diff --git a/backend/locales/en.json b/backend/locales/en.json
index 4ba7a5f2c..6532f453a 100644
--- a/backend/locales/en.json
+++ b/backend/locales/en.json
@@ -1696,6 +1696,7 @@
   "editor.assets.uploadFailed": "File upload failed.",
   "editor.backToEditor": "Back to Editor",
   "editor.blockNotEnabled": "This block is not enabled for this site, and will be removed when the page is saved. Blocks are managed in the administration area.",
+  "editor.blockParams.title": "Edit Block Parameters - {name}",
   "editor.blockPicker.blockUnavailable": "This block is not available on this site. Blocks are managed in the administration area.",
   "editor.blockPicker.insert": "Insert Block",
   "editor.blockPicker.loadFailed": "Failed to load the list of blocks.",
@@ -1763,6 +1764,7 @@
   "editor.markup.blockquoteWarning": "Warning Blockquote",
   "editor.markup.bold": "Bold",
   "editor.markup.distractionFreeMode": "Distraction Free Mode",
+  "editor.markup.editBlock": "Edit Block Parameters",
   "editor.markup.editTable": "Edit in Table Editor",
   "editor.markup.header": "Header",
   "editor.markup.headerLevel": "Header {level}",
diff --git a/frontend/src/components/BlockParamsDialog.vue b/frontend/src/components/BlockParamsDialog.vue
new file mode 100644
index 000000000..22652e0f7
--- /dev/null
+++ b/frontend/src/components/BlockParamsDialog.vue
@@ -0,0 +1,99 @@
+<template>
+  <w-dialog v-model="dialogVisible" @hide="onDialogHide">
+    <w-card style="width: 550px">
+      <w-card-section class="card-header">
+        <w-icon
+          :name="`img:/_assets/icons/ultraviolet-${definition.isCustom ? 'plugin' : definition.icon}.svg`"
+          size="sm"
+          class="mr-2" />
+        <!-- -> The block is named in the title rather than over the form: one line of chrome above a
+                short form is enough, and which block this is belongs with what is being done to it. -->
+        <span>{{ t('editor.blockParams.title', { name: definition.name }) }}</span>
+      </w-card-section>
+      <w-card-section>
+        <block-props-form :fields="definition.props ?? []" :values="state.values" />
+      </w-card-section>
+      <w-card-actions class="card-actions">
+        <w-space />
+        <w-btn
+          class="acrylic-btn"
+          flat
+          :label="t(`common.actions.cancel`)"
+          color="grey"
+          padding="xs md"
+          @click="onDialogCancel" />
+        <w-btn
+          unelevated
+          :label="t(`common.actions.apply`)"
+          color="primary"
+          padding="xs md"
+          :disabled="!canApply"
+          @click="apply" />
+      </w-card-actions>
+    </w-card>
+  </w-dialog>
+</template>
+
+<script setup>
+import { computed, reactive } from 'vue'
+import { useI18n } from 'vue-i18n'
+
+import { dialogComponentEmits, useDialogComponent } from '@/composables/dialog'
+import { blockPropsFilled } from '@/helpers/blocks'
+
+import BlockPropsForm from '@/components/BlockPropsForm.vue'
+
+/**
+ * What a block already in the page was given, for changing.
+ *
+ * The same form the picker fills in for a new block — see `BlockPropsForm` — over the values read
+ * back out of the page source. What comes back is only that: the caller knows which line the block
+ * was on and what else was written on it, and is the one to put the answer back.
+ *
+ * The values are copied on the way in, so closing without applying leaves the page as it was.
+ */
+
+// PROPS
+
+const props = defineProps({
+  /** The block as the API describes it: its name, its icon and the props it declares. */
