diff --git a/CHANGELOG.md b/CHANGELOG.md
index e569caa9..b818f0db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,27 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.295] — 2026-05-04
+
+### Fixed (1 PR — closes #1618 / #1463)
+
+- **YAML, JSON, and diff/patch fenced code blocks now render multi-line, not collapsed to a single line** (closes #1618 / #1463, reported by @Zixim) — PR #484 (v0.50.237) introduced a JSON/YAML tree-viewer that routes `lang === 'json'` and `lang === 'yaml'` blocks through `<div class="code-tree-wrap">…<pre class="tree-raw-view">…</pre></div>` instead of bare `<pre>`. Same release added the diff/patch coloring path that emits `<pre class="diff-block">`. The `_pre_stash` regex at `static/ui.js:1914` matched only literal `<pre>` (no attributes): `<pre>[\s\S]*?<\/pre>`. Both new shapes failed to match, fell through to the paragraph-wrap pass, and `\n` characters inside the code blocks got replaced with `<br>` tags inside `<code>`. By the time Prism ran, there were no newlines left for it to highlight against. PR #1516 (v0.50.279) had attempted a CSS-only fix on Prism's token white-space — that rule is in `style.css` and reaches the browser, but it was the wrong layer: the rule preserves newlines inside `.token` spans, but the spans were built from a string that had no newlines left. **Fix:** relax the `_pre_stash` regex to accept any attribute on `<pre>` (`<pre>` → `<pre[^>]*>`). One regex character. Pulls JSON, YAML, AND diff/patch blocks into the stash so paragraph-wrap can't mangle them. Bash, Python, Go, etc. were never affected because they emit bare `<pre>` and matched the existing regex. Reporter @Zixim noted the bug persisted from v0.50.279 → v0.50.291 → v0.50.292 despite the previous "fix"; this lands the actual fix at the actual layer.
+
+  > **Note on the previous diagnosis:** the maintainer comment on #1618 asserting the fix had landed was based on `git show v0.50.291:static/style.css` confirming the CSS rule's presence — but a presence check on a rule is not a behavioral check that the rule does anything useful. Live-rendering YAML through `renderMd()` in the browser was the test that decided whether the maintainer reply or the user was correct. Apologies to @Zixim for the wrong call. Class of bug now documented in `webui-rendermd-pipeline` skill § Bug 10.
+
+### Tests
+
+4245 → **4254 passing** (+9 regression tests on `tests/test_issue1618_yaml_json_diff_newline_preserve.py`). 0 regressions. Full suite in ~115s.
+
+- **2 source-string tests** pin the regex shape (`<pre[^>]*>`) and structural integrity of the surrounding `_pre_stash` block.
+- **7 behavioral tests** drive the actual `static/ui.js` `renderMd()` via a node-driver and assert that YAML, JSON, diff, yml-alias (sanity), bash (sanity), mermaid (sanity), and a multi-line YAML scenario all preserve their `\n` characters in the rendered `<pre>` inner content. Six of these tests fail on master without the fix and pass with it — the sanity checks (yml/bash/mermaid) pass on both because their code paths emit bare `<pre>` or `<div class="mermaid-block">` and were never affected.
+- Plus widened the source-scan window in 3 pre-existing `tests/test_745_code_block_newlines.py` assertions from 400 to 1500 chars (the new comment block above the fixed regex pushed the regex past the previous scan window — `pytest-pitfalls` § D documents this exact pattern).
+
+### Pre-release verification
+
+- Self-built fix (nesquena-hermes), pending Opus advisor pre-merge pass and independent review APPROVED by nesquena.
+- **Verified the bug reproduces on master**: the 6 behavioral tests fail on `origin/master` (304a422) with the literal-`<pre>`-only regex, then pass after the one-character relax. The 3 sanity checks (yml/bash/mermaid) pass on both — confirming the fix doesn't break unaffected paths.
+- **Live browser render** confirms the rendered YAML now multi-lines correctly with `\n` characters in `<code>` textContent (was `'foo:  bar: 1  baz:    - 2    - 3'` pre-fix, now `'foo:\n  bar: 1\n  baz:\n    - 2\n    - 3'` post-fix).
