feat: improve error handling for unprocessable state deltas

[timon0305]

Summary

Improves error handling when the backend sends state deltas that the frontend cannot process, addressing issue #6019.

Changes

Frontend (reflex/.templates/web/utils/state.js)

• Added try-catch wrapper around event handler
• Validates that dispatch[substate] exists before calling
• Emits client_error event back to backend with actionable error messages
• Stops processing further updates on error to prevent cascading failures

Backend (reflex/app.py)

• Added on_client_error() handler to receive and log frontend errors in terminal
• Added debug logging in emit_update() to show which substates are being sent
• Provides actionable error messages (rebuild frontend, check api_url)

Documentation (reflex/state.py)

• Enhanced StateUpdate class docstring to explain error handling behavior
• Documents common causes of frontend/backend mismatch

Impact

• ✅ Frontend errors now appear in backend terminal logs (where devs look first)
• ✅ Clear, actionable error messages guide users to solutions
• ✅ Prevents silent failures and broken-looking apps
• ✅ Better debugging with substate logging

Testing

• All 3540 unit tests pass
• No breaking changes
• Backward compatible

Fixes #6019

[greptile-apps]

Greptile Overview

Greptile Summary

This PR improves error handling when the backend sends state deltas that the frontend cannot process. The implementation adds proper error detection, validation, and reporting between frontend and backend.

Key Changes:

• Frontend now validates dispatch[substate] exists before calling, preventing silent failures
• Frontend emits client_error events back to backend when state updates fail
• Backend logs frontend errors in terminal with actionable guidance
• Added debug logging to show which substates are being sent
• Enhanced documentation explaining error handling behavior

Minor Issues:

• String literals for event names and error types ("client_error", "dispatch_function_missing", "state_update_processing_error") should be extracted into constants per the project's coding standards
• Error detection logic using error.message.includes("dispatch function") is fragile and should use error types instead

Impact:
The changes provide much better visibility into frontend/backend state mismatches, helping developers quickly identify and fix configuration issues.

Confidence Score: 4/5

• This PR is safe to merge with minor style improvements recommended.
• The implementation is solid and addresses the issue effectively. Error handling logic is correct, backward compatible, and well-documented. The only concerns are minor style issues around hardcoded string literals that should be extracted into constants per project standards. No breaking changes, all tests pass.
• No files require special attention - the style recommendations are minor improvements rather than critical issues.

Important Files Changed

Filename	Overview
reflex/.templates/web/utils/state.js	Added try-catch error handling for state updates with validation and backend error reporting. String literals should be extracted to constants for maintainability.
reflex/app.py	Added on_client_error() handler to receive frontend errors and debug logging for substate updates. Clean implementation with proper error messages.
reflex/state.py	Enhanced StateUpdate docstring with comprehensive error handling documentation. Clear and helpful for developers.

Sequence Diagram

sequenceDiagram
    participant Frontend as Frontend (state.js)
    participant Backend as Backend (app.py)
    participant Console as Terminal/Logs
    
    Backend->>Frontend: emit("event", state update)
    Frontend->>Frontend: Validate dispatch[substate] exists
    
    alt Dispatch function missing
        Frontend->>Frontend: Log error to console
        Frontend->>Backend: emit("client_error", error details)
        Backend->>Console: console.error() with actionable message
        Frontend->>Frontend: throw Error (stop processing)
    else Dispatch function exists
        Frontend->>Frontend: dispatch[substate](delta)
        Frontend->>Frontend: applyClientStorageDelta()
        Frontend->>Frontend: queueEvents()
    end
    
    alt Other processing error
        Frontend->>Frontend: catch error
        Frontend->>Backend: emit("client_error", error details)
        Backend->>Console: console.error() with error type
        Frontend->>Frontend: Set event_processing = false
    end

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[greptile-apps]

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