diff --git a/workspace-server/internal/provisioner/cp_provisioner.go b/workspace-server/internal/provisioner/cp_provisioner.go
index 269816909..ef1f8595c 100644
--- a/workspace-server/internal/provisioner/cp_provisioner.go
+++ b/workspace-server/internal/provisioner/cp_provisioner.go
@@ -241,9 +241,12 @@ func (p *CPProvisioner) Start(ctx context.Context, cfg WorkspaceConfig) (string,
 	// Cap body read at 64 KiB — the CP only ever returns small JSON
 	// responses; an unbounded read could be weaponized into log-flood
 	// DoS by a compromised upstream.
-	respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 64<<10))
+	respBody, readErr := io.ReadAll(io.LimitReader(resp.Body, 64<<10))
+	if readErr != nil {
+		return "", fmt.Errorf("cp provisioner: read response body: %w", readErr)
+	}
 	var result cpProvisionResponse
-	json.Unmarshal(respBody, &result)
+	unmarshalErr := json.Unmarshal(respBody, &result)
 
 	if resp.StatusCode != http.StatusCreated {
 		// Prefer the structured {"error":"..."} field. Do NOT fall back
@@ -257,6 +260,10 @@ func (p *CPProvisioner) Start(ctx context.Context, cfg WorkspaceConfig) (string,
 		return "", fmt.Errorf("cp provisioner: provision failed (%d): %s", resp.StatusCode, errMsg)
 	}
 
+	if unmarshalErr != nil {
+		return "", fmt.Errorf("cp provisioner: decode 201 response: %w", unmarshalErr)
+	}
+
 	log.Printf("CP provisioner: workspace %s → EC2 instance %s (%s)", cfg.WorkspaceID, result.InstanceID, result.State)
 	provlog.Event("provision.ec2_started", map[string]any{
 		"workspace_id": cfg.WorkspaceID,
@@ -409,7 +416,11 @@ func (p *CPProvisioner) Stop(ctx context.Context, workspaceID string) error {
 		// Read a bounded slice of the body so the error message gives ops
 		// enough to triage without risking a multi-MB log line on a
 		// pathological response. 512 bytes covers any sane error envelope.
-		body, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
+		body, readErr := io.ReadAll(io.LimitReader(resp.Body, 512))
+		if readErr != nil {
+			return fmt.Errorf("cp provisioner: stop %s: unexpected %d (read body failed: %w)",
+				workspaceID, resp.StatusCode, readErr)
+		}
 		return fmt.Errorf("cp provisioner: stop %s: unexpected %d: %s",
 			workspaceID, resp.StatusCode, strings.TrimSpace(string(body)))
 	}
diff --git a/workspace-server/internal/provisioner/cp_provisioner_test.go b/workspace-server/internal/provisioner/cp_provisioner_test.go
index 44e27660a..23c747882 100644
--- a/workspace-server/internal/provisioner/cp_provisioner_test.go
+++ b/workspace-server/internal/provisioner/cp_provisioner_test.go
@@ -442,6 +442,26 @@ func TestStart_SymlinkTemplatePathError(t *testing.T) {
 	}
 }
 
+// TestStart_Malformed201SurfacesError — when CP returns 201 Created with
+// unparseable JSON, Start must return an error instead of silently
+// returning an empty instance_id. CR2 blocker from review #5552.
+func TestStart_Malformed201SurfacesError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusCreated)
+		_, _ = io.WriteString(w, `{"instance_id": broken-json`)
+	}))
+	defer srv.Close()
+
+	p := &CPProvisioner{baseURL: srv.URL, orgID: "org-1", httpClient: srv.Client()}
+	_, err := p.Start(context.Background(), WorkspaceConfig{WorkspaceID: "ws-1", Runtime: "py"})
+	if err == nil {
+		t.Fatal("expected error on malformed 201, got nil")
+	}
+	if !strings.Contains(err.Error(), "decode 201 response") {
+		t.Errorf("error should mention decode 201 response, got %q", err.Error())
+	}
+}
+
 // TestStop_SendsBothAuthHeaders — verify #118/#130 compliance on the
 // teardown path. Any call to /cp/workspaces/:id must carry both the
 // platform-wide shared secret AND the per-tenant admin token, or the