diff --git a/PROGRESS.md b/PROGRESS.md
index bac4167..6b3a635 100644
--- a/PROGRESS.md
+++ b/PROGRESS.md
@@ -382,7 +382,7 @@ vcpkg's imgui 1.91.9 removed the `sdl2-binding` feature (only `sdl3-binding` exi
 
 ## What's Next
 
-Sprint 4 complete. Step 126 (build system integration) done.
+Sprint 5 in progress. Step 129 (dependency management UI) done. Next: Step 130 (library API indexing via LSP).
 
 ---
 
@@ -472,3 +472,6 @@ Sprint 4 complete. Step 126 (build system integration) done.
 | 2026-02-09 | Codex | Step 125: Agents panel enhancements (permissions, disconnect, request logging). 2/2 tests pass. |
 | 2026-02-09 | Codex | Added integration test for Step 125 agent mutation flow; updated testing policy in ARCHITECTURE.md. 3/3 integration tests pass. |
 | 2026-02-09 | Codex | Step 126: Build system detection, build commands, and error parsing/jump. 7/7 tests pass. |
+| 2026-02-09 | Codex | Step 127: Package registry abstraction stubs for major ecosystems. 4/4 tests pass. |
+| 2026-02-09 | Codex | Step 128: Dependency file parsing with Import population. 6/6 tests pass. |
+| 2026-02-09 | Codex | Step 129: Dependency management UI panel with add/remove/update, source targeting, and writeback support for requirements.txt/package.json/Cargo.toml/go.mod/vcpkg.json. 8/8 tests pass. |
diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index b29128e..44c9bc0 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -728,6 +728,10 @@ add_executable(step128_test tests/step128_test.cpp)
 target_include_directories(step128_test PRIVATE src)
 target_link_libraries(step128_test PRIVATE nlohmann_json::nlohmann_json)
 
+add_executable(step129_test tests/step129_test.cpp)
+target_include_directories(step129_test PRIVATE src)
+target_link_libraries(step129_test PRIVATE nlohmann_json::nlohmann_json)
+
 find_package(SDL2 CONFIG REQUIRED)
 find_package(OpenGL REQUIRED)
 find_package(glad CONFIG REQUIRED)
diff --git a/editor/src/DependencyPanel.h b/editor/src/DependencyPanel.h
new file mode 100644
index 0000000..8a33e40
--- /dev/null
+++ b/editor/src/DependencyPanel.h
@@ -0,0 +1,477 @@
+#pragma once
+#include "imgui.h"
+#include "DependencyParser.h"
+#include "PackageRegistry.h"
+#include <filesystem>
+#include <fstream>
+#include <cstdio>
+#include <sstream>
+#include <vector>
+#include <string>
+#include <nlohmann/json.hpp>
+
+struct DependencyFile {
+    std::string path;
+    std::string label;
+};
+
+struct DependencyPanelState {
+    std::vector<DependencySpec> deps;
+    std::vector<DependencyFile> sources;
+    int selected = -1;
+    int sourceSelected = -1;
+    int editVersionIndex = -1;
+    char searchBuf[128] = {};
+    char versionBuf[32] = {};
+    char editVersionBuf[32] = {};
+    char addBuf[128] = {};
+    bool showAddPopup = false;
+    PackageEcosystem addEcosystem = PackageEcosystem::Python;
+    PackageInfo lastLookup;
+    std::string lastWorkspaceRoot;
+};
+
+static std::vector<std::string> dependencyFileNames() {
+    return {
+        "requirements.txt",
+        "package.json",
+        "Cargo.toml",
+        "go.mod",
+        "pom.xml",
+        "build.gradle",
+        "vcpkg.json",
+        "CMakeLists.txt",
+        "setup.py",
+        "pyproject.toml"
+    };
+}
+
+static std::string sourceBase(const std::string& source) {
+    auto pos = source.find(':');
+    if (pos == std::string::npos) return source;
+    return source.substr(0, pos);
+}
+
+static std::string sourceGroup(const std::string& source) {
+    auto pos = source.find(':');
+    if (pos == std::string::npos) return "";
+    return source.substr(pos + 1);
+}
+
+static PackageEcosystem ecosystemForSource(const std::string& source) {
+    std::string base = sourceBase(source);
+    if (base == "requirements.txt" || base == "setup.py" || base == "pyproject.toml")
+        return PackageEcosystem::Python;
+    if (base == "package.json")
+        return PackageEcosystem::Npm;
+    if (base == "Cargo.toml")
