diff --git a/cmake/CliFboss2.cmake b/cmake/CliFboss2.cmake
index d0891c04e7737..6a5872c93f967 100644
--- a/cmake/CliFboss2.cmake
+++ b/cmake/CliFboss2.cmake
@@ -577,6 +577,8 @@ add_library(fboss2_config_lib
   fboss/cli/fboss2/commands/config/CmdConfigReload.cpp
   fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h
   fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.cpp
+  fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h
+  fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.cpp
   fboss/cli/fboss2/session/ConfigSession.h
   fboss/cli/fboss2/session/ConfigSession.cpp
   fboss/cli/fboss2/CmdListConfig.cpp
diff --git a/cmake/CliFboss2Test.cmake b/cmake/CliFboss2Test.cmake
index 3d4391cb82dee..e50f1dd335b8e 100644
--- a/cmake/CliFboss2Test.cmake
+++ b/cmake/CliFboss2Test.cmake
@@ -5,6 +5,7 @@ add_executable(fboss2_cmd_test
   fboss/cli/fboss2/test/TestMain.cpp
   fboss/cli/fboss2/test/CmdConfigAppliedInfoTest.cpp
   fboss/cli/fboss2/test/CmdConfigReloadTest.cpp
+  fboss/cli/fboss2/test/CmdConfigSessionDiffTest.cpp
   fboss/cli/fboss2/test/CmdConfigSessionTest.cpp
   fboss/cli/fboss2/test/CmdSetPortStateTest.cpp
   fboss/cli/fboss2/test/CmdShowAclTest.cpp
diff --git a/fboss/cli/fboss2/BUCK b/fboss/cli/fboss2/BUCK
index 592c7bc87e675..eb56fe91079a5 100644
--- a/fboss/cli/fboss2/BUCK
+++ b/fboss/cli/fboss2/BUCK
@@ -772,12 +772,14 @@ cpp_library(
         "commands/config/CmdConfigAppliedInfo.cpp",
         "commands/config/CmdConfigReload.cpp",
         "commands/config/session/CmdConfigSessionCommit.cpp",
+        "commands/config/session/CmdConfigSessionDiff.cpp",
         "session/ConfigSession.cpp",
     ],
     headers = [
         "commands/config/CmdConfigAppliedInfo.h",
         "commands/config/CmdConfigReload.h",
         "commands/config/session/CmdConfigSessionCommit.h",
+        "commands/config/session/CmdConfigSessionDiff.h",
         "session/ConfigSession.h",
     ],
     exported_deps = [
diff --git a/fboss/cli/fboss2/CmdHandlerImplConfig.cpp b/fboss/cli/fboss2/CmdHandlerImplConfig.cpp
index d4217e43fa286..6c93601afdc7a 100644
--- a/fboss/cli/fboss2/CmdHandlerImplConfig.cpp
+++ b/fboss/cli/fboss2/CmdHandlerImplConfig.cpp
@@ -13,6 +13,7 @@
 #include "fboss/cli/fboss2/commands/config/CmdConfigAppliedInfo.h"
 #include "fboss/cli/fboss2/commands/config/CmdConfigReload.h"
 #include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h"
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h"
 
 namespace facebook::fboss {
 
@@ -21,5 +22,7 @@ CmdHandler<CmdConfigAppliedInfo, CmdConfigAppliedInfoTraits>::run();
 template void CmdHandler<CmdConfigReload, CmdConfigReloadTraits>::run();
 template void
 CmdHandler<CmdConfigSessionCommit, CmdConfigSessionCommitTraits>::run();
+template void
+CmdHandler<CmdConfigSessionDiff, CmdConfigSessionDiffTraits>::run();
 
 } // namespace facebook::fboss
diff --git a/fboss/cli/fboss2/CmdListConfig.cpp b/fboss/cli/fboss2/CmdListConfig.cpp
index e59256deb5fbc..231cef7c5990c 100644
--- a/fboss/cli/fboss2/CmdListConfig.cpp
+++ b/fboss/cli/fboss2/CmdListConfig.cpp
@@ -14,6 +14,7 @@
 #include "fboss/cli/fboss2/commands/config/CmdConfigAppliedInfo.h"
 #include "fboss/cli/fboss2/commands/config/CmdConfigReload.h"
 #include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h"
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h"
 
