diff --git a/Buffer.cc b/Buffer.cc
index b2a0899..bceb1da 100644
--- a/Buffer.cc
+++ b/Buffer.cc
@@ -6,6 +6,9 @@
 #include "Buffer.h"
 #include "UndoSystem.h"
 #include "UndoTree.h"
+// For reconstructing highlighter state on copies
+#include "HighlighterRegistry.h"
+#include "NullHighlighter.h"
 
 
 Buffer::Buffer()
@@ -40,9 +43,32 @@ Buffer::Buffer(const Buffer &other)
 	mark_set_       = other.mark_set_;
 	mark_curx_      = other.mark_curx_;
 	mark_cury_      = other.mark_cury_;
+	// Copy syntax/highlighting flags
+	version_        = other.version_;
+	syntax_enabled_ = other.syntax_enabled_;
+	filetype_       = other.filetype_;
 	// Fresh undo system for the copy
 	undo_tree_ = std::make_unique<UndoTree>();
 	undo_sys_  = std::make_unique<UndoSystem>(*this, *undo_tree_);
+
+	// Recreate a highlighter engine for this copy based on filetype/syntax state
+	if (syntax_enabled_) {
+		// Allocate engine and install an appropriate highlighter
+		highlighter_ = std::make_unique<kte::HighlighterEngine>();
+		if (!filetype_.empty()) {
+			auto hl = kte::HighlighterRegistry::CreateFor(filetype_);
+			if (hl) {
+				highlighter_->SetHighlighter(std::move(hl));
+			} else {
+				// Unsupported filetype -> NullHighlighter keeps syntax pipeline active
+				highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+			}
+		} else {
+			// No filetype -> keep syntax enabled but use NullHighlighter
+			highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+		}
+		// Fresh engine has empty caches; nothing to invalidate
+	}
 }
 
 
@@ -65,9 +91,28 @@ Buffer::operator=(const Buffer &other)
 	mark_set_       = other.mark_set_;
 	mark_curx_      = other.mark_curx_;
 	mark_cury_      = other.mark_cury_;
+	version_        = other.version_;
+	syntax_enabled_ = other.syntax_enabled_;
+	filetype_       = other.filetype_;
 	// Recreate undo system for this instance
 	undo_tree_ = std::make_unique<UndoTree>();
 	undo_sys_  = std::make_unique<UndoSystem>(*this, *undo_tree_);
+
+	// Recreate highlighter engine consistent with syntax settings
+	highlighter_.reset();
+	if (syntax_enabled_) {
+		highlighter_ = std::make_unique<kte::HighlighterEngine>();
+		if (!filetype_.empty()) {
+			auto hl = kte::HighlighterRegistry::CreateFor(filetype_);
+			if (hl) {
+				highlighter_->SetHighlighter(std::move(hl));
+			} else {
+				highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+			}
+		} else {
+			highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+		}
+	}
 	return *this;
 }
 
@@ -91,6 +136,11 @@ Buffer::Buffer(Buffer &&other) noexcept
 	  undo_tree_(std::move(other.undo_tree_)),
 	  undo_sys_(std::move(other.undo_sys_))
 {
+	// Move syntax/highlighting state
+	version_        = other.version_;
+	syntax_enabled_ = other.syntax_enabled_;
+	filetype_       = std::move(other.filetype_);
+	highlighter_    = std::move(other.highlighter_);
 	// Update UndoSystem's buffer reference to point to this object
 	if (undo_sys_) {
 		undo_sys_->UpdateBufferReference(*this);
@@ -122,6 +172,12 @@ Buffer::operator=(Buffer &&other) noexcept
 	undo_tree_      = std::move(other.undo_tree_);
 	undo_sys_       = std::move(other.undo_sys_);
 