+  definition: {
+    type: Object,
+    required: true
+  },
+  /** What the page currently gives it, by prop name. */
+  values: {
+    type: Object,
+    required: true
+  }
+})
+
+// EMITS
+
+defineEmits([...dialogComponentEmits])
+
+// DIALOG
+
+const { dialogVisible, onDialogHide, onDialogOK, onDialogCancel } = useDialogComponent()
+
+// I18N
+
+const { t } = useI18n()
+
+// DATA
+
+const state = reactive({
+  values: { ...props.values }
+})
+
+// COMPUTED
+
+// -> A required prop emptied out would leave a block that cannot draw anything
+const canApply = computed(() => blockPropsFilled(props.definition, state.values))
+
+// METHODS
+
+function apply() {
+  onDialogOK({ ...state.values })
+}
+</script>
diff --git a/frontend/src/components/BlockPickerOverlay.vue b/frontend/src/components/BlockPickerOverlay.vue
index 340be1613..20108928c 100644
--- a/frontend/src/components/BlockPickerOverlay.vue
+++ b/frontend/src/components/BlockPickerOverlay.vue
@@ -88,45 +88,10 @@
               </div>
               <template v-else>
                 <div class="w-section-header">{{ state.selected.name }}</div>
-                <!--
-                  A block with nothing to fill in is not a broken form: it is inserted as it stands. A
-                  custom block reports no props at all, since only the compiled manifest carries them.
-                -->
-                <div
-                  v-if="state.selected.props.length < 1"
-                  class="text-caption mt-4 px-4 text-black/60 dark:text-white/70">
-                  {{ t('editor.blockPicker.noProps') }}
-                </div>
-                <w-form v-else class="gap-4 px-4 pt-4">
-                  <template v-for="prop of state.selected.props" :key="prop.name">
-                    <w-select
-                      v-if="prop.type === `select`"
-                      v-model="state.values[prop.name]"
-                      :options="prop.options ?? []"
-                      outlined
-                      dense
-                      options-dense
-                      :label="prop.label ?? prop.name"
-                      :aria-label="prop.label ?? prop.name"
-                      :required="prop.required"
-                      :hint="prop.hint" />
-                    <w-toggle
-                      v-else-if="prop.type === `boolean`"
-                      v-model="state.values[prop.name]"
-                      dense
-                      :label="prop.label ?? prop.name" />
-                    <w-input
-                      v-else
-                      v-model="state.values[prop.name]"
-                      outlined
-                      dense
-                      :type="prop.type === `number` ? `number` : `text`"
-                      :label="prop.label ?? prop.name"
-                      :aria-label="prop.label ?? prop.name"
-                      :required="prop.required"
-                      :hint="prop.hint" />
-                  </template>
-                </w-form>
+                <block-props-form
+                  class="px-4 pt-4"
+                  :fields="state.selected.props"
+                  :values="state.values" />
                 <!-- -> The markup itself, since that is what lands in the page -->
                 <div class="w-section-header mt-6">{{ t('editor.blockPicker.markdown') }}</div>
                 <!-- The same 16px all round, so it sits inside the panel the way the fields do -->
@@ -145,7 +110,9 @@ import { computed, onMounted, reactive } from 'vue'
 import { useI18n } from 'vue-i18n'
 
