I've been running a campaign for months now and I have one player who is constantly trying to modify his weapons, most times during combat, for example: attaching a rope to a crossbow bolt in order to make it function like a harpoon. Another example was tying a rope to a broken sword to make it a weapon with reach. The way I have been handling it is I have allowed him to do it (I don't like stifling creativity) but it costs him his action in combat to make the modification. In the case of the crossbow, I had him roll at a disadvantage (a hand crossbow bolt with a rope attached, just didn't seem feasible to me). My question for my fellow DM's is should there be a skill check he has to make in order to do these modifications? I have heard that Matt Mercer has his players do something like that or requires them to be proficient with tinkers tools (I'm not positive on this). How would you handle this situation? Other then just allowing it or saying no. 

Making the thing cost an action sounds like a good idea.

If the modification is very advanced you could also require skill checks to see how well the modification is made.

The lower the roll is the more likely the weapon will missfire or something. 

Then the player will learn that these improvised weapons might have a "cost" to it and isnt just something they put on top of their weapon without any chance of penalty.

I like the idea of a misfire or something going wrong with the modification. Now I just have to figure out what skill check to use, and I may require tinker's tools in order to make the modifications.

I would consider having the "new" weapon act as an improvised weapon for the player. Yes, they can make a modification to the weapon as they want, but since they are using the weapon in a way they are not used to, they lose their proficiency bonus to attack. If the player is really invested in this, then they always have the option of taking the "Tavern Brawler" feat to gain proficiency in improvised weapons. 

With most of Matt Mercer's stuff they have to craft it ahead of time.  For doing it in the heat of the moment, I could see it costing a full action to attach a rope to a broken blade or something similar and they get to do a skill check... say Sleight of Hand (Intelligence) to allow them to use a proficiency they might have but with a different modifier they aren't expecting.  They could then try to attack with it as an improvised reach weapon next turn.

In general I would require a class feature (possibly some Artificer subclass) to do that kind of thing in a combat useful timeframe.

Quote from Kaboom979 >>

I would consider having the "new" weapon act as an improvised weapon for the player. Yes, they can make a modification to the weapon as they want, but since they are using the weapon in a way they are not used to, they lose their proficiency bonus to attack. If the player is really invested in this, then they always have the option of taking the "Tavern Brawler" feat to gain proficiency in improvised weapons. 

 Yeah I've done this in combat, where it counts as an improvised weapon and they lose any proficiency they would normally have.

Making it cost an action to quickly craft an improvised weapon is a good way of doing it, in addition to skill checks. Depending on how successful they are with their check determines a threshold of failure. If they roll a nat20+modifiers, then there would be a very low threshold of failure (for example, 5% chance on a d100). If they roll a nat1, then there would be a very high threshold of failure (95% chance on a d100). It wouldn't make it impossible to craft and use an improvised weapon, but it makes it more dependent on the character's abilities and skills to do so without time and meticulous research done beforehand.

Think about the weight of the rope and how deep the bolt would penetrate into the target.

That would affect the range and I would even go as far as asking for a good damage roll to support enough weight. 

This would only be possible with a silk rope IMO.

Giving reach to a sword like that is crazy, only a blunt weapon or something with spikes should be effective in that case and only as improvised weaponry.

+1 for treating these as improvised weapons. 1 action to improvise it, skill roll of some sort needed, and then it is just an improvised weapon. This is while doing it in combat.

Out of combat if he has an appropriate tool proficiency, then I can see him crafting/modding weapons to suit his needs. But this would take the normal sorts of crafting time (days).

Some of the time, the answer should just be "no".

For example, a rope tied to a crossbow bolt isn't a harpoon, because the rope weighs more than the bolt. It will get a few feet and fall to the ground. A ballista bolt would be fine, as would using the crossbow bolt with a thin cord.

Of course, sometimes the answer should just be "yes". Creativity should be encouraged.

A rope tied to a broken blade and swung around the head is a great idea for a broken weapon. It sounds nasty. Perhaps nasty to everyone within 10ft of the fighter. :-)

Quote from Greenstone_Walker >>

Some of the time, the answer should just be "no".

For example, a rope tied to a crossbow bolt isn't a harpoon, because the rope weighs more than the bolt. It will get a few feet and fall to the ground. A ballista bolt would be fine, as would using the crossbow bolt with a thin cord.

Of course, sometimes the answer should just be "yes". Creativity should be encouraged.

A rope tied to a broken blade and swung around the head is a great idea for a broken weapon. It sounds nasty. Perhaps nasty to everyone within 10ft of the fighter. :-)

 Agreed, in my mind, the 50 ft. rope, almost every adventurer has, is about a half-inch to three-quarter-inch in diameter (which fits actually rather ok'ish with manila hemp ropes). Using this with a rather small crossbow bolt or arrow, will not work.

1. Making him burn an action in combat to do so. Good DMing and the monsters thank you.
2. Skill check. Not needed if the weapon goes away after combat. If he wants to make a real weapon. A few skill checks and down time days. But don’t bump up the damage, or give too much reach.

creativity is great but it needs to make sense as others have stated a crossbow bolt is not going to have enough velocity in order to support someone climbing a rope or pulling a large creature in a harpoon kinda way.  Broken sword with a rope sounds cool to me.  I wouldn't tell them no let them do it but that doesn't mean they will be successful.  Let them shoot the improvised crossbow bolt rope then when they try to climb across the bolt comes immediately dislodged