diff --git a/CLAUDE.md b/CLAUDE.md
index 83a0bc1..5764d09 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -73,6 +73,7 @@ Worker messages:
 - **Visualizer modes:** in `box` mode every call is a `Frame` box at its call site. In `substitution` mode (the default; prop `visualizerMode`, embed `?visualizer=`), a call with `shortForm` renders as `Frame` scope `'inline'`: its return expression with parameters substituted, in parentheses in operator context.
 - **Layout and scrolling:** nested boxes show their variables below their code (CSS). The scroll effect follows the active box horizontally. It sets `scrollLeft`/`scrollTop` on the visualizer view only; don't use `scrollIntoView`, which also scrolls the LiaScript page around the embed and moves the step buttons.
 - **Console and Stop:** output, REPL inputs, Tests reports and notes form one transcript (`ConsoleEntry` list after `output`). While Python is busy for more than `STOP_BUTTON_DELAY`, the run button is **Stop**: `stopPython` terminates the worker and calls `startWorker` again, so the REPL namespace is gone after a stop. The embed reports this to its parent (protocol version 4, `glpython:busy`/`glpython:stop`).
+- **One Monaco model for all files:** a tab switch replaces the editor's text. `updateCode` ignores flush events (`setValue`), since `@monaco-editor/react` shows a read-only file (`ProjectFile.readOnly`) with `setValue` before its `onChange` knows the new tab - the text would land in the previous file.
 - **UI strings** come in via the `t: CoreStrings` prop. `src/i18n.ts` is their single source: `coreStrings` holds German and English, and `resolveUiLocale` picks the language. The embed uses them directly; the glpython IDE imports them and adds only its shell strings. New strings have been added as *optional* `CoreStrings` keys with an English fallback, so older hosts keep compiling; add both languages to `coreStrings`. Python values, reprs and error messages are never translated.
 - **Two layouts:** with `compact` (the embed) the original tree (`main-split` > `editor-column` + graphics aside), unchanged and styled by `src/embed/style.css`; without it (the IDE) the stacked layout (`workspace-stack`: `stack-top` with the editor panel and, while drawing, the graphics, then `stack-divider` and `stack-console`). Both are assembled from the same JSX pieces (`actionsNode`, `visualizerBarNode`, `editorWrapNode`, `consoleNode`, …) just before the return. The buttons can be portaled into the host's bar (`actionsContainer`).
 - **Embedded/reduced layouts:** the knobs are `compact`, `capabilities` (`WorkspaceCapabilities`, including `repl`) and `command` (`WorkspaceCommand`, an immutable `{id, type}` used by hosts to trigger run/visualize/test/stop/reset without the workspace's own buttons; `reset` restarts Python and clears the console). Defaults must preserve the full-workspace behavior.
diff --git a/README.md b/README.md
index 2947880..0058c26 100644
--- a/README.md
+++ b/README.md
@@ -255,6 +255,13 @@ the embed's Reset does. The run button keeps its width when it becomes
 the editor. `capabilities={{ closeFileTabs: false }}` drops the ✕ on the file
 tabs, for a host that shows every file of the project as a tab.
 
+A file with `readOnly: true` (`ProjectFile`) is one to read, not to change,
+e.g. a course's library that learners use as a black box: the editor shows it
+read-only (on an iPad the keyboard doesn't open), its tab has a lock
+(`readOnlyFile`, also the message when typing into it), and
+`updateProjectFile`, `renameProjectFile` and `deleteProjectFile` leave it
+as it is. It runs, imports and visualizes like any other file.
+
 ### Compact embeds
 
 `GlPythonWorkspace` accepts optional `compact` and `capabilities` props. The
diff --git a/scripts/check-embed-protocol.mjs b/scripts/check-embed-protocol.mjs
index 26993df..48b28d1 100644
--- a/scripts/check-embed-protocol.mjs
+++ b/scripts/check-embed-protocol.mjs
@@ -5,6 +5,9 @@ import {
   GLPYTHON_MAX_SOURCE_LENGTH,
   parseParentMessage,
   parseParentOrigin,
+  deleteProjectFile,
+  renameProjectFile,
+  updateProjectFile,
 } from '../dist/index.js'
 
 assert.equal(parseParentOrigin('https://course.example'), 'https://course.example')
@@ -131,4 +134,21 @@ const readyMessage = {
 }
 assert.equal(readyMessage.capabilities.input, false)
 
+// Not the protocol, but the one check against the built library: a
+// read-only file (a course's library) is never changed, renamed or deleted by
+// the project helpers; an ordinary file still is.
