diff --git a/include/sta/ReportFieldExtension.hh b/include/sta/ReportFieldExtension.hh
new file mode 100644
index 00000000..159315c2
--- /dev/null
+++ b/include/sta/ReportFieldExtension.hh
@@ -0,0 +1,57 @@
+// OpenSTA, Static Timing Analyzer
+// Copyright (c) 2026, Parallax Software, Inc.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+//
+// The origin of this software must not be misrepresented; you must not
+// claim that you wrote the original software.
+//
+// Altered source versions must be plainly marked as such, and must not be
+// misrepresented as being the original software.
+//
+// This notice may not be removed or altered from any source distribution.
+
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <string_view>
+
+#include "NetworkClass.hh"
+#include "SearchClass.hh"
+
+namespace sta {
+
+// External producers register extensions to add custom columns to
+// report_checks output. ReportPath takes ownership of the registered
+// pointer and deletes it at shutdown.
+class ReportFieldExtension
+{
+ public:
+  virtual ~ReportFieldExtension() = default;
+
+  // Identifier matched against `-fields` tokens.
+  virtual std::string_view name() const = 0;
+  // Column header.
+  virtual std::string_view title() const = 0;
+  // Column width in characters.
+  virtual size_t width() const = 0;
+
+  // Per-row value. Return "" to leave the cell blank.
+  virtual std::string value(const Path *path,
+                            const Pin *pin,
+                            const Instance *inst) const = 0;
+};
+
+}  // namespace sta
diff --git a/include/sta/Sta.hh b/include/sta/Sta.hh
index 5237ec9a..a5f026e7 100644
--- a/include/sta/Sta.hh
+++ b/include/sta/Sta.hh
@@ -71,6 +71,7 @@ class SearchPred;
 class Scene;
 class ClkSkews;
 class ReportField;
+class ReportFieldExtension;
 class EquivCells;
 class StaSimObserver;
 class GraphLoop;
@@ -989,7 +990,8 @@ public:
                            bool report_slew,
                            bool report_fanout,
                            bool report_variation,
-                           bool report_src_attr);
+                           bool report_src_attr,
+                           const StringSeq &extension_names);
   ReportField *findReportPathField(std::string_view name);
   void setReportPathDigits(int digits);
   void setReportPathNoSplit(bool no_split);
@@ -997,6 +999,9 @@ public:
   void reportPathEnds(PathEndSeq *ends);
   ReportPath *reportPath() { return report_path_; }
   void reportPath(const Path *path);
+  // Register an extension that adds a custom column to report_checks.
+  // Sta takes ownership and deletes at destruction.
+  void registerReportFieldExtension(ReportFieldExtension *ext);
 
   // Report clk skews for clks.
   void reportClkSkew(ConstClockSeq &clks,
diff --git a/search/ReportPath.cc b/search/ReportPath.cc
index 50d05574..06513108 100644
--- a/search/ReportPath.cc
+++ b/search/ReportPath.cc
@@ -53,6 +53,7 @@
 #include "PortDelay.hh"
 #include "PortDirection.hh"
 #include "Report.hh"
+#include "ReportFieldExtension.hh"
 #include "Scene.hh"
 #include "Sdc.hh"
 #include "Search.hh"
@@ -96,7 +97,9 @@ ReportField::ReportField(std::string_view name,
 }
 
 ReportField::~ReportField()
-= default;
+{
+  delete extension_;
+}
 
 void
 ReportField::setProperties(std::string_view title,
@@ -128,7 +131,8 @@ ReportPath::ReportPath(StaState *sta) :
 {
   makeFields();
   setDigits(2);
-  setReportFields(false, false, false, false, false, false, false, false);
+  setReportFields(false, false, false, false, false, false, false, false,
+                  StringSeq{});
 }
 
 ReportPath::~ReportPath()
@@ -226,7 +230,8 @@ ReportPath::setReportFields(bool report_input_pin,
                             bool report_slew,
                             bool report_fanout,
                             bool report_variation,
-                            bool report_src_attr)
+                            bool report_src_attr,
+                            const StringSeq &extension_names)
 {
   report_input_pin_ = report_input_pin;
   report_hier_pins_ = report_hier_pins;
@@ -239,6 +244,62 @@ ReportPath::setReportFields(bool report_input_pin,
   field_src_attr_->setEnabled(report_src_attr);
   // for debug
   field_case_->setEnabled(false);
+
+  // Reset extension-backed fields: disable, drop slot assignment.
+  for (ReportField *field : fields_) {
+    if (field->extension() != nullptr) {
+      field->setEnabled(false);
+      field->setExtensionIndex(-1);
+    }
+  }
+  // Enable each requested extension name and assign a buffer slot.
+  int slot = 0;
+  for (const std::string &name : extension_names) {
+    ReportField *field = findField(name);
+    if (field == nullptr || field->extension() == nullptr) {
+      report_->warn(168, "no field extension registered for '{}'.", name);
+      continue;
+    }
+    field->setEnabled(true);
+    field->setExtensionIndex(slot++);
+  }
+  enabled_extension_count_ = slot;
+}
+
+void
+ReportPath::registerFieldExtension(ReportFieldExtension *ext)
+{
+  if (findField(ext->name()) != nullptr) {
+    // Already registered (e.g. re-init in a long-lived Sta). Silently dedupe.
+    delete ext;
+    return;
+  }
+  ReportField *field = makeField(ext->name(),
+                                 ext->title(),
+                                 static_cast<int>(ext->width()),
+                                 true,
+                                 nullptr,
+                                 false);
+  field->setExtension(ext);
+}
+
+void
+ReportPath::collectExtensionValues(const Path *path,
+                                   const Pin *pin,
+                                   const Instance *inst,
+                                   std::vector<std::string> &values) const
+{
+  if (enabled_extension_count_ == 0) {
+    values.clear();
+    return;
+  }
+  values.resize(enabled_extension_count_);
+  for (const ReportField *field : fields_) {
+    ReportFieldExtension *ext = field->extension();
+    if (ext == nullptr || field->extensionIndex() < 0)
+      continue;
+    values[field->extensionIndex()] = ext->value(path, pin, inst);
+  }
 }
 
