diff --git a/packages/settings/settings-local/src/index.ts b/packages/settings/settings-local/src/index.ts
index b305f61fe7..b9df71f9e1 100644
--- a/packages/settings/settings-local/src/index.ts
+++ b/packages/settings/settings-local/src/index.ts
@@ -9,11 +9,11 @@ import { Context, Service } from 'cordis'
 import z from 'schemastery'
 import { watch as chokidarWatch } from 'chokidar'
 import { randomBytes } from 'node:crypto'
-import { mkdir, readFile, rename, rm, writeFile } from 'node:fs/promises'
+import { mkdir, readFile, rename, rm, stat, writeFile } from 'node:fs/promises'
 import { dirname, extname, join, resolve } from 'node:path'
 import { Document, parseDocument } from 'yaml'
 import { resolveDshHome } from '@deepseek-ai/dsh-paths'
-import { Settings, type SettingsNamespace } from '@deepseek-ai/dsh-settings'
+import { Settings, deepEqualJson, type SettingsNamespace } from '@deepseek-ai/dsh-settings'
 
 /** Plugin config: file location and hot-reload behavior. */
 export interface Config {
@@ -64,11 +64,53 @@ export function resolveSpec(config: Config): ResolvedSpec {
   }
 }
 
+/** Whether a parsed YAML value is a map for diffing purposes. */
+function isMapLike(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value)
+}
+
+/**
+ * Apply the difference between one node's stored and next value as minimal
+ * `setIn`/`deleteIn` edits, recursing through maps, so every untouched node —
+ * and the key node of every changed pair — keeps its comments, anchors, and
+ * formatting. Non-map values (arrays and scalars) replace wholesale when
+ * unequal, taking any comments inside them along.
+ */
+function patchNode(document: Document, path: readonly string[], current: unknown, next: unknown): void {
+  if (isMapLike(current) && isMapLike(next)) {
+    for (const key of Object.keys(current)) {
+      if (!(key in next)) document.deleteIn([...path, key])
+    }
+    for (const [key, value] of Object.entries(next)) {
+      patchNode(document, [...path, key], current[key], value)
+    }
+    return
+  }
+  if (!deepEqualJson(current, next)) document.setIn([...path], next)
+}
+
 /** Whether a filesystem error means absence; every non-ENOENT failure must surface. */
 function isENOENT(error: unknown): boolean {
   return (error as NodeJS.ErrnoException | null)?.code === 'ENOENT'
 }
 
+/** Whether an exclusive create failed because the path already exists. */
+function isEEXIST(error: unknown): boolean {
+  return (error as NodeJS.ErrnoException | null)?.code === 'EEXIST'
+}
+
+/**
+ * Writer-lock protocol constants. These are robustness invariants of the
+ * cross-process write protocol, not deployment tunables: a holder rewrites one
+ * small document in milliseconds, so contention resolves well inside the
+ * retry deadline, and a lock older than the stale age can only belong to a
+ * crashed holder.
+ */
+const LOCK_RETRY_INITIAL_MS = 20
+const LOCK_RETRY_MAX_MS = 200
+const LOCK_TIMEOUT_MS = 2_000
+const LOCK_STALE_MS = 5_000
+
 /** File-backed settings provider (`settings.yaml`/`.json`). */
 export class SettingsLocal extends Settings {
   static Config: z<Config> = z.object({
@@ -85,10 +127,13 @@ export class SettingsLocal extends Settings {
    * this cache are no-ops, which is also the self-write suppression.
    */
   private text: string | undefined
-  /** Serializes watcher-triggered reloads so reads never interleave. */
-  private refreshTask: Promise<void> = Promise.resolve()
-  /** Serializes whole-document writes across namespace queues; settled tail. */
-  private persistChain: Promise<void> = Promise.resolve()
+  /**
+   * Single exclusive operation chain: watcher reloads and document writes run
+   * one at a time in queue order (settled tail), so a write can never render
+   * from text a concurrent reload is busy replacing, and a reload can never
+   * read a half-committed write.
+   */
+  private operations: Promise<void> = Promise.resolve()
   /** Set at dispose: refuse new watcher events and let in-flight work no-op. */
   private closed = false
 
@@ -125,32 +170,107 @@ export class SettingsLocal extends Settings {
 
   protected persist(ns: SettingsNamespace, section: Record<string, unknown>): Promise<void> {
     // One document backs every namespace, so writes from different namespace
-    // queues must serialize here: each render must see the text the previous
-    // write committed, or the loser's section silently vanishes from disk.