+
 ## [v0.50.294] — 2026-05-04
 
 ### Fixed (3 PRs — streaming stability trio + models cache version stamp + session race + readonly fs guard — closes #1430, #1470, #1623, #1624, #1625, #1633)
diff --git a/docs/pr-media/issue-1618/after.png b/docs/pr-media/issue-1618/after.png
new file mode 100644
index 00000000..f6714777
Binary files /dev/null and b/docs/pr-media/issue-1618/after.png differ
diff --git a/docs/pr-media/issue-1618/before.png b/docs/pr-media/issue-1618/before.png
new file mode 100644
index 00000000..44ab5bd0
Binary files /dev/null and b/docs/pr-media/issue-1618/before.png differ
diff --git a/static/ui.js b/static/ui.js
index d4bed673..842a080a 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -1927,7 +1927,15 @@ function renderMd(raw){
   // with <br>. Token \x00E (next free after B D F G L M C O A).
   // Fixes #745: code blocks collapse to single line when not preceded by blank line.
   const _pre_stash=[];
-  s=s.replace(/(<div class="pre-header">[\s\S]*?<\/div>)?<pre>[\s\S]*?<\/pre>|<div class="(mermaid-block|katex-block)"[\s\S]*?<\/div>/g,m=>{
+  // #1463 / #1618: regex must match <pre> with ANY attributes — PR #484 added
+  // <pre class="tree-raw-view"> for JSON/YAML and <pre class="diff-block"> for
+  // diff/patch which the literal-<pre> shape missed. Newlines inside those
+  // blocks were falling through to the paragraph wrap below and getting
+  // converted to <br>, causing the YAML/JSON/diff collapse. PR #1516's CSS
+  // fix targeted the wrong layer (Prism token white-space) — by the time it
+  // ran, the \n had already been replaced. The CSS rule is kept as defense
+  // in depth.
+  s=s.replace(/(<div class="pre-header">[\s\S]*?<\/div>)?<pre[^>]*>[\s\S]*?<\/pre>|<div class="(mermaid-block|katex-block)"[\s\S]*?<\/div>/g,m=>{
     _pre_stash.push(m);
     return '\x00E'+(_pre_stash.length-1)+'\x00';
   });
diff --git a/tests/test_745_code_block_newlines.py b/tests/test_745_code_block_newlines.py
index 08a564b5..9482d40d 100644
--- a/tests/test_745_code_block_newlines.py
+++ b/tests/test_745_code_block_newlines.py
@@ -66,7 +66,7 @@ class TestCodeBlockNewlinePreservation:
         src = get_ui_js()
         # Find the replacement regex used to populate _pre_stash
         stash_block_idx = src.index('_pre_stash=[]')
-        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        stash_block = src[stash_block_idx:stash_block_idx + 1500]
         assert 'pre-header' in stash_block, \
             "pre-stash regex must match <div class=\"pre-header\"> wrappers"
 
@@ -74,7 +74,7 @@ class TestCodeBlockNewlinePreservation:
         """The stash regex must also cover mermaid-block divs."""
         src = get_ui_js()
         stash_block_idx = src.index('_pre_stash=[]')
-        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        stash_block = src[stash_block_idx:stash_block_idx + 1500]
         assert 'mermaid-block' in stash_block, \
             "pre-stash regex must cover mermaid-block divs"
 
@@ -82,7 +82,7 @@ class TestCodeBlockNewlinePreservation:
         """The stash regex must also cover katex-block divs."""
         src = get_ui_js()
         stash_block_idx = src.index('_pre_stash=[]')
-        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        stash_block = src[stash_block_idx:stash_block_idx + 1500]
         assert 'katex-block' in stash_block, \
             "pre-stash regex must cover katex-block divs"
 
diff --git a/tests/test_issue1618_yaml_json_diff_newline_preserve.py b/tests/test_issue1618_yaml_json_diff_newline_preserve.py
new file mode 100644
index 00000000..73c5db9f
--- /dev/null
+++ b/tests/test_issue1618_yaml_json_diff_newline_preserve.py
@@ -0,0 +1,322 @@
+"""Tests for issue #1618 / #1463 — YAML/JSON code blocks render flattened.
+
+Bug shape (live-verified in the browser May 04 2026):
+
+    ```yaml
+    foo:
+      bar: 1
+      baz:
+    ```
+
+renders as a single line `foo:  bar: 1  baz:` with no newlines, while:
+
+    ```yml
+    foo:
+      bar: 1
+      baz:
+    ```
+
+renders correctly multi-line. PR #1516 (v0.50.279) shipped a CSS-only fix
+targeting Prism token white-space; the rule is in `style.css` and reaches
+the browser, but the bug persists because the actual newline destruction
+happens earlier in the pipeline, before Prism runs.
+
+Root cause:
+  - PR #484 (v0.50.237, JSON/YAML tree-viewer) routes those two languages
+    through `<div class="code-tree-wrap">…<pre class="tree-raw-view">`
+    instead of bare `<pre>`.
