diff --git a/internal/state/sessionstore/save.go b/internal/state/sessionstore/save.go
index bdb1769157..e7197f5909 100644
--- a/internal/state/sessionstore/save.go
+++ b/internal/state/sessionstore/save.go
@@ -99,6 +99,7 @@ type sessionPersistState struct {
 	// and heal a stale ledger.
 	saveVerified bool
 	ok           bool
+	disk         persistedDisk
 }
 
 type sessionSaveMode int
@@ -272,7 +273,7 @@ func (s *Session) saveLocked(path string, mode sessionSaveMode) error {
 	// stalest capture written last would then read the newer transcript it
 	// lost the race to as a bogus stale-prefix conflict.
 	msgs, version, rewriteVersion := s.SnapshotWithVersion()
-	digest, contentBytes, err := digestAndSizeSessionMessages(msgs)
+	digest, contentBytes, prefixDigest, err := digestSizeAndPrefix(context.Background(), msgs, s.diskPrefixLen(path))
 	if err != nil {
 		return err
 	}
@@ -292,7 +293,7 @@ func (s *Session) saveLocked(path string, mode sessionSaveMode) error {
 	}
 	repairLog := false
 	ownedRewrite := mode == sessionSaveRewrite || mode == sessionSaveRewriteCompact
-	decision, err := s.checkSnapshotWrite(path, msgs, digest, version, ownedRewrite)
+	decision, err := s.snapshotDecision(path, msgs, digest, prefixDigest, version, ownedRewrite)
 	if err != nil {
 		return err
 	}
@@ -1119,28 +1120,8 @@ func digestAndSizeSessionMessages(msgs []provider.Message) ([sha256.Size]byte, i
 }
 
 func digestAndSizeSessionMessagesContext(ctx context.Context, msgs []provider.Message) ([sha256.Size]byte, int64, error) {
-	h := sha256.New()
-	size := int64(0)
-	for _, m := range msgs {
-		if err := ctx.Err(); err != nil {
-			return [sha256.Size]byte{}, 0, err
-		}
-		m = messageForSessionIdentity(m)
-		b, err := json.Marshal(m)
-		if err != nil {
-			return [sha256.Size]byte{}, 0, err
-		}
-		if _, err := h.Write(b); err != nil {
-			return [sha256.Size]byte{}, 0, err
-		}
-		if _, err := h.Write([]byte{'\n'}); err != nil {
-			return [sha256.Size]byte{}, 0, err
-		}
-		size += int64(len(b)) + 1
-	}
-	var out [sha256.Size]byte
-	copy(out[:], h.Sum(nil))
-	return out, size, nil
+	digest, size, _, err := digestSizeAndPrefix(ctx, msgs, -1)
+	return digest, size, err
 }
 
 func messagesHavePrefix(full, prefix []provider.Message) bool {
diff --git a/internal/state/sessionstore/save_fast_baseline.go b/internal/state/sessionstore/save_fast_baseline.go
new file mode 100644
index 0000000000..5cd8e96456
--- /dev/null
+++ b/internal/state/sessionstore/save_fast_baseline.go
@@ -0,0 +1,125 @@
+package sessionstore
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/json"
+	"os"
+
+	"reasonix/internal/contract/provider"
+	"reasonix/internal/state/store"
+)
+
+type fileStampOnDisk struct {
+	size, mod int64
+	here      bool
+}
+
+// baselineStamp prints the four files a save is judged against: the
+// checkpoint, the event log, the ledger sidecar and the damage marker.
+type baselineStamp [4]fileStampOnDisk
+
+func stampBaseline(path string) baselineStamp {
+	var out baselineStamp
+	for i, name := range []string{path, store.SessionEventLog(path), store.SessionMeta(path), store.SessionEventLogDamaged(path)} {
+		if info, err := os.Stat(name); err == nil {
+			out[i] = fileStampOnDisk{size: info.Size(), mod: info.ModTime().UnixNano(), here: true}
+		}
+	}
+	return out
+}
+
+// persistedDisk is what this runtime knows about disk right after its own
+// completed save: the files' print and how many messages they hold.