 import { notify } from '@/composables/notify'
-import { blockMarkdown } from '@/helpers/blocks'
+import { blockMarkdown, blockPropsFilled } from '@/helpers/blocks'
+
+import BlockPropsForm from '@/components/BlockPropsForm.vue'
 
 import { useSiteStore } from '@/stores/site'
 
@@ -186,15 +153,10 @@ const blocks = computed(() => state.blocks.filter((block) => block.isEnabled))
 
 const markdown = computed(() => (state.selected ? blockMarkdown(state.selected, state.values) : ''))
 
-const canInsert = computed(() => {
-  if (!state.selected) {
-    return false
-  }
-  // -> A required prop with nothing in it would insert a block that cannot draw anything
-  return state.selected.props
-    .filter((prop) => prop.required)
-    .every((prop) => String(state.values[prop.name] ?? '').length > 0)
-})
+// -> A required prop with nothing in it would insert a block that cannot draw anything
+const canInsert = computed(
+  () => Boolean(state.selected) && blockPropsFilled(state.selected, state.values)
+)
 
 // METHODS
 
diff --git a/frontend/src/components/BlockPropsForm.vue b/frontend/src/components/BlockPropsForm.vue
new file mode 100644
index 000000000..6d24e369a
--- /dev/null
+++ b/frontend/src/components/BlockPropsForm.vue
@@ -0,0 +1,75 @@
+<template>
+  <div v-if="fields.length < 1" class="text-caption text-black/60 dark:text-white/70">
+    {{ t('editor.blockPicker.noProps') }}
+  </div>
+  <w-form v-else class="gap-4">
+    <template v-for="field of fields" :key="field.name">
+      <w-select
+        v-if="field.type === `select`"
+        v-model="values[field.name]"
+        :options="field.options ?? []"
+        outlined
+        dense
+        options-dense
+        :label="field.label ?? field.name"
+        :aria-label="field.label ?? field.name"
+        :required="field.required"
+        :hint="field.hint" />
+      <w-toggle
+        v-else-if="field.type === `boolean`"
+        v-model="values[field.name]"
+        dense
+        :label="field.label ?? field.name" />
+      <w-input
+        v-else
+        v-model="values[field.name]"
+        outlined
+        dense
+        :type="field.type === `number` ? `number` : `text`"
+        :label="field.label ?? field.name"
+        :aria-label="field.label ?? field.name"
+        :required="field.required"
+        :hint="field.hint" />
+    </template>
+  </w-form>
+</template>
+
+<script setup>
+import { useI18n } from 'vue-i18n'
+
+/**
+ * The form a block's props make: one field per prop, in the order the block declares them.
+ *
+ * Shared by the block picker, which fills it in for a block about to be inserted, and the parameters
+ * dialog the editor's lens opens over one already in the page. The two ask the same thing of an
+ * author and must offer the same controls, so the fields are described once here.
+ *
+ * A block with nothing to fill in is not a broken form: it is inserted, or left, as it stands. A
+ * custom block reports no props at all, since only the compiled manifest carries them.
+ *
+ * It writes into the `values` object it is given rather than emitting: what a caller wants back is
+ * "what is in the form now", and both of them already keep that object as their own state — a
+ * `v-model` per field would be the same object, one indirection further away.
+ *
+ * Padding is the caller's: this sits in a panel in one and a card in the other.
+ */
+
+// PROPS
+
+defineProps({
+  /** The props the block declares, as the API describes them. */
+  fields: {
+    type: Array,
+    required: true
+  },
+  /** Values by prop name, written into as the author types. */
+  values: {
+    type: Object,
+    required: true
+  }
+})
+
+// I18N
+
+const { t } = useI18n()
+</script>
diff --git a/frontend/src/components/EditorMarkdown.vue b/frontend/src/components/EditorMarkdown.vue
index 0aaee20b4..d7f2367be 100644
--- a/frontend/src/components/EditorMarkdown.vue
+++ b/frontend/src/components/EditorMarkdown.vue
@@ -309,6 +309,7 @@
 <script setup>
 import {
   computed,
+  defineAsyncComponent,
   reactive,
   ref,
   shallowRef,
@@ -325,6 +326,7 @@ import { notify } from '@/composables/notify'
 import { useMinWidth } from '@/composables/screen'
 import { assetPath } from '@/helpers/assets'
 import { blockMarkdown } from '@/helpers/blocks'
+import { blockOpeningLine, blockValues, findBlocks } from '@/helpers/markdownBlocks'
 import { findEditableTables } from '@/helpers/markdownTable'
 
 import EditorCodeBlockMenu from '@/components/EditorCodeBlockMenu.vue'
@@ -384,6 +386,15 @@ let md
 let pasteCaptureNode = null
 /** The "Edit Table" lens provider, which is registered against the language rather than this editor. */
 let tableLensProvider = null
+/** The "Edit Block Parameters" lens provider, registered the same way. */
+let blockLensProvider = null
+/**
+ * The blocks this site has, as the API describes them — their props included.
+ *
+ * Read once with the list of disabled ones, since it is the same request. What the lens needs from it
+ * is the props: a block whose definition is not here is one this editor cannot offer a form for.
+ */
+let siteBlocks = []
 const monacoRef = ref(null)
 const editorPreviewContainerRef = ref(null)
 
