diff --git a/cmd/gasm/asmhdr.go b/cmd/gasm/asmhdr.go
new file mode 100644
index 0000000..275d02c
--- /dev/null
+++ b/cmd/gasm/asmhdr.go
@@ -0,0 +1,354 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/constant"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+
+	"sourcedock.dev/petrbalvin/gasm-devkit/arch"
+)
+
+// go_asm.h is the header the Go compiler writes for every package that
+// carries assembly (the compiler's -asmhdr output): "#define const_NAME
+// value" for each package constant, and for each named struct type
+// "#define TYPE__size size" plus one "#define TYPE_field offset" per field.
+// GOROOT assembly includes it, and a standalone assembler has no compiler
+// to have produced it, so gasm generates the equivalent itself: the package
+// the .s file lives in is parsed and type-checked here, with the target
+// architecture's own sizes, and the same defines are written out.
+//
+// The emitter mirrors cmd/compile's dumpasmhdr exactly: constants come out
+// as "const_NAME", struct entries as "NAME__size" followed by the fields in
+// declaration order, blank names are skipped, and float and complex
+// constants are omitted (the assembler carries integers, bools and strings
+// only).  Aliases to structs are emitted, generic types are not: they have
+// no fixed size.  A define the assembly references but this header does not
+// carry surfaces later as the assembler's own "undefined" diagnostic naming
+// the define, which is the honest failure.
+
+// goAsmInclude matches the #include "go_asm.h" directive, tolerant of
+// whitespace, so the wiring knows which files need a generated header
+// before the preprocessor runs and would report the header as missing.
+var goAsmInclude = regexp.MustCompile(`(?m)^\s*#\s*include\s+"go_asm\.h"`)
+
+// needsGoAsmHeader reports whether src includes go_asm.h.
+func needsGoAsmHeader(src string) bool {
+	return goAsmInclude.MatchString(src)
+}
+
+// goAsmHeaderResolved reports whether the include of go_asm.h from a file in
+// asmDir already resolves: to a header in the package directory itself, or
+// in one of the -I directories, the way the preprocessor searches.  Only an
+// unresolved include is generated for; a header someone placed by hand is
+// the tool the author chose, and it also wins the preprocessor's own search
+// order, so generating a second copy would be dead weight at best.
+func goAsmHeaderResolved(asmDir string, dirs []string) bool {
+	candidates := []string{filepath.Join(asmDir, "go_asm.h")}
+	for _, d := range dirs {
+		candidates = append(candidates, filepath.Join(d, "go_asm.h"))
+	}
+	for _, candidate := range candidates {
+		if st, err := os.Stat(candidate); err == nil && !st.IsDir() {
+			return true
+		}
+	}
+	return false
+}
+
+// generateGoAsmHeader type-checks the Go package in pkgDir for goarch,
+// writes its go_asm.h equivalent into dir, and returns dir.  The caller
+// owns the directory and its removal.
+func generateGoAsmHeader(pkgDir, goarch, dir string) (string, error) {
+	imp := newSourceImporter(goarch)
+	if imp.sizes == nil {
+		return "", fmt.Errorf("go_asm.h: unknown GOARCH %q", goarch)
+	}
+	bp, err := imp.ctxt.ImportDir(pkgDir, 0)
+	if err != nil {
+		return "", fmt.Errorf("go_asm.h for GOARCH %s in %s: %w", goarch, pkgDir, err)
+	}
+	files, errs := imp.parse(bp)
+	if len(errs) > 0 {
+		return "", fmt.Errorf("go_asm.h for GOARCH %s in %s: %s", goarch, pkgDir, errorList(errs))
+	}
+	_, info, errs := imp.checkPackage(bp, files)
+	if len(errs) > 0 {
+		return "", fmt.Errorf("go_asm.h for GOARCH %s in %s: package does not type-check: %s", goarch, pkgDir, errorList(errs))
+	}
+
+	var b strings.Builder
+	fmt.Fprintf(&b, "// generated by gasm from package %s (GOARCH %s)\n\n", bp.Name, goarch)
+	// Files in the build's own order and declarations in source order: the
+	// same walk the compiler's reader makes, so the header reads the same
+	// way the toolchain's does.  Order carries no meaning to the assembler
+	// (defines form a table), only to a human diffing against one.
