@ferdinand
This was very educational thank you !

Call Commands ≠ Modelling Commands
Symbol = Name of Constant value or data structure that is used with an immutable intend
Script Log -> not your API dictionary (I was influenced by a very old post in an old forum probably from Noseman when he mentioned that if in any case someone was interested in finding what function or method call C4D is using for a specific tool/attribute we can see if from the Script Log)

One more thing...
I've never seen a Call Command with two IDs. What's the common second one ?
In your screenshot CallCammands have all two IDs. Mine has always one.

@ferdinand
Thank you for replying.

By "method" I meant the programmatic term which some times coincides with the term "function".
In my case I was expecting it to return the c4d.MCOMMAND_SPLIT but then remembered it's actually an ID.

Isn't using the Script Log a good way to quickly and easy find all the appropriate function/class calls of objects/attributes/tools instead of manually searching the API documentation ?

For example I get from the Script Log

c4d.CallCommand(14046) # Split

Is there an Index page where I can see to what method that number belongs to ?

Thank you very much for the hands-on example @ferdinand !

About "Islands" and "selections" terms.

As far as I know "Islands" are groups of polygons that are isolated from the rest of the object's geometry, similarly to the "Fill Bucket" of MS Painter in 2D if we choose a polygon of the mesh and apply the Grow Selection to it infinitely many times it will never reach the "Island" polygons.

Selections, well, they are selections we all know what those are but when you asked me if I was bridging Islands or Selections I didn't (and still don't) why it should matter. Doesn't the Bridge behave the same when connecting polygons be it Islands or Selections ? After all Selections are like islands in the sense that we isolate them from the rest of the mesh to apply modifications to them.

EDIT:
Ah ! the

settings.SetBool(c4d.MDATA_BRIDGE_FIRST_SELECT_ONLY, True)
settings.SetBool(c4d.MDATA_BRIDGE_SECOND_SELECT_ONLY, True)

They were at the bottom of the toolbridge.h with the rest of the UI-related Attributes that I didn't pay attention at all.... there's even a comment line.. how blind am I ?

About the Server Error
The zip containing my project I was trying to upload was 11,6 KB so I don't know what's wrong with that... tried again and failed. Tried a C4D file also and still failed.
The exact message:
Something went wrong while parsing server response

I am dropping the files in this text field to upload, should I be using a different method ? I don't want to externally link to them (like Dropbox or something else)

EDIT #2
Ah ! By islands you meant two different objects !

EDIT #3
I'm having constant crashes when trying to bridge polygon selections from the same object ...
Are there specific attributes that need to be left empty when bridging the same object with itself ?
For now I'll try to disconnect the target polygon selection and treat it as a different object in order to bridge it...

@ferdinand

At this point I have no idea what the difference between "Polygon Islands" and "Polygon Selections" is ....
But yes, I have tried this with both two different objects and a single unified mesh of the two objects and didn't work. Exactly the same way I would do it manually as a user but this time I set polygon selections on the object and link them on the plugin generator to handle them.

"""
Minimal example of trying to use ID_MODELING_BRIDGE_TOOL programmatically
to bridge two polygon selections on a single merged object.

SETUP:
- Two cylinder objects with polygon selection tags
- Each tag selects polygons on one end cap
- We merge them with Connect, then try to bridge the selections

PROBLEM:
- With OBJINDEX1 only: C4D crashes
- With OBJINDEX1 and OBJINDEX3: C4D crashes  
- With all four OBJINDEX: Returns False (no crash, but doesn't work)
"""

import c4d
from c4d import utils

def BridgePolygonSelections(doc):
    """
    Attempt to bridge two polygon selections on a merged object
    """
    
    # Get two source objects with polygon selection tags
    obj1 = doc.SearchObject("Cylinder")      # Has tag "Polygon Selection" with 9 polys selected
    obj2 = doc.SearchObject("Cylinder 2")    # Has tag "Polygon Selection" with 12 polys selected
    
    if not obj1 or not obj2:
        print("Objects not found")
        return
    
    # Clone the objects
    clone1 = obj1.GetClone()
    clone2 = obj2.GetClone()
    
    # Get the polygon selection tags
    tag1 = clone1.GetTag(c4d.Tpolygonselection)
    tag2 = clone2.GetTag(c4d.Tpolygonselection)
    
    if not tag1 or not tag2:
        print("Selection tags not found")
        return
    