@@ -618,6 +629,49 @@ function editTable(line) {
   })
 }
 
+/** The block as this site describes it, or undefined for one it does not list. */
+function blockDefinition(name) {
+  return siteBlocks.find((block) => block.block === name)
+}
+
+/**
+ * The parameters dialog, over a block already in the page — what the lens above one opens.
+ *
+ * The block is looked up again here rather than taken from the lens, for the reason `editTable` gives:
+ * a lens is provided once and then moves with the text, so the line it carries is from whenever the
+ * document last settled. The name it was drawn for is carried along and has to match too — where a
+ * table spans lines and can be found by containment, a block's opening line is a single line, and an
+ * edit above it would otherwise put a form for one block over another.
+ */
+function editBlock(line, name) {
+  const found = findBlocks(editor.getModel().getValue()).find(
+    (entry) => entry.line === line && entry.block === name
+  )
+  const definition = found && blockDefinition(found.block)
+  if (!definition) {
+    return
+  }
+  dialog({
+    component: defineAsyncComponent(() => import('./BlockParamsDialog.vue')),
+    componentProps: { definition, values: blockValues(found, definition) }
+  }).onOk((values) => {
+    /*
+      The opening line and nothing else, so the body between the fences is left exactly as it was —
+      which for a tabset is every tab in it. One undo takes the whole change back, and the caret lands
+      on the line that moved rather than wherever it was before the dialog opened.
+    */
+    const model = editor.getModel()
+    editor.executeEdits('block', [
+      {
+        range: new Range(found.line, 1, found.line, model.getLineMaxColumn(found.line)),
+        text: blockOpeningLine(found, definition, values)
+      }
+    ])
+    editor.setPosition(new Position(found.line, 1))
+    editor.focus()
+  })
+}
+
 /**
  * The table the overlay built: over the lines it was read from, or at the cursor when it is a new one.
  *
@@ -900,23 +954,25 @@ async function toggleMarkup({ start, end }) {
 }
 
 /**
- * Read which blocks this site has switched off, once, before the first preview is drawn.
+ * Read the blocks this site has, once, before the first preview is drawn.
  *
  * Order matters more than it looks: a component only has to be fetched once to be defined for the
  * rest of the session, so a list that arrives after the first render is too late to keep a disabled
- * block from drawing.
+ * block from drawing. The lens over a block wants the same list a moment later, and asking twice for
+ * it would be asking the same question twice.
  */
-async function loadDisabledBlocks() {
+async function loadSiteBlocks() {
   try {
-    const blocks = (await API_CLIENT.get(`sites/${siteStore.id}/blocks`).json()) ?? []
+    siteBlocks = (await API_CLIENT.get(`sites/${siteStore.id}/blocks`).json()) ?? []
     disabledBlockTags.value = new Set(
-      blocks.filter((block) => !block.isEnabled).map((block) => `block-${block.block}`)
+      siteBlocks.filter((block) => !block.isEnabled).map((block) => `block-${block.block}`)
     )
   } catch (err) {
     /*
       Left empty, which draws everything as it did before. The preview being too generous is the
       better failure: the server strips a disabled block on save either way, so the cost is a preview
-      that flatters the page, against hiding blocks the site really does have.
+      that flatters the page, against hiding blocks the site really does have. The lens is the other
+      way round — with no definitions to build a form from, it simply does not appear.
     */
     console.warn(`Could not read which blocks this site has enabled: ${err.message}`)
   }
@@ -1178,7 +1234,7 @@ onMounted(async () => {
   })
 
   // -> Awaited here so it is settled well before the first preview render at the end of this hook
-  await loadDisabledBlocks()
+  await loadSiteBlocks()
 
   md = new MarkdownRenderer(editorStore.editors.markdown)
 
@@ -1252,6 +1308,34 @@ onMounted(async () => {
     }
   })
 
