diff --git a/docs/audits/claude-history-window-budget/README.md b/docs/audits/claude-history-window-budget/README.md
new file mode 100644
index 00000000000..2d5a9bddb47
--- /dev/null
+++ b/docs/audits/claude-history-window-budget/README.md
@@ -0,0 +1,25 @@
+# Claude history-window read quota
+
+Restart reconciliation's Claude history reader declares a 16 MiB source quota, but the original `stat` followed by unrestricted `readFile` admits later growth. Reuse `readNodeFileWithinLimit` to enforce the same quota while reading from one descriptor. Overflow preserves the existing inconsistent-history result; it cannot establish that a submitted message was absent.
+
+## Reproduce
+
+```sh
+ORCA_BACKGROUND_LAUNCH=1 node docs/audits/claude-history-window-budget/reproduce.mjs
+```
+
+The seven-case fixture calls the actual history reader, branch proof and bounded reader against real temporary files. It injects an append immediately after the relevant size snapshot. The baseline reverses only `fix.patch` in a temporary Vite transform, leaving the checkout unchanged. Each child has a 45-second timeout and a 512 MiB old-space ceiling. No application window or provider process starts.
+
+| Case                                                      | Before                                         | After                                                                   |
+| --------------------------------------------------------- | ---------------------------------------------- | ----------------------------------------------------------------------- |
+| 305-byte valid source grows to 17 MiB after size snapshot | Reads 17,825,792 bytes and accepts the history | Stops after 16,777,217 bytes, rejects history and closes its descriptor |
+| Stable valid source                                       | Accepted                                       | Accepted                                                                |
+| Exactly 16 MiB                                            | Accepted                                       | Accepted                                                                |
+| Initially 16 MiB plus one byte                            | Rejected before content read                   | Same                                                                    |
+| Growth within quota                                       | Reads the additional bytes and accepts         | Same                                                                    |
+| Read error                                                | Inconsistent history                           | Same; descriptor closed                                                 |
+| Missing anchor                                            | No source read                                 | Same                                                                    |
+
+Before: one failing fixed-behavior assertion and six passing controls. After: seven passing cases. `results.json` records observed read bytes, size snapshots, descriptor counts, source hashes, exit codes and timeout status. These are read-budget measurements, not RSS or retained-heap measurements; the byte quota does not describe all parser allocations.
+
+This reader is **absent from v1.4.198**, so the finding cannot explain #19768 or #19831. It fixes a current-source quota race. The separately unrestricted `proveClaudeTranscriptBranch` file reader has no declared byte quota and is outside this change. The related legacy-import quota race is already addressed by #20976.
diff --git a/docs/audits/claude-history-window-budget/fix.patch b/docs/audits/claude-history-window-budget/fix.patch
new file mode 100644
index 00000000000..5952bc249d2
--- /dev/null
+++ b/docs/audits/claude-history-window-budget/fix.patch
@@ -0,0 +1,30 @@
+diff --git a/src/main/claude/claude-structured-history-window.ts b/src/main/claude/claude-structured-history-window.ts
+index 1cfd48a8a6..f7a541fa31 100644
+--- a/src/main/claude/claude-structured-history-window.ts
++++ b/src/main/claude/claude-structured-history-window.ts
+@@ -12,8 +12,8 @@
+ // of those makes absence meaningless. Failing it reports an inconsistent
+ // boundary rather than an empty window, because the two decide opposite things.
+ 
+-import { readFile, stat } from 'node:fs/promises'
+ import { join } from 'node:path'
++import { readNodeFileWithinLimit } from '../../shared/node-bounded-file-reader'
+ import type { AgentSessionJournalIdentity } from '../../shared/agent-session-journal-types'
+ import { resolveSessionFilePath } from '../native-chat/session-file-resolver'
+ import type {
+@@ -284,10 +284,11 @@ export async function readClaudeProviderHistoryWindow(input: {
+   }
+   let contents: string
+   try {
+-    if ((await stat(input.transcriptPath)).size > MAX_HISTORY_WINDOW_SOURCE_BYTES) {
+-      return INCONSISTENT
+-    }
+-    contents = await readFile(input.transcriptPath, 'utf8')
++    const read = await readNodeFileWithinLimit(
++      input.transcriptPath,
++      MAX_HISTORY_WINDOW_SOURCE_BYTES
++    )
++    contents = read.buffer.toString('utf8')
+   } catch {
+     return INCONSISTENT
+   }
diff --git a/docs/audits/claude-history-window-budget/reproduce.mjs b/docs/audits/claude-history-window-budget/reproduce.mjs
new file mode 100644
index 00000000000..aefb721afeb
--- /dev/null
+++ b/docs/audits/claude-history-window-budget/reproduce.mjs
@@ -0,0 +1,161 @@
+import { createHash } from 'node:crypto'
+import { mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
+import { createRequire } from 'node:module'
+import { tmpdir } from 'node:os'
+import { join, resolve } from 'node:path'
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import { applyPatch, parsePatch, reversePatch } from 'diff'
+import { build } from 'esbuild'
+
+if (process.env.ORCA_BACKGROUND_LAUNCH !== '1') {
+  throw new Error('Run with ORCA_BACKGROUND_LAUNCH=1.')
