Support specifying designated assignees/approvers for Human-in-the-Loop nodes

Start by locating the Human-in-the-Loop node configuration UI, workflow payload/schema handling, and node execution permission checks described in the issue. Trace how the optional approvers field would move from configuration through execution, then identify existing approval tests or add coverage for designated users and the unchanged unrestricted behavior when the field is empty.

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

Currently, in the Human-in-the-Loop (HITL) node of Dify, any user with access can approve or review the execution. In many enterprise workflows or multi-user collaboration scenarios, sensitive tasks (e.g., financial approvals, content moderation, or escalated support) need to be restricted to specific users or roles rather than opened to all users.

We propose adding an optional Assignees / Approvers parameter to both the HITL node UI configuration and its underlying APIs:

1. Node Configuration UI:

• Add an optional input field Approvers / Assignees in the HITL node configuration panel.

• Support selecting or specifying one or multiple user IDs / email addresses / user roles.

2. API & Workflow Engine Support:

• Update the workflow payload/schema to accept an optional approvers field (e.g., a list of strings: ["user_id_1", "user_id_2"]).

• Modify the node execution logic to restrict approval permissions to only the specified user(s) when this field is populated.

3. Fallback / Default Behavior:

• Optional Field: If the field is left empty, maintain the current default behavior where any authorized user can perform the approval action.

2. Additional context or comments

Use Cases: Enterprise approval flows, role-based access control (RBAC) in workflow automation, multi-tenant/multi-tier user moderation.

Expected API Field Example:

{
  "node_type": "human-in-the-loop",
  "inputs": {
    "approvers": ["user_123", "user_456"]
  }
}

3. Can you help us with this feature?

• I am interested in contributing to this feature.