diff --git a/numpower.c b/numpower.c
index f2e7507..f4cd122 100644
--- a/numpower.c
+++ b/numpower.c
@@ -297,6 +297,108 @@ PHP_METHOD(NDArray, gpu) {
 #endif
 }
 
+ZEND_BEGIN_ARG_INFO(arginfo_setdtype, 0)
+    ZEND_ARG_INFO(0, dtype)
+ZEND_END_ARG_INFO();
+/**
+ * @brief Sets the data type of this NDArray in place.
+ *
+ * ```
+ * setDataType(string $dtype): void
+ * ```
+ *
+ * Re-casts the array to the requested dtype, releasing the previous
+ * storage (unless still referenced by a live slice). This method mutates
+ * `$this` and returns nothing, consistent with the other in-place method
+ * `fill()`. The device (CPU or GPU) is preserved. If the target dtype
+ * equals the current dtype, the call is a true no-op with no allocation.
+ *
+ * Supported dtypes: float4, float8, float16, float32, float64, float128,
+ * int8, uint8, int16, uint16, int32, uint32, int64, uint64.
+ *
+ * @param dtype Target dtype alias.
+ *
+ * @throws \Error If the dtype is unknown.
+ */
+PHP_METHOD(NDArray, setDataType) {
+    char *dtype;
+    size_t dtypeLen = 0;
+    zval *obj_zval = getThis();
+
+    ZEND_PARSE_PARAMETERS_START(1, 1)
+        Z_PARAM_STRING(dtype, dtypeLen)
+    ZEND_PARSE_PARAMETERS_END();
+
+    NDArray *ndarray = NDArrayFactory_restoreFromZval(obj_zval);
+    if (ndarray == NULL) {
+        return;
+    }
+
+    const char *canonical = type_canonicalize(dtype);
+    if (canonical == NULL) {
+        zend_throw_error(NULL,
+            "Invalid data type '%s'. Supported: float4, float8, float16, "
+            "float32, float64, float128, int8, uint8, int16, uint16, "
+            "int32, uint32, int64, uint64", dtype);
+        return;
+    }
+
+    /* Fast path: already this dtype — true no-op, no allocation, no swap. */
+    if (is_type(NDArray_TYPE(ndarray), canonical)) {
+        return;
+    }
+
+    /* NDArray_AsType always returns a fresh, caller-owned, unbuffered
+       NDArray; the source `ndarray` is left intact. We then swap the new
+       array into the same buffer slot (preserving the object's uuid) and
+       release the previous occupant. NDArray_FREE is refcount-aware: if a
+       live slice still references the old array, its data survives until
+       the last view is freed. */
+    NDArray *casted = NDArray_AsType(ndarray, canonical);
+    if (casted == NULL) {
+        return; /* Original NDArray remains untouched. */
+    }
+
+    int uuid = NDArray_UUID(ndarray);
+    NDArray *prev = buffer_replace(uuid, casted);
+    if (prev) {
+        NDArray_FREE(prev);
+    }
+}
+
+ZEND_BEGIN_ARG_INFO(arginfo_datatype, 0)
+ZEND_END_ARG_INFO();
+/**
+ * @brief Returns the data type of this NDArray.
+ *
+ * ```
+ * dataType(): string
+ * ```
+ *
+ * One of the supported dtypes: float4, float8, float16, float32,
+ * float64, float128, int8, uint8, int16, uint16, int32, uint32,
+ * int64, uint64.
+ *
+ * Pair with `setDataType(string $dtype): void`.
+ */
+PHP_METHOD(NDArray, dataType) {
+    zval *obj_zval = getThis();
+
+    ZEND_PARSE_PARAMETERS_START(0, 0)
+    ZEND_PARSE_PARAMETERS_END();
+
+    NDArray *ndarray = NDArrayFactory_restoreFromZval(obj_zval);
+    if (ndarray == NULL) {
+        return;
+    }
+
+    /* NDArray_TYPE() points at a canonical static string from src/types.h
+       (e.g. "float32"); RETVAL_STRINGL copies the bytes into a fresh
+       PHP string zval so the descriptor's storage is not affected. */
+    const char *type = NDArray_TYPE(ndarray);
+    RETVAL_STRINGL(type, (size_t)strlen(type));
+}
+
 /**
  * @brief Fills the NDArray with a specified value.