 namespace facebook::fboss {
 
@@ -30,11 +31,17 @@ const CommandTree& kConfigCommandTree() {
           "session",
           "Manage config session",
           {{
-              "commit",
-              "Commit the current config session",
-              commandHandler<CmdConfigSessionCommit>,
-              argTypeHandler<CmdConfigSessionCommitTraits>,
-          }},
+               "commit",
+               "Commit the current config session",
+               commandHandler<CmdConfigSessionCommit>,
+               argTypeHandler<CmdConfigSessionCommitTraits>,
+           },
+           {
+               "diff",
+               "Show diff between configs (session vs live, session vs revision, or revision vs revision)",
+               commandHandler<CmdConfigSessionDiff>,
+               argTypeHandler<CmdConfigSessionDiffTraits>,
+           }},
       },
 
       {"config",
diff --git a/fboss/cli/fboss2/CmdSubcommands.cpp b/fboss/cli/fboss2/CmdSubcommands.cpp
index 87485bcd711c5..199f4b00a6c6b 100644
--- a/fboss/cli/fboss2/CmdSubcommands.cpp
+++ b/fboss/cli/fboss2/CmdSubcommands.cpp
@@ -219,6 +219,10 @@ CLI::App* CmdSubcommands::addCommand(
         case utils::ObjectArgTypeId::OBJECT_ARG_TYPE_FAN_PWM:
           subCmd->add_option("pwm", args, "Fan PWM (0..100) or 'disable'");
           break;
+        case utils::ObjectArgTypeId::OBJECT_ARG_TYPE_ID_REVISION_LIST:
+          subCmd->add_option(
+              "revisions", args, "Revision(s) in the form 'rN' or 'current'");
+          break;
         case utils::ObjectArgTypeId::OBJECT_ARG_TYPE_ID_UNINITIALIZE:
         case utils::ObjectArgTypeId::OBJECT_ARG_TYPE_ID_NONE:
           break;
diff --git a/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.cpp b/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.cpp
new file mode 100644
index 0000000000000..efb0fb447f8aa
--- /dev/null
+++ b/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.cpp
@@ -0,0 +1,149 @@
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h"
+#include "fboss/cli/fboss2/session/ConfigSession.h"
+
+#include <folly/Subprocess.h>
+#include <filesystem>
+
+namespace fs = std::filesystem;
+
+namespace facebook::fboss {
+
+namespace {
+
+// Helper function to resolve a revision specifier to a file path
+// Note: Revision format validation is done in RevisionList constructor
+std::string resolveRevisionPath(
+    const std::string& revision,
+    const std::string& cliConfigDir,
+    const std::string& systemConfigPath) {
+  if (revision == "current") {
+    return systemConfigPath;
+  }
+
+  // Build the path (revision is already validated to be in "rN" format)
+  std::string revisionPath = cliConfigDir + "/agent-" + revision + ".conf";
+
+  // Check if the file exists
+  if (!fs::exists(revisionPath)) {
+    throw std::invalid_argument(
+        "Revision " + revision + " does not exist at " + revisionPath);
+  }
+
+  return revisionPath;
+}
+
+// Helper function to execute diff and return the result
+std::string executeDiff(
+    const std::string& path1,
+    const std::string& path2,
+    const std::string& label1,
+    const std::string& label2) {
+  try {
+    folly::Subprocess proc(
+        std::vector<std::string>{
+            "/usr/bin/diff",
+            "-u",
+            "--label",
+            label1,
+            "--label",
+            label2,
+            path1,
+            path2},
+        folly::Subprocess::Options().pipeStdout().pipeStderr());
+
+    auto result = proc.communicate();
+    int returnCode = proc.wait().exitStatus();
+
+    // diff returns 0 if files are identical, 1 if different, 2 on error
+    if (returnCode == 0) {
+      return "No differences between " + label1 + " and " + label2 + ".";
+    } else if (returnCode == 1) {
+      // Files differ - return the diff output
+      return result.first;
+    } else {
+      // Error occurred
+      throw std::runtime_error("diff command failed: " + result.second);