-    // The stored tail is settled on both outcomes, so chaining needs no catch.
-    const task = this.persistChain.then(() => this.persistSection(ns, section))
-    this.persistChain = task.then(() => undefined, () => undefined)
+    // queues serialize with each other and with watcher reloads on the one
+    // operation chain: each render must see the text the previous operation
+    // committed, or a sibling section silently vanishes from disk.
+    return this.enqueue(() => this.persistSection(ns, section))
+  }
+
+  /** Queue one exclusive document operation behind every earlier one. */
+  private enqueue<T>(operation: () => Promise<T>): Promise<T> {
+    const task = this.operations.then(operation)
+    this.operations = task.then(() => undefined, () => undefined)
     return task
   }
 
+  /** Queue a reload; only an invariant violation escaping a commit can reject it. */
+  private queueRefresh(): void {
+    void this.enqueue(() => this.refresh()).catch((error: unknown) => {
+      // Only an invariant violation escaping the commit path can reject a
+      // refresh; keep the operation queue alive and surface it as an error so
+      // one poisoned commit cannot silently end hot reloading forever.
+      this.ctx.logger.error('settings-local: reload commit failed at %s', this.spec.filename)
+      this.ctx.logger.error(error)
+    })
+  }
+
   private async persistSection(ns: SettingsNamespace, section: Record<string, unknown>): Promise<void> {
-    const output = this.spec.format === 'yaml'
-      ? this.renderYaml(ns, section)
-      : this.renderJson(ns, section)
     await mkdir(dirname(this.spec.filename), { recursive: true })
-    // Exclusive-create (`wx`) a random-suffix sibling: the open refuses to
-    // follow any planted symlink at a guessable temp path, and the fresh inode
-    // carries owner-only permissions that survive the rename — a document that
-    // may hold personal values is never world-readable and never a symlink.
-    const temp = `${this.spec.filename}.${randomBytes(6).toString('hex')}.tmp`
-    try {
-      await writeFile(temp, output, { mode: 0o600, flag: 'wx' })
-      await rename(temp, this.spec.filename)
-    } catch (error) {
-      await rm(temp, { force: true })
-      throw error
+    await this.withWriterLock(async () => {
+      // Read-modify-write: fold in any on-disk state this process has not
+      // observed yet — an external edit still inside the watcher debounce
+      // window, a change the watcher missed, or another process's write — so
+      // the render below can never resurrect a stale document. An unparsable
+      // on-disk document fails the write loud instead of silently overwriting
+      // a user's manual edit.
+      await this.reconcileFromDisk()
+      const output = this.spec.format === 'yaml'
+        ? this.renderYaml(ns, section)
+        : this.renderJson(ns, section)
+      // Exclusive-create (`wx`) a random-suffix sibling: the open refuses to
+      // follow any planted symlink at a guessable temp path, and the fresh inode
+      // carries owner-only permissions that survive the rename — a document that
+      // may hold personal values is never world-readable and never a symlink.
+      const temp = `${this.spec.filename}.${randomBytes(6).toString('hex')}.tmp`
+      try {
+        await writeFile(temp, output, { mode: 0o600, flag: 'wx' })
+        await rename(temp, this.spec.filename)
+      } catch (error) {
+        await rm(temp, { force: true })
+        throw error
+      }
+      this.text = output
+    })
+  }
+
+  /**
+   * Hold the cross-process writer lock around one read-render-rename cycle.
+   * The lock is a `wx`-created sibling (`<file>.lock`); the rename-based
+   * commit keeps readers lock-free, so only writers contend. A lock older
+   * than {@link LOCK_STALE_MS} is a crashed holder and is broken with a
+   * warning; a live holder past {@link LOCK_TIMEOUT_MS} fails the write.
+   */
+  private async withWriterLock<T>(operation: () => Promise<T>): Promise<T> {
+    const lockPath = `${this.spec.filename}.lock`
+    const deadline = Date.now() + LOCK_TIMEOUT_MS
+    let delay = LOCK_RETRY_INITIAL_MS
+    for (;;) {
+      try {
+        await writeFile(lockPath, `${process.pid}\n`, { mode: 0o600, flag: 'wx' })
+        break
+      } catch (error) {
+        if (!isEEXIST(error)) throw error
+      }
+      const ageMs = await this.lockAgeMs(lockPath)
+      // The holder released between the failed create and the stat: the lock
+      // is free right now, so retry without burning backoff or deadline.
