diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3cfa482..504d602 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Added
+
+- Support compile-time automatic instrumentation of packages
+  that don't import `errtrace` by specifying the flag `unsafe-packages`.
+  This still requires at least one import of `errtrace` in the binary.
+
 ## 0.4.0 - 2025-07-21
 
 This release supports compile-time rewriting of source files via `toolexec`.
diff --git a/cmd/errtrace/main.go b/cmd/errtrace/main.go
index b463f1a..718ffcc 100644
--- a/cmd/errtrace/main.go
+++ b/cmd/errtrace/main.go
@@ -26,9 +26,6 @@
 //	-l    list files that would be modified without making any changes.
 package main
 
-// TODO
-//   - -toolexec: run as a tool executor, fit for use with 'go build -toolexec'
-
 import (
 	"bytes"
 	"encoding/json"
@@ -52,6 +49,8 @@ import (
 	"braces.dev/errtrace"
 )
 
+const errtracePkgImport = "braces.dev/errtrace"
+
 func main() {
 	cmd := &mainCmd{
 		Stdin:  os.Stdin,
@@ -413,6 +412,17 @@ type parsedFile struct {
 	importsErrtrace bool // includes blank imports
 	inserts         []insert
 	unusedOptouts   []int // list of line numbers
+
+	// Used for toolexec unsafe mode, when rewriting packages that don't import
+	// errtrace, so we use go:linkname wrappers.
+	errtraceUnsafePrefix string
+}
+
+func (f *parsedFile) errtracePkgPrefix() string {
+	if f.errtraceUnsafePrefix == "" {
+		return fmt.Sprintf("%s.", f.errtracePkg)
+	}
+	return f.errtraceUnsafePrefix
 }
 
 func (cmd *mainCmd) parseFile(filename string, src []byte, opts rewriteOpts) (parsedFile, error) {
@@ -426,7 +436,7 @@ func (cmd *mainCmd) parseFile(filename string, src []byte, opts rewriteOpts) (pa
 	var importsErrtrace bool   // whether there's any errtrace import, including blank imports
 	needErrtraceImport := true // whether to add a new import.
 	for _, imp := range f.Imports {
-		if imp.Path.Value == `"braces.dev/errtrace"` {
+		if imp.Path.Value == `"`+errtracePkgImport+`"` {
 			importsErrtrace = true
 			if imp.Name != nil {
 				if imp.Name.Name == "_" {
@@ -574,15 +584,20 @@ func (cmd *mainCmd) rewriteFile(f parsedFile, out *bytes.Buffer) error {
 				_, _ = io.WriteString(out, "import ")
 			}
 
-			if f.errtracePkg == "errtrace" {
+			if f.errtraceUnsafePrefix != "" {
+				// unsafe mode uses go:link, which requires importing "unsafe" instead
+				// of errtrace.  Since duplicate blank imports are allowed by Go, we can
+				// always add it, without checking if it has been imported.
+				fmt.Fprint(out, `_ "unsafe"`)
+			} else if f.errtracePkg == "errtrace" {
 				// Don't use named imports if we're using the default name.
-				fmt.Fprintf(out, "%q", "braces.dev/errtrace")
+				fmt.Fprintf(out, "%q", errtracePkgImport)
 			} else {
-				fmt.Fprintf(out, "%s %q", f.errtracePkg, "braces.dev/errtrace")
+				fmt.Fprintf(out, "%s %q", f.errtracePkg, errtracePkgImport)
 			}
 
 		case *insertWrapOpen:
-			fmt.Fprintf(out, "%s.Wrap", f.errtracePkg)
+			fmt.Fprintf(out, "%sWrap", f.errtracePkgPrefix())
 			if it.N > 1 {
 				fmt.Fprintf(out, "%d", it.N)
 			}
@@ -605,7 +620,7 @@ func (cmd *mainCmd) rewriteFile(f parsedFile, out *bytes.Buffer) error {
 
 			// Last return is an error, wrap it.
 			last := &vars[len(vars)-1]
-			*last = fmt.Sprintf("%s.Wrap(%v)", f.errtracePkg, *last)
+			*last = fmt.Sprintf("%sWrap(%v)", f.errtracePkgPrefix(), *last)
 
 			fmt.Fprintf(out, "; return %s }", strings.Join(vars, ", "))
 
