diff --git a/examples/desktop/docs/qa-step-card-merge/01-fused-collapsed.png b/examples/desktop/docs/qa-step-card-merge/01-fused-collapsed.png
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index 0000000000..5c26950290
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diff --git a/examples/desktop/docs/qa-step-card-merge/02-fused-expanded.png b/examples/desktop/docs/qa-step-card-merge/02-fused-expanded.png
new file mode 100644
index 0000000000..aff38b4a90
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diff --git a/examples/desktop/docs/qa-step-card-merge/03-fused-edit-rerun.png b/examples/desktop/docs/qa-step-card-merge/03-fused-edit-rerun.png
new file mode 100644
index 0000000000..d20d45a69f
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diff --git a/examples/desktop/scripts/qa-cdp-shoot-step-card-merge.mjs b/examples/desktop/scripts/qa-cdp-shoot-step-card-merge.mjs
new file mode 100644
index 0000000000..32e86a9993
--- /dev/null
+++ b/examples/desktop/scripts/qa-cdp-shoot-step-card-merge.mjs
@@ -0,0 +1,258 @@
+// scripts/qa-cdp-shoot-step-card-merge.mjs — feat/step-card-merge shoot.
+//
+// Boots an isolated Electron on CDP :9522 with its own --user-data-dir
+// and $DSH_DESKTOP_HOME so the user's live desktop demo (pid 7810/7816
+// on ~/.dsh-desktop) is never touched. Seeds a turn with thinking +
+// multiple tool calls, captures three screenshots:
+//
+//   01-fused-collapsed.png — chat flow, fused step/tool cards collapsed
+//   02-fused-expanded.png  — first fused card opened, showing args +
+//                            result + inputs/outputs/events panes + the
+//                            edit-and-re-run trigger revealed on open
+//   03-fused-edit-rerun.png — edit-and-re-run panel opened inside the
+//                            expanded fused card (textarea visible)
+//
+// Isolation follows scripts/qa-cdp-shoot-context-topright.mjs precedent.
+
+import { spawn } from 'node:child_process'
+import { existsSync, mkdirSync, writeFileSync, rmSync } from 'node:fs'
+import { resolve, join } from 'node:path'
+import { setTimeout as sleep } from 'node:timers/promises'
+import { tmpdir } from 'node:os'
+
+const WORKTREE = resolve(process.env.DSH_WORKTREE || process.cwd())
+const PARENT = resolve(process.env.DSH_REPO || '/Users/ziya/harness/dsh-desktop-demo')
+const ELECTRON = join(PARENT, 'node_modules/.bin/electron')
+const CDP_PORT = Number(process.env.DSH_STEP_CARD_MERGE_PORT || 9522)
+const USER_DATA = join(tmpdir(), 'dsh-step-card-merge-userdata')
+const DSH_HOME = join(tmpdir(), 'dsh-step-card-merge-home')
+const OUTDIR = join(WORKTREE, 'docs/qa-step-card-merge')
+
+if (!existsSync(ELECTRON)) {
+  console.error(`electron binary not found at ${ELECTRON}`)
+  process.exit(2)
+}
+mkdirSync(OUTDIR, { recursive: true })
+for (const dir of [USER_DATA, DSH_HOME]) {
+  try { rmSync(dir, { recursive: true, force: true }) } catch {}
+  mkdirSync(dir, { recursive: true })
+}
+writeFileSync(join(DSH_HOME, 'config.json'), JSON.stringify({
+  role: 'coding', approvalMode: 'never',
+}))
+writeFileSync(join(DSH_HOME, '.onboarded'), new Date().toISOString())
+
+async function bootElectron() {
+  const child = spawn(ELECTRON, [
+    `--remote-debugging-port=${CDP_PORT}`,
+    `--user-data-dir=${USER_DATA}`,
+    '--disable-gpu',
+    '--no-sandbox',
+    '.',
+  ], {
+    cwd: WORKTREE,
+    env: {
+      ...process.env,
+      DSH_DESKTOP_HOME: DSH_HOME,
+      DSH_MAXIMIZE: '1',
+      // Deliberately DO NOT set DSH_QA=1 — the QA fixture bootstrap seeds
+      // its own bench/session content which overlays our seeded turns and
+      // scrolls the fused cards off-screen. We drive the renderer seam
+      // directly instead of relying on the fixture path.
+    },
+    stdio: ['ignore', 'pipe', 'pipe'],
+  })
+  const logs = []
+  child.stdout.on('data', d => logs.push(String(d)))
+  child.stderr.on('data', d => logs.push(String(d)))
+  for (let i = 0; i < 40; i++) {
+    await sleep(500)
+    try {
+      const r = await fetch(`http://localhost:${CDP_PORT}/json/list`)
+      if (r.ok) return { child, logs }
+    } catch {}
+  }
+  child.kill('SIGKILL')
+  console.error('electron CDP did not come up. logs:\n' + logs.join(''))
+  process.exit(3)
+}
+
+async function newCdp() {
+  const targets = await (await fetch(`http://localhost:${CDP_PORT}/json/list`)).json()
+  const target = targets.find(t => t.type === 'page')
+  if (!target) throw new Error('no page target on port ' + CDP_PORT)
+  const ws = new WebSocket(target.webSocketDebuggerUrl)
+  await new Promise((ok, err) => { ws.onopen = ok; ws.onerror = e => err(e) })
+  let id = 1
+  const pending = new Map()
+  ws.onmessage = ev => {
+    const msg = JSON.parse(typeof ev.data === 'string' ? ev.data : String(ev.data))
+    if (msg.id != null && pending.has(msg.id)) {
+      const [ok, err] = pending.get(msg.id); pending.delete(msg.id)
+      if (msg.error) err(new Error(msg.error.message)); else ok(msg.result)
+    }
+  }
+  const call = (m, p = {}, ms = 15000) => new Promise((ok, err) => {
+    const _id = id++
+    const t = setTimeout(() => { pending.delete(_id); err(new Error('cdp timeout: ' + m)) }, ms)
+    pending.set(_id, [v => { clearTimeout(t); ok(v) }, e => { clearTimeout(t); err(e) }])
+    ws.send(JSON.stringify({ id: _id, method: m, params: p }))
+  })
+  const evj = async expr => {
+    const r = await call('Runtime.evaluate', {
+      expression: `(async()=>{try{return (${expr})}catch(e){return {__err:String(e)}}})()`,
+      returnByValue: true, awaitPromise: true,
+    })
+    if (r.exceptionDetails) throw new Error(r.exceptionDetails.exception?.description || r.exceptionDetails.text)
+    return r.result?.value
+  }
+  return { ws, call, evj }
+}
+
+// Seed: user prompt → 2 turns. First turn has one thinking chunk and a
+// single tool call (read). Second turn has thinking + 3 parallel tool
+// calls (read, grep, bash) so we can observe the "<first> +N" summary
+// on the multi-call fused card.