+	for _, f := range files {
+		for _, decl := range f.Decls {
+			gd, ok := decl.(*ast.GenDecl)
+			if !ok {
+				continue
+			}
+			for _, spec := range gd.Specs {
+				switch gd.Tok {
+				case token.CONST:
+					vs, ok := spec.(*ast.ValueSpec)
+					if !ok {
+						continue
+					}
+					for _, name := range vs.Names {
+						emitConst(&b, info.Defs[name], name.Name)
+					}
+				case token.TYPE:
+					ts, ok := spec.(*ast.TypeSpec)
+					if !ok {
+						continue
+					}
+					emitStruct(&b, imp.sizes, info.Defs[ts.Name], ts.Name.Name)
+				}
+			}
+		}
+	}
+
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", fmt.Errorf("go_asm.h for GOARCH %s in %s: %w", goarch, pkgDir, err)
+	}
+	out := filepath.Join(dir, "go_asm.h")
+	if err := os.WriteFile(out, []byte(b.String()), 0o644); err != nil {
+		return "", fmt.Errorf("go_asm.h for GOARCH %s in %s: %w", goarch, pkgDir, err)
+	}
+	return dir, nil
+}
+
+// emitConst writes one const define, skipping what the toolchain skips:
+// blank names, and float and complex values the assembler has no syntax for.
+func emitConst(b *strings.Builder, obj types.Object, name string) {
+	c, ok := obj.(*types.Const)
+	if !ok || name == "_" {
+		return
+	}
+	switch c.Val().Kind() {
+	case constant.Float, constant.Complex, constant.Unknown:
+		return
+	}
+	fmt.Fprintf(b, "#define const_%s %s\n", name, c.Val().ExactString())
+}
+
+// emitStruct writes one named struct type's size and field offsets,
+// skipping what the toolchain skips: blank names, non-struct types, and
+// generic types, whose size depends on their instantiation.
+func emitStruct(b *strings.Builder, sizes types.Sizes, obj types.Object, name string) {
+	tn, ok := obj.(*types.TypeName)
+	if !ok || name == "_" {
+		return
+	}
+	t := types.Unalias(tn.Type())
+	// Generic types are spelled *types.Named with a type-parameter list;
+	// a plain struct type or an instantiated one carries none.
+	if named, ok := t.(*types.Named); ok && named.TypeParams().Len() > 0 {
+		return
+	}
+	st, ok := t.Underlying().(*types.Struct)
+	if !ok {
+		return
+	}
+	fmt.Fprintf(b, "#define %s__size %d\n", name, sizes.Sizeof(t))
+	fields := make([]*types.Var, st.NumFields())
+	for i := range st.NumFields() {
+		fields[i] = st.Field(i)
+	}
+	for i, off := range sizes.Offsetsof(fields) {
+		fld := fields[i]
+		if fld.Name() == "_" {
+			continue
+		}
+		fmt.Fprintf(b, "#define %s_%s %d\n", name, fld.Name(), off)
+	}
+}
+
+// errorList renders at most three errors, enough to say what is wrong
+// without burying the diagnostic the caller actually reads.
+func errorList(errs []error) string {
+	if len(errs) > 3 {
+		errs = errs[:3]
+	}
+	msgs := make([]string, len(errs))
+	for i, err := range errs {
+		msgs[i] = err.Error()
+	}
+	return strings.Join(msgs, "; ")
+}
+
+// sourceImporter type-checks imported packages from source with the target
+// architecture's sizes.  go/importer's "source" importer pins the host
+// GOARCH, which would lay out imported types (internal/cpu, internal/abi)
+// for the wrong target on a cross-architecture header, so the recursion is
+// carried here with one build context and one sizes instance per
+// architecture.
+type sourceImporter struct {
+	fset  *token.FileSet
+	ctxt  *build.Context
+	sizes types.Sizes
+	pkgs  map[string]*types.Package
+}
+
+// newSourceImporter returns the importer for one target architecture.
+// Cgo is disabled so the file set is deterministic and independent of the
+// host's C toolchain: cgo-tagged files drop out of the build exactly as
+// they do from a CGO_ENABLED=0 build, whose assembly is what gasm targets.
