diff --git a/src/assets/site.css b/src/assets/site.css
index efc9084..9e695fb 100644
--- a/src/assets/site.css
+++ b/src/assets/site.css
@@ -647,6 +647,9 @@ tbody th a:hover code { text-decoration: underline; }
    at once for one a chat sent with the source in the link. What is said to one alone is not shown
    to the other; the class is on <html> before the page paints (index.njk). */
 .linked .when-unlinked, :root:not(.linked) .when-linked { display: none; }
+/* A link whose source could not be read: what is said of the source it carried gives way to what
+   is said of the link, from when build-page.ts has tried it. */
+.unread .when-read, :root:not(.unread) .when-unread { display: none; }
 
 /* A bar at the top, then bands the width of the window, each holding a column of the page's width. */
 :root {
diff --git a/src/h5p/build/build-page.ts b/src/h5p/build/build-page.ts
index 385f939..70dff9f 100644
--- a/src/h5p/build/build-page.ts
+++ b/src/h5p/build/build-page.ts
@@ -26,7 +26,8 @@ import { formatSource, jsonProblem, place, type Target } from './json-scan'
  * a sample that plays (`SAMPLE`), and finds the sample's source in the box, which the first paste
  * replaces whole (`prefillSample`). The markup says which words are for whom, and the class
  * `linked` on `<html>` shows them, set before the page paints by the script in `index.njk` and
- * kept in step with the address here.
+ * kept in step with the address here. A link whose source cannot be read is said to be damaged,
+ * with the ways to bring the source (`linkUnread`).
  */
 
 type Opened = Awaited<ReturnType<typeof openPack>>
@@ -41,6 +42,8 @@ const sourceField = $<HTMLTextAreaElement>('source')
 const sourceLabel = $('source-label')
 const sourceState = $('source-state')
 const sourceFile = $<HTMLInputElement>('source-file')
+const linkState = $('link-state')
+const openSource = $<HTMLButtonElement>('open-source')
 const filesStep = $('files-step')
 const fileList = $<HTMLUListElement>('file-list')
 const filesInput = $<HTMLInputElement>('files')
@@ -117,6 +120,28 @@ function setStatus(text: string, state: 'idle' | 'building' | 'ready' | 'error')
   status.dataset.state = state
 }
 
+/**
+ * A link whose source cannot be read. A chat writes the link out in its reply, and a compressed
+ * source survives no wrong character: a link ChatGPT gave held 43 characters of its own in place
+ * of 37, near its start, and was the command's output everywhere else. The page then said the link
+ * had carried the source, over an empty box, with the reason under Build, out of sight in a window
+ * 800px high. So it says the link is damaged where a person who opened it looks. The class `unread`
+ * on `<html>` shows what the hero says of such a link in place of what it says of a source the link
+ * carried, for the rest of the visit, since the link carried none whatever comes after. The words
+ * above the box are the ways to bring the source, and stay until one is here (`checkSource`).
+ */
+function linkUnread(unread: boolean): void {
+  document.documentElement.classList.toggle('unread', unread)
+  showLinkState(unread)
+}
+
+/** The words above the box, and with them the step's marker, which site.css draws from their state. */
+function showLinkState(show: boolean): void {
+  linkState.hidden = !show
+  if (show) linkState.dataset.state = 'error'
+  else delete linkState.dataset.state
+}
+
 /** Where a problem is in the source, for the button that goes there: its place in the text, or none. */
 function target({ path, at }: Problem): Target {
   return at === undefined ? { path } : { at }
@@ -316,6 +341,8 @@ player.addEventListener('error', (event) => {
  */
 function checkSource(): void {
   const text = sourceField.value
+  // A source came after a link that could not be read, from a file or a paste: the ways to bring one go.
+  if (!linkState.hidden && compactSource(text)) showLinkState(false)
   const problem = jsonProblem(text)
   sourceState.dataset.state = problem ? 'error' : text.trim() ? 'ok' : ''
   if (!problem) {
@@ -406,6 +433,8 @@ sourceFile.addEventListener('change', async () => {
   if (!file) return
   setSource(await file.text())
 })
+// The same file field, from the words above the box on a link that could not be read.
+openSource.addEventListener('click', () => sourceFile.click())
 