+const SEED = `(async () => {
+  const R = window.__dshRenderer
+  if (!R) return { __err: 'renderer seam missing' }
+  const sid = 'step-merge-' + Date.now()
+  R.ensureSession(sid, { title: 'step-card-merge demo', header: { model: 'deepseek-r1' } })
+  await R.selectSession(sid)
+  const emit = (ev) => R.onSessionEvent(sid, ev)
+  const t0 = Date.now()
+  let seq = 1
+  const at = (o) => t0 + o
+  // Turn 1: single-tool step
+  emit({ type: 'user/message', seq: seq++, time: at(0),
+         data: { content: [{ type: 'text', text: 'Read the design doc header for me.' }] } })
+  emit({ type: 'turn/start', seq: seq++, time: at(5), data: { turnId: 't0', model: 'deepseek-r1' } })
+  emit({ type: 'step/start', seq: seq++, time: at(10), data: { turn: 0, step: 0 } })
+  emit({ type: 'assistant/message', seq: seq++, time: at(20), data: {
+    content: [{ type: 'text', text: 'thinking: open the design doc' }],
+    usage: { inputTokens: 210, outputTokens: 8, cacheReadTokens: 1024 },
+  } })
+  emit({ type: 'tool/call', seq: seq++, time: at(60),
+         data: { callId: 'c-1', name: 'read', arguments: JSON.stringify({ path: 'docs/DESIGN.md', limit: 40 }) } })
+  emit({ type: 'tool/result', seq: seq++, time: at(140),
+         data: { callId: 'c-1', content: [{ type: 'text', text: '# design doc\\n\\n(header excerpt)' }] } })
+  emit({ type: 'step/end', seq: seq++, time: at(150), data: {} })
+  emit({ type: 'turn/end', seq: seq++, time: at(160), data: { turnId: 't0', reason: 'completed',
+    usage: { total_tokens: 240 }, durationMs: 160 } })
+  // Turn 2: multi-tool step
+  emit({ type: 'user/message', seq: seq++, time: at(200),
+         data: { content: [{ type: 'text', text: 'now grep for TODOs and list changed files.' }] } })
+  emit({ type: 'turn/start', seq: seq++, time: at(210), data: { turnId: 't1', model: 'deepseek-r1' } })
+  emit({ type: 'step/start', seq: seq++, time: at(215), data: { turn: 1, step: 0 } })
+  emit({ type: 'assistant/message', seq: seq++, time: at(220), data: {
+    content: [{ type: 'text', text: 'thinking: run three tools in parallel' }],
+    usage: { inputTokens: 320, outputTokens: 14, cacheReadTokens: 2048 },
+  } })
+  emit({ type: 'tool/call', seq: seq++, time: at(230),
+         data: { callId: 'c-2', name: 'read', arguments: JSON.stringify({ path: 'src/renderer/renderer.js', limit: 20 }) } })
+  emit({ type: 'tool/call', seq: seq++, time: at(232),
+         data: { callId: 'c-3', name: 'grep', arguments: JSON.stringify({ pattern: 'TODO', path: 'src' }) } })
+  emit({ type: 'tool/call', seq: seq++, time: at(234),
+         data: { callId: 'c-4', name: 'bash', arguments: JSON.stringify({ command: 'git status -s' }) } })
+  emit({ type: 'tool/result', seq: seq++, time: at(310),
+         data: { callId: 'c-2', content: [{ type: 'text', text: '// top of renderer.js' }] } })
+  emit({ type: 'tool/result', seq: seq++, time: at(320),
+         data: { callId: 'c-3', content: [{ type: 'text', text: 'src/renderer/foo.js:42:  // TODO polish' }] } })
+  emit({ type: 'tool/result', seq: seq++, time: at(330),
+         data: { callId: 'c-4', content: [{ type: 'text', text: ' M src/renderer/style.css' }] } })
+  emit({ type: 'step/end', seq: seq++, time: at(340), data: {} })
+  emit({ type: 'turn/end', seq: seq++, time: at(350), data: { turnId: 't1', reason: 'completed',
+    usage: { total_tokens: 512 }, durationMs: 150 } })
+  return { sid, count: seq - 1 }
+})()`
+
+async function shoot(cdp, name) {
+  const shot = await cdp.call('Page.captureScreenshot', { format: 'png', fromSurface: false })
+  const buf = Buffer.from(shot.data, 'base64')
+  writeFileSync(join(OUTDIR, name), buf)
+  console.log(' shot', name, buf.length, 'bytes')
+}
+
+async function main() {
+  const { child } = await bootElectron()
+  try {
+    await sleep(1500)
+    const cdp = await newCdp()
+    await cdp.call('Page.enable')
+    await cdp.call('Emulation.setDeviceMetricsOverride', {
+      width: 1400, height: 1000, deviceScaleFactor: 2, mobile: false,
+    })
+    for (let i = 0; i < 20; i++) {
+      const ready = await cdp.evj(`!!(window.__dshRenderer && window.__dshRenderer.onSessionEvent)`)
+      if (ready) break
+      await sleep(250)
+    }
+    const seedRes = await cdp.evj(SEED)
+    console.log('seed:', JSON.stringify(seedRes))
+    await sleep(800)
+
+    // Verify fusion DOM contract before shooting so we don't waste a
+    // screenshot on a broken build.