+    }
+  } catch (const std::exception& ex) {
+    throw std::runtime_error(
+        "Failed to execute diff command: " + std::string(ex.what()));
+  }
+}
+
+} // namespace
+
+CmdConfigSessionDiffTraits::RetType CmdConfigSessionDiff::queryClient(
+    const HostInfo& /* hostInfo */,
+    const utils::RevisionList& revisions) {
+  auto& session = ConfigSession::getInstance();
+
+  std::string systemConfigPath = session.getSystemConfigPath();
+  std::string sessionConfigPath = session.getSessionConfigPath();
+  std::string cliConfigDir = session.getCliConfigDir();
+
+  // Mode 1: No arguments - diff session vs current live config
+  if (revisions.empty()) {
+    if (!session.sessionExists()) {
+      return "No config session exists. Make a config change first.";
+    }
+
+    return executeDiff(
+        systemConfigPath,
+        sessionConfigPath,
+        "current live config",
+        "session config");
+  }
+
+  // Mode 2: One argument - diff session vs specified revision
+  if (revisions.size() == 1) {
+    if (!session.sessionExists()) {
+      return "No config session exists. Make a config change first.";
+    }
+
+    std::string revisionPath =
+        resolveRevisionPath(revisions[0], cliConfigDir, systemConfigPath);
+    std::string label =
+        revisions[0] == "current" ? "current live config" : revisions[0];
+
+    return executeDiff(
+        revisionPath, sessionConfigPath, label, "session config");
+  }
+
+  // Mode 3: Two arguments - diff between two revisions
+  if (revisions.size() == 2) {
+    std::string path1 =
+        resolveRevisionPath(revisions[0], cliConfigDir, systemConfigPath);
+    std::string path2 =
+        resolveRevisionPath(revisions[1], cliConfigDir, systemConfigPath);
+
+    std::string label1 =
+        revisions[0] == "current" ? "current live config" : revisions[0];
+    std::string label2 =
+        revisions[1] == "current" ? "current live config" : revisions[1];
+
+    return executeDiff(path1, path2, label1, label2);
+  }
+
+  // More than 2 arguments is an error
+  throw std::invalid_argument(
+      "Too many arguments. Expected 0, 1, or 2 revision specifiers.");
+}
+
+void CmdConfigSessionDiff::printOutput(const RetType& diffOutput) {
+  std::cout << diffOutput;
+  if (!diffOutput.empty() && diffOutput.back() != '\n') {
+    std::cout << std::endl;
+  }
+}
+
+} // namespace facebook::fboss
diff --git a/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h b/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h
new file mode 100644
index 0000000000000..10655c6485cbd
--- /dev/null
+++ b/fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h
@@ -0,0 +1,39 @@
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#pragma once
+
+#include "fboss/cli/fboss2/CmdHandler.h"
+#include "fboss/cli/fboss2/utils/CmdClientUtils.h"
+#include "fboss/cli/fboss2/utils/CmdUtils.h"
+
+namespace facebook::fboss {
+
+struct CmdConfigSessionDiffTraits : public WriteCommandTraits {
+  static constexpr utils::ObjectArgTypeId ObjectArgTypeId =
+      utils::ObjectArgTypeId::OBJECT_ARG_TYPE_ID_REVISION_LIST;
+  using ObjectArgType = utils::RevisionList;
+  using RetType = std::string;
+};
+
+class CmdConfigSessionDiff
+    : public CmdHandler<CmdConfigSessionDiff, CmdConfigSessionDiffTraits> {
+ public:
+  using ObjectArgType = CmdConfigSessionDiffTraits::ObjectArgType;
+  using RetType = CmdConfigSessionDiffTraits::RetType;
+
+  RetType queryClient(
+      const HostInfo& hostInfo,
+      const utils::RevisionList& revisions);
+
+  void printOutput(const RetType& diffOutput);
+};
+
+} // namespace facebook::fboss
diff --git a/fboss/cli/fboss2/session/ConfigSession.cpp b/fboss/cli/fboss2/session/ConfigSession.cpp
index ecf11bb06a86b..e9232a33b216f 100644
--- a/fboss/cli/fboss2/session/ConfigSession.cpp
+++ b/fboss/cli/fboss2/session/ConfigSession.cpp
@@ -197,10 +197,24 @@ ConfigSession::ConfigSession(
   initializeSession();
 }
 
