diff --git a/ext/standard/substr_compare.php b/ext/standard/substr_compare.php
index 0ffbebdad8a..32499aff11a 100644
--- a/ext/standard/substr_compare.php
+++ b/ext/standard/substr_compare.php
@@ -10,6 +10,8 @@
 use PHPCompiler\JIT\Builtin\StringSubstrCompare;
 use PHPCompiler\JIT\Builtin\TypeErrorRaise;
 use PHPCompiler\JIT\Context;
+use PHPCompiler\JIT\ExceptionBridge;
+use PHPCompiler\JIT\JitBoolArg;
 use PHPCompiler\JIT\JitOperandTypeLabel;
 use PHPCompiler\JIT\JitStringArg;
 use PHPCompiler\JIT\JitStringBuiltinArg;
@@ -48,10 +50,16 @@ public function execute(Frame $frame): void
                 $length = self::requireIntArg($frame->calledArgs[3], 'substr_compare', 4, 'length');
             }
         }
+        // Z_PARAM_BOOL $case_insensitive — strict TypeError; soft-null DEP+coerce (#29756).
         $caseInsensitive = false;
         if (5 === $argc) {
-            $ci = $frame->calledArgs[4]->resolveIndirect();
-            $caseInsensitive = $ci->toBool();
+            $caseInsensitive = VmMath::parseBoolBuiltinArgForFrame(
+                $frame,
+                4,
+                'substr_compare',
+                5,
+                'case_insensitive'
+            );
         }
         if (null === $frame->returnVar) {
             return;
@@ -105,13 +113,27 @@ public function call(Context $context, JITVariable ...$args): Value
                 $lengthVal = self::lowerStrictIntArg($context, $args[3], 'substr_compare', 4, 'length');
             }
         }
+        // Z_PARAM_BOOL $case_insensitive — strict TypeError; soft-null DEP+coerce (#29756).
+        // Compile-time null under strict: emit catchable TypeError and stop (do not continue
+        // after terminator — lowerCoerceZParamBool would soft-null-DEP into a dead block).
         $ci = $i32->constInt(0, false);
         if (5 === $argc) {
-            if (JITVariable::TYPE_NATIVE_BOOL !== $args[4]->type) {
-                throw new \LogicException('substr_compare() case_insensitive must be a boolean in this compiler build');
+            if ($context->callerStrictTypes && self::isCompileTimeNull($args[4])) {
+                ExceptionBridge::emitTypeErrorAndAbort(
+                    $context,
+                    'substr_compare(): Argument #5 ($case_insensitive) must be of type bool, null given'
+                );
+
+                return $i64->constInt(0, false);
             }
             $ci = $context->builder->zExt(
-                $this->jitBool($context, $args[4], 'substr_compare() case_insensitive'),
+                JitBoolArg::lowerCoerceZParamBool(
+                    $context,
+                    $args[4],
+                    'substr_compare',
+                    'case_insensitive',
+                    5
+                ),
                 $i32
             );
         }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index a168cf66dfd..932be57536f 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -205,6 +205,10 @@
             <file>./test/compliance/RoundNullModeForward84JITTest.php</file>
             <file>./test/compliance/SubstrCompareNullOffsetVMTest.php</file>
             <file>./test/compliance/SubstrCompareNullOffsetJITTest.php</file>
+            <file>./test/compliance/SubstrCompareNullCiStrictVMTest.php</file>
+            <file>./test/compliance/SubstrCompareNullCiStrictJITTest.php</file>
+            <file>./test/compliance/SubstrCompareNullCiCoerceVMTest.php</file>
+            <file>./test/compliance/SubstrCompareNullCiCoerceJITTest.php</file>
             <file>./test/compliance/MetaphoneMaxPhonemesValueErrorVMTest.php</file>
             <file>./test/compliance/MetaphoneMaxPhonemesValueErrorJITTest.php</file>
             <file>./test/compliance/FilePutContentsEmptyPathValueErrorVMTest.php</file>
diff --git a/test/compliance/SubstrCompareNullCiCoerceJITTest.php b/test/compliance/SubstrCompareNullCiCoerceJITTest.php
new file mode 100644
index 00000000000..1d0a4f1d899
--- /dev/null
+++ b/test/compliance/SubstrCompareNullCiCoerceJITTest.php
