diff --git a/.github/actions/test-helloworld/action.yml b/.github/actions/test-helloworld/action.yml
index 9020e7887b..54474a8447 100644
--- a/.github/actions/test-helloworld/action.yml
+++ b/.github/actions/test-helloworld/action.yml
@@ -58,6 +58,6 @@ runs:
         func main() {
         }
         EOL
-        llgo build -v -target esp32-coreboard-v2 -o demo.out .
-        test -f demo.out.elf && echo "ESP32 cross-compilation test passed: demo.out.elf generated"
+        llgo build -v -target esp32-coreboard-v2 -o demo.elf .
+        test -f demo.elf && echo "ESP32 cross-compilation test passed: demo.elf generated"
         exit $?
diff --git a/.github/workflows/llgo.yml b/.github/workflows/llgo.yml
index d37781368e..350c6dfa83 100644
--- a/.github/workflows/llgo.yml
+++ b/.github/workflows/llgo.yml
@@ -347,7 +347,7 @@ jobs:
           echo "Testing cross-compilation wasm with $(go env GOVERSION)"
 
           # Compile for wasm architecture
-          GOOS=wasip1 GOARCH=wasm llgo build -o hello -tags=nogc -v ./helloc
+          GOOS=wasip1 GOARCH=wasm llgo build -o hello.wasm -tags=nogc -v ./helloc
 
           # Check file type
           file hello.wasm
diff --git a/_demo/embed/export/verify_export.sh b/_demo/embed/export/verify_export.sh
index 41dd62b15f..275eb3c239 100755
--- a/_demo/embed/export/verify_export.sh
+++ b/_demo/embed/export/verify_export.sh
@@ -6,7 +6,7 @@ echo "Building for embedded target..."
 
 # Build for embedded target as executable
 # Use llgo directly instead of llgo.sh to avoid go.mod version check
-llgo build -o test-verify --target=esp32 .
+llgo build -o test-verify.elf --target=esp32 .
 
 echo "Checking exported symbols..."
 
@@ -44,7 +44,7 @@ echo ""
 
 echo "Testing that non-embedded target rejects different export names..."
 # Build without --target should fail with panic
-if llgo build -o test-notarget . 2>&1 | grep -q 'export comment has wrong name "LPSPI2_IRQHandler"'; then
+if llgo build -o test-notarget.elf . 2>&1 | grep -q 'export comment has wrong name "LPSPI2_IRQHandler"'; then
     echo "✅ Correctly rejected different export name on non-embedded target"
 else
     echo "❌ Should have panicked with 'export comment has wrong name' error"
diff --git a/_demo/embed/test_esp32c3_startup.sh b/_demo/embed/test_esp32c3_startup.sh
index 647cb234d2..26558d862b 100755
--- a/_demo/embed/test_esp32c3_startup.sh
+++ b/_demo/embed/test_esp32c3_startup.sh
@@ -99,7 +99,7 @@ EOF
 
 echo "==> Building for ESP32-C3 target (ELF + BIN)..."
 cd "$TEMP_DIR"
-llgo build -target=esp32c3 -o test -obin "$TEST_GO"
+llgo build -target=esp32c3 -o test.elf -obin "$TEST_GO"
 
 if [ ! -f "$TEST_ELF" ]; then
     echo "✗ FAIL: Build failed, $TEST_ELF not found"
diff --git a/_demo/go/export/test.sh b/_demo/go/export/test.sh
index 2b292381b7..74a9d10afd 100755
--- a/_demo/go/export/test.sh
+++ b/_demo/go/export/test.sh
@@ -163,18 +163,22 @@ if [[ "$run_build_mode_tests" == true ]]; then
 
 # Test 1: c-shared mode
 print_status "=== Test 1: Building with -buildmode c-shared ==="
-if $LLGO_SCRIPT build -buildmode c-shared -o export .; then
+if [[ "$OSTYPE" == "darwin"* ]]; then
+    SHARED_LIB="libexport.dylib"
+else
+    SHARED_LIB="libexport.so"
+fi
+
+if $LLGO_SCRIPT build -buildmode c-shared -o "$SHARED_LIB" .; then
     print_status "Build succeeded"
 