+const project = {
+  id: 'p', name: 'p', mainFile: 'auswerter.py',
+  files: {
+    'parser.py': { path: 'parser.py', label: 'parser', kind: 'python', code: 'def parse(text): ...', readOnly: true },
+    'auswerter.py': { path: 'auswerter.py', label: 'auswerter', kind: 'python', code: 'from parser import parse' },
+  },
+}
+assert.equal(updateProjectFile(project, 'parser.py', 'kaputt'), project)
+assert.equal(renameProjectFile(project, 'parser.py', 'p2.py'), project)
+assert.equal(deleteProjectFile(project, 'parser.py'), project)
+assert.equal(updateProjectFile(project, 'auswerter.py', 'x = 1').files['auswerter.py'].code, 'x = 1')
+assert.equal(updateProjectFile(project, 'auswerter.py', 'x = 1').files['parser.py'].readOnly, true)
+assert.deepEqual(Object.keys(renameProjectFile(project, 'auswerter.py', 'main.py').files), ['parser.py', 'main.py'])
+
 console.log('Embed protocol checks passed.')
diff --git a/src/GlPythonWorkspace.tsx b/src/GlPythonWorkspace.tsx
index 7ed0515..8a1f6a0 100644
--- a/src/GlPythonWorkspace.tsx
+++ b/src/GlPythonWorkspace.tsx
@@ -2,7 +2,7 @@ import { createPortal } from 'react-dom'
 import { Fragment, useEffect, useLayoutEffect, useMemo, useRef, useState, type CSSProperties, type KeyboardEvent, type PointerEvent as ReactPointerEvent, type ReactNode } from 'react'
 import Editor, { useMonaco } from '@monaco-editor/react'
 import type { Monaco, OnMount } from '@monaco-editor/react'
-import { AlertTriangle, BookOpen, Copy, Square, FileCode2, FlaskConical, PanelRightClose, PanelRightOpen, ChevronLeft, ChevronRight, Ellipsis, Play, RotateCcw, SkipBack, SkipForward, Sparkles, SquareTerminal, Terminal, Workflow, X } from 'lucide-react'
+import { AlertTriangle, BookOpen, Copy, Square, FileCode2, FlaskConical, Lock, PanelRightClose, PanelRightOpen, ChevronLeft, ChevronRight, Ellipsis, Play, RotateCcw, SkipBack, SkipForward, Sparkles, SquareTerminal, Terminal, Workflow, X } from 'lucide-react'
 import { projectImportsGturtle, type Project } from './project'
 import { GraphicsWindow, type TurtleCommand } from './GraphicsWindow'
 import { buildAnnotationsUpTo, buildEvaluationChain, buildExecutedLinesUpTo, buildLocalsUpTo, buildSubstitutionsUpTo, collapseDeepCallChain, hasEvaluationChain, hiddenRangesWithin, removedLines, computeLineOffsets, computeOpenFramesUpTo, indexAstNodes, lineNumberAtOffset, locateNodeRenderedRange, nodeRange, renderFrameSource, substitutionModeSteps, type AstNode, type CollapsedCalls, type OpenFrame, type TraceResult, type TraceStep, type VisualizerMode } from './visualizer'
@@ -59,6 +59,9 @@ type CoreStrings = {
   // visualizerCollapsedCalls.
   consoleSize?: string
   editorPlaceholder?: string
+  // A read-only file (ProjectFile.readOnly): the lock's label on its tab, and
+  // the message when typing into it. Optional, like editorPlaceholder.