+    const contract = await cdp.evj(`(() => {
+      const fused = document.querySelectorAll('.tool-block.trace-card-fused')
+      const standalone = document.querySelectorAll('.trace-card:not(.trace-card-fused)')
+      const drawers = document.querySelectorAll('.turn-trace-drawer')
+      const multi = document.querySelector('.tool-block.trace-card-fused[data-step-index="0"][data-step-turn="1"]')
+      const multiName = multi ? multi.querySelector('.tool-family-name').textContent : null
+      return {
+        fusedCount: fused.length,
+        standaloneTraceCount: standalone.length,
+        drawerCount: drawers.length,
+        multiCallSummary: multiName,
+      }
+    })()`)
+    console.log('contract:', JSON.stringify(contract))
+
+    // Make sure we're on the Chat tab so the seeded turns are on-screen,
+    // then scroll to the top of the stream so shot 1 captures the whole
+    // "collapsed fused" flow (thinking bubbles + tool row + turn footer).
+    await cdp.evj(`window.__dshTabs && window.__dshTabs.switchTo && window.__dshTabs.switchTo('chat')`)
+    await sleep(300)
+    await cdp.evj(`(() => {
+      const first = document.querySelector('.tool-block.trace-card-fused')
+      if (first && first.scrollIntoView) first.scrollIntoView({ block: 'center' })
+    })()`)
+    await sleep(300)
+
+    // Shot 1: collapsed
+    await shoot(cdp, '01-fused-collapsed.png')
+
+    // Open the first fused card + shoot expanded (args, result, panes + edit&re-run trigger visible)
+    await cdp.evj(`(() => {
+      const c = document.querySelector('.tool-block.trace-card-fused')
+      if (c && c.tagName === 'DETAILS') c.open = true
+      c && c.scrollIntoView({ block: 'center' })
+    })()`)
+    await sleep(400)
+    await shoot(cdp, '02-fused-expanded.png')
+
+    // Open the edit-and-re-run panel inside the same expanded fused card.
+    const editState = await cdp.evj(`(() => {
+      const c = document.querySelector('.tool-block.trace-card-fused')
+      if (!c) return { __err: 'no fused card' }
+      const trigger = c.querySelector('.tool-edit-rerun-trigger')
+      if (!trigger) return { __err: 'no edit trigger inside fused card' }
+      trigger.click()
+      const panel = c.querySelector('.tool-edit-rerun-panel')
+      return { visible: !!(panel && !panel.hidden), triggerText: trigger.textContent }
+    })()`)
+    console.log('edit-rerun:', JSON.stringify(editState))
+    // Scroll the whole fused card into view so the shot captures the
+    // summary + args/result + edit panel, not just the panel textarea.
+    await cdp.evj(`(() => {
+      const c = document.querySelector('.tool-block.trace-card-fused')
+      if (c && c.scrollIntoView) c.scrollIntoView({ block: 'start' })
+    })()`)
+    await sleep(300)
+    await shoot(cdp, '03-fused-edit-rerun.png')
+  } finally {
+    try { child.kill('SIGKILL') } catch {}
+  }
+}
+
+main().catch(err => { console.error(err); process.exit(1) })
diff --git a/examples/desktop/src/renderer/renderer.js b/examples/desktop/src/renderer/renderer.js
index 127e660f72..da08e54a43 100644
--- a/examples/desktop/src/renderer/renderer.js
+++ b/examples/desktop/src/renderer/renderer.js
@@ -1444,8 +1444,21 @@ function finishTurnContainer(sessionId, { footerSpec, traceCard, traceSummaryTex
   const hasSignal = tf && typeof tf.specHasAnySignal === 'function'
     ? (!!tf.specHasAnySignal(rawSpec) || !!tf.specHasAnySignal(footerSpec))
     : !!footerSpec
-  const hasTraceCard = !!(traceCard && traceCard.parentNode)
-  if (!hasSignal && !hasTraceCard) {
+  // Fusion: a fused step card is already positioned inline at the tool
+  // call's narrative slot. Lifting it into the footer drawer would tear
+  // it out of the stream — treat it as "no drawer needed here". The
+  // turn-flow glyph already scrolls to it via `dsh-open-turn-trace`
+  // (see the listener below).
+  const isFusedCard = !!(traceCard && traceCard.dataset && traceCard.dataset.stepFused === '1')
+  const hasTraceCard = !isFusedCard && !!(traceCard && traceCard.parentNode)
+  // Fused card path: the trailing standalone .trace-card is suppressed
+  // (the card now lives inline at the tool call's position), so we can't
+  // rely on `hasTraceCard` alone. Keep the footer up when the turn has
+  // at least one fused step so the "turn ended" chip row + flow glyph +
+  // click-to-scroll behavior remain reachable.
