diff --git a/data/transactions/logic/README.md b/data/transactions/logic/README.md
index 3ba9037fc1..da4fdefac9 100644
--- a/data/transactions/logic/README.md
+++ b/data/transactions/logic/README.md
@@ -751,6 +751,12 @@ Account fields used in the `acct_params_get` opcode.
 
 ### Box Access
 
+Box opcodes that create, delete, or resize boxes affect the minimum
+balance requirement of the calling application's account.  The change
+is immediate, and can be observed after exection by using
+`min_balance`.  If the account does not possess the new minimum
+balance, the opcode fails.
+
 All box related opcodes fail immediately if used in a
 ClearStateProgram. This behavior is meant to discourage Smart Contract
 authors from depending upon the availability of boxes in a ClearState
@@ -763,13 +769,15 @@ are sure to be _available_.
 
 | Opcode | Description |
 | - | -- |
-| `box_create` | create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1 |
+| `box_create` | create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1 |
 | `box_extract` | read C bytes from box A, starting at offset B. Fail if A does not exist, or the byte range is outside A's size. |
 | `box_replace` | write byte-array C into box A, starting at offset B. Fail if A does not exist, or the byte range is outside A's size. |
+| `box_splice` | set box A to contain its previous bytes up to index B, followed by D, followed by the original bytes of A that began at index B+C. |
 | `box_del` | delete box named A if it exists. Return 1 if A existed, 0 otherwise |
 | `box_len` | X is the length of box A if A exists, else 0. Y is 1 if A exists, else 0. |
 | `box_get` | X is the contents of box A if A exists, else ''. Y is 1 if A exists, else 0. |
 | `box_put` | replaces the contents of box A with byte-array B. Fails if A exists and len(B) != len(box A). Creates A if it does not exist |
+| `box_resize` | change the size of box named A to be of length B, adding zero bytes to end or removing bytes from the end, as needed. Fail if the name A is empty, A is not an existing box, or B exceeds 32,768. |
 
 ### Inner Transactions
 
diff --git a/data/transactions/logic/README_in.md b/data/transactions/logic/README_in.md
index e98d6c2441..31f8fb05be 100644
--- a/data/transactions/logic/README_in.md
+++ b/data/transactions/logic/README_in.md
@@ -406,6 +406,12 @@ Account fields used in the `acct_params_get` opcode.
 
 ### Box Access
 
+Box opcodes that create, delete, or resize boxes affect the minimum
+balance requirement of the calling application's account.  The change
+is immediate, and can be observed after exection by using
+`min_balance`.  If the account does not possess the new minimum
+balance, the opcode fails.
+
 All box related opcodes fail immediately if used in a
 ClearStateProgram. This behavior is meant to discourage Smart Contract
 authors from depending upon the availability of boxes in a ClearState
diff --git a/data/transactions/logic/TEAL_opcodes_v10.md b/data/transactions/logic/TEAL_opcodes_v10.md
index 4ec00a52bb..7df81c33a8 100644
--- a/data/transactions/logic/TEAL_opcodes_v10.md
+++ b/data/transactions/logic/TEAL_opcodes_v10.md
@@ -1487,7 +1487,7 @@ The notation A,B indicates that A and B are interpreted as a uint128 value, with
 
 - Bytecode: 0xb9
 - Stack: ..., A: boxName, B: uint64 → ..., bool
-- create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
+- create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
 - Availability: v8
 - Mode: Application
 
@@ -1641,6 +1641,24 @@ Fields
 | 1 | BlkTimestamp | uint64 |  |
 
 
+## box_splice
+
+- Bytecode: 0xd2
+- Stack: ..., A: boxName, B: uint64, C: uint64, D: []byte → ...
+- set box A to contain its previous bytes up to index B, followed by D, followed by the original bytes of A that began at index B+C.
+- Availability: v10
+- Mode: Application
+
+Boxes are of constant length. If C < len(D), then len(D)-C bytes will be removed from the end. If C > len(D), zero bytes will be appended to the end to reach the box length.
+
+## box_resize
+
+- Bytecode: 0xd3
+- Stack: ..., A: boxName, B: uint64 &rarr; ...
+- change the size of box named A to be of length B, adding zero bytes to end or removing bytes from the end, as needed. Fail if the name A is empty, A is not an existing box, or B exceeds 32,768.
+- Availability: v10
+- Mode: Application
+
 ## ec_add
 