+}
+
+const root = fileURLToPath(new URL('../../../', import.meta.url))
+const patch = await readFile(new URL('./fix.patch', import.meta.url), 'utf8')
+const beforeSources = {}
+const sourceHashes = {}
+for (const parsed of parsePatch(patch)) {
+  const path = parsed.newFileName.replace(/^b\//, '')
+  const absolute = resolve(root, path)
+  const current = await readFile(absolute, 'utf8')
+  const before = applyPatch(current, reversePatch(parsed))
+  if (before === false) {
+    throw new Error(`Source changed; review the proof patch: ${path}`)
+  }
+  beforeSources[absolute.replaceAll('\\', '/')] = before
+  sourceHashes[path] = {
+    before: createHash('sha256').update(before).digest('hex'),
+    after: createHash('sha256').update(current).digest('hex')
+  }
+}
+
+for (const path of [
+  'src/main/claude/claude-history-window-source-budget.test.ts',
+  'src/shared/node-bounded-file-reader.ts',
+  'src/main/claude/claude-transcript-branch-proof.ts'
+]) {
+  sourceHashes[path] = {
+    current: createHash('sha256')
+      .update(await readFile(resolve(root, path)))
+      .digest('hex')
+  }
+}
+
+const scratch = await mkdtemp(join(tmpdir(), 'orca-claude-history-window-budget-'))
+const require = createRequire(import.meta.url)
+let runnerModuleId
+try {
+  const runnerPath = join(scratch, 'run-process.cjs')
+  await build({
+    absWorkingDir: root,
+    entryPoints: [resolve(root, 'src/shared/child-process/run-process.ts')],
+    outfile: runnerPath,
+    bundle: true,
+    platform: 'node',
+    format: 'cjs',
+    logLevel: 'silent'
+  })
+  runnerModuleId = require.resolve(runnerPath)
+  const { runProcess } = require(runnerModuleId)
+  const baselineConfig = join(scratch, 'before.config.mjs')
+  const fixedConfig = join(scratch, 'after.config.mjs')
+  const includes = ['src/main/claude/claude-history-window-source-budget.test.ts']
+  const configImport = JSON.stringify(pathToFileURL(resolve(root, 'config/vitest.config.ts')).href)
+  await writeFile(
+    baselineConfig,
+    `import base from ${configImport};
+const beforeSources = ${JSON.stringify(beforeSources)};
+export default {...base, test: {...base.test, include: ${JSON.stringify(includes)}}, plugins: [{
+  name: 'claude-history-window-budget-before-fix', enforce: 'pre',
+  transform(_code, id) {
+    const before = beforeSources[id.replaceAll('\\\\', '/').split('?')[0]];
+    return before === undefined ? null : {code: before, map: null};
+  }
+}]};\n`
+  )
+
+  await writeFile(
+    fixedConfig,
+    `import base from ${configImport};\nexport default {...base, test: {...base.test, include: ${JSON.stringify(includes)}}};\n`
+  )
+
+  async function run(label, config) {
+    const report = join(scratch, `${label}.json`)
+    const observationPath = join(scratch, `${label}.observations.jsonl`)
+    const result = await runProcess({
+      program: process.execPath,
+      args: [
+        resolve(root, 'node_modules/vitest/vitest.mjs'),
+        'run',
+        '--config',
+        config,
+        '--reporter=json',
+        `--outputFile=${report}`
+      ],
+      cwd: root,
+      env: {
+        ...process.env,
+        ORCA_HISTORY_BUDGET_PROOF_OUTPUT: observationPath,
+        NODE_OPTIONS: '--max-old-space-size=512'
+      },
+      timeoutMs: 45_000,
+      maxOutputBytes: 4 * 1024 * 1024
+    })
+    let parsed
+    try {
+      parsed = JSON.parse(await readFile(report, 'utf8'))
+    } catch (error) {
+      throw new Error(`${label} runner failed: ${result.stderr || result.stdout}`, { cause: error })
+    }
+    return {
+      observations: (await readFile(observationPath, 'utf8'))
+        .trim()
+        .split('\n')
+        .map((line) => JSON.parse(line)),
+      exitCode: result.code,
+      timedOut: result.timedOut,
+      passed: parsed.numPassedTests,
+      failed: parsed.numFailedTests,
+      failedCases: parsed.testResults.flatMap((suite) =>
+        suite.assertionResults
+          .filter((test) => test.status === 'failed')
+          .map((test) => test.fullName)
+      )
+    }
+  }
+
+  const before = await run('before', baselineConfig)