@@ -625,7 +640,7 @@ func (cmd *mainCmd) rewriteFile(f parsedFile, out *bytes.Buffer) error {
 				if i > 0 {
 					_, _ = out.WriteString(", ")
 				}
-				fmt.Fprintf(out, "%s.Wrap(%s)", f.errtracePkg, name)
+				fmt.Fprintf(out, "%sWrap(%s)", f.errtracePkgPrefix(), name)
 			}
 			_, _ = out.WriteString("; ")
 
diff --git a/cmd/errtrace/testdata/toolexec-test/main.go b/cmd/errtrace/testdata/toolexec-test/main.go
index fa384ee..533d4fa 100644
--- a/cmd/errtrace/testdata/toolexec-test/main.go
+++ b/cmd/errtrace/testdata/toolexec-test/main.go
@@ -14,5 +14,5 @@ func main() {
 }
 
 func callP1() error {
-	return p1.WrapP2() // @trace
+	return p1.WrapP2OnlyErr() // @trace
 }
diff --git a/cmd/errtrace/testdata/toolexec-test/p1/p1.go b/cmd/errtrace/testdata/toolexec-test/p1/p1.go
index 217045c..174ac9b 100644
--- a/cmd/errtrace/testdata/toolexec-test/p1/p1.go
+++ b/cmd/errtrace/testdata/toolexec-test/p1/p1.go
@@ -6,7 +6,15 @@ import (
 	"braces.dev/errtrace/cmd/errtrace/testdata/toolexec-test/p2"
 )
 
-// WrapP2 wraps an error return from p2.
-func WrapP2() error {
-	return fmt.Errorf("test2: %w", p2.CallP3())
+// WrapP2OnlyErr only returns the error from WrapP2.
+func WrapP2OnlyErr() error {
+	if _, err := WrapP2(); err != nil {
+		return fmt.Errorf("test2: %w", err) // @unsafe-trace
+	}
+	return nil
+}
+
+// WrapRet2 calls WrapP2, but has a multi-return.
+func WrapP2() (string, error) {
+	return p2.CallP3() // @unsafe-trace
 }
diff --git a/cmd/errtrace/testdata/toolexec-test/p2/p2.go b/cmd/errtrace/testdata/toolexec-test/p2/p2.go
index 32e99ee..5569473 100644
--- a/cmd/errtrace/testdata/toolexec-test/p2/p2.go
+++ b/cmd/errtrace/testdata/toolexec-test/p2/p2.go
@@ -7,6 +7,6 @@ import (
 )
 
 // CallP3 calls p3, and wraps the error.
-func CallP3() error {
-	return errtrace.Wrap(p3.ReturnErr()) // @trace
+func CallP3() (string, error) {
+	return errtrace.Wrap2(p3.ReturnStrErr()) // @trace
 }
diff --git a/cmd/errtrace/testdata/toolexec-test/p3/p3.go b/cmd/errtrace/testdata/toolexec-test/p3/p3.go
index b60a12f..9a06a84 100644
--- a/cmd/errtrace/testdata/toolexec-test/p3/p3.go
+++ b/cmd/errtrace/testdata/toolexec-test/p3/p3.go
@@ -4,7 +4,7 @@ import (
 	"errors"
 )
 
-// ReturnErr returns an error.
-func ReturnErr() error {
-	return errors.New("test") // @trace
+// ReturnStrErr returns an error.
+func ReturnStrErr() (string, error) {
+	return "", errors.New("test") // @trace
 }
diff --git a/cmd/errtrace/testdata/toolexec-unsafe-no-import/main.go b/cmd/errtrace/testdata/toolexec-unsafe-no-import/main.go
new file mode 100644
index 0000000..81be4a6
--- /dev/null
+++ b/cmd/errtrace/testdata/toolexec-unsafe-no-import/main.go
@@ -0,0 +1,13 @@
+package main
+
+import "fmt"
+
+func main() {
+	if err := getErr(); err != nil {
+		fmt.Printf("%+v\n", err)
+	}
+}
+
+func getErr() error {
+	return fmt.Errorf("err")
+}
diff --git a/cmd/errtrace/toolexec.go b/cmd/errtrace/toolexec.go
index 76be1c3..7ed95a3 100644
--- a/cmd/errtrace/toolexec.go
+++ b/cmd/errtrace/toolexec.go
@@ -18,6 +18,9 @@ import (
 	"braces.dev/errtrace"
 )
 