 - Syntax: `ec_add G` ∋ G: [EC](#field-group-ec)
diff --git a/data/transactions/logic/TEAL_opcodes_v8.md b/data/transactions/logic/TEAL_opcodes_v8.md
index b8efb37fa3..71d756a1dc 100644
--- a/data/transactions/logic/TEAL_opcodes_v8.md
+++ b/data/transactions/logic/TEAL_opcodes_v8.md
@@ -1485,7 +1485,7 @@ The notation A,B indicates that A and B are interpreted as a uint128 value, with
 
 - Bytecode: 0xb9
 - Stack: ..., A: boxName, B: uint64 &rarr; ..., bool
-- create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
+- create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
 - Availability: v8
 - Mode: Application
 
diff --git a/data/transactions/logic/TEAL_opcodes_v9.md b/data/transactions/logic/TEAL_opcodes_v9.md
index 54f053686f..f2ff330591 100644
--- a/data/transactions/logic/TEAL_opcodes_v9.md
+++ b/data/transactions/logic/TEAL_opcodes_v9.md
@@ -1485,7 +1485,7 @@ The notation A,B indicates that A and B are interpreted as a uint128 value, with
 
 - Bytecode: 0xb9
 - Stack: ..., A: boxName, B: uint64 &rarr; ..., bool
-- create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
+- create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1
 - Availability: v8
 - Mode: Application
 
diff --git a/data/transactions/logic/assembler_test.go b/data/transactions/logic/assembler_test.go
index 19c75d201a..4fdcaae615 100644
--- a/data/transactions/logic/assembler_test.go
+++ b/data/transactions/logic/assembler_test.go
@@ -430,7 +430,13 @@ pushbytess "1" "2" "1"
 const v8Nonsense = v7Nonsense + switchNonsense + frameNonsense + matchNonsense + boxNonsense
 
 const v9Nonsense = v8Nonsense
-const v10Nonsense = v9Nonsense + pairingNonsense
+
+const spliceNonsence = `
+  box_splice
+  box_resize
+`
+
+const v10Nonsense = v9Nonsense + pairingNonsense + spliceNonsence
 
 const v6Compiled = "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"
 
@@ -447,7 +453,10 @@ const matchCompiled = "83030102018e02fff500008203013101320131"
 const v8Compiled = v7Compiled + switchCompiled + frameCompiled + matchCompiled + boxCompiled
 
 const v9Compiled = v8Compiled
-const v10Compiled = v9Compiled + pairingCompiled
+
+const spliceCompiled = "d2d3"
+
+const v10Compiled = v9Compiled + pairingCompiled + spliceCompiled
 
 var nonsense = map[uint64]string{
 	1:  v1Nonsense,
@@ -527,7 +536,7 @@ func TestAssemble(t *testing.T) {
 	}
 }
 
-var experiments = []uint64{pairingVersion}
+var experiments = []uint64{pairingVersion, spliceVersion}
 
 // TestExperimental forces a conscious choice to promote "experimental" opcode
 // groups. This will fail when we increment vFuture's LogicSigVersion. If we had
diff --git a/data/transactions/logic/box.go b/data/transactions/logic/box.go
index ad371c6a1f..4938f95aa9 100644
--- a/data/transactions/logic/box.go
+++ b/data/transactions/logic/box.go
@@ -35,6 +35,8 @@ const (
 	BoxWriteOperation
 	// BoxDeleteOperation deletes a box
 	BoxDeleteOperation
+	// BoxResizeOperation resizes a box
+	BoxResizeOperation
 )
 
 func (cx *EvalContext) availableBox(name string, operation BoxOperation, createSize uint64) ([]byte, bool, error) {
@@ -81,6 +83,13 @@ func (cx *EvalContext) availableBox(name string, operation BoxOperation, createS
 			cx.available.dirtyBytes += writeSize
 		}
 		dirty = true
+	case BoxResizeOperation:
+		newSize := createSize
+		if dirty {
+			cx.available.dirtyBytes -= uint64(len(content))
+		}
+		cx.available.dirtyBytes += newSize
+		dirty = true
 	case BoxDeleteOperation:
 		if dirty {
 			cx.available.dirtyBytes -= uint64(len(content))
@@ -199,6 +208,34 @@ func opBoxReplace(cx *EvalContext) error {
 	return cx.Ledger.SetBox(cx.appID, name, bytes)
 }
 
