diff --git a/coordinator/internal/agent/setupui/server.go b/coordinator/internal/agent/setupui/server.go
index 7d9cca7..16becaf 100644
--- a/coordinator/internal/agent/setupui/server.go
+++ b/coordinator/internal/agent/setupui/server.go
@@ -300,6 +300,25 @@ type statusView struct {
 	TokenSet      bool   `json:"token_set"`
 }
 
+// ensureVenvTaskRunner rewrites the saved config so its task runner uses the
+// managed venv python when one exists and the config does not already pin one.
+func (s *Server) ensureVenvTaskRunner() {
+	raw, err := os.ReadFile(s.cfgPath)
+	if err != nil {
+		return
+	}
+	var file agent.ConfigFile
+	if json.Unmarshal(raw, &file) != nil || len(file.TaskRunner) > 0 {
+		return
+	}
+	if venv := s.venvPython(); venv != "" {
+		file.TaskRunner = []string{venv, "-m", "scimesh.worker.task"}
+		if payload, err := json.MarshalIndent(file, "", "  "); err == nil {
+			_ = os.WriteFile(s.cfgPath, append(payload, '\n'), 0o600)
+		}
+	}
+}
+
 func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
 	view := statusView{ConfigPath: s.cfgPath, LogPath: s.logPath, Running: s.sup.Alive(), Pid: s.sup.Pid()}
 	if raw, err := os.ReadFile(s.cfgPath); err == nil {
@@ -365,6 +384,14 @@ func (s *Server) handleSaveConfig(w http.ResponseWriter, r *http.Request) {
 	if file.CPUCount < 1 {
 		file.CPUCount = 1
 	}
+	// The wizard UI bakes the venv python into the runner after an install;
+	// an API-driven or scripted flow may not, so the server guarantees it:
+	// workloads execute through scimesh's task runner, which lives in the venv.
+	if len(file.TaskRunner) == 0 {
+		if venv := s.venvPython(); venv != "" {
+			file.TaskRunner = []string{venv, "-m", "scimesh.worker.task"}
+		}
+	}
 	if err := agent.SaveConfigFile(s.cfgPath, file); err != nil {
 		s.log.Error("save wizard config", "err", err)
 		writeJSON(w, http.StatusInternalServerError, map[string]string{"error": "could not write the config file"})
@@ -393,6 +420,7 @@ func (s *Server) handleStart(w http.ResponseWriter, r *http.Request) {
 		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "no configuration saved yet"})
 		return
 	}
+	s.ensureVenvTaskRunner()
 	pid, err := s.sup.Start(s.cfgPath, s.logPath)
 	if err != nil {
 		writeJSON(w, http.StatusConflict, map[string]string{"error": err.Error()})
@@ -514,6 +542,20 @@ func (s *Server) handleInstallRuntime(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, installRuntimeResponse{OK: true, Python: venvPython, Installed: true})
 }
 
+// venvPython returns the managed venv python when the runtime installer has
+// created one, so the task runner can be pointed at it automatically.
+func (s *Server) venvPython() string {
+	for _, candidate := range []string{
+		filepath.Join(s.dir, "venv", "bin", "python"),
+		filepath.Join(s.dir, "venv", "Scripts", "python.exe"),
+	} {
+		if _, err := os.Stat(candidate); err == nil {
+			return candidate
+		}
+	}
+	return ""
+}
+
 // installScimeshWithPip installs the package with the venv's own pip,
 // streaming into the agent log so a long build is not silent.
 func installScimeshWithPip(ctx context.Context, venvPython, pkg string) error {
diff --git a/coordinator/internal/agent/setupui/server_test.go b/coordinator/internal/agent/setupui/server_test.go
index 44e1ab7..ab617f6 100644
--- a/coordinator/internal/agent/setupui/server_test.go
+++ b/coordinator/internal/agent/setupui/server_test.go
@@ -409,3 +409,48 @@ func TestInstallRuntimeWheelDownloadFailureIsExplained(t *testing.T) {
 		t.Errorf("error = %v, want a SCIMESH_PIP_PACKAGE hint", data["error"])
 	}
 }
+
+func TestStartPinsTheVenvTaskRunner(t *testing.T) {
+	sup := &fakeSup{}
+	server, base := newTestServer(t, sup)
+	postJSON(t, base, "/api/config", map[string]any{
+		"coordinator_url": "http://coord:8080", "token": "t", "work_dir": ".",
+	})
+	// Simulate the runtime installer: create the venv python marker.
+	venvPython := filepath.Join(server.dir, "venv", "bin", "python")
+	_ = os.MkdirAll(filepath.Dir(venvPython), 0o755)
+	_ = os.WriteFile(venvPython, []byte("#!/bin/sh\nexit 0\n"), 0o755)
+
+	rec, _ := postJSON(t, base, "/api/start", map[string]any{})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("start: got %d, want 200", rec.Code)
+	}
+	config, err := agent.LoadConfigFile(server.cfgPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(config.TaskRunner) != 3 || config.TaskRunner[0] != venvPython || config.TaskRunner[1] != "-m" || config.TaskRunner[2] != "scimesh.worker.task" {
+		t.Errorf("task runner = %v, want the venv python runner", config.TaskRunner)
+	}
+}
+
+func TestSaveConfigPinsVenvRunnerWhenPresent(t *testing.T) {
+	server, base := newTestServer(t, &fakeSup{})
+	venvPython := filepath.Join(server.dir, "venv", "bin", "python")
+	_ = os.MkdirAll(filepath.Dir(venvPython), 0o755)
+	_ = os.WriteFile(venvPython, []byte("#!/bin/sh\nexit 0\n"), 0o755)
+
+	rec, _ := postJSON(t, base, "/api/config", map[string]any{
+		"coordinator_url": "http://coord:8080", "token": "t", "work_dir": ".",
+	})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("config: got %d", rec.Code)
+	}
+	config, err := agent.LoadConfigFile(server.cfgPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(config.TaskRunner) != 3 || config.TaskRunner[0] != venvPython {
+		t.Errorf("task runner = %v, want the venv python", config.TaskRunner)
+	}
+}