+// Note: Choose a prefix that is not likely to clash with user symbols.
+const errtraceUnsafePrefix = "__errtrace_"
+
 func (cmd *mainCmd) handleToolExec(args []string) (exitCode int, handled bool) {
 	// In toolexec mode, we're passed the original command + arguments.
 	if len(args) == 0 {
@@ -51,6 +54,7 @@ func (cmd *mainCmd) handleToolExec(args []string) (exitCode int, handled bool) {
 
 type toolExecParams struct {
 	RequiredPkgSelectors []string
+	UnsafePkgSelectors   []string
 
 	Tool     string
 	ToolArgs []string
@@ -64,9 +68,11 @@ func (p *toolExecParams) Parse(w io.Writer, args []string) error {
 		logln(w, `usage with go build/run/test: -toolexec="errtrace [options]"`)
 		flag.PrintDefaults()
 	}
-	var requiredPkgs string
+	var requiredPkgs, unsafePkgs string
 	p.flags.StringVar(&requiredPkgs, "required-packages", "", "comma-separated list of package selectors "+
 		"that are expected to be import errtrace if they return errors.")
+	p.flags.StringVar(&unsafePkgs, "unsafe-packages", "", "comma-separated list of package selectors "+
+		"to rewrite using unsafe go:link, regardless of whether they import errtrace.")
 
 	// Flag parsing stops at the first non-flag argument (no "-").
 	if err := p.flags.Parse(args); err != nil {
@@ -81,6 +87,7 @@ func (p *toolExecParams) Parse(w io.Writer, args []string) error {
 	p.Tool = remArgs[0]
 	p.ToolArgs = remArgs[1:]
 	p.RequiredPkgSelectors = strings.Split(requiredPkgs, ",")
+	p.UnsafePkgSelectors = strings.Split(unsafePkgs, ",")
 	return nil
 }
 
@@ -106,6 +113,27 @@ func (p *toolExecParams) requiredPackage(pkg string) bool {
 	return false
 }
 
+func (p *toolExecParams) unsafeRewriteStd() bool {
+	// stdlib requires an explicit opt-in.
+	// Since there's known issues with error checks in the stdlib
+	// which can break with error wrapping, we call it std-unsafe.
+	return slices.Contains(p.UnsafePkgSelectors, "std-unsafe")
+}
+
+func (p *toolExecParams) unsafeRewrite(pkg string) bool {
+	if pkg == errtracePkgImport {
+		// Never rewrite the errtrace package, which leads to circular deps.
+		return false
+	}
+
+	for _, selector := range p.UnsafePkgSelectors {
+		if packageSelectorMatch(selector, pkg) {
+			return true
+		}
+	}
+	return false
+}
+
 func (cmd *mainCmd) toolExecVersion(p toolExecParams) int {
 	version, err := binaryVersion()
 	if err != nil {
@@ -145,8 +173,8 @@ func (cmd *mainCmd) toolExecRewrite(pkg string, p toolExecParams) (exitCode int)
 		return cmd.runOriginal(p)
 	}
 
-	// We only modify files that import errtrace, so stdlib is never eliglble.
-	if isStdLib(p.ToolArgs) {
+	// We only modify files that import errtrace, so stdlib is only eligible in unsafe mode.
+	if isStdLib(p.ToolArgs) && !p.unsafeRewriteStd() {
 		return cmd.runOriginal(p)
 	}
 