+  readOnlyFile?: string
   // Accessible name of the REPL's input line, and the note shown after a
   // "def"/"class" typed into the REPL. Optional like visualizerCollapsedCalls.
   replInputLabel?: string
@@ -1151,7 +1154,11 @@ function GlPythonWorkspace(props: GlPythonWorkspaceProps) {
     setInputValue('')
   }
 
-  const updateCode = (code: string | undefined) => {
+  // Only the learner's own edits. A "flush" is Monaco's setValue, which
+  // @monaco-editor/react uses to show a read-only file - possibly before its
+  // onChange knows the new tab, so the text would land in the previous file.
+  const updateCode = (code: string | undefined, event?: { isFlush: boolean }) => {
+    if (event?.isFlush) return
     if (coverageMarks?.path === activeFile) setCoverageMarks(null)
     onFileCodeChange(activeFile, code ?? '')
   }
@@ -2080,8 +2087,8 @@ function GlPythonWorkspace(props: GlPythonWorkspaceProps) {
   const breadcrumbsNode = <div className="breadcrumbs"><span>{project.name}</span><span>/</span><strong>{selectedFile.path}</strong></div>
   const visualizerBarNode = <div className="visualizer-bar">{visualTraceSteps.length > 0 ? <><div className="visualizer-controls"><button type="button" className="icon-button small" aria-label={t.visualizerFirstStep ?? t.visualizerRestart} title={t.visualizerFirstStep ?? t.visualizerRestart} onClick={visualFirstStep} disabled={clampedStepIndex === -1}><SkipBack size={15} /></button><button type="button" className="icon-button small" aria-label={t.visualizerPrevStep} title={t.visualizerPrevStep} onClick={visualStepBack} disabled={clampedStepIndex === -1}><ChevronLeft size={15} /></button><button type="button" className="icon-button small" aria-label={t.visualizerNextStep} title={t.visualizerNextStep} onClick={visualStepForward} disabled={clampedStepIndex >= visualTraceSteps.length - 1}><ChevronRight size={15} /></button><button type="button" className="icon-button small" aria-label={t.visualizerLastStep ?? 'Last step'} title={t.visualizerLastStep ?? 'Last step'} onClick={visualLastStep} disabled={clampedStepIndex >= visualTraceSteps.length - 1}><SkipForward size={15} /></button></div><div className="visualizer-scrubber-track"><input className="visualizer-scrubber" type="range" min={-1} max={Math.max(0, visualTraceSteps.length - 1)} value={clampedStepIndex} onChange={(event) => { setVisualStepIndex(Number(event.target.value)) }} />{errorTraceStep && <span className="visualizer-scrubber-error" title={errorTraceStep.label} style={{ '--marker-position': (visualTraceSteps.indexOf(errorTraceStep) + 1) / Math.max(1, visualTraceSteps.length) } as CSSProperties} />}</div><span className="visualizer-step-count">{clampedStepIndex === -1 ? t.visualizerBeforeStart : t.visualizerStepOf(clampedStepIndex + 1, visualTraceSteps.length)}</span>{visualTrace?.truncated && <span className="visualizer-truncated-hint"><AlertTriangle size={13} />{t.visualizerTruncated}</span>}</> : <span className="visualizer-step-count">{t.visualizerNoSteps}</span>}<div className="visualizer-mode" role="group" aria-label={t.visualizerModeLabel ?? 'Display'}><button type="button" aria-pressed={!substitutionMode} onClick={() => changeVisualizerMode('box')}>{t.visualizerModeBox ?? 'Box'}</button><button type="button" aria-pressed={substitutionMode} onClick={() => changeVisualizerMode('substitution')}>{t.visualizerModeSubstitution ?? 'Substitution'}</button></div><OptionsMenu label={t.visualizerOptions ?? 'Options'}><button type="button" role="menuitemcheckbox" aria-checked={animationsEnabled} className="visualizer-options-item" onClick={() => setAnimationsEnabled((enabled) => !enabled)}><span className="visualizer-options-check" aria-hidden="true" />{t.visualizerAnimate}</button>{workspaceCapabilities.export && <><button type="button" role="menuitemcheckbox" aria-checked={chainComments} className="visualizer-options-item" onClick={() => changeChainComments(!chainComments)}><span className="visualizer-options-check" aria-hidden="true" />{t.copyChainComments ?? 