    # CRITICAL: Rename tags so Connect doesn't merge them into one
    tag1.SetName("bridge_source")
    tag2.SetName("bridge_target")
    
    # Extract polygon indices from original tags
    source_indices = []
    target_indices = []
    
    bs1 = tag1.GetBaseSelect()
    bs2 = tag2.GetBaseSelect()
    
    for i in range(clone1.GetPolygonCount()):
        if bs1.IsSelected(i):
            source_indices.append(i)
    
    for i in range(clone2.GetPolygonCount()):
        if bs2.IsSelected(i):
            target_indices.append(i)
    
    print(f"Source: {len(source_indices)} polygons")
    print(f"Target: {len(target_indices)} polygons")
    
    # Create temp document
    temp_doc = c4d.documents.BaseDocument()
    
    # Create Connect object and insert clones under it
    connect = c4d.BaseObject(1011010)
    temp_doc.InsertObject(connect)
    clone1.InsertUnder(connect)
    clone2.InsertUnder(connect)
    
    # Execute passes to build Connect cache
    temp_doc.ExecutePasses(None, True, True, True, c4d.BUILDFLAGS_NONE)
    
    # Make Connect editable to get merged polygon object
    result = utils.SendModelingCommand(
        command=c4d.MCOMMAND_CURRENTSTATETOOBJECT,
        list=[connect],
        mode=c4d.MODELINGCOMMANDMODE_ALL,
        doc=temp_doc
    )
    
    if not result:
        print("Failed to make Connect editable")
        return
    
    merged_obj = result[0]
    print(f"Merged object: {merged_obj.GetPolygonCount()} polygons")
    
    # Find the two selection tags on merged object
    source_tag = None
    target_tag = None
    
    tag = merged_obj.GetFirstTag()
    while tag:
        if tag.GetName() == "bridge_source":
            source_tag = tag
        elif tag.GetName() == "bridge_target":
            target_tag = tag
        tag = tag.GetNext()
    
    if not source_tag or not target_tag:
        print("Tags not found on merged object")
        return
    
    # Extract actual polygon indices from merged object's tags
    merged_source = []
    merged_target = []
    
    bs_src = source_tag.GetBaseSelect()
    bs_tgt = target_tag.GetBaseSelect()
    
    for i in range(merged_obj.GetPolygonCount()):
        if bs_src.IsSelected(i):
            merged_source.append(i)
        if bs_tgt.IsSelected(i):
            merged_target.append(i)
    
    print(f"Merged source indices: {merged_source}")
    print(f"Merged target indices: {merged_target}")
    
    # Build bridge settings
    settings = c4d.BaseContainer()
    settings[c4d.MDATA_BRIDGE_RESTRICT] = True
    
    # QUESTION: How to set OBJINDEX for polygon mode?
    # Option 1: Only OBJINDEX1 (CRASHES C4D)
    # settings[c4d.MDATA_BRIDGE_OBJINDEX1] = merged_obj
    
    # Option 2: OBJINDEX1 and OBJINDEX3 (CRASHES C4D)
    settings[c4d.MDATA_BRIDGE_OBJINDEX1] = merged_obj
    settings[c4d.MDATA_BRIDGE_OBJINDEX3] = merged_obj
    
    # Option 3: All four OBJINDEX (Returns False, doesn't crash)
    # settings[c4d.MDATA_BRIDGE_OBJINDEX1] = merged_obj
    # settings[c4d.MDATA_BRIDGE_OBJINDEX2] = merged_obj
    # settings[c4d.MDATA_BRIDGE_OBJINDEX3] = merged_obj
    # settings[c4d.MDATA_BRIDGE_OBJINDEX4] = merged_obj
    
    # ELEMENT parameters - first and last polygon of each selection
    settings[c4d.MDATA_BRIDGE_ELEMENT1] = merged_source[0]   # 48
    settings[c4d.MDATA_BRIDGE_ELEMENT2] = merged_source[-1]  # 56
    settings[c4d.MDATA_BRIDGE_ELEMENT3] = merged_target[0]   # 19
    settings[c4d.MDATA_BRIDGE_ELEMENT4] = merged_target[-1]  # 30
    
    # Other settings
    settings[c4d.MDATA_BRIDGE_SUBDIVISION] = 0
    settings[c4d.MDATA_BRIDGE_TENSION] = 0.0
    settings[c4d.MDATA_BRIDGE_BOTH_MODE] = 0
    
    print(f"Executing bridge...")
    
