Fusion Procedure Guide

Fusion scripting has two halves that beginners often mix together:

1. The Fusion Monster declares what counts as its Fusion Materials.
2. A Spell, Trap, monster effect, or procedure effect collects materials and performs the Fusion Summon.

Those halves meet at Card.CheckFusionMaterial and Duel.SelectFusionMaterial. The Fusion Monster's material procedure tells the core what combinations are legal; the effect that Summons it decides where the materials may come from and how those materials move.

For a guided walkthrough of the actual player prompts, optional summons, Chain Material routes, and changing material availability, continue with Fusion Summon Choices.

The Two Layers

Layer	Where it usually lives	What it does
Fusion Monster material requirement	The Fusion Monster script in initial_effect	Registers EFFECT_FUSION_MATERIAL with helpers such as aux.AddFusionProcCode2 or aux.AddFusionProcFunRep.
Fusion Summon effect	A Spell/Trap/monster effect that Summons from the Extra Deck	Gathers material candidates, checks Fusion Monsters, lets the player choose materials, moves them, and Special Summons with SUMMON_TYPE_FUSION.

If a Fusion Monster has no material procedure, generic Fusion Spells cannot know how to Summon it. If a Fusion Spell has no Summon operation, the Fusion Monster material procedure will never be used.

Fusion Monster Material Procedures

Fusion Monsters normally start by enabling their proper-Summon rule, then registering a material procedure:

function s.initial_effect(c)
    c:EnableReviveLimit()
    aux.AddFusionProcFunRep(c,s.ffilter,2,true)
end

function s.ffilter(c)
    return c:IsFusionAttribute(ATTRIBUTE_DARK)
        and c:IsOnField()
        and not c:IsType(TYPE_TOKEN)
end

Starving Venom Fusion Dragon (c41209827.lua) uses that pattern. Its material helper means "2 DARK monsters on the field, excluding Tokens." The helper registers an EFFECT_TYPE_SINGLE effect with EFFECT_FUSION_MATERIAL; it does not perform a Summon by itself.

For fixed names, local scripts use code helpers:

function c10248389.initial_effect(c)
    c:EnableReviveLimit()
    aux.AddFusionProcCode2(c,97023549,11460577,false,false)
end

The common material-definition helpers are:

Helper	Use it when the material text says...
aux.AddFusionProcCode2	Two specific card names.
aux.AddFusionProcCode3 / Code4	Three or four specific card names.
aux.AddFusionProcFun2	One material matching filter A plus one material matching filter B.
aux.AddFusionProcFunRep	A fixed number of materials all matching the same filter.
aux.AddFusionProcFunRep2	A variable number of materials matching one filter.
aux.AddFusionProcMix	A custom list of fixed code/filter slots.
aux.AddFusionProcMixRep	Fixed slots plus a repeated variable slot.
aux.AddFusionProcShaddoll	The local Shaddoll two-material pattern.

Many helpers accept sub and insf:

Parameter	Meaning
sub	Allows Fusion Substitute material checks when the procedure supports them.
insf	Allows the procedure check with no material group, used by Instant Fusion-style handling.

Most beginner scripts should copy the sub and insf values from a close local Fusion Monster. The flags affect legality in the material condition, not only the player prompt.

Fusion-Specific Material Checks

Inside Fusion Material filters, prefer Fusion-aware card checks:

return c:IsFusionSetCard(0x9d)

Fusion material identity is not always the same as ordinary identity. The core has functions such as:

• Card.IsFusionCode
• Card.IsFusionSetCard
• Card.IsFusionType
• Card.IsFusionAttribute

Those checks can see Fusion-only code, set, type, and Attribute changes. For Fusion Material text, they are usually safer than ordinary IsCode, IsSetCard, IsType, or IsAttribute.

The lower-level legality check is Card.IsCanBeFusionMaterial. It answers whether a candidate card may be used as Fusion Material for a Fusion Monster and Summon type. Material procedures call it internally, and manual Fusion effects often use it before adding unusual material locations.

A Standard Fusion Spell Flow

Polymerization (c24094653.lua) is the clean local model for a standard Fusion Spell. It activates like an ordinary Spell, but the target and operation do the Fusion work.