'Comments'}</button><button type="button" role="menuitem" className="visualizer-options-item visualizer-chain-copy" onClick={copyEvaluationChain} disabled={!canCopyChain}><Copy size={14} aria-hidden="true" />{chainFeedback === 'copied' ? (t.copyChainDone ?? 'Copied') : chainFeedback === 'failed' ? (t.copyChainFailed ?? 'Could not copy') : (t.copyChain ?? 'Copy \u2192 chain')}</button></>}</OptionsMenu>{workspaceCapabilities.exitVisualizer && <button type="button" className="icon-button small visualizer-exit" aria-label={t.exitVisualizer} title={t.exitVisualizer} onClick={exitVisualizer}><X size={16} /></button>}</div>
   const visualizerWarningNode = <>{isVisualizing && visualTrace?.error && !errorTraceStep && <div className="visualizer-warning"><AlertTriangle size={14} />{t.visualizerError(visualTrace.error)}</div>}</>
-  const editorTabsNode = <div className="editor-tabs">{openFiles.map((path) => { const file = project.files[path]; return <button className={`editor-tab ${activeFile === path ? 'active' : ''}`} key={path} onClick={() => onOpenFile(path)}><span className={`tab-file-icon ${file.kind === 'python' ? 'python' : 'notes'}`}>{file.kind === 'python' ? <FileCode2 size={14} /> : <BookOpen size={14} />}</span><span>{file.path}</span>{workspaceCapabilities.closeFileTabs && <span className="tab-close" role="button" aria-label={t.closeFile(file.path)} onClick={(event) => { event.stopPropagation(); onCloseFile(path) }}><X size={13} /></span>}</button> })}</div>
-  const editorWrapNode = <div className="editor-wrap">{isVisualizing ? <div className={`visualizer-view${animationsEnabled ? ' visualizer-animated' : ''}`} ref={visualizerScrollRef}><div className="visualizer-canvas" ref={visualizerCanvasRef}><Frame frameId={0} path={visualRootPath ?? activeFile} scope={{ kind: 'module' }} openFrames={framesForRender} nodeIndexByFile={astIndexByFile} visualSources={visualSources} trace={visualTrace?.trace ?? []} uptoStep={currentTraceStep?.step ?? -1} currentStep={currentTraceStep} nextStep={nextTraceStep} monaco={monaco} valueKindLabels={valueKindLabels} hiddenRanges={visualTrace?.hidden ?? {}} declareTarget={nextStepDeclareTarget} pendingCallLocals={pendingCallLocals} errorStep={isAtErrorStep ? errorTraceStep : null} collapsedCalls={collapsedCalls} collapsedCallsLabel={collapsedCallsLabel} substitutionMode={substitutionMode} boxHint={t.visualizerBoxHint ?? 'The box shows which return is executed.'} insideBox={false} activeHost={activeHost} errorHost={errorHost} finalNote={finalNote} errorHint={isAtErrorStep && visualTrace?.errorHint && t.errorHint?.(visualTrace.errorHint, 'other') ? <ErrorHintNote hint={visualTrace.errorHint} t={t} /> : null} />{animationsEnabled && closingBoxes.length > 0 && <div className="visualizer-closing-boxes">{closingBoxes.map((box) => <div key={box.id} className="visual-closing-box-ghost" style={{ left: box.left, top: box.top, width: box.width, height: box.height }} dangerouslySetInnerHTML={{ __html: box.html }} />)}</div>}{(connectorPaths.length > 0 || flights.length > 0) && <svg className="visualizer-connectors"><defs><marker id="visualizer-arrowhead" viewBox="0 0 10 10" refX="8" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path d="M 0 0 L 10 5 L 0 10 z" /></marker></defs>{connectorPaths.map((connector, index) => <path key={index} d={connector.d} markerEnd="url(#visualizer-arrowhead)" />)}{animationsEnabled && flights.map((flight) => <path