feat: add slack manifest diff command

[mwbrooks]

Changelog

Added the slack manifest diff command, which prints differences between the project manifest and the app settings.

Summary

This pull request adds a read-only slack manifest diff command that compares the project manifest against the app settings on Slack and prints any differences.

• Useful for CI/CD guardrails, pre-check before deploys, and debugging manifest conflicts.
• Read-only - no API mutations, no file writes.
• 91.5% test coverage of internal/manifest.
• Carved out from PR feat: add two-way manifest sync between project and app settings #543 to make that PR easier to review. The two-way slack manifest sync command will build on these primitives.
• Ships without an experiment gate because it's a stand-alone read-only command. Happy to gate it, if reviewers want.
• Exits 0 even when differences are found, to match slack manifest validate (which exits 0 even when the manifest is invalid). Standardizing exit-code semantics for scripting use-cases is a separate, project-wide conversation that should change validate and diff together.

Preview

2026-06-15-manifest-diff.mov

Testing

Testing outside of a project will error:

$ ./bin/slack manifest diff

🚫 This is an invalid Slack app project directory (invalid_app_directory)

Setup a new a project:

$ ./bin/slack create my-app --template slack-samples/bolt-js-starter-template
$ cd my-app/

Testing inside of a project with no App IDs will error:

$ ../bin/slack manifest diff

🚫 A valid installation of this app is required to take this action (installation_required)

Setup a new App ID for testing:

$ ../bin/slack install -E local

Testing a new install should return no differences:

$ ../bin/slack manifest diff

📚 App Manifest
   Project manifest and app settings match

Testing different values for project and app settings:

$ vim manifest.json
# Set "features.bot_user.always_online" to false

$ ../bin/slack manifest diff

📚 App Manifest
   Found 1 difference between project and app settings

  features.bot_user.always_online
    Project:      false
    App settings: true

Testing project missing a value:

$ vim manifest.json
# Delete the line `"home_tab_enabled": true,`

$ ../bin/slack manifest diff

📚 App Manifest
   Found 2 differences between project and app settings

  features.app_home.home_tab_enabled
    Project:      (not set)
    App settings: true

  features.bot_user.always_online
    Project:      false
    App settings: true

Testing app settings missing a value:

$ vim manifest.json
# Add the following line to the top-level of the manifest object:
# 
# "outgoing_domains": ["example.com"],

$ ../bin/slack manifest diff

📚 App Manifest
   Found 3 differences between project and app settings

  features.app_home.home_tab_enabled
    Project:      (not set)
    App settings: true

  features.bot_user.always_online
    Project:      false
    App settings: true

  outgoing_domains
    Project:      ["example.com"]
    App settings: (not set)

Clean up:

$ lack delete -f
$ cd ../
$ rm -rf my-app/

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 86.23853% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.86%. Comparing base (d7dfda7) to head (2093f53).

Files with missing lines	Patch %	Lines
internal/manifest/diff.go	83.50%	8 Missing and 8 partials ⚠️
cmd/manifest/diff.go	79.48%	4 Missing and 4 partials ⚠️
internal/manifest/flatten.go	86.66%	2 Missing and 2 partials ⚠️
internal/manifest/diff_display.go	96.07%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
+ Coverage   71.67%   71.86%   +0.18%     
==========================================
  Files         226      230       +4     
  Lines       19176    19394     +218     
==========================================
+ Hits        13744    13937     +193     
- Misses       4221     4233      +12     
- Partials     1211     1224      +13     

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