-ConfigSession& ConfigSession::getInstance() {
-  static ConfigSession instance;
+namespace {
+std::unique_ptr<ConfigSession>& getInstancePtr() {
+  static std::unique_ptr<ConfigSession> instance;
   return instance;
 }
+} // namespace
+
+ConfigSession& ConfigSession::getInstance() {
+  auto& instance = getInstancePtr();
+  if (!instance) {
+    instance = std::make_unique<ConfigSession>();
+  }
+  return *instance;
+}
+
+void ConfigSession::setInstance(std::unique_ptr<ConfigSession> newInstance) {
+  getInstancePtr() = std::move(newInstance);
+}
 
 std::string ConfigSession::getSessionConfigPath() const {
   return sessionConfigPath_;
@@ -210,6 +224,10 @@ std::string ConfigSession::getSystemConfigPath() const {
   return systemConfigPath_;
 }
 
+std::string ConfigSession::getCliConfigDir() const {
+  return cliConfigDir_;
+}
+
 bool ConfigSession::sessionExists() const {
   return fs::exists(sessionConfigPath_);
 }
diff --git a/fboss/cli/fboss2/session/ConfigSession.h b/fboss/cli/fboss2/session/ConfigSession.h
index b07e9962c1702..5f962dad5c874 100644
--- a/fboss/cli/fboss2/session/ConfigSession.h
+++ b/fboss/cli/fboss2/session/ConfigSession.h
@@ -93,6 +93,9 @@ class ConfigSession {
   // Get the path to the system config file (/etc/coop/agent.conf)
   std::string getSystemConfigPath() const;
 
+  // Get the path to the CLI config directory (/etc/coop/cli)
+  std::string getCliConfigDir() const;
+
   // Atomically commit the session to /etc/coop/cli/agent-rN.conf,
   // update the symlink /etc/coop/agent.conf to point to it, and reload config.
   // Returns the revision number that was committed if the commit was
@@ -124,6 +127,9 @@ class ConfigSession {
       const std::string& systemConfigPath,
       const std::string& cliConfigDir);
 
