diff --git a/0xtest/unit/test_str_builder.c b/0xtest/unit/test_str_builder.c
new file mode 100644
index 0000000..c35aebb
--- /dev/null
+++ b/0xtest/unit/test_str_builder.c
@@ -0,0 +1,85 @@
+/*!
+ * @copyright
+ * Copyright (c) Microsoft Corporation.
+ * Licensed under the MIT license.
+ */
+
+#include "0xc/std/str_builder.h"
+#include "0xc/sys/check.h"
+#include "0xc/sys/unit.h"
+
+#include <string.h>
+
+static void
+UNIT_TEST_FUNC(str_builder_init)(
+        __unused const unit_t *un)
+{
+    char buff[10] = "";
+    xc_str_builder_t sb;
+    bool result = false;
+
+    result = xc_str_builder_init(&sb, buff, sizeof(buff));
+    checkint("valid initialization succeeds", result, true);
+    checkint("length is initialized to 0", sb.length, 0);
+    checkint("capacity is correctly set", sb.capacity, 10);
+
+    result = xc_str_builder_init(NULL, buff, 10);
+    checkint("null builder fails safely", result, false);
+
+    result = xc_str_builder_init(&sb, NULL, 10);
+    checkint("null buffer fails safely", result, false);
+
+    result = xc_str_builder_init(&sb, buff, 0);
+    checkint("zero capacity fails safely", result, false);
+}
+
+static void
+UNIT_TEST_FUNC(str_builder_append_overflow)(
+        __unused const unit_t *un)
+{
+    char buff[12] = "";
+    xc_str_builder_t sb;
+    bool result = false;
+
+    xc_str_builder_init(&sb, buff, sizeof(buff));
+
+    result = xc_str_builder_append(&sb, "Hello");
+    checkint("normal append succeeds", result, true);
+    checkint("length updated correctly", sb.length, 5);
+    checkcstr("buffer contains Hello", sb.buffer, "Hello");
+
+    result = xc_str_builder_append(&sb, " World");
+    checkint("second append succeeds", result, true);
+    checkint("length updated to 11", sb.length, 11);
+    checkcstr("buffer contains Hello World", sb.buffer, "Hello World");
+
+    /* OVERFLOW TEST: Capacity is 12. Length is 11. */
+    result = xc_str_builder_append(&sb, "!");
+    checkint("overflow append fails safely", result, false);
+    checkint("length remains 11", sb.length, 11);
+    checkcstr("buffer untouched by overflow", sb.buffer, "Hello World");
+}
+
+static void
+UNIT_TEST_FUNC(str_builder_format_truncation)(
+        __unused const unit_t *un)
+{
+    char buff[16] = "";
+    xc_str_builder_t sb;
+    bool result = false;
+
+    xc_str_builder_init(&sb, buff, sizeof(buff));
+
+    result = xc_str_builder_appendf(&sb, "Age: %d", 25);
+    checkint("formatted append succeeds", result, true);
+    checkcstr("buffer formatted correctly", sb.buffer, "Age: 25");
+
+    /* TRUNCATION TEST: Try to format a string that exceeds remaining space */
+    result = xc_str_builder_appendf(&sb, " is a very long string");
+    checkint("truncating format fails safely", result, false);
+    checkcstr("buffer untouched by truncation", sb.buffer, "Age: 25");
+}
+
+UNIT_TEST(str_builder_init);
+UNIT_TEST(str_builder_append_overflow);
+UNIT_TEST(str_builder_format_truncation);
\ No newline at end of file
diff --git a/src/0xc/std/str_builder.c b/src/0xc/std/str_builder.c
new file mode 100644
index 0000000..800bb91
--- /dev/null
+++ b/src/0xc/std/str_builder.c
@@ -0,0 +1,130 @@
+/*!
+ * @copyright
+ * Copyright (c) Microsoft Corporation.
+ * Licensed under the MIT license.
+ */
+
+#include "str_builder.h"
+#include <stdio.h>
+#include <string.h> 
+
+bool xc_str_builder_init(
+    xc_str_builder_t *sb,
+    char *buffer,
+    size_t capacity )   
+{ 
+    if (!sb || !buffer || !capacity) {
+        return false;
+    }
+
+    sb->buffer = buffer;
+    sb->capacity = capacity;
+    sb->length = 0;
+    sb->buffer[0] = '\0';
+
+    return true;
+}
+
+bool xc_str_builder_append(
+    xc_str_builder_t *sb, 
+    const char *suffix ) 
+{
+    if (!sb || !suffix) {
+        return false;
+    }
+
+    size_t suffix_len = strlen(suffix);
+    if (sb->length + suffix_len >= sb->capacity) {
+        return false;
+    }
+
+    memcpy(sb->buffer + sb->length, suffix, suffix_len);
+    sb->length += suffix_len;
+    sb->buffer[sb->length] = '\0';
+
+    return true;
+}
+
+bool xc_str_builder_appendf(
+    xc_str_builder_t *sb,
+    const char *format,
+    ... ) 
+{
+    if (!sb || !format) {
+        return false;
+    }
+
+    size_t remaining = sb->capacity - sb->length;
+    if (!remaining) {
+        return false;
+    }
+
+    va_list args;
+    va_start(args, format);
+        
+    int written = vsnprintf(sb->buffer + sb->length, remaining, format, args);
+        
+    va_end(args);
+        
+    if (written < 0) {
+        return false;
+    }
+    
+    // If it truncated, UNDO the partial write!