+        return PackageEcosystem::Rust;
+    if (base == "go.mod")
+        return PackageEcosystem::Go;
+    if (base == "pom.xml" || base == "build.gradle")
+        return PackageEcosystem::Java;
+    return PackageEcosystem::Cpp;
+}
+
+static const char* ecosystemLabel(PackageEcosystem eco) {
+    switch (eco) {
+        case PackageEcosystem::Python: return "Python (PyPI)";
+        case PackageEcosystem::Npm: return "JavaScript (npm)";
+        case PackageEcosystem::Rust: return "Rust (crates.io)";
+        case PackageEcosystem::Go: return "Go (pkg.go.dev)";
+        case PackageEcosystem::Java: return "Java (Maven)";
+        case PackageEcosystem::Cpp: return "C/C++ (vcpkg/Conan)";
+        default: return "Unknown";
+    }
+}
+
+static std::vector<DependencyFile> discoverDependencyFiles(const std::string& workspaceRoot) {
+    std::vector<DependencyFile> out;
+    if (workspaceRoot.empty()) return out;
+    std::filesystem::path root(workspaceRoot);
+    for (const auto& name : dependencyFileNames()) {
+        auto path = root / name;
+        if (std::filesystem::exists(path)) {
+            DependencyFile file;
+            file.path = path.string();
+            file.label = name;
+            out.push_back(file);
+        }
+    }
+    return out;
+}
+
+static std::string resolveSourcePath(const std::string& workspaceRoot, const std::string& source) {
+    std::filesystem::path root(workspaceRoot);
+    std::string base = sourceBase(source);
+    return (root / base).string();
+}
+
+static std::vector<DependencySpec> depsForSource(const std::vector<DependencySpec>& deps,
+                                                 const std::string& base) {
+    std::vector<DependencySpec> out;
+    for (const auto& dep : deps) {
+        if (sourceBase(dep.source) == base) {
+            out.push_back(dep);
+        }
+    }
+    return out;
+}
+
+static std::string formatRequirementLine(const DependencySpec& dep) {
+    if (dep.version.empty()) return dep.name;
+    if (!dep.version.empty()) {
+        char c = dep.version[0];
+        if (c == '>' || c == '<' || c == '=' || c == '~' || c == '^') {
+            return dep.name + dep.version;
+        }
+    }
+    return dep.name + "==" + dep.version;
+}
+
+static bool writeRequirementsFile(const std::string& path,
+                                  const std::vector<DependencySpec>& deps,
+                                  std::string& logOut) {
+    std::ofstream out(path);
+    if (!out.is_open()) {
+        logOut += "[deps] Failed to write " + path + "\n";
+        return false;
+    }
+    for (const auto& dep : deps) {
+        if (dep.name.empty()) continue;
+        out << formatRequirementLine(dep) << "\n";
+    }
+    logOut += "[deps] Wrote " + path + "\n";
+    return true;
+}
+
+static bool writePackageJson(const std::string& path,
+                             const std::vector<DependencySpec>& deps,
+                             std::string& logOut) {
+    nlohmann::json j;
+    if (std::filesystem::exists(path)) {
+        std::ifstream in(path);
+        if (in.is_open()) {
+            try { in >> j; } catch (...) { j = nlohmann::json::object(); }
+        }
+    }
+    if (!j.is_object()) j = nlohmann::json::object();
+
+    nlohmann::json depsObj = nlohmann::json::object();
+    nlohmann::json devObj = nlohmann::json::object();
+    for (const auto& dep : deps) {
+        if (dep.name.empty()) continue;
+        std::string group = sourceGroup(dep.source);
+        std::string version = dep.version.empty() ? "*" : dep.version;
+        if (group == "devDependencies") {
+            devObj[dep.name] = version;
+        } else {
+            depsObj[dep.name] = version;
+        }
+    }
+    j["dependencies"] = depsObj;
+    j["devDependencies"] = devObj;
+
+    std::ofstream out(path);
+    if (!out.is_open()) {
+        logOut += "[deps] Failed to write " + path + "\n";
+        return false;
+    }
+    out << j.dump(2);