     # Check generated files (different extensions on different platforms)
     if [[ "$OSTYPE" == "darwin"* ]]; then
         # macOS
         check_file "libexport.dylib" "Dynamic library (libexport.dylib)"
-        SHARED_LIB="libexport.dylib"
     else
         # Linux and others
         check_file "libexport.so" "Dynamic library (libexport.so)"
-        SHARED_LIB="libexport.so"
     fi
 
     check_file "libexport.h" "C header (libexport.h)"
@@ -216,7 +220,7 @@ fi
 
 # Test 2: c-archive mode
 print_status "=== Test 2: Building with -buildmode c-archive ==="
-if $LLGO_SCRIPT build -buildmode c-archive -o export .; then
+if $LLGO_SCRIPT build -buildmode c-archive -o libexport.a .; then
     print_status "Build succeeded"
 
     # Check generated files
@@ -314,6 +318,7 @@ else
     print_error "Go export demo execution failed"
     print_error "Error output:"
     cat /tmp/go_export_output.log | sed 's/^/    /'
+    build_failures=$((build_failures + 1))
 fi
 
 # Cleanup temporary file
diff --git a/_demo/go/export/use/Makefile b/_demo/go/export/use/Makefile
index ee7dcee847..da66ac8688 100644
--- a/_demo/go/export/use/Makefile
+++ b/_demo/go/export/use/Makefile
@@ -48,7 +48,7 @@ all: build-go $(TARGET)
 # Build the Go library first
 build-go:
 	@echo $(BUILD_MSG)
-	cd .. && ../../../dev/llgo.sh build -buildmode $(BUILDMODE) $(LLGOFLAGS) -o export .
+	cd .. && ../../../dev/llgo.sh build -buildmode $(BUILDMODE) $(LLGOFLAGS) -o $(notdir $(LIBRARY)) .
 
 # Build the C executable
 $(TARGET): $(SOURCES) $(LIBRARY) $(HEADER)
diff --git a/doc/Embedded_Cmd.md b/doc/Embedded_Cmd.md
index dc4dd762c8..a258567df8 100644
--- a/doc/Embedded_Cmd.md
+++ b/doc/Embedded_Cmd.md
@@ -2,7 +2,7 @@
 
 ## Flags
 
-- `-o <file>` - Specify output file name
+- `-o <file>` - Specify output file name (used as-is; no prefix or extension is added)
 - `-target <platform>` - Specify target platform for cross-compilation
 - `-obin` - Generate binary format output (requires `-target`)
 - `-ohex` - Generate Intel HEX format output (requires `-target`)
diff --git a/internal/build/build.go b/internal/build/build.go
index 6f870695c1..8458a321c2 100644
--- a/internal/build/build.go
+++ b/internal/build/build.go
@@ -733,8 +733,7 @@ func Build(inv Invocation) ([]Package, error) {
 
 			// Generate C headers for c-archive and c-shared modes before linking
 			if ctx.buildConf.BuildMode == BuildModeCArchive || ctx.buildConf.BuildMode == BuildModeCShared {
-				libname := strings.TrimSuffix(filepath.Base(outFmts.Out), conf.AppExt)
-				headerPath := filepath.Join(filepath.Dir(outFmts.Out), libname) + ".h"
+				libname, headerPath := cHeaderOutputPath(outFmts.Out)
 				pkgs := cHeaderPackages(allPkgs)
 				headerErr := header.GenHeaderFile(prog, pkgs, libname, headerPath, verbose)
 				if headerErr != nil {
@@ -801,6 +800,11 @@ func Build(inv Invocation) ([]Package, error) {
 	return allPkgs, nil
 }
 
+func cHeaderOutputPath(output string) (libname, headerPath string) {
+	libname = strings.TrimSuffix(filepath.Base(output), filepath.Ext(output))
+	return libname, filepath.Join(filepath.Dir(output), libname) + ".h"
+}
+
 // cHeaderPackages excludes the patched standard runtime implementation. Its
 // //export callbacks are linker implementation details and may use internal C
 // types that are deliberately not representable in a public generated header.
diff --git a/internal/build/build_test.go b/internal/build/build_test.go
index 05c28144ff..d2d38135c0 100644
--- a/internal/build/build_test.go
+++ b/internal/build/build_test.go
@@ -981,13 +981,33 @@ func TestRunPrintfWithStdioNobuf(t *testing.T) {
 	mockRun([]string{"../../cl/_testdata/printf"}, &Config{Mode: ModeRun})
 }
 