The target branch checks whether a Fusion Monster can be Summoned:

function c24094653.filter2(c,e,tp,m,f,chkf)
    return c:IsType(TYPE_FUSION) and (not f or f(c))
        and c:IsCanBeSpecialSummoned(e,SUMMON_TYPE_FUSION,tp,false,false)
        and c:CheckFusionMaterial(m,nil,chkf)
end

function c24094653.target(e,tp,eg,ep,ev,re,r,rp,chk)
    if chk==0 then
        local chkf=tp
        local mg1=Duel.GetFusionMaterial(tp)
        local res=Duel.IsExistingMatchingCard(c24094653.filter2,tp,
            LOCATION_EXTRA,0,1,nil,e,tp,mg1,nil,chkf)
        if not res then
            local ce=Duel.GetChainMaterial(tp)
            if ce~=nil then
                local fgroup=ce:GetTarget()
                local mg2=fgroup(ce,e,tp)
                local mf=ce:GetValue()
                res=Duel.IsExistingMatchingCard(c24094653.filter2,tp,
                    LOCATION_EXTRA,0,1,nil,e,tp,mg2,mf,chkf)
            end
        end
        return res
    end
    Duel.SetOperationInfo(0,CATEGORY_SPECIAL_SUMMON,nil,1,tp,LOCATION_EXTRA)
end

The important pieces are:

Piece	Job
chkf=tp	Tells the Fusion Material check to respect the player's Extra Monster Zone and Main Monster Zone availability.
Duel.GetFusionMaterial	Gets the player's normal Fusion Material pool.
Card.CheckFusionMaterial	Asks the chosen Fusion Monster's registered material procedure whether the pool can satisfy it.
Duel.GetChainMaterial	Optional fallback for Chain Material-style effects.
f / mf	Extra Chain Material filter supplied by the Chain Material effect.

The target branch does not move cards. It only proves that at least one legal Fusion Summon is currently possible.

Operation Order

The operation branch repeats the Fusion search at resolution, because the field may have changed after activation:

local mg1=Duel.GetFusionMaterial(tp):Filter(c24094653.filter1,nil,e)
local sg1=Duel.GetMatchingGroup(c24094653.filter2,tp,
    LOCATION_EXTRA,0,nil,e,tp,mg1,nil,chkf)

Then it selects the Fusion Monster, selects the materials, records them, moves them, and Summons:

local mat1=Duel.SelectFusionMaterial(tp,tc,mg1,nil,chkf)
tc:SetMaterial(mat1)
Duel.SendtoGrave(mat1,REASON_EFFECT+REASON_MATERIAL+REASON_FUSION)
Duel.BreakEffect()
Duel.SpecialSummon(tc,SUMMON_TYPE_FUSION,tp,tp,false,false,POS_FACEUP)
tc:CompleteProcedure()

Keep that order:

1. Duel.SelectFusionMaterial asks the player for a legal material group.
2. Card.SetMaterial records the materials on the Fusion Monster.
3. The material movement function sends, banishes, or returns the materials with REASON_EFFECT+REASON_MATERIAL+REASON_FUSION.
4. Duel.BreakEffect separates the material movement and summon in effect timing; it is not merely a visual pause.
5. Duel.SpecialSummon uses SUMMON_TYPE_FUSION.
6. Card.CompleteProcedure marks the monster as properly Fusion Summoned.

The local core maps SUMMON_TYPE_FUSION to REASON_FUSION for material event handling. The explicit REASON_FUSION flag in the movement call is still the local script pattern for Fusion Material movement.

Using The Local Fusion Effect Helper

procedure.lua also provides a configurable helper for Fusion Summon effects: Auxiliary.AddFusionEffectProcUltimate.

It creates an activation effect whose default behavior is close to Polymerization:

• params.mat_location defaults to LOCATION_HAND+LOCATION_MZONE;
• params.mat_filter defaults to Auxiliary.FMaterialFilter, which checks Card.IsCanBeFusionMaterial;
• params.mat_operation defaults to sending materials to the GY with REASON_EFFECT+REASON_MATERIAL+REASON_FUSION;
• params.grave_operation defaults to banishing GY materials;
• params.deck_operation and params.extra_operation default to the main material operation unless overridden;
• params.reg=false returns the effect without registering it.

The smaller wrapper Auxiliary.AddFusionEffectProc fills the common filter, mat_location, mat_filter, and mat_operation fields before calling the ultimate helper.

Many current local card scripts write the Fusion flow manually, especially older or highly custom Fusion Spells. The helper is still useful for new scripts when the card text matches its parameter table.

Deck And Extra Material

Deck Fusion cards add another material group before checking the Extra Deck. Red-Eyes Fusion (c6172122.lua) and Branded Fusion (c44362883.lua) both use the same local shape:

local mg1=Duel.GetFusionMaterial(tp)
local mg2=Duel.GetMatchingGroup(s.filter0,tp,LOCATION_DECK,0,nil)
mg1:Merge(mg2)

Their Deck material filters still check Fusion-material legality and movement:

function s.filter0(c)
    return c:IsType(TYPE_MONSTER)
        and c:IsCanBeFusionMaterial()
        and c:IsAbleToGrave()
end

Deck Fusion cards usually need extra legality checks. Red-Eyes Fusion uses aux.FCheckAdditional to require a "Red-Eyes" material, and Branded Fusion uses it to require a material with the listed code and at most two materials:

function c44362883.fcheck(tp,sg,fc)
    return sg:GetCount()<=2 and sg:IsExists(Card.IsFusionCode,1,nil,68468459)
end

aux.FCheckAdditional is a temporary global hook. Set it only around the Fusion Material check or selection that needs it, then clear it immediately.