+    logOut += "[deps] Wrote " + path + "\n";
+    return true;
+}
+
+static bool writeCargoToml(const std::string& path,
+                           const std::vector<DependencySpec>& deps,
+                           std::string& logOut) {
+    std::vector<std::string> prefix;
+    bool foundDeps = false;
+    if (std::filesystem::exists(path)) {
+        std::ifstream in(path);
+        std::string line;
+        while (std::getline(in, line)) {
+            if (line.rfind("[dependencies]", 0) == 0) {
+                foundDeps = true;
+                break;
+            }
+            prefix.push_back(line);
+        }
+    }
+    std::ofstream out(path);
+    if (!out.is_open()) {
+        logOut += "[deps] Failed to write " + path + "\n";
+        return false;
+    }
+    for (const auto& line : prefix) {
+        out << line << "\n";
+    }
+    if (!foundDeps) out << "\n";
+    out << "[dependencies]\n";
+    for (const auto& dep : deps) {
+        if (dep.name.empty()) continue;
+        std::string version = dep.version.empty() ? "*" : dep.version;
+        out << dep.name << " = \"" << version << "\"\n";
+    }
+    logOut += "[deps] Wrote " + path + "\n";
+    return true;
+}
+
+static bool writeGoMod(const std::string& path,
+                       const std::vector<DependencySpec>& deps,
+                       std::string& logOut) {
+    std::string moduleLine;
+    std::string goLine;
+    if (std::filesystem::exists(path)) {
+        std::ifstream in(path);
+        std::string line;
+        while (std::getline(in, line)) {
+            if (line.rfind("module ", 0) == 0) moduleLine = line;
+            if (line.rfind("go ", 0) == 0) goLine = line;
+        }
+    }
+    if (moduleLine.empty()) moduleLine = "module example.com/project";
+    if (goLine.empty()) goLine = "go 1.20";
+
+    std::ofstream out(path);
+    if (!out.is_open()) {
+        logOut += "[deps] Failed to write " + path + "\n";
+        return false;
+    }
+    out << moduleLine << "\n" << goLine << "\n\n";
+    out << "require (\n";
+    for (const auto& dep : deps) {
+        if (dep.name.empty()) continue;
+        std::string version = dep.version.empty() ? "v0.0.0" : dep.version;
+        out << "    " << dep.name << " " << version << "\n";
+    }
+    out << ")\n";
+    logOut += "[deps] Wrote " + path + "\n";
+    return true;
+}
+
+static bool writeVcpkgJson(const std::string& path,
+                           const std::vector<DependencySpec>& deps,
+                           std::string& logOut) {
+    nlohmann::json j;
+    if (std::filesystem::exists(path)) {
+        std::ifstream in(path);
+        if (in.is_open()) {
+            try { in >> j; } catch (...) { j = nlohmann::json::object(); }
+        }
+    }
+    if (!j.is_object()) j = nlohmann::json::object();
+    nlohmann::json depsArr = nlohmann::json::array();
+    for (const auto& dep : deps) {
+        if (dep.name.empty()) continue;
+        if (dep.version.empty()) {
+            depsArr.push_back(dep.name);
+        } else {
+            depsArr.push_back({{"name", dep.name}, {"version", dep.version}});
+        }
+    }
+    j["dependencies"] = depsArr;
+    std::ofstream out(path);
+    if (!out.is_open()) {
+        logOut += "[deps] Failed to write " + path + "\n";
+        return false;
+    }
+    out << j.dump(2);
+    logOut += "[deps] Wrote " + path + "\n";
+    return true;
+}
+
+static bool writeDependenciesForSource(const std::string& source,
+                                       const std::string& workspaceRoot,
+                                       const std::vector<DependencySpec>& deps,
+                                       std::string& logOut) {
+    if (workspaceRoot.empty()) {
+        logOut += "[deps] No workspace root set.\n";
+        return false;
+    }
+    std::string base = sourceBase(source);
+    std::string path = resolveSourcePath(workspaceRoot, base);
+    auto filtered = depsForSource(deps, base);
+