+func TestCHeaderOutputPath(t *testing.T) {
+	tests := []struct {
+		output         string
+		wantLibname    string
+		wantHeaderPath string
+	}{
+		{output: "libmylib.a", wantLibname: "libmylib", wantHeaderPath: "libmylib.h"},
+		{output: "build/custom.so", wantLibname: "custom", wantHeaderPath: "build/custom.h"},
+		{output: "/tmp/custom.dylib", wantLibname: "custom", wantHeaderPath: "/tmp/custom.h"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.output, func(t *testing.T) {
+			libname, headerPath := cHeaderOutputPath(tt.output)
+			if libname != tt.wantLibname || headerPath != tt.wantHeaderPath {
+				t.Fatalf("cHeaderOutputPath(%q) = (%q, %q), want (%q, %q)", tt.output, libname, headerPath, tt.wantLibname, tt.wantHeaderPath)
+			}
+		})
+	}
+}
+
 func TestTestOutputFileLogic(t *testing.T) {
 	// Test output file path determination logic for test mode
 	tests := []struct {
 		name        string
 		pkgName     string
 		conf        *Config
-		multiPkg    bool
 		wantBase    string
 		wantDir     string
 		description string
@@ -996,34 +1016,14 @@ func TestTestOutputFileLogic(t *testing.T) {
 			name:        "compile only without -o",
 			pkgName:     "mypackage.test",
 			conf:        &Config{Mode: ModeTest, CompileOnly: true},
-			multiPkg:    false,
 			wantBase:    "mypackage.test",
 			wantDir:     ".",
 			description: "-c without -o: write pkg.test in current directory",
 		},
-		{
-			name:        "with -o absolute file path",
-			pkgName:     "mypackage",
-			conf:        &Config{Mode: ModeTest, OutFile: "/tmp/mytest.test", AppExt: ".test"},
-			multiPkg:    false,
-			wantBase:    "mytest",
-			wantDir:     "/tmp",
-			description: "-o with absolute file path: use specified file",
-		},
-		{
-			name:        "with -o relative file path",
-			pkgName:     "mypackage",
-			conf:        &Config{Mode: ModeTest, OutFile: "my.test", AppExt: ".test"},
-			multiPkg:    false,
-			wantBase:    "my",
-			wantDir:     ".",
-			description: "-o with relative file path: use specified file in current dir",
-		},
 		{
 			name:        "with -o directory",
 			pkgName:     "mypackage.test",
 			conf:        &Config{Mode: ModeTest, OutFile: "/tmp/build/", AppExt: ".test"},
-			multiPkg:    false,
 			wantBase:    "mypackage.test",
 			wantDir:     "/tmp/build/",
 			description: "-o with directory: write pkg.test in that directory",
@@ -1032,7 +1032,6 @@ func TestTestOutputFileLogic(t *testing.T) {
 			name:        "default test mode",
 			pkgName:     "mypackage",
 			conf:        &Config{Mode: ModeTest, AppExt: ".test"},
-			multiPkg:    false,
 			wantBase:    "mypackage",
 			wantDir:     "",
 			description: "default test mode: use temp file",
@@ -1041,7 +1040,7 @@ func TestTestOutputFileLogic(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			baseName, dir := determineBaseNameAndDir(tt.pkgName, tt.conf, tt.multiPkg)
+			baseName, dir := determineBaseNameAndDir(tt.pkgName, tt.conf)
 			if baseName != tt.wantBase {
 				t.Errorf("%s: got baseName=%q, want %q", tt.description, baseName, tt.wantBase)
 			}
diff --git a/internal/build/outputs.go b/internal/build/outputs.go
index 901268e364..aeb2013540 100644
--- a/internal/build/outputs.go
+++ b/internal/build/outputs.go
@@ -54,36 +54,17 @@ func setOutFmt(conf *Config, formatName string) {
 	}
 }
 
