diff --git a/src/main/library-path-config.ts b/src/main/library-path-config.ts
new file mode 100644
index 0000000..ab0ea3f
--- /dev/null
+++ b/src/main/library-path-config.ts
@@ -0,0 +1,139 @@
+import * as fs from 'fs';
+import * as path from 'path';
+
+export type LibraryPathPreparationStatus =
+    | 'explicit-override'
+    | 'configured'
+    | 'bootstrapped'
+    | 'invalid-config'
+    | 'write-failed';
+
+export interface LibraryPathPreparation {
+    status: LibraryPathPreparationStatus;
+    environmentDlcDir?: string;
+    error?: string;
+}
+
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+    return typeof value === 'object' && value !== null && !Array.isArray(value);
+}
+
+/**
+ * Normalize a configured library path and accept it only when it already
+ * names a directory.
+ */
+export function normalizeExistingLibraryDirectory(rawPath?: string): string | undefined {
+    const candidate = (rawPath || '').trim();
+    if (!candidate) return undefined;
+
+    try {
+        return fs.statSync(candidate).isDirectory() ? candidate : undefined;
+    } catch {
+        return undefined;
+    }
+}
+
+/**
+ * Apply the prepared library-path contract to the Python child environment.
+ * Starting from a copy of process.env means an invalid or stale parent value
+ * must be removed explicitly when config.json owns the path.
+ */
+export function applyLibraryPathToPythonEnvironment(
+    environment: Record<string, string>,
+    preparation: LibraryPathPreparation,
+): void {
+    if (preparation.environmentDlcDir) {
+        environment.DLC_DIR = preparation.environmentDlcDir;
+    } else {
+        delete environment.DLC_DIR;
+    }
+}
+
+/**
+ * Prepare the library path contract for the Python backend.
+ *
+ * An explicit DLC_DIR remains an administrator-owned environment override.
+ * Normal desktop launches instead keep the selected path in config.json so
+ * the backend can re-read a Settings change on the next manual scan without
+ * requiring a process restart.
+ *
+ * Failure statuses (invalid-config, write-failed) still export the resolved
+ * fallback as DLC_DIR: the backend has NO built-in default (_get_dlc_dir
+ * returns None), so removing the env var when config.json cannot own the
+ * path would strand the user with an empty library and only a console warn.
+ * Config-owned dynamic behaviour applies exactly when config.json is usable.
+ */
+export function prepareLibraryPathForPython(
+    configDir: string,
+    resolvedDlcDir: string,
+    explicitDlcDir?: string,
+): LibraryPathPreparation {
+    const override = normalizeExistingLibraryDirectory(explicitDlcDir);
+    if (override) {
+        return {
+            status: 'explicit-override',
+            environmentDlcDir: override,
+        };
+    }
+
+    const configFile = path.join(configDir, 'config.json');
+    let config: Record<string, unknown> = {};
+
+    if (fs.existsSync(configFile)) {
+        try {
+            const parsed: unknown = JSON.parse(fs.readFileSync(configFile, 'utf8'));
+            if (!isPlainObject(parsed)) {
+                return {
+                    status: 'invalid-config',
+                    environmentDlcDir: resolvedDlcDir,
+                    error: 'config.json is not a JSON object',
+                };
+            }
+            config = parsed;
+        } catch (err) {
+            return {
+                status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
+                error: err instanceof Error ? err.message : String(err),
+            };
+        }
+
+        const configured = config.dlc_dir;
+        if (typeof configured === 'string' && configured.trim()) {
+            if (normalizeExistingLibraryDirectory(configured)) {
+                return { status: 'configured' };
+            }
+            return {
+                status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
+                error: 'config.json dlc_dir is not an existing directory',
+            };
+        }
+        if (configured !== undefined && configured !== null && configured !== '') {
+            return {
