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feat(cli): DO NOT MERGE cdk synth --watch re-synthesizes on file changes - #1851

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feat(cli): DO NOT MERGE cdk synth --watch re-synthesizes on file changes#1851
iankhou wants to merge 1 commit into
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worktree-synth-watch-flag

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@iankhou

@iankhou iankhou commented Aug 19, 2026

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Do not merge. Waiting on consolidation of CLI and Toolkit Lib synth implementations, which are still separate now.

Description

Adds a --watch flag to cdk synth, allowing developers to synth on file changes.

Very similar to cdk validate --watch (which also synths on file changes, but 1. returns the validation report, 2. performs online validation by default)

CLI synth and Toolkit synth do not currently follow the same code path, but implement the same behavior (migration needed).

  • toolkit-lib: new public Toolkit.watchSynth() and WatchSynthOptions, built on the existing generic _watch loop (chokidar, change batching, error resilience, dispose/waitForEnd). The body of Toolkit.synth is extracted into a private _synth(assembly, options) so both can share it, matching the existing validate/_validate split.

  • CLI: new watch option on the synth command, regenerated parser files, and a CdkToolkit.synthWatch() that requires the cdk.json "watch" key (WatchConfigMissing otherwise) and passes an uncached assembly source so every iteration re-synthesizes.

Testing

Unit tests
Integ tests pending

Checklist

  • This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
    • Release notes for the new version:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

Adds a --watch flag to `cdk synth` that continuously observes the project
files (configured via the "watch" key of cdk.json, like `cdk deploy --watch`)
and re-synthesizes the selected stacks whenever a change is detected. Never
deploys, and never prints templates to stdout.

Mirrors the `cdk validate --watch` precedent end to end:

- toolkit-lib: new public `Toolkit.watchSynth()` and `WatchSynthOptions`,
  built on the existing generic `_watch` loop (chokidar, change batching,
  error resilience, dispose/waitForEnd). The body of `Toolkit.synth` is
  extracted into a private `_synth(assembly, options)` so both share it,
  matching the existing `validate`/`_validate` split.
- CLI: new `watch` option on the synth command, regenerated parser files,
  and a `CdkToolkit.synthWatch()` that requires the cdk.json "watch" key
  (WatchConfigMissing otherwise) and passes an uncached assembly source so
  every iteration re-synthesizes.
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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

@iankhou
iankhou deployed to no-approval August 19, 2026 15:08 — with GitHub Actions Active
@aws-cdk-automation
aws-cdk-automation requested a review from a team August 19, 2026 15:08
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Codecov Report

❌ Patch coverage is 88.67925% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.91%. Comparing base (88f6ad1) to head (d8708cb).
⚠️ Report is 13 commits behind head on main.

Files with missing lines Patch % Lines
packages/aws-cdk/lib/cli/cli.ts 0.00% 6 Missing ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##             main    #1851      +/-   ##
==========================================
+ Coverage   90.31%   90.91%   +0.59%     
==========================================
  Files          80       80              
  Lines       12159    12302     +143     
  Branches     1725     1759      +34     
==========================================
+ Hits        10982    11184     +202     
+ Misses       1143     1081      -62     
- Partials       34       37       +3     
Flag Coverage Δ
suite.unit 90.91% <88.67%> (+0.59%) ⬆️

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@iankhou iankhou changed the title feat(cli): cdk synth --watch re-synthesizes on file changes feat(cli): DO NOT MERGE cdk synth --watch re-synthesizes on file changes Aug 20, 2026
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2 participants