+func opBoxSplice(cx *EvalContext) error {
+	last := len(cx.Stack) - 1 // replacement
+	replacement := cx.Stack[last].Bytes
+	length := cx.Stack[last-1].Uint
+	start := cx.Stack[last-2].Uint
+	name := string(cx.Stack[last-3].Bytes)
+
+	err := argCheck(cx, name, 0)
+	if err != nil {
+		return err
+	}
+
+	contents, exists, err := cx.availableBox(name, BoxWriteOperation, 0 /* size is already known */)
+	if err != nil {
+		return err
+	}
+	if !exists {
+		return fmt.Errorf("no such box %#x", name)
+	}
+
+	bytes, err := spliceCarefully(contents, replacement, start, length)
+	if err != nil {
+		return err
+	}
+	cx.Stack = cx.Stack[:last-3]
+	return cx.Ledger.SetBox(cx.appID, name, bytes)
+}
+
 func opBoxDel(cx *EvalContext) error {
 	last := len(cx.Stack) - 1 // name
 	name := string(cx.Stack[last].Bytes)
@@ -222,6 +259,48 @@ func opBoxDel(cx *EvalContext) error {
 	return nil
 }
 
+func opBoxResize(cx *EvalContext) error {
+	last := len(cx.Stack) - 1 // size
+	prev := last - 1          // name
+
+	name := string(cx.Stack[prev].Bytes)
+	size := cx.Stack[last].Uint
+
+	err := argCheck(cx, name, size)
+	if err != nil {
+		return err
+	}
+
+	contents, exists, err := cx.availableBox(name, BoxResizeOperation, size)
+	if err != nil {
+		return err
+	}
+
+	if !exists {
+		return fmt.Errorf("no such box %#x", name)
+	}
+	appAddr := cx.GetApplicationAddress(cx.appID)
+	_, err = cx.Ledger.DelBox(cx.appID, name, appAddr)
+	if err != nil {
+		return err
+	}
+	var resized []byte
+	if size > uint64(len(contents)) {
+		resized = make([]byte, size)
+		copy(resized, contents)
+	} else {
+		resized = contents[:size]
+	}
+	err = cx.Ledger.NewBox(cx.appID, name, resized, appAddr)
+	if err != nil {
+		return err
+	}
+
+	cx.Stack = cx.Stack[:prev]
+	return err
+
+}
+
 func opBoxLen(cx *EvalContext) error {
 	last := len(cx.Stack) - 1 // name
 	name := string(cx.Stack[last].Bytes)
@@ -292,3 +371,34 @@ func opBoxPut(cx *EvalContext) error {
 	appAddr := cx.GetApplicationAddress(cx.appID)
 	return cx.Ledger.NewBox(cx.appID, name, value, appAddr)
 }
+
+// spliceCarefully is used to make a NEW byteslice copy of original, with
+// replacement written over the bytes from start to start+length. Returned slice
+// is always the same size as original. Zero bytes are "shifted in" or high
+// bytes are "shifted out" as needed.
+func spliceCarefully(original []byte, replacement []byte, start uint64, olen uint64) ([]byte, error) {
+	if start > uint64(len(original)) {
+		return nil, fmt.Errorf("replacement start %d beyond length: %d", start, len(original))
+	}
+	oend := start + olen
+	if oend < start {
+		return nil, fmt.Errorf("splice end exceeds uint64")
+	}
+
+	if oend > uint64(len(original)) {
+		return nil, fmt.Errorf("splice end %d beyond original length: %d", oend, len(original))
+	}
+
+	// Do NOT use the append trick to make a copy here.
+	// append(nil, []byte{}...) would return a nil, which means "not a bytearray" to AVM.
+	clone := make([]byte, len(original))
+	copy(clone[:start], original)
+	copied := copy(clone[start:], replacement)
+	if copied != len(replacement) {
+		return nil, fmt.Errorf("splice inserted bytes too long")
+	}
+	// If original is "too short" we get zeros at the end. If original is "too
+	// long" we lose some bytes. Fortunately, that's what we want.
+	copy(clone[int(start)+copied:], original[oend:])
+	return clone, nil
+}
diff --git a/data/transactions/logic/box_test.go b/data/transactions/logic/box_test.go
index 5f08878a93..6649627440 100644
--- a/data/transactions/logic/box_test.go
+++ b/data/transactions/logic/box_test.go
@@ -41,11 +41,15 @@ func TestBoxNewDel(t *testing.T) {
 			ep, txn, ledger := MakeSampleEnv()
 
 			createSelf := fmt.Sprintf(`byte "self"; int %d; box_create;`, size)
+			growSelf := fmt.Sprintf(`byte "self"; int %d; box_resize; int 1`, size+5)
 			createOther := fmt.Sprintf(`byte "other"; int %d; box_create;`, size)
 
 			ledger.NewApp(txn.Sender, 888, basics.AppParams{})
 