+func newSourceImporter(goarch string) *sourceImporter {
+	ctxt := new(build.Context)
+	*ctxt = build.Default
+	ctxt.GOARCH = goarch
+	ctxt.CgoEnabled = false
+	return &sourceImporter{
+		fset:  token.NewFileSet(),
+		ctxt:  ctxt,
+		sizes: types.SizesFor("gc", goarch),
+		pkgs:  map[string]*types.Package{},
+	}
+}
+
+// Import type-checks one imported package and memoises it.  "unsafe" must
+// resolve to go/types' own package, never to the source in GOROOT/src/unsafe:
+// the source declares Sizeof and Offsetof as ordinary functions over
+// ArbitraryType, and checking against that signature rejects half the
+// unsafe arithmetic the gc compiler accepts, which is exactly the divergence
+// srcimporter guards against the same way.
+func (im *sourceImporter) Import(path string) (*types.Package, error) {
+	if path == "unsafe" {
+		return types.Unsafe, nil
+	}
+	if p, ok := im.pkgs[path]; ok {
+		return p, nil
+	}
+	bp, err := im.ctxt.Import(path, "", 0)
+	if err != nil {
+		return nil, err
+	}
+	files, errs := im.parse(bp)
+	if len(errs) > 0 {
+		return nil, errors.New(errorList(errs))
+	}
+	pkg, _, _ := im.checkPackage(bp, files)
+	im.pkgs[path] = pkg
+	return pkg, nil
+}
+
+// parse reads the build package's Go files.  Import-level failures (no Go
+// files for the target, unreadable files) come back as errors, and the
+// type-check decides the rest.
+func (im *sourceImporter) parse(bp *build.Package) ([]*ast.File, []error) {
+	if len(bp.GoFiles) == 0 {
+		return nil, []error{fmt.Errorf("no Go source files for GOOS=%s GOARCH=%s", im.ctxt.GOOS, im.ctxt.GOARCH)}
+	}
+	var (
+		files []*ast.File
+		errs  []error
+	)
+	for _, name := range bp.GoFiles {
+		f, err := parser.ParseFile(im.fset, filepath.Join(bp.Dir, name), nil, parser.SkipObjectResolution)
+		if err != nil {
+			errs = append(errs, err)
+			continue
+		}
+		files = append(files, f)
+	}
+	return files, errs
+}
+
+// checkPackage type-checks one package's files with the importer's sizes,
+// recording every error: a header from a package that does not type-check
+// could silently mis-state an offset, so the caller refuses the header
+// rather than trusting it.  The returned Defs map backs the root package's
+// emission walk; imports only need the checked package itself.
+func (im *sourceImporter) checkPackage(bp *build.Package, files []*ast.File) (*types.Package, *types.Info, []error) {
+	var errs []error
+	conf := &types.Config{
+		Importer: im,
+		Sizes:    im.sizes,
+		Error:    func(err error) { errs = append(errs, err) },
+	}
+	info := &types.Info{Defs: map[*ast.Ident]types.Object{}}
+	pkg, _ := conf.Check(bp.ImportPath, im.fset, files, info)
+	return pkg, info, errs
+}
+
+// asmhdrCache generates one go_asm.h per package directory and target
+// architecture under one temp root, for callers that assemble many files
+// (the corpus audit).  Failures are cached too: a package that does not
+// type-check must not be re-checked once per file.
+type asmhdrCache struct {
+	root string
+	dirs map[string]string // "pkgDir\x00goarch" -> directory holding go_asm.h
+	errs map[string]error
+}
+
+func newAsmhdrCache() (*asmhdrCache, error) {
+	root, err := os.MkdirTemp("", "gasm-asmhdr")
+	if err != nil {
+		return nil, err
+	}
+	return &asmhdrCache{root: root, dirs: map[string]string{}, errs: map[string]error{}}, nil
+}
+
+// dirFor returns the directory holding the generated go_asm.h for pkgDir
+// and goarch, generating it on first use.
+func (c *asmhdrCache) dirFor(pkgDir, goarch string) (string, error) {
+	key := pkgDir + "\x00" + goarch
+	if dir, ok := c.dirs[key]; ok {
+		return dir, nil
+	}
+	if err, ok := c.errs[key]; ok {
+		return "", err
+	}
+	dir := filepath.Join(c.root, fmt.Sprintf("h%d_%s", len(c.dirs), goarch))
+	if _, err := generateGoAsmHeader(pkgDir, goarch, dir); err != nil {
+		c.errs[key] = err
+		return "", err
+	}
+	c.dirs[key] = dir
+	return dir, nil
+}
+
+// close removes the temp root.