+    if ((size_t)written >= remaining) {
+        sb->buffer[sb->length] = '\0';
+        return false;
+    }
+        
+    sb->length += (size_t)written;
+        
+    return true;
+}
+
+bool xc_str_builder_append_char(
+    xc_str_builder_t *sb,
+    char c ) 
+{
+    if (!sb) {
+        return false;
+    }
+
+    if (sb->length + 1 >= sb->capacity) {
+        return false;
+    }
+
+    sb->buffer[sb->length] = c;
+    sb->length++;
+    sb->buffer[sb->length] = '\0';
+
+    return true;
+}
+
+void xc_str_builder_clear(
+    xc_str_builder_t *sb ) 
+{
+    if (!sb) {
+        return;
+    }
+
+    sb->length = 0;
+    sb->buffer[0] = '\0';
+}
+
+bool xc_str_builder_truncate(
+    xc_str_builder_t *sb,
+    size_t new_length ) 
+{
+    if (!sb) {
+        return false;
+    }
+
+    if (new_length > sb->length) {
+        return false;
+    }
+
+    sb->length = new_length;
+    sb->buffer[sb->length] = '\0';
+
+    return true;
+}
\ No newline at end of file
diff --git a/src/0xc/std/str_builder.h b/src/0xc/std/str_builder.h
new file mode 100644
index 0000000..3e09e37
--- /dev/null
+++ b/src/0xc/std/str_builder.h
@@ -0,0 +1,145 @@
+/*!
+ * @copyright
+ * Copyright (c) Microsoft Corporation.
+ * Licensed under the MIT license.
+ *
+ * @header
+ * Bounds-tracked string builder functions.
+ */
+#ifndef __ZX_STD_STR_BUILDER_H
+#define __ZX_STD_STR_BUILDER_H
+
+#include <0xc/std/api.h>
+
+// IWYU pragma: begin_exports
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stddef.h>
+// IWYU pragma: end_exports
+
+__API_HEADER_BEGIN(c, nonnull, single)
+
+// MARK: Public
+
+/*!
+ * @typedef xc_str_builder_t
+ * Wraps a statically-allocated character array to provide bounds-tracked
+ * string building and concatenation.
+ */
+typedef struct {
+    char *buffer;
+    size_t capacity;
+	size_t length;
+} xc_str_builder_t;
+
+/*!
+ * @function xc_str_builder_init
+ * Initializes the builder, wrapping an existing buffer safely.
+ *
+ * @param SB
+ * The string builder instance to initialize.
+ *
+ * @param BUFFER
+ * The statically-allocated destination buffer.
+ *
+ * @param CAPACITY
+ * The total capacity of the buffer (including the null terminator).
+ *
+ * @result
+ * Returns true if successfully initialized, false if arguments are invalid.
+ */
+bool xc_str_builder_init(
+	xc_str_builder_t *sb, 
+	char *buffer, 
+	size_t capacity
+);
+
+/*!
+ * @function xc_str_builder_append
+ * Appends a null-terminated string to the builder.
+ *
+ * @param SB
+ * The string builder instance.
+ *
+ * @param SUFFIX
+ * The null-terminated string to append.
+ *
+ * @result
+ * Returns true if the string was successfully appended. Returns false if the
+ * operation would exceed the buffer's capacity.
+ */
+bool xc_str_builder_append(
+	xc_str_builder_t *sb, 
+	const char *suffix
+);
+
+/*!
+ * @function xc_str_builder_appendf
+ * Safely formats and appends data without silent truncation.
+ *
+ * @param SB
+ * The string builder instance.
+ *
+ * @param FORMAT
+ * The format string, followed by format arguments.
+ *
+ * @result
+ * Returns true if the formatted string was completely appended. Returns false 
+ * if truncation occurred or arguments were invalid, leaving the buffer intact.
+ */
+bool xc_str_builder_appendf(
+	xc_str_builder_t *sb, 
+	const char *format, 
+	...
+);
+
+/*!
+ * @function xc_str_builder_append_char
+ * Appends a single character safely using O(1) performance.
+ *
+ * @param SB
+ * The string builder instance.
+ *
+ * @param C
+ * The character to append.
+ *
+ * @result
+ * Returns true if the character was appended, false if capacity was exceeded.
+ */
+bool xc_str_builder_append_char(
+	xc_str_builder_t *sb, 
+	char c
+);
+
+/*!
+ * @function xc_str_builder_clear
+ * Resets the string builder length to 0, allowing reuse of the buffer.
+ *
+ * @param SB
+ * The string builder instance.
+ */
+void xc_str_builder_clear(
+	xc_str_builder_t *sb
+);
+
+/*!
+ * @function xc_str_builder_truncate
+ * Safely shortens the string to the specified length.
+ *
+ * @param SB
+ * The string builder instance.
+ *
+ * @param NEW_LENGTH
+ * The length to shorten the string to.
+ *
+ * @result
+ * Returns true on success, false if the new length exceeds current length.
+ */
+bool xc_str_builder_truncate(
+	xc_str_builder_t *sb, 
+	size_t new_length
+);
+
+__API_HEADER_END(c, nonnull, single)
+
+#endif // __ZX_STD_STR_BUILDER_H
\ No newline at end of file