Different Material Movement

Not all Fusion effects send materials to the GY. The local helper has default material operations for common movement choices:

Operation	Movement
Auxiliary.FMaterialToGrave	Duel.SendtoGrave(mat,REASON_EFFECT+REASON_MATERIAL+REASON_FUSION)
Auxiliary.FMaterialRemove	Duel.Remove(mat,POS_FACEUP,REASON_EFFECT+REASON_MATERIAL+REASON_FUSION)
Auxiliary.FMaterialToDeck	Duel.SendtoDeck(mat,nil,2,REASON_EFFECT+REASON_MATERIAL+REASON_FUSION)

Manual scripts follow the same idea. Choose movement based on card text, and make sure the candidate material filter matches that movement. A card that will banish materials should check IsAbleToRemove; a card that sends materials to the GY should check IsAbleToGrave.

Chain Material

Polymerization-style scripts include a Chain Material fallback:

local ce=Duel.GetChainMaterial(tp)
if ce~=nil then
    local fgroup=ce:GetTarget()
    local mg2=fgroup(ce,e,tp)
    local mf=ce:GetValue()
    res=Duel.IsExistingMatchingCard(s.filter2,tp,LOCATION_EXTRA,0,1,nil,
        e,tp,mg2,mf,chkf)
end

If the selected Fusion Monster is Summoned through that Chain Material path, the script does not move the materials itself. It calls the Chain Material operation:

local mat2=Duel.SelectFusionMaterial(tp,tc,mg2,nil,chkf)
local fop=ce:GetOperation()
fop(ce,e,tp,tc,mat2)

For beginner scripts, keep the Chain Material block intact when copying a standard Fusion Spell. Removing only half of it can make activation checks and resolution disagree.

Contact Fusion

Contact Fusion is related, but it is not the same as a normal Fusion Spell. The local helper Auxiliary.AddContactFusionProcedure registers a Special Summon procedure on the Fusion Monster. Its material filter uses Card.IsCanBeFusionMaterial(fc,SUMMON_TYPE_SPECIAL), and its operation uses a material operation such as sending materials to the Deck.

That deserves its own focused guide later. For this page, the key distinction is simple: normal Fusion effects Summon with SUMMON_TYPE_FUSION; Contact Fusion procedures are custom Special Summon procedures that still use Fusion Material checks.

Common Pitfalls

Mistake	Better habit
Writing a Fusion Spell before the Fusion Monsters have material procedures.	Put EnableReviveLimit and an AddFusionProc... helper on the Fusion Monster first.
Using ordinary IsCode or IsSetCard inside Fusion Material filters.	Prefer IsFusionCode, IsFusionSetCard, IsFusionType, and IsFusionAttribute for Fusion Material identity.
Moving materials before SetMaterial.	Call tc:SetMaterial(mat) before sending, banishing, or returning the materials.
Sending materials without REASON_MATERIAL+REASON_FUSION.	Include REASON_EFFECT+REASON_MATERIAL+REASON_FUSION unless a local source-backed card pattern says otherwise.
Reusing the target-branch material group in the operation.	Rebuild and re-filter the material group at resolution.
Forgetting immunity checks in the operation.	Filter ordinary materials with not c:IsImmuneToEffect(e) or a local equivalent before selecting them.
Leaving aux.FCheckAdditional set.	Set it around the relevant check or selection, then clear it immediately.
Treating Contact Fusion as a normal SUMMON_TYPE_FUSION Spell.	Use the local Contact Fusion procedure pattern and SUMMON_TYPE_SPECIAL material checks.

Useful API Pages

• Auxiliary.AddFusionEffectProcUltimate
• Auxiliary.AddFusionEffectProc
• Auxiliary.AddFusionProcCode2
• Auxiliary.AddFusionProcFunRep
• Auxiliary.AddFusionProcMix
• Auxiliary.AddContactFusionProcedure
• Duel.GetFusionMaterial
• Duel.SelectFusionMaterial
• Duel.SetFusionMaterial
• Duel.GetChainMaterial
• Card.CheckFusionMaterial
• Card.IsCanBeFusionMaterial
• Card.IsFusionCode
• Card.IsFusionSetCard
• Card.IsFusionType
• Card.IsFusionAttribute
• Card.SetMaterial
• Card.CompleteProcedure
• Duel.SendtoGrave
• Duel.Remove
• Duel.SendtoDeck
• Duel.SpecialSummon