fix(desktop): apply the focus code recipe to fenced human prompts too

Quality review caught a real bug in the previous commit. The
`CodeBlockVariantContext` provider was mounted inside `MessageActivity`,
which only handles assistant items — `UserMessageBubble` returns before
it. A fence inside a human prompt therefore kept the legacy 16px muted
frame, nested inside the new 12px prompt bubble.

The recipe is a property of the *surface*, not of a role: a fenced block
in a prompt should read the same as one in a reply. The provider moves up
to `AgentSessionTranscriptList`, alongside the variant and turn-meta
providers, so every descendant markdown render inherits it. It still
renders no DOM, so `default`/`compactPreview` markup stays byte-identical
and the baseline fixture is unchanged.

Added a fenced-prompt test, which the existing fenced test could not
catch because it only rendered an assistant item. Proven non-vacuous two
ways: pinning the boundary provider to `default` fails both fenced tests,
and re-adding a role-scoped `default` provider around only the user
bubble's markdown fails the new prompt test alone.

Also renamed `channel-message code blocks are untouched by the focus
recipe` to `the default transcript variant keeps the legacy code chrome`.
It renders the `default` transcript variant, not a channel message row, so
the old name claimed a contract it did not prove.

AGENTS.md gains the `assert.equal(el, null)` note: that form serializes
the matched element's whole subtree to build a failure diff and OOMs the
runner instead of printing the message, which made a genuine failure
unreadable. Use `assert.ok(x === null)`.

Verified at this tree: full desktop suite 5428 passing / 0 failing,
`tsc --noEmit` clean, `biome check` at main's baseline, px-text /
pubkey-truncation / file-size clean. Screenshots captured light and dark
by temporarily pinning `transcriptVariant="conversation"` in
`AgentSessionThreadPanel`; that pin is dev-00's slice and is not in this
commit.

Signed-off-by: ss-dev-01 <11939edb7df583f855dbef923f2358f1184538f88ca452e19e7e35e42ad6d796@buzz.block.builderlab.xyz>
Co-authored-by: Bradley Axen <baxen@squareup.com>
Signed-off-by: Bradley Axen <baxen@squareup.com>

AGENTS.md

@@ -256,6 +256,24 @@ scripts build the required E2E bridge before running Playwright.
 
 See [TESTING.md](TESTING.md) for the full multi-agent E2E guide.
 
+### Never assert a DOM element against `null` with `assert.equal`
+
+In the desktop jsdom tests (`desktop/src/**/*.test.mjs`), write
+
+```js
+assert.ok(container.querySelector(sel) === null, "no header row");
+```
+
+**not** `assert.equal(container.querySelector(sel), null, ...)`. Both pass
+identically, but when the `assert.equal` form *fails*, node serializes the
+matched element and its entire subtree to build a diff. On a real transcript
+that exhausts memory and the runner dies with SIGKILL after ~100s instead of
+printing the assertion message — so a genuine regression is unreadable and
+looks like a hang or an OOM in unrelated code. The `assert.ok(x === null)` form
+fails in milliseconds with the message you wrote.
+
+Comparing `getAttribute(...)` to `null` is fine — attributes are strings.
+
 ### PR Screenshots
 
 > **Do NOT use `buzz upload`, the relay media endpoint, or any third-party

desktop/src/features/agents/ui/AgentSessionTranscriptList.conversation.test.mjs

@@ -435,6 +435,26 @@ function fencedCodeItems() {
   ];
 }
 
+/** One *human prompt* whose body is a fenced code block. */
+function fencedCodePromptItems() {
+  return [
+    {
+      channelId: "chan-1",
+      sessionId: "sess-1",
+      turnId: "turn-1",
+      id: "msg:user",
+      type: "message",
+      renderClass: "message",
+      role: "user",
+      title: TRIGGER_TITLE,
+      text: "fix this\n\n```ts\nconst a = 1;\nconst b = 2;\n```\n",
+      timestamp: "2026-06-14T19:00:00.000Z",
+      messageId: "event-1",
+      authorPubkey: AUTHOR,
+    },
+  ];
+}
+
 /**
  * Same mount, but the caller can swap the list props afterwards. Needed for the
  * contracts that are only visible across a rerender: a streaming thought
@@ -619,11 +639,42 @@ test("conversation frames fenced code with berd's header row", async () => {
   assert.equal(code.querySelectorAll("[data-line]").length, 2);
 });
 
