diff --git a/pymage/internal/cli/cli.go b/pymage/internal/cli/cli.go
index a7fa8a2..b41ed19 100644
--- a/pymage/internal/cli/cli.go
+++ b/pymage/internal/cli/cli.go
@@ -72,6 +72,7 @@ type buildFlags struct {
 	insecure    bool
 	cacheDir    string
 	noCache     bool
+	buildSdists bool
 }
 
 func buildCmd() *cobra.Command {
@@ -119,6 +120,7 @@ func buildCmd() *cobra.Command {
 	fs.BoolVar(&f.insecure, "insecure", false, "use plain HTTP for the registry")
 	fs.StringVar(&f.cacheDir, "cache-dir", "", "cache root directory (default: per-user cache dir)")
 	fs.BoolVar(&f.noCache, "no-cache", false, "disable all build caches (layers, downloaded wheels, interpreter detection)")
+	fs.BoolVar(&f.buildSdists, "build-sdists", false, "allow building wheels from source distributions when no compatible wheel exists (runs the dependency's build code on this host; single-arch for compiled packages)")
 	return cmd
 }
 
@@ -256,7 +258,7 @@ func buildOne(ctx context.Context, f *buildFlags, lk *lock.Lock, platform *v1.Pl
 		return nil, nil, err
 	}
 
-	wheels, err := wheelhouse.ResolveContext(ctx, reqs, f.findLinks, target, wheelCache)
+	wheels, err := wheelhouse.ResolveContext(ctx, reqs, f.findLinks, target, wheelCache, f.buildSdists)
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/pymage/internal/cli/defaults.go b/pymage/internal/cli/defaults.go
index 11f6b9c..e81f42c 100644
--- a/pymage/internal/cli/defaults.go
+++ b/pymage/internal/cli/defaults.go
@@ -75,6 +75,9 @@ func applyDefaults(cmd *cobra.Command, f *buildFlags) error {
 	if !changed("package") && cfg.Package != "" {
 		f.pkg = cfg.Package
 	}
+	if !changed("build-sdists") && cfg.BuildSdists {
+		f.buildSdists = true
+	}
 	if !changed("python") && cfg.Python != "" {
 		f.pythonTag = cfg.Python
 	}
diff --git a/pymage/internal/project/project.go b/pymage/internal/project/project.go
index 107269d..815ab5c 100644
--- a/pymage/internal/project/project.go
+++ b/pymage/internal/project/project.go
@@ -48,6 +48,7 @@ type Config struct {
 	NoCache        bool              `toml:"no-cache"`
 	Extras         []string          `toml:"extras"`
 	Package        string            `toml:"package"`
+	BuildSdists    bool              `toml:"build-sdists"`
 }
 
 const defaultBase = "cgr.dev/chainguard/python:latest"
diff --git a/pymage/internal/wheelhouse/sdist.go b/pymage/internal/wheelhouse/sdist.go
index 090d603..dbcc67b 100644
--- a/pymage/internal/wheelhouse/sdist.go
+++ b/pymage/internal/wheelhouse/sdist.go
@@ -21,7 +21,9 @@ import (
 // returns it as a ResolvedWheel. The built wheel is cached (keyed by the sdist
 // hash and target) so it isn't rebuilt on subsequent builds.
 //
-// Building runs the host's `pip wheel`, so a Python toolchain must be present.
+// This is only reached when the caller opted in (allowSdist), because building
+// runs the dependency's own build code on the host with no isolation. Building
+// runs the host's `pip wheel`, so a Python toolchain must be present.
 // A pure-python sdist yields a `py3-none-any` wheel usable on any target; a
 // compiled sdist is built for the host platform only, so cross-arch builds of
 // such packages will fail the compatibility check with a clear error.
diff --git a/pymage/internal/wheelhouse/sdist_test.go b/pymage/internal/wheelhouse/sdist_test.go
index 7d76886..82c0636 100644
--- a/pymage/internal/wheelhouse/sdist_test.go
+++ b/pymage/internal/wheelhouse/sdist_test.go
@@ -1,8 +1,10 @@
 package wheelhouse
 
 import (
+	"context"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/imjasonh/terraform-playground/pymage/internal/lock"
@@ -56,6 +58,25 @@ func TestSelectBuiltWheelWrongVersion(t *testing.T) {
 	}
 }
 