+      if (ageMs === undefined) continue
+      if (ageMs > LOCK_STALE_MS) {
+        this.ctx.logger.warn('settings-local: breaking a stale writer lock at %s', lockPath)
+        await rm(lockPath, { force: true })
+        continue
+      }
+      if (Date.now() >= deadline) {
+        throw new Error(`settings-local: timed out waiting for the writer lock at ${lockPath}`)
+      }
+      await new Promise(resolve => setTimeout(resolve, delay))
+      delay = Math.min(delay * 2, LOCK_RETRY_MAX_MS)
+    }
+    try {
+      return await operation()
+    } finally {
+      await rm(lockPath, { force: true })
+    }
+  }
+
+  /** Age of the writer lock, or `undefined` when it vanished after a failed create. */
+  private async lockAgeMs(lockPath: string): Promise<number | undefined> {
+    try {
+      return Date.now() - (await stat(lockPath)).mtimeMs
+    } catch (error) {
+      if (!isENOENT(error)) throw error
+      return undefined
     }
-    this.text = output
   }
 
   override async* [Service.init](): AsyncGenerator<() => Promise<void> | void, void, void> {
@@ -168,13 +288,14 @@ export class SettingsLocal extends Settings {
     })
     watcher.on('all', () => {
       if (this.closed) return
-      this.refreshTask = this.refreshTask.then(() => this.refresh()).catch((error: unknown) => {
-        // Only an invariant violation escaping the commit path can reject a
-        // refresh; keep the reload queue alive and surface it as an error so
-        // one poisoned commit cannot silently end hot reloading forever.
-        this.ctx.logger.error('settings-local: reload commit failed at %s', this.spec.filename)
-        this.ctx.logger.error(error)
-      })
+      this.queueRefresh()
+    })
+    watcher.on('ready', () => {
+      // The base init's load raced the watcher's own setup: a change written
+      // between that read and the watcher becoming active never fires an
+      // event. One reconcile at ready closes the gap.
+      if (this.closed) return
+      this.queueRefresh()
     })
     watcher.on('error', (error) => {
       this.ctx.logger.warn('settings-local: watcher error on %s', this.spec.filename)
@@ -182,10 +303,10 @@ export class SettingsLocal extends Settings {
     })
     yield async () => {
       // Quiesce: stop accepting events, close the watcher, then wait out any
-      // queued or in-flight refresh so nothing publishes after disposal.
+      // queued or in-flight operation so nothing publishes after disposal.
       this.closed = true
       await watcher.close()
-      await this.refreshTask
+      await this.operations
     }
   }
 
@@ -212,46 +333,62 @@ export class SettingsLocal extends Settings {
    * Re-read the document after a watcher event. Unchanged content (including
    * this provider's own writes) is a no-op; an unreadable or unparsable
    * document keeps the last good sections and warns — a live hot-reload must
-   * never take the process down.
+   * never take the process down. An invariant violation escaping a commit is
+   * not a reload failure and propagates to the queue's error surface.
    */
   private async refresh(): Promise<void> {
     if (this.closed) return
-    let text: string
+    try {
+      await this.reconcileFromDisk()
+    } catch (error) {
+      if ((error as { code?: unknown } | null)?.code === 'INVARIANT') throw error
+      this.ctx.logger.warn('settings-local: reload failed at %s; keeping the last good document', this.spec.filename)
+      this.ctx.logger.warn(error)
+    }
+  }
+
+  /**
+   * Compare the on-disk text against the cache and publish any difference
+   * into the seam. Absence publishes the empty document; an unreadable or
+   * unparsable file throws, so each caller picks its policy — a reload warns
+   * and keeps the last good document, a write fails loud.