+  const after = await run('after', fixedConfig)
+  const passed =
+    before.exitCode === 1 &&
+    after.exitCode === 0 &&
+    !before.timedOut &&
+    !after.timedOut &&
+    before.failed === 1 &&
+    before.passed === 6 &&
+    after.passed === 7 &&
+    after.failed === 0
+  console.log(
+    JSON.stringify(
+      {
+        comparison:
+          'Actual history reader and bounded file reader with real files; growth injected after stat; before reverses only fix.patch in a temporary Vite transform',
+        sourceHashes,
+        before,
+        after,
+        passed
+      },
+      null,
+      2
+    )
+  )
+  if (!passed) {
+    process.exitCode = 1
+  }
+} finally {
+  if (runnerModuleId) {
+    delete require.cache[runnerModuleId]
+  }
+  await rm(scratch, { recursive: true, force: true })
+}
diff --git a/docs/audits/claude-history-window-budget/results.json b/docs/audits/claude-history-window-budget/results.json
new file mode 100644
index 00000000000..716ddba13f8
--- /dev/null
+++ b/docs/audits/claude-history-window-budget/results.json
@@ -0,0 +1,137 @@
+{
+  "comparison": "Actual history reader and bounded file reader with real files; growth injected after stat; before reverses only fix.patch in a temporary Vite transform",
+  "sourceHashes": {
+    "src/main/claude/claude-structured-history-window.ts": {
+      "before": "43a9868f6bb87bfda620ace31f680f36b3ecd88851ad98c24be5320a9814358b",
+      "after": "77098508b3f5ab5cf4a53163a14da2fa74b1de38859c4717496f7a97e2cd50ea"
+    },
+    "src/main/claude/claude-history-window-source-budget.test.ts": {
+      "current": "88f7a9a9ad9669c636bd7369fdd9a1263325848971f67b499278bf78eb17681e"
+    },
+    "src/shared/node-bounded-file-reader.ts": {
+      "current": "2cffa451b72d059226e0c2d4b6d2a5203cb8ddafb358d5e828dcd3436d197ecb"
+    },
+    "src/main/claude/claude-transcript-branch-proof.ts": {
+      "current": "3224594befe1edae514724caf5eff6a4cda85212e2985be4920b6164b088b82c"
+    }
+  },
+  "before": {
+    "observations": [
+      {
+        "test": "reads a stable history without changing prompt evidence",
+        "bytesRead": 305,
+        "observedStatBytes": 305,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "accepts a complete source at the existing byte limit",
+        "bytesRead": 16777216,
+        "observedStatBytes": 16777216,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "rejects an initially oversized source before reading its contents",
+        "bytesRead": 0,
+        "observedStatBytes": 16777217,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "refuses concurrent growth beyond the existing source quota",
+        "bytesRead": 17825792,
+        "observedStatBytes": 305,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "accepts concurrent growth that stays within the quota",
+        "bytesRead": 369,
+        "observedStatBytes": 305,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "preserves the inconsistent result on a read error",
+        "bytesRead": 0,
+        "observedStatBytes": 305,
+        "opens": 0,
+        "closes": 0
+      },
+      {
+        "test": "does not open a source without an anchor",
+        "bytesRead": 0,
+        "observedStatBytes": 0,
+        "opens": 0,
+        "closes": 0
+      }
+    ],
+    "exitCode": 1,
+    "timedOut": false,
+    "passed": 6,
+    "failed": 1,
+    "failedCases": [
+      "Claude provider history source budget refuses concurrent growth beyond the existing source quota"
+    ]
+  },
+  "after": {
+    "observations": [
+      {
+        "test": "reads a stable history without changing prompt evidence",
+        "bytesRead": 305,
+        "observedStatBytes": 305,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "accepts a complete source at the existing byte limit",
+        "bytesRead": 16777216,
+        "observedStatBytes": 16777216,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "rejects an initially oversized source before reading its contents",
+        "bytesRead": 0,
+        "observedStatBytes": 16777217,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "refuses concurrent growth beyond the existing source quota",
