diff --git a/AGENTS.md b/AGENTS.md
index f29daa4..f2a449b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -298,10 +298,17 @@ someone spells it or pins it.
   has — for the fallback to refuse at zero rather than walk again; counting every list twice
   halved the list limit, counting the directive form once doubled the parser's frames per level
   (the maintainer, 2026-09-18).
+- Nothing spreads an unbounded array into a call — a node's siblings, a code block's held lines, a
+  mark run's segments: the argument list caps near 125k and throws a `RangeError` where a `Result`
+  is owed. A walk pushes one at a time. A literal spread (`[...value]`) is not the same thing and
+  is fine (4c).
 - A reader takes the text and an index — a sticky regex whose `lastIndex` the caller sets on the
   line before it reads, `indexOf` — never a fresh slice per character, and a per-character walk
   hoists the scan that does not vary with the character. The pipeline persona feeds documents
-  nobody typed, and a megabyte through a quadratic walk is a minute rather than a millisecond.
+  nobody typed, and a megabyte through a quadratic walk is a minute rather than a millisecond. A
+  scan may keep what it read for a later walk of the same text, and the fallback where it kept
+  nothing must be the same reader over the same text at the same index, so the two cannot disagree
+  — which is what makes the kept value a memo rather than a second spelling (4c).
 - No casts: `as`, `as unknown as`, non-null `!`. A boundary owes a type guard validating the
   fields it claims (`isAdfDocument`); past it everything is typed. Make invalid states
   unrepresentable.
diff --git a/src/markdown/commonmark-grammar.ts b/src/markdown/commonmark-grammar.ts
index 9047d0f..53c9f1a 100644
--- a/src/markdown/commonmark-grammar.ts
+++ b/src/markdown/commonmark-grammar.ts
@@ -174,7 +174,6 @@ export function isThematicBreak(line: string): boolean {
   return holdsThematicBreak(thematicBreakTail(line), line)
 }
 
-// A break runs to the line's end, so one scan of that run answers every level the list walk opens.
 export function thematicBreakTail(text: string): ThematicBreakTail | undefined {
   let marker: string | undefined
   let markers = 0
@@ -195,6 +194,8 @@ export function thematicBreakTail(text: string): ThematicBreakTail | undefined {
   return { longest: text.length - first, marker, shortest: text.length - third }
 }
 
+// `text` is a suffix of what `tail` was read from, or opens with a space: a length alone names a
+// suffix, and the marker check is what refuses one starting mid-run — `--- ---` holds ` ---`.
 export function holdsThematicBreak(tail: ThematicBreakTail | undefined, text: string): boolean {
   if (tail === undefined) return false
   return text.length >= tail.shortest && text.length <= tail.longest && text.charAt(0) === tail.marker
diff --git a/src/markdown/directive-syntax.ts b/src/markdown/directive-syntax.ts
index 41e7fcb..5ae76af 100644
--- a/src/markdown/directive-syntax.ts
+++ b/src/markdown/directive-syntax.ts
@@ -17,7 +17,7 @@ export type DirectiveLine =
   | { argument: string | undefined; attributes: DirectiveAttributes; kind: 'opener'; name: string }
   | { kind: 'closer'; name: string }
 
-// spans: the directives the content holds, at their offset in it, so reading the slot back never scans them again.
+// spans keys index content, so the two travel together: separate them and every offset is wrong.
 export type DirectiveSpan = { attributes: DirectiveAttributes; content: string | undefined; length: number; name: string; spans: NestedSpans }
 
 export type NestedSpans = ReadonlyMap<number, DirectiveSpan>
@@ -254,10 +254,11 @@ function readDirectiveContent(text: string, start: number, depth: number): Read<
       cursor = span
       continue
     }
-    const nested = keepNestedSpan(text, cursor, depth, start, spans)
+    const nested = readNestedDirective(text, cursor, depth + 1)
     if (nested?.fault !== undefined) return { fault: nested.fault }
     if (nested !== undefined) {
-      cursor = nested.value
+      spans.set(cursor - start, nested.value)
+      cursor += nested.value.length
       continue
     }
     if (character === ']' && brackets === 0) return { value: { end: cursor, spans } }
@@ -268,14 +269,6 @@ function readDirectiveContent(text: string, start: number, depth: number): Read<
   return { fault: malformedDirective(`an inline directive [content] is unclosed; ${directiveEscape}`) }
 }
 