+			TestApp(t, growSelf, ep, "no such box")
+
 			TestApp(t, createSelf, ep)
+			TestApp(t, growSelf, ep)
 			ledger.DelBoxes(888, "self")
 
 			TestApp(t, createSelf+`assert;`+createSelf+`!`, ep)
@@ -77,10 +81,13 @@ func TestBoxNewBad(t *testing.T) {
 	ledger.NewApp(txn.Sender, 888, basics.AppParams{})
 	TestApp(t, `byte "self"; int 999; box_create`, ep, "write budget")
 
-	// In test proto, you get 100 I/O budget per boxref
+	// In test proto, you get 100 I/O budget per boxref, and 1000 is the
+	// absolute biggest box.
 	ten := [10]transactions.BoxRef{}
 	txn.Boxes = append(txn.Boxes, ten[:]...) // write budget is now 11*100 = 1100
 	TestApp(t, `byte "self"; int 999; box_create`, ep)
+	TestApp(t, `byte "self"; int 1000; box_resize; int 1`, ep)
+	TestApp(t, `byte "self"; int 1001; box_resize; int 1`, ep, "box size too large")
 	ledger.DelBoxes(888, "self")
 	TestApp(t, `byte "self"; int 1000; box_create`, ep)
 	ledger.DelBoxes(888, "self")
@@ -139,6 +146,73 @@ func TestBoxReadWrite(t *testing.T) {
 		"no such box")
 	TestApp(t, `byte "junk"; int 1; byte 0x3031; box_replace`, ep,
 		"invalid Box reference")
+
+	TestApp(t, `byte "self"; int 1; int 2; byte 0x3031; box_splice`, ep,
+		"no such box")
+	TestApp(t, `byte "junk"; int 1; int 2; byte 0x3031; box_splice`, ep,
+		"invalid Box reference")
+}
+
+func TestBoxSplice(t *testing.T) {
+	partitiontest.PartitionTest(t)
+	t.Parallel()
+
+	ep, txn, ledger := MakeSampleEnv()
+
+	ledger.NewApp(txn.Sender, 888, basics.AppParams{})
+	// extract some bytes until past the end, confirm the begin as zeros, and
+	// when it fails.
+	TestApp(t, `byte "self"; int 4; box_create;`, ep)
+
+	// replace two bytes with two bytes. would usually use box_replace
+	TestApp(t, `byte "self"; int 1; int 2; byte 0x5555; box_splice;
+                byte "self"; box_get; assert; byte 0x00555500; ==`, ep)
+
+	// replace first 55 with two 44s.
+	TestApp(t, `byte "self"; int 1; int 1; byte 0x4444; box_splice;
+                byte "self"; box_get; assert; byte 0x00444455; ==`, ep)
+
+	// replace second 44 with two 33s. (loses the 55)
+	TestApp(t, `byte "self"; int 2; int 1; byte 0x3333; box_splice;
+                byte "self"; box_get; assert; byte 0x00443333; ==`, ep)
+
+	// replace 0044 with 22. (shifts in a 0x00)
+	TestApp(t, `byte "self"; int 0; int 2; byte 0x22; box_splice;
+                byte "self"; box_get; assert; byte 0x22333300; ==`, ep)
+
+	// dumb: try to replace 00 with 1111, but growing is illegal
+	TestApp(t, `byte "self"; int 3; int 1; byte 0x1111; box_splice;
+                byte "self"; box_get; assert; byte 0x2233331111; ==`, ep,
+		"inserted bytes too long")
+
+	// dumber: try to replace 00__ with 1111, but placing outside bounds is illegal
+	TestApp(t, `byte "self"; int 3; int 2; byte 0x1111; box_splice;
+                byte "self"; box_get; assert; byte 0x2233331111; ==`, ep,
+		"splice end 5 beyond original length")
+
+	// try to replace AT end (fails because it would extend)
+	TestApp(t, `byte "self"; int 4; int 0; byte 0x1111; box_splice;
+                byte "self"; box_get; assert; byte 0x223333001111; ==`, ep,
+		"splice inserted bytes too long")
+
+	// so it's ok if you splice in nothing
+	TestApp(t, `byte "self"; int 4; int 0; byte 0x; box_splice;
+                byte "self"; box_get; assert; byte 0x22333300; ==`, ep)
+
+	// try to replace BEYOND end (fails no matter what)
+	TestApp(t, `byte "self"; int 5; int 0; byte 0x1111; box_splice;
+                byte "self"; box_get; assert; byte 0x22333300001111; ==`, ep,
+		"replacement start 5 beyond length")
+
+	// even doing nothing is illegal beyond the end
+	TestApp(t, `byte "self"; int 5; int 0; byte 0x; box_splice;
+                byte "self"; box_get; assert; byte 0x22333300; ==`, ep,
+		"replacement start 5 beyond length")
+
+	// overflow doesn't work
+	TestApp(t, `byte "self"; int 2; int 18446744073709551615; byte 0x; box_splice;
+                byte "self"; box_get; assert; byte 0x22333300; ==`, ep,
+		"splice end exceeds uint64")
 }
 