+  // Set the singleton instance (for testing only)
+  static void setInstance(std::unique_ptr<ConfigSession> instance);
+
  private:
   std::string sessionConfigPath_;
   std::string systemConfigPath_;
diff --git a/fboss/cli/fboss2/test/BUCK b/fboss/cli/fboss2/test/BUCK
index f15b5acd5fe8b..797d2da0b80c8 100644
--- a/fboss/cli/fboss2/test/BUCK
+++ b/fboss/cli/fboss2/test/BUCK
@@ -57,6 +57,7 @@ cpp_unittest(
     srcs = [
         "CmdConfigAppliedInfoTest.cpp",
         "CmdConfigReloadTest.cpp",
+        "CmdConfigSessionDiffTest.cpp",
         "CmdConfigSessionTest.cpp",
         "CmdGetPcapTest.cpp",
         "CmdSetPortStateTest.cpp",
diff --git a/fboss/cli/fboss2/test/CmdConfigSessionDiffTest.cpp b/fboss/cli/fboss2/test/CmdConfigSessionDiffTest.cpp
new file mode 100644
index 0000000000000..8488e18815854
--- /dev/null
+++ b/fboss/cli/fboss2/test/CmdConfigSessionDiffTest.cpp
@@ -0,0 +1,376 @@
+// (c) Facebook, Inc. and its affiliates. Confidential and proprietary.
+
+#include <boost/filesystem.hpp>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include <sstream>
+
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionDiff.h"
+#include "fboss/cli/fboss2/session/ConfigSession.h"
+#include "fboss/cli/fboss2/test/CmdHandlerTestBase.h"
+#include "fboss/cli/fboss2/test/TestableConfigSession.h"
+#include "fboss/cli/fboss2/utils/CmdUtils.h"
+#include "fboss/cli/fboss2/utils/PortMap.h"
+
+#include <filesystem>
+#include <fstream>
+
+namespace fs = std::filesystem;
+
+using namespace ::testing;
+
+namespace facebook::fboss {
+
+class CmdConfigSessionDiffTestFixture : public CmdHandlerTestBase {
+ public:
+  fs::path testHomeDir_;
+  fs::path testEtcDir_;
+  fs::path systemConfigPath_;
+  fs::path sessionConfigPath_;
+  fs::path cliConfigDir_;
+
+  void SetUp() override {
+    CmdHandlerTestBase::SetUp();
+
+    // Create unique test directories for each test to avoid conflicts when
+    // running tests in parallel
+    auto tempBase = fs::temp_directory_path();
+    auto uniquePath = boost::filesystem::unique_path(
+        "fboss2_config_session_diff_test_%%%%-%%%%-%%%%-%%%%");
+    testHomeDir_ = tempBase / (uniquePath.string() + "_home");
+    testEtcDir_ = tempBase / (uniquePath.string() + "_etc");
+
+    // Clean up any previous test artifacts (shouldn't exist with unique names)
+    std::error_code ec;
+    fs::remove_all(testHomeDir_, ec);
+    fs::remove_all(testEtcDir_, ec);
+
+    // Create fresh test directories
+    fs::create_directories(testHomeDir_);
+    fs::create_directories(testEtcDir_);
+
+    // Set up paths
+    systemConfigPath_ = testEtcDir_ / "agent.conf";
+    sessionConfigPath_ = testHomeDir_ / ".fboss2" / "agent.conf";
+    cliConfigDir_ = testEtcDir_ / "coop" / "cli";
+
+    // Create CLI config directory
+    fs::create_directories(cliConfigDir_);
+
+    // Create a default system config
+    createTestConfig(
+        systemConfigPath_,
+        R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 2,
+        "speed": 100000
+      }
+    ]
+  }
+})");
+  }
+
+  void TearDown() override {
+    // Reset the singleton to ensure tests don't interfere with each other
+    TestableConfigSession::setInstance(nullptr);
+    // Clean up test directories (use error_code to avoid exceptions)
+    std::error_code ec;
+    fs::remove_all(testHomeDir_, ec);
+    fs::remove_all(testEtcDir_, ec);
+    CmdHandlerTestBase::TearDown();
+  }
+
+  void createTestConfig(const fs::path& path, const std::string& content) {
+    fs::create_directories(path.parent_path());
+    std::ofstream file(path);
+    file << content;
+    file.close();
+  }
+
+  std::string readFile(const fs::path& path) {
+    std::ifstream file(path);
+    std::stringstream buffer;
+    buffer << file.rdbuf();
+    return buffer.str();
+  }
+
+  void initializeTestSession() {
+    // Ensure system config exists before initializing session
+    // (ConfigSession constructor calls initializeSession which will copy it)
+    if (!fs::exists(systemConfigPath_)) {
+      createTestConfig(
+          systemConfigPath_,
+          R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 2,
+        "speed": 100000
+      }
+    ]
+  }
+})");
+    }
+
+    // Ensure the parent directory of session config exists
+    // (initializeSession will try to copy system config to session config)
+    fs::create_directories(sessionConfigPath_.parent_path());
+
+    // Initialize ConfigSession singleton with test paths
+    // The constructor will automatically call initializeSession()
+    TestableConfigSession::setInstance(
+        std::make_unique<TestableConfigSession>(
+            sessionConfigPath_.string(),
+            systemConfigPath_.string(),
+            cliConfigDir_.string()));
+  }
+};
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffNoSession) {
+  // Initialize the session (which creates the session config file)
+  initializeTestSession();
+
+  // Then delete the session config file to simulate "no session" case
+  fs::remove(sessionConfigPath_);
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList emptyRevisions(std::vector<std::string>{});
+
+  auto result = cmd.queryClient(localhost(), emptyRevisions);
+
+  EXPECT_EQ(result, "No config session exists. Make a config change first.");
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffIdenticalConfigs) {
+  initializeTestSession();
+
+  // initializeTestSession() already creates the session config by copying
+  // the system config, so we don't need to do it again
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList emptyRevisions(std::vector<std::string>{});
+
+  auto result = cmd.queryClient(localhost(), emptyRevisions);
+
+  EXPECT_NE(
+      result.find(
+          "No differences between current live config and session config"),