+   */
+  private async reconcileFromDisk(): Promise<void> {
+    let text: string | undefined
     try {
       text = await readFile(this.spec.filename, 'utf8')
     } catch (error) {
-      if (!isENOENT(error)) {
-        this.ctx.logger.warn('settings-local: reload failed at %s; keeping the last good document', this.spec.filename)
-        this.ctx.logger.warn(error)
-        return
-      }
-      if (this.text === undefined || this.isClosed()) return
+      if (!isENOENT(error)) throw error
+      text = undefined
+    }
+    if (text === this.text || this.isClosed()) return
+    if (text === undefined) {
       this.text = undefined
       this.publish({})
       return
     }
-    if (text === this.text || this.isClosed()) return
-    let doc: Record<string, unknown>
-    try {
-      doc = this.parse(text)
-    } catch (error) {
-      this.ctx.logger.warn('settings-local: reload failed at %s; keeping the last good document', this.spec.filename)
-      this.ctx.logger.warn(error)
-      return
-    }
+    const doc = this.parse(text)
     this.text = text
     this.publish(doc)
   }
 
-  /** Render the next YAML text by patching one namespace in the comment-preserving document. */
+  /**
+   * Render the next YAML text by patching one namespace in the
+   * comment-preserving document. The next section lands as a leaf-level diff
+   * against the stored one — only changed values set, only removed keys
+   * delete — so comments inside the section survive edits to their siblings,
+   * not just comments outside it.
+   */
   private renderYaml(ns: SettingsNamespace, section: Record<string, unknown>): string {
     if (this.text === undefined) {
       return new Document({ [ns]: section }).toString()
     }
     // this.text only ever caches content that parsed successfully, so this
-    // re-parse (for the mutable comment-preserving tree) cannot fail.
+    // re-parse (for the mutable comment-preserving tree) cannot fail, and
+    // parse() already rejected any non-map root.
     const document = parseDocument(this.text)
-    document.set(ns, section)
+    const root: unknown = document.toJS()
+    patchNode(document, [ns], isMapLike(root) ? root[ns] : undefined, section)
     return document.toString()
   }
 
diff --git a/packages/settings/settings-local/tests/concurrency.spec.ts b/packages/settings/settings-local/tests/concurrency.spec.ts
new file mode 100644
index 0000000000..ab09866819
--- /dev/null
+++ b/packages/settings/settings-local/tests/concurrency.spec.ts
@@ -0,0 +1,103 @@
+// Cross-instance and writer-lock behavior: two providers on one document are
+// the in-process equivalent of two dsh processes sharing a harness home —
+// neither knows the other's cache, so only the read-modify-write cycle under
+// the `<file>.lock` sibling keeps both namespaces alive on disk.
+import { afterEach, describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import z from 'schemastery'
+import { chmod, mkdtemp, readFile, rm, utimes, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { settingsNamespace } from '@deepseek-ai/dsh-settings'
+import { SettingsLocal } from '../src/index.ts'
+
+const AlphaSchema: z<{ value: number }> = z.object({ value: z.number().default(0) })
+const BetaSchema: z<{ value: number }> = z.object({ value: z.number().default(0) })
+
+const cleanups: Array<() => Promise<void>> = []
+
+afterEach(async () => {
+  while (cleanups.length > 0) await cleanups.pop()!()
+})
+
+async function tempDir(): Promise<string> {
+  const dir = await mkdtemp(join(tmpdir(), 'dsh-settings-lock-'))
+  cleanups.push(() => rm(dir, { recursive: true, force: true }))
+  return dir
+}
+
+async function boot(config: ConstructorParameters<typeof SettingsLocal>[1]): Promise<Context> {
+  const ctx = new Context()
+  const fiber = ctx.plugin(SettingsLocal, config)
+  cleanups.push(async () => { await fiber.dispose() })
+  await fiber
+  return ctx
+}
+
+describe('cross-instance writes', () => {
+  it('keeps both namespaces when two providers write the same document concurrently', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const first = await boot({ path, watch: false })
+    const second = await boot({ path, watch: false })
+    const alpha = first.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    const beta = second.settings.register(settingsNamespace('beta'), BetaSchema)
+    const rounds = [1, 2, 3, 4, 5]
+    await Promise.all([
+      (async () => { for (const value of rounds) await alpha.update({ value }) })(),
+      (async () => { for (const value of rounds) await beta.update({ value }) })(),
+    ])
+    const text = await readFile(path, 'utf8')
+    expect(text).toContain('alpha:')
+    expect(text).toContain('beta:')
+    // A third instance resolves both final values from the shared document.