+                status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
+                error: 'config.json dlc_dir is not a string',
+            };
+        }
+    }
+
+    config.dlc_dir = resolvedDlcDir;
+    const tmpFile = configFile + '.tmp';
+    try {
+        fs.mkdirSync(configDir, { recursive: true });
+        fs.writeFileSync(tmpFile, JSON.stringify(config, null, 2), 'utf8');
+        fs.renameSync(tmpFile, configFile);
+    } catch (err) {
+        try {
+            if (fs.existsSync(tmpFile)) fs.rmSync(tmpFile, { force: true });
+        } catch { /* best-effort temporary-file cleanup */ }
+        return {
+            status: 'write-failed',
+            environmentDlcDir: resolvedDlcDir,
+            error: err instanceof Error ? err.message : String(err),
+        };
+    }
+
+    return { status: 'bootstrapped' };
+}
diff --git a/src/main/python.ts b/src/main/python.ts
index 25b99fb..3733afa 100644
--- a/src/main/python.ts
+++ b/src/main/python.ts
@@ -11,6 +11,11 @@ import * as net from 'net';
 import * as os from 'os';
 import { getActiveSoundfontPath, getDesktopConfig } from './soundfont-manager';
 import { isDebugEnabled } from './debug-log';
+import {
+    applyLibraryPathToPythonEnvironment,
+    normalizeExistingLibraryDirectory,
+    prepareLibraryPathForPython,
+} from './library-path-config';
 
 let pythonProcess: ChildProcess | null = null;
 // A backend that is being *gracefully* stopped (SIGTERM sent, async SIGKILL
@@ -423,8 +428,8 @@ function getPluginsDir(): string {
     return pluginsDir;
 }
 
-function getDLCDir(): string {
-    if (process.env.DLC_DIR && fs.existsSync(process.env.DLC_DIR)) return process.env.DLC_DIR;
+function getDLCDir(explicitDlcDir = normalizeExistingLibraryDirectory(process.env.DLC_DIR)): string {
+    if (explicitDlcDir) return explicitDlcDir;
 
     // Read from shared config
     const configFile = path.join(getConfigDir(), 'config.json');
@@ -496,7 +501,8 @@ export async function startPython(): Promise<void> {
     }
     serverPort = await findPort(PREFERRED_PORT);
     const configDir = getConfigDir();
-    const dlcDir = getDLCDir();
+    const explicitDlcDir = normalizeExistingLibraryDirectory(process.env.DLC_DIR);
+    const dlcDir = getDLCDir(explicitDlcDir);
     // Ensure the resolved library folder exists before the server starts. The
     // Python side only seeds starter content (and scans) when DLC_DIR.is_dir()
     // is true, and it can't bootstrap the folder itself (the seed's mkdir runs
@@ -508,6 +514,15 @@ export async function startPython(): Promise<void> {
     } catch (err) {
         console.warn(`[python] could not create DLC dir ${dlcDir}:`, err);
     }
+    // Preserve a caller-supplied DLC_DIR as an explicit administrator override.
+    // Normal desktop launches bootstrap the initial/default path into config.json
+    // instead. The backend re-reads that file for every scan, so a path saved in
+    // Settings takes effect immediately rather than being shadowed by the
+    // startup path until the Python process restarts.
+    const libraryPath = prepareLibraryPathForPython(configDir, dlcDir, explicitDlcDir);
+    if (libraryPath.error) {
+        console.warn(`[python] could not prepare dynamic library path (${libraryPath.status}): ${libraryPath.error}`);
+    }
     const pluginsDir = getPluginsDir();
     const slopsmithPlugins = path.join(slopsmithDir, 'plugins');
 
@@ -558,7 +573,6 @@ export async function startPython(): Promise<void> {
         ...process.env as Record<string, string>,
         PYTHONPATH: pythonPathEnv,
         CONFIG_DIR: configDir,
-        DLC_DIR: dlcDir,
         SLOPSMITH_PLUGINS_DIR: pluginsDir,
         HOME: homeDir,
         XDG_CACHE_HOME: cacheBase,
@@ -572,6 +586,9 @@ export async function startPython(): Promise<void> {
             : path.join(__dirname, '..', '..', 'resources', 'bin') + path.delimiter
         ) + (process.env.PATH || ''),
     };
+    // `...process.env` may carry an empty/invalid value. Do not let it shadow
+    // config.json in the normal dynamic-settings path.