+      std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffDifferentConfigs) {
+  initializeTestSession();
+
+  // Create session directory and modify the config
+  fs::create_directories(sessionConfigPath_.parent_path());
+  createTestConfig(
+      sessionConfigPath_,
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "description": "Modified port",
+        "state": 2,
+        "speed": 100000
+      }
+    ]
+  }
+})");
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList emptyRevisions(std::vector<std::string>{});
+
+  auto result = cmd.queryClient(localhost(), emptyRevisions);
+
+  // Should show a diff with the added "description" field
+  EXPECT_NE(result.find("description"), std::string::npos);
+  EXPECT_NE(result.find("Modified port"), std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffSessionVsRevision) {
+  initializeTestSession();
+
+  // Create a revision file
+  createTestConfig(
+      cliConfigDir_ / "agent-r1.conf",
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 1
+      }
+    ]
+  }
+})");
+
+  // Create session with different content
+  fs::create_directories(sessionConfigPath_.parent_path());
+  createTestConfig(
+      sessionConfigPath_,
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 2
+      }
+    ]
+  }
+})");
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList revisions(std::vector<std::string>{"r1"});
+
+  auto result = cmd.queryClient(localhost(), revisions);
+
+  // Should show a diff between r1 and session (state changed from 1 to 2)
+  EXPECT_NE(result.find("-        \"state\": 1"), std::string::npos);
+  EXPECT_NE(result.find("+        \"state\": 2"), std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffTwoRevisions) {
+  initializeTestSession();
+
+  // Create two different revision files
+  createTestConfig(
+      cliConfigDir_ / "agent-r1.conf",
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 2
+      }
+    ]
+  }
+})");
+
+  createTestConfig(
+      cliConfigDir_ / "agent-r2.conf",
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "description": "Added description",
+        "state": 2
+      }
+    ]
+  }
+})");
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList revisions(std::vector<std::string>{"r1", "r2"});
+
+  auto result = cmd.queryClient(localhost(), revisions);
+
+  // Should show the added "description" field
+  EXPECT_NE(result.find("description"), std::string::npos);
+  EXPECT_NE(result.find("Added description"), std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffWithCurrentKeyword) {
+  initializeTestSession();
+
+  // Create a revision file
+  createTestConfig(
+      cliConfigDir_ / "agent-r1.conf",
+      R"({
+  "sw": {
+    "ports": [
+      {
+        "logicalID": 1,
+        "name": "eth1/1/1",
+        "state": 1
+      }
+    ]
+  }
+})");
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList revisions(std::vector<std::string>{"r1", "current"});
+
+  auto result = cmd.queryClient(localhost(), revisions);
+
+  // Should show a diff between r1 and current system config (state changed from
+  // 1 to 2, speed added)
+  EXPECT_NE(result.find("-        \"state\": 1"), std::string::npos);
+  EXPECT_NE(result.find("+        \"state\": 2"), std::string::npos);
+  EXPECT_NE(result.find("+        \"speed\": 100000"), std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffNonexistentRevision) {
+  initializeTestSession();
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList revisions(std::vector<std::string>{"r999"});
+
+  // Should throw an exception for nonexistent revision
+  EXPECT_THROW(cmd.queryClient(localhost(), revisions), std::invalid_argument);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, diffTooManyArguments) {
+  initializeTestSession();
+
+  auto cmd = CmdConfigSessionDiff();
+  utils::RevisionList revisions(std::vector<std::string>{"r1", "r2", "r3"});
+
+  // Should throw an exception for too many arguments
+  EXPECT_THROW(cmd.queryClient(localhost(), revisions), std::invalid_argument);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, printOutput) {
+  auto cmd = CmdConfigSessionDiff();
+  std::string diffOutput =
+      "--- a/config\n+++ b/config\n@@ -1 +1 @@\n-old\n+new";
+
+  // Redirect cout to capture output
+  std::stringstream buffer;
+  std::streambuf* old = std::cout.rdbuf(buffer.rdbuf());
+
+  cmd.printOutput(diffOutput);
+
+  // Restore cout
+  std::cout.rdbuf(old);
+
+  std::string output = buffer.str();
+
+  EXPECT_NE(output.find("--- a/config"), std::string::npos);
+  EXPECT_NE(output.find("+++ b/config"), std::string::npos);
+  EXPECT_NE(output.find("-old"), std::string::npos);
+  EXPECT_NE(output.find("+new"), std::string::npos);
+}
+
+TEST_F(CmdConfigSessionDiffTestFixture, printOutputNoDifferences) {
+  auto cmd = CmdConfigSessionDiff();
+  std::string noDiffMessage =
+      "No differences between current live config and session config.";
+
+  // Redirect cout to capture output
+  std::stringstream buffer;
+  std::streambuf* old = std::cout.rdbuf(buffer.rdbuf());
+
+  cmd.printOutput(noDiffMessage);
+
+  // Restore cout
+  std::cout.rdbuf(old);
+
+  std::string output = buffer.str();
+
+  EXPECT_NE(output.find("No differences"), std::string::npos);
+}
+
+} // namespace facebook::fboss
diff --git a/fboss/cli/fboss2/test/TestableConfigSession.h b/fboss/cli/fboss2/test/TestableConfigSession.h
index 4838656d3db37..1f2179dcf6c10 100644
--- a/fboss/cli/fboss2/test/TestableConfigSession.h
+++ b/fboss/cli/fboss2/test/TestableConfigSession.h
@@ -23,6 +23,9 @@ class TestableConfigSession : public ConfigSession {
       const std::string& systemConfigPath,
       const std::string& cliConfigDir)
       : ConfigSession(sessionConfigPath, systemConfigPath, cliConfigDir) {}
+
+  // Expose protected setInstance() for testing
+  using ConfigSession::setInstance;
 };
 