 func TestBoxAcrossTxns(t *testing.T) {
@@ -167,22 +241,37 @@ func TestDirtyTracking(t *testing.T) {
 	partitiontest.PartitionTest(t)
 	t.Parallel()
 
-	ep, txn, ledger := MakeSampleEnv()
+	ep, txn, ledger := MakeSampleEnv() // has two box refs, "self", "other" = 200 budget
 
 	ledger.NewApp(txn.Sender, 888, basics.AppParams{})
 	TestApp(t, `byte "self"; int 200; box_create`, ep)
+	TestApp(t, `byte "self"; int 201; box_resize; int 1`, ep, "write budget")
 	TestApp(t, `byte "other"; int 201; box_create`, ep, "write budget")
 	// deleting "self" doesn't give extra write budget to create big "other"
-	TestApp(t, `byte "self"; box_del; !; byte "other"; int 201; box_create`, ep,
+	TestApp(t, `byte "self"; box_del; assert; byte "other"; int 201; box_create`, ep,
 		"write budget")
 
 	// though it cancels out a creation that happened here
 	TestApp(t, `byte "self"; int 200; box_create; assert
                       byte "self"; box_del; assert
-                      byte "self"; int 200; box_create;
+                      byte "other"; int 200; box_create;
+                     `, ep)
+	ledger.DelBoxes(888, "self", "other")
+
+	// create 200, but shrink it, then the write budget frees up
+	TestApp(t, `byte "self"; int 200; box_create; assert
+                      byte "self"; int 150; box_resize;
+                      byte "other"; int 50; box_create;
                      `, ep)
+	ledger.DelBoxes(888, "self", "other")
 
+	// confirm that the exactly right amount freed up
+	TestApp(t, `byte "self"; int 200; box_create; assert
+                      byte "self"; int 150; box_resize;
+                      byte "other"; int 51; box_create;
+                     `, ep, "write budget")
 	ledger.DelBoxes(888, "self", "other")
+
 	// same, but create a different box than deleted
 	TestApp(t, `byte "self"; int 200; box_create; assert
                       byte "self"; box_del; assert
@@ -217,6 +306,7 @@ func TestBoxUnavailableWithClearState(t *testing.T) {
 		"box_len":     `byte "self"; box_len`,
 		"box_put":     `byte "put"; byte "self"; box_put`,
 		"box_replace": `byte "self"; int 0; byte "new"; box_replace`,
+		"box_resize":  `byte "self"; int 10; box_resize`,
 	}
 
 	for name, program := range tests {
@@ -523,6 +613,8 @@ func TestEarlyPanics(t *testing.T) {
 		"box_len":     `byte "%s"; box_len`,
 		"box_put":     `byte "%s"; byte "hello"; box_put`,
 		"box_replace": `byte "%s"; int 0; byte "new"; box_replace`,
+		"box_splice":  `byte "%s"; int 0; int 2; byte "new"; box_splice`,
+		"box_resize":  `byte "%s"; int 2; box_resize`,
 	}
 
 	for name, program := range tests {
diff --git a/data/transactions/logic/doc.go b/data/transactions/logic/doc.go
index c060d82623..bfecb01a67 100644
--- a/data/transactions/logic/doc.go
+++ b/data/transactions/logic/doc.go
@@ -286,13 +286,15 @@ var opDescByName = map[string]OpDesc{
 	"frame_bury": {"replace the Nth (signed) value from the frame pointer in the stack with A", "", []string{"frame slot"}},
 	"popn":       {"remove N values from the top of the stack", "", []string{"stack depth"}},
 