@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/**
+ * @group llvm
+ */
+/** JIT: substr_compare(null $case_insensitive) soft-null DEP+coerce (#29756). */
+final class SubstrCompareNullCiCoerceJITTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'substr_compare_null_ci_coerce_jit.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/substr_compare_null_ci_coerce_jit.phpt',
+            'substr_compare_null_ci_coerce_jit.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/jit.php');
+    }
+}
diff --git a/test/compliance/SubstrCompareNullCiCoerceVMTest.php b/test/compliance/SubstrCompareNullCiCoerceVMTest.php
new file mode 100644
index 00000000000..7d1084f635a
--- /dev/null
+++ b/test/compliance/SubstrCompareNullCiCoerceVMTest.php
@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/** VM: substr_compare(null $case_insensitive) soft-null DEP+coerce (#29756). */
+final class SubstrCompareNullCiCoerceVMTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'substr_compare_null_ci_coerce.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/substr_compare_null_ci_coerce.phpt',
+            'substr_compare_null_ci_coerce.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/vm.php');
+    }
+}
diff --git a/test/compliance/SubstrCompareNullCiStrictJITTest.php b/test/compliance/SubstrCompareNullCiStrictJITTest.php
new file mode 100644
index 00000000000..758d7b24a79
--- /dev/null
+++ b/test/compliance/SubstrCompareNullCiStrictJITTest.php
@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/**
+ * @group llvm
+ */
+/** JIT: substr_compare(null $case_insensitive) under strict_types TypeError (#29756). */
+final class SubstrCompareNullCiStrictJITTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'substr_compare_null_ci_strict_jit.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/substr_compare_null_ci_strict_jit.phpt',
+            'substr_compare_null_ci_strict_jit.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/jit.php');
+    }
+}
diff --git a/test/compliance/SubstrCompareNullCiStrictVMTest.php b/test/compliance/SubstrCompareNullCiStrictVMTest.php
new file mode 100644
index 00000000000..2c6a6ad4935
--- /dev/null
+++ b/test/compliance/SubstrCompareNullCiStrictVMTest.php
@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+require_once __DIR__.'/../BaseTest.php';
+
+/** VM: substr_compare(null $case_insensitive) under strict_types TypeError (#29756). */
+final class SubstrCompareNullCiStrictVMTest extends BaseTest
+{
+    protected static string $DIR = __DIR__;
+
+    public static function providePHPTests(): \Generator
+    {
+        yield 'substr_compare_null_ci_strict.phpt' => self::parsePHPT(
+            __DIR__.'/cases/stdlib/substr_compare_null_ci_strict.phpt',
+            'substr_compare_null_ci_strict.phpt'
+        );
+    }
+
+    public function setUp(): void
+    {
+        $this->BIN = realpath(__DIR__.'/../../bin/vm.php');
+    }
+}
diff --git a/test/compliance/cases/stdlib/substr_compare_null_ci_coerce.phpt b/test/compliance/cases/stdlib/substr_compare_null_ci_coerce.phpt
new file mode 100644
index 00000000000..2ebd977004f
--- /dev/null
+++ b/test/compliance/cases/stdlib/substr_compare_null_ci_coerce.phpt
@@ -0,0 +1,23 @@
+--TEST--
+stdlib substr_compare(null $case_insensitive) soft-null DEP+coerce (#29756, ext/standard/string.c Z_PARAM_BOOL)
+--FILE--
+<?php
+set_error_handler(static function (int $no, string $msg): bool {
+    if (E_DEPRECATED === $no) {
+        echo "DEP\n";
+
+        return true;
+    }
+
+    return false;
+});
+try {
+    $r = substr_compare('abc', 'ab', 0, 2, null);
+    echo (0 === $r ? 'OK' : 'BAD '.var_export($r, true)), "\n";
+} catch (Throwable $e) {
+    echo get_class($e), ':', $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+DEP
+OK
diff --git a/test/compliance/cases/stdlib/substr_compare_null_ci_coerce_jit.phpt b/test/compliance/cases/stdlib/substr_compare_null_ci_coerce_jit.phpt
new file mode 100644
index 00000000000..6abb3cfd99d
--- /dev/null
+++ b/test/compliance/cases/stdlib/substr_compare_null_ci_coerce_jit.phpt