+type persistedDisk struct {
+	stamp baselineStamp
+	n     int
+	armed bool
+}
+
+// armDiskBaseline records the print of the files this save just wrote. It runs
+// under the save locks, so no cooperating writer can land between the write and
+// the print.
+func (s *Session) armDiskBaseline(path string, digest [sha256.Size]byte, n int) {
+	stamp := stampBaseline(path)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if !s.persisted.ok || !s.persisted.saveVerified || s.persisted.path != canonicalSessionSavePath(path) || s.persisted.digest != digest {
+		return
+	}
+	s.persisted.disk = persistedDisk{stamp: stamp, n: n, armed: true}
+}
+
+// diskPrefixLen is how many leading messages the next save must account for to
+// use fastSnapshotDecision, or -1 when this runtime has no print to trust.
+func (s *Session) diskPrefixLen(path string) int {
+	st := s.persistState(path)
+	if !st.ok || !st.saveVerified || !st.revisionKnown || !st.disk.armed {
+		return -1
+	}
+	return st.disk.n
+}
+
+// fastSnapshotDecision answers checkSnapshotWrite without reading the
+// transcript back. The files are exactly as this runtime left them, so disk
+// holds the persisted messages, and prefix — the digest of next's first n
+// messages, taken in the pass that digested all of it — says whether next still
+// begins with them. Anything else falls to the full comparison.
+func (s *Session) fastSnapshotDecision(path string, next []provider.Message, prefix [sha256.Size]byte) (snapshotWriteDecision, bool) {
+	st := s.persistState(path)
+	if !st.ok || !st.saveVerified || !st.revisionKnown || !st.disk.armed {
+		return snapshotWriteDecision{}, false
+	}
+	n := st.disk.n
+	if n > len(next) || prefix != st.digest || stampBaseline(path) != st.disk.stamp {
+		return snapshotWriteDecision{}, false
+	}
+	for _, m := range next {
+		if len(m.Images) > 0 || len(m.UnavailableImages) > 0 {
+			return snapshotWriteDecision{}, false
+		}
+	}
+	return snapshotWriteDecision{
+		revision:   st.revision,
+		upToDate:   len(next) == n,
+		appendOnly: len(next) > n,
+		appendFrom: n,
+	}, true
+}
+
+// digestSizeAndPrefix is digestAndSizeSessionMessagesContext that also reports
+// the digest of the first k messages, accumulated in the same pass. k < 0 asks
+// for none.
+func digestSizeAndPrefix(ctx context.Context, msgs []provider.Message, k int) (digest [sha256.Size]byte, size int64, prefix [sha256.Size]byte, err error) {
+	h := sha256.New()
+	if k == 0 {
+		copy(prefix[:], h.Sum(nil))
+	}
+	for i, m := range msgs {
+		if err := ctx.Err(); err != nil {
+			return digest, 0, prefix, err
+		}
+		m = messageForSessionIdentity(m)
+		b, err := json.Marshal(m)
+		if err != nil {
+			return digest, 0, prefix, err
+		}
+		h.Write(b)
+		h.Write([]byte{'\n'})
+		size += int64(len(b)) + 1
+		if i+1 == k {
+			copy(prefix[:], h.Sum(nil))
+		}
+	}
+	copy(digest[:], h.Sum(nil))
+	return digest, size, prefix, nil
+}
+
+// snapshotDecision is checkSnapshotWrite, answered from the print when the
+// files are as this runtime left them.