+    if (base == "requirements.txt") return writeRequirementsFile(path, filtered, logOut);
+    if (base == "package.json") return writePackageJson(path, filtered, logOut);
+    if (base == "Cargo.toml") return writeCargoToml(path, filtered, logOut);
+    if (base == "go.mod") return writeGoMod(path, filtered, logOut);
+    if (base == "vcpkg.json") return writeVcpkgJson(path, filtered, logOut);
+
+    logOut += "[deps] Writeback not implemented for " + base + "\n";
+    return false;
+}
+
+static void refreshDependencies(DependencyPanelState& state,
+                                const std::string& workspaceRoot,
+                                std::string& logOut) {
+    state.deps.clear();
+    state.sources = discoverDependencyFiles(workspaceRoot);
+    state.selected = -1;
+    state.editVersionIndex = -1;
+    state.lastWorkspaceRoot = workspaceRoot;
+    if (workspaceRoot.empty()) return;
+    for (const auto& file : state.sources) {
+        auto parsed = DependencyParser::parseFile(file.path);
+        state.deps.insert(state.deps.end(), parsed.begin(), parsed.end());
+    }
+    if (!state.sources.empty()) {
+        if (state.sourceSelected < 0 || state.sourceSelected >= (int)state.sources.size()) {
+            state.sourceSelected = 0;
+        }
+    } else {
+        state.sourceSelected = -1;
+    }
+    logOut += "[deps] Loaded " + std::to_string(state.deps.size()) + " dependencies.\n";
+}
+
+static void renderDependencyPanel(DependencyPanelState& state,
+                                  const std::string& workspaceRoot,
+                                  std::string& logOut) {
+    if (workspaceRoot.empty()) {
+        ImGui::TextDisabled("Open a workspace to manage dependencies.");
+        return;
+    }
+    if (state.lastWorkspaceRoot != workspaceRoot || state.deps.empty()) {
+        refreshDependencies(state, workspaceRoot, logOut);
+    }
+
+    ImGui::InputText("Search", state.searchBuf, sizeof(state.searchBuf));
+    ImGui::SameLine();
+    if (ImGui::Button("Refresh")) {
+        refreshDependencies(state, workspaceRoot, logOut);
+    }
+    ImGui::SameLine();
+    if (ImGui::Button("Add Package")) {
+        state.showAddPopup = true;
+        state.addBuf[0] = '\0';
+        state.versionBuf[0] = '\0';
+    }
+
+    std::string targetLabel = "(create requirements.txt)";
+    if (state.sourceSelected >= 0 && state.sourceSelected < (int)state.sources.size()) {
+        targetLabel = state.sources[state.sourceSelected].label;
+    }
+    if (ImGui::BeginCombo("Target File", targetLabel.c_str())) {
+        for (int i = 0; i < (int)state.sources.size(); ++i) {
+            bool selected = (state.sourceSelected == i);
+            if (ImGui::Selectable(state.sources[i].label.c_str(), selected)) {
+                state.sourceSelected = i;
+            }
+        }
+        if (ImGui::Selectable("(create requirements.txt)", state.sourceSelected == -1)) {
+            state.sourceSelected = -1;
+        }
+        ImGui::EndCombo();
+    }
+
+    if (state.showAddPopup) {
+        ImGui::OpenPopup("Add Dependency");
+        state.showAddPopup = false;
+    }
+    if (ImGui::BeginPopupModal("Add Dependency", nullptr, ImGuiWindowFlags_AlwaysAutoResize)) {
+        ImGui::InputText("Package", state.addBuf, sizeof(state.addBuf));
+        ImGui::InputText("Version (optional)", state.versionBuf, sizeof(state.versionBuf));
+        if (ImGui::BeginCombo("Ecosystem", ecosystemLabel(state.addEcosystem))) {
+            for (int i = 0; i < 6; ++i) {
+                auto eco = static_cast<PackageEcosystem>(i);
+                bool selected = (state.addEcosystem == eco);
+                if (ImGui::Selectable(ecosystemLabel(eco), selected)) {
+                    state.addEcosystem = eco;
+                }
+                if (selected) ImGui::SetItemDefaultFocus();
+            }
+            ImGui::EndCombo();
+        }
+        if (ImGui::Button("Search Registry")) {