-// buildOutFmts creates OutFmtDetails based on package, configuration and multi-package status
 // determineBaseNameAndDir extracts the base name and directory from configuration
-func determineBaseNameAndDir(pkgName string, conf *Config, multiPkg bool) (baseName, dir string) {
+func determineBaseNameAndDir(pkgName string, conf *Config) (baseName, dir string) {
 	switch conf.Mode {
 	case ModeInstall:
 		return pkgName, conf.BinPath
 	case ModeBuild:
-		if !multiPkg && conf.OutFile != "" {
-			dir = filepath.Dir(conf.OutFile)
-			baseName = strings.TrimSuffix(filepath.Base(conf.OutFile), conf.AppExt)
-			if dir == "." {
-				dir = ""
-			}
-			return baseName, dir
-		}
 		return pkgName, ""
 	case ModeTest:
 		if conf.OutFile != "" {
-			// Handle -o flag for test mode
-			if strings.HasSuffix(conf.OutFile, "/") || isDir(conf.OutFile) {
-				// If OutFile ends in / or is a directory, write pkg.test in that directory
-				// pkgName for test packages already includes .test suffix
-				return pkgName, conf.OutFile
-			}
-			// Otherwise, use the specified file path
-			dir = filepath.Dir(conf.OutFile)
-			baseName = strings.TrimSuffix(filepath.Base(conf.OutFile), conf.AppExt)
-			// Don't convert "." to "" for test mode with explicit output
-			// This preserves the information that user specified an output file
-			return baseName, dir
+			// Explicit file paths are handled by buildOutFmts before reaching here.
+			return pkgName, conf.OutFile
 		}
 		if conf.CompileOnly {
 			// -c without -o: write pkg.test in current directory
@@ -103,6 +84,15 @@ func isDir(path string) bool {
 	return err == nil && info.IsDir()
 }
 
+// explicitOutFile reports whether the user controls the final output path verbatim.
+func explicitOutFile(conf *Config, multiPkg bool) bool {
+	if conf.OutFile == "" || multiPkg {
+		return false
+	}
+	return conf.Mode == ModeBuild ||
+		(conf.Mode == ModeTest && !strings.HasSuffix(conf.OutFile, "/") && !isDir(conf.OutFile))
+}
+
 // applyPrefix applies build mode specific naming conventions
 func applyPrefix(baseName string, buildMode BuildMode, target string, goos string) string {
 	// Determine the effective OS for naming conventions
@@ -140,7 +130,9 @@ func applyPrefix(baseName string, buildMode BuildMode, target string, goos strin
 
 // buildOutputPath creates the final output path from baseName, dir and other parameters
 func buildOutputPath(baseName, dir string, conf *Config, multiPkg bool, appExt string) (string, error) {
-	baseName = applyPrefix(baseName, conf.BuildMode, conf.Target, conf.Goos)
+	if conf.OutFile == "" {
+		baseName = applyPrefix(baseName, conf.BuildMode, conf.Target, conf.Goos)
+	}
 
 	if dir != "" {
 		// dir == "." means current directory (explicit user specification)
@@ -161,18 +153,21 @@ func buildOutputPath(baseName, dir string, conf *Config, multiPkg bool, appExt s
 	}
 }
 
+// buildOutFmts creates OutFmtDetails based on package, configuration and multi-package status.
 func buildOutFmts(pkgName string, conf *Config, multiPkg bool, crossCompile *crosscompile.Export) (*OutFmtDetails, error) {
 	details := &OutFmtDetails{}
 
-	// Determine base name and directory
-	baseName, dir := determineBaseNameAndDir(pkgName, conf, multiPkg)
-
-	// Build output path
-	outputPath, err := buildOutputPath(baseName, dir, conf, multiPkg, conf.AppExt)
-	if err != nil {
-		return nil, err
+	if explicitOutFile(conf, multiPkg) {
+		// An explicit -o file name is authoritative: do not rewrite its prefix or extension.
+		details.Out = conf.OutFile
+	} else {
+		baseName, dir := determineBaseNameAndDir(pkgName, conf)
+		outputPath, err := buildOutputPath(baseName, dir, conf, multiPkg, conf.AppExt)
+		if err != nil {
+			return nil, err
+		}
+		details.Out = outputPath
 	}
-	details.Out = outputPath
 	if conf.PCLNMode == PCLNExternal {
 		details.PCLN = pclnSidecarPath(details.Out)
 	}
diff --git a/internal/build/outputs_test.go b/internal/build/outputs_test.go
index 53686d8950..f8137ac7ae 100644
--- a/internal/build/outputs_test.go
+++ b/internal/build/outputs_test.go
@@ -4,6 +4,8 @@
 package build
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -451,7 +453,18 @@ func TestBuildOutFmtsBuildModes(t *testing.T) {
 			target:      "",
 			goos:        "linux",
 			appExt:      ".a",
-			expectedOut: "libcustom.a",
+			expectedOut: "custom.a",
+		},
+		{
+			name:        "c_archive_build_with_outfile_without_extension",
+			pkgName:     "mylib",
+			buildMode:   BuildModeCArchive,
+			outFile:     "custom",
+			mode:        ModeBuild,
+			target:      "",
+			goos:        "linux",
+			appExt:      ".a",
+			expectedOut: "custom",
 		},
 		{
 			name:        "c_archive_build_with_path",
@@ -462,7 +475,7 @@ func TestBuildOutFmtsBuildModes(t *testing.T) {
 			target:      "",
 			goos:        "linux",
 			appExt:      ".a",
-			expectedOut: "build/libcustom.a",
+			expectedOut: "build/custom.a",
 		},
 