key={`flight-${flight.id}`} className="visual-flight-arrow" d={flight.d} markerEnd="url(#visualizer-arrowhead)" />)}</svg>}{animationsEnabled && flights.length > 0 && <div className="visualizer-flights">{flights.map((flight) => <span key={flight.id} className="visual-flight-value" style={{ offsetPath: `path('${flight.d}')` }}>{flight.value}</span>)}</div>}</div></div> : <Editor key="normal-editor" height="100%" language={selectedFile.kind === 'python' ? GLPYTHON_PYTHON : 'markdown'} theme={GLPYTHON_THEME} beforeMount={(instance) => { registerGlpythonPython(instance); defineGlpythonTheme(instance) }} value={selectedFile.code} onChange={updateCode} onMount={setNormalEditor} options={{ minimap: { enabled: false }, bracketPairColorization: { enabled: false }, fontSize: 15, lineHeight: 24, padding: { top: 22 }, fontFamily: 'ui-monospace, SFMono-Regular, Menlo, Consolas, monospace', scrollBeyondLastLine: false, smoothScrolling: true, automaticLayout: true, tabSize: 4, insertSpaces: true, detectIndentation: false, placeholder: t.editorPlaceholder ?? 'Type code here' }} />}</div>
+  const editorTabsNode = <div className="editor-tabs">{openFiles.map((path) => { const file = project.files[path]; return <button className={`editor-tab ${activeFile === path ? 'active' : ''}`} key={path} onClick={() => onOpenFile(path)}><span className={`tab-file-icon ${file.kind === 'python' ? 'python' : 'notes'}`}>{file.kind === 'python' ? <FileCode2 size={14} /> : <BookOpen size={14} />}</span><span>{file.path}</span>{file.readOnly && <span className="tab-read-only" role="img" aria-label={t.readOnlyFile ?? 'Read-only: this file can be read but not changed.'} title={t.readOnlyFile ?? 'Read-only: this file can be read but not changed.'}><Lock size={12} /></span>}{workspaceCapabilities.closeFileTabs && <span className="tab-close" role="button" aria-label={t.closeFile(file.path)} onClick={(event) => { event.stopPropagation(); onCloseFile(path) }}><X size={13} /></span>}</button> })}</div>
+  const editorWrapNode = <div className="editor-wrap">{isVisualizing ? <div className={`visualizer-view${animationsEnabled ? ' visualizer-animated' : ''}`} ref={visualizerScrollRef}><div className="visualizer-canvas" ref={visualizerCanvasRef}><Frame frameId={0} path={visualRootPath ?? activeFile} scope={{ kind: 'module' }} openFrames={framesForRender} nodeIndexByFile={astIndexByFile} visualSources={visualSources} trace={visualTrace?.trace ?? []} uptoStep={currentTraceStep?.step ?? -1} currentStep={currentTraceStep} nextStep={nextTraceStep} monaco={monaco} valueKindLabels={valueKindLabels} hiddenRanges={visualTrace?.hidden ?? {}} declareTarget={nextStepDeclareTarget} pendingCallLocals={pendingCallLocals} errorStep={isAtErrorStep ? errorTraceStep : null} collapsedCalls={collapsedCalls} collapsedCallsLabel={collapsedCallsLabel} substitutionMode={substitutionMode} boxHint={t.visualizerBoxHint ?? 'The box shows which return is executed.'} insideBox={false} activeHost={activeHost} errorHost={errorHost} finalNote={finalNote} errorHint={isAtErrorStep && visualTrace?.errorHint && t.errorHint?.(visualTrace.errorHint, 'other') ? <ErrorHintNote hint={visualTrace.errorHint} t={t} /> : null} />{animationsEnabled && closingBoxes.length > 0 && <div className="visualizer-closing-boxes">{closingBoxes.map((box) => <div key={box.id} className="visual-closing-box-ghost" style={{ left: box.left, top: box.top, width: box.width, height: box.height }} dangerouslySetInnerHTML={{ __html: box.html }} />)}</div>}{(connectorPaths.length > 0 || flights.length > 0) && <svg className="visualizer-connectors"><defs><marker id="visualizer-arrowhead" viewBox="0 0 10 10" refX="8" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path d="M 0 0 L 10 5 L 0 10 z" /></marker></defs>{connectorPaths.map((connector, index) => <path