diff --git a/core/services/worker/ephemeral_capacity.go b/core/services/worker/ephemeral_capacity.go
new file mode 100644
index 000000000..32ba77574
--- /dev/null
+++ b/core/services/worker/ephemeral_capacity.go
@@ -0,0 +1,628 @@
+// SPDX-License-Identifier: MIT
+
+package worker
+
+import (
+	"fmt"
+	"io"
+	"math"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"github.com/mudler/LocalAI/pkg/xsysinfo"
+)
+
+const ephemeralCapacityWriteChunk int64 = 64 << 10
+
+// EphemeralCapacityError reports the values used to reject a reservation.
+type EphemeralCapacityError struct {
+	RequestedBytes int64
+	UsageBytes     int64
+	LimitBytes     int64
+	AvailableBytes int64
+	HeadroomBytes  int64
+}
+
+func (e *EphemeralCapacityError) Error() string {
+	return fmt.Sprintf(
+		"ephemeral capacity exceeded: requested=%d usage=%d limit=%d available=%d headroom=%d",
+		e.RequestedBytes,
+		e.UsageBytes,
+		e.LimitBytes,
+		e.AvailableBytes,
+		e.HeadroomBytes,
+	)
+}
+
+// EphemeralReservationConflictError reports an attempt to change an active
+// reservation without first committing or releasing it.
+type EphemeralReservationConflictError struct {
+	Path           string
+	ActiveBytes    int64
+	RequestedBytes int64
+}
+
+func (e *EphemeralReservationConflictError) Error() string {
+	return fmt.Sprintf(
+		"ephemeral path %q already has an active reservation of %d bytes; requested %d bytes",
+		e.Path,
+		e.ActiveBytes,
+		e.RequestedBytes,
+	)
+}
+
+type ephemeralCapacityState uint8
+
+const (
+	ephemeralCapacityExisting ephemeralCapacityState = iota
+	ephemeralCapacityActive
+	ephemeralCapacityWriting
+	ephemeralCapacityCommitted
+)
+
+type ephemeralCapacityEntry struct {
+	state       ephemeralCapacityState
+	baseline    int64
+	reserved    int64
+	pending     int64
+	inflight    int64
+	openWriters int
+}
+
+// EphemeralCapacityGuard accounts files and in-flight writes below a fixed set
+// of ephemeral roots.
+type EphemeralCapacityGuard struct {
+	mu            sync.Mutex
+	changed       *sync.Cond
+	roots         []string
+	byteLimit     int64
+	minFreeBytes  int64
+	usage         int64
+	entries       map[string]ephemeralCapacityEntry
+	commitWaiters map[string]int
+}
+
+// NewEphemeralCapacityGuard creates a guard and accounts regular files already
+// present below roots. Directory walks do not follow symbolic links.
+func NewEphemeralCapacityGuard(roots []string, byteLimit, minFreeBytes int64) (*EphemeralCapacityGuard, error) {
+	if len(roots) == 0 {
+		return nil, fmt.Errorf("at least one ephemeral root is required")
+	}
+	if byteLimit < 0 {
+		return nil, fmt.Errorf("ephemeral byte limit must not be negative")
+	}
+	if minFreeBytes < 0 {
+		return nil, fmt.Errorf("ephemeral free-space headroom must not be negative")
+	}
+
+	guard := &EphemeralCapacityGuard{
+		roots:         make([]string, 0, len(roots)),
+		byteLimit:     byteLimit,
+		minFreeBytes:  minFreeBytes,
+		entries:       make(map[string]ephemeralCapacityEntry),
+		commitWaiters: make(map[string]int),
+	}
+	guard.changed = sync.NewCond(&guard.mu)
+	seenRoots := make(map[string]struct{}, len(roots))
+	for _, root := range roots {
+		cleanRoot, err := cleanEphemeralAbsolutePath(root)
+		if err != nil {
+			return nil, fmt.Errorf("resolving ephemeral root: %w", err)
+		}
+		if _, found := seenRoots[cleanRoot]; found {
+			continue
+		}
+		if err := rejectEphemeralSymlinkComponents(cleanRoot); err != nil {
+			return nil, fmt.Errorf("validating ephemeral root %q: %w", cleanRoot, err)
+		}
+		seenRoots[cleanRoot] = struct{}{}
+		guard.roots = append(guard.roots, cleanRoot)
+	}
+
+	for _, root := range guard.roots {
+		if err := guard.accountExistingFiles(root); err != nil {
+			return nil, err
+		}
+	}
+	return guard, nil
+}
+
+// Reserve atomically reserves size additional bytes for path. An existing or
+// committed file remains charged until the new write is committed.