+    const third = await boot({ path, watch: false })
+    expect(third.settings.register(settingsNamespace('alpha'), AlphaSchema).get()).toEqual({ value: 5 })
+    expect(third.settings.register(settingsNamespace('beta'), BetaSchema).get()).toEqual({ value: 5 })
+  })
+})
+
+describe('writer lock', () => {
+  it('waits for a busy writer lock instead of failing', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    await writeFile(`${path}.lock`, 'holder\n')
+    const release = setTimeout(() => { void rm(`${path}.lock`, { force: true }) }, 120)
+    cleanups.push(async () => { clearTimeout(release) })
+    await scope.update({ value: 7 })
+    expect(await readFile(path, 'utf8')).toContain('value: 7')
+  })
+
+  it('breaks a stale writer lock with a warning and writes through', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    await writeFile(`${path}.lock`, 'crashed-holder\n')
+    const past = (Date.now() - 60_000) / 1000
+    await utimes(`${path}.lock`, past, past)
+    await scope.update({ value: 9 })
+    expect(await readFile(path, 'utf8')).toContain('value: 9')
+  })
+
+  it('times out on a lock a live holder never releases', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    await writeFile(`${path}.lock`, 'busy-holder\n')
+    await expect(scope.update({ value: 1 })).rejects.toThrow(/timed out waiting for the writer lock/)
+  }, 10_000)
+
+  it('surfaces a non-contention lock failure as the write error', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    await chmod(dir, 0o500)
+    cleanups.push(() => chmod(dir, 0o700))
+    await expect(scope.update({ value: 1 })).rejects.toThrow(/EACCES|permission/)
+  })
+})
diff --git a/packages/settings/settings-local/tests/local.spec.ts b/packages/settings/settings-local/tests/local.spec.ts
index 4c3c24ccd9..0b753675f5 100644
--- a/packages/settings/settings-local/tests/local.spec.ts
+++ b/packages/settings/settings-local/tests/local.spec.ts
@@ -204,6 +204,102 @@ describe('persist', () => {
     expect(written).toContain('theme: light')
   })
 
+  it('keeps comments inside the section when a sibling key changes', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, [
+      'ui-theme:',
+      '  # chosen during onboarding',
+      '  theme: light',
+      '  fontSize: 12',
+      '',
+    ].join('\n'))
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    await scope.update({ fontSize: 18 })
+    const written = await readFile(path, 'utf8')
+    expect(written).toContain('# chosen during onboarding')
+    expect(written).toContain('theme: light')
+    expect(written).toContain('fontSize: 18')
+  })
+
+  it('keeps a changed key\'s own-line comment while replacing its value', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, [
+      'ui-theme:',
+      '  # chosen during onboarding',
+      '  theme: light',
+      '',
+    ].join('\n'))
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    await scope.update({ theme: 'dark' })
+    const written = await readFile(path, 'utf8')
+    expect(written).toContain('# chosen during onboarding')
+    expect(written).toContain('theme: dark')
+  })
+
+  it('deletes only the removed key on replace, keeping sibling comments', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, [
+      'ui-theme:',
+      '  # chosen during onboarding',
+      '  theme: light',
+      '  fontSize: 12',
+      '',
+    ].join('\n'))
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    await scope.replace({ theme: 'light' })
+    const written = await readFile(path, 'utf8')
+    expect(written).toContain('# chosen during onboarding')
+    expect(written).toContain('theme: light')
+    expect(written).not.toContain('fontSize')
+  })
+
+  it('keeps an unchanged array\'s comments and replaces a changed array wholesale', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const TagsSchema: z<{ tags: string[]; label: string }> = z.object({
+      tags: z.array(z.string()).default([]),
+      label: z.string().default(''),
+    })
+    await writeFile(path, [
+      'workspace:',
+      '  tags:',
+      '    # pinned by hand',
+      '    - alpha',
+      '  label: draft',
+      '',
+    ].join('\n'))
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('workspace'), TagsSchema)
+    await scope.update({ label: 'final' })
+    const untouched = await readFile(path, 'utf8')
+    expect(untouched).toContain('# pinned by hand')
+    expect(untouched).toContain('label: final')
+    // A changed array replaces wholesale; comments inside it go with it.