 void
@@ -2451,11 +2512,13 @@ ReportPath::reportPathLine(const Path *path,
   std::string src_attr;
   if (inst)
     src_attr = network_->getAttribute(inst, "src");
+  collectExtensionValues(path, pin, inst, extension_values_buf_);
   // Don't show capacitance field for input pins.
   if (is_driver && field_capacitance_->enabled())
     cap = graph_delay_calc_->loadCap(pin, rf, scene, min_max);
   reportLine(what, cap, slew, field_blank_, incr, field_blank_,
-             time, false, early_late, rf, src_attr, line_case);
+             time, false, early_late, rf, src_attr, line_case,
+             extension_values_buf_);
 }
 
 void
@@ -2733,6 +2796,7 @@ ReportPath::reportPath6(const Path *path,
     std::string src_attr;
     if (inst)
       src_attr = network_->getAttribute(inst, "src");
+    collectExtensionValues(path1, pin, inst, extension_values_buf_);
     // Always show the search start point (register clk pin).
     // Skip reporting the clk tree unless it is requested.
     if (is_clk_start
@@ -2825,7 +2889,7 @@ ReportPath::reportPath6(const Path *path,
         const std::string what = descriptionField(vertex);
         reportLine(what, cap, slew, fanout,
                    incr, field_blank_, time, false, min_max, rf, src_attr,
-                   line_case);
+                   line_case, extension_values_buf_);
 
         if (report_net_) {
           const std::string what2 = descriptionNet(pin);
@@ -2844,7 +2908,7 @@ ReportPath::reportPath6(const Path *path,
           const std::string what = descriptionField(vertex);
           reportLine(what, field_blank_, slew, field_blank_,
                      incr, field_blank_, time, false, min_max, rf, src_attr,
-                     line_case);
+                     line_case, extension_values_buf_);
           prev_time = time;
         }
       }
@@ -3182,7 +3246,8 @@ ReportPath::reportLine(std::string_view what,
                        const EarlyLate *early_late,
                        const RiseFall *rf,
                        std::string_view src_attr,
-                       std::string_view line_case) const
+                       std::string_view line_case,
+                       std::span<const std::string> extension_values) const
 {
   std::string line;
   size_t field_index = 0;
@@ -3234,6 +3299,13 @@ ReportPath::reportLine(std::string_view what,
       }
       else if (field == field_case_)
         line += line_case;
+      else if (field->extensionIndex() >= 0) {
+        size_t idx = static_cast<size_t>(field->extensionIndex());
+        if (idx < extension_values.size() && !extension_values[idx].empty())
+          reportField(extension_values[idx], field, line);
+        else
+          reportFieldBlank(field, line);
+      }
 
       first_field = false;
     }
diff --git a/search/ReportPath.hh b/search/ReportPath.hh
index b8ce652d..1acb60c7 100644
--- a/search/ReportPath.hh
+++ b/search/ReportPath.hh
@@ -25,6 +25,7 @@
 #pragma once
 
 #include <cstddef>
+#include <span>
 #include <string>
 #include <string_view>
 #include <vector>
@@ -50,6 +51,7 @@ namespace sta {
 class Scene;
 class PathExpanded;
 class ReportField;
+class ReportFieldExtension;
 
 using ReportFieldSeq = std::vector<ReportField*>;
 