+	// Move syntax/highlighting state
+	version_        = other.version_;
+	syntax_enabled_ = other.syntax_enabled_;
+	filetype_       = std::move(other.filetype_);
+	highlighter_    = std::move(other.highlighter_);
+
 	// Update UndoSystem's buffer reference to point to this object
 	if (undo_sys_) {
 		undo_sys_->UpdateBufferReference(*this);
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a02d475..32d0bf2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -13,6 +13,7 @@ set(BUILD_TESTS OFF CACHE BOOL "Enable building test programs.")
 option(KTE_USE_PIECE_TABLE "Use PieceTable instead of GapBuffer implementation" ON)
 set(KTE_FONT_SIZE "18.0" CACHE STRING "Default font size for GUI")
 option(KTE_UNDO_DEBUG "Enable undo instrumentation logs" OFF)
+option(KTE_ENABLE_TREESITTER "Enable optional Tree-sitter highlighter adapter" OFF)
 
 if (CMAKE_HOST_UNIX)
     message(STATUS "Build system is POSIX.")
@@ -37,6 +38,9 @@ else ()
 endif ()
 add_compile_definitions(KGE_PLATFORM=${CMAKE_HOST_SYSTEM_NAME})
 add_compile_definitions(KTE_VERSION_STR="v${KTE_VERSION}")
+if (KTE_ENABLE_TREESITTER)
+    add_compile_definitions(KTE_ENABLE_TREESITTER)
+endif ()
 
 message(STATUS "Build system: ${CMAKE_HOST_SYSTEM_NAME}")
 
@@ -80,6 +84,11 @@ set(COMMON_SOURCES
         LispHighlighter.cc
 )
 
+if (KTE_ENABLE_TREESITTER)
+    list(APPEND COMMON_SOURCES
+            TreeSitterHighlighter.cc)
+endif ()
+
 set(COMMON_HEADERS
         GapBuffer.h
         PieceTable.h
@@ -116,6 +125,11 @@ set(COMMON_HEADERS
         LispHighlighter.h
 )
 
+if (KTE_ENABLE_TREESITTER)
+    list(APPEND COMMON_HEADERS
+            TreeSitterHighlighter.h)
+endif ()
+
 # kte (terminal-first) executable
 add_executable(kte
         main.cc
@@ -132,6 +146,18 @@ endif ()
 
 target_link_libraries(kte ${CURSES_LIBRARIES})
 
+if (KTE_ENABLE_TREESITTER)
+    # Users can provide their own tree-sitter include/lib via cache variables
+    set(TREESITTER_INCLUDE_DIR "" CACHE PATH "Path to tree-sitter include directory")
+    set(TREESITTER_LIBRARY "" CACHE FILEPATH "Path to tree-sitter library (.a/.dylib)")
+    if (TREESITTER_INCLUDE_DIR)
+        target_include_directories(kte PRIVATE ${TREESITTER_INCLUDE_DIR})
+    endif ()
+    if (TREESITTER_LIBRARY)
+        target_link_libraries(kte ${TREESITTER_LIBRARY})
+    endif ()
+endif ()
+
 install(TARGETS kte
         RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
 )
@@ -157,6 +183,14 @@ if (BUILD_TESTS)
 
 
     target_link_libraries(test_undo ${CURSES_LIBRARIES})
+    if (KTE_ENABLE_TREESITTER)
+        if (TREESITTER_INCLUDE_DIR)
+            target_include_directories(test_undo PRIVATE ${TREESITTER_INCLUDE_DIR})
+        endif ()
+        if (TREESITTER_LIBRARY)
+            target_link_libraries(test_undo ${TREESITTER_LIBRARY})
+        endif ()
+    endif ()
 endif ()
 
 if (${BUILD_GUI})
diff --git a/Editor.cc b/Editor.cc
index 5695a29..1aa82f2 100644
--- a/Editor.cc
+++ b/Editor.cc
@@ -1,6 +1,8 @@
 #include <algorithm>
 #include <utility>
 #include <filesystem>
+#include "HighlighterRegistry.h"
+#include "NullHighlighter.h"
 