+    await scope.update({ tags: ['beta'] })
+    const replaced = await readFile(path, 'utf8')
+    expect(replaced).not.toContain('# pinned by hand')
+    expect(replaced).toContain('- beta')
+  })
+
+  it('keeps a comment-only document\'s comment when the first section lands', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    // Parses to a null root: the document exists but holds no sections yet.
+    await writeFile(path, '# reserved for future settings\n')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    await scope.update({ theme: 'light' })
+    const written = await readFile(path, 'utf8')
+    expect(written).toContain('# reserved for future settings')
+    expect(written).toContain('theme: light')
+  })
+
   it('creates a json document from scratch', async () => {
     const dir = await tempDir()
     const path = join(dir, 'settings.json')
diff --git a/packages/settings/settings-local/tests/lock-race.spec.ts b/packages/settings/settings-local/tests/lock-race.spec.ts
new file mode 100644
index 0000000000..09eb025654
--- /dev/null
+++ b/packages/settings/settings-local/tests/lock-race.spec.ts
@@ -0,0 +1,100 @@
+// Writer-lock races that cannot be timed from outside: a contender whose lock
+// vanishes between the failed exclusive create and the stat, a stat failing
+// for a reason other than absence, and a temp-file write failing mid-cycle.
+// The fs/promises seam is partially mocked to inject exactly one failure at a
+// chosen path suffix; everything else passes through to the real filesystem.
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { Context } from 'cordis'
+import z from 'schemastery'
+import { access, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { settingsNamespace } from '@deepseek-ai/dsh-settings'
+import { SettingsLocal } from '../src/index.ts'
+
+const state = vi.hoisted(() => ({
+  /** One-shot failure injections keyed by operation, matched on a path suffix. */
+  failures: [] as Array<{ op: 'writeFile' | 'stat'; suffix: string; code: string }>,
+}))
+
+vi.mock('node:fs/promises', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('node:fs/promises')>()
+  const inject = (op: 'writeFile' | 'stat', path: unknown): void => {
+    const index = state.failures.findIndex(f => f.op === op && String(path).endsWith(f.suffix))
+    if (index === -1) return
+    const [failure] = state.failures.splice(index, 1)
+    throw Object.assign(new Error(`${failure!.code}: injected ${op} failure`), { code: failure!.code })
+  }
+  return {
+    ...actual,
+    writeFile: (async (path: unknown, ...rest: never[]) => {
+      inject('writeFile', path)
+      return (actual.writeFile as (path: unknown, ...args: never[]) => Promise<void>)(path, ...rest)
+    }) as typeof actual.writeFile,
+    stat: (async (path: unknown, ...rest: never[]) => {
+      inject('stat', path)
+      return (actual.stat as (path: unknown, ...args: never[]) => Promise<unknown>)(path, ...rest)
+    }) as typeof actual.stat,
+  }
+})
+
+const AlphaSchema: z<{ value: number }> = z.object({ value: z.number().default(0) })
+
+const cleanups: Array<() => Promise<void>> = []
+
+afterEach(async () => {
+  state.failures.length = 0
+  while (cleanups.length > 0) await cleanups.pop()!()
+})
+
+async function tempDir(): Promise<string> {
+  const dir = await mkdtemp(join(tmpdir(), 'dsh-settings-lockrace-'))
+  cleanups.push(() => rm(dir, { recursive: true, force: true }))
+  return dir
+}
+
+async function boot(config: ConstructorParameters<typeof SettingsLocal>[1]): Promise<Context> {
+  const ctx = new Context()
+  const fiber = ctx.plugin(SettingsLocal, config)
+  cleanups.push(async () => { await fiber.dispose() })
+  await fiber
+  return ctx
+}
+
+describe('writer-lock races', () => {
+  it('retries immediately when the contending lock vanished before the stat', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    // The exclusive create loses to a holder that releases before the stat:
+    // no lock file actually exists, so the stat sees honest absence and the
+    // very next attempt takes the lock.