+  - The `_pre_stash` regex at static/ui.js:1914 matched only literal `<pre>`
+    with NO attributes (`<pre>[\\s\\S]*?<\\/pre>`).
+  - `<pre class="tree-raw-view">` doesn't match → falls through to the
+    paragraph wrap pass which replaces `\\n` with `<br>`.
+  - By the time Prism runs and the CSS rule applies, the `\\n` characters
+    that the rule was meant to preserve are already gone.
+
+Same bug affects:
+  - `lang === 'yaml'` (issue #1463 / #1618 — the canonical case)
+  - `lang === 'json'` (same code path at static/ui.js:1621)
+  - `lang === 'diff'` / `lang === 'patch'` (`<pre class="diff-block">`,
+    same shape, same regex miss — emits at static/ui.js:1619)
+
+Fix: relax the `_pre_stash` regex to accept any attribute on `<pre>`:
+    `<pre>[\\s\\S]*?<\\/pre>`  →  `<pre[^>]*>[\\s\\S]*?<\\/pre>`
+
+These tests pin both the source-level invariant (regex shape) and the
+end-to-end behavior via a node-driver that exercises the actual
+static/ui.js renderMd() function.
+"""
+
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+UI_JS_PATH = REPO_ROOT / "static" / "ui.js"
+NODE = shutil.which("node")
+
+
+# ─────────────────────────────────────────────────────────────────────────
+# § A — Source-string invariants (run without node, fast)
+# ─────────────────────────────────────────────────────────────────────────
+
+
+def test_pre_stash_regex_matches_pre_with_attributes():
+    """static/ui.js _pre_stash regex must match <pre> with ANY attributes.
+
+    The narrow shape `<pre>[\\s\\S]*?<\\/pre>` (literal <pre> with no
+    attributes) misses every <pre class="..."> emitted by the JSON/YAML
+    tree-viewer pass and the diff/patch coloring pass — those blocks fall
+    through to paragraph wrap, which converts \\n to <br>.
+    """
+    src = UI_JS_PATH.read_text(encoding="utf-8")
+
+    # The fix introduces `<pre[^>]*>` (any attributes) in the _pre_stash regex.
+    # The exact regex line is documented in static/ui.js:1914.
+    assert "<pre[^>]*>[\\s\\S]*?<\\/pre>" in src, (
+        "_pre_stash regex must use <pre[^>]*> to match <pre> with any attributes "
+        "(#1463/#1618). The narrow shape <pre>[\\s\\S]*?<\\/pre> misses every "
+        "<pre class=\"tree-raw-view\"> from the JSON/YAML tree-viewer (PR #484) "
+        "and <pre class=\"diff-block\"> from diff/patch — newlines inside those "
+        "blocks fall through to paragraph wrap and become <br> tags."
+    )
+
+    # Defense against accidental regression: the literal-only shape must NOT
+    # be present anywhere in the _pre_stash region of the file.
+    pre_stash_idx = src.find("const _pre_stash=[]")
+    assert pre_stash_idx > 0, "_pre_stash declaration not found"
+    pre_stash_line = src[pre_stash_idx:pre_stash_idx + 1500]
+    assert "<pre>[\\s\\S]*?<\\/pre>" not in pre_stash_line, (
+        "_pre_stash regex must not contain the literal-<pre>-only shape — "
+        "use <pre[^>]*> to match attributes."
+    )
+
+
+def test_pre_stash_still_captures_pre_header_and_optional_div():
+    """The fix must keep the rest of the _pre_stash regex intact —
+    specifically the optional <div class="pre-header"> prefix and the
+    mermaid-block / katex-block alternation."""
+    src = UI_JS_PATH.read_text(encoding="utf-8")
+
+    pre_stash_idx = src.find("const _pre_stash=[]")
+    pre_stash_block = src[pre_stash_idx:pre_stash_idx + 1500]
+
+    assert '(<div class="pre-header">[\\s\\S]*?<\\/div>)?<pre[^>]*>' in pre_stash_block, (
+        "Optional <div class=\"pre-header\"> prefix must still precede the "
+        "<pre[^>]*> match"
+    )
+    assert '<div class="(mermaid-block|katex-block)"' in pre_stash_block, (
+        "Mermaid/katex block alternation must remain in the regex"
+    )
+
+
+# ─────────────────────────────────────────────────────────────────────────
+# § B — Behavioural tests via node-driver (skipped if node not on PATH)
+# ─────────────────────────────────────────────────────────────────────────
+
+pytestmark_node = pytest.mark.skipif(NODE is None, reason="node not on PATH")
+
+
+# Reuses the same driver shape as tests/test_renderer_js_behaviour.py.
