diff --git a/frontend/src/components/StreamTranscript.incremental.test.tsx b/frontend/src/components/StreamTranscript.incremental.test.tsx
new file mode 100644
index 0000000..81a05a8
--- /dev/null
+++ b/frontend/src/components/StreamTranscript.incremental.test.tsx
@@ -0,0 +1,68 @@
+import { cleanup, render, screen } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { StreamTranscript } from './StreamTranscript'
+
+const eventLine = (text: string) =>
+  JSON.stringify({ type: 'assistant', message: { content: [{ type: 'text', text }] } })
+
+afterEach(() => {
+  cleanup()
+  vi.restoreAllMocks()
+})
+
+describe('StreamTranscript incremental parsing', () => {
+  it('parses only newly appended JSONL lines', () => {
+    const first = eventLine('first row')
+    const second = eventLine('second row')
+    const parseSpy = vi.spyOn(JSON, 'parse')
+    const { rerender } = render(<StreamTranscript text={`${first}\n`} />)
+
+    expect(screen.getByText('first row')).toBeTruthy()
+
+    rerender(<StreamTranscript text={`${first}\n${second}\n`} />)
+
+    expect(screen.getByText('second row')).toBeTruthy()
+    expect(screen.getAllByText(/^(first|second) row$/).map((node) => node.textContent)).toEqual([
+      'first row',
+      'second row',
+    ])
+    expect(parseSpy.mock.calls.filter(([input]) => input === first)).toHaveLength(1)
+    expect(parseSpy.mock.calls.filter(([input]) => input === second)).toHaveLength(1)
+  })
+
+  it('does not parse identical props again', () => {
+    const line = eventLine('only row')
+    const text = `${line}\n`
+    const parseSpy = vi.spyOn(JSON, 'parse')
+    const { rerender } = render(<StreamTranscript text={text} />)
+
+    rerender(<StreamTranscript text={text} />)
+
+    expect(screen.getAllByText('only row')).toHaveLength(1)
+    expect(parseSpy.mock.calls.filter(([input]) => input === line)).toHaveLength(1)
+  })
+
+  it('buffers split lines and flushes an unterminated final line once', () => {
+    const split = eventLine('split row')
+    const final = eventLine('final row')
+    const splitAt = Math.floor(split.length / 2)
+    const { rerender } = render(<StreamTranscript text={split.slice(0, splitAt)} />)
+
+    expect(screen.queryByText('split row')).toBeNull()
+
+    rerender(<StreamTranscript text={`${split}\n`} />)
+    expect(screen.getAllByText('split row')).toHaveLength(1)
+
+    rerender(<StreamTranscript text={`${split}\n${final}`} />)
+    expect(screen.queryByText('final row')).toBeNull()
+
+    rerender(<StreamTranscript text={`${split}\n${final}`} complete />)
+    expect(screen.getAllByText('split row')).toHaveLength(1)
+    expect(screen.getAllByText('final row')).toHaveLength(1)
+
+    rerender(<StreamTranscript text={`${split}\n${final}`} complete />)
+    expect(screen.getAllByText('split row')).toHaveLength(1)
+    expect(screen.getAllByText('final row')).toHaveLength(1)
+  })
+})
diff --git a/frontend/src/components/StreamTranscript.tsx b/frontend/src/components/StreamTranscript.tsx
index da93a17..fcca2ac 100644
--- a/frontend/src/components/StreamTranscript.tsx
+++ b/frontend/src/components/StreamTranscript.tsx
@@ -1,4 +1,4 @@
-import { useEffect, useMemo, useRef } from 'react'
+import { useEffect, useRef, useState } from 'react'
 
 /**
  * Render the harness transcript (Claude or Codex) as readable rows.
@@ -32,7 +32,15 @@ import { useEffect, useMemo, useRef } from 'react'
  * stderr leakage stays legible.
  */
 export function StreamTranscript({ text, complete = false }: { text: string; complete?: boolean }) {
-  const rows = useMemo(() => parseStream(text, complete), [text, complete])
+  const [parseState, setParseState] = useState(() =>
+    appendStreamText(emptyParseState, text, complete),
+  )
+  let rows = parseState.rows
+  if (parseState.receivedLength !== text.length || parseState.tailFlushed !== complete) {
+    const nextParseState = appendStreamText(parseState, text, complete)
+    setParseState(nextParseState)
+    rows = nextParseState.rows
+  }
 
   // Stick-to-bottom: when the transcript renders inside its own scroll box
   // (the detail page caps `.ins-xscript .stream-transcript`), follow new rows
@@ -92,6 +100,47 @@ type Row =
   | { kind: 'unknown'; label: string; detail: string }
   | { kind: 'raw'; line: string }
 
+interface IncrementalParseState {
+  rows: Row[]
+  receivedLength: number
+  partialLine: string
+  tailFlushed: boolean
+}
+
+const emptyParseState: IncrementalParseState = {
+  rows: [],
+  receivedLength: 0,
+  partialLine: '',
+  tailFlushed: false,
+}
+
+function appendStreamText(
+  previous: IncrementalParseState,
+  text: string,
+  complete: boolean,
+): IncrementalParseState {
+  const suffix = text.slice(previous.receivedLength)
+  if (suffix === '' && complete === previous.tailFlushed) return previous
+
+  const pending = previous.partialLine + suffix
+  const lastNewline = pending.lastIndexOf('\n')
+  const terminated = lastNewline === -1 ? [] : pending.slice(0, lastNewline).split('\n')
+  let partialLine = lastNewline === -1 ? pending : pending.slice(lastNewline + 1)
+  const newRows = rowsForLines(terminated)
+
+  if (complete && partialLine !== '') {
+    newRows.push(...rowsForLines([partialLine]))
+    partialLine = ''
+  }
+
+  return {
+    rows: newRows.length === 0 ? previous.rows : [...previous.rows, ...newRows],
+    receivedLength: text.length,
+    partialLine,
+    tailFlushed: complete,
+  }
+}
+
 // Exported for unit tests; not a component (HMR fast-refresh warning is moot).
 // eslint-disable-next-line react-refresh/only-export-components
 export function parseStream(text: string, complete: boolean): Row[] {
@@ -107,8 +156,12 @@ export function parseStream(text: string, complete: boolean): Row[] {
     // complete list is already correct.
   }
 
+  return rowsForLines(completeLines)
+}
+
+function rowsForLines(lines: string[]): Row[] {
   const rows: Row[] = []
-  for (const line of completeLines) {
+  for (const line of lines) {
     if (!line.trim()) continue
     const event = tryParse(line)
     if (event === null) {