+            state.lastLookup = PackageRegistry::query(state.addEcosystem, state.addBuf);
+            logOut += "[deps] Registry: " + state.lastLookup.name + " - " +
+                      state.lastLookup.description + "\n";
+        }
+        if (!state.lastLookup.name.empty()) {
+            ImGui::TextWrapped("Found: %s", state.lastLookup.name.c_str());
+        }
+        ImGui::Separator();
+        if (ImGui::Button("Add")) {
+            if (state.addBuf[0] == '\0') {
+                logOut += "[deps] Add aborted: empty package name.\n";
+            } else {
+                DependencySpec dep;
+                dep.name = state.addBuf;
+                dep.version = state.versionBuf;
+                if (state.sourceSelected >= 0 && state.sourceSelected < (int)state.sources.size()) {
+                    dep.source = state.sources[state.sourceSelected].label;
+                } else {
+                    dep.source = "requirements.txt";
+                }
+                state.deps.push_back(dep);
+                if (writeDependenciesForSource(dep.source, workspaceRoot, state.deps, logOut)) {
+                    refreshDependencies(state, workspaceRoot, logOut);
+                }
+            }
+            ImGui::CloseCurrentPopup();
+        }
+        ImGui::SameLine();
+        if (ImGui::Button("Cancel")) {
+            ImGui::CloseCurrentPopup();
+        }
+        ImGui::EndPopup();
+    }
+
+    ImGui::Separator();
+    ImGui::BeginChild("##depList", ImVec2(0, 0), false);
+    for (int i = 0; i < (int)state.deps.size(); ++i) {
+        const auto& dep = state.deps[i];
+        std::string label = dep.name;
+        if (!dep.version.empty()) label += " (" + dep.version + ")";
+        if (!dep.source.empty()) label += " [" + dep.source + "]";
+        if (state.searchBuf[0] != '\0') {
+            std::string q = state.searchBuf;
+            if (label.find(q) == std::string::npos) continue;
+        }
+        if (ImGui::Selectable(label.c_str(), state.selected == i)) {
+            state.selected = i;
+        }
+    }
+    ImGui::EndChild();
+
+    if (state.selected >= 0 && state.selected < (int)state.deps.size()) {
+        auto& dep = state.deps[state.selected];
+        ImGui::Separator();
+        ImGui::Text("Selected: %s", dep.name.c_str());
+        if (state.editVersionIndex != state.selected) {
+            std::snprintf(state.editVersionBuf, sizeof(state.editVersionBuf), "%s", dep.version.c_str());
+            state.editVersionIndex = state.selected;
+        }
+        ImGui::InputText("Version", state.editVersionBuf, sizeof(state.editVersionBuf));
+        if (ImGui::Button("Apply Version")) {
+            dep.version = state.editVersionBuf;
+            if (writeDependenciesForSource(dep.source, workspaceRoot, state.deps, logOut)) {
+                refreshDependencies(state, workspaceRoot, logOut);
+            }
+        }
+        ImGui::SameLine();
+        if (ImGui::Button("Remove")) {
+            std::string src = dep.source;
+            state.deps.erase(state.deps.begin() + state.selected);
+            state.selected = -1;
+            if (writeDependenciesForSource(src, workspaceRoot, state.deps, logOut)) {
+                refreshDependencies(state, workspaceRoot, logOut);
+            }
+        }
+        ImGui::SameLine();
+        if (ImGui::Button("Update to Latest")) {
+            PackageEcosystem eco = ecosystemForSource(dep.source);
+            auto info = PackageRegistry::query(eco, dep.name);
+            if (!info.versions.empty()) {
+                dep.version = info.versions.back();
+                if (writeDependenciesForSource(dep.source, workspaceRoot, state.deps, logOut)) {
+                    refreshDependencies(state, workspaceRoot, logOut);
+                }
+            } else {
+                logOut += "[deps] No version info for " + dep.name + "\n";
+            }
+        }
+    }
+}