-test("channel-message code blocks are untouched by the focus recipe", async () => {
+test("conversation applies the code recipe to a fenced human prompt too", async () => {
+  // Regression guard for a real bug quality caught. The provider was first
+  // mounted inside `MessageActivity`, which only handles assistant items — the
+  // user bubble returns before it, so a fence inside a prompt kept the legacy
+  // 16px muted frame nested inside the new 12px bubble. The recipe is a
+  // property of the *surface*, not of a role, so the provider now sits at the
+  // transcript boundary and both roles inherit it.
+  const { container } = await renderTranscriptWithCodeChrome("conversation", {
+    items: fencedCodePromptItems(),
+  });
+  const bubble = container.querySelector(
+    '[data-testid="transcript-user-message"]',
+  );
+  assert.ok(bubble, "the prompt should render");
+  assert.ok(
+    bubble.querySelector('[data-testid="markdown-code-block-header"]'),
+    "a fence inside the prompt gets berd's header row",
+  );
+  const frame = bubble.querySelector("pre");
+  assert.match(frame.className, /rounded-lg/);
+  assert.doesNotMatch(
+    frame.className,
+    /rounded-2xl/,
+    "the legacy 16px frame must not nest inside the 12px bubble",
+  );
+  assert.doesNotMatch(frame.className, /pr-12/);
+});
+
+test("the default transcript variant keeps the legacy code chrome", async () => {
   // The markdown renderer is shared with channel messages, so `focusProse` is
-  // opt-in per surface. Rendering the same fenced block through `default` must
-  // still produce the original chrome: no header row, 16px radius, muted fill,
-  // and the absolutely-positioned copy button.
+  // opt-in per surface. Rendering the same fenced block through the `default`
+  // transcript variant must still produce the original chrome: no header row,
+  // 16px radius, muted fill, and the absolutely-positioned copy button.
+  //
+  // This proves the *variant gate*, not the channel-message row itself — those
+  // rows are covered by the markdown tests in `shared/ui/markdown`.
   const { container } = await renderTranscriptWithCodeChrome("default", {
     items: fencedCodeItems(),
   });

desktop/src/features/agents/ui/AgentSessionTranscriptList.tsx

@@ -16,6 +16,7 @@ import { useStableArrayShallow } from "@/shared/hooks/useStableReference";
 import { cn } from "@/shared/lib/cn";
 import { AnimatedCount } from "@/shared/ui/AnimatedCount";
 import { FuzzyLogo } from "@/shared/ui/buzz-logo/FuzzyLogo";
+import { CodeBlockVariantContext } from "@/shared/ui/markdown/CodeBlock";
 import type { TranscriptItem } from "./agentSessionTypes";
 import { TurnLivenessIndicator } from "./TurnLivenessIndicator";
 import {
@@ -229,41 +230,53 @@ export function AgentSessionTranscriptList({
         ref={autoTail ? contentRef : undefined}
         role="log"
       >
-        <AgentSessionTranscriptVariantProvider value={variant}>
-          <AgentSessionTranscriptTurnMetaProvider value={turnMeta}>
-            {displayBlocks.map((block) => {
-              const blockKey = getDisplayBlockKey(block);
-              return (
-                <motion.div
-                  animate={ROW_ENTER_TO}
-                  data-message-id={blockKey}
-                  initial={
-                    animationsDisabled || !hasCompletedInitialRenderRef.current
-                      ? false
-                      : ROW_ENTER_FROM
-                  }
-                  key={blockKey}
-                  layout={layoutAnimationsEnabled ? "position" : false}
-                  transition={ROW_ENTER_SPRING}
-                >
-                  {/* content-visibility stays on a non-animated child: motion
+        {/* The berd code-block recipe is a property of the *surface*, not of a
+            role: a fenced block in a human prompt must get the same chrome as
+            one in an agent reply. Provided at the transcript boundary so every
+            descendant markdown render inherits it. A context provider emits no
+            DOM, so `default`/`compactPreview` markup is unaffected. */}
+        <CodeBlockVariantContext.Provider
+          value={isConversation ? "focusProse" : "default"}
+        >
+          <AgentSessionTranscriptVariantProvider value={variant}>
+            <AgentSessionTranscriptTurnMetaProvider value={turnMeta}>
+              {displayBlocks.map((block) => {
+                const blockKey = getDisplayBlockKey(block);
+                return (
+                  <motion.div
+                    animate={ROW_ENTER_TO}
+                    data-message-id={blockKey}
+                    initial={
+                      animationsDisabled ||
+                      !hasCompletedInitialRenderRef.current
+                        ? false
+                        : ROW_ENTER_FROM
+                    }
+                    key={blockKey}
+                    layout={layoutAnimationsEnabled ? "position" : false}
+                    transition={ROW_ENTER_SPRING}
+                  >
+                    {/* content-visibility stays on a non-animated child: motion
                     measures the outer wrapper for layout animations, which
                     would otherwise force skipped offscreen rows to render. */}
-                  <div className="content-visibility-auto">
-                    <TranscriptDisplayBlockView
-                      agentAvatarUrl={agentAvatarUrl}
-                      agentName={agentName}
-                      agentPubkey={agentPubkey}
-                      block={block}
-                      profiles={profiles}
-                    />
-                  </div>
-                </motion.div>
-              );
-            })}
-            {isTurnLive && !isCompactPreview ? <TurnLivenessIndicator /> : null}
-          </AgentSessionTranscriptTurnMetaProvider>
-        </AgentSessionTranscriptVariantProvider>
+                    <div className="content-visibility-auto">
+                      <TranscriptDisplayBlockView
+                        agentAvatarUrl={agentAvatarUrl}
+                        agentName={agentName}
+                        agentPubkey={agentPubkey}
+                        block={block}
+                        profiles={profiles}
+                      />
+                    </div>
+                  </motion.div>
+                );
+              })}
+              {isTurnLive && !isCompactPreview ? (
+                <TurnLivenessIndicator />
+              ) : null}
+            </AgentSessionTranscriptTurnMetaProvider>
+          </AgentSessionTranscriptVariantProvider>
+        </CodeBlockVariantContext.Provider>
       </div>
     </motion.div>
   );