+  /*
+    "Edit Block Parameters" over every block in the page, for the same reason the table has one: what
+    a block was given is a list of quoted attributes on one line, which is a poor thing to edit by
+    hand and an easy thing to offer a form for.
+
+    It appears only over a block this editor holds a definition for and that has something to fill in.
+    A child block -- a `::block-tab` inside a tabset -- is one it never does: those are left out of
+    the list the API answers with, having no switch of their own to be listed against.
+  */
+  const editBlockCommand = editor.addCommand(0, (_accessor, line, block) => editBlock(line, block))
+  blockLensProvider = monaco.languages.registerCodeLensProvider('markdown', {
+    provideCodeLenses(model) {
+      return {
+        lenses: findBlocks(model.getValue())
+          .filter((found) => blockDefinition(found.block)?.props?.length > 0)
+          .map((found) => ({
+            range: new Range(found.line, 1, found.line, 1),
+            command: {
+              id: editBlockCommand,
+              title: t('editor.markup.editBlock'),
+              arguments: [found.line, found.block]
+            }
+          })),
+        dispose() {}
+      }
+    }
+  })
+
   // -> Define Formatting Actions
   editor.addAction({
     contextMenuGroupId: 'markdown.extension.editing',
@@ -1484,6 +1568,7 @@ onBeforeUnmount(() => {
   monacoRef.value?.removeEventListener('drop', onEditorDrop)
   // -> Registered against the markdown language, not this editor, so nothing else takes it down
   tableLensProvider?.dispose()
+  blockLensProvider?.dispose()
   // -> Before the editor goes: the binding is holding the model, and leaving the room is what takes
   //    this author's avatar out of everyone else's header
   stopCollabSession()
diff --git a/frontend/src/helpers/blocks.js b/frontend/src/helpers/blocks.js
index 1004537eb..6fb6b1f9a 100644
--- a/frontend/src/helpers/blocks.js
+++ b/frontend/src/helpers/blocks.js
@@ -3,32 +3,49 @@
  *
  * Shared rather than living in the block picker, because the picker is not the only way a block gets
  * inserted — the toolbar has a shortcut for the tabset, which has to produce exactly what picking
- * Tabs from the list would have produced.
+ * Tabs from the list would have produced, and the "Edit Block Parameters" lens rewrites the opening
+ * line of a block already in the page.
  *
  * `::block-name{prop="value"}` is what the renderer turns into `<block-name prop="value">`, the
  * element the component registers itself as.
+ */
+
+/**
+ * What an author filled in, as MDC attributes — one `name="value"` per prop worth writing out.
+ *
+ * Separate from `blockMarkdown` because editing an existing block reuses only this half: its body is
+ * whatever the author has since written between the two fences, and rebuilding the whole block from
+ * the definition would throw that away.
+ *
+ * @param {{ props?: Array }} block A block as the API describes it.
+ * @param {Record<string, unknown>} [values] What the author filled in, by prop name.
+ * @returns {string[]} The attributes, in the order the block declares its props.
+ */
+export function blockAttributes(block, values = {}) {
+  /*
+    Only what is worth writing out: anything given a value that is not already the block's own
+    default. A block reading its default from its own code does not need to be told it in every page.
+  */
+  const written = (block.props ?? []).filter((prop) => {
+    const value = values[prop.name]
+    if (value === undefined || value === null || value === '') {
+      return false
+    }
+    return String(value) !== String(prop.default ?? '')
+  })
+  // -> A double quote in a value would close the attribute; MDC has no escape for it, so it goes
+  return written.map((prop) => `${prop.name}="${String(values[prop.name]).replaceAll('"', "'")}"`)
+}
+
+/**
+ * A block, opening and closing lines included.
  *
  * @param {{ block: string, props?: Array, template?: string }} block A block as the API describes it.
  * @param {Record<string, unknown>} [values] What the author filled in, by prop name.
  * @returns {string} The markup, opening and closing lines included.
  */
 export function blockMarkdown(block, values = {}) {
-  const attributes = (block.props ?? [])
-    .filter((prop) => {
-      /*
-        Only what is worth writing out: anything given a value that is not already the block's own
-        default. A block reading its default from its own code does not need to be told it in every
-        page.
-      */
-      const value = values[prop.name]
-      if (value === undefined || value === null || value === '') {
-        return false
-      }
-      return String(value) !== String(prop.default ?? '')
-    })
-    // -> A double quote in a value would close the attribute; MDC has no escape for it, so it goes
-    .map((prop) => `${prop.name}="${String(values[prop.name]).replaceAll('"', "'")}"`)
-    .join(' ')
+  const attributes = blockAttributes(block, values).join(' ')
   const suffix = attributes ? `{${attributes}}` : ''
 
