feat(ui): reconnect to in-progress tasks on page load

[onematchfox]

When a user reloads the chat page while the agent is still running, detect the active task and resubscribe to its SSE stream via tasks/resubscribe rather than showing a stale UI. Handles clean stream end, terminal errors, transient network failures with retry/backoff, and AbortError cancellation.

Current behaviour:

current.mp4

With reconnect:

with-fix.mp4

[Copilot]

Pull request overview

This PR adds a UI reconnect path so that if a user reloads the chat page while an agent task is still running, the client detects the in-progress task and resubscribes to its SSE stream via tasks/resubscribe instead of leaving the UI stale.

Changes:

• Added KagentA2AClient.resubscribeStream() to open an SSE stream for an existing task via JSON-RPC tasks/resubscribe.
• Updated ChatInterface initialization to detect active (submitted/working) tasks and automatically resubscribe on page load.
• Refactored shared stream consumption logic into a reusable consumeStream() helper and added a best-effort DB reload after resubscribe completion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
ui/src/lib/a2aClient.ts	Adds a new client method to resubscribe to an existing task’s SSE stream via tasks/resubscribe.
ui/src/components/chat/ChatInterface.tsx	Detects in-progress tasks on load, resubscribes to the stream, and centralizes SSE consumption/timeout handling.

Comments suppressed due to low confidence (2)

ui/src/components/chat/ChatInterface.tsx:468

• streamResubscribedTask unconditionally calls setChatStatus("ready") in finally. This can override statuses set by handleMessageEvent during the stream (notably input_required for HITL interrupts, or error/auth_required), leaving the UI in the wrong state. Align this cleanup with sendApprovalDecision’s pattern (only reset transient states like thinking/submitted/working, or derive the final status from the reloaded tasks).

    } finally {
      abortControllerRef.current = null;
      setChatStatus("ready");
      setIsStreaming(false);
      setStreamingContent("");
    }

ui/src/components/chat/ChatInterface.tsx:462

• The PR description mentions handling transient network failures with retry/backoff, but streamResubscribedTask currently makes a single tasks/resubscribe attempt and then falls back to reloadSessionFromDB() on any non-terminal error. If retries/backoff are intended, add a retry loop (respecting AbortError and terminal errors) before giving up, otherwise the UI may stop following a still-running task after a brief network blip.

    } catch (error: unknown) {
      if (error instanceof Error && error.name !== "AbortError" && !isTerminalError(error)) {
        console.error("Resubscribe failed:", error);
      }
      // Terminal, AbortError, or unexpected error — reload whatever state we have.
      if (!(error instanceof Error && error.name === "AbortError")) {
        await reloadSessionFromDB();
      }

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[onematchfox]

The project has no component-level tests for ChatInterface at all at present.