fix: register platform deprecation flags in serve.py lifespan

[neelasha-writer]

Problem

The deprecation banner in BuilderApp.vue (introduced in PR #1288) checks:

wf.featureFlags.value.includes("beforeDeprecationCutoffAbv2")

However, writer.Config.feature_flags defaults to an empty list in core.py, and PR #1288 didn't add the deprecation flag keys there. This means the LaunchDarkly client never registers those flags for evaluation, so featureFlags.value in the Vue frontend never includes them — the banner never shows.

Fix

After app_runner.load() in the lifespan context manager in serve.py, we now merge the two platform-level deprecation flags into writer.Config.feature_flags:

_PLATFORM_FLAGS = {"beforeDeprecationCutoffAbv2", "afterDeprecationCutoffAbv2"}
writer.Config.feature_flags = list(set(writer.Config.feature_flags) | _PLATFORM_FLAGS)

This runs after the user's main.py executes (so user-defined flags are preserved), and ensures the LaunchDarkly client evaluates the deprecation flags for every agent builder session.

Out of scope

• The x-is-org-admin header injection still needs a platform/proxy configuration change to correctly populate wf.isOrganizationAdmin.value in the frontend. That is a separate infrastructure task.

Made with Cursor

Summary by CodeRabbit

• Backend / Chores
  • Platform-level deprecation feature flags are now automatically registered during app startup, merged with any application-defined flags, and deduplicated.
• UI/UX
  • Removed deprecation overlay elements from the Agent Builder interface.
  • Updated post-cutoff messaging for non-admin users to guide them to migrate their agent to the new Agent Builder with their Admin.
  • Improved banner text layout responsiveness by adjusting its maximum width.
• Tests
  • Updated feature flag assertions to treat flags as an unordered set and expanded expected flags.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: eaada1c2-2a15-4659-a78c-09db171f4319

📥 Commits

Reviewing files that changed from the base of the PR and between 6bf8969 and 9e52cee.

📒 Files selected for processing (1)

• tests/backend/test_serve.py

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📝 Walkthrough

Walkthrough

During FastAPI startup, serve.py adds two platform deprecation feature flags into writer.Config.feature_flags. The builder UI removes its deprecation overlay elements and updates the non-admin deprecation banner copy and width.

Changes

LaunchDarkly deprecation flag registration

Layer / File(s)	Summary
Platform deprecation flag injection at startup src/writer/serve.py, tests/backend/test_serve.py	Adds a top-level writer module import and, during ASGI lifespan startup, merges two platform deprecation feature flag keys into writer.Config.feature_flags with deduplication; the feature-flag test now expects the expanded unordered set.

Builder deprecation UI updates

Layer / File(s)	Summary
Builder overlay removal src/ui/src/builder/BuilderApp.vue	Removes the conditional deprecation overlay elements from the template and deletes the scoped CSS rules for the overlay and blocker classes.
Deprecation banner copy src/ui/src/builder/BuilderDeprecationBanner.vue	Updates the non-admin post-cutoff banner body text to direct users to work with their Admin to migrate to the new Agent Builder and changes the copy width constraint to a responsive value.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• writer/writer-framework#1288: Directly overlaps with the builder deprecation UI changes in BuilderApp.vue and BuilderDeprecationBanner.vue.

Suggested reviewers

• yavorsky

Poem

🐇 The flags hopped in at startup light,
The builder shed its dimmer night.
A banner changed its words to say,
“Ask your Admin to lead the way.”
The copy stretched with room to spare,
While bunny ears bounced everywhere.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: registering platform deprecation flags during serve.py lifespan startup.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-ACTION-5011-register-deprecation-feature-flags

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