 } // namespace facebook::fboss
diff --git a/fboss/cli/fboss2/utils/CmdUtils.cpp b/fboss/cli/fboss2/utils/CmdUtils.cpp
index fba8013585a84..3a3b43ec16096 100644
--- a/fboss/cli/fboss2/utils/CmdUtils.cpp
+++ b/fboss/cli/fboss2/utils/CmdUtils.cpp
@@ -473,4 +473,41 @@ getUncachedSwitchReachabilityInfo(
   return reachabilityMatrix;
 }
 
+RevisionList::RevisionList(std::vector<std::string> v) {
+  // Validate each revision specifier
+  for (const auto& revision : v) {
+    if (revision == "current") {
+      // "current" is always valid
+      data_.push_back(revision);
+      continue;
+    }
+
+    // Must be in the form "rN" where N is a positive integer
+    if (revision.empty() || revision[0] != 'r') {
+      throw std::invalid_argument(
+          "Invalid revision specifier: '" + revision +
+          "'. Expected 'rN' or 'current'");
+    }
+
+    // Extract the number part after 'r'
+    std::string revNum = revision.substr(1);
+    if (revNum.empty()) {
+      throw std::invalid_argument(
+          "Invalid revision specifier: '" + revision +
+          "'. Expected 'rN' or 'current'");
+    }
+
+    // Validate that it's all digits
+    for (char c : revNum) {
+      if (!std::isdigit(c)) {
+        throw std::invalid_argument(
+            "Invalid revision number: '" + revision +
+            "'. Expected 'rN' or 'current'");
+      }
+    }
+
+    data_.push_back(revision);
+  }
+}
+
 } // namespace facebook::fboss::utils
diff --git a/fboss/cli/fboss2/utils/CmdUtils.h b/fboss/cli/fboss2/utils/CmdUtils.h
index 03ce47b283bf9..fed01fabc6855 100644
--- a/fboss/cli/fboss2/utils/CmdUtils.h
+++ b/fboss/cli/fboss2/utils/CmdUtils.h
@@ -417,6 +417,15 @@ class MirrorList : public BaseObjectArgType<std::string> {
       ObjectArgTypeId::OBJECT_ARG_TYPE_ID_MIRROR_LIST;
 };
 
+class RevisionList : public BaseObjectArgType<std::string> {
+ public:
+  /* implicit */ RevisionList() : BaseObjectArgType() {}
+  /* implicit */ RevisionList(std::vector<std::string> v);
+
+  const static ObjectArgTypeId id =
+      ObjectArgTypeId::OBJECT_ARG_TYPE_ID_REVISION_LIST;
+};
+
 // Called after CLI11 is initlized but before parsing, for any final
 // initialization steps
 void postAppInit(int argc, char* argv[], CLI::App& app);
diff --git a/fboss/cli/fboss2/utils/CmdUtilsCommon.h b/fboss/cli/fboss2/utils/CmdUtilsCommon.h
index 5987c6bd72e81..340b77dde69ee 100644
--- a/fboss/cli/fboss2/utils/CmdUtilsCommon.h
+++ b/fboss/cli/fboss2/utils/CmdUtilsCommon.h
@@ -59,6 +59,7 @@ enum class ObjectArgTypeId : uint8_t {
   OBJECT_ARG_TYPE_ID_MIRROR_LIST,
   OBJECT_ARG_TYPE_LINK_DIRECTION,
   OBJECT_ARG_TYPE_FAN_PWM,
+  OBJECT_ARG_TYPE_ID_REVISION_LIST,
 };
 
 template <typename T>