diff --git a/editor/src/DependencyParser.h b/editor/src/DependencyParser.h
index 0f55de8..8398be6 100644
--- a/editor/src/DependencyParser.h
+++ b/editor/src/DependencyParser.h
@@ -84,18 +84,18 @@ private:
         if (!in.is_open()) return out;
         nlohmann::json j;
         try { in >> j; } catch (...) { return out; }
-        auto collect = [&](const std::string& key) {
+        auto collect = [&](const std::string& key, const std::string& sourceTag) {
             if (!j.contains(key)) return;
             for (auto it = j[key].begin(); it != j[key].end(); ++it) {
                 DependencySpec dep;
                 dep.name = it.key();
                 dep.version = it.value().get<std::string>();
-                dep.source = "package.json";
+                dep.source = sourceTag;
                 out.push_back(dep);
             }
         };
-        collect("dependencies");
-        collect("devDependencies");
+        collect("dependencies", "package.json:dependencies");
+        collect("devDependencies", "package.json:devDependencies");
         return out;
     }
 
diff --git a/editor/src/EditorState.h b/editor/src/EditorState.h
index ee0448f..cb9fd9a 100644
--- a/editor/src/EditorState.h
+++ b/editor/src/EditorState.h
@@ -43,6 +43,7 @@
 #include "TerminalPanel.h"
 #include "WebSocketServer.h"
 #include "BuildSystem.h"
+#include "DependencyPanel.h"
 #include "IncrementalOptimizer.h"
 #include "ast/Serialization.h"
 #include "ast/Generator.h"
@@ -163,6 +164,8 @@ struct EditorState {
     std::vector<BuildError> buildErrors;
     std::string lastBuildOutput;
     std::string lastBuildCommand;
+    bool              showDependencyPanel = true;
+    DependencyPanelState dependencyPanel;
 
     // Custom editor widget state
     bool              showWhitespace = false;
diff --git a/editor/src/main.cpp b/editor/src/main.cpp
index b0d1ed1..44149e5 100644
--- a/editor/src/main.cpp
+++ b/editor/src/main.cpp
@@ -302,6 +302,7 @@ int main(int, char**) {
                 ImGui::MenuItem("Show Outline", nullptr, &state.showOutline);
                 ImGui::MenuItem("Terminal", state.keys.getBinding("view.toggleTerminal").toString().c_str(),
                                 &state.showTerminalPanel);
+                ImGui::MenuItem("Dependencies", nullptr, &state.showDependencyPanel);
                 ImGui::MenuItem("Settings", nullptr, &state.showSettingsPanel);
                 ImGui::MenuItem("LSP Servers...", nullptr, &state.showLspSettings);
                 if (state.active()) {
@@ -550,6 +551,17 @@ int main(int, char**) {
             ImGui::End();
         }
 
+        // ---------------------------------------------------------------
+        //  Dependencies Panel
+        // ---------------------------------------------------------------
+        if (state.showDependencyPanel) {
+            ImGui::Begin("Dependencies", &state.showDependencyPanel);
+            ImGui::PushFont(uiFont);
+            renderDependencyPanel(state.dependencyPanel, state.workspaceRoot, state.outputLog);
+            ImGui::PopFont();
+            ImGui::End();
+        }
+
         // ---------------------------------------------------------------
         //  Find / Replace bar (floating at top of editor)
         // ---------------------------------------------------------------
diff --git a/editor/tests/step129_test.cpp b/editor/tests/step129_test.cpp
new file mode 100644
index 0000000..8f4a77b
--- /dev/null
+++ b/editor/tests/step129_test.cpp
@@ -0,0 +1,104 @@
+// Step 129 TDD Test: Dependency management UI writeback
+#include "DependencyPanel.h"
+#include <iostream>
+#include <filesystem>
+#include <fstream>
+#include <sstream>
+#include <nlohmann/json.hpp>
+
+static void expect(bool cond, const std::string& name, int& passed, int& failed) {
+    if (cond) {
+        std::cout << "Test " << (passed + failed + 1) << " PASS: " << name << "\n";
+        ++passed;
+    } else {
+        std::cout << "Test " << (passed + failed + 1) << " FAIL: " << name << "\n";