+func (c *asmhdrCache) close() { os.RemoveAll(c.root) }
+
+// ensureGoAsmHeader prepares the include directory a file that includes
+// go_asm.h needs: the generated header for the package in path's directory,
+// for the file's target architecture.  It reports a usage error when the
+// architecture cannot be determined, and passes through the generator's
+// diagnostics, which name the package.
+func ensureGoAsmHeader(path string, target arch.Arch, cache *asmhdrCache) (string, func(), error) {
+	if path == "-" {
+		return "", nil, errors.New("cannot generate go_asm.h for standard input (no package directory)")
+	}
+	if target == arch.Unknown {
+		return "", nil, errors.New("a file that includes go_asm.h needs a target architecture: name the file _<arch>.s or pass -GOARCH")
+	}
+	if cache != nil {
+		dir, err := cache.dirFor(filepath.Dir(path), goarchName(target))
+		return dir, func() {}, err
+	}
+	root, err := os.MkdirTemp("", "gasm-asmhdr")
+	if err != nil {
+		return "", nil, err
+	}
+	dir, err := generateGoAsmHeader(filepath.Dir(path), goarchName(target), root)
+	if err != nil {
+		os.RemoveAll(root)
+		return "", nil, err
+	}
+	return dir, func() { os.RemoveAll(root) }, nil
+}
diff --git a/cmd/gasm/asmhdr_test.go b/cmd/gasm/asmhdr_test.go
new file mode 100644
index 0000000..370010e
--- /dev/null
+++ b/cmd/gasm/asmhdr_test.go
@@ -0,0 +1,311 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// writePkg lays out a minimal Go package in a temp directory.
+func writePkg(t *testing.T, files map[string]string) string {
+	t.Helper()
+	dir := t.TempDir()
+	for name, src := range files {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(src), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return dir
+}
+
+// generateFor generates the header for dir and returns its text.
+func generateFor(t *testing.T, dir, goarch string) string {
+	t.Helper()
+	hdrDir, err := generateGoAsmHeader(dir, goarch, t.TempDir())
+	if err != nil {
+		t.Fatalf("generateGoAsmHeader(%q, %s): %v", dir, goarch, err)
+	}
+	b, err := os.ReadFile(filepath.Join(hdrDir, "go_asm.h"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(b)
+}
+
+func TestGenerateGoAsmHeaderShape(t *testing.T) {
+	dir := writePkg(t, map[string]string{"sample.go": `package sample
+
+const bufSize = 1024
+
+const (
+	a = iota * 8
+	b
+	c
+)
+
+const (
+	strConst = "hello"
+	boolConst = true
+	floatConst = 1.5
+	_ = "the blank identifier is skipped"
+)
+
+const shift = 1 << 20
+
+type reader struct {
+	r    int64
+	w    int64
+	_    [4]byte
+	name string
+}
+
+type scalar int
+
+type aliased struct {
+	k uint32
+	v uint32
+}
+
+type alias = aliased
+`})
+	hdr := generateFor(t, dir, "amd64")
+	want := []string{
+		"#define const_bufSize 1024",
+		// iota resolves through go/types, one define per name.
+		"#define const_a 0",
+		"#define const_b 8",
+		"#define const_c 16",
+		`#define const_strConst "hello"`,
+		"#define const_boolConst true",
+		// Floats are the toolchain's own skip, as are blank names.
+		"#define const_shift 1048576",
+		// The blank field still occupies its bytes: the pad after w runs to
+		// the string's 8-byte alignment.
+		"#define reader__size 40",
+		"#define reader_r 0",
+		"#define reader_w 8",
+		"#define reader_name 24",
+		// Non-struct named types carry no defines; aliases to structs do.