+  const hasFusedStep = Array.isArray(turnSteps) && turnSteps.some(
+    s => s && Array.isArray(s._toolBlocks) && s._toolBlocks.length > 0)
+  if (!hasSignal && !hasTraceCard && !hasFusedStep) {
     // Nothing to draw — leave the assistant-turn container clean.
     ct.section.dataset.turnStatus = 'sealed'
     state.currentTurn = null
@@ -1493,7 +1506,7 @@ function finishTurnContainer(sessionId, { footerSpec, traceCard, traceSummaryTex
     })
   }
   let drawer = null
-  if (traceCard && traceCard.parentNode) {
+  if (!isFusedCard && traceCard && traceCard.parentNode) {
     // Lift the trace card out of `streamEl` into a details drawer inside
     // the footer. Preserves whatever internal state the card already has
     // (chunk fold, per-line payload expander). When the tri-view module
@@ -1556,6 +1569,28 @@ function finishTurnContainer(sessionId, { footerSpec, traceCard, traceSummaryTex
         try { drawer.scrollIntoView({ block: 'nearest' }) } catch (_) { /* jsdom */ }
       }
     })
+  } else {
+    // Fused-only turn: no trailing drawer was built (the step card lives
+    // in-stream at its tool-call position). The glyph still fires, so
+    // fall back to opening + scrolling to the first fused card inside
+    // this turn's section, plus a brief `.flash-ring` accent so the eye
+    // tracks the jump. Matches the drawer path's "open + scroll" gesture.
+    footer.addEventListener('dsh-open-turn-trace', () => {
+      const section = ct.section
+      if (!section || typeof section.querySelector !== 'function') return
+      const fused = section.querySelector('.tool-block.trace-card-fused')
+      if (!fused) return
+      fused.open = true
+      if (typeof fused.scrollIntoView === 'function') {
+        try { fused.scrollIntoView({ block: 'nearest' }) } catch (_) { /* jsdom */ }
+      }
+      if (fused.classList && typeof fused.classList.add === 'function') {
+        fused.classList.add('flash-ring')
+        setTimeout(() => {
+          try { fused.classList.remove('flash-ring') } catch (_) { /* jsdom */ }
+        }, 2000)
+      }
+    })
   }
   ct.section.appendChild(footer)
   ct.section.dataset.turnStatus = 'sealed'
@@ -2041,6 +2076,13 @@ function beginTraceStep(meta, event) {
     inputs: meta.pendingTraceInputs || [],
     outputs: [],
     events: [],
+    // Tool-blocks emitted within this step's window — populated by the
+    // tool/call handler. finishTraceStep fuses these with the step
+    // meta (usage badge + duration + panes) instead of appending a
+    // trailing standalone .trace-card, so the tool row IS the step card
+    // at its narrative position. Text-only steps leave this empty and
+    // fall through to the historical .trace-card render.
+    _toolBlocks: [],
   }
   meta.pendingTraceInputs = []
   // "streaming-first": drop a placeholder card
@@ -2123,6 +2165,20 @@ function finishTraceStep(meta, endSeq, endTime) {
     }
     rec._streamingNode = null
   }
+  // Fusion path: if the step emitted any tool blocks, upgrade the first
+  // one into the step card in place (adds usage badge / duration / step
+  // marker to its summary, absorbs sibling tool-blocks, appends the
+  // trace panes to its body). The narrative position of the tool call
+  // is preserved and the trailing standalone .trace-card is not emitted.
+  // Text-only steps fall through to the historical renderTraceCard path.
+  if (Array.isArray(rec._toolBlocks) && rec._toolBlocks.length > 0) {
+    const fused = fuseStepIntoToolBlock(rec)
+    if (fused) {
+      meta.lastTurnTraceCard = fused
+      return fused
+    }
+    // fall through if fusion failed (defensive)
+  }
   // Return the just-appended trace card so callers (turn/end handler)
   // can lift it into the turn-footer drawer.
   //
@@ -2142,6 +2198,128 @@ function finishTraceStep(meta, endSeq, endTime) {
   return card
 }
 
+// Fuse the step's aggregated record (usage badge / duration / step marker
+// / trace panes) into the first tool-block emitted during the step.
+// The tool-block stays at its narrative position in the turn body — no
+// standalone .trace-card is appended at the stream tail for tool steps.
+//
+// Multi-call step: the first tool-block becomes the outer card; sibling
+// tool-blocks are moved inside its body (above the trace panes) and the
+// summary shows `<first-tool> +N`.
+//
+// Returns the fused DOM node (also usable as the drawer traceCard via
+// `finishTurnContainer`, though we suppress the drawer for fused steps
+// so the reader doesn't see the same card twice).
+function fuseStepIntoToolBlock(rec) {
+  const agg = window.__dshTraceAgg
+  if (!agg || !rec || !Array.isArray(rec._toolBlocks) || rec._toolBlocks.length === 0) return null
+  const first = rec._toolBlocks[0]
+  const el = first && first.el
+  if (!el || !el.querySelector) return null
+  const doc = el.ownerDocument || document
+  const summary = el.querySelector(':scope > summary')
+  if (!summary) return null
+
+  // Mark the block as a fused step card so callers (finishTurnContainer)
+  // know not to lift it into a trailing drawer + tests can select it.
+  el.classList.add('trace-card-fused')
+  el.dataset.stepFused = '1'
+  if (rec.startSeq !== null) el.dataset.startSeq = String(rec.startSeq)
+  if (rec.endSeq !== null) el.dataset.endSeq = String(rec.endSeq)
+  if (rec.turn !== null) el.dataset.stepTurn = String(rec.turn)
+  if (rec.step !== null) el.dataset.stepIndex = String(rec.step)
+
+  // Multi-call: absorb sibling tool-blocks into this card's body BEFORE
+  // the panes. Update the summary's tool-name to show `<first> +N`.
+  const extras = rec._toolBlocks.slice(1)
+  if (extras.length > 0) {
+    const nameEl = summary.querySelector('.tool-family-name')
+    if (nameEl && typeof first.name === 'string') {
+      nameEl.textContent = `${first.name} +${extras.length}`
+      nameEl.title = extras.map(b => b.name).join(', ')
+    }
+  }
+
+  // Rehost each sibling tool-block as a nested `.fused-call-row` inside
+  // the outer card. `<details>` inside `<details>` is legal DOM; we
+  // keep the child tool-block's own open/close independent so per-call
+  // args/result stay explorable. Insertion order matches call order.