+    applyLibraryPathToPythonEnvironment(pythonEnv, libraryPath);
 
     // Debug mode: raise the Slopsmith server's log level and tee its
     // structured logs to a file. lib/logging_setup.py reads LOG_LEVEL and
diff --git a/tests/library-path-config.test.js b/tests/library-path-config.test.js
new file mode 100644
index 0000000..051df4d
--- /dev/null
+++ b/tests/library-path-config.test.js
@@ -0,0 +1,216 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { loadTs } = require('./_load-ts');
+
+const {
+    applyLibraryPathToPythonEnvironment,
+    normalizeExistingLibraryDirectory,
+    prepareLibraryPathForPython,
+} = loadTs('src/main/library-path-config.ts');
+
+function tmpConfigDir() {
+    return fs.mkdtempSync(path.join(os.tmpdir(), 'feedback-library-path-'));
+}
+
+test('normal desktop startup bootstraps the fallback into config instead of DLC_DIR', () => {
+    const configDir = tmpConfigDir();
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Music\\fee[dB]ack');
+
+    assert.deepEqual(result, { status: 'bootstrapped' });
+    assert.deepEqual(
+        JSON.parse(fs.readFileSync(path.join(configDir, 'config.json'), 'utf8')),
+        { dlc_dir: 'C:\\Music\\fee[dB]ack' },
+    );
+    assert.equal(result.environmentDlcDir, undefined);
+});
+
+test('bootstrap merges the fallback into an existing config without losing settings', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, JSON.stringify({ master_difficulty: 75 }));
+
+    const result = prepareLibraryPathForPython(configDir, 'D:\\Songs');
+
+    assert.equal(result.status, 'bootstrapped');
+    assert.deepEqual(
+        JSON.parse(fs.readFileSync(configFile, 'utf8')),
+        { master_difficulty: 75, dlc_dir: 'D:\\Songs' },
+    );
+});
+
+test('an existing saved library stays config-owned and can change between scans', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    const savedSongs = path.join(configDir, 'Saved Songs');
+    fs.mkdirSync(savedSongs);
+    fs.writeFileSync(configFile, JSON.stringify({ dlc_dir: savedSongs }));
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.deepEqual(result, { status: 'configured' });
+    assert.deepEqual(
+        JSON.parse(fs.readFileSync(configFile, 'utf8')),
+        { dlc_dir: savedSongs },
+    );
+    assert.equal(result.environmentDlcDir, undefined);
+});
+
+test('a missing saved library is reported without overwriting the intended path', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    const missingSongs = path.join(configDir, 'Disconnected Songs');
+    fs.writeFileSync(configFile, JSON.stringify({ dlc_dir: missingSongs }));
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.match(result.error, /not an existing directory/);
+    assert.deepEqual(JSON.parse(fs.readFileSync(configFile, 'utf8')), { dlc_dir: missingSongs });
+});
+
+test('a saved library file is rejected without overwriting the configured value', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    const file = path.join(configDir, 'not-a-library');
+    fs.writeFileSync(file, 'x');
+    fs.writeFileSync(configFile, JSON.stringify({ dlc_dir: file }));
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.match(result.error, /not an existing directory/);
+    assert.deepEqual(JSON.parse(fs.readFileSync(configFile, 'utf8')), { dlc_dir: file });
+});
+
+test('an explicit valid DLC_DIR remains an environment override', () => {
+    const configDir = tmpConfigDir();
+    const managedSongs = path.join(configDir, 'Managed Songs');
+    fs.mkdirSync(managedSongs);
+    const result = prepareLibraryPathForPython(
+        configDir,
+        'C:\\Default Songs',
+        ` ${managedSongs} `,
+    );
+
+    assert.deepEqual(result, {
+        status: 'explicit-override',
+        environmentDlcDir: managedSongs,
+    });
+    assert.equal(fs.existsSync(path.join(configDir, 'config.json')), false);