+func (g *EphemeralCapacityGuard) Reserve(path string, size int64) error {
+	if size < 0 {
+		return fmt.Errorf("reservation size must not be negative")
+	}
+	cleanPath, root, err := g.registeredPath(path)
+	if err != nil {
+		return err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[cleanPath]
+	if found && entry.isActive() {
+		if entry.reserved == size {
+			return nil
+		}
+		return &EphemeralReservationConflictError{
+			Path: cleanPath, ActiveBytes: entry.reserved, RequestedBytes: size,
+		}
+	}
+	if err := g.checkCapacityLocked(root, size); err != nil {
+		return err
+	}
+	entry.state = ephemeralCapacityActive
+	entry.reserved = size
+	entry.pending = size
+	entry.inflight = 0
+	g.entries[cleanPath] = entry
+	g.usage += size
+	return nil
+}
+
+// Commit replaces the path's baseline and reservation with the regular file's
+// actual size. It waits for every bounded writer for the path to close.
+func (g *EphemeralCapacityGuard) Commit(path string) error {
+	cleanPath, root, err := g.registeredPath(path)
+	if err != nil {
+		return err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for {
+		entry, found := g.entries[cleanPath]
+		if !found {
+			return fmt.Errorf("ephemeral path %q has no reservation", cleanPath)
+		}
+		if entry.state == ephemeralCapacityExisting || entry.state == ephemeralCapacityCommitted {
+			return nil
+		}
+		if entry.openWriters == 0 {
+			break
+		}
+		g.commitWaiters[cleanPath]++
+		g.changed.Wait()
+		g.commitWaiters[cleanPath]--
+		if g.commitWaiters[cleanPath] == 0 {
+			delete(g.commitWaiters, cleanPath)
+		}
+	}
+
+	info, err := os.Lstat(cleanPath)
+	if err != nil {
+		return fmt.Errorf("stating committed ephemeral file %q: %w", cleanPath, err)
+	}
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("committed ephemeral path %q is not a regular file", cleanPath)
+	}
+	entry := g.entries[cleanPath]
+	charged := entry.baseline + entry.reserved
+	if info.Size() > charged {
+		additional := info.Size() - charged
+		if err := g.checkCapacityLocked(root, additional); err != nil {
+			return err
+		}
+		charged += additional
+		g.usage += additional
+	}
+	g.usage -= charged - info.Size()
+	entry.state = ephemeralCapacityCommitted
+	entry.baseline = info.Size()
+	entry.reserved = 0
+	entry.pending = 0
+	entry.inflight = 0
+	g.entries[cleanPath] = entry
+	return nil
+}
+
+// Release forgets all accounting for path. It is safe to call repeatedly.
+func (g *EphemeralCapacityGuard) Release(path string) error {
+	cleanPath, _, err := g.registeredPath(path)
+	if err != nil {
+		return err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for {
+		entry, found := g.entries[cleanPath]
+		if !found {
+			return nil
+		}
+		if entry.openWriters == 0 {
+			g.releaseLocked(cleanPath)
+			return nil
+		}
+		g.changed.Wait()
+	}
+}
+
+// Account records bytes found by recovery after startup. Existing active
+// reservations are left unchanged so recovery cannot erase in-flight charges.
