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The following page details an example app that deploys a LangChain deep agent inside a SvelteKit project, built for Cloudflare Workers with @sveltejs/adapter-cloudflare: streaming chat UI, subagent detail views, thread history, and the Agent Streaming Protocol exposed under /api/threads/.... No separate backend process is required. Source: js-sveltekit in the deployment cookbook.

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Deploy to Cloudflare

1

Install and build

cd js-sveltekit
cp .env.example .env   # set OPENAI_API_KEY for local dev
pnpm install
pnpm build
2

Configure secrets

npx wrangler login
npx wrangler secret put OPENAI_API_KEY
3

Deploy

pnpm run deploy
svelte.config.js uses adapter-cloudflare(). wrangler.jsonc points Wrangler at .svelte-kit/cloudflare/_worker.js and serves assets from .svelte-kit/cloudflare, matching the SvelteKit Cloudflare adapter docs. The build script appends the ThreadSession Durable Object export to that generated Worker entry because Durable Object classes must be exported by the Worker module. nodejs_compat and nodejs_compat_populate_process_env are enabled because the LangChain runtime and tracing integrations expect Node-compatible APIs and environment access. Optionally enable LangSmith tracing by adding the variables from .env.example as Worker secrets or vars.

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Required API endpoints

The app exposes the Agent Streaming Protocol under /api/threads/.... SvelteKit route handlers live in src/routes/api/threads/.

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Minimum (streaming chat)

MethodPathPurpose
POST/api/threads/:threadId/commandsAccept protocol commands (run.start, …) and start agent runs
POST/api/threads/:threadId/streamSSE stream of protocol events for a run
GET / POST/api/threads/:threadId/stateRead and bootstrap checkpointed thread state

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Optional (sidebar)

MethodPathPurpose
GET/api/threadsList threads known to the checkpointer
DELETE/api/threads/:threadIdDelete a thread’s session and checkpoints
POST/api/threads/:threadId/historyPaginated checkpoint history

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Cloudflare backend design

ConcernImplementation
FrontendSvelteKit client routes and components
API layerSvelteKit server endpoints in src/routes/api/threads/
RuntimeWorkers V8 + nodejs_compat
SSE replayPer-thread Durable Object (ThreadSession)
Agent runsWorker isolate; protocol events POSTed to the DO
Static assetsWorkers Static Assets via adapter-cloudflare
Secretswrangler secret / local .env

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Production persistence

Out of the box, the agent uses an in-memory MemorySaver checkpointer (src/lib/server/agent/index.ts). The per-thread SSE replay/session log lives in a Durable Object so streaming clients reconnect to one coordination point instead of a process-local map. The checkpointer is still isolate-local demo state. Cloudflare isolates are ephemeral and may scale horizontally, so checkpointed conversation state is not durable across deploys, cold starts, or isolates. For production:
  1. Swap in a durable checkpointer (for example Postgres via Hyperdrive, or a custom Durable Object-backed store).
  2. Persist long-lived replay/history if clients need to reconnect after the Durable Object has been evicted from memory.

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Local development

cp .env.example .env   # set OPENAI_API_KEY
pnpm install
pnpm dev
Open http://localhost:5173. 
pnpm build      # production build for Cloudflare
pnpm preview    # preview the production build locally
pnpm typecheck  # svelte-check over the project
For Cloudflare-style local testing after a build, run: 
npx wrangler dev .svelte-kit/cloudflare/_worker.js

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Project layout

  • src/lib/server/agent/ — deep agent (createDeepAgent) with researcher and math-whiz subagents and mock tools.
  • src/lib/server/durable-objects/thread-session.ts — per-thread Durable Object event log for SSE replay.
  • src/lib/server/protocol/ — Agent Streaming Protocol helpers: checkpointer-backed state/history, run publishing, serialization, and registry.
  • src/routes/api/threads/ — SvelteKit route handlers for the protocol endpoints.
  • src/lib/chat/threads-client.ts — browser thread bootstrap and sidebar helpers.
  • src/lib/components/ — Svelte chat UI using @langchain/svelte.
  • svelte.config.js — SvelteKit configured with @sveltejs/adapter-cloudflare.
  • scripts/export-durable-objects.mjs — postbuild patch that re-exports the Durable Object class from the generated Worker entry.
  • wrangler.jsonc — Cloudflare Workers Static Assets and Durable Object config.

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See also

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