   /*
@@ -42,3 +59,19 @@ export function blockMarkdown(block, values = {}) {
   }
   return `::block-${block.block}${suffix}\n::`
 }
+
+/**
+ * Whether every prop the block insists on has been given something.
+ *
+ * Asked by both the picker's Insert button and the parameters dialog's Apply: a required prop left
+ * empty is a block that cannot draw anything.
+ *
+ * @param {{ props?: Array }} block A block as the API describes it.
+ * @param {Record<string, unknown>} values What the author filled in, by prop name.
+ * @returns {boolean}
+ */
+export function blockPropsFilled(block, values) {
+  return (block.props ?? [])
+    .filter((prop) => prop.required)
+    .every((prop) => String(values[prop.name] ?? '').length > 0)
+}
diff --git a/frontend/src/helpers/markdownBlocks.js b/frontend/src/helpers/markdownBlocks.js
new file mode 100644
index 000000000..867d7621e
--- /dev/null
+++ b/frontend/src/helpers/markdownBlocks.js
@@ -0,0 +1,153 @@
+import { blockAttributes } from '@/helpers/blocks'
+
+/**
+ * The blocks already in a page's source, read back and rewritten.
+ *
+ * The counterpart to `blocks.js`, which writes a block out: this finds the ones a page already
+ * carries so the editor can offer to edit their parameters, reads what they were given back into the
+ * form's shape, and writes the answer over the line it came from.
+ *
+ * Only the OPENING line is ever read or replaced. Everything a block's props can say is on that line,
+ * and what sits between the fences is the author's — page content, or the blocks of a tabset. Building
+ * the whole block again from its definition, the way inserting one does, would throw that away.
+ */
+
+/** The opening or closing line of a fenced block, indented up to the three spaces markdown allows. */
+const FENCE = /^ {0,3}(`{3,}|~{3,})/
+
+/**
+ * A block component opening a line: `::block-name`, with its attributes if it was given any.
+ *
+ * Anchored to the start of the line because that is MDC's own rule for a block — `:block-name{…}`
+ * mid-sentence is an inline component, which has no body and is not what the picker writes. Three or
+ * more colons is the same block fenced to hold blocks of its own, so the count is captured and put
+ * back rather than assumed.
+ */
+const OPENING = /^(:{2,})block-([a-z0-9-]+)[ \t]*(?:\{(.*)\})?[ \t]*$/
+
+/**
+ * One entry in an attribute list: `name`, `name=value`, `name="value"`, or a `.class` / `#id`
+ * shorthand. Ordered so a quoted value wins over the unquoted reading, which would stop at the space.
+ */
+const ATTRIBUTE = /([.#][^\s"'=]+)|([^\s"'=]+)(?:=(?:"([^"]*)"|'([^']*)'|([^\s}]*)))?/g
+
+/**
+ * Split an attribute list into what it says.
+ *
+ * `name` is null for a `.class` or `#id`, which belongs to no prop; `value` is null for a bare name,
+ * which MDC reads as true. `raw` is what was written, kept so that anything this block does not
+ * declare survives a rewrite untouched — see `blockOpeningLine`.
+ *
+ * @param {string} source The inside of the braces.
+ * @returns {Array<{ name: string|null, value: string|null, raw: string }>}
+ */
+function parseAttributes(source) {
+  return [...source.matchAll(ATTRIBUTE)].map((match) => ({
+    name: match[1] ? null : match[2],
+    value: match[1] ? null : (match[3] ?? match[4] ?? match[5] ?? null),
+    raw: match[0]
+  }))
+}
+
+/**
+ * Every block in the source, in the order they appear. Line numbers are 1-based, to be handed
+ * straight to the editor.
+ *
+ * A block inside a fenced code block is a code sample and not a block, so those are skipped — the
+ * same reading `findEditableTables` takes of the same lines. Nesting needs no tracking of its own:
+ * every opening line stands on its own, whatever it is written inside.