 #include "Editor.h"
 #include "HighlighterRegistry.h"
@@ -218,11 +220,32 @@ Editor::OpenFile(const std::string &path, std::string &err)
 bool
 Editor::SwitchTo(std::size_t index)
 {
-	if (index >= buffers_.size()) {
-		return false;
-	}
-	curbuf_ = index;
-	return true;
+    if (index >= buffers_.size()) {
+        return false;
+    }
+    curbuf_ = index;
+    // Robustness: ensure a valid highlighter is installed when switching buffers
+    Buffer &b = buffers_[curbuf_];
+    if (b.SyntaxEnabled()) {
+        b.EnsureHighlighter();
+        if (auto *eng = b.Highlighter()) {
+            if (!eng->HasHighlighter()) {
+                // Try to set based on existing filetype; fall back to NullHighlighter
+                if (!b.Filetype().empty()) {
+                    auto hl = kte::HighlighterRegistry::CreateFor(b.Filetype());
+                    if (hl) {
+                        eng->SetHighlighter(std::move(hl));
+                    } else {
+                        eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+                    }
+                } else {
+                    eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
+                }
+                eng->InvalidateFrom(0);
+            }
+        }
+    }
+    return true;
 }
 
 
diff --git a/HighlighterRegistry.cc b/HighlighterRegistry.cc
index 66141fd..b2c67a8 100644
--- a/HighlighterRegistry.cc
+++ b/HighlighterRegistry.cc
@@ -3,9 +3,22 @@
 
 #include <algorithm>
 #include <filesystem>
+#include <vector>
+#include <cctype>
 
 // Forward declare simple highlighters implemented in this project
 namespace kte {
+
+// Registration storage
+struct RegEntry {
+    std::string ft; // normalized
+    HighlighterRegistry::Factory factory;
+};
+
+static std::vector<RegEntry> &registry() {
+    static std::vector<RegEntry> reg;
+    return reg;
+}
 class JSONHighlighter; class MarkdownHighlighter; class ShellHighlighter;
 class GoHighlighter; class PythonHighlighter; class RustHighlighter; class LispHighlighter;
 }
@@ -45,6 +58,10 @@ std::string HighlighterRegistry::Normalize(std::string_view ft)
 std::unique_ptr<LanguageHighlighter> HighlighterRegistry::CreateFor(std::string_view filetype)
 {
     std::string ft = Normalize(filetype);
+    // Prefer externally registered factories
+    for (const auto &e : registry()) {
+        if (e.ft == ft && e.factory) return e.factory();
+    }
     if (ft == "cpp") return std::make_unique<CppHighlighter>();
     if (ft == "json") return std::make_unique<JSONHighlighter>();
     if (ft == "markdown") return std::make_unique<MarkdownHighlighter>();
@@ -91,3 +108,50 @@ std::string HighlighterRegistry::DetectForPath(std::string_view path, std::strin
 }
 
 } // namespace kte
+
+// Extensibility API implementations
+namespace kte {
+
+void HighlighterRegistry::Register(std::string_view filetype, Factory factory, bool override_existing)
+{
+    std::string ft = Normalize(filetype);
+    for (auto &e : registry()) {
+        if (e.ft == ft) {
+            if (override_existing) e.factory = std::move(factory);
+            return;
+        }
+    }
+    registry().push_back(RegEntry{ft, std::move(factory)});
+}
+
+bool HighlighterRegistry::IsRegistered(std::string_view filetype)
+{
+    std::string ft = Normalize(filetype);
+    for (const auto &e : registry()) if (e.ft == ft) return true;
+    return false;
+}
+
+std::vector<std::string> HighlighterRegistry::RegisteredFiletypes()
+{
+    std::vector<std::string> out;
+    out.reserve(registry().size());
+    for (const auto &e : registry()) out.push_back(e.ft);
+    return out;
+}
+
+#ifdef KTE_ENABLE_TREESITTER
+// Forward declare adapter factory
+std::unique_ptr<LanguageHighlighter> CreateTreeSitterHighlighter(const char* filetype,
+                                                                 const void* (*get_lang)());
+
+void HighlighterRegistry::RegisterTreeSitter(std::string_view filetype,
+                                             const TSLanguage* (*get_language)())
+{
+    std::string ft = Normalize(filetype);
+    Register(ft, [ft, get_language]() {
+        return CreateTreeSitterHighlighter(ft.c_str(), reinterpret_cast<const void* (*)()>(get_language));
+    }, /*override_existing=*/true);
+}
+#endif
+
+} // namespace kte
diff --git a/HighlighterRegistry.h b/HighlighterRegistry.h
index f4985bd..5eb39f7 100644
--- a/HighlighterRegistry.h
+++ b/HighlighterRegistry.h
@@ -1,10 +1,11 @@
-// HighlighterRegistry.h - create/detect language highlighters
+// HighlighterRegistry.h - create/detect language highlighters and allow external registration
 #pragma once
 
