--- a/src/main/native-chat/agent-session-wire/agent-session-delta-coalescer.ts
+++ b/src/main/native-chat/agent-session-wire/agent-session-delta-coalescer.ts
@@ -39,0 +40,2 @@
+  /** The caller byte-bounds protected metadata; only ordinary streams use the count cap. */
+  isProtected?: (key: string) => boolean
@@ -86,2 +88 @@
-  const streamOrder = new Map<string, number>()
-  let nextOrder = 0
+  const evictable = new Set<string>()
@@ -133,2 +134,2 @@
-        if (streams.size >= maxStreams) {
-          const oldest = [...streamOrder.entries()].sort((a, b) => a[1] - b[1])[0]?.[0]
+        while (!deps.isProtected?.(key) && evictable.size >= maxStreams) {
+          const oldest = evictable.values().next().value
@@ -135,0 +137,4 @@
+            if (deps.isProtected?.(oldest)) {
+              evictable.delete(oldest)
+              continue
+            }
@@ -146 +151 @@
-            streamOrder.delete(oldest)
+            evictable.delete(oldest)
@@ -147,0 +153 @@
+          break
@@ -156 +162,5 @@
-        streamOrder.set(key, nextOrder++)
+        if (!deps.isProtected?.(key)) {
+          evictable.add(key)
+        }
+      } else if (deps.isProtected?.(key)) {
+        evictable.delete(key)
@@ -158 +168,2 @@
-      stream.observedBytes += Buffer.byteLength(delta, 'utf8')
+      const deltaBytes = Buffer.byteLength(delta, 'utf8')
+      stream.observedBytes += deltaBytes
@@ -165,0 +177 @@
+          deltaBytes,
@@ -187 +199 @@
-        streamOrder.delete(key)
+        evictable.delete(key)
@@ -194 +206 @@
-      streamOrder.clear()
+      evictable.clear()
@@ -213,0 +226 @@
+  deltaBytes: number,
@@ -217,2 +230 @@
-  const deltaBuffer = Buffer.from(delta, 'utf8')
-  if (deltaBuffer.byteLength <= available) {
+  if (deltaBytes <= available) {
@@ -221 +233,3 @@
-    current.push(delta)
+    if (delta.length > 0) {
+      current.push(delta)
+    }
@@ -224 +238 @@
-      retainedBytes: currentBytes + deltaBuffer.byteLength,
+      retainedBytes: currentBytes + deltaBytes,
@@ -232 +246 @@
-    deltaBuffer
+    Buffer.from(delta, 'utf8')