    # Execute bridge
    bridge_result = utils.SendModelingCommand(
        command=c4d.ID_MODELING_BRIDGE_TOOL,
        list=[merged_obj],
        mode=c4d.MODELINGCOMMANDMODE_POLYGONSELECTION,
        bc=settings,
        doc=temp_doc
    )
    
    print(f"Bridge result: {bridge_result}")
    
    if bridge_result:
        print("SUCCESS!")
    else:
        print("Bridge returned False")

# Run it
if __name__ == '__main__':
    doc = c4d.documents.GetActiveDocument()
    BridgePolygonSelections(doc)

I would post the full zip but I get a server error again... something must have broken on this forum with uploaded files as I had the same issue when trying to upload a screenshot

@ferdinand I'm still having trouble with this.

I'm trying to bridge polygon selections on a single merged object, but it keeps returning False.

My setup:

• Single polygon object (2 cylinders under a Connect, then Connect+Delete)
• Two polygon selection tags with different names on the merged object
• Polygon mode (MODELINGCOMMANDMODE_POLYGONSELECTION)

All settings:

python  settings[MDATA_BRIDGE_RESTRICT] = True
settings[MDATA_BRIDGE_OBJINDEX1] = merged_obj
settings[MDATA_BRIDGE_OBJINDEX2] = merged_obj
settings[MDATA_BRIDGE_OBJINDEX3] = merged_obj
settings[MDATA_BRIDGE_OBJINDEX4] = merged_obj
settings[MDATA_BRIDGE_ELEMENT1] = 48  # First polygon of source selection
settings[MDATA_BRIDGE_ELEMENT2] = 56  # Last polygon of source selection
settings[MDATA_BRIDGE_ELEMENT3] = 19  # First polygon of target selection
settings[MDATA_BRIDGE_ELEMENT4] = 30  # Last polygon of target selection
//+ subdivision, tension, mode, etc.

The bridge tool returns False. The selections are verified to exist on the object (9 polygons at indices 48-56, and 12 polygons at indices 19-30).

Questions:

• Do all MDATA_BRIDGE_OBJINDEX# need to be filled when bridging in polygon mode ?
• Do I need to apply the selections to the active BaseSelect, or should they only be in tags?
• Are there other required parameters for polygon mode ?
• Does RESTRICT=True have any meaning when using the tool programmatically ?
• If MDATA_BRIDGE_DELETE is "Keep Original Polygons" when bridging polygons then what is the MDATA_BRIDGE_KEEP_POLYGONS ?

Thanks!

Oh ! we can bridge independent objects ? That simplifies things as I thought I would have to Connect+Delete first and then bridge...
Thank you for the explanation of the attribute calls.

Hello, I'm trying to use the Bridge Tool with the SendModelingCommand but I'm having trouble as there are attributes I don't know how to use. Could someone explain to me what these are for ?

MDATA_BRIDGE_ELEMENT1 = 1100, //LONG
MDATA_BRIDGE_ELEMENT2 = 1101, //LONG
MDATA_BRIDGE_ELEMENT3 = 1102, //LONG
MDATA_BRIDGE_ELEMENT4 = 1103, //LONG
MDATA_BRIDGE_OBJINDEX1 = 1104, //LINK
MDATA_BRIDGE_OBJINDEX2 = 1105, //LINK
MDATA_BRIDGE_OBJINDEX3 = 1106, //LINK
MDATA_BRIDGE_OBJINDEX4 = 1107, //LINK
MDATA_BRIDGE_DELETE = 1108, //BOOL
MDATA_BRIDGE_ISO = 1109, //BOOL

Also, given the type of elements I'm bridging and subsequently the modeling mode in which I will be using the Tool should I look out for specific things like the sequence of edges or points between the two parts to be connected ? Should I be treating Edge Selections like a sequence of point IDs or can I use them with their own IDs ?

Thank you for your time.

Ah ! They are called Proxy Tags and I can see the actual tag with the MSG_GETREALTAGDATA ! Thank you ️

I managed to solve this and make the Python Effector look like a plugin. I programmatically add the object, its icon and the script to it. But because it's impossible to add the User Data, I added them manually and then saved the project file containing the Effector object. Then added a simple condition if the scene file is present in the plugin folder merge the object to the current scene, else add the object without the User Data.

I think this solution is acceptable and elegant to present something as a complete plugin and not as a C4D scene file (usually a demonstration file) as I have encounter so far.