 #include <functional>
 #include <memory>
 #include <string>
 #include <string_view>
+#include <vector>
 
 #include "LanguageHighlighter.h"
 
@@ -12,6 +13,8 @@ namespace kte {
 
 class HighlighterRegistry {
 public:
+    using Factory = std::function<std::unique_ptr<LanguageHighlighter>()>;
+
     // Create a highlighter for normalized filetype id (e.g., "cpp", "json", "markdown", "shell", "go", "python", "rust", "lisp").
     static std::unique_ptr<LanguageHighlighter> CreateFor(std::string_view filetype);
 
@@ -21,6 +24,26 @@ public:
 
     // Normalize various aliases/extensions to canonical ids.
     static std::string Normalize(std::string_view ft);
+
+    // Extensibility: allow external code to register highlighters at runtime.
+    // The filetype key is normalized via Normalize(). If a factory is already registered for the
+    // normalized key and override=false, the existing factory is kept.
+    static void Register(std::string_view filetype, Factory factory, bool override_existing = true);
+
+    // Returns true if a factory is registered for the (normalized) filetype.
+    static bool IsRegistered(std::string_view filetype);
+
+    // Return a list of currently registered (normalized) filetypes. Primarily for diagnostics/tests.
+    static std::vector<std::string> RegisteredFiletypes();
+
+#ifdef KTE_ENABLE_TREESITTER
+    // Forward declaration to avoid hard dependency when disabled.
+    struct TSLanguage;
+    // Convenience: register a Tree-sitter-backed highlighter for a filetype.
+    // The getter should return a non-null language pointer for the grammar.
+    static void RegisterTreeSitter(std::string_view filetype,
+                                   const TSLanguage* (*get_language)());
+#endif
 };
 