+func (g *EphemeralCapacityGuard) Account(path string, size int64) error {
+	if size < 0 {
+		return fmt.Errorf("accounted size must not be negative")
+	}
+	cleanPath, _, err := g.registeredPath(path)
+	if err != nil {
+		return err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[cleanPath]
+	if found && entry.isActive() {
+		return &EphemeralReservationConflictError{
+			Path: cleanPath, ActiveBytes: entry.reserved, RequestedBytes: size,
+		}
+	}
+	newUsage := g.usage
+	if found {
+		newUsage -= entry.baseline
+	}
+	if size > math.MaxInt64-newUsage {
+		return fmt.Errorf("ephemeral usage exceeds supported size")
+	}
+	entry = ephemeralCapacityEntry{state: ephemeralCapacityExisting, baseline: size}
+	g.entries[cleanPath] = entry
+	g.usage = newUsage + size
+	return nil
+}
+
+// ReleaseTree forgets accounting for files at or below path. Recovery cleanup
+// can call this after it successfully removes a stale request tree.
+func (g *EphemeralCapacityGuard) ReleaseTree(path string) error {
+	cleanPath, _, err := g.registeredPath(path)
+	if err != nil {
+		return err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for g.hasOpenWriterLocked(cleanPath) {
+		g.changed.Wait()
+	}
+	for entryPath := range g.entries {
+		if ephemeralPathAtOrBelow(entryPath, cleanPath) {
+			g.releaseLocked(entryPath)
+		}
+	}
+	return nil
+}
+
+// HasActiveReservation reports whether path itself or a descendant has an
+// active reservation. Recovery cleanup uses it to avoid active request trees.
+func (g *EphemeralCapacityGuard) HasActiveReservation(path string) bool {
+	cleanPath, _, err := g.registeredPath(path)
+	if err != nil {
+		return false
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for entryPath, entry := range g.entries {
+		if entry.isActive() && ephemeralPathAtOrBelow(entryPath, cleanPath) {
+			return true
+		}
+	}
+	return false
+}
+
+func (g *EphemeralCapacityGuard) commitWaiterCount(path string) int {
+	cleanPath, _, err := g.registeredPath(path)
+	if err != nil {
+		return 0
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.commitWaiters[cleanPath]
+}
+
+// NewWriter returns a writer that admits unknown-length input in bounded
+// chunks. Callers must close it before committing or releasing the path.
+func (g *EphemeralCapacityGuard) NewWriter(path string, destination io.Writer) (*EphemeralCapacityWriter, error) {
+	if destination == nil {
+		return nil, fmt.Errorf("ephemeral writer destination is nil")
+	}
+	cleanPath, root, err := g.registeredPath(path)
+	if err != nil {
+		return nil, err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[cleanPath]
+	if !found || !entry.isActive() {
+		if err := g.checkCapacityLocked(root, 0); err != nil {
+			return nil, err
+		}
+		entry.state = ephemeralCapacityActive
+		entry.reserved = 0
+		entry.pending = 0
+		entry.inflight = 0
+	}
+	entry.state = ephemeralCapacityWriting
+	entry.openWriters++
+	g.entries[cleanPath] = entry
+	return &EphemeralCapacityWriter{guard: g, path: cleanPath, destination: destination}, nil
+}
+
+// EphemeralCapacityWriter bounds writes through an EphemeralCapacityGuard.
+// Close finalizes its accounting lifecycle without closing the destination.