key={index} d={connector.d} markerEnd="url(#visualizer-arrowhead)" />)}{animationsEnabled && flights.map((flight) => <path key={`flight-${flight.id}`} className="visual-flight-arrow" d={flight.d} markerEnd="url(#visualizer-arrowhead)" />)}</svg>}{animationsEnabled && flights.length > 0 && <div className="visualizer-flights">{flights.map((flight) => <span key={flight.id} className="visual-flight-value" style={{ offsetPath: `path('${flight.d}')` }}>{flight.value}</span>)}</div>}</div></div> : <Editor key="normal-editor" height="100%" language={selectedFile.kind === 'python' ? GLPYTHON_PYTHON : 'markdown'} theme={GLPYTHON_THEME} beforeMount={(instance) => { registerGlpythonPython(instance); defineGlpythonTheme(instance) }} value={selectedFile.code} onChange={updateCode} onMount={setNormalEditor} options={{ minimap: { enabled: false }, bracketPairColorization: { enabled: false }, fontSize: 15, lineHeight: 24, padding: { top: 22 }, fontFamily: 'ui-monospace, SFMono-Regular, Menlo, Consolas, monospace', scrollBeyondLastLine: false, smoothScrolling: true, automaticLayout: true, tabSize: 4, insertSpaces: true, detectIndentation: false, placeholder: t.editorPlaceholder ?? 'Type code here', readOnly: selectedFile.readOnly === true, domReadOnly: selectedFile.readOnly === true, readOnlyMessage: { value: t.readOnlyFile ?? 'Read-only: this file can be read but not changed.' } }} />}</div>
   const consoleNode = <>{!hideOutputPanel && <section className="output-panel"><div className="output-heading"><div className="output-title"><SquareTerminal size={16} /><span>{t.output}</span>{runtimeState !== 'ready' && <span className={`runtime-badge${runtimeState === 'failed' ? ' runtime-badge--failed' : ''}`}><span className="pulse" /> {t.pyodideRuntime}</span>}</div>{Object.keys(project.files).length > 1 && <span className="output-hint">{t.runsFile(project.mainFile)}</span>}</div><div className="console" ref={outputRef} role="log" onScroll={(event) => { const element = event.currentTarget; consoleAtBottomRef.current = element.scrollHeight - element.scrollTop - element.clientHeight < 24 }}>{isVisualizing ? <pre>{isAtErrorStep ? <>{visualStdout && !visualStdout.endsWith('\n') ? `${visualStdout}\n` : visualStdout}<span className="output-error">{visualTrace?.error}</span></> : visualStdout || t.noOutput}</pre> : <><pre>{output}<ErrorHintNote hint={outputErrorHint} t={t} /></pre>{consoleEntries.map((entry) => entry.kind === 'tests' ? <div key={entry.id} data-console-entry={entry.id}><TestReportView report={entry.report} t={t} onVisualizeExample={workspaceCapabilities.visualization ? visualizeExample : null} /></div> : entry.kind === 'note' ? <p key={entry.id} className="console-note" role="note" data-console-entry={entry.id}>{entry.text}</p> : <pre key={entry.id} data-console-entry={entry.id}><ReplTranscriptEntry entry={entry} definitionHint={t.replDefinitionHint ?? 'Definitions belong in the file.'} t={t} onVisualize={workspaceCapabilities.visualization && entry.result && !entry.result.unparsed && !entry.result.untraced && !entry.stale ? () => visualizeReplEntry(entry.id) : undefined} busy={isBusy} /></pre>)}</>}</div>{workspaceCapabilities.repl && <form className="repl-line" onSubmit={(event) => { event.preventDefault(); submitRepl() }}><span className="repl-prompt" aria-hidden="true">&gt;&gt;&gt;</span><input className="repl-input" value={replInput} onChange={(event) => { setReplInput(event.target.value); setReplHistoryIndex(null) }} onKeyDown={handleReplKeyDown} aria-label={t.replInputLabel ?? 'Python input'} spellCheck={false} autoComplete="off" autoCapitalize="off" /></form>}</section>}</>
   // In the stacked layout the graphics show only while the program draws -
