feat(cli): route fern sdk preview through remote generation, add --local and --push-diff

[devin-ai-integration]

Description

Routes fern sdk preview through remote generation by default (matching fern generate), adds --local flag to opt into local Docker execution, and adds --push-diff flag for pushing preview branches to SDK repos.

Command	Generation	Publish
fern sdk preview	Remote	Preview registry
fern sdk preview --output ./path	Remote	Disk only (downloaded from S3)
fern sdk preview --local --output ./path --output https://registry	Local Docker	Disk + custom registry
fern sdk preview --push-diff	Remote	Preview registry + push preview branch
fern sdk preview --local	Local Docker	Preview registry
fern sdk preview --local --output ./path	Local Docker	Disk only

Key design: decoupled preview semantics

The CLI needs preview mode internally (lenient env var substitution, skip version check, skip dynamic IR upload), but sending preview: true to the generation service causes dry-run mode which prevents actual publishing.

Solution: New fiddlePreview parameter that overrides what's sent to the generation service as the preview field. For fern sdk preview, we send fiddlePreview: false so the service doesn't dry-run. For fern generate --preview, fiddlePreview is undefined and falls back to the existing behavior (absolutePathToPreview != null, which is true for preview — preserving dry-run exactly as-is).

This avoids any server-side dry-run logic changes, eliminating regression risk for fern generate --preview.

Changes Made

• cli.ts: Add --local and --push-diff boolean options. --local opts into local Docker generation; --push-diff cannot be combined with --local. Pass both to sdkPreview().
• sdkPreview.ts: Route based on --local flag (useRemoteGeneration = !local) instead of presence of --output. Pass absolutePathToPreview: absolutePathToOutput to remote generation so --output downloads files from S3. Validate --push-diff + --local incompatibility. Warn when --push-diff falls back due to missing github config.
• overrideOutputForPreview.ts: Consolidate local/remote output override into single function with optional pushDiff param. Extract createNpmOverrideOutputMode helper. When pushDiff=true and generator has github config, use githubV2(push) with npm publishInfo. Add unit tests for getGithubOwnerRepo, overrideGroupOutputForDownload, and overrideGroupOutputForPreview.
• createAndStartJob.ts: Add fiddlePreview and pushPreviewBranch params. Override preview field sent to the generation service via fiddlePreview ?? absolutePathToPreview != null. pushPreviewBranch is threaded but commented out until the server-side SDK is bumped.
• runRemoteGenerationForAPIWorkspace.ts / runRemoteGenerationForGenerator.ts: Thread fiddlePreview and pushPreviewBranch through the remote generation chain.
• versions.yml: Add 4.66.0 changelog entry.

Safety

• --push-diff is gated with an error until the server-side SDK is bumped — prevents sending a github push job without the preview branch flag, which would write to the default branch.
• fern generate --preview is completely unaffected — fiddlePreview is only set in the sdkPreview.ts code path.

Human review checklist

• Confirm RemoteTaskHandler correctly downloads generated files to absolutePathToOutput when set — verified end-to-end with fern sdk preview --output ./out.
• The --local flag is independent of --output: --local alone still publishes to the preview registry via Docker. Verify this matches product intent.
• fiddlePreview: false is the critical behavioral change — it prevents the generation service from setting dryRun=true. If the service's dry-run logic changes in the future, this coupling should be revisited.
• Multi-output (--output ./path --output https://registry) is fully stubbed and routed for both local and remote paths, but the custom-registry-URL flow through remote generation has not been verified end-to-end.

Testing

• fern sdk preview → package published to preview registry via remote (no dry-run)
• fern sdk preview --output ./out → remote generation, files downloaded to disk
• fern sdk preview --local → local Docker, package published to preview registry
• fern sdk preview --local --output ./out → local Docker, files on disk
• fern sdk preview --local --output ./out --output https://registry → local Docker, files on disk + custom registry
• fern sdk preview --push-diff → errors with "not yet available" (gated until server update)
• fern generate --preview → existing behavior preserved (dry-run)
• fern generate → completely unaffected

Link to Devin session: https://app.devin.ai/sessions/92d936472d7a4bf187ae6b52721f36fd

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review to trigger a review and subscribe this PR to future pushes, or @claude review once for a one-time review.

_{Tip: disable this comment in your organization's Code Review settings.}

[github-actions]

🌱 Seed Test Selector

Select languages to run seed tests for:

• Python
• TypeScript
• Java
• Go
• Ruby
• C#
• PHP
• Swift
• Rust
• OpenAPI

---

How to use: Click the ⋯ menu above → "Edit" → check the boxes you want → click "Update comment". Tests will run automatically and snapshots will be committed to this PR.