+type EphemeralCapacityWriter struct {
+	mu          sync.Mutex
+	guard       *EphemeralCapacityGuard
+	path        string
+	destination io.Writer
+	closed      bool
+}
+
+var _ io.WriteCloser = (*EphemeralCapacityWriter)(nil)
+
+func (w *EphemeralCapacityWriter) Write(payload []byte) (int, error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.closed {
+		return 0, fmt.Errorf("ephemeral capacity writer is closed")
+	}
+
+	total := 0
+	for len(payload) > 0 {
+		chunkLength := min(len(payload), int(ephemeralCapacityWriteChunk))
+		grown, err := w.guard.beginWrite(w.path, int64(chunkLength))
+		if err != nil {
+			return total, err
+		}
+		written, writeErr := w.destination.Write(payload[:chunkLength])
+		if written < 0 || written > chunkLength {
+			w.guard.settleWrite(w.path, int64(chunkLength), 0, grown)
+			return total, fmt.Errorf("ephemeral destination returned invalid write count %d", written)
+		}
+		w.guard.settleWrite(w.path, int64(chunkLength), int64(written), grown)
+		total += written
+		payload = payload[written:]
+		if writeErr != nil {
+			return total, writeErr
+		}
+		if written != chunkLength {
+			return total, io.ErrShortWrite
+		}
+	}
+	return total, nil
+}
+
+func (w *EphemeralCapacityWriter) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.closed {
+		return nil
+	}
+	w.closed = true
+	w.guard.closeWriter(w.path)
+	return nil
+}
+
+func (e ephemeralCapacityEntry) isActive() bool {
+	return e.state == ephemeralCapacityActive || e.state == ephemeralCapacityWriting
+}
+
+func (g *EphemeralCapacityGuard) accountExistingFiles(root string) error {
+	err := filepath.WalkDir(root, func(path string, dirEntry os.DirEntry, walkErr error) error {
+		if walkErr != nil {
+			return walkErr
+		}
+		if dirEntry.Type()&os.ModeSymlink != 0 {
+			if dirEntry.IsDir() {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		info, err := dirEntry.Info()
+		if err != nil {
+			return err
+		}
+		if !info.Mode().IsRegular() {
+			return nil
+		}
+		cleanPath := filepath.Clean(path)
+		if _, found := g.entries[cleanPath]; found {
+			return nil
+		}
+		if info.Size() > math.MaxInt64-g.usage {
+			return fmt.Errorf("ephemeral usage exceeds supported size")
+		}
+		g.entries[cleanPath] = ephemeralCapacityEntry{
+			state: ephemeralCapacityExisting, baseline: info.Size(),
+		}
+		g.usage += info.Size()
+		return nil
+	})
+	if os.IsNotExist(err) {
+		return nil
+	}
+	if err != nil {
+		return fmt.Errorf("accounting ephemeral root %q: %w", root, err)
+	}
+	return nil
+}
+
+func (g *EphemeralCapacityGuard) beginWrite(path string, size int64) (int64, error) {
+	_, root, err := g.registeredPath(path)
+	if err != nil {
+		return 0, err
+	}
+
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[path]
+	if !found || entry.state != ephemeralCapacityWriting || entry.openWriters == 0 {
+		return 0, fmt.Errorf("ephemeral path %q has no active writer", path)
+	}
+	availableReservation := entry.pending - entry.inflight
+	grow := max(int64(0), size-availableReservation)
+	if err := g.checkCapacityLocked(root, grow); err != nil {
+		return 0, err
+	}
+	entry.reserved += grow
+	entry.pending += grow
+	entry.inflight += size
+	g.entries[path] = entry
+	g.usage += grow
+	return grow, nil
+}
+
+func (g *EphemeralCapacityGuard) settleWrite(path string, attempted, written, grown int64) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[path]
+	if !found {
+		return
+	}
+	entry.inflight -= attempted
+	entry.pending -= written
+	unusedGrowth := min(grown, attempted-written)
+	entry.reserved -= unusedGrowth