   // for a visualization: if it draws at all, so the panel doesn't appear in
diff --git a/src/i18n.ts b/src/i18n.ts
index b4584e7..616f2dd 100644
--- a/src/i18n.ts
+++ b/src/i18n.ts
@@ -69,6 +69,7 @@ export const coreStrings: Record<UiLocale, CoreStrings> = {
     stop: 'Stop',
     stoppedNote: 'Stopped. Python was restarted.',
     consoleSize: 'Console size',
+    readOnlyFile: 'Read-only: this file can be read but not changed.',
     editorPlaceholder: 'Type code here',
     pyodideRuntime: 'Pyodide runtime',
     runsFile: (mainFile) => `Runs ${mainFile}`,
@@ -146,6 +147,7 @@ export const coreStrings: Record<UiLocale, CoreStrings> = {
     stop: 'Stoppen',
     stoppedNote: 'Gestoppt. Python wurde neu gestartet.',
     consoleSize: 'Grösse der Konsole',
+    readOnlyFile: 'Schreibgeschützt: Diese Datei kann gelesen, aber nicht geändert werden.',
     editorPlaceholder: 'Code hier eingeben',
     pyodideRuntime: 'Pyodide-Laufzeit',
     runsFile: (mainFile) => `Führt ${mainFile} aus`,
diff --git a/src/project.ts b/src/project.ts
index 74acf6d..364d62c 100644
--- a/src/project.ts
+++ b/src/project.ts
@@ -5,6 +5,10 @@ export type ProjectFile = {
   label: string
   code: string
   kind: ProjectFileKind
+  // A file to read, not to change - e.g. a course's library that learners
+  // use as a black box. The editor shows it read-only, and the helpers below
+  // don't change, rename or delete it.
+  readOnly?: boolean
 }
 
 export type Project = {
@@ -53,7 +57,7 @@ export function projectImportsGturtle(project: Project): boolean {
 
 export function updateProjectFile(project: Project, path: string, code: string): Project {
   const file = project.files[path]
-  if (!file) return project
+  if (!file || file.readOnly) return project
 
   return {
     ...project,
@@ -73,7 +77,7 @@ export function addProjectFile(project: Project, file: ProjectFile): Project {
 
 export function deleteProjectFile(project: Project, path: string): Project {
   const { [path]: deletedFile, ...remainingFiles } = project.files
-  if (!deletedFile) return project
+  if (!deletedFile || deletedFile.readOnly) return project
 
   const nextMainFile = project.mainFile === path
     ? Object.keys(remainingFiles).find((filePath) => remainingFiles[filePath].kind === 'python') ?? Object.keys(remainingFiles)[0]
@@ -84,7 +88,7 @@ export function deleteProjectFile(project: Project, path: string): Project {
 
 export function renameProjectFile(project: Project, path: string, nextPath: string): Project {
   const file = project.files[path]
-  if (!file || project.files[nextPath]) return project
+  if (!file || file.readOnly || project.files[nextPath]) return project
 
   const { [path]: renamedFile, ...remainingFiles } = project.files
   const nextFile = { ...renamedFile, path: nextPath, label: nextPath.replace(/\.[^/.]+$/, '') }
diff --git a/src/style.css b/src/style.css
index b52b1da..22a01ac 100644
--- a/src/style.css
+++ b/src/style.css
@@ -67,6 +67,7 @@
 .editor-tab { min-width: 130px; display: flex; align-items: center; gap: 8px; padding: 0 12px; border: 0; border-right: 1px solid var(--glp-border); color: var(--glp-text-muted); background: var(--glp-surface); font: 13px var(--glp-font-code); cursor: pointer; }
 .editor-tab.active { color: var(--glp-text); background: var(--glp-bg); box-shadow: inset 0 2px 0 var(--glp-accent); }
 .tab-file-icon { display: inline-flex; color: var(--glp-text-muted); }
+.tab-read-only { display: inline-flex; color: var(--glp-text-muted); }
 .editor-tab.active .tab-file-icon { color: var(--glp-accent); }
 .tab-close { display: inline-flex; margin-left: auto; padding: 3px; color: var(--glp-text-muted); border-radius: var(--glp-radius); }
 .tab-close:hover { color: var(--glp-text); background: var(--glp-border); }