snehaapratap · Nov 2, 2026
@@ -78,13 +78,14 @@ On a real project: [age](https://github.com/FiloSottile/age) at commit `b74dce4`
 
 ## Usage
 
-    godebugdiff [--from VERSION] [--to VERSION] [--tests] [--json] [--fail] [DIR]
+    godebugdiff [--from VERSION] [--to VERSION] [--tests] [--only PACKAGES] [--json] [--fail] [DIR]
 
 - `DIR` is the module root, the directory with go.mod. It defaults to `.`.
 - `--to` defaults to the installed toolchain's version, such as 1.27 for go1.27.3. It can't be newer than the toolchain, because older toolchains don't know about later settings.
 - `--from` defaults to the go line in go.mod. Pass the base branch's version to review a change that already raised it.
 - `--tests` adds test binaries. A module with no main package, such as a library, is checked through its test binaries automatically.
-- `--json` prints the full result. `--fail` exits 1 when a change reaches a package one of your binaries imports. Exit code 2 means bad usage, or the go command couldn't load the module.
+- `--only net/http,crypto/tls` includes only settings with those exact package names in text and JSON output. `--fail` considers only included settings. Package names can be separated by commas with optional whitespace; subpackages are not included automatically.
+- `--json` prints the result. `--fail` exits 1 when a change reaches a package one of your binaries imports. Exit code 2 means bad usage, or the go command couldn't load the module.
 
 In CI, on a pull request that changes go.mod:
 

@@ -1,5 +1,8 @@
 # Roadmap
 
+## Available
+- Filter settings by exact package names with `--only` in text and JSON output
+
 ## Next
 - A Markdown output for pull request comments
 - Read the go line from go.work in workspace mode, instead of only noting it

@@ -34,6 +34,7 @@ from --from to --to, and which binaries import the affected packages.
 
   --from VERSION  go line to compare from (default: the one in go.mod)
   --to VERSION    go line to compare to (default: the installed toolchain)
+  --only PACKAGES comma-separated exact package names to include
   --tests         also check test binaries (always on for modules with no main package)
   --json          print the result as JSON
   --fail          exit 1 when a change reaches a package a binary imports
@@ -82,6 +83,7 @@ type Result struct {
 	TestsOnly  bool     `json:"testsOnly"`
 	Changes    []Change `json:"changes"`
 	Notes      []string `json:"notes,omitempty"`
+	Only       []string `json:"only,omitempty"`
 	reachCount int
 }
 
@@ -96,6 +98,7 @@ func run(args []string, stdout, stderr io.Writer) int {
 	from := fs.String("from", "", "")
 	to := fs.String("to", "", "")
 	tests := fs.Bool("tests", false, "")
+	only := fs.String("only", "", "")
 	asJSON := fs.Bool("json", false, "")
 	fail := fs.Bool("fail", false, "")
 	showVersion := fs.Bool("version", false, "")
@@ -119,6 +122,10 @@ func run(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stderr, "godebugdiff:", err)
 		return 2
 	}
+	if err := filterPackages(res, *only); err != nil {
+		fmt.Fprintln(stderr, "godebugdiff:", err)
+		return 2
+	}
 	if *asJSON {
 		enc := json.NewEncoder(stdout)
 		enc.SetIndent("", "  ")
@@ -132,11 +139,43 @@ func run(args []string, stdout, stderr io.Writer) int {
 	return 0
 }
 