+	entry.pending -= unusedGrowth
+	g.usage -= unusedGrowth
+	g.entries[path] = entry
+}
+
+func (g *EphemeralCapacityGuard) closeWriter(path string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	entry, found := g.entries[path]
+	if !found || entry.openWriters == 0 {
+		return
+	}
+	entry.openWriters--
+	if entry.openWriters == 0 {
+		entry.state = ephemeralCapacityActive
+	}
+	g.entries[path] = entry
+	g.changed.Broadcast()
+}
+
+func (g *EphemeralCapacityGuard) checkCapacityLocked(root string, requested int64) error {
+	available, err := ephemeralAvailableBytes(root)
+	if err != nil {
+		return err
+	}
+	exceedsLimit := requested > g.byteLimit-g.usage
+	pending := g.pendingLocked()
+	freeAfterHeadroom := available - min(available, g.minFreeBytes)
+	exceedsAvailable := pending > freeAfterHeadroom || requested > freeAfterHeadroom-pending
+	if exceedsLimit || exceedsAvailable {
+		return &EphemeralCapacityError{
+			RequestedBytes: requested,
+			UsageBytes:     g.usage,
+			LimitBytes:     g.byteLimit,
+			AvailableBytes: available,
+			HeadroomBytes:  g.minFreeBytes,
+		}
+	}
+	return nil
+}
+
+func (g *EphemeralCapacityGuard) pendingLocked() int64 {
+	var pending int64
+	for _, entry := range g.entries {
+		if entry.pending > math.MaxInt64-pending {
+			return math.MaxInt64
+		}
+		pending += entry.pending
+	}
+	return pending
+}
+
+func (g *EphemeralCapacityGuard) hasOpenWriterLocked(path string) bool {
+	for entryPath, entry := range g.entries {
+		if entry.openWriters > 0 && ephemeralPathAtOrBelow(entryPath, path) {
+			return true
+		}
+	}
+	return false
+}
+
+func (g *EphemeralCapacityGuard) releaseLocked(path string) {
+	entry, found := g.entries[path]
+	if !found {
+		return
+	}
+	g.usage -= entry.baseline + entry.reserved
+	delete(g.entries, path)
+}
+
+func (g *EphemeralCapacityGuard) registeredPath(path string) (string, string, error) {
+	cleanPath, err := cleanEphemeralAbsolutePath(path)
+	if err != nil {
+		return "", "", err
+	}
+	root := ""
+	for _, candidate := range g.roots {
+		if ephemeralPathAtOrBelow(cleanPath, candidate) && len(candidate) > len(root) {
+			root = candidate
+		}
+	}
+	if root == "" {
+		return "", "", fmt.Errorf("path %q is outside registered ephemeral roots", cleanPath)
+	}
+	if err := rejectEphemeralSymlinkComponents(cleanPath); err != nil {
+		return "", "", fmt.Errorf("validating ephemeral path %q: %w", cleanPath, err)
+	}
+	return cleanPath, root, nil
+}
+
+func cleanEphemeralAbsolutePath(path string) (string, error) {
+	if path == "" {
+		return "", fmt.Errorf("ephemeral path is empty")
+	}
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return "", fmt.Errorf("resolving ephemeral path %q: %w", path, err)
+	}
+	return filepath.Clean(absPath), nil
+}
+
+func rejectEphemeralSymlinkComponents(path string) error {
+	volume := filepath.VolumeName(path)
+	remainder := strings.TrimPrefix(path, volume)
+	current := volume + string(filepath.Separator)
+	remainder = strings.TrimPrefix(remainder, string(filepath.Separator))
+	for _, component := range strings.Split(remainder, string(filepath.Separator)) {
+		if component == "" {
+			continue
+		}
+		current = filepath.Join(current, component)
+		info, err := os.Lstat(current)
+		if os.IsNotExist(err) {
+			return nil
+		}
+		if err != nil {
+			return fmt.Errorf("checking path component %q: %w", current, err)
+		}
+		if info.Mode()&os.ModeSymlink != 0 {
+			return fmt.Errorf("path component %q is a symlink", current)
+		}
+	}
+	return nil
+}
+
+func ephemeralPathAtOrBelow(path, parent string) bool {