-	"box_create":  {"create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1", "Newly created boxes are filled with 0 bytes. `box_create` will fail if the referenced box already exists with a different size. Otherwise, existing boxes are unchanged by `box_create`.", nil},
+	"box_create":  {"create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1", "Newly created boxes are filled with 0 bytes. `box_create` will fail if the referenced box already exists with a different size. Otherwise, existing boxes are unchanged by `box_create`.", nil},
 	"box_extract": {"read C bytes from box A, starting at offset B. Fail if A does not exist, or the byte range is outside A's size.", "", nil},
 	"box_replace": {"write byte-array C into box A, starting at offset B. Fail if A does not exist, or the byte range is outside A's size.", "", nil},
+	"box_splice":  {"set box A to contain its previous bytes up to index B, followed by D, followed by the original bytes of A that began at index B+C.", "Boxes are of constant length. If C < len(D), then len(D)-C bytes will be removed from the end. If C > len(D), zero bytes will be appended to the end to reach the box length.", nil},
 	"box_del":     {"delete box named A if it exists. Return 1 if A existed, 0 otherwise", "", nil},
 	"box_len":     {"X is the length of box A if A exists, else 0. Y is 1 if A exists, else 0.", "", nil},
 	"box_get":     {"X is the contents of box A if A exists, else ''. Y is 1 if A exists, else 0.", "For boxes that exceed 4,096 bytes, consider `box_create`, `box_extract`, and `box_replace`", nil},
 	"box_put":     {"replaces the contents of box A with byte-array B. Fails if A exists and len(B) != len(box A). Creates A if it does not exist", "For boxes that exceed 4,096 bytes, consider `box_create`, `box_extract`, and `box_replace`", nil},
+	"box_resize":  {"change the size of box named A to be of length B, adding zero bytes to end or removing bytes from the end, as needed. Fail if the name A is empty, A is not an existing box, or B exceeds 32,768.", "", nil},
 }
 
 // OpDoc returns a description of the op
@@ -351,7 +353,7 @@ var OpGroups = map[string][]string{
 	"Loading Values":          {"intcblock", "intc", "intc_0", "intc_1", "intc_2", "intc_3", "pushint", "pushints", "bytecblock", "bytec", "bytec_0", "bytec_1", "bytec_2", "bytec_3", "pushbytes", "pushbytess", "bzero", "arg", "arg_0", "arg_1", "arg_2", "arg_3", "args", "txn", "gtxn", "txna", "txnas", "gtxna", "gtxnas", "gtxns", "gtxnsa", "gtxnsas", "global", "load", "loads", "store", "stores", "gload", "gloads", "gloadss", "gaid", "gaids"},
 	"Flow Control":            {"err", "bnz", "bz", "b", "return", "pop", "popn", "dup", "dup2", "dupn", "dig", "bury", "cover", "uncover", "frame_dig", "frame_bury", "swap", "select", "assert", "callsub", "proto", "retsub", "switch", "match"},
 	"State Access":            {"balance", "min_balance", "app_opted_in", "app_local_get", "app_local_get_ex", "app_global_get", "app_global_get_ex", "app_local_put", "app_global_put", "app_local_del", "app_global_del", "asset_holding_get", "asset_params_get", "app_params_get", "acct_params_get", "log", "block"},
-	"Box Access":              {"box_create", "box_extract", "box_replace", "box_del", "box_len", "box_get", "box_put"},
+	"Box Access":              {"box_create", "box_extract", "box_replace", "box_splice", "box_del", "box_len", "box_get", "box_put", "box_resize"},
 	"Inner Transactions":      {"itxn_begin", "itxn_next", "itxn_field", "itxn_submit", "itxn", "itxna", "itxnas", "gitxn", "gitxna", "gitxnas"},
 }
 
