[wrangler] Add WRANGLER_COMMAND environment variable to custom build commands

[petebacondarwin]

Fixes #2634.

When using a custom build command in wrangler.toml, users can now detect whether wrangler dev or wrangler deploy triggered the build by reading the WRANGLER_COMMAND environment variable.

This variable will be set to "dev", "deploy", "versions upload", or "types" depending on which command invoked the build. This allows customizing the build process based on the deployment context.

Example usage in a build script:

if [ "$WRANGLER_COMMAND" = "dev" ]; then
  echo "Building for development..."
else
  echo "Building for production..."
fi

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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s): docs: document WRANGLER_COMMAND environment variable for custom builds cloudflare-docs#28207
  • Documentation not necessary because: The changeset includes documentation of the new feature, and a separate docs PR can be created to add this to the custom builds documentation page.

A picture of a cute animal (not mandatory, but encouraged)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 511eae8

The changes in this PR will be included in the next version bump.

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[claude]

Claude finished @petebacondarwin's task —— View job

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Changeset Review Complete

✅ All changesets look good

The changeset in .changeset/wrangler-command-env-var.md is well-formatted and follows all guidelines:

1. Version Type: Correctly marked as minor (new feature - new environment variable)
2. Changelog Quality: Clear, descriptive title and body with practical usage example
3. Markdown Headers: No h1/h2/h3 headers used ✓
4. Analytics: No analytics changes
5. Experimental features: Not applicable

The changeset provides excellent documentation including:

• Clear explanation of what the feature does
• Complete list of possible values ("dev", "deploy", "versions upload", "types")
• Practical bash script example showing how to use it

---

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commit: 511eae8

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.