+		"#define aliased__size 8",
+		"#define aliased_k 0",
+		"#define aliased_v 4",
+		"#define alias__size 8",
+		"#define alias_k 0",
+		"#define alias_v 4",
+	}
+	for _, w := range want {
+		if !strings.Contains(hdr, w+"\n") {
+			t.Errorf("header misses %q\ngot:\n%s", w, hdr)
+		}
+	}
+	for _, banned := range []string{"#define const_floatConst", "#define _ ", "#define scalar"} {
+		if strings.Contains(hdr, banned) {
+			t.Errorf("header must not carry %s\ngot:\n%s", banned, hdr)
+		}
+	}
+}
+
+func TestGenerateGoAsmHeaderPerArch(t *testing.T) {
+	dir := writePkg(t, map[string]string{
+		"common.go": `package perarch
+
+type layout struct {
+	a int32
+	p uintptr
+}
+`,
+		// The build-tagged file set is part of the contract: a per-arch
+		// package is exactly how internal/cpu declares its layouts.
+		"const_amd64.go": `//go:build amd64
+
+package perarch
+
+const flavour = 1
+`,
+		"const_arm64.go": `//go:build arm64
+
+package perarch
+
+const flavour = 2
+`,
+	})
+	amd64 := generateFor(t, dir, "amd64")
+	arm64 := generateFor(t, dir, "arm64")
+	if !strings.Contains(amd64, "#define const_flavour 1\n") {
+		t.Errorf("amd64 header misses const_flavour 1:\n%s", amd64)
+	}
+	if !strings.Contains(arm64, "#define const_flavour 2\n") {
+		t.Errorf("arm64 header misses const_flavour 2:\n%s", arm64)
+	}
+	if strings.Contains(arm64, "#define const_flavour 1\n") {
+		t.Errorf("arm64 header must not carry the amd64 file's value")
+	}
+	// SizesFor makes the layout the target's: uintptr is 4 bytes wide on
+	// 386 and 8 on amd64, which must move p and grow the struct.
+	if !strings.Contains(amd64, "#define layout__size 16\n") || !strings.Contains(amd64, "#define layout_p 8\n") {
+		t.Errorf("amd64 layout wrong:\n%s", amd64)
+	}
+	w386 := generateFor(t, dir, "386")
+	if !strings.Contains(w386, "#define layout__size 8\n") || !strings.Contains(w386, "#define layout_p 4\n") {
+		t.Errorf("386 layout wrong:\n%s", w386)
+	}
+}
+
+func TestGenerateGoAsmHeaderErrors(t *testing.T) {
+	t.Run("type error", func(t *testing.T) {
+		dir := writePkg(t, map[string]string{"bad.go": `package bad
+
+const x = undefinedIdent
+`})
+		_, err := generateGoAsmHeader(dir, "amd64", t.TempDir())
+		if err == nil {
+			t.Fatal("generation must fail for a package that does not type-check")
+		}
+		if !strings.Contains(err.Error(), dir) {
+			t.Errorf("error must name the package directory: %v", err)
+		}
+		if !strings.Contains(err.Error(), "type-check") {
+			t.Errorf("error must say the package does not type-check: %v", err)
+		}
+	})
+	t.Run("no go files", func(t *testing.T) {
+		dir := t.TempDir()
+		_, err := generateGoAsmHeader(dir, "amd64", t.TempDir())
+		if err == nil {
+			t.Fatal("generation must fail without Go files")
+		}
+		if !strings.Contains(err.Error(), dir) {
+			t.Errorf("error must name the package directory: %v", err)
+		}
+	})
+}
+
+func TestNeedsGoAsmHeader(t *testing.T) {
+	yes := "#include \"go_asm.h\"\n#include \"textflag.h\"\n"
+	no := "#include \"textflag.h\"\n#include \"funcdata.h\"\n"
+	if !needsGoAsmHeader(yes) {
+		t.Error("needsGoAsmHeader(missing on a go_asm.h include)")
+	}
+	if needsGoAsmHeader(no) {
+		t.Error("needsGoAsmHeader claims other headers need generation")
+	}
+}
+
+func TestGoAsmHeaderResolved(t *testing.T) {
+	dir := t.TempDir()
+	if goAsmHeaderResolved(dir, nil) {
+		t.Error("resolved with no header anywhere")
+	}
+	other := t.TempDir()
+	if goAsmHeaderResolved(dir, []string{other}) {
+		t.Error("resolved with an empty -I directory")
+	}
+	if err := os.WriteFile(filepath.Join(dir, "go_asm.h"), nil, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if !goAsmHeaderResolved(dir, nil) {
+		t.Error("not resolved with the header in the package directory")
+	}
+}
+
+func TestOtherGOOSFile(t *testing.T) {
+	for path, want := range map[string]bool{
+		"/x/sys_windows_amd64.s": true,
+		"/x/rt0_js_wasm.s":       true,
+		"/x/sys_darwin_arm64.s":  true,
+		"/x/sys_linux_amd64.s":   false,
+		"/x/time_linux_amd64.s":  false,
+		"/x/memmove_amd64.s":     false,
+		"/x/generic.s":           false,
+	} {
+		if got := otherGOOSFile(path); got != want {
+			t.Errorf("otherGOOSFile(%q) = %v, want %v", path, got, want)
+		}
+	}
+}
+
+// TestRunCorpusAuditGoAsm covers the audit wiring end to end: a package
+// beside its kernel, the kernel living off the generated defines, and the
+// histogram recording a generation failure as its own reason.