+	return path == parent || strings.HasPrefix(path, parent+string(filepath.Separator))
+}
+
+func ephemeralAvailableBytes(path string) (int64, error) {
+	diskInfo, err := xsysinfo.GetDiskInfo(path)
+	if err != nil {
+		return 0, fmt.Errorf("reading ephemeral filesystem availability: %w", err)
+	}
+	if diskInfo.Available > math.MaxInt64 {
+		return math.MaxInt64, nil
+	}
+	return int64(diskInfo.Available), nil
+}
diff --git a/core/services/worker/ephemeral_capacity_test.go b/core/services/worker/ephemeral_capacity_test.go
new file mode 100644
index 000000000..41a44db93
--- /dev/null
+++ b/core/services/worker/ephemeral_capacity_test.go
@@ -0,0 +1,317 @@
+// SPDX-License-Identifier: MIT
+
+package worker
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+type capacityGatedFileWriter struct {
+	file    *os.File
+	entered chan struct{}
+	resume  chan struct{}
+	once    sync.Once
+}
+
+func (w *capacityGatedFileWriter) Write(p []byte) (int, error) {
+	w.once.Do(func() {
+		close(w.entered)
+		<-w.resume
+	})
+	return w.file.Write(p)
+}
+
+type capacityShortWriter struct{}
+
+func (capacityShortWriter) Write(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	return 1, nil
+}
+
+var _ = Describe("EphemeralCapacityGuard", func() {
+	It("accounts existing regular files without following symlinks", func() {
+		root := GinkgoT().TempDir()
+		outside := filepath.Join(GinkgoT().TempDir(), "outside.bin")
+		Expect(os.WriteFile(filepath.Join(root, "existing.bin"), make([]byte, 6), 0o600)).To(Succeed())
+		Expect(os.WriteFile(outside, make([]byte, 100), 0o600)).To(Succeed())
+		Expect(os.Symlink(outside, filepath.Join(root, "outside-link"))).To(Succeed())
+
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+
+		err = guard.Reserve(filepath.Join(root, "next.bin"), 5)
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(capacityErr.RequestedBytes).To(Equal(int64(5)))
+		Expect(capacityErr.UsageBytes).To(Equal(int64(6)))
+		Expect(capacityErr.LimitBytes).To(Equal(int64(10)))
+		Expect(capacityErr.AvailableBytes).To(BeNumerically(">", 0))
+		Expect(capacityErr.HeadroomBytes).To(BeZero())
+	})
+
+	It("serializes competing reservations", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 1, 0)
+		Expect(err).NotTo(HaveOccurred())
+
+		start := make(chan struct{})
+		results := make(chan error, 32)
+		var wait sync.WaitGroup
+		for i := range 32 {
+			wait.Add(1)
+			go func(index int) {
+				defer wait.Done()
+				<-start
+				results <- guard.Reserve(filepath.Join(root, string(rune('a'+index))), 1)
+			}(i)
+		}
+		close(start)
+		wait.Wait()
+		close(results)
+
+		succeeded := 0
+		for result := range results {
+			if result == nil {
+				succeeded++
+				continue
+			}
+			var capacityErr *EphemeralCapacityError
+			Expect(errors.As(result, &capacityErr)).To(BeTrue())
+		}
+		Expect(succeeded).To(Equal(1))
+	})
+
+	It("makes only an equal active reservation idempotent", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "nested", "payload.bin")
+
+		Expect(guard.Reserve(path, 4)).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "nested", ".", "payload.bin"), 4)).To(Succeed())
+		err = guard.Reserve(path, 5)
+		var conflictErr *EphemeralReservationConflictError
+		Expect(errors.As(err, &conflictErr)).To(BeTrue())
+		Expect(conflictErr.ActiveBytes).To(Equal(int64(4)))
+		Expect(conflictErr.RequestedBytes).To(Equal(int64(5)))
+		Expect(guard.Reserve(filepath.Join(root, "other.bin"), 6)).To(Succeed())
+		Expect(guard.Release(filepath.Join(root, "nested", ".", "payload.bin"))).To(Succeed())