diff --git a/data/transactions/logic/eval.go b/data/transactions/logic/eval.go
index 2db51b24a7..0e1faec0de 100644
--- a/data/transactions/logic/eval.go
+++ b/data/transactions/logic/eval.go
@@ -3962,7 +3962,7 @@ func replaceCarefully(original []byte, replacement []byte, start uint64) ([]byte
 		return nil, fmt.Errorf("replacement start %d beyond length: %d", start, len(original))
 	}
 	end := start + uint64(len(replacement))
-	if end < start { // impossible because it is sum of two avm value lengths
+	if end < start { // impossible because it is sum of two avm value (or box) lengths
 		return nil, fmt.Errorf("replacement end exceeds uint64")
 	}
 
diff --git a/data/transactions/logic/langspec_v10.json b/data/transactions/logic/langspec_v10.json
index 7ee54fb98d..affe46601e 100644
--- a/data/transactions/logic/langspec_v10.json
+++ b/data/transactions/logic/langspec_v10.json
@@ -4242,7 +4242,7 @@
       ],
       "Size": 1,
       "DocCost": "1",
-      "Doc": "create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
+      "Doc": "create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
       "DocExtra": "Newly created boxes are filled with 0 bytes. `box_create` will fail if the referenced box already exists with a different size. Otherwise, existing boxes are unchanged by `box_create`.",
       "IntroducedVersion": 8,
       "Groups": [
@@ -4644,6 +4644,39 @@
         "State Access"
       ]
     },
+    {
+      "Opcode": 210,
+      "Name": "box_splice",
+      "Args": [
+        "boxName",
+        "uint64",
+        "uint64",
+        "[]byte"
+      ],
+      "Size": 1,
+      "DocCost": "1",
+      "Doc": "set box A to contain its previous bytes up to index B, followed by D, followed by the original bytes of A that began at index B+C.",
+      "DocExtra": "Boxes are of constant length. If C \u003c len(D), then len(D)-C bytes will be removed from the end. If C \u003e len(D), zero bytes will be appended to the end to reach the box length.",
+      "IntroducedVersion": 10,
+      "Groups": [
+        "Box Access"
+      ]
+    },
+    {
+      "Opcode": 211,
+      "Name": "box_resize",
+      "Args": [
+        "boxName",
+        "uint64"
+      ],
+      "Size": 1,
+      "DocCost": "1",
+      "Doc": "change the size of box named A to be of length B, adding zero bytes to end or removing bytes from the end, as needed. Fail if the name A is empty, A is not an existing box, or B exceeds 32,768.",
+      "IntroducedVersion": 10,
+      "Groups": [
+        "Box Access"
+      ]
+    },
     {
       "Opcode": 224,
       "Name": "ec_add",
diff --git a/data/transactions/logic/langspec_v8.json b/data/transactions/logic/langspec_v8.json
index 3b496ddcb8..4963f4c85a 100644
--- a/data/transactions/logic/langspec_v8.json
+++ b/data/transactions/logic/langspec_v8.json
@@ -4238,7 +4238,7 @@
       ],
       "Size": 1,
       "DocCost": "1",
-      "Doc": "create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
+      "Doc": "create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
       "DocExtra": "Newly created boxes are filled with 0 bytes. `box_create` will fail if the referenced box already exists with a different size. Otherwise, existing boxes are unchanged by `box_create`.",
       "IntroducedVersion": 8,
       "Groups": [
diff --git a/data/transactions/logic/langspec_v9.json b/data/transactions/logic/langspec_v9.json
index c52d36862d..50418be824 100644
--- a/data/transactions/logic/langspec_v9.json
+++ b/data/transactions/logic/langspec_v9.json
@@ -4238,7 +4238,7 @@
       ],
       "Size": 1,
       "DocCost": "1",
-      "Doc": "create a box named A, of length B. Fail if A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
+      "Doc": "create a box named A, of length B. Fail if the name A is empty or B exceeds 32,768. Returns 0 if A already existed, else 1",
       "DocExtra": "Newly created boxes are filled with 0 bytes. `box_create` will fail if the referenced box already exists with a different size. Otherwise, existing boxes are unchanged by `box_create`.",
       "IntroducedVersion": 8,
       "Groups": [
diff --git a/data/transactions/logic/opcodeExplain.go b/data/transactions/logic/opcodeExplain.go
index 4b4f965a65..3643ade21f 100644
--- a/data/transactions/logic/opcodeExplain.go
+++ b/data/transactions/logic/opcodeExplain.go
@@ -197,6 +197,12 @@ func opBoxReplaceStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, bas
 	return BoxState, AppStateWrite, cx.appID, basics.Address{}, string(cx.Stack[pprev].Bytes)
 }
 