+func TestRunCorpusAuditGoAsm(t *testing.T) {
+	dir := t.TempDir()
+	write := func(name, src string) {
+		t.Helper()
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(src), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	write("pkg.go", `package corpus
+
+const pageSize = 4096
+
+type header struct {
+	magic uint64
+	flags uint64
+}
+`)
+	write("kern_amd64.s", "#include \"go_asm.h\"\nTEXT \xc2\xb7f(SB), NOSPLIT, $0-16\n\tMOVQ\t$const_pageSize, AX\n\tMOVQ\t$header__size, BX\n\tRET\n")
+	// The defines live in the file's own package; a kernel in a directory
+	// without Go files has no package to generate from.
+	if err := os.MkdirAll(filepath.Join(dir, "sub"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	write(filepath.Join("sub", "lonely_arm64.s"), "#include \"go_asm.h\"\nTEXT \xc2\xb7g(SB), NOSPLIT, $0-0\n\tRET\n")
+
+	stats, err := runCorpusAudit(dir, nil)
+	if err != nil {
+		t.Fatalf("runCorpusAudit: %v", err)
+	}
+	get := func(name string) *corpusTally {
+		for i, tg := range stats.targets {
+			if tg.name == name {
+				return stats.tallies[i]
+			}
+		}
+		t.Fatalf("no tally for %s", name)
+		return nil
+	}
+	if a := get("amd64"); a.attempted != 1 || a.assembled != 1 {
+		t.Errorf("amd64 = %d/%d, want 1/1", a.assembled, a.attempted)
+	}
+	// lonely_arm64.s is an arm64 file whose package cannot be generated.
+	if a := get("arm64"); a.attempted != 1 || a.assembled != 0 {
+		t.Errorf("arm64 = %d/%d, want 0/1", a.assembled, a.attempted)
+	}
+	if r := get("arm64").reasons["go_asm.h generation failed"]; r != 1 {
+		t.Errorf("arm64 go_asm.h failure count = %d, want 1", r)
+	}
+}
+
+// TestGenerateGoAsmHeaderRuntime pins the generator against the real thing:
+// the runtime package, whose header the toolchain's own -asmhdr output was
+// sampled from.  Skipped in short mode: it type-checks the whole package.
+func TestGenerateGoAsmHeaderRuntime(t *testing.T) {
+	if testing.Short() {
+		t.Skip("type-checks the whole runtime package")
+	}
+	dir, err := generateGoAsmHeader("/usr/local/go/src/runtime", "amd64", t.TempDir())
+	if err != nil {
+		t.Fatalf("generateGoAsmHeader(runtime): %v", err)
+	}
+	b, err := os.ReadFile(dir + "/go_asm.h")
+	if err != nil {
+		t.Fatal(err)
+	}
+	hdr := string(b)
+	for _, want := range []string{
+		"#define const_hashSize 8\n",
+		"#define const_avxSupported 1\n",
+		"#define const_pageSize 8192\n",
+		"#define g_stackguard0 16\n",
+		"#define m__size ",
+	} {
+		if !strings.Contains(hdr, want) {
+			t.Errorf("runtime header misses %q", want)
+		}
+	}
+}
diff --git a/cmd/gasm/audit.go b/cmd/gasm/audit.go
index 9df6635..d38337f 100644
--- a/cmd/gasm/audit.go
+++ b/cmd/gasm/audit.go
@@ -418,7 +418,9 @@ func cmdAuditCorpus(args []string, dirs includeDirs) error {
 	// The toolchain's shipped headers (funcdata.h and friends) define the
 	// macros GOROOT files include; a corpus audit measures those files, so
 	// the header directory joins the search path automatically.  go_asm.h
-	// is compiler-generated per package and stays unresolvable on purpose.
