feat: add blocks preview command to open blocks in Block Kit Builder

[WilliamBergamin]

Changelog

Add the slack blocks preview command to open Block Kit blocks in the Block Kit Builder.

Summary

This adds a new blocks preview command. It takes a set of Block Kit blocks and opens them in the Block Kit Builder in a web browser.

Blocks are passed with the --blocks flag. The value is a JSON array of blocks or a JSON object with a blocks array. Pass - to read the blocks from standard input.

The command is hidden from the help output unless an AI coding tool is detected. It is meant to be used by AI tools that generate Block Kit layouts.

Preview

(No UI change. The command prints the Builder URL and opens it in the browser.)

Testing

Build the CLI first:

make build

Then run the examples below with SLACK_DISABLE_TELEMETRY=true.

Expected to work:

1. Preview a bare blocks array passed as a flag value.

   ./bin/slack blocks preview --blocks '[{"type":"divider"}]'
   

2. Preview a full blocks object passed as a flag value.

   ./bin/slack blocks preview --blocks '{"blocks":[{"type":"section","text":{"type":"mrkdwn","text":"Hello"}}]}'
   

3. Preview blocks read from a file on standard input.

   echo '[{"type":"divider"}]' > blocks.json
   ./bin/slack blocks preview --blocks - < blocks.json
   

Expected to error:

4. Pass a value that is not valid JSON. This returns an unable_to_parse_json error.

   ./bin/slack blocks preview --blocks 'not json'
   

5. Pass valid JSON that is not a blocks payload. This returns an invalid_blocks error.

   ./bin/slack blocks preview --blocks '{"foo":"bar"}'
   

Unit tests:

make test testdir=cmd/blocks

Notes

• The command is gated on AI-tool detection, so it stays hidden from the normal help output and docs.
• The URL path segment /block-kit-builder/{id}/builder was verified in an authenticated browser.

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 89.33333% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.91%. Comparing base (2592815) to head (006f027).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/blocks/preview.go	88.80%	7 Missing and 7 partials ⚠️
internal/slackdeps/os.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
+ Coverage   71.78%   71.91%   +0.13%     
==========================================
  Files         227      229       +2     
  Lines       19283    19433     +150     
==========================================
+ Hits        13842    13976     +134     
- Misses       4225     4234       +9     
- Partials     1216     1223       +7     

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