+        ++failed;
+    }
+}
+
+static std::string readFile(const std::filesystem::path& path) {
+    std::ifstream in(path);
+    std::ostringstream ss;
+    ss << in.rdbuf();
+    return ss.str();
+}
+
+int main() {
+    int passed = 0;
+    int failed = 0;
+
+    std::filesystem::path dir = std::filesystem::current_path() / "dep_panel_tmp";
+    std::filesystem::remove_all(dir);
+    std::filesystem::create_directories(dir);
+
+    // requirements.txt writeback
+    {
+        std::ofstream out(dir / "requirements.txt");
+        out << "numpy==1.26.0\n";
+        out.close();
+
+        DependencyPanelState panel;
+        std::string log;
+        refreshDependencies(panel, dir.string(), log);
+        expect(panel.deps.size() == 1, "requirements parsed", passed, failed);
+
+        DependencySpec dep;
+        dep.name = "requests";
+        dep.version = "2.0.0";
+        dep.source = "requirements.txt";
+        panel.deps.push_back(dep);
+        bool ok = writeDependenciesForSource("requirements.txt", dir.string(), panel.deps, log);
+        expect(ok, "requirements writeback ok", passed, failed);
+        std::string content = readFile(dir / "requirements.txt");
+        expect(content.find("numpy==1.26.0") != std::string::npos, "requirements keep numpy", passed, failed);
+        expect(content.find("requests==2.0.0") != std::string::npos, "requirements add requests", passed, failed);
+    }
+
+    // package.json writeback with devDependencies
+    {
+        std::ofstream out(dir / "package.json");
+        out << "{ \"dependencies\": { \"react\": \"^18.2.0\" }, "
+               "\"devDependencies\": { \"vite\": \"^5.0.0\" } }";
+        out.close();
+
+        DependencyPanelState panel;
+        std::string log;
+        refreshDependencies(panel, dir.string(), log);
+
+        for (auto it = panel.deps.begin(); it != panel.deps.end(); ) {
+            if (it->name == "react") {
+                it->version = "^19.0.0";
+                ++it;
+            } else if (it->name == "vite") {
+                it = panel.deps.erase(it);
+            } else {
+                ++it;
+            }
+        }
+        bool ok = writeDependenciesForSource("package.json", dir.string(), panel.deps, log);
+        expect(ok, "package.json writeback ok", passed, failed);
+
+        nlohmann::json j;
+        {
+            std::ifstream in(dir / "package.json");
+            in >> j;
+        }
+        expect(j["dependencies"]["react"] == "^19.0.0", "package.json updates dependency", passed, failed);
+        bool hasVite = j.contains("devDependencies") && j["devDependencies"].contains("vite");
+        expect(!hasVite, "package.json removes devDependency", passed, failed);
+    }
+
+    // edge case: unsupported writeback
+    {
+        std::string log;
+        std::vector<DependencySpec> deps;
+        deps.push_back({"Boost", "", "CMakeLists.txt"});
+        bool ok = writeDependenciesForSource("CMakeLists.txt", dir.string(), deps, log);
+        expect(!ok, "unsupported writeback returns false", passed, failed);
+    }
+
+    std::filesystem::remove_all(dir);
+
+    std::cout << "\n=== Step 129 Results: " << passed << " passed, " << failed << " failed ===\n";
+    return failed == 0 ? 0 : 1;
+}
diff --git a/sprint5_plan.md b/sprint5_plan.md
index 37f035e..b70da1a 100644
--- a/sprint5_plan.md
+++ b/sprint5_plan.md
@@ -47,7 +47,7 @@ The foundation: import libraries, parse their APIs, index their symbols.
   Populate `Import` AST nodes from parsed dependencies.
   *New:* `DependencyParser.h`, modifies `ast/ASTNode.h` (Import concept)
 
-- [ ] **Step 129: Dependency management UI**
+- [x] **Step 129: Dependency management UI**
   New dockable panel: "Dependencies". Lists all imported packages with version.
   "Add Package" button → search dialog (queries PackageRegistry). Version
   selector dropdown. "Remove" button. "Update" shows available newer versions.