+func TestSdistRequiresOptIn(t *testing.T) {
+	// An sdist-only requirement (no wheels) must error unless opted in, rather
+	// than silently building (which runs the dependency's build code).
+	req := lock.Requirement{
+		Name:    "timeout-decorator",
+		Version: "0.5.0",
+		Sdist:   &lock.SdistRef{URL: "https://example/timeout_decorator-0.5.0.tar.gz", SHA256: "deadbeef", Filename: "timeout_decorator-0.5.0.tar.gz"},
+	}
+	target := wheel.Target{OS: "linux", Arch: "amd64", PyMajor: 3, PyMinor: 12}
+
+	_, err := ResolveContext(context.Background(), []lock.Requirement{req}, nil, target, t.TempDir(), false)
+	if err == nil {
+		t.Fatal("expected error when sdist build is not opted in")
+	}
+	if !strings.Contains(err.Error(), "--build-sdists") {
+		t.Fatalf("error should point at the opt-in flag, got: %v", err)
+	}
+}
+
 func TestBuiltPathKey(t *testing.T) {
 	c := &wheelCache{dir: "/tmp/c"}
 	target := wheel.Target{OS: "linux", Arch: "amd64", PyMajor: 3, PyMinor: 12}
diff --git a/pymage/internal/wheelhouse/wheelhouse.go b/pymage/internal/wheelhouse/wheelhouse.go
index 20461d6..4a64ef5 100644
--- a/pymage/internal/wheelhouse/wheelhouse.go
+++ b/pymage/internal/wheelhouse/wheelhouse.go
@@ -47,7 +47,7 @@ type candidate struct {
 // per-user cache directory. Results are sorted by (normalized name, version)
 // for deterministic ordering.
 func Resolve(reqs []lock.Requirement, dirs []string, target wheel.Target) ([]ResolvedWheel, error) {
-	return ResolveContext(context.Background(), reqs, dirs, target, defaultWheelCacheDir())
+	return ResolveContext(context.Background(), reqs, dirs, target, defaultWheelCacheDir(), false)
 }
 
 // defaultWheelCacheDir returns the per-user wheel download cache, or "" if the
@@ -62,8 +62,10 @@ func defaultWheelCacheDir() string {
 }
 
 // ResolveContext is Resolve with an explicit context and on-disk wheel cache
-// directory for lock-file downloads.
-func ResolveContext(ctx context.Context, reqs []lock.Requirement, dirs []string, target wheel.Target, wheelCacheDir string) ([]ResolvedWheel, error) {
+// directory for lock-file downloads. When allowSdist is true, packages with no
+// compatible wheel may be built from their source distribution (which executes
+// the dependency's build code on this host); otherwise such packages error.
+func ResolveContext(ctx context.Context, reqs []lock.Requirement, dirs []string, target wheel.Target, wheelCacheDir string, allowSdist bool) ([]ResolvedWheel, error) {
 	var index map[string][]candidate
 	if len(dirs) > 0 {
 		var err error
@@ -80,7 +82,7 @@ func ResolveContext(ctx context.Context, reqs []lock.Requirement, dirs []string,
 
 	out := make([]ResolvedWheel, 0, len(reqs))
 	for _, req := range reqs {
-		rw, err := resolveOne(ctx, req, index, target, cache)
+		rw, err := resolveOne(ctx, req, index, target, cache, allowSdist)
 		if err != nil {
 			return nil, err
 		}
@@ -97,7 +99,7 @@ func ResolveContext(ctx context.Context, reqs []lock.Requirement, dirs []string,
 	return out, nil
 }
 
-func resolveOne(ctx context.Context, req lock.Requirement, index map[string][]candidate, target wheel.Target, cache *wheelCache) (ResolvedWheel, error) {
+func resolveOne(ctx context.Context, req lock.Requirement, index map[string][]candidate, target wheel.Target, cache *wheelCache, allowSdist bool) (ResolvedWheel, error) {
 	key := lock.NormalizeName(req.Name) + "\x00" + req.Version
 	if index != nil {
 		if cands := index[key]; len(cands) > 0 {
@@ -141,6 +143,14 @@ func resolveOne(ctx context.Context, req lock.Requirement, index map[string][]ca
 		if req.Sdist == nil {
 			return ResolvedWheel{}, err
 		}
+		// No compatible lock wheel; consider building from the sdist below.
+	}
+
+	// Building from an sdist runs the dependency's own build code (setup.py /
+	// build backend) on this host with no container isolation, so it is gated
+	// behind explicit opt-in.
+	if !allowSdist {
+		return ResolvedWheel{}, fmt.Errorf("wheelhouse: %s==%s has no compatible pre-built wheel and would have to be built from its source distribution, which runs arbitrary build code from the dependency on this machine; pass --build-sdists (or set build-sdists in [tool.pymage]) to allow it, or supply a pre-built wheel via --find-links", req.Name, req.Version)
 	}
 	return buildFromSdist(ctx, req, target, cache)
 }