diff --git a/internal/runtime/registry.go b/internal/runtime/registry.go
index 3302ffe61d..7295b73cf4 100644
--- a/internal/runtime/registry.go
+++ b/internal/runtime/registry.go
@@ -2,6 +2,7 @@ package runtime
 
 import (
 	"fmt"
+	"slices"
 	"strings"
 
 	"github.com/fullsend-ai/fullsend/internal/config"
@@ -29,19 +30,43 @@ func Resolve(name string) (Backend, error) {
 }
 
 // ResolveFromConfig selects the runtime backend from org config defaults.
+// The runtime name is validated against [config.ValidRuntimes] before
+// resolution so that stub runtimes registered in [Resolve] for dev/testing
+// cannot be activated through config files.
 func ResolveFromConfig(cfg config.OrgConfigReader) (Backend, error) {
 	rt := "claude"
 	if cfg != nil && cfg.OrgRepoDefaults().Runtime != "" {
 		rt = cfg.OrgRepoDefaults().Runtime
 	}
+	if err := validateConfigRuntime(rt); err != nil {
+		return Backend{}, err
+	}
 	return Resolve(rt)
 }
 
 // ResolveFromPerRepoConfig selects the runtime backend from per-repo config.
+// The runtime name is validated against [config.ValidRuntimes] before
+// resolution so that stub runtimes registered in [Resolve] for dev/testing
+// cannot be activated through config files.
 func ResolveFromPerRepoConfig(cfg config.PerRepoConfigReader) (Backend, error) {
 	rt := "claude"
 	if cfg != nil && cfg.ConfigRuntime() != "" {
 		rt = cfg.ConfigRuntime()
 	}
+	if err := validateConfigRuntime(rt); err != nil {
+		return Backend{}, err
+	}
 	return Resolve(rt)
 }
+
+// validateConfigRuntime checks that rt is in the set of user-facing
+// runtimes allowed in config files.  Stub runtimes (e.g. "opencode")
+// are intentionally excluded from [config.ValidRuntimes] so they
+// cannot be activated through org or per-repo config.
+func validateConfigRuntime(rt string) error {
+	valid := config.ValidRuntimes()
+	if !slices.Contains(valid, rt) {
+		return fmt.Errorf("invalid runtime %q in config: must be one of %s", rt, strings.Join(valid, ", "))
+	}
+	return nil
+}
diff --git a/internal/runtime/registry_test.go b/internal/runtime/registry_test.go
index e0b29d13b6..a79950078c 100644
--- a/internal/runtime/registry_test.go
+++ b/internal/runtime/registry_test.go
@@ -69,14 +69,6 @@ func TestResolveFromPerRepoConfig(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, "dummy", dummyBackend.Runtime.Name())
 
-	// opencode is not in ValidRuntimes() but is resolvable via Resolve().
-	// A hand-written config bypassing validation can reach the stub.
-	ocCfg := config.NewPerRepoConfig(nil, "")
-	ocCfg.SetRuntime("opencode")
-	ocBackend, err := ResolveFromPerRepoConfig(ocCfg)
-	require.NoError(t, err)
-	assert.Equal(t, "opencode", ocBackend.Runtime.Name())
-
 	// pi is user-selectable (#6464).
 	piCfg := config.NewPerRepoConfig(nil, "")
 	piCfg.SetRuntime("pi")
@@ -89,3 +81,44 @@ func TestResolveFromPerRepoConfig(t *testing.T) {
 	_, err = ResolveFromPerRepoConfig(invalidCfg)
 	require.Error(t, err)
 }
+
+func TestResolveFromPerRepoConfig_RejectsStubRuntimes(t *testing.T) {
+	t.Parallel()
+
+	// Stub runtimes like "opencode" are resolvable via Resolve() for
+	// dev/testing, but must be rejected when coming through config.
+	for _, name := range []string{"opencode"} {
+		ocCfg := config.NewPerRepoConfig(nil, "")
+		ocCfg.SetRuntime(name)
+		_, err := ResolveFromPerRepoConfig(ocCfg)
+		require.Error(t, err, "stub runtime %q should fail via config path", name)
+		assert.Contains(t, err.Error(), "invalid runtime")
+	}
+
+	// Direct Resolve() still works for dev/testing.
+	rt, err := Resolve("opencode")
+	require.NoError(t, err)
+	assert.Equal(t, "opencode", rt.Runtime.Name())
+}
+
+func TestResolveFromConfig_RejectsStubRuntimes(t *testing.T) {
+	t.Parallel()
+
+	// Org config with a stub runtime should fail at resolution time.
+	cfg, parseErr := config.ParseOrgConfig([]byte(`version: "1"
+dispatch:
+  platform: github-actions
+defaults:
+  roles: [triage]
+  runtime: opencode
+repos: {}
+`))
+	// ParseOrgConfig calls Validate() which also rejects "opencode",
+	// so this may fail at parse time.  If parsing succeeds (e.g. because
+	// Validate() is not called), ResolveFromConfig must still reject it.
+	if parseErr == nil {
+		_, err := ResolveFromConfig(cfg)
+		require.Error(t, err, "stub runtime %q should fail via org config path", "opencode")
+		assert.Contains(t, err.Error(), "invalid runtime")
+	}
+}