+  for (const extra of extras) {
+    if (!extra || !extra.el) continue
+    const child = extra.el
+    if (child.parentNode) child.parentNode.removeChild(child)
+    child.classList.add('fused-call-row')
+    el.appendChild(child)
+  }
+
+  // Right-cluster on the summary: usage badge, duration pill, fold glyph.
+  // Insert BEFORE any existing `.tool-duration` / `.tool-edit-rerun-trigger`
+  // so the pill+glyph stay at the tail; margin-left:auto on the badge
+  // pushes the whole cluster right per LangSmith parity.
+  const stepUsage = agg.sumUsageForStep ? agg.sumUsageForStep(rec) : null
+  const badgeText = agg.usageBadgeText ? agg.usageBadgeText(stepUsage) : ''
+  const anchor = summary.querySelector('.tool-duration')
+    || summary.querySelector('.tool-edit-rerun-trigger')
+    || null
+  if (badgeText) {
+    const badge = doc.createElement('span')
+    badge.className = 'trace-usage-badge fused-usage-badge'
+    badge.textContent = badgeText
+    if (stepUsage) {
+      let total = 0
+      for (const k of ['inputTokens','outputTokens','cacheReadTokens','cacheWriteTokens','reasoningTokens']) {
+        const v = stepUsage[k]
+        if (Number.isFinite(v)) total += v
+      }
+      badge.title = tokenBreakdownTooltip(stepUsage, total)
+    } else {
+      badge.title = 'Sum of `data.usage` across every assistant/message in this step'
+    }
+    if (anchor && anchor.parentNode === summary) summary.insertBefore(badge, anchor)
+    else summary.appendChild(badge)
+  }
+  const dur = doc.createElement('span')
+  dur.className = 'trace-duration fused-duration'
+  dur.textContent = rec.durationMs !== null ? `${rec.durationMs}ms` : ''
+  if (anchor && anchor.parentNode === summary) summary.insertBefore(dur, anchor)
+  else summary.appendChild(dur)
+  // Right-side fold glyph (LangSmith parity — same ∨ marker as the
+  // standalone trace-card). Toggles the outer `<details>`.
+  const foldGlyph = doc.createElement('span')
+  foldGlyph.className = 'trace-card-fold-glyph fused-fold-glyph mono'
+  foldGlyph.setAttribute('aria-hidden', 'true')
+  foldGlyph.textContent = '∨'
+  foldGlyph.title = 'Fold / unfold this step\'s subtree'
+  foldGlyph.addEventListener('click', function (e) {
+    if (e && e.stopPropagation) e.stopPropagation()
+    if (e && e.preventDefault) e.preventDefault()
+    el.open = !el.open
+  })
+  summary.appendChild(foldGlyph)
+
+  // Body: appended AFTER the existing args/result rows so the tool's
+  // own detail sits above the step-aggregate panes. This mirrors the
+  // standalone trace-card body (meta strip + inputs/outputs/events).
+  const fusedBody = doc.createElement('div')
+  fusedBody.className = 'trace-body fused-trace-body'
+  fusedBody.appendChild(renderTraceStepMetaStrip(rec, stepUsage))
+  const stepModelName = (rec && rec.header && typeof rec.header.model === 'string' && rec.header.model)
+    || modelFromRecEvents(rec)
+    || null
+  const barCtx = (Number.isFinite(rec.startTime) && Number.isFinite(rec.durationMs) && rec.durationMs > 0)
+    ? { startTime: rec.startTime, durationMs: rec.durationMs, stepModel: stepModelName }
+    : (stepModelName ? { stepModel: stepModelName } : null)
+  fusedBody.appendChild(renderTracePane('inputs', rec.inputs, 'events consumed by this step', barCtx))
+  fusedBody.appendChild(renderTracePane('outputs', rec.outputs, 'events produced by this step', barCtx))
+  fusedBody.appendChild(renderTracePane('events', rec.events, 'every SessionEvent inside this step', barCtx))
+  el.appendChild(fusedBody)
+  // stash the step record on the fused node so anything that walks the
+  // DOM for aggregates (turn-footer tri-view, QA probes) finds the same
+  // shape it would on a standalone .trace-card.
+  el._rec = rec
+  return el
+}
+
 function renderTraceCard(rec) {
   const agg = window.__dshTraceAgg
   if (!agg) return null
@@ -5042,6 +5220,23 @@ function onSessionEvent(sessionId, event) {
           sessionId,
         })
       }
+      // step/tool fusion: register the tool-block on the currently-open
+      // trace record so finishTraceStep can upgrade it into the step
+      // card in place instead of appending a trailing standalone
+      // .trace-card. On the FIRST tool block of the step, also retire
+      // the streaming placeholder — its role (marking "step in flight")
+      // is now filled by the tool-block itself.
+      const rec = meta.currentTraceRecord
+      if (rec && Array.isArray(rec._toolBlocks) && toolBlockEl) {
+        if (rec._toolBlocks.length === 0 && rec._streamingNode) {
+          if (typeof rec._streamingNode.remove === 'function') rec._streamingNode.remove()
+          else if (rec._streamingNode.parentNode && rec._streamingNode.parentNode.removeChild) {
+            rec._streamingNode.parentNode.removeChild(rec._streamingNode)
+          }
+          rec._streamingNode = null
+        }
+        rec._toolBlocks.push({ callId, name, el: toolBlockEl })
+      }
       return
     }
     case 'tool/result': {
diff --git a/examples/desktop/src/renderer/style.css b/examples/desktop/src/renderer/style.css
index b06cb340e1..7c5e5789d4 100644
--- a/examples/desktop/src/renderer/style.css
+++ b/examples/desktop/src/renderer/style.css
@@ -10404,6 +10404,18 @@ textarea:focus-visible {
   animation: traceDeepLinkFlash 1.4s ease-out;
 }
 
+/* Fused step card fallback: when a turn's only trace lives as a fused
+ * tool-block (no trailing drawer), the turn-flow glyph click scrolls
+ * to the card and briefly rings it so the eye tracks the jump. Runs
+ * for ~2s, then the JS strips the class. */
+@keyframes flashRing {
+  0%   { box-shadow: 0 0 0 2px var(--accent, #4c9aff); }
+  100% { box-shadow: 0 0 0 2px transparent; }
+}
+.flash-ring {
+  animation: flashRing 2s ease-out;
+}
+
 /* Task #203: full-session trace overlay (opened from Devtools drawer). */
 .devtools-full-trace-overlay {
   position: fixed; inset: 0; z-index: 40;
@@ -12904,3 +12916,58 @@ button.artifact-version:hover {
   font-size: 12px;
   padding: 12px 16px;
 }
+
+/* -----------------------------------------------------------------------
+ * Step / tool-block fusion (feat/step-card-merge, 2026-07-19).