+    state.failures.push({ op: 'writeFile', suffix: '.lock', code: 'EEXIST' })
+    await scope.update({ value: 3 })
+    expect(await readFile(path, 'utf8')).toContain('value: 3')
+  })
+
+  it('propagates a stat failure that does not mean absence', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    state.failures.push({ op: 'writeFile', suffix: '.lock', code: 'EEXIST' })
+    state.failures.push({ op: 'stat', suffix: '.lock', code: 'EACCES' })
+    await expect(scope.update({ value: 3 })).rejects.toThrow(/EACCES/)
+  })
+
+  it('cleans up the temp file and releases the lock when the write fails mid-cycle', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, 'alpha:\n  value: 1\n')
+    const ctx = await boot({ path, watch: false })
+    const scope = ctx.settings.register(settingsNamespace('alpha'), AlphaSchema)
+    state.failures.push({ op: 'writeFile', suffix: '.tmp', code: 'ENOSPC' })
+    await expect(scope.update({ value: 9 })).rejects.toThrow(/ENOSPC/)
+    // The document is untouched and the writer lock was released on the way out.
+    expect(await readFile(path, 'utf8')).toContain('value: 1')
+    await expect(access(`${path}.lock`)).rejects.toThrow()
+  })
+})
diff --git a/packages/settings/settings-local/tests/watcher.spec.ts b/packages/settings/settings-local/tests/watcher.spec.ts
index 439f213473..7b289b22a3 100644
--- a/packages/settings/settings-local/tests/watcher.spec.ts
+++ b/packages/settings/settings-local/tests/watcher.spec.ts
@@ -1,7 +1,7 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { Context } from 'cordis'
 import z from 'schemastery'
-import { chmod, mkdtemp, rm, writeFile } from 'node:fs/promises'
+import { chmod, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { settingsNamespace } from '@deepseek-ai/dsh-settings'
@@ -155,6 +155,7 @@ describe('watcher pipeline', () => {
     await fiber.dispose()
     disposed = true
     instance!.watcher.emit('all', 'change', path)
+    instance!.watcher.emit('ready')
     await new Promise(resolve => setTimeout(resolve, 100))
     expect(postDisposeCommits).toBe(0)
   })
@@ -169,4 +170,56 @@ describe('watcher pipeline', () => {
     await new Promise(resolve => setTimeout(resolve, 50))
     expect(scope.get()).toEqual({ theme: 'dark' })
   })
+
+  it('folds an unobserved external edit into a write instead of overwriting it', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, 'ui-theme:\n  theme: light\n')
+    const ctx = await boot({ path, debounceMs: 5 })
+    const theme = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    const editor = ctx.settings.register(settingsNamespace('editor'), z.object({
+      tabWidth: z.number().default(2),
+    }))
+    // The external edit has landed on disk but its watcher event has not
+    // fired yet (a debounce window, or a missed event): the write must fold
+    // it in, not resurrect the stale document.
+    await writeFile(path, 'ui-theme:\n  theme: light\neditor:\n  tabWidth: 8\n')
+    await theme.update({ theme: 'darker' })
+    const text = await readFile(path, 'utf8')
+    expect(text).toContain('tabWidth: 8')
+    expect(text).toContain('theme: darker')
+    // The fold published the unobserved section before the write committed.
+    expect(editor.get()).toEqual({ tabWidth: 8 })
+  })
+
+  it('reconciles at watcher ready so a change during setup is not missed', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, 'ui-theme:\n  theme: light\n')
+    const ctx = await boot({ path, debounceMs: 5 })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    // Written after the initial load but before the watcher became active:
+    // no 'all' event will ever fire for it.
+    await writeFile(path, 'ui-theme:\n  theme: written-before-ready\n')
+    const [instance] = await fakeInstances()
+    instance!.watcher.emit('ready')
+    await vi.waitFor(() => {
+      expect(scope.get().theme).toBe('written-before-ready')
+    })
+  })
+
+  it('fails a write loud when the on-disk document turned invalid unobserved', async () => {
+    const dir = await tempDir()
+    const path = join(dir, 'settings.yaml')
+    await writeFile(path, 'ui-theme:\n  theme: light\n')
+    const ctx = await boot({ path, debounceMs: 5 })
+    const scope = ctx.settings.register(settingsNamespace('ui-theme'), ThemeSchema)
+    const broken = 'ui-theme: [unclosed\n  flow: {\n'
+    await writeFile(path, broken)
+    await expect(scope.update({ theme: 'darker' })).rejects.toThrow(/invalid document/)
+    // The user's manual edit stays on disk untouched and the cache keeps the
+    // last good value.
+    expect(await readFile(path, 'utf8')).toBe(broken)
+    expect(scope.get()).toEqual({ theme: 'light' })
+  })
 })