+ *
+ * @param {string} text The page source.
+ * @returns {Array<{ block: string, line: number, fence: string, attributes: Array }>}
+ */
+export function findBlocks(text) {
+  const lines = text.split('\n')
+  const blocks = []
+  let fence = null
+
+  for (let index = 0; index < lines.length; index++) {
+    const edge = FENCE.exec(lines[index])
+    if (fence) {
+      if (edge && edge[1][0] === fence[0] && edge[1].length >= fence.length) {
+        fence = null
+      }
+      continue
+    }
+    if (edge) {
+      fence = edge[1]
+      continue
+    }
+
+    const opening = OPENING.exec(lines[index])
+    if (opening) {
+      blocks.push({
+        block: opening[2],
+        line: index + 1,
+        fence: opening[1],
+        attributes: parseAttributes(opening[3] ?? '')
+      })
+    }
+  }
+  return blocks
+}
+
+/**
+ * What the form should open on: the block's props, filled in from what the page gave them.
+ *
+ * A prop the source says nothing about starts at the block's own default, which is what the block
+ * will do if left alone — the same footing the picker starts a new block on.
+ *
+ * @param {{ attributes: Array }} found A block from `findBlocks`.
+ * @param {{ props?: Array }} definition The same block as the API describes it.
+ * @returns {Record<string, unknown>} Values by prop name.
+ */
+export function blockValues(found, definition) {
+  const written = new Map(
+    found.attributes.filter((attribute) => attribute.name).map((a) => [a.name, a.value])
+  )
+  return Object.fromEntries(
+    (definition.props ?? []).map((prop) => {
+      if (!written.has(prop.name)) {
+        return [prop.name, prop.default ?? '']
+      }
+      const value = written.get(prop.name)
+      switch (prop.type) {
+        /*
+          -> A bare `hideToolbar` is true, and so is any value but the word false — which is exactly
+             how the blocks themselves read a boolean attribute, since MDC writes every prop as a
+             string and an attribute that is merely present would otherwise be true whatever it says.
+        */
+        case 'boolean':
+          return [prop.name, value === null ? true : value !== 'false']
+        case 'number': {
+          const number = Number(value)
+          return [prop.name, Number.isFinite(number) ? number : (prop.default ?? '')]
+        }
+        default:
+          return [prop.name, value ?? '']
+      }
+    })
+  )
+}
+
+/**
+ * The opening line to write back, from what the form now holds.
+ *
+ * Anything in the original list that the block does not declare is carried over as it was written:
+ * a `.class`, or an attribute belonging to a version of the block that had a prop this one has not.
+ * None of them survive being saved — the renderer allows a block exactly the attributes its
+ * definition declares — but dropping them here would edit a line the author is still writing.
+ *
+ * @param {{ block: string, fence: string, attributes: Array }} found A block from `findBlocks`.
+ * @param {{ props?: Array }} definition The same block as the API describes it.
+ * @param {Record<string, unknown>} values What the form holds, by prop name.
+ * @returns {string} The line, with no trailing newline.
+ */
+export function blockOpeningLine(found, definition, values) {
+  const declared = new Set((definition.props ?? []).map((prop) => prop.name))
+  const kept = found.attributes
+    .filter((attribute) => !attribute.name || !declared.has(attribute.name))
+    .map((attribute) => attribute.raw)
+  const attributes = [...blockAttributes(definition, values), ...kept].join(' ')
+  return `${found.fence}block-${found.block}${attributes ? `{${attributes}}` : ''}`
+}