  *
@@ -7422,6 +7524,8 @@ static const zend_function_entry class_NDArray_methods[] = {
     ZEND_ME(NDArray, gpu, arginfo_gpu, ZEND_ACC_PUBLIC)
     ZEND_ME(NDArray, cpu, arginfo_cpu, ZEND_ACC_PUBLIC)
     ZEND_ME(NDArray, isGPU, arginfo_is_gpu, ZEND_ACC_PUBLIC)
+    ZEND_ME(NDArray, setDataType, arginfo_setdtype, ZEND_ACC_PUBLIC)
+    ZEND_ME(NDArray, dataType, arginfo_datatype, ZEND_ACC_PUBLIC)
 
     ZEND_ME(NDArray, size, arginfo_size, ZEND_ACC_PUBLIC)
     ZEND_ME(NDArray, count, arginfo_count, ZEND_ACC_PUBLIC)
diff --git a/tests/types/029-setdatatype.phpt b/tests/types/029-setdatatype.phpt
new file mode 100644
index 0000000..c0d5a55
--- /dev/null
+++ b/tests/types/029-setdatatype.phpt
@@ -0,0 +1,125 @@
+--TEST--
+NDArray::setDataType() mutates dtype in place; dataType() returns current type
+--FILE--
+<?php
+/* setDataType() must:
+     - mutate $this in place (no change to object identity)
+     - return void (null)
+     - convert element values and PHP types to the target dtype
+     - preserve the device (a CPU array stays CPU)
+     - be a no-op when the target dtype equals the current one
+     - throw on an unknown dtype, leaving the array and its dataType() unchanged
+
+   dataType() must:
+     - return the current dtype as a string (e.g. 'float32')
+     - reflect the immediately-preceding setDataType() call */
+
+/* 1. float32 -> int32: values truncate, elements become ints;
+      in-place: same PHP object identity, alias observes the change */
+$a = new NDArray([1.5, 2.5, 3.7], 'float32');
+$aliasA = $a;                                /* alias must share the object */
+$idA = spl_object_id($a);
+$pre = $a->dataType();
+$ret = $a->setDataType('int32');
+$php = $a->toArray();
+$post = $a->dataType();
+$aliasOK = (spl_object_id($aliasA) === $idA)
+    && ($aliasA->dataType() === 'int32')
+    && ($aliasA->toArray() === [1, 2, 3])
+    && (array_map('gettype', $aliasA->toArray()) === ['integer','integer','integer']);
+$ok = ($pre === 'float32') && $post === 'int32'
+     && (spl_object_id($a) === $idA)         /* same PHP object after setDataType */
+     && ($ret === null) && $php === [1, 2, 3]
+     && array_map('gettype', $php) === ['integer', 'integer', 'integer']
+     && !$a->isGPU()
+     && $aliasOK;
+echo "float32->int32: pre=$pre post=$post ret=", ($ret === null ? 'null' : 'non-null'),
+     " values=", json_encode($php),
+     " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 2. int32 -> float64: elements become PHP floats (values preserved) */
+$b = new NDArray([1, 2, 3], 'int32');
+$before = $b->dataType();
+$b->setDataType('float64');
+$after = $b->dataType();
+$php = $b->toArray();
+$ok = $before === 'int32' && $after === 'float64'
+     && array_map('gettype', $php) === ['double', 'double', 'double']
+     && $php === [1.0, 2.0, 3.0];
+echo "int32->float64: pre=$before post=$after",
+     " values=", json_encode($php),
+     " types=", json_encode(array_map('gettype', $php)),
+     " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 3. unknown dtype throws; dataType() and values unchanged */
+$c = new NDArray([1.0, 2.0], 'float64');
+$idC = spl_object_id($c);
+$beforeType = $c->dataType();
+$beforeVals = $c->toArray();
+$threw = false; $tclass = '';
+try { $c->setDataType('badtype'); }
+catch (Throwable $t) { $threw = true; $tclass = get_class($t); }
+$ok = $threw
+     && (spl_object_id($c) === $idC)         /* object NOT replaced on throw */
+     && $c->dataType() === $beforeType
+     && $c->toArray() === $beforeVals
+     && array_map('gettype', $c->toArray()) === ['double', 'double'];
+echo "badtype: threw=", $tclass, " pre=$beforeType post=", $c->dataType(),
+     " vals=", json_encode($c->toArray()),
+     " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 4. same-dtype cast is a true no-op: dtype and values unchanged */
+$d = new NDArray([4, 5, 6], 'int32');
+$idD = spl_object_id($d);
+$d->setDataType('int32');
+$ok = (spl_object_id($d) === $idD)           /* no-op must not replace object */
+     && $d->dataType() === 'int32'
+     && $d->toArray() === [4, 5, 6]
+     && array_map('gettype', $d->toArray()) === ['integer','integer','integer']
+     && !$d->isGPU();
+echo "no-op: type=", $d->dataType(), " vals=", json_encode($d->toArray()),