@@ -160,43 +188,25 @@ func (cmd *mainCmd) toolExecRewrite(pkg string, p toolExecParams) (exitCode int)
 }
 
 func (cmd *mainCmd) rewriteCompile(pkg string, p toolExecParams) (exitCode int, _ error) {
-	var canRewrite, needRewrite bool
-	parsed := make(map[string]parsedFile)
-	for _, arg := range p.ToolArgs {
-		if !isGoFile(arg) {
-			continue
-		}
-
-		contents, err := os.ReadFile(arg)
-		if err != nil {
-			return -1, errtrace.Wrap(err)
-		}
-
-		f, err := cmd.parseFile(arg, contents, rewriteOpts{})
-		if err != nil {
-			return -1, errtrace.Wrap(err)
-		}
-		parsed[arg] = f
-
-		// TODO: Support an "unsafe" mode to rewrite packages without errtrace imports.
-		if f.importsErrtrace {
-			canRewrite = true
-		}
-		if len(f.inserts) > 0 {
-			needRewrite = true
-		}
+	parsed, err := cmd.parsePkg(pkg, p.ToolArgs)
+	if err != nil {
+		return -1, errtrace.Wrap(err)
 	}
 
-	if !needRewrite {
+	if !parsed.needsRewrite {
 		return cmd.runOriginal(p), nil
 	}
 
-	if !canRewrite {
-		if p.requiredPackage(pkg) {
+	var unsafeForceImport bool
+	if !parsed.importsErrtrace {
+		unsafeForceImport = p.unsafeRewrite(pkg)
+		if !unsafeForceImport && p.requiredPackage(pkg) {
 			logf(cmd.Stderr, "errtrace required package %v missing errtrace import, needs rewrite", pkg)
 			return 1, nil
 		}
-		return cmd.runOriginal(p), nil
+		if !unsafeForceImport {
+			return cmd.runOriginal(p), nil
+		}
 	}
 
 	// Use a temporary directory per-package that is rewritten.
@@ -206,22 +216,36 @@ func (cmd *mainCmd) rewriteCompile(pkg string, p toolExecParams) (exitCode int,
 	}
 	defer os.RemoveAll(tempDir) //nolint:errcheck // best-effort removal of temp files.
 
+	// If a package doesn't already import errtrace, add `go:linkname` to the
+	// package to link to errtrace symbols. Only required once per-package.
+	addLinkName := unsafeForceImport
+
 	newArgs := make([]string, 0, len(p.ToolArgs))
 	for _, arg := range p.ToolArgs {
-		f, ok := parsed[arg]
+		f, ok := parsed.files[arg]
 		if !ok || len(f.inserts) == 0 {
 			newArgs = append(newArgs, arg)
 			continue
 		}
 
+		if unsafeForceImport {
+			f.errtraceUnsafePrefix = errtraceUnsafePrefix
+		}
+
 		// Add a //line directive so the original filepath is used in errors and panics.
-		var out bytes.Buffer
-		_, _ = fmt.Fprintf(&out, "//line %v:1\n", arg)
+		out := &bytes.Buffer{}
+		_, _ = fmt.Fprintf(out, "//line %v:1\n", arg)
 
-		if err := cmd.rewriteFile(f, &out); err != nil {
+		if err := cmd.rewriteFile(f, out); err != nil {
 			return -1, errtrace.Wrap(err)
 		}
 
+		if addLinkName {
+			_, _ = fmt.Fprintf(out, "\n\n//go:linkname %vWrap %v.Wrap\n", errtraceUnsafePrefix, errtracePkgImport)
+			_, _ = fmt.Fprintf(out, "func %vWrap(err error) error\n", errtraceUnsafePrefix)
+			addLinkName = false
+		}
+
 		// TODO: Handle clashes with the same base name in different directories (E.g., with bazel).
 		newFile := filepath.Join(tempDir, filepath.Base(arg))
 		if err := os.WriteFile(newFile, out.Bytes(), 0o666); err != nil {
@@ -235,6 +259,52 @@ func (cmd *mainCmd) rewriteCompile(pkg string, p toolExecParams) (exitCode int,
 	return cmd.runOriginal(p), nil
 }
 