+        "bytesRead": 16777217,
+        "observedStatBytes": 305,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "accepts concurrent growth that stays within the quota",
+        "bytesRead": 369,
+        "observedStatBytes": 305,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "preserves the inconsistent result on a read error",
+        "bytesRead": 0,
+        "observedStatBytes": 305,
+        "opens": 1,
+        "closes": 1
+      },
+      {
+        "test": "does not open a source without an anchor",
+        "bytesRead": 0,
+        "observedStatBytes": 0,
+        "opens": 0,
+        "closes": 0
+      }
+    ],
+    "exitCode": 0,
+    "timedOut": false,
+    "passed": 7,
+    "failed": 0,
+    "failedCases": []
+  },
+  "passed": true
+}
diff --git a/src/main/claude/claude-history-window-source-budget.test.ts b/src/main/claude/claude-history-window-source-budget.test.ts
new file mode 100644
index 00000000000..5e6e5332744
--- /dev/null
+++ b/src/main/claude/claude-history-window-source-budget.test.ts
@@ -0,0 +1,188 @@
+import { join } from 'node:path'
+import { tmpdir } from 'node:os'
+import type * as FsPromises from 'node:fs/promises'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+const state = vi.hoisted(() => ({
+  path: '',
+  growth: '',
+  readError: false,
+  bytesRead: 0,
+  closes: 0,
+  opens: 0,
+  observedStatBytes: 0
+}))
+
+vi.mock('node:fs/promises', async (importOriginal) => {
+  const fs = await importOriginal<typeof FsPromises>()
+  const afterStat = async (path: string, size: number) => {
+    if (path !== state.path) {
+      return
+    }
+    state.observedStatBytes = size
+    if (state.growth) {
+      const growth = state.growth
+      state.growth = ''
+      await fs.appendFile(path, growth)
+    }
+  }
+  return {
+    ...fs,
+    stat: async (path: string) => {
+      const snapshot = await fs.stat(path)
+      await afterStat(path, snapshot.size)
+      return snapshot
+    },
+    readFile: async (path: string, encoding: BufferEncoding) => {
+      if (path === state.path && state.readError) {
+        throw new Error('Injected read failure')
+      }
+      const result = await fs.readFile(path, encoding)
+      if (path === state.path) {
+        state.bytesRead += Buffer.byteLength(result)
+      }
+      return result
+    },
+    open: async (path: string, flags: string) => {
+      const handle = await fs.open(path, flags)
+      if (path !== state.path) {
+        return handle
+      }
+      state.opens += 1
+      return {
+        stat: async () => {
+          const snapshot = await handle.stat()
+          await afterStat(path, snapshot.size)
+          return snapshot
+        },
+        read: async (buffer: Buffer, offset: number, length: number, position: number) => {
+          if (state.readError) {
+            throw new Error('Injected read failure')
+          }
+          const result = await handle.read(buffer, offset, length, position)
+          state.bytesRead += result.bytesRead
+          return result
+        },
+        close: async () => {
+          state.closes += 1
+          await handle.close()
+        }
+      }
+    }
+  }
+})
+
+import { appendFile, mkdtemp, rm, writeFile } from 'node:fs/promises'
+import { readClaudeProviderHistoryWindow } from './claude-structured-history-window'
+
+const LIMIT = 16 * 1024 * 1024
+const SOURCE = `${[
+  {
+    type: 'user',
+    uuid: 'anchor',
+    parentUuid: null,
+    sessionId: 'provider',
+    message: { role: 'user', content: 'before' }
+  },
+  {
+    type: 'user',
+    uuid: 'latest',
+    parentUuid: 'anchor',
+    sessionId: 'provider',
+    message: { role: 'user', content: 'after' }
+  },
+  { type: 'last-prompt', sessionId: 'provider', leafUuid: 'latest' }
+]
+  .map((row) => JSON.stringify(row))
+  .join('\n')}\n`
+let directory = ''
+
+const read = (previousLeafUuid: string | null = 'anchor') =>
+  readClaudeProviderHistoryWindow({
+    transcriptPath: state.path,
+    providerSessionId: 'provider',
+    previousLeafUuid,
+    sessionId: 'orca',
+    turnInFlight: false
