feat(sdk): add vue, svelte, angular implementation of useStream hook

[Christian Bromann (christian-bromann)]

This PR introduces first-class Vue, Svelte, and Angular integrations for the LangGraph useStream hook — bringing the same streaming chat experience previously available only in React to three additional frontend frameworks. It also restructures the SDK internals to extract a framework-agnostic UI core, splits the React integration into its own dedicated package, and adds a client-side submission queue system.

New Packages

Package	Framework	Reactivity Model	Peer Dependencies
@langchain/react	React 18/19	Hooks (useState, useSyncExternalStore)	react, react-dom
@langchain/vue	Vue 3+	Composition API (ref, computed, watch)	vue
@langchain/svelte	Svelte 4/5	Stores (writable, derived) / Runes	svelte
@langchain/angular	Angular 18-20	Signals (signal, computed, effect)	@angular/core

All four packages expose a unified useStream API adapted to each framework's idiomatic reactivity system, providing:

• Real-time streaming of LangGraph agent state via SSE
• Optimistic updates for instant UI feedback
• Interrupt handling for human-in-the-loop workflows
• Thread management including history, branching, and thread switching
• Subagent tracking for multi-agent graphs
• Tool call lifecycle tracking
• Submission queue for batching rapid user inputs (new feature)
• Custom transport support via FetchStreamTransport or user-defined implementations

SDK Core Refactoring (@langchain/langgraph-sdk)

The React-specific streaming implementation has been extracted from the core SDK into the new @langchain/react package. In its place, the core SDK now exports a framework-agnostic UI module (@langchain/langgraph-sdk/ui) containing shared primitives:

• StreamManager — Core stream lifecycle and event processing
• MessageTupleManager — Message state normalization between dict and class formats
• SubmitQueue — Client-side queue that drains submissions sequentially when the agent is idle, with configurable error handling ("continue" / "stop")
• FetchStreamTransport — Pluggable SSE transport abstraction using the Fetch API
• extractInterrupts — Unified interrupt extraction from stream state and thread state
• getBranchContext / getMessagesMetadataMap — Thread history branching utilities
• SubagentManager — Tracking and management of nested agent runs

The existing @langchain/langgraph-sdk/react entrypoint remains backward-compatible — it re-exports from @langchain/react with a thin wrapper that preserves the existing message dictionary format.

Removed

• @langchain/langgraph-sdk-validation — The standalone validation package has been removed. Its test coverage has been absorbed into each framework package's own test suite, providing more targeted and framework-specific validation.

Example Applications

Three new example apps demonstrate the integration for each framework:

• examples/ui-angular/ — Angular standalone component with signals-based chat
• examples/ui-svelte/ — Svelte 5 app with runes-based chat
• examples/ui-vue/ — Vue 3 Composition API chat

All examples follow the same pattern: a simple chat UI backed by a single-node StateGraph agent, showcasing message streaming with optimistic updates.

Testing

Each framework package includes comprehensive browser-based integration tests using Vitest Browser Mode with WebDriverIO:

• Runtime tests — Message submission, streaming, interrupts, branching, queue management, thread switching, stop/cancel, subgraph streaming, metadata, custom transport, and message removal
• Type tests (*.test-d.ts) — Extensive type-level tests for useStream, createAgent, createDeepAgent, and LangGraph schema inference, ensuring correct generic type resolution across all framework adapters
• Visual regression — Screenshot-based snapshot tests for UI state verification

The core SDK also adds unit tests for the new SubmitQueue (libs/sdk/src/ui/queue.test.ts).

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commit: 5ffc2f7

[pawel-twardziak]

shouldn't be return this.queue = this.queue.then(() => this.enqueue(action, options));?
return is missing