diff --git a/include/asm/format.hpp b/include/asm/format.hpp
index 7f9d793c..8a85c200 100644
--- a/include/asm/format.hpp
+++ b/include/asm/format.hpp
@@ -37,8 +37,8 @@ public:
 	void useCharacter(int c);
 	void finishCharacters();
 
-	std::string formatString(std::string const &value) const;
-	std::string formatNumber(uint32_t value) const;
+	void appendString(std::string &str, std::string const &value) const;
+	void appendNumber(std::string &str, uint32_t value) const;
 };
 
 #endif // RGBDS_FORMAT_SPEC_H
diff --git a/src/asm/format.cpp b/src/asm/format.cpp
index cc93a155..92ff1191 100644
--- a/src/asm/format.cpp
+++ b/src/asm/format.cpp
@@ -3,9 +3,7 @@
 #include "asm/format.hpp"
 
 #include <algorithm>
-#include <assert.h>
 #include <inttypes.h>
-#include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -105,7 +103,7 @@ void FormatSpec::finishCharacters() {
 		state = FORMAT_INVALID;
 }
 
-std::string FormatSpec::formatString(std::string const &value) const {
+void FormatSpec::appendString(std::string &str, std::string const &value) const {
 	int useType = type;
 	if (isEmpty()) {
 		// No format was specified
@@ -127,8 +125,7 @@ std::string FormatSpec::formatString(std::string const &value) const {
 	size_t totalLen = width > valueLen ? width : valueLen;
 	size_t padLen = totalLen - valueLen;
 
-	std::string str;
-	str.reserve(totalLen);
+	str.reserve(str.length() + totalLen);
 	if (alignLeft) {
 		str.append(value);
 		str.append(padLen, ' ');
@@ -141,11 +138,9 @@ std::string FormatSpec::formatString(std::string const &value) const {
 		error("Formatted string value too long\n");
 		str.resize(MAXSTRLEN);
 	}
-
-	return str;
 }
 
-std::string FormatSpec::formatNumber(uint32_t value) const {
+void FormatSpec::appendNumber(std::string &str, uint32_t value) const {
 	int useType = type;
 	bool usePrefix = prefix;
 	if (isEmpty()) {
@@ -224,8 +219,7 @@ std::string FormatSpec::formatNumber(uint32_t value) const {
 	size_t totalLen = width > numLen ? width : numLen;
 	size_t padLen = totalLen - numLen;
 
-	std::string str;
-	str.reserve(totalLen);
+	str.reserve(str.length() + totalLen);
 	if (alignLeft) {
 		if (signChar)
 			str += signChar;
@@ -256,6 +250,4 @@ std::string FormatSpec::formatNumber(uint32_t value) const {
 		error("Formatted numeric value too long\n");
 		str.resize(MAXSTRLEN);
 	}
-
-	return str;
 }
diff --git a/src/asm/lexer.cpp b/src/asm/lexer.cpp
index eb746cc1..18fd1c8f 100644
--- a/src/asm/lexer.cpp
+++ b/src/asm/lexer.cpp
@@ -1170,20 +1170,20 @@ static char const *readInterpolation(size_t depth) {
 	// Don't return before `lexerState->disableInterpolation` is reset!
 	lexerState->disableInterpolation = disableInterpolation;
 
-	static char buf[MAXSTRLEN + 1];
-
 	Symbol const *sym = sym_FindScopedValidSymbol(fmtBuf);
 
 	if (!sym) {
 		error("Interpolated symbol \"%s\" does not exist\n", fmtBuf.c_str());
 	} else if (sym->type == SYM_EQUS) {
-		std::string str = fmt.formatString(*sym->getEqus());
-		memcpy(buf, str.c_str(), str.length() + 1);
-		return buf;
+		static std::string buf;
+		buf.clear();
+		fmt.appendString(buf, *sym->getEqus());
+		return buf.c_str();
 	} else if (sym->isNumeric()) {
-		std::string str = fmt.formatNumber(sym->getConstantValue());
-		memcpy(buf, str.c_str(), str.length() + 1);
-		return buf;
+		static std::string buf;
+		buf.clear();
+		fmt.appendNumber(buf, sym->getConstantValue());
+		return buf.c_str();
 	} else {
 		error("Only numerical and string symbols can be interpolated\n");
 	}
diff --git a/src/asm/parser.y b/src/asm/parser.y
index 8fe0dc1d..b3e5dda9 100644
--- a/src/asm/parser.y
+++ b/src/asm/parser.y
@@ -2696,11 +2696,11 @@ static std::string strfmt(
 			// Will warn after formatting is done.
 			str += '%';
 		} else if (auto *n = std::get_if<uint32_t>(&args[argIndex]); n) {
-			str.append(fmt.formatNumber(*n));
+			fmt.appendNumber(str, *n);
 		} else {
 			assert(std::holds_alternative<std::string>(args[argIndex]));
 			auto &s = std::get<std::string>(args[argIndex]);
-			str.append(fmt.formatString(s));
+			fmt.appendString(str, s);
 		}
 
 		argIndex++;