecondfile

- name: Current SHA1
  set_fact:
    mf2sha1: "{{ mysecondfile.stat.checksum }}"

- name: Compilation Changed
  debug:
    msg: "File Compare"
  failed_when:  mf2sha1 != mf1sha1

your "diff" task is missing the shell keyword, Ansible thinks you want to use the diff module instead.

also i think diff (as name of the variable to register the tasks result) leads ansible to confusion, change to diff_result or something.

code (example):

  tasks:
  - local_action: shell diff /etc/hosts /etc/fstab
    failed_when: "diff_output.rc > 1"
    register: diff_output

  - debug:
      var: diff_output

hope it helps