+	// is compiler-generated per package, so it is not resolved from here:
+	// files that include it get one generated per target architecture,
+	// which runCorpusAudit arranges.
 	if out, err := exec.Command("go", "env", "GOROOT").Output(); err == nil {
 		pkgInclude := filepath.Join(strings.TrimSpace(string(out)), "pkg", "include")
 		if fi, err := os.Stat(pkgInclude); err == nil && fi.IsDir() {
@@ -462,9 +464,15 @@ var goPortSuffixes = []string{
 	"s390x", "wasm",
 }
 
-// otherPortFile reports whether the file's name carries a Go-architecture
-// suffix gasm does not support.
+// otherPortFile reports whether the file belongs to a build no supported
+// target ever compiles: either its name carries a Go-architecture suffix
+// gasm does not support, or, for a file with no architecture suffix at all,
+// it names another GOOS, which go/build drops from the file set
+// (rt0_js_wasm.s is a javascript build, not a generic one).
 func otherPortFile(path string) bool {
+	if otherGOOSFile(path) {
+		return true
+	}
 	base := path
 	if i := strings.LastIndexByte(base, '/'); i >= 0 {
 		base = base[i+1:]
@@ -477,6 +485,29 @@ func otherPortFile(path string) bool {
 	return false
 }
 
+// goOSNames are the GOOS values go/build recognises in file names.
+var goOSNames = map[string]bool{
+	"aix": true, "darwin": true, "dragonfly": true, "freebsd": true,
+	"ios": true, "js": true, "linux": true, "netbsd": true,
+	"openbsd": true, "plan9": true, "solaris": true, "wasip1": true,
+	"windows": true,
+}
+
+// otherGOOSFile reports whether the file's name names a GOOS other than the
+// host's, by go/build's file-name rules.
+func otherGOOSFile(path string) bool {
+	base := path
+	if i := strings.LastIndexByte(base, '/'); i >= 0 {
+		base = base[i+1:]
+	}
+	for seg := range strings.SplitSeq(strings.TrimSuffix(base, ".s"), "_") {
+		if goOSNames[seg] && seg != runtime.GOOS {
+			return true
+		}
+	}
+	return false
+}
+
 func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 	files, err := asmFiles(root)
 	if err != nil {
@@ -497,12 +528,20 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 	// its name allows assembles it.
 	full, generic, otherPort := 0, 0, 0
 
+	// Header generation is created on first use, so a corpus with no
+	// go_asm.h includes never pays for a temp directory.
+	var hdr *asmhdrCache
+	defer func() {
+		if hdr != nil {
+			hdr.close()
+		}
+	}()
+
 	for _, path := range files {
 		src, err := readSource(path)
 		if err != nil {
 			return nil, err
 		}
-		f, errs := parser.ParseWithOptions(path, src, parser.Options{Expand: true, IncludeDirs: dirs})
 
 		var wanted []int // indexes into targets
 		if a := arch.FromFilename(path); a != arch.Unknown {
@@ -513,10 +552,11 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 			}
 		} else if otherPortFile(path) {
 			// A file named for a Go port gasm does not support (arm,
-			// 386, s390x, ...) is compiled by no supported-arch build,
-			// so it is neither generic nor a per-arch attempt: counting
-			// it as generic would make the headline unreachably low
-			// for reasons no supported target can fix.
+			// 386, s390x, ...) or for another GOOS is compiled by no
+			// supported-arch build, so it is neither generic nor a
+			// per-arch attempt: counting it as generic would make the
+			// headline unreachably low for reasons no supported target
+			// can fix.
 			otherPort++
 		} else {
 			generic++
@@ -525,6 +565,54 @@ func runCorpusAudit(root string, dirs includeDirs) (*corpusStats, error) {
 			}
 		}
 
+		// A file that includes go_asm.h parses against a per-target header:
+		// the defines differ per architecture (internal/cpu's layout, for
+		// one), so the parse cannot be shared the way a header-free file's
+		// can.  A generation failure is a failure for every target, named
+		// for the package rather than a bare "include not found".  A header
+		// already resolvable in the package directory or the -I list is
+		// left alone.