+_DRIVER_SRC = r"""
+const fs = require('fs');
+const src = fs.readFileSync(process.argv[2], 'utf8');
+global.window = {};
+global.document = { createElement: () => ({ innerHTML: '', textContent: '' }) };
+const esc = s => String(s ?? '').replace(/[&<>"']/g, c => (
+  {'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;',"'":'&#39;'}[c]));
+const _IMAGE_EXTS=/\.(png|jpg|jpeg|gif|webp|bmp|ico|avif)$/i;
+const _SVG_EXTS=/\.svg$/i;
+const _AUDIO_EXTS=/\.(mp3|ogg|wav|m4a|aac|flac|wma|opus|webm)$/i;
+const _VIDEO_EXTS=/\.(mp4|webm|mkv|mov|avi|ogv|m4v)$/i;
+
+function extractFunc(name) {
+  const re = new RegExp('function\\s+' + name + '\\s*\\(');
+  const start = src.search(re);
+  if (start < 0) throw new Error(name + ' not found');
+  let i = src.indexOf('{', start);
+  let depth = 1; i++;
+  while (depth > 0 && i < src.length) {
+    if (src[i] === '{') depth++;
+    else if (src[i] === '}') depth--;
+    i++;
+  }
+  return src.slice(start, i);
+}
+eval(extractFunc('renderMd'));
+
+let buf = '';
+process.stdin.on('data', c => { buf += c; });
+process.stdin.on('end', () => { process.stdout.write(renderMd(buf)); });
+"""
+
+
+@pytest.fixture(scope="module")
+def driver_path(tmp_path_factory):
+    p = tmp_path_factory.mktemp("issue1618_driver") / "driver.js"
+    p.write_text(_DRIVER_SRC, encoding="utf-8")
+    return str(p)
+
+
+def _render(driver_path, markdown: str) -> str:
+    """Run renderMd against the actual ui.js and return the rendered HTML."""
+    result = subprocess.run(
+        [NODE, driver_path, str(UI_JS_PATH)],
+        input=markdown,
+        capture_output=True,
+        text=True,
+        timeout=10,
+    )
+    if result.returncode != 0:
+        raise RuntimeError(f"node driver failed: {result.stderr}")
+    return result.stdout
+
+
+def _extract_pre_inner(html: str) -> str:
+    """Extract the content of the first <pre ...>...</pre> block."""
+    import re
+    m = re.search(r"<pre[^>]*>([\s\S]*?)</pre>", html)
+    if not m:
+        return ""
+    return m.group(1)
+
+
+# ── The core regression: YAML newlines must survive ────────────────────
+
+
+@pytestmark_node
+def test_yaml_block_preserves_newlines(driver_path):
+    """YAML code blocks must render multi-line, not flatten to a single line.
+
+    This is the exact symptom Zixim reported on #1618: a YAML block renders
+    with all newlines collapsed to spaces. The fix is the relaxed _pre_stash
+    regex; without it, the block falls through to paragraph wrap and \\n
+    becomes <br> inside <code>, which Prism then can't recover from.
+    """
+    md = "```yaml\nfoo:\n  bar: 1\n  baz:\n    - 2\n    - 3\n```"
+    out = _render(driver_path, md)
+
+    # The block must end up wrapped in code-tree-wrap (PR #484's shape)
+    assert "code-tree-wrap" in out, (
+        "YAML blocks should still route through the tree-viewer wrapper"
+    )
+
+    # Inner <pre>...</pre> must contain literal \n characters (preserved
+    # newlines), NOT <br> tags.
+    pre_inner = _extract_pre_inner(out)
+    assert pre_inner, f"No <pre> block found in rendered output: {out!r}"
+    assert "\n" in pre_inner, (
+        f"YAML <pre> block lost its newlines (#1463/#1618).  "
+        f"<pre> inner content: {pre_inner!r}.  "
+        f"Likely cause: _pre_stash regex doesn't match <pre class=\"tree-raw-view\">, "
+        f"so the block falls through to the paragraph wrap pass which converts \\n to <br>."
+    )
+    assert "<br>" not in pre_inner, (
+        f"YAML <pre> block contains <br> tags — newlines were converted by paragraph "
+        f"wrap.  This means the _pre_stash regex did not capture the block.  "
+        f"<pre> inner content: {pre_inner!r}"
+    )
+
+
+@pytestmark_node
+def test_json_block_preserves_newlines(driver_path):
+    """JSON code blocks have the same shape as YAML (PR #484) and must also
+    preserve newlines."""