+		Expect(guard.Release(path)).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "replacement.bin"), 4)).To(Succeed())
+	})
+
+	It("retains committed bytes when the same path starts another reservation", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "payload.bin")
+
+		Expect(guard.Reserve(path, 4)).To(Succeed())
+		Expect(os.WriteFile(path, make([]byte, 4), 0o600)).To(Succeed())
+		Expect(guard.Commit(path)).To(Succeed())
+		Expect(guard.HasActiveReservation(path)).To(BeFalse())
+		Expect(guard.Reserve(path, 6)).To(Succeed())
+		Expect(guard.HasActiveReservation(path)).To(BeTrue())
+
+		err = guard.Reserve(filepath.Join(root, "overflow.bin"), 1)
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(capacityErr.UsageBytes).To(Equal(int64(10)))
+	})
+
+	It("retains startup-accounted bytes when the path is reserved", func() {
+		root := GinkgoT().TempDir()
+		path := filepath.Join(root, "payload.bin")
+		Expect(os.WriteFile(path, make([]byte, 4), 0o600)).To(Succeed())
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+
+		Expect(guard.Reserve(path, 6)).To(Succeed())
+		err = guard.Reserve(filepath.Join(root, "overflow.bin"), 1)
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(capacityErr.UsageBytes).To(Equal(int64(10)))
+	})
+
+	It("commits the regular file's actual size", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "payload.bin")
+
+		Expect(guard.Reserve(path, 10)).To(Succeed())
+		Expect(os.WriteFile(path, []byte("four"), 0o600)).To(Succeed())
+		Expect(guard.Commit(path)).To(Succeed())
+		Expect(guard.Commit(filepath.Clean(path))).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "six.bin"), 6)).To(Succeed())
+	})
+
+	It("preserves configured filesystem headroom", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 1<<30, 1<<62)
+		Expect(err).NotTo(HaveOccurred())
+
+		err = guard.Reserve(filepath.Join(root, "payload.bin"), 1)
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(capacityErr.RequestedBytes).To(Equal(int64(1)))
+		Expect(capacityErr.AvailableBytes).To(BeNumerically(">", 0))
+		Expect(capacityErr.HeadroomBytes).To(Equal(int64(1 << 62)))
+	})
+
+	It("reserves bounded chunks before forwarding unknown-length input", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, ephemeralCapacityWriteChunk+1, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "payload.bin")
+		file, err := os.Create(path)
+		Expect(err).NotTo(HaveOccurred())
+		DeferCleanup(file.Close)
+		writer, err := guard.NewWriter(path, file)
+		Expect(err).NotTo(HaveOccurred())
+
+		n, err := writer.Write(make([]byte, ephemeralCapacityWriteChunk+2))
+		Expect(n).To(Equal(int(ephemeralCapacityWriteChunk)))
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(writer.Close()).To(Succeed())
+		info, err := file.Stat()
+		Expect(err).NotTo(HaveOccurred())
+		Expect(info.Size()).To(Equal(ephemeralCapacityWriteChunk))
+	})
+
+	It("waits for an open bounded writer before committing", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "payload.bin")
+		file, err := os.Create(path)
+		Expect(err).NotTo(HaveOccurred())
+		DeferCleanup(file.Close)
+		gated := &capacityGatedFileWriter{
+			file: file, entered: make(chan struct{}), resume: make(chan struct{}),
+		}
+		DeferCleanup(func() {
+			select {
+			case <-gated.resume:
+			default:
+				close(gated.resume)
+			}
+		})
+		writer, err := guard.NewWriter(path, gated)
+		Expect(err).NotTo(HaveOccurred())
+
+		writeDone := make(chan error, 1)
+		go func() {
+			_, writeErr := writer.Write([]byte("1234567"))
+			writeDone <- writeErr
+		}()
+		Eventually(gated.entered).Should(BeClosed())