+		if len(wanted) > 0 && needsGoAsmHeader(src) && !goAsmHeaderResolved(filepath.Dir(path), dirs) {
+			if hdr == nil {
+				if hdr, err = newAsmhdrCache(); err != nil {
+					return nil, err
+				}
+			}
+			pkgDir := filepath.Dir(path)
+			ok := true
+			for _, i := range wanted {
+				tg, t := targets[i], tallies[i]
+				t.attempted++
+				hdrDir, err := hdr.dirFor(pkgDir, goarchName(tg.a))
+				if err != nil {
+					ok = false
+					t.fail(path, corpusReason(err))
+					continue
+				}
+				f, errs := parser.ParseWithOptions(path, src, parser.Options{
+					Expand:      true,
+					IncludeDirs: append(slices.Clone(dirs), hdrDir),
+				})
+				if len(errs) > 0 {
+					ok = false
+					t.fail(path, corpusReason(errs[0]))
+					continue
+				}
+				if _, err := assembleFile(tg.a, f); err != nil {
+					ok = false
+					t.fail(path, corpusReason(err))
+					continue
+				}
+				t.assembled++
+			}
+			if ok && len(wanted) > 0 {
+				full++
+			}
+			continue
+		}
+
+		f, errs := parser.ParseWithOptions(path, src, parser.Options{Expand: true, IncludeDirs: dirs})
+
 		ok := true
 		for _, i := range wanted {
 			tg, t := targets[i], tallies[i]
@@ -581,6 +669,8 @@ func printCorpusStats(s *corpusStats) {
 func corpusReason(err error) string {
 	msg := err.Error()
 	switch {
+	case strings.Contains(msg, "go_asm.h for GOARCH"):
+		return "go_asm.h generation failed"
 	case strings.Contains(msg, "unsupported"), strings.Contains(msg, "cannot encode"):
 		return "instruction not encodable"
 	case strings.Contains(msg, "undefined label"):
diff --git a/cmd/gasm/main.go b/cmd/gasm/main.go
index fa7c2f9..0ba70d7 100644
--- a/cmd/gasm/main.go
+++ b/cmd/gasm/main.go
@@ -503,6 +503,12 @@ system toolchain; goobj emits the Go toolchain's own object format, which
 cmd/link consumes directly (it requires -p, the package path, and the
 installed Go toolchain: the object preamble is captured from go tool asm
 and the format version from go version).
+
+A file that includes go_asm.h gets that header generated automatically from
+the package it lives in (the .go files beside it, type-checked for the
+target architecture, the toolchain's own defines), so GOROOT assembly
+assembles without a compiler.  A package that has no Go files for the
+target or does not type-check is a hard error naming the package.
 `)
 	out := fs.String("o", "", "write the output to this file")
 	format := fs.String("format", "raw", "output format: raw (concatenated image), elf or goobj (Go object)")
@@ -538,6 +544,23 @@ and the format version from go version).
 		fmt.Fprintln(os.Stderr, "gasm:", err)
 		return 1
 	}
+	// A file that includes go_asm.h cannot assemble without the package's
+	// defines, and without a compiler nothing else has generated them, so
+	// gasm produces the equivalent itself: automatic, because the compiler
+	// behaves the same way and a flag would only ever be forgotten.  A
+	// generation failure is fatal and names the package: assembling against
+	// a missing header would fail later with a bare "undefined" instead.
+	// A go_asm.h that already resolves (placed by hand, or passed with -I)
+	// is left alone.
+	if needsGoAsmHeader(src) && !goAsmHeaderResolved(filepath.Dir(path), dirs) {
+		hdrDir, cleanup, err := ensureGoAsmHeader(path, targetArch, nil)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "gasm asm:", err)
+			return 1
+		}
+		defer cleanup()
+		dirs = append(dirs, hdrDir)
+	}
 	f, errs := parser.ParseWithOptions(path, src, parser.Options{Expand: true, IncludeDirs: dirs})
 	for _, e := range errs {
 		fmt.Fprintf(os.Stderr, "%s: %v\n", path, e)