 } // namespace kte
diff --git a/TreeSitterHighlighter.cc b/TreeSitterHighlighter.cc
new file mode 100644
index 0000000..0c63327
--- /dev/null
+++ b/TreeSitterHighlighter.cc
@@ -0,0 +1,46 @@
+#include "TreeSitterHighlighter.h"
+
+#ifdef KTE_ENABLE_TREESITTER
+
+#include "Buffer.h"
+#include <utility>
+
+namespace kte {
+
+TreeSitterHighlighter::TreeSitterHighlighter(const TSLanguage* lang, std::string filetype)
+    : language_(lang), filetype_(std::move(filetype))
+{
+}
+
+TreeSitterHighlighter::~TreeSitterHighlighter()
+{
+    disposeParser();
+}
+
+void TreeSitterHighlighter::ensureParsed(const Buffer& /*buf*/) const
+{
+    // Intentionally a stub to avoid pulling the Tree-sitter API and library by default.
+    // In future, when linking against tree-sitter, initialize parser_, set language_,
+    // and build tree_ from the buffer contents.
+}
+
+void TreeSitterHighlighter::disposeParser() const
+{
+    // Stub; nothing to dispose when not actually creating parser/tree
+}
+
+void TreeSitterHighlighter::HighlightLine(const Buffer &/*buf*/, int /*row*/, std::vector<HighlightSpan> &/*out*/) const
+{
+    // For now, no-op. When tree-sitter is wired, map nodes to TokenKind spans per line.
+}
+
+std::unique_ptr<LanguageHighlighter> CreateTreeSitterHighlighter(const char* filetype,
+                                                                 const void* (*get_lang)())
+{
+    const auto* lang = reinterpret_cast<const TSLanguage*>(get_lang ? get_lang() : nullptr);
+    return std::make_unique<TreeSitterHighlighter>(lang, filetype ? std::string(filetype) : std::string());
+}
+
+} // namespace kte
+
+#endif // KTE_ENABLE_TREESITTER
diff --git a/TreeSitterHighlighter.h b/TreeSitterHighlighter.h
new file mode 100644
index 0000000..7cd53d9
--- /dev/null
+++ b/TreeSitterHighlighter.h
@@ -0,0 +1,48 @@
+// TreeSitterHighlighter.h - optional adapter for Tree-sitter (behind KTE_ENABLE_TREESITTER)
+#pragma once
+
+#ifdef KTE_ENABLE_TREESITTER
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "LanguageHighlighter.h"
+
+// Forward-declare Tree-sitter C API to avoid hard coupling in headers if includes are not present
+extern "C" {
+struct TSLanguage;
+struct TSParser;
+struct TSTree;
+}
+
+namespace kte {
+
+// A minimal adapter that uses Tree-sitter to parse the whole buffer and then, for now,
+// does very limited token classification. This acts as a scaffold for future richer
+// queries. If no queries are provided, it currently produces no spans (safe fallback).
+class TreeSitterHighlighter : public LanguageHighlighter {
+public:
+    explicit TreeSitterHighlighter(const TSLanguage* lang, std::string filetype);
+    ~TreeSitterHighlighter() override;
+
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+
+private:
+    const TSLanguage* language_{nullptr};
+    std::string filetype_;
+    // Lazy parser to avoid startup cost; mutable to allow creation in const method
+    mutable TSParser* parser_{nullptr};
+    mutable TSTree* tree_{nullptr};
+
+    void ensureParsed(const Buffer& buf) const;
+    void disposeParser() const;
+};
+
+// Factory used by HighlighterRegistry when registering via RegisterTreeSitter.
+std::unique_ptr<LanguageHighlighter> CreateTreeSitterHighlighter(const char* filetype,
+                                                                 const void* (*get_lang)());
+
+} // namespace kte
+
+#endif // KTE_ENABLE_TREESITTER
diff --git a/docs/syntax.md b/docs/syntax.md
index 6d55ddc..bc1a98b 100644
--- a/docs/syntax.md
+++ b/docs/syntax.md
@@ -50,3 +50,21 @@ Renderer integration
 
 - Terminal and GUI renderers request line spans via `Highlighter()->GetLine(buf, row, buf.Version())`.
 - Search highlight and cursor overlays take precedence over syntax colors.
+
+Extensibility (Phase 4)
+-----------------------
+
+- Public registration API: external code can register custom highlighters by filetype.
+  - Use `HighlighterRegistry::Register("mylang", []{ return std::make_unique<MyHighlighter>(); });`
+  - Registered factories are preferred over built-ins for the same filetype key.
+  - Filetype keys are normalized via `HighlighterRegistry::Normalize()`.
+- Optional Tree-sitter adapter: disabled by default to keep dependencies minimal.
+  - Enable with CMake option `-DKTE_ENABLE_TREESITTER=ON` and provide
+    `-DTREESITTER_INCLUDE_DIR=...` and `-DTREESITTER_LIBRARY=...` if needed.
+  - Register a Tree-sitter-backed highlighter for a language (example assumes you link a grammar):
+    ```c++
+    extern "C" const TSLanguage* tree_sitter_c();
+    kte::HighlighterRegistry::RegisterTreeSitter("c", &tree_sitter_c);
+    ```
+  - Current adapter is a stub scaffold; it compiles and integrates cleanly when enabled, but
+    intentionally emits no spans until Tree-sitter node-to-token mapping is implemented.