+func opBoxSpliceStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, basics.AppIndex, basics.Address, string) {
+	name := len(cx.Stack) - 4 // name, start, length, replacement
+
+	return BoxState, AppStateWrite, cx.appID, basics.Address{}, string(cx.Stack[name].Bytes)
+}
+
 func opBoxDelStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, basics.AppIndex, basics.Address, string) {
 	last := len(cx.Stack) - 1 // name
 
@@ -210,6 +216,12 @@ func opBoxPutStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, basics.
 	return BoxState, AppStateWrite, cx.appID, basics.Address{}, string(cx.Stack[prev].Bytes)
 }
 
+func opBoxResizeStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, basics.AppIndex, basics.Address, string) {
+	name := len(cx.Stack) - 2 // name, size
+
+	return BoxState, AppStateWrite, cx.appID, basics.Address{}, string(cx.Stack[name].Bytes)
+}
+
 func opAppLocalGetStateChange(cx *EvalContext) (AppStateEnum, AppStateOpEnum, basics.AppIndex, basics.Address, string) {
 	last := len(cx.Stack) - 1 // state key
 	prev := last - 1          // account
diff --git a/data/transactions/logic/opcodes.go b/data/transactions/logic/opcodes.go
index dc2d44bc09..fa3fd22625 100644
--- a/data/transactions/logic/opcodes.go
+++ b/data/transactions/logic/opcodes.go
@@ -75,6 +75,7 @@ const sharedResourcesVersion = 9 // apps can access resources from other transac
 // moved from vFuture to a new consensus version. If they remain unready, bump
 // their version, and fixup TestAssemble() in assembler_test.go.
 const pairingVersion = 10 // bn256 opcodes. will add bls12-381, and unify the available opcodes.
+const spliceVersion = 10  // box splicing/resizing
 
 // Unlimited Global Storage opcodes
 const boxVersion = 8 // box_*
@@ -721,6 +722,8 @@ var OpSpecs = []OpSpec{
 	// randomness support
 	{0xd0, "vrf_verify", opVrfVerify, proto("b83:bT"), randomnessVersion, field("s", &VrfStandards).costs(5700)},
 	{0xd1, "block", opBlock, proto("i:a"), randomnessVersion, field("f", &BlockFields)},
+	{0xd2, "box_splice", opBoxSplice, proto("Niib:").appStateExplain(opBoxSpliceStateChange), spliceVersion, only(ModeApp)},
+	{0xd3, "box_resize", opBoxResize, proto("Ni:").appStateExplain(opBoxResizeStateChange), spliceVersion, only(ModeApp)},
 
 	{0xe0, "ec_add", opEcAdd, proto("bb:b"), pairingVersion,
 		costByField("g", &EcGroups, []int{
diff --git a/data/transactions/logic/teal.tmLanguage.json b/data/transactions/logic/teal.tmLanguage.json
index 56cf7b5dc8..ef80fd048a 100644
--- a/data/transactions/logic/teal.tmLanguage.json
+++ b/data/transactions/logic/teal.tmLanguage.json
@@ -72,7 +72,7 @@
         },
         {
           "name": "keyword.other.unit.teal",
-          "match": "^(box_create|box_del|box_extract|box_get|box_len|box_put|box_replace|acct_params_get|app_global_del|app_global_get|app_global_get_ex|app_global_put|app_local_del|app_local_get|app_local_get_ex|app_local_put|app_opted_in|app_params_get|asset_holding_get|asset_params_get|balance|block|log|min_balance)\\b"
+          "match": "^(box_create|box_del|box_extract|box_get|box_len|box_put|box_replace|box_resize|box_splice|acct_params_get|app_global_del|app_global_get|app_global_get_ex|app_global_put|app_local_del|app_local_get|app_local_get_ex|app_local_put|app_opted_in|app_params_get|asset_holding_get|asset_params_get|balance|block|log|min_balance)\\b"
         },
         {
           "name": "keyword.operator.teal",