+		commitDone := make(chan error, 1)
+		go func() { commitDone <- guard.Commit(path) }()
+		Eventually(func() int { return guard.commitWaiterCount(path) }).Should(Equal(1))
+		Expect(commitDone).NotTo(Receive())
+
+		close(gated.resume)
+		Expect(<-writeDone).To(Succeed())
+		Expect(commitDone).NotTo(Receive())
+		Expect(writer.Close()).To(Succeed())
+		Eventually(commitDone).Should(Receive(Succeed()))
+
+		err = guard.Reserve(filepath.Join(root, "other.bin"), 4)
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(capacityErr.UsageBytes).To(Equal(int64(7)))
+	})
+
+	It("does not share pending capacity between concurrent writers", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		path := filepath.Join(root, "payload.bin")
+		file, err := os.Create(path)
+		Expect(err).NotTo(HaveOccurred())
+		DeferCleanup(file.Close)
+		gated := &capacityGatedFileWriter{
+			file: file, entered: make(chan struct{}), resume: make(chan struct{}),
+		}
+		first, err := guard.NewWriter(path, gated)
+		Expect(err).NotTo(HaveOccurred())
+		var secondDestination bytes.Buffer
+		second, err := guard.NewWriter(path, &secondDestination)
+		Expect(err).NotTo(HaveOccurred())
+
+		firstDone := make(chan error, 1)
+		go func() {
+			_, writeErr := first.Write([]byte("1234567"))
+			firstDone <- writeErr
+		}()
+		Eventually(gated.entered).Should(BeClosed())
+
+		n, err := second.Write([]byte("7654321"))
+		Expect(n).To(BeZero())
+		var capacityErr *EphemeralCapacityError
+		Expect(errors.As(err, &capacityErr)).To(BeTrue())
+		Expect(secondDestination.Len()).To(BeZero())
+
+		close(gated.resume)
+		Expect(<-firstDone).To(Succeed())
+		Expect(first.Close()).To(Succeed())
+		Expect(second.Close()).To(Succeed())
+	})
+
+	It("rolls back bytes the destination writer does not accept", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 5, 0)
+		Expect(err).NotTo(HaveOccurred())
+		writer, err := guard.NewWriter(filepath.Join(root, "payload.bin"), capacityShortWriter{})
+		Expect(err).NotTo(HaveOccurred())
+
+		n, err := writer.Write([]byte("123"))
+		Expect(n).To(Equal(1))
+		Expect(err).To(MatchError(io.ErrShortWrite))
+		Expect(writer.Close()).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "other.bin"), 4)).To(Succeed())
+	})
+
+	It("rejects paths outside roots and through symlinks", func() {
+		root := GinkgoT().TempDir()
+		outside := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 100, 0)
+		Expect(err).NotTo(HaveOccurred())
+
+		Expect(guard.Reserve(filepath.Join(outside, "payload.bin"), 1)).To(
+			MatchError(ContainSubstring("outside registered ephemeral roots")),
+		)
+		Expect(os.Symlink(outside, filepath.Join(root, "escape"))).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "escape", "payload.bin"), 1)).To(
+			MatchError(ContainSubstring("symlink")),
+		)
+	})
+
+	It("supports recovery tree accounting without dropping active reservations", func() {
+		root := GinkgoT().TempDir()
+		guard, err := NewEphemeralCapacityGuard([]string{root}, 10, 0)
+		Expect(err).NotTo(HaveOccurred())
+		active := filepath.Join(root, "active", "payload.bin")
+		stale := filepath.Join(root, "stale", "payload.bin")
+
+		Expect(guard.Reserve(active, 4)).To(Succeed())
+		Expect(guard.Account(stale, 3)).To(Succeed())
+		Expect(guard.HasActiveReservation(root)).To(BeTrue())
+		Expect(guard.ReleaseTree(filepath.Join(root, "stale"))).To(Succeed())
+		Expect(guard.Reserve(filepath.Join(root, "replacement.bin"), 6)).To(Succeed())
+		Expect(guard.ReleaseTree(root)).To(Succeed())
+		Expect(guard.HasActiveReservation(root)).To(BeFalse())
+	})
+})