+ *
+ * A step whose outputs include tool/call events no longer appends a
+ * trailing standalone .trace-card at the stream tail — instead, the
+ * step meta (usage badge / duration / trace panes) is fused INTO the
+ * first tool-block emitted during the step, right where the call
+ * originally landed in the narrative flow. See fuseStepIntoToolBlock
+ * in renderer.js for the DOM shape.
+ *
+ * Visual grammar (Lane A, LangSmith parity):
+ *   collapsed  = one low-key row →  [glyph][name][gist] … [usage][dur][∨]
+ *   expanded   = args + result + edit&re-run panel + inputs/outputs/events
+ *                panes stacked in one card, single left border.
+ * ----------------------------------------------------------------------- */
+.tool-block.trace-card-fused > summary .fused-usage-badge {
+  /* Anchor the right-cluster on the fused summary — mirrors the standalone
+   * .trace-card's `margin-left:auto` on .trace-usage-badge. */
+  margin-left: auto;
+}
+.tool-block.trace-card-fused > summary .fused-duration {
+  margin-left: 6px;
+}
+.tool-block.trace-card-fused > summary .fused-fold-glyph {
+  margin-left: 6px;
+  color: var(--muted);
+  cursor: pointer;
+  user-select: none;
+  transition: transform 120ms ease;
+}
+.tool-block.trace-card-fused[open] > summary .fused-fold-glyph {
+  transform: rotate(180deg);
+}
+/* Panes appear last inside the fused card body; add a hairline separator
+ * so the reader's eye splits "this call" from "the whole step". */
+.tool-block.trace-card-fused > .fused-trace-body {
+  margin-top: 6px;
+  padding-top: 6px;
+  border-top: 1px dashed var(--border);
+}
+/* Nested tool-blocks for multi-call steps: reduce visual weight so the
+ * outer card owns the frame. Fused-call-row keeps its own detail toggle
+ * and edit&re-run affordance. */
+.tool-block.trace-card-fused .fused-call-row {
+  margin: 4px 0 4px 12px;
+  border-left-width: 1px;
+  background: transparent;
+}
+/* edit & re-run affordance is only visible while the tool-block is open
+ * (design confirm §a: no hover-only reveal, keep the collapsed row
+ * quiet). */
+.tool-block:not([open]) > summary .tool-edit-rerun-trigger {
+  display: none;
+}
diff --git a/examples/desktop/test/renderer-step-tool-fusion.test.js b/examples/desktop/test/renderer-step-tool-fusion.test.js
new file mode 100644
index 0000000000..45ad1aebd9
--- /dev/null
+++ b/examples/desktop/test/renderer-step-tool-fusion.test.js
@@ -0,0 +1,253 @@
+// feat/step-card-merge (2026-07-19): the tool call row and the trailing
+// standalone step card get fused into a single unit at the tool call's
+// narrative position. This covers the DOM contract fuseStepIntoToolBlock
+// (renderer.js) produces + the finishTurnContainer guard that stops the
+// fused card from getting lifted into a trailing turn-trace-drawer.
+//
+// Related design memo: the assistant turn's chat stream should show ONE
+// entry per tool call (the tool-block, now upgraded to a step card),
+// not two — the historical grammar dropped a `.tool-block` at the call
+// position AND a `.trace-card` at the stream tail. The trailing card is
+// suppressed for tool-only steps; text-only steps still emit it.
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert')
+const { loadRenderer } = require('./renderer-harness.js')
+
+// Small helpers: the test harness has a naive selector matcher that
+// doesn't grok combinators or `:scope`. Walk children explicitly.
+function directChild(el, predicate) {
+  if (!el || !el.children) return null
+  for (const c of el.children) if (predicate(c)) return c
+  return null
+}
+function directSummary(el) {
+  return directChild(el, c => c && c.tagName === 'SUMMARY')
+}
+function firstDescendant(el, cls) {
+  return el && el.querySelector ? el.querySelector('.' + cls) : null
+}
+function playStream(renderer, sid, events) {
+  for (const ev of events) renderer.onSessionEvent(sid, ev)
+}
+
+// Single-tool step in a single-step turn. Should produce:
+//   - one .tool-block (also .trace-card-fused, data-step-fused=1)
+//   - zero standalone .trace-card
+//   - one .turn-footer with the flow glyph but NO .turn-trace-drawer
+function makeSingleToolTurn() {
+  return [
+    { seq: 1, time: 1000, type: 'turn/start', data: {} },
+    { seq: 2, time: 1005, type: 'step/start', data: { turn: 0, step: 0 } },
+    { seq: 3, time: 1020, type: 'tool/call', data: {
+      callId: 'c-a', name: 'read', arguments: JSON.stringify({ path: 'foo.ts' }),
+    } },
+    { seq: 4, time: 1080, type: 'tool/result', data: {
+      callId: 'c-a', content: [{ type: 'text', text: 'file contents' }],
+    } },
+    // usage rides on assistant/message; include one so the fused summary
+    // gets a usage badge. Real DeepSeek traffic always emits at least one
+    // assistant/message per step (the model reply that triggers the call).