+});
+
+test('an explicit DLC_DIR rejects whitespace, files, and missing paths', () => {
+    const root = tmpConfigDir();
+    const file = path.join(root, 'not-a-directory');
+    fs.writeFileSync(file, 'x');
+
+    assert.equal(normalizeExistingLibraryDirectory('   '), undefined);
+    assert.equal(normalizeExistingLibraryDirectory(file), undefined);
+    assert.equal(normalizeExistingLibraryDirectory(path.join(root, 'missing')), undefined);
+});
+
+test('an explicit DLC_DIR is trimmed before directory validation', () => {
+    const root = tmpConfigDir();
+    const directory = path.join(root, 'Managed Songs');
+    fs.mkdirSync(directory);
+
+    assert.equal(normalizeExistingLibraryDirectory(`  ${directory}  `), directory);
+});
+
+test('a corrupt config is never overwritten during bootstrap', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, '{broken');
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.match(result.error, /JSON/);
+    assert.equal(fs.readFileSync(configFile, 'utf8'), '{broken');
+});
+
+test('invalid-config exports the resolved fallback so the backend keeps a library', () => {
+    // The backend has no built-in default (_get_dlc_dir returns None): with
+    // the env var deleted AND config.json unusable, scans and starter
+    // seeding would silently skip. Failure statuses must keep the pre-#117
+    // env fallback.
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, JSON.stringify({ dlc_dir: path.join(configDir, 'gone') }));
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+
+    const environment = {};
+    applyLibraryPathToPythonEnvironment(environment, result);
+    assert.deepEqual(environment, { DLC_DIR: 'C:\\Default Songs' });
+});
+
+test('corrupt config exports the resolved fallback without being overwritten', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, '{broken');
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+    assert.equal(fs.readFileSync(configFile, 'utf8'), '{broken');
+});
+
+test('write-failed exports the resolved fallback so the backend keeps a library', () => {
+    // Force the config write to fail by making config.json's parent a file.
+    const root = tmpConfigDir();
+    const configDir = path.join(root, 'not-a-dir');
+    fs.writeFileSync(configDir, 'x');
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'write-failed');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+});
+
+test('Python environment omits DLC_DIR when config owns the library path', () => {
+    const environment = { DLC_DIR: 'C:\\Stale Parent Value', KEEP: 'yes' };
+
+    applyLibraryPathToPythonEnvironment(environment, { status: 'configured' });
+
+    assert.deepEqual(environment, { KEEP: 'yes' });
+});
+
+test('Python environment exports only a validated explicit DLC_DIR', () => {
+    const configDir = tmpConfigDir();
+    const managedSongs = path.join(configDir, 'Managed Songs');
+    fs.mkdirSync(managedSongs);
+    const preparation = prepareLibraryPathForPython(
+        configDir,
+        'C:\\Default Songs',
+        ` ${managedSongs} `,
+    );
+    const environment = { DLC_DIR: 'C:\\Stale Parent Value', KEEP: 'yes' };
+
+    applyLibraryPathToPythonEnvironment(environment, preparation);
+
+    assert.deepEqual(environment, { DLC_DIR: managedSongs, KEEP: 'yes' });
+});
+
+test('Python environment removes an invalid explicit DLC_DIR', () => {
+    const configDir = tmpConfigDir();
+    const fallback = path.join(configDir, 'Fallback Songs');
+    fs.mkdirSync(fallback);
+    const file = path.join(configDir, 'not-a-directory');
+    fs.writeFileSync(file, 'x');
+    const preparation = prepareLibraryPathForPython(configDir, fallback, file);
+    const environment = { DLC_DIR: file };
+
+    applyLibraryPathToPythonEnvironment(environment, preparation);
+
+    assert.deepEqual(environment, {});
+});