 // The label is for the textarea, which the editor hides: a click on it goes to the editor.
 sourceLabel.addEventListener('click', () => editor?.focus())
@@ -537,9 +566,12 @@ async function fromLink(): Promise<void> {
   try {
     text = await sourceFromFragment(location.hash)
   } catch (error) {
-    setStatus(error instanceof Error ? error.message : String(error), 'error')
+    // Why it cannot be read is for whoever looks into it; the person is told what to do.
+    console.warn(error instanceof Error ? error.message : String(error))
+    linkUnread(true)
     return
   }
+  linkUnread(false)
   if (text === null) return
   carried = compactSource(text) ?? ''
   setSource(text)
diff --git a/src/h5p/build/index.njk b/src/h5p/build/index.njk
index bbc01e0..3a55f75 100644
--- a/src/h5p/build/index.njk
+++ b/src/h5p/build/index.njk
@@ -30,7 +30,10 @@ csp: "default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' da
     (#source=), gets the build at once. One who came with none gets a landing page: what it makes,
     a sample that plays, how to make their own. What is said to one alone is marked when-linked or
     when-unlinked, and site.css shows it by the class this line puts on <html> before the page
-    paints; build-page.ts keeps the class in step with the address from then on. -#}
+    paints; build-page.ts keeps the class in step with the address from then on. A link whose
+    source cannot be read is a third case, which build-page.ts finds once it has tried: it puts
+    `unread` on <html>, and what is said of a source the link carried is marked when-read, what is
+    said in its place when-unread. -#}
 <script>if (new URLSearchParams(location.hash.slice(1)).has("source")) document.documentElement.classList.add("linked")</script>
 {% include "topbar.njk" %}
 
@@ -51,11 +54,20 @@ csp: "default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' da
           <a class="button button-solid button-large" href="#make">Make your own</a>
           <a class="button button-large" href="#build-it">Paste your chat's JSON</a>
         </div>
-        <p class="lead when-linked">The link you opened carries an activity an AI chat wrote.</p>
-        <p class="when-linked">
+        <p class="lead when-linked when-read">The link you opened carries an activity an AI chat wrote.</p>
+        <p class="when-linked when-read">
           This page builds it in your browser, where you play it and download a
           <code>.h5p</code> file that Moodle or any other H5P site takes. Nothing is uploaded.
         </p>
+        {#- A link whose source cannot be read: a chat writes the link out itself, and one wrong
+            character leaves nothing of a compressed source. The page says so here, first, in place
+            of saying the link carried an activity, and what to do above the source's box. These
+            two stay for the visit, so they say what stays true once the source has come another way. -#}
+        <p class="lead when-unread">The link you opened arrived damaged, so the activity in it cannot be read.</p>
+        <p class="when-unread">
+          This page builds an activity in your browser from its source, which
+          <a href="#source-heading">step 1</a> says how to bring here. Nothing is uploaded.
+        </p>
       </div>
       {#- The sample is /h5p/build/sample.h5p, which Vite builds from sample.json beside this file.
           build-page.ts gives the player its address only for a visitor who came with no source. -#}
@@ -157,7 +169,16 @@ csp: "default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' da
       <section class="step" aria-labelledby="source-heading">
         <h3 id="source-heading"><span class="step-marker"><span class="step-number">1</span></span> Source</h3>
         <p class="when-unlinked">The source is your activity as the chat wrote it. The sample's is here to start with: paste yours over it, or open it from a file.</p>
-        <p class="when-linked">The source is your activity as the chat wrote it. The link carried it here.</p>
+        <p class="when-linked">The source is your activity as the chat wrote it. <span class="when-read">The link carried it here.</span></p>
+        {#- Shown by build-page.ts for a link whose source cannot be read, until a source is here: above
+            the box, where a person who opened the link looks, with each way to bring the source. -#}
+        <p class="status" id="link-state" role="status" hidden>
+          The source this link carried cannot be read: a chat can get characters wrong when it
+          writes out a long link. Ask it to give the link again. If it gave you a
+          <code>.source.json</code> file,
+          <button type="button" class="link-button" id="open-source">open that file here</button>
+          and choose Build below. Or ask it for the source as JSON and paste it into the box.
+        </p>
         <div class="field">
           <label for="source" id="source-label">The source, as JSON</label>
           <textarea id="source" rows="12" spellcheck="false" autocomplete="off" placeholder="The source your chat wrote goes here. It starts with { and ends with }."></textarea>