From 46c69bc35ccbc89f48fb2a64784d12025d7a9102 Mon Sep 17 00:00:00 2001
From: Luke Parker <10430890+Hona@users.noreply.github.com>
Date: Thu, 8 Oct 2026 08:24:05 +1000
Subject: [PATCH] fix(session-ui): show latest thinking summary heading while
 streaming (#53815)

---
 .../src/timeline/projection.test.ts           | 41 +++++++++++++++-
 .../session-ui/src/timeline/projection.ts     | 49 ++++++++-----------
 2 files changed, 61 insertions(+), 29 deletions(-)

diff --git a/packages/session-ui/src/timeline/projection.test.ts b/packages/session-ui/src/timeline/projection.test.ts
index 9d7e8248c4e..d33515443e8 100644
--- a/packages/session-ui/src/timeline/projection.test.ts
+++ b/packages/session-ui/src/timeline/projection.test.ts
@@ -6,7 +6,14 @@ import type {
   SessionMessageInfo,
 } from "@opencode/client/promise"
 import { createStore } from "solid-js/store"
-import { createTimelineProjection, reuseTimelineRows, Timeline, TimelineRow, type PartGroup } from "./projection"
+import {
+  createTimelineProjection,
+  reasoningHeading,
+  reuseTimelineRows,
+  Timeline,
+  TimelineRow,
+  type PartGroup,
+} from "./projection"
 
 const context = (key: string, partIDs: string[], identity: { userMessageID?: string; messageID?: string } = {}) =>
   new TimelineRow.AssistantPart({
@@ -325,3 +332,35 @@ describe("createTimelineProjection", () => {
     ])
   })
 })
+
+describe("reasoningHeading", () => {
+  test("returns the most recent thinking summary heading as sections stream in", () => {
+    const sections = [
+      "**Designing Interactive Experience**\nI'm focusing on crafting a fun, tactile, and interactive visual toy.",
+      "**Analyzing Reading Recommendations**\nI'm currently dissecting whether Luke Parker should read the *Throne of Glass* series.",
+      "**Constructing Reader Tools**\nI'm now building out interactive tools.",
+    ]
+
+    expect(reasoningHeading(sections[0])).toBe("Designing Interactive Experience")
+    expect(reasoningHeading(sections.slice(0, 2).join("\n\n"))).toBe("Analyzing Reading Recommendations")
+    expect(reasoningHeading(sections.join("\n\n"))).toBe("Constructing Reader Tools")
+  })
+
+  test("keeps the previous heading while the next bold heading is still streaming", () => {
+    const text = [
+      "**Designing Interactive Experience**\nI'm focusing on crafting a fun, tactile, and interactive visual toy.",
+      "**Analyzing Reading",
+    ].join("\n\n")
+
+    expect(reasoningHeading(text)).toBe("Designing Interactive Experience")
+  })
+
+  test("returns the latest heading across markdown heading styles and ignores inline bold spans", () => {
+    expect(reasoningHeading("## First step\n\nDetails.\n\n## Second step\n\nMore details.")).toBe("Second step")
+    expect(reasoningHeading("<h2>First step</h2>\n<p>Body</p>\n<h3>Second step</h3>")).toBe("Second step")
+    expect(reasoningHeading("First step\n===\n\nSecond step\n---")).toBe("Second step")
+    expect(
+      reasoningHeading("**Designing Interactive Experience**\n\n**Note** that this line ends with **bold**"),
+    ).toBe("Designing Interactive Experience")
+  })
+})
diff --git a/packages/session-ui/src/timeline/projection.ts b/packages/session-ui/src/timeline/projection.ts
index 2cfdecc9915..e2e2dfad8fd 100644
--- a/packages/session-ui/src/timeline/projection.ts
+++ b/packages/session-ui/src/timeline/projection.ts
@@ -869,39 +869,32 @@ function toolGroupType(
 
 export function reasoningHeading(text: string): string | undefined {
   const markdown = text.replace(/\r\n?/g, "\n")
-  const html = markdown.match(/<h[1-6][^>]*>([\s\S]*?)<\/h[1-6]>/i)
+  const html = lastHeading(markdown, /<h[1-6][^>]*>([\s\S]*?)<\/h[1-6]>/gi, (value) => value.replace(/<[^>]+>/g, " "))
+  const atx = lastHeading(markdown, /^\s{0,3}#{1,6}[ \t]+(.+?)(?:[ \t]+#+[ \t]*)?$/gm)
+  const setext = lastHeading(markdown, /^([^\n]+)\n(?:=+|-+)\s*$/gm)
+  const strong = lastHeading(markdown, /^\s*(?:\*\*((?:(?!\*\*).)+)\*\*|__((?:(?!__).)+)__)\s*$/gm)
 
-  if (html?.[1]) {
-    const value = cleanHeading(html[1].replace(/<[^>]+>/g, " "))
+  const latest = [html, atx, strong].reduce<HeadingMatch | undefined>(
+    (best, current) => (!best || (current && current.index >= best.index) ? current : best),
+    undefined,
+  )
 
-    if (value) return value
-  }
+  return (latest ?? setext)?.value
+}
 
-  const atx = markdown.match(/^\s{0,3}#{1,6}[ \t]+(.+?)(?:[ \t]+#+[ \t]*)?$/m)
+type HeadingMatch = { index: number; value: string }
 
-  if (atx?.[1]) {
-    const value = cleanHeading(atx[1])
+function lastHeading(
+  markdown: string,
+  pattern: RegExp,
+  transform?: (value: string) => string,
+): HeadingMatch | undefined {
+  return Array.from(markdown.matchAll(pattern)).reduce<HeadingMatch | undefined>((best, match) => {
+    const raw = match[1] ?? match[2] ?? ""
+    const value = cleanHeading(transform ? transform(raw) : raw)
 
-    if (value) return value
-  }
-
-  const setext = markdown.match(/^([^\n]+)\n(?:=+|-+)\s*$/m)
-
-  if (setext?.[1]) {
-    const value = cleanHeading(setext[1])
-
-    if (value) return value
-  }
-
-  const strong = markdown.match(/^\s*(?:\*\*|__)(.+?)(?:\*\*|__)\s*$/m)
-
-  if (strong?.[1]) {
-    const value = cleanHeading(strong[1])
-
-    if (value) return value
-  }
-
-  return undefined
+    return value ? { index: match.index, value } : best
+  }, undefined)
 }
 
 function cleanHeading(value: string) {