desktop/src/features/agents/ui/activityRenderClasses/MessageActivity.tsx

@@ -1,6 +1,5 @@
 import type { UserProfileLookup } from "@/features/profile/lib/identity";
 import { Markdown } from "@/shared/ui/markdown";
-import { CodeBlockVariantContext } from "@/shared/ui/markdown/CodeBlock";
 import { UserAvatar } from "@/shared/ui/UserAvatar";
 import { useAgentSessionTranscriptVariant } from "../agentSessionTranscriptContext";
 import { formatTranscriptTimestampTitle } from "../agentSessionUtils";
@@ -99,25 +98,19 @@ function MessageItem({
           }
           title={formatTranscriptTimestampTitle(item.timestamp)}
         >
-          {/* A context provider renders no DOM, so wrapping unconditionally
-              keeps `default`/`compactPreview` markup byte-identical. */}
-          <CodeBlockVariantContext.Provider
-            value={isConversation ? "focusProse" : "default"}
-          >
-            <Markdown
-              className={
-                isCompactPreview
-                  ? "text-xs leading-4"
-                  : isConversation
-                    ? // Focus mode reads as prose: no box, comfortable line
-                      // height, and full-fidelity markdown/code from the shared
-                      // renderer.
-                      "leading-relaxed"
-                    : "leading-5"
-              }
-              content={text || " "}
-            />
-          </CodeBlockVariantContext.Provider>
+          <Markdown
+            className={
+              isCompactPreview
+                ? "text-xs leading-4"
+                : isConversation
+                  ? // Focus mode reads as prose: no box, comfortable line
+                    // height, and full-fidelity markdown/code from the shared
+                    // renderer.
+                    "leading-relaxed"
+                  : "leading-5"
+            }
+            content={text || " "}
+          />
         </div>
       </div>
     </div>

desktop/src/shared/ui/markdown/CodeBlock.tsx

@@ -28,7 +28,10 @@ import { getReactNodeText } from "./utils";
  * border, and no shadow.
  *
  * It is opt-in because this renderer is shared with channel messages —
- * restyling the default would change every code block in the app.
+ * restyling the default would change every code block in the app. The variant
+ * is a property of the *surface*, not of a role: `AgentSessionTranscriptList`
+ * provides it once at the transcript boundary, so a fence in a human prompt
+ * gets the same chrome as one in an agent reply.
  *
  * Delivered through context rather than a `MarkdownCodeBlock` prop for the same
  * reason as `VideoReviewMarkdownContext`: the component map handed to