How to add dynamic private key (auth) to AI Agent-triggered HTTP tool nodes?

Hello team,

I'm currently developing a workflow using callin.io's AI Agent with Langchain and HTTP request tool nodes. My objective is to dynamically inject a privatekey obtained from an authentication request into the headers of subsequent tool API calls, but I'm encountering some difficulties.

Goal

When a user selects a tool (e.g., update_meter_status) via the AI Agent, callin.io should perform the following:

1. Invoke the authorization_service to retrieve a privatekey.
2. Include this privatekey within the headers of the actual tool request.
3. Ensure the authentication node (specifically for Private Key) is not displayed in the AI's tool selection menu.

What I’ve Tried (and why it failed)

• Direct expression:
  ={{ $node["Private Key"].json[0].privatekey }}
  Fails unless the node has already been executed, resulting in an `undefined` or `unexecuted` error.
• Inline `$http()` in expression:
  ={{ $http({...}).json[0].privatekey }}
  Not supported, as `$http` is not permitted within expressions.
• Merge node:
  Attempted to combine AI input with the Private Key before the tool call.
  Does not trigger both branches; the second input typically remains unexecuted.
• Code node to combine data:
  Manually merged the inputs.
  Encountered the same issue where one branch remains unexecuted.
• Renaming node as `_internal_` to hide from menu
  The node still appears in the AI tool list.

What I’m Looking For

• A method to automatically execute an internal authentication node prior to any AI-selected tool being invoked.
• The ability to keep this authentication node hidden from the tool menu.
• The capability to reuse the obtained privatekey in all subsequent HTTP headers.

Are there any straightforward solutions for this pattern? Perhaps utilizing workflow context, a credentials trick, or hidden pre-execution hooks?

I greatly appreciate any guidance you can offer. I'm happy to share the full workflow JSON if needed!

Thanks

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