+    { seq: 5, time: 1100, type: 'assistant/message', data: {
+      content: [{ type: 'text', text: 'looking at foo.ts' }],
+      usage: { inputTokens: 40, outputTokens: 12, cacheReadTokens: 200 },
+    } },
+    { seq: 6, time: 1120, type: 'step/end', data: {} },
+    { seq: 7, time: 1125, type: 'turn/end', data: { reason: 'completed' } },
+  ]
+}
+
+test('fused: single tool step upgrades its tool-block into the step card', async () => {
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-fuse', { title: 't', header: {} })
+  await renderer.selectSession('s-fuse')
+  playStream(renderer, 's-fuse', makeSingleToolTurn())
+
+  const fused = document.querySelectorAll('.tool-block.trace-card-fused')
+  assert.equal(fused.length, 1, 'exactly one fused tool-block acting as the step card')
+  const el = fused[0]
+  assert.equal(el.dataset.stepFused, '1', 'stepFused marker present for downstream QA/tests')
+
+  // Trailing standalone .trace-card is suppressed for a tool-only step.
+  const standalone = document.querySelectorAll('.trace-card:not(.trace-card-fused)')
+  assert.equal(standalone.length, 0, 'no trailing standalone .trace-card for tool-only step')
+
+  // Streaming placeholder must be gone (retired on first tool/call).
+  const placeholders = document.querySelectorAll('.trace-card-streaming')
+  assert.equal(placeholders.length, 0, 'streaming placeholder retired')
+
+  // Summary carries the fused usage badge + duration pill + fold glyph.
+  const summary = directSummary(el)
+  assert.ok(summary, 'fused card has a summary')
+  const summaryChildren = summary.children.map(c => c.className)
+  assert.ok(summaryChildren.some(cn => cn.includes('fused-usage-badge')),
+    'usage badge attached to fused summary; got children ' + JSON.stringify(summaryChildren))
+  const dur = directChild(summary, c => c.className && c.className.includes('fused-duration'))
+  assert.ok(dur && /ms$/.test(dur.textContent), 'duration pill shows ms (got ' + (dur && dur.textContent) + ')')
+  assert.ok(directChild(summary, c => c.className && c.className.includes('fused-fold-glyph')),
+    'right-side fold glyph present')
+
+  // Fused body includes the meta strip + three trace panes.
+  const body = directChild(el, c => c.className && c.className.includes('fused-trace-body'))
+  assert.ok(body, 'fused trace body appended inside the tool-block')
+  assert.ok(firstDescendant(body, 'trace-step-meta'), 'step meta strip lives in fused body')
+  const panes = Array.from(body.querySelectorAll('.trace-pane'))
+  assert.ok(panes.length >= 3, 'three panes rendered inside fused body — got ' + panes.length)
+})
+
+test('fused: single tool step does not build a trailing turn-trace-drawer', async () => {
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-drw', { title: 't', header: {} })
+  await renderer.selectSession('s-drw')
+  playStream(renderer, 's-drw', makeSingleToolTurn())
+
+  const drawers = document.querySelectorAll('.turn-trace-drawer')
+  assert.equal(drawers.length, 0,
+    'fused card stays inline — footer drawer is suppressed for tool-only steps')
+
+  // Turn footer still exists so the "turn ended" glyph + chips are reachable.
+  const footers = document.querySelectorAll('.turn-footer')
+  assert.equal(footers.length, 1, 'turn footer preserved (glyph + optional chips)')
+})
+
+test('fused: multi-tool step lists sibling calls inside the fused card + summary shows "<first> +N"', async () => {
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-mul', { title: 't', header: {} })
+  await renderer.selectSession('s-mul')
+  const events = [
+    { seq: 1, time: 1000, type: 'turn/start', data: {} },
+    { seq: 2, time: 1005, type: 'step/start', data: { turn: 0, step: 0 } },
+    { seq: 3, time: 1010, type: 'tool/call', data: {
+      callId: 'c-a', name: 'read', arguments: JSON.stringify({ path: 'a.ts' }),
+    } },
+    { seq: 4, time: 1015, type: 'tool/call', data: {
+      callId: 'c-b', name: 'grep', arguments: JSON.stringify({ pattern: 'foo' }),
+    } },
+    { seq: 5, time: 1020, type: 'tool/call', data: {
+      callId: 'c-c', name: 'bash', arguments: JSON.stringify({ cmd: 'ls' }),
+    } },
+    { seq: 6, time: 1050, type: 'tool/result', data: { callId: 'c-a', content: [{ type: 'text', text: '.' }] } },
+    { seq: 7, time: 1051, type: 'tool/result', data: { callId: 'c-b', content: [{ type: 'text', text: '.' }] } },
+    { seq: 8, time: 1052, type: 'tool/result', data: { callId: 'c-c', content: [{ type: 'text', text: '.' }] } },
+    { seq: 9, time: 1070, type: 'assistant/message', data: {
+      content: [{ type: 'text', text: 'ok' }],
+      usage: { inputTokens: 20, outputTokens: 5 },
+    } },
+    { seq: 10, time: 1080, type: 'step/end', data: {} },
+    { seq: 11, time: 1085, type: 'turn/end', data: { reason: 'completed' } },
+  ]
+  playStream(renderer, 's-mul', events)
+
+  // Exactly one fused card, three tool-blocks total (first outer + two absorbed).
+  const fused = document.querySelectorAll('.tool-block.trace-card-fused')
+  assert.equal(fused.length, 1, 'multi-call step still fuses into one outer card')
+  const outer = fused[0]
+  assert.equal(outer.getAttribute('data-tool-name'), 'read', 'outer card is the first tool by call order')
+  // Absorbed sibling blocks live inside the outer card with the marker class.
