TclCaller in FME 2026.2 and backports

Consideration 

In FME 2026.2 and specified versions containing the backported security updates, the TclCaller transformer may fail, producing this error on failure:

This may occur when the security tier is set to the Default Hardening tier, which is configurable in the FME Workbench user interface in FME Form, FME Form CLI, or FME Flow System Configurations. 

Why This Happens

As a result of security changes, the ability to specify some TCL and FME Functions in parameter and attribute values has been disabled or reduced, impacting some transformers and advanced scripting features. FME Hub items using this transformer may also be impacted. 

This applies to the updated versions and instances that have been patched according to the instructions in the advisory: 
 

FME Form versions:

• 2026.2
• 2026.1.0.1, 2026.1.1.1, 2026.1.2.1
• 2025.2.0.1, 2025.2.1.1, 2025.2.2.1, 2025.2.3.1, 2025.2.4.2
• 2025.1.0.2, 2025.1.1.1, 2025.1.2.1, 2025.1.3.5
• 2025.0.0.1, 2025.0.1.1, 2025.0.2.1, 2025.0.3.2
• 2024.2.0.1, 2024.2.1.1, 2024.2.2.1, 2024.2.3.1, 2024.2.4.1, 2024.2.5.2

FME Flow versions: 

• 2026.2
• 2026.1.2.1
• 2025.2.4.2
• 2025.1.3.5
• 2025.0.3.2
• 2024.2.5.2
• FME Flow instances with the hotfix patch and upgraded engine installed per the resolution option for backported security fixes described in the FME September 2026 Security Updates:
  • 2026.1.0.0 through 2026.1.2.0
  • 2025.2.0.0 through 2025.2.4.1
  • 2025.1.0.0 through 2025.1.3.4
  • 2025.0.0.0 through 2025.0.3.1
  • 2024.2.0.0 through 2024.2.5.1

How to Resolve It

TclCaller has been deprecated since FME 2024.0. We recommend rewriting old Tcl workflows using Python and running them using the PythonCaller transformer. AI Assist inside the PythonCaller can be helpful as a tool for rewriting the Tcl scripts. To learn more, see AI Assist for Python. 

Alternatively, changing the security tier to the Minimal Hardening tier will allow TclCaller to work again. 

Minimal Hardening is not recommended for most users.