fix(ups): formalize subjects

[MasterPtato]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[MasterPtato]

This change is part of the following stack:

• chore: deploy patch #4324
  • chore: flatten runtime config #4326
    • fix(api): make actor destroy wait for ack #4327
      • fix(udb): add txn tagging #4328
        • chore: bump rivetkit protocol version #4329
          • fix(ups): formalize subjects #4334 ◀
            • fix(api): properly dedup name list responses #4337
              • fix(pb): clear actor reschedule ts on alloc #4338
            • fix(ups): add metrics #4368

_{Change managed by git-spice.}

[railway-app]

🚅 Deployed to the rivet-pr-4334 environment in rivet-frontend

Service	Status	Web	Updated (UTC)
website	😴 Sleeping (View Logs)	Web	Mar 3, 2026 at 11:44 pm
frontend-inspector	😴 Sleeping (View Logs)	Web	Mar 3, 2026 at 6:15 pm
frontend-cloud	❌ Build Failed (View Logs)	Web	Mar 2, 2026 at 8:42 pm
mcp-hub	✅ Success (View Logs)	Web	Mar 2, 2026 at 8:39 pm
ladle	❌ Build Failed (View Logs)	Web	Mar 2, 2026 at 8:38 pm

[pkg-pr-new]

More templates

• actor-actions
• actor-actions-vercel
• ai-agent
• ai-agent-vercel
• example-ai-and-user-generated-actors-freestyle
• chat-room
• chat-room-vercel
• example-cloudflare-workers
• example-cloudflare-workers-hono
• collaborative-document
• collaborative-document-vercel
• cross-actor-actions
• cross-actor-actions-vercel
• example-cursors
• example-cursors-raw-websocket
• example-cursors-raw-websocket-vercel
• example-cursors-vercel
• custom-serverless
• example-elysia
• example-experimental-durable-streams-ai-agent
• example-experimental-durable-streams-ai-agent-vercel
• geo-distributed-database
• geo-distributed-database-vercel
• hello-world
• hello-world-vercel
• example-hono
• hono-react
• hono-react-vercel
• example-kitchen-sink
• example-kitchen-sink-vercel
• multi-region-vercel
• multiplayer-game
• example-multiplayer-game-patterns
• example-multiplayer-game-patterns-vercel
• multiplayer-game-vercel
• native-websockets
• native-websockets-vercel
• next-js
• per-tenant-database
• per-tenant-database-vercel
• example-raw-fetch-handler
• example-raw-fetch-handler-vercel
• example-raw-websocket-handler
• example-raw-websocket-handler-proxy
• example-raw-websocket-handler-proxy-vercel
• example-raw-websocket-handler-vercel
• example-react
• example-react-vercel
• sandbox
• sandbox-coding-agent
• sandbox-coding-agent-vercel
• sandbox-vercel
• scheduling
• scheduling-vercel
• example-sqlite-drizzle
• example-sqlite-raw
• state
• state-vercel
• example-stream
• example-stream-vercel
• example-trpc

@rivetkit/cloudflare-workers

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/cloudflare-workers@4334

@rivetkit/framework-base

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/framework-base@4334

@rivetkit/next-js

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/next-js@4334

@rivetkit/react

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/react@4334

rivetkit

pnpm add https://pkg.pr.new/rivet-dev/rivet/rivetkit@4334

@rivetkit/sql-loader

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/sql-loader@4334

@rivetkit/sqlite-vfs

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/sqlite-vfs@4334

@rivetkit/traces

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/traces@4334

@rivetkit/workflow-engine

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/workflow-engine@4334

@rivetkit/virtual-websocket

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/virtual-websocket@4334

@rivetkit/engine-runner

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/engine-runner@4334

@rivetkit/engine-runner-protocol

pnpm add https://pkg.pr.new/rivet-dev/rivet/@rivetkit/engine-runner-protocol@4334

commit: d0fb826

[claude]

PR Review: fix(ups): formalize subjects. Overall clean, well-scoped refactor. Issues: (1) No Subject impl for owned String, only for &str and &String. (2) publish_with_backoff takes &impl Subject then immediately calls .as_cow() - cleaner to just take &str. (3) MsgSubject::root() allocates via Cow::Owned on every call - could use Cow::Borrowed if M::subject() returned static str. Minor: Helper struct comment should be a complete sentence per style guide. Positives: spawn error propagation fix in message_async is correct, MsgSubject clone is zero-alloc, subject_cow computed once before chunk loop is good.