@@ -68,7 +70,13 @@ public:
                        bool report_slew,
                        bool report_fanout,
                        bool report_variation,
-                       bool report_src_attr);
+                       bool report_src_attr,
+                       const StringSeq &extension_names);
+
+  // Register an extension that adds a custom column to report_checks.
+  // ReportPath takes ownership and deletes at destruction.
+  void registerFieldExtension(ReportFieldExtension *ext);
+
   int digits() const { return digits_; }
   void setDigits(int digits);
   void setNoSplit(bool no_split);
@@ -280,6 +288,10 @@ protected:
                       const Delay &incr,
                       const Arrival &time,
                       std::string_view line_case) const;
+  void collectExtensionValues(const Path *path,
+                              const Pin *pin,
+                              const Instance *inst,
+                              std::vector<std::string> &values) const;
   void reportCommonClkPessimism(const PathEnd *end,
                                 Arrival &clk_arrival) const ;
   void reportClkUncertainty(const PathEnd *end,
@@ -380,7 +392,8 @@ protected:
                   const EarlyLate *early_late,
                   const RiseFall *rf,
                   std::string_view src_attr,
-                  std::string_view line_case) const;
+                  std::string_view line_case,
+                  std::span<const std::string> extension_values = {}) const;
   void reportLineTotal(std::string_view what,
                        const Delay &incr,
                        const EarlyLate *early_late) const;
@@ -502,6 +515,8 @@ protected:
   ReportField *field_src_attr_;
   ReportField *field_edge_;
   ReportField *field_case_;
+  mutable std::vector<std::string> extension_values_buf_;
+  int enabled_extension_count_{0};
 
   std::string plus_zero_;
   std::string minus_zero_;
@@ -533,6 +548,11 @@ public:
   const std::string &blank() const { return blank_; }
   void setEnabled(bool enabled);
   bool enabled() const { return enabled_; }
+  int extensionIndex() const { return extension_index_; }
+  void setExtensionIndex(int index) { extension_index_ = index; }
+  ReportFieldExtension *extension() const { return extension_; }
+  // ReportField takes ownership of the extension and deletes it.
+  void setExtension(ReportFieldExtension *ext) { extension_ = ext; }
 
 protected:
   std::string name_;
@@ -542,6 +562,8 @@ protected:
   Unit *unit_;
   bool enabled_;
   std::string blank_;
+  int extension_index_{-1};
+  ReportFieldExtension *extension_{nullptr};
 };
 
 } // namespace sta
diff --git a/search/Search.i b/search/Search.i
index c378a5e8..df1b55c8 100644
--- a/search/Search.i
+++ b/search/Search.i
@@ -414,7 +414,8 @@ set_report_path_fields(bool report_input_pin,
                        bool report_slew,
                        bool report_fanout,
                        bool report_variation,
-                       bool report_src_attr)
+                       bool report_src_attr,
+                       StringSeq extension_names)
 {
   Sta::sta()->setReportPathFields(report_input_pin,
                                   report_hier_pins,
@@ -423,7 +424,8 @@ set_report_path_fields(bool report_input_pin,
                                   report_slew,
                                   report_fanout,
                                   report_variation,
-                                  report_src_attr);
+                                  report_src_attr,
+                                  extension_names);
 }
 
 void
diff --git a/search/Search.tcl b/search/Search.tcl
index 11394292..e4dd296a 100644
--- a/search/Search.tcl
+++ b/search/Search.tcl
@@ -336,7 +336,7 @@ define_cmd_args "report_checks" \
      [-sort_by_slack]\
      [-path_group group_name]\
      [-format full|full_clock|full_clock_expanded|short|end|slack_only|summary|json]\
-     [-fields capacitance|slew|fanout|input_pin|net|src_attr]\
+     [-fields capacitance|slew|fanout|input_pin|net|src_attr|<extension_name>...]\
      [-digits digits]\
      [-no_line_splits]\
      [> filename] [>> filename]}
@@ -724,6 +724,7 @@ proc parse_report_path_options { cmd args_var default_format
   set report_fanout 0
   set report_variation 0
   set report_src_attr 0
+  set extension_names {}
   if { [info exists path_options(-fields)] } {
     foreach field $path_options(-fields) {
       if { [string match "input*" $field] } {
@@ -743,13 +744,14 @@ proc parse_report_path_options { cmd args_var default_format
       } elseif { [string match "src*" $field] } {
         set report_src_attr 1
       } else {
-        sta_warn 168 "unknown field $field."
+        lappend extension_names $field
       }
     }
   }
 
   set_report_path_fields $report_input_pin $report_hier_pins $report_net \
-    $report_cap $report_slew $report_fanout $report_variation $report_src_attr
+    $report_cap $report_slew $report_fanout $report_variation $report_src_attr \
+    $extension_names
 
   set_report_path_no_split [info exists path_options(-no_line_splits)]
 }
diff --git a/search/Sta.cc b/search/Sta.cc
index c5a40139..27798c26 100644
--- a/search/Sta.cc
+++ b/search/Sta.cc
@@ -2823,11 +2823,19 @@ Sta::setReportPathFields(bool report_input_pin,
 			 bool report_slew,
 			 bool report_fanout,
                          bool report_variation,
-			 bool report_src_attr)
+			 bool report_src_attr,
+			 const StringSeq &extension_names)
 {
   report_path_->setReportFields(report_input_pin, report_hier_pins, report_net,
                                 report_cap, report_slew, report_fanout,
-                                report_variation, report_src_attr);
+                                report_variation, report_src_attr,
+                                extension_names);
+}
+
+void
+Sta::registerReportFieldExtension(ReportFieldExtension *ext)
+{
+  report_path_->registerFieldExtension(ext);
 }
 
 ReportField *