fix: fix assign value stand as default

[fatelei]

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Summary

fix #30650

1. Workflow starts executing
   ↓
2. ResponseStreamCoordinator initializes
   - Register Answer node template: "{{#conversation.conv_var#}}"
   - Parse template into segments: [TextSegment, VariableSegment]
   ↓
3. Attempt to send streaming output (try_flush)
   - Iterate through template segments
   - Encounter VariableSegment: conversation.conv_var
   - Read current value from variable_pool → "default" (not yet updated at this point)
   - Send NodeRunStreamChunkEvent("default")
   ↓
4. Event flow
   - NodeRunStreamChunkEvent → QueueTextChunkEvent
   - Frontend receives TextChunkStreamResponse → message event
   ↓
5. Workflow continues execution
   - Variable Assigner node executes, updates conversation.conv_var = "123"
   - Answer node finishes execution, outputs "123"
   ↓
6. Problem
   - Frontend has already displayed "default"
   - Need to send message_replace event to update

Screenshots

Before	After
...	...

before

Screen.Recording.2026-01-06.at.21.53.16.mov

after

Screen.Recording.2026-01-06.at.21.51.21.mov

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue in the advanced chat workflow where the 'Answer' node could display an incorrect initial value if its underlying variables were updated later in the execution pipeline. The changes introduce logic to detect such discrepancies and send a 'message_replace' event to the frontend, ensuring that the final, correct answer is always presented to the user, thereby improving the accuracy of streamed responses.

Highlights

• Problem Identification: The pull request addresses a bug where the 'Answer' node in a workflow might initially stream a default or outdated value to the frontend.
• Root Cause: This issue occurs because variables used in the 'Answer' node's template might be updated later in the workflow by a 'Variable Assigner' node, after the initial stream has already begun.
• Solution Implementation: A new mechanism is introduced to detect when the final output of an 'Answer' node differs from the value already streamed.
• Frontend Update: If a discrepancy is found, a 'message_replace' event is sent to the frontend, ensuring the user interface displays the correct, updated value.

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[gemini-code-assist]

Code Review

This pull request introduces a fix for an issue where the UI displays a default variable value that is later updated by a variable_assigner node. The proposed solution correctly identifies when the final output of an ANSWER node differs from the initially streamed content and sends a message_replace event to update the UI. The implementation is clean and directly addresses the problem. I have one suggestion to improve maintainability regarding the reason for the message replacement.