+    md = '```json\n{\n  "a": 1,\n  "b": [2, 3]\n}\n```'
+    out = _render(driver_path, md)
+
+    assert "code-tree-wrap" in out
+    pre_inner = _extract_pre_inner(out)
+    assert pre_inner
+    assert "\n" in pre_inner, (
+        f"JSON <pre> block lost newlines.  Inner: {pre_inner!r}"
+    )
+    assert "<br>" not in pre_inner
+
+
+@pytestmark_node
+def test_diff_block_preserves_newlines(driver_path):
+    """Diff/patch blocks emit <pre class=\"diff-block\"> (static/ui.js:1619).
+    Same regex-miss shape as YAML/JSON. Newlines must survive."""
+    md = "```diff\n-removed line\n+added line\n unchanged\n```"
+    out = _render(driver_path, md)
+
+    assert "diff-block" in out
+    pre_inner = _extract_pre_inner(out)
+    assert pre_inner
+    assert "\n" in pre_inner, (
+        f"Diff <pre> block lost newlines.  Inner: {pre_inner!r}"
+    )
+    assert "<br>" not in pre_inner
+
+
+@pytestmark_node
+def test_yml_alias_already_worked_still_works(driver_path):
+    """Sanity check: ` ```yml ` (the Prism alias) renders bare <pre> and
+    was never affected by the bug. This must continue to work after the
+    regex relaxation."""
+    md = "```yml\nfoo:\n  bar: 1\n```"
+    out = _render(driver_path, md)
+    pre_inner = _extract_pre_inner(out)
+    assert "\n" in pre_inner
+    assert "<br>" not in pre_inner
+
+
+@pytestmark_node
+def test_bash_block_unaffected_baseline(driver_path):
+    """Sanity: bash blocks emit bare <pre> and were never affected by the bug.
+    They must continue to render correctly post-fix."""
+    md = "```bash\necho one\necho two\n```"
+    out = _render(driver_path, md)
+    pre_inner = _extract_pre_inner(out)
+    assert "\n" in pre_inner
+    assert "<br>" not in pre_inner
+
+
+# ── End-to-end Zixim-scenario reproducer ───────────────────────────────
+
+
+@pytestmark_node
+def test_yaml_block_renders_multiline_html_shape(driver_path):
+    """The specific shape Zixim reported: 5-line YAML block must produce
+    exactly 5 newline-separated logical lines in the <pre> inner content.
+
+    Pre-fix this collapsed to a single space-joined string. Post-fix the
+    line count should equal the original input line count.
+    """
+    md = "```yaml\nname: hermes\nport: 8787\nfeatures:\n  - chat\n  - tasks\n```"
+    out = _render(driver_path, md)
+
+    pre_inner = _extract_pre_inner(out)
+    # Split on \n to count rendered lines. Empty trailing line tolerated.
+    rendered_lines = [l for l in pre_inner.split("\n") if l.strip()]
+
+    assert len(rendered_lines) == 5, (
+        f"YAML block should preserve 5 lines, got {len(rendered_lines)}: {rendered_lines}.  "
+        f"Full <pre> inner content: {pre_inner!r}"
+    )
+
+
+# ── Mermaid/katex blocks unaffected ────────────────────────────────────
+
+
+@pytestmark_node
+def test_mermaid_block_unaffected_by_regex_relaxation(driver_path):
+    """Mermaid blocks come through a different alternation in the same regex
+    (`<div class=\"(mermaid-block|katex-block)\"...`). Confirm they still get
+    captured into _pre_stash and aren't paragraph-wrapped."""
+    md = "```mermaid\ngraph TD\n  A --> B\n  B --> C\n```"
+    out = _render(driver_path, md)
+
+    # Mermaid block emits <div class="mermaid-block"> (no <pre>).
+    assert "mermaid-block" in out
+    # The mermaid div should not be wrapped in <p>...</p>.
+    assert "<p><div class=\"mermaid-block\"" not in out
+    # Internal newlines inside data-mermaid-id should not be relevant —
+    # mermaid content is in the data-attr / esc()'d innerText. But the
+    # surrounding paragraph-wrap-bypass MUST still work.
+    assert "<p>" not in out or out.find("<p>") > out.find("mermaid-block"), (
+        "Mermaid block should bypass paragraph wrap"
+    )