+  })
+
+beforeEach(async () => {
+  directory = await mkdtemp(join(tmpdir(), 'orca-history-source-budget-'))
+  Object.assign(state, {
+    path: join(directory, 'session.jsonl'),
+    growth: '',
+    readError: false,
+    bytesRead: 0,
+    opens: 0,
+    closes: 0,
+    observedStatBytes: 0
+  })
+  await writeFile(state.path, SOURCE)
+})
+
+afterEach(async (context) => {
+  try {
+    const output = process.env.ORCA_HISTORY_BUDGET_PROOF_OUTPUT
+    if (output) {
+      await appendFile(
+        output,
+        `${JSON.stringify({
+          test: context.task.name,
+          bytesRead: state.bytesRead,
+          observedStatBytes: state.observedStatBytes,
+          opens: state.opens,
+          closes: state.closes
+        })}\n`
+      )
+    }
+    expect(state.closes).toBe(state.opens)
+  } finally {
+    await rm(directory, { recursive: true, force: true })
+  }
+})
+
+describe('Claude provider history source budget', () => {
+  it('reads a stable history without changing prompt evidence', async () => {
+    const result = await read()
+    expect(result.boundaryConsistent).toBe(true)
+    expect(result.items.map((item) => item.providerItemId)).toEqual(['latest'])
+  })
+
+  it('accepts a complete source at the existing byte limit', async () => {
+    await writeFile(state.path, SOURCE + ' '.repeat(LIMIT - Buffer.byteLength(SOURCE)))
+    expect((await read()).boundaryConsistent).toBe(true)
+    expect(state.bytesRead).toBe(LIMIT)
+  })
+
+  it('rejects an initially oversized source before reading its contents', async () => {
+    await writeFile(state.path, SOURCE + ' '.repeat(LIMIT + 1 - Buffer.byteLength(SOURCE)))
+    expect((await read()).boundaryConsistent).toBe(false)
+    expect(state.bytesRead).toBe(0)
+  })
+
+  it('refuses concurrent growth beyond the existing source quota', async () => {
+    state.growth = ' '.repeat(LIMIT + 1024 * 1024 - Buffer.byteLength(SOURCE))
+    const result = await read()
+    expect(state.observedStatBytes).toBe(Buffer.byteLength(SOURCE))
+    expect(result.boundaryConsistent).toBe(false)
+    expect(result.items).toEqual([])
+    expect(state.bytesRead).toBeLessThanOrEqual(LIMIT + 1)
+  })
+
+  it('accepts concurrent growth that stays within the quota', async () => {
+    state.growth = ' \n'.repeat(32)
+    expect((await read()).boundaryConsistent).toBe(true)
+    expect(state.bytesRead).toBe(Buffer.byteLength(SOURCE) + 64)
+  })
+
+  it('preserves the inconsistent result on a read error', async () => {
+    state.readError = true
+    expect((await read()).boundaryConsistent).toBe(false)
+  })
+
+  it('does not open a source without an anchor', async () => {
+    expect((await read(null)).boundaryConsistent).toBe(false)
+    expect(state.opens).toBe(0)
+    expect(state.bytesRead).toBe(0)
+  })
+})
diff --git a/src/main/claude/claude-structured-history-window.ts b/src/main/claude/claude-structured-history-window.ts
index 1cfd48a8a63..f7a541fa317 100644
--- a/src/main/claude/claude-structured-history-window.ts
+++ b/src/main/claude/claude-structured-history-window.ts
@@ -12,8 +12,8 @@
 // of those makes absence meaningless. Failing it reports an inconsistent
 // boundary rather than an empty window, because the two decide opposite things.
 
-import { readFile, stat } from 'node:fs/promises'
 import { join } from 'node:path'
+import { readNodeFileWithinLimit } from '../../shared/node-bounded-file-reader'
 import type { AgentSessionJournalIdentity } from '../../shared/agent-session-journal-types'
 import { resolveSessionFilePath } from '../native-chat/session-file-resolver'
 import type {
@@ -284,10 +284,11 @@ export async function readClaudeProviderHistoryWindow(input: {
   }
   let contents: string
   try {
-    if ((await stat(input.transcriptPath)).size > MAX_HISTORY_WINDOW_SOURCE_BYTES) {
-      return INCONSISTENT
-    }
-    contents = await readFile(input.transcriptPath, 'utf8')
+    const read = await readNodeFileWithinLimit(
+      input.transcriptPath,
+      MAX_HISTORY_WINDOW_SOURCE_BYTES
+    )
+    contents = read.buffer.toString('utf8')
   } catch {
     return INCONSISTENT
   }