+type parsePkgState struct {
+	pkg             string
+	files           map[string]parsedFile
+	importsErrtrace bool
+	needsRewrite    bool
+}
+
+func (cmd *mainCmd) parsePkg(pkg string, toolArgs []string) (*parsePkgState, error) {
+	s := &parsePkgState{
+		pkg:   pkg,
+		files: make(map[string]parsedFile),
+	}
+
+	for _, arg := range toolArgs {
+		if !isGoFile(arg) {
+			continue
+		}
+
+		contents, err := os.ReadFile(arg)
+		if err != nil {
+			return nil, errtrace.Wrap(err)
+		}
+
+		f, err := cmd.parseFile(arg, contents, rewriteOpts{
+			// WrapN is not compatible with unsafe rewrites, as `go:linkname`
+			// can't be used for generic functions like WrapN.
+			// We don't need WrapN, as it's is meant for direct source file changes,
+			// while toolexec writes ephemeral temp files.
+			NoWrapN: true,
+		})
+		if err != nil {
+			return nil, errtrace.Wrap(err)
+		}
+		s.files[arg] = f
+
+		if f.importsErrtrace {
+			s.importsErrtrace = true
+		}
+		if len(f.inserts) > 0 {
+			s.needsRewrite = true
+		}
+	}
+
+	return s, nil
+}
+
 func isCompile(arg string) bool {
 	if runtime.GOOS == "windows" {
 		arg = strings.TrimSuffix(arg, ".exe")
diff --git a/cmd/errtrace/toolexec_test.go b/cmd/errtrace/toolexec_test.go
index 9cf64b9..d87e86b 100644
--- a/cmd/errtrace/toolexec_test.go
+++ b/cmd/errtrace/toolexec_test.go
@@ -90,6 +90,27 @@ func TestToolExec(t *testing.T) {
 			},
 			wantTraces: wantTraces,
 		},
+		{
+			name: "toolexec with unsafe-packages ...",
+			goArgs: func(t testing.TB) []string {
+				return []string{"-toolexec", errTraceCmd + " -unsafe-packages=...", "."}
+			},
+			wantTraces: append(wantTraces, tracePaths(t, testProgDir, "@unsafe-trace")...),
+		},
+		{
+			name: "toolexec with unsafe-packages test/...",
+			goArgs: func(t testing.TB) []string {
+				return []string{"-toolexec", errTraceCmd + " -unsafe-packages=braces.dev/errtrace/cmd/errtrace/testdata/toolexec-test/...", "."}
+			},
+			wantTraces: append(wantTraces, tracePaths(t, testProgDir, "@unsafe-trace")...),
+		},
+		{
+			name: "toolexec with unsafe-packages test/p1",
+			goArgs: func(t testing.TB) []string {
+				return []string{"-toolexec", errTraceCmd + " -unsafe-packages=braces.dev/errtrace/cmd/errtrace/testdata/toolexec-test/p1", "."}
+			},
+			wantTraces: append(wantTraces, tracePaths(t, testProgDir, "@unsafe-trace")...),
+		},
 	}
 
 	for _, tt := range tests {
@@ -116,6 +137,7 @@ func TestToolExec(t *testing.T) {
 			verifyTraces := func(t testing.TB, stdout string) {
 				gotLines := fileLines(stdout)
 				sort.Strings(gotLines)
+				sort.Strings(tt.wantTraces)
 
 				if d := diff.Diff(tt.wantTraces, gotLines); d != "" {
 					t.Errorf("diff in traces:\n%s", d)
@@ -152,6 +174,20 @@ func TestToolExec(t *testing.T) {
 			})
 		})
 	}
+
+	// When using -unsafe-packages, packages that don't import errtrace can be
+	// rewritten to use Wrap, but at least one package in the binary still needs
+	// to import errtrace, otherwise, the final link will fail.
+	t.Run("unsafe no errtrace import", func(t *testing.T) {
+		args := []string{"run", "-toolexec", errTraceCmd + " -unsafe-packages=...", "."}
+		_, stderr, err := runGo(t, "./testdata/toolexec-unsafe-no-import", args...)
+		if err == nil {
+			t.Fatal("run should fail")
+		}
+		if want := "relocation target braces.dev/errtrace.Wrap not defined"; !strings.Contains(stderr, want) {
+			t.Fatalf("stderr missing expected error: %v, got:\n%s", want, stderr)
+		}
+	})
 }
 
 func tracePaths(t testing.TB, path string, traceMarker string) []string {