+func (s *Session) snapshotDecision(path string, msgs []provider.Message, digest, prefix [sha256.Size]byte, version uint64, ownedRewrite bool) (snapshotWriteDecision, error) {
+	if decision, ok := s.fastSnapshotDecision(path, msgs, prefix); ok {
+		return decision, nil
+	}
+	return s.checkSnapshotWrite(path, msgs, digest, version, ownedRewrite)
+}
diff --git a/internal/state/sessionstore/save_fast_baseline_test.go b/internal/state/sessionstore/save_fast_baseline_test.go
new file mode 100644
index 0000000000..870dd2cb7c
--- /dev/null
+++ b/internal/state/sessionstore/save_fast_baseline_test.go
@@ -0,0 +1,86 @@
+package sessionstore
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"reasonix/internal/contract/provider"
+)
+
+func seededSession(t *testing.T, path string, n int) *Session {
+	t.Helper()
+	s := NewSession("sys")
+	for i := range n / 2 {
+		s.Add(provider.Message{Role: provider.RoleUser, Content: fmt.Sprintf("turn %d: %s", i, strings.Repeat("please keep going. ", 20))})
+		s.Add(provider.Message{Role: provider.RoleAssistant, Content: strings.Repeat("a plain english sentence about the work. ", 40)})
+	}
+	if err := s.Save(path); err != nil {
+		t.Fatalf("seed save: %v", err)
+	}
+	return s
+}
+
+func allocatedBy(f func()) uint64 {
+	var before, after runtime.MemStats
+	runtime.GC()
+	runtime.ReadMemStats(&before)
+	f()
+	runtime.ReadMemStats(&after)
+	return after.TotalAlloc - before.TotalAlloc
+}
+
+// A runtime that wrote the last save and finds the files untouched has nothing
+// to reload: one more message costs about one encoding pass, not a replay of the
+// event log plus a decode and re-encode of what disk already holds.
+func TestSavingOneMoreMessageDoesNotReplayTheLog(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "s.jsonl")
+	s := seededSession(t, path, 2000)
+	if err := s.Save(path); err != nil {
+		t.Fatal(err)
+	}
+	_, size, err := digestAndSizeSessionMessages(s.Snapshot())
+	if err != nil {
+		t.Fatal(err)
+	}
+	s.Add(provider.Message{Role: provider.RoleUser, Content: "next"})
+	got := allocatedBy(func() {
+		if err := s.Save(path); err != nil {
+			t.Fatal(err)
+		}
+	})
+	if limit := uint64(size) * 4; got > limit {
+		t.Fatalf("one-message save allocated %d bytes for a %d byte transcript (limit %d)", got, size, limit)
+	}
+}
+
+func TestSavingAfterAnotherRuntimeWroteStillSeesIt(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "s.jsonl")
+	a := seededSession(t, path, 40)
+	b, err := LoadSession(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	b.Add(provider.Message{Role: provider.RoleUser, Content: "written by the other runtime"})
+	if err := b.Save(path); err != nil {
+		t.Fatalf("other runtime save: %v", err)
+	}
+	a.Add(provider.Message{Role: provider.RoleUser, Content: "written by the stale runtime"})
+	if err := a.Save(path); err == nil {
+		after, lerr := LoadSession(path)
+		if lerr != nil {
+			t.Fatal(lerr)
+		}
+		var kept int
+		for _, m := range after.Snapshot() {
+			if strings.HasPrefix(m.Content, "written by the other runtime") {
+				kept++
+			}
+		}
+		if kept != 1 {
+			t.Fatalf("the stale runtime's save dropped the other runtime's message (kept %d)", kept)
+		}
+	}
+}
diff --git a/internal/state/sessionstore/save_listing_projection.go b/internal/state/sessionstore/save_listing_projection.go
index 0996e58b02..156140d4fd 100644
--- a/internal/state/sessionstore/save_listing_projection.go
+++ b/internal/state/sessionstore/save_listing_projection.go
@@ -10,6 +10,7 @@ import (
 func (s *Session) markPersistedWithListing(path string, digest [sha256.Size]byte, version uint64, revision int64, rewriteVersion int, msgs []provider.Message) {
 	s.markPersisted(path, digest, version, revision, rewriteVersion)
 	persistSessionListingProjection(path, msgs)
+	s.armDiskBaseline(path, digest, len(msgs))
 }
 
 func persistSessionListingProjection(path string, msgs []provider.Message) {