+// filterPackages applies the same exact-package selection to text, JSON and --fail.
+func filterPackages(r *Result, only string) error {
+	if only == "" {
+		return nil
+	}
+	selected := make(map[string]bool)
+	var packages []string
+	for _, name := range strings.Split(only, ",") {
+		name = strings.TrimSpace(name)
+		if name == "" {
+			return fmt.Errorf("--only requires comma-separated non-empty package names")
+		}
+		if !selected[name] {
+			packages = append(packages, name)
+			selected[name] = true
+		}
+	}
+	r.Only = packages
+	changes := make([]Change, 0)
+	r.reachCount = 0
+	for _, c := range r.Changes {
+		if selected[c.Package] {
+			changes = append(changes, c)
+			if len(c.ReachedBy) > 0 {
+				r.reachCount++
+			}
+		}
+	}
+	r.Changes = changes
+	return nil
+}
+
 // reorder moves flags before the directory argument, so
 // `godebugdiff DIR --to 1.24` works like `godebugdiff --to 1.24 DIR`.
 func reorder(args []string) []string {
 	var flags, rest []string
-	valued := map[string]bool{"from": true, "to": true}
+	valued := map[string]bool{"from": true, "to": true, "only": true}
 	for i := 0; i < len(args); i++ {
 		a := args[i]
 		if a == "--" {
@@ -753,13 +792,20 @@ func Print(w io.Writer, r *Result) {
 		count = strings.TrimSuffix(kind, "aries") + "ary"
 	}
 	fmt.Fprintf(w, "go %s -> %s in %s (%s, %d %s)\n", r.From, r.To, r.Module, r.Toolchain, len(r.Binaries), count)
+	if len(r.Only) > 0 {
+		fmt.Fprintf(w, "Packages: %s (exact matches; other changes omitted)\n", strings.Join(r.Only, ", "))
+	}
 	if len(r.Binaries) == 0 {
 		fmt.Fprintf(w, "\nNo main packages or tests found, so there's nothing to compare.\n")
 		printNotes(w, r)
 		return
 	}
 	if len(r.Changes) == 0 {
-		fmt.Fprintf(w, "\nNo GODEBUG defaults change.\n")
+		if len(r.Only) > 0 {
+			fmt.Fprintf(w, "\nNo GODEBUG defaults change in the selected packages.\n")
+		} else {
+			fmt.Fprintf(w, "\nNo GODEBUG defaults change.\n")
+		}
 		printNotes(w, r)
 		return
 	}

@@ -246,3 +246,45 @@ func run1(t *testing.T, name string, args ...string) string {
 	}
 	return string(out)
 }
+
+func TestFilterPackages(t *testing.T) {
+	r := &Result{
+		Binaries: []string{"example.com/app"},
+		Changes: []Change{
+			{Name: "http", Package: "net/http", ReachedBy: []string{"example.com/app"}},
+			{Name: "child", Package: "net/http/httputil", ReachedBy: []string{"example.com/app"}},
+			{Name: "tls", Package: "crypto/tls"},
+			{Name: "unknown", Package: ""},
+		},
+		reachCount: 2,
+	}
+	if err := filterPackages(r, " net/http,crypto/tls,net/http "); err != nil {
+		t.Fatal(err)
+	}
+	if len(r.Changes) != 2 || r.Changes[0].Name != "http" || r.Changes[1].Name != "tls" || r.reachCount != 1 {
+		t.Fatalf("filter did not preserve exact matches and recount reach: %+v", r)
+	}
+	if !reflect.DeepEqual(r.Only, []string{"net/http", "crypto/tls"}) {
+		t.Fatalf("filter names = %v", r.Only)
+	}
+	if err := filterPackages(r, "net"); err != nil || len(r.Changes) != 0 || r.reachCount != 0 {
+		t.Fatalf("prefix must not match subpackages: %+v, %v", r, err)
+	}
+	var output bytes.Buffer
+	Print(&output, r)
+	if !strings.Contains(output.String(), "No GODEBUG defaults change in the selected packages.") {
+		t.Fatalf("empty filtered result needs scoped wording: %s", output.String())
+	}
+}
+
+func TestFilterPackagesValidation(t *testing.T) {
+	for _, input := range []string{",", "net/http,", " ,net/http"} {
+		if err := filterPackages(&Result{}, input); err == nil {
+			t.Errorf("expected empty package name in %q to be rejected", input)
+		}
+	}
+	r := &Result{Changes: []Change{{Name: "keep"}}, reachCount: 1}
+	if err := filterPackages(r, ""); err != nil || len(r.Changes) != 1 || r.reachCount != 1 {
+		t.Fatalf("omitted filter changed the result: %+v, %v", r, err)
+	}
+}
@@ -309,3 +309,20 @@ func TestErrors(t *testing.T) {
 		t.Errorf("removed setting: %d %s", code, stderr)
 	}
 }
+
+func TestOnlyPackages(t *testing.T) {
+	needGo(t, 24)
+	r, code, stderr := diff(t, "../examples/shop", "--to", "1.24", "--only", "net/http,crypto/tls", "--fail")
+	if code != 1 || len(r.Changes) == 0 {
+		t.Fatalf("expected selected changes and exit 1, got %d: %s", code, stderr)
+	}
+	for _, change := range r.Changes {
+		if change.Package != "net/http" && change.Package != "crypto/tls" {
+			t.Errorf("unexpected package in filtered report: %s", change.Package)
+		}
+	}
+	r, code, stderr = diff(t, "../examples/shop", "--to", "1.24", "--only=example.com/not-imported", "--fail")
+	if code != 0 || len(r.Changes) != 0 {
+		t.Fatalf("excluded changes must not trigger --fail: %d, %+v, %s", code, r.Changes, stderr)
+	}
+}