@@ -0,0 +1,23 @@
+--TEST--
+JIT: substr_compare(null $case_insensitive) soft-null DEP+coerce (#29756, ext/standard/string.c Z_PARAM_BOOL)
+--FILE--
+<?php
+set_error_handler(static function (int $no, string $msg): bool {
+    if (E_DEPRECATED === $no) {
+        echo "DEP\n";
+
+        return true;
+    }
+
+    return false;
+});
+try {
+    $r = substr_compare('abc', 'ab', 0, 2, null);
+    echo (0 === $r ? 'OK' : 'BAD '.var_export($r, true)), "\n";
+} catch (Throwable $e) {
+    echo get_class($e), ':', $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+DEP
+OK
diff --git a/test/compliance/cases/stdlib/substr_compare_null_ci_strict.phpt b/test/compliance/cases/stdlib/substr_compare_null_ci_strict.phpt
new file mode 100644
index 00000000000..000f84bb38c
--- /dev/null
+++ b/test/compliance/cases/stdlib/substr_compare_null_ci_strict.phpt
@@ -0,0 +1,21 @@
+--TEST--
+stdlib substr_compare(null $case_insensitive) under strict_types TypeError (#29756, ext/standard/string.c Z_PARAM_BOOL)
+--FILE--
+<?php
+declare(strict_types=1);
+
+try {
+    substr_compare('abc', 'ab', 0, 2, null);
+    echo "uncaught\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    substr_compare('abc', 'ab', 0, 2, 1);
+    echo "uncaught-int\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+--EXPECT--
+substr_compare(): Argument #5 ($case_insensitive) must be of type bool, null given
+substr_compare(): Argument #5 ($case_insensitive) must be of type bool, int given
diff --git a/test/compliance/cases/stdlib/substr_compare_null_ci_strict_jit.phpt b/test/compliance/cases/stdlib/substr_compare_null_ci_strict_jit.phpt
new file mode 100644
index 00000000000..c7b73150457
--- /dev/null
+++ b/test/compliance/cases/stdlib/substr_compare_null_ci_strict_jit.phpt
@@ -0,0 +1,21 @@
+--TEST--
+JIT: substr_compare(null $case_insensitive) under strict_types TypeError (#29756, ext/standard/string.c Z_PARAM_BOOL)
+--FILE--
+<?php
+declare(strict_types=1);
+
+try {
+    substr_compare('abc', 'ab', 0, 2, null);
+    echo "uncaught\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    substr_compare('abc', 'ab', 0, 2, 1);
+    echo "uncaught-int\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+--EXPECT--
+substr_compare(): Argument #5 ($case_insensitive) must be of type bool, null given
+substr_compare(): Argument #5 ($case_insensitive) must be of type bool, int given
diff --git a/test/fixtures/aot/cases/substr_compare_null_ci_strict.phpt b/test/fixtures/aot/cases/substr_compare_null_ci_strict.phpt
new file mode 100644
index 00000000000..75ed32ba55f
--- /dev/null
+++ b/test/fixtures/aot/cases/substr_compare_null_ci_strict.phpt
@@ -0,0 +1,14 @@
+--TEST--
+AOT: substr_compare(null $case_insensitive) under strict_types TypeError (#29756, ext/standard/string.c Z_PARAM_BOOL)
+--FILE--
+<?php
+declare(strict_types=1);
+
+try {
+    echo substr_compare('abc', 'ab', 0, 2, null), "\n";
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+substr_compare(): Argument #5 ($case_insensitive) must be of type bool, null given
diff --git a/test/repro/issue_substr_compare_null_ci.php b/test/repro/issue_substr_compare_null_ci.php
new file mode 100644
index 00000000000..5a4dc4b94a2
--- /dev/null
+++ b/test/repro/issue_substr_compare_null_ci.php
@@ -0,0 +1,9 @@
+<?php
+// #29756 — substr_compare(..., null) $case_insensitive under strict_types → TypeError
+declare(strict_types=1);
+
+try {
+    var_dump(substr_compare('abc', 'ab', 0, 2, null));
+} catch (Throwable $e) {
+    echo get_class($e), ': ', $e->getMessage(), "\n";
+}
diff --git a/test/repro/issue_substr_compare_null_ci_aot.php b/test/repro/issue_substr_compare_null_ci_aot.php
new file mode 100644
index 00000000000..37ba17fe659
--- /dev/null
+++ b/test/repro/issue_substr_compare_null_ci_aot.php
@@ -0,0 +1,9 @@
+<?php
+// #29756 — AOT: substr_compare(..., null) $case_insensitive under strict_types → TypeError
+declare(strict_types=1);
+
+try {
+    echo substr_compare('abc', 'ab', 0, 2, null), "\n";
+} catch (Throwable $e) {
+    echo get_class($e), ': ', $e->getMessage(), "\n";
+}