+     " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 5. 0-D scalar (shape []) stays an NDArray; __toString is "7\n" */
+$e = new NDArray(7.0, 'float64');
+$pre = $e->dataType();
+$e->setDataType('int32');
+$post = $e->dataType();
+$ok = ($e instanceof NDArray) && $e->shape() === []
+     && ($pre === 'float64') && ($post === 'int32')
+     && trim((string)$e) === '7';
+echo "0-d: shape=", json_encode($e->shape()), " pre=$pre post=$post",
+     " str=", json_encode((string)$e), " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 6. device preserved: a CPU array stays CPU after setDataType */
+$f = new NDArray([1.0, 2.0, 3.0], 'float64');
+$f->setDataType('float32');
+$ok = ($f instanceof NDArray) && !$f->isGPU()
+     && $f->toArray() === [1.0, 2.0, 3.0]
+     && $f->dataType() === 'float32';
+echo "cpu: isGPU=", ($f->isGPU() ? 1 : 0), " type=", $f->dataType(),
+     " vals=", json_encode($f->toArray()), " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+
+/* 7. wrong argument counts reject */
+try {
+    (new NDArray([1, 2, 3], 'float32'))->setDataType();
+    echo "no-arg setDataType: NO-THROW\n";
+} catch (Throwable $t) {
+    echo "no-arg setDataType threw: ", get_class($t), "\n";
+}
+try {
+    (new NDArray([1, 2, 3], 'float32'))->dataType(123);
+    echo "dataType(123): NO-THROW\n";
+} catch (Throwable $t) {
+    echo "dataType(123) threw: ", get_class($t), "\n";
+}
+?>
+--EXPECT--
+float32->int32: pre=float32 post=int32 ret=null values=[1,2,3] ok=OK
+int32->float64: pre=int32 post=float64 values=[1,2,3] types=["double","double","double"] ok=OK
+badtype: threw=Error pre=float64 post=float64 vals=[1,2] ok=OK
+no-op: type=int32 vals=[4,5,6] ok=OK
+0-d: shape=[] pre=float64 post=int32 str="7\n" ok=OK
+cpu: isGPU=0 type=float32 vals=[1,2,3] ok=OK
+no-arg setDataType threw: ArgumentCountError
+dataType(123) threw: ArgumentCountError
diff --git a/tests/types/030-setdatatype-gpu.phpt b/tests/types/030-setdatatype-gpu.phpt
new file mode 100644
index 0000000..200193e
--- /dev/null
+++ b/tests/types/030-setdatatype-gpu.phpt
@@ -0,0 +1,50 @@
+--TEST--
+NDArray::setDataType() on GPU array stays on GPU and converts values in place
+--SKIPIF--
+<?php try { (new NDArray([1.0]))->gpu(); } catch (Error $e) { die('skip ' . $e->getMessage()); } ?>
+--FILE--
+<?php
+/* When the array is on the GPU, setDataType() must keep it on the GPU
+   and convert the values in place (the same object stays on device).
+   ->cpu() is used only to read values back — it returns a new array and
+   leaves $g on the GPU, so the isGPU() assertion is still meaningful. */
+
+$cases = [
+    ['float32', 'int32',   [1.5, 2.5, 3.5], [1, 2, 3]],
+    ['int32',   'float64', [1, 2, 3],      [1, 2, 3]],
+    ['float64', 'float32', [1.5, 2.5, 0.5], [1.5, 2.5, 0.5]],
+    ['int32',   'int16',   [1, 2, 3],      [1, 2, 3]],
+];
+
+foreach ($cases as [$src, $dst, $vals, $expect]) {
+    $g = (new NDArray($vals, $src))->gpu();
+    $pre = $g->dataType();
+    $g->setDataType($dst);
+    $post = $g->dataType();
+    $on_gpu = $g->isGPU();              /* still after setDataType() */
+    $back = $g->cpu()->toArray();       /* cpu() returns a new array; $g stays GPU */
+    $ok = ($g instanceof NDArray) && $pre === $src && $post === $dst && $on_gpu && $back === $expect;
+    echo "$src->$dst: pre=$pre post=$post",
+         " isGPU=", ($g->isGPU() ? 1 : 0),
+         " vals=", json_encode($back),
+         " ok=", ($ok ? 'OK' : 'BAD'), "\n";
+}
+
+/* unknown dtype on GPU throws too, leaving the array intact on GPU */
+$g2 = (new NDArray([1.0, 2.0], 'float32'))->gpu();
+$pre2 = $g2->dataType();
+try {
+    $g2->setDataType('nope');
+    echo "badtype: NO-THROW\n";
+} catch (Throwable $t) {
+    echo "badtype: pre=$pre2 post=", $g2->dataType(),
+         " isGPU=", ($g2->isGPU() ? 1 : 0),
+         " vals=", json_encode($g2->cpu()->toArray()), "\n";
+}
+?>
+--EXPECT--
+float32->int32: pre=float32 post=int32 isGPU=1 vals=[1,2,3] ok=OK
+int32->float64: pre=int32 post=float64 isGPU=1 vals=[1,2,3] ok=OK
+float64->float32: pre=float64 post=float32 isGPU=1 vals=[1.5,2.5,0.5] ok=OK
+int32->int16: pre=int32 post=int16 isGPU=1 vals=[1,2,3] ok=OK
+badtype: pre=float32 post=float32 isGPU=1 vals=[1,2]