+  const absorbed = outer.querySelectorAll('.fused-call-row')
+  assert.equal(absorbed.length, 2, 'two sibling tool-blocks absorbed into the fused card')
+  const absorbedNames = Array.from(absorbed).map(el => el.getAttribute('data-tool-name'))
+  assert.deepEqual(absorbedNames, ['grep', 'bash'], 'absorbed rows retain their tool-name in call order')
+
+  // Summary shows `<first> +N` with the total-N excluding the outer.
+  const summary = directSummary(outer)
+  const nameEl = directChild(summary, c => c.className && c.className.includes('tool-family-name'))
+  assert.ok(nameEl && nameEl.textContent === 'read +2',
+    'summary tool-name reads "<first> +N" — got ' + (nameEl && nameEl.textContent))
+})
+
+test('fused: text-only step (no tool/call) still emits a standalone trace-card', async () => {
+  // Guardrail: the fusion path is opt-in — a step without any tool/call
+  // must retain the historical trailing .trace-card so text-only reasoning
+  // remains reachable via the turn-footer drawer.
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-txt', { title: 't', header: {} })
+  await renderer.selectSession('s-txt')
+  const events = [
+    { seq: 1, time: 1000, type: 'turn/start', data: {} },
+    { seq: 2, time: 1005, type: 'step/start', data: { turn: 0, step: 0 } },
+    { seq: 3, time: 1050, type: 'assistant/message', data: {
+      content: [{ type: 'text', text: 'a plain response' }],
+      usage: { inputTokens: 5, outputTokens: 2 },
+    } },
+    { seq: 4, time: 1080, type: 'step/end', data: {} },
+    { seq: 5, time: 1085, type: 'turn/end', data: { reason: 'completed' } },
+  ]
+  playStream(renderer, 's-txt', events)
+
+  const fused = document.querySelectorAll('.tool-block.trace-card-fused')
+  assert.equal(fused.length, 0, 'no fused card without a tool/call')
+  // The standalone trace-card gets lifted into the turn drawer at turn/end.
+  // Look at the drawer for its presence.
+  const drawer = document.querySelector('.turn-trace-drawer')
+  assert.ok(drawer, 'text-only step still emits a trailing trace-card lifted into the drawer')
+})
+
+// Glyph fallback contract: a turn whose only trace lives as a fused
+// tool-block (no trailing drawer built) MUST still respond to the
+// turn-flow-glyph's `dsh-open-turn-trace` event — open the fused card
+// + scroll it into view + briefly ring it with `.flash-ring` so the
+// reader tracks the jump. Without this the glyph would be a silent
+// no-op on tool-only turns.
+test('fused: glyph fallback opens + scrolls to + flash-rings the fused card when no drawer exists', async () => {
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-glyph', { title: 't', header: {} })
+  await renderer.selectSession('s-glyph')
+  playStream(renderer, 's-glyph', makeSingleToolTurn())
+
+  // Sanity: drawer suppressed, fused card present, footer built.
+  assert.equal(document.querySelectorAll('.turn-trace-drawer').length, 0,
+    'precondition: no drawer built for fused-only turn')
+  const footer = document.querySelector('.turn-footer')
+  assert.ok(footer, 'turn footer built')
+  const fused = document.querySelector('.tool-block.trace-card-fused')
+  assert.ok(fused, 'fused card exists')
+
+  // Track scroll invocation on the fused card so we can assert the
+  // fallback actually reached the scroll step.
+  let scrolled = 0
+  fused.scrollIntoView = function () { scrolled += 1 }
+  fused.open = false
+
+  // Fire the glyph's event through the harness's listener hook.
+  assert.equal(typeof footer._fire, 'function',
+    'harness footer exposes _fire for synthetic event dispatch')
+  footer._fire('dsh-open-turn-trace', {})
+
+  assert.equal(fused.open, true, 'fused card opened by fallback')
+  assert.equal(scrolled, 1, 'fallback scrolled the fused card into view')
+  assert.ok(fused.classList.contains('flash-ring'),
+    'fallback applied .flash-ring accent for 2s visual cue')
+})
+
+// Twin of the above: when a text-only step already built a drawer,
+// the fallback path must NOT fire — the historical drawer-open path
+// remains authoritative. Guards against a regression where both
+// listeners run and the fused-selector accidentally matches something.
+test('fused: text-only turn keeps the drawer path — no flash-ring on the stream', async () => {
+  const { renderer, document } = await loadRenderer()
+  renderer.ensureSession('s-txt2', { title: 't', header: {} })
+  await renderer.selectSession('s-txt2')
+  playStream(renderer, 's-txt2', [
+    { seq: 1, time: 1000, type: 'turn/start', data: {} },
+    { seq: 2, time: 1005, type: 'step/start', data: { turn: 0, step: 0 } },
+    { seq: 3, time: 1050, type: 'assistant/message', data: {
+      content: [{ type: 'text', text: 'a plain response' }],
+      usage: { inputTokens: 5, outputTokens: 2 },
+    } },
+    { seq: 4, time: 1080, type: 'step/end', data: {} },
+    { seq: 5, time: 1085, type: 'turn/end', data: { reason: 'completed' } },
+  ])
+  const footer = document.querySelector('.turn-footer')
+  const drawer = document.querySelector('.turn-trace-drawer')
+  assert.ok(footer && drawer, 'precondition: drawer path in play')
+  drawer.open = false
+  footer._fire('dsh-open-turn-trace', {})
+  assert.equal(drawer.open, true, 'drawer opened by drawer path')
+  // No fused card to ring — the accent class must not be applied
+  // anywhere in the tree.
+  const ringed = document.querySelectorAll('.flash-ring')
+  assert.equal(ringed.length, 0, 'flash-ring not applied on drawer-path turn')
+})