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phys_constraint is a point entity available in all Source games.

Entity Description

It forces one entity to keep its position in relation to another. The two objects need not be touching, or even near to each other.

If a phys_constraint is attached to a func_train, it will not hold. Instead, parent a func_brush to the train and attach the phys_constraint to that.



Entity 1 <targetname>
Entity 2 <targetname>
The entities to constrain. Leave one or the other field blank to constrain to the world.
Note:Only one entity will be constrained, even if several share the given targetname.
Constraint System Manager <targetname>
A phys_constraintsystem that this constraint should be a part of. This avoids the "jiggling" caused by constraints applied to the same set of entities fighting with each other.
Force Limit to Break (lbs) <float>
Impact force required to break the constraint, in pounds. 0 means infinite. A way of calculating this is to set it to the weight of an object that would break the constraint if it were resting on its objects.
Torque Limit to Break (lbs * distance) <float>
Torque required to break the constraint, in pounds*inches. 0 means infinite. A way of calculating this is to multiply any reference mass by the resting distance (from the center of mass of the object) needed to break the constraint.
Play Sound on Break <string>
A sound played when the constraint is broken.
Follow teleport distance <float>
If one constrained object teleports more than this many units away, the other will teleport with it.


Name <string>
The targetname that other entities refer to this entity by.
Entity Scripts <scriptlist> (New with Left 4 Dead 2)
Space delimited list of VScript files (without file extension) that are executed after all entities have spawned. The scripts are all executed in the same script scope, later ones overwriting any identical variables and functions.
Script think function <string> (New with Left 4 Dead 2)
Name of a function in this entity's script which will be called automatically every 100 milliseconds (ten times a second) for the duration of the script. It can be used to create timers or to simulate autonomous behavior. The return value (if present) will set the time until the next call.
Note:Try to avoid expensive operations in this function, as it may cause performance problems.


  • 1: No Collision until break
  • 4: Start inactive
  • 8: Change mass to keep stable attachment to world
  • 16: Do not connect entities until turned on



Force the constraint to break.
Enable the constraint; do this if the constrained objects don't exist when it spawns. Note that broken constraints cannot be turned back on as they have been deleted.
Disable the constraint.


Removes this entity and any entities parented to it from the world.
Functions the same as Kill, although entities with a large number of children are killed marginally faster.
AddOutput  <string>
Evaluates a keyvalue/output on this entity. It can be potentially very dangerous, use with care.
Format: <key> <value>
Format: <output name> <targetname>:<inputname>:<parameter>:<delay>:<max times to fire, -1 means infinite>
FireUser1 to FireUser4
Fire the OnUser outputs; see User Inputs and Outputs.
Use  !FGD
Same as a player invoking +use; may not do anything. Can also be invoked by creating an output that does not specify an input.
This input is not included in Valve's FGDs.
RunScriptFile  <script> (New with Left 4 Dead 2)
Execute a VScript file from disk, without file extension. The script contents are merged with the script scope of the receiving entity.
RunScriptCode  <string> (New with Left 4 Dead 2)
Execute a string of VScript source code in the scope of the entity receiving the input. String quotation may be needed when fired via console.
In <Left 4 Dead 2>, the code is executed in the script scope of the entity that fires the output, not the one receiving the input.
Warning:Never try to pass string parameters to a script function with this input. It will corrupt the VMF structure because of the nested quotation marks, which then must be removed manually with a text editor.
CallScriptFunction  <string> (New with Left 4 Dead 2) !FGD
Execute a VScript function in the scope of the receiving entity.
Note:This input is missing from some games FGD files.
SetLocalOrigin  <origin> (New with Alien Swarm) !FGD
Set this entity's origin in the map.
Note:This input is missing from some games FGD files.
SetLocalAngles  <angles> (New with Alien Swarm) !FGD
Set this entity's angles in the map.
Note:This input is missing from some games FGD files.



Fired when the constraint breaks.


OnUser1 to OnUser4
These Outputs each fire in response to the firing of the like-numbered FireUser1 to FireUser4 Input; see User Inputs and Outputs.
OnKilled  (New with Left 4 Dead)
This Output fires when the entity is killed and removed from the game.