-function keepNestedSpan(text: string, cursor: number, depth: number, start: number, spans: Map<number, DirectiveSpan>): Read<number> | undefined {
-  const nested = readNestedDirective(text, cursor, depth + 1)
-  if (nested === undefined) return undefined
-  if (nested.fault !== undefined) return { fault: nested.fault }
-  spans.set(cursor - start, nested.value)
-  return { value: cursor + nested.value.length }
-}
-
 // `undefined` where the span crosses the line ending an inline directive may not cross.
 function readCodeSpanEnd(text: string, index: number): number | undefined {
   const opener = backtickRun(text, index)
diff --git a/src/markdown/emit/adf-to-markdown.test.ts b/src/markdown/emit/adf-to-markdown.test.ts
index ca07e0a..e5a2969 100644
--- a/src/markdown/emit/adf-to-markdown.test.ts
+++ b/src/markdown/emit/adf-to-markdown.test.ts
@@ -736,3 +736,8 @@ test('carries whitespace CommonMark strips in the reserved text directive', () =
   // CommonMark strips spaces and tabs alone, so the whitespace beside them is plain text.
   assert.equal(emitted({ text: '\va\f', type: 'text' }), '\va\f\n')
 })
+
+test("joins a mark run's segments as a walk rather than as one call's arguments", () => {
+  const run = Array.from({ length: 200000 }, (): AdfNode => ({ marks: [{ type: 'strong' }], text: 'a', type: 'text' }))
+  assert.equal(markdown(adfToMarkdown(document({ content: run, type: 'paragraph' }))), `**${'a'.repeat(200000)}**\n`)
+})
diff --git a/src/markdown/emit/inline-line.ts b/src/markdown/emit/inline-line.ts
index 77311f5..0cddd44 100644
--- a/src/markdown/emit/inline-line.ts
+++ b/src/markdown/emit/inline-line.ts
@@ -166,7 +166,7 @@ function emitRun(nodes: readonly AdfNode[], depth: number, firstIndex: number, c
     const emitted = run.kind === 'plain' ? emitLeaf(run.node, runContext, run.index) : emitMarkedRun(run.nodes, run.mark, depth, run.index, runContext)
     if (!emitted.ok) return emitted
     if (emitted.value.carry !== undefined) return emitted
-    segments.push(...emitted.value.segments)
+    for (const segment of emitted.value.segments) segments.push(segment)
   }
   return success({ segments })
 }
diff --git a/src/markdown/parse/blocks.test.ts b/src/markdown/parse/blocks.test.ts
index f4061be..2866051 100644
--- a/src/markdown/parse/blocks.test.ts
+++ b/src/markdown/parse/blocks.test.ts
@@ -118,3 +118,7 @@ test('closes the containers a closer names past as unclosed, and crosses no list
   assert.deepEqual(kinds('!adf:rule {localId=a-1}\nPart.\n!adf:/rule\n'), ['fault', 'paragraph'])
   assert.deepEqual(kinds('!adf:rule {localId=a-1}\n!adf:/rule\n!adf:/rule\n'), ['fault', 'fault'])
 })
+
+test("releases an indented code block's held blank lines as a walk rather than as one call's arguments", () => {
+  assert.deepEqual(kinds(`    a\n${'\n'.repeat(200000)}    b\n`), ['code'])
+})
diff --git a/src/markdown/parse/blocks.ts b/src/markdown/parse/blocks.ts
index 09edb33..b011452 100644
--- a/src/markdown/parse/blocks.ts
+++ b/src/markdown/parse/blocks.ts
@@ -365,7 +365,8 @@ function readIndentedCodeLine(leaf: Extract<OpenLeaf, { kind: 'indented-code' }>
     return true
   }
   if (leadingColumns(line) < indentedCodeColumns) return false
-  leaf.lines.push(...leaf.held, removeColumns(line, indentedCodeColumns).text)
+  for (const held of leaf.held) leaf.lines.push(held)
+  leaf.lines.push(removeColumns(line, indentedCodeColumns).text)
   leaf.held.length = 0
   return true
 }
diff --git a/todo-history.md b/todo-history.md
index 65ed0be..07ab2b7 100644
--- a/todo-history.md
+++ b/todo-history.md
@@ -534,6 +534,11 @@ The done `todo.md` items in full, as they were written. `todo.md` keeps a one-li
       38 ms where the tail scan alone would have left it quadratic.
       The sixth site the sweep found went to 18 rather than landing here (the maintainer,
       2026-09-18).
+      **Widened** (the systems-architect, 2026-09-18): the guard's spread was a class rather than a
+      site, and two more threw out of the public API — `readIndentedCodeLine` releasing the blank
+      lines an indented code block held (200k of them at 200 kB), and `emitRun` joining a mark
+      run's segments (200k nodes under one mark). Both fixed here with the same loop and a test
+      each, and §11 gained the rule so the spelling cannot walk back in.
 - [x] **5a — Rename to `@larvit/adf-codec` (`0.1.0`).** Before the first publish, the name being
       the published identity: `package.json` `name` and `repository`, the Gitea repo and its
       remote, the README title, §6's published-as line, the checkout directory.