 		// C-Shared tests
@@ -499,6 +512,17 @@ func TestBuildOutFmtsBuildModes(t *testing.T) {
 			appExt:      ".dylib",
 			expectedOut: "libmylib.dylib",
 		},
+		{
+			name:        "c_shared_build_with_outfile",
+			pkgName:     "mylib",
+			buildMode:   BuildModeCShared,
+			outFile:     "custom.so",
+			mode:        ModeBuild,
+			target:      "",
+			goos:        "darwin",
+			appExt:      ".dylib",
+			expectedOut: "custom.so",
+		},
 		{
 			name:        "c_shared_embedded_target",
 			pkgName:     "mylib",
@@ -545,6 +569,28 @@ func TestBuildOutFmtsBuildModes(t *testing.T) {
 			appExt:      "",
 			expectedOut: "myapp",
 		},
+		{
+			name:        "exe_preserve_outfile_lib_prefix",
+			pkgName:     "myapp",
+			buildMode:   BuildModeExe,
+			outFile:     "libmyapp",
+			mode:        ModeBuild,
+			target:      "",
+			goos:        "linux",
+			appExt:      "",
+			expectedOut: "libmyapp",
+		},
+		{
+			name:        "exe_preserve_outfile_extension",
+			pkgName:     "myapp",
+			buildMode:   BuildModeExe,
+			outFile:     "myapp.bin",
+			mode:        ModeBuild,
+			target:      "",
+			goos:        "windows",
+			appExt:      ".exe",
+			expectedOut: "myapp.bin",
+		},
 	}
 
 	for _, tt := range tests {
@@ -573,6 +619,53 @@ func TestBuildOutFmtsBuildModes(t *testing.T) {
 	}
 }
 
+func TestBuildOutFmtsExplicitOutFile(t *testing.T) {
+	tests := []struct {
+		name        string
+		pkgName     string
+		conf        *Config
+		multiPkg    bool
+		wantOut     string
+		wantTempExt string
+	}{
+		{
+			name:    "test explicit file path",
+			pkgName: "mypackage.test",
+			conf: &Config{
+				Mode: ModeTest, OutFile: "custom.test", AppExt: ".test",
+			},
+			wantOut: "custom.test",
+		},
+		{
+			name:    "multiple build packages ignore explicit output file",
+			pkgName: "mylib",
+			conf: &Config{
+				Mode: ModeBuild, OutFile: "custom.a", BuildMode: BuildModeCArchive, AppExt: ".a",
+			},
+			multiPkg:    true,
+			wantTempExt: ".a",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := buildOutFmts(tt.pkgName, tt.conf, tt.multiPkg, &crosscompile.Export{})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if tt.wantOut != "" && got.Out != tt.wantOut {
+				t.Fatalf("buildOutFmts().Out = %q, want %q", got.Out, tt.wantOut)
+			}
+			if tt.wantTempExt != "" {
+				t.Cleanup(func() { _ = os.Remove(got.Out) })
+				if got.Out == tt.conf.OutFile || !strings.HasPrefix(filepath.Base(got.Out), tt.pkgName+"-") || !strings.HasSuffix(got.Out, tt.wantTempExt) {
+					t.Fatalf("buildOutFmts().Out = %q, want a temporary %s file for %q", got.Out, tt.wantTempExt, tt.pkgName)
+				}
+			}
+		})
+	}
+}
+
 func TestApplyBuildModeNaming(t *testing.T) {
 	tests := []struct {
 		name      string