diff --git a/app-prefixable/src/context/sync.tsx b/app-prefixable/src/context/sync.tsx
index 0b303b00..4255ea40 100644
--- a/app-prefixable/src/context/sync.tsx
+++ b/app-prefixable/src/context/sync.tsx
@@ -1,6 +1,6 @@
 import { createContext, useContext, onCleanup, batch, type ParentProps } from "solid-js"
 import { createStore, reconcile, produce } from "solid-js/store"
-import type { Session, Message, Part, Provider } from "../sdk/client"
+import type { Session, Message, Part, Provider, TextPart } from "../sdk/client"
 import { useSDK } from "./sdk"
 import { useServer } from "./server"
 import { createSSEParser } from "../utils/sse"
@@ -58,6 +58,95 @@ function sortParts(parts: Part[]): Part[] {
   return [...withId, ...withoutId]
 }
 
+function toolEnd(part: Extract<Part, { type: "tool" }>): number {
+  const state = part.state
+  if (state.status === "completed") return state.time.end
+  if (state.status === "error") return state.time.end
+  return 0
+}
+
+function toolStart(part: Extract<Part, { type: "tool" }>): number {
+  const state = part.state
+  if (state.status === "running") return state.time.start
+  if (state.status === "completed") return state.time.start
+  if (state.status === "error") return state.time.start
+  return 0
+}
+
+function toolRank(part: Extract<Part, { type: "tool" }>): number {
+  const state = part.state
+  if (state.status === "pending") return 1
+  if (state.status === "running") return 2
+  return 3
+}
+
+function mergePart(existing: Part, synced: Part): Part {
+  if (existing.type !== "tool") {
+    if (synced.type !== "tool") {
+      const getTimeValue = (part: Part): number => {
+        if (part.type === "text" || part.type === "reasoning") {
+          const t = (part as TextPart).time
+          return t?.end ?? t?.start ?? 0
+        }
+        if (part.type === "retry") {
+          return (part as { time?: { created?: number } }).time?.created ?? 0
+        }
+        return 0
+      }
+      const existingEnd = getTimeValue(existing)
+      const syncedEnd = getTimeValue(synced)
+      if (existingEnd > syncedEnd) return existing
+      if (syncedEnd > existingEnd) return synced
+      return existing
+    }
+    return synced
+  }
+  if (synced.type !== "tool") return synced
+
+  const existingEnd = toolEnd(existing)
+  const syncedEnd = toolEnd(synced)
+  if (existingEnd > syncedEnd) return existing
+  if (syncedEnd > existingEnd) return synced
+
+  const existingStart = toolStart(existing)
+  const syncedStart = toolStart(synced)
+  if (existingStart > syncedStart) return existing
+  if (syncedStart > existingStart) return synced
+
+  const existingRank = toolRank(existing)
+  const syncedRank = toolRank(synced)
+  if (existingRank > syncedRank) return existing
+  if (syncedRank > existingRank) return synced
+
+  return existing
+}
+
+function mergeMessage(existing: MessageWithParts, synced: MessageWithParts): MessageWithParts {
+  const existingWithId = existing.parts.filter((p) => !!p.id)
+  const existingWithoutId = existing.parts.filter((p) => !p.id)
+  const map = new Map(existingWithId.map((part) => [part.id, part]))
+  const merged = synced.parts.map((part) => {
+    const current = map.get(part.id)
+    if (!current) return part
+    return mergePart(current, part)
+  })
+  const ids = new Set(merged.filter((p) => !!p.id).map((part) => part.id))
+
+  for (const part of existingWithId) {
+    if (ids.has(part.id)) continue
+    merged.push(part)
+  }
+
+  for (const part of existingWithoutId) {
+    merged.push(part)
+  }
+
+  return {
+    info: synced.info,
+    parts: sortParts(merged),
+  }
+}
+
 function errorText(err: unknown) {
   if (err instanceof Error && err.message.trim()) return err.message
   return "Failed to bootstrap app state from API."
@@ -246,17 +335,38 @@ export function SyncProvider(props: ParentProps) {
       setStore("message", session.id, reconcile([]))
     }
 
+    function isNewer(a: Part, b: Part): boolean {
+      if (a.type !== "tool" || b.type !== "tool") return false
+      const aState = a.state as Extract<Part, { type: "tool" }>["state"]
+      const bState = b.state as Extract<Part, { type: "tool" }>["state"]
+      if (aState.status === "pending" || bState.status === "pending") return false
+      const aEnd = aState.time?.end ?? aState.time?.start
+      const bEnd = bState.time?.end ?? bState.time?.start
+      if (!aEnd) return false
+      if (!bEnd) return true
+      return aEnd > bEnd
+    }
+
     // Message part events - the main real-time update mechanism
     if (event.type === "message.part.updated") {
       const part = props.part as Part
       if (!part?.sessionID || !part?.messageID) return
 
-      // Update or insert the part
+      const hasId = !!part.id
       setStore("part", part.messageID, (existing: Part[] | undefined) => {
         if (!existing) return sortParts([part])
-        const idx = existing.findIndex((p) => p.id === part.id)
-        if (idx === -1) return sortParts([...existing, part])
-        return existing.map((p, i) => (i === idx ? part : p))
+
+        if (hasId) {
+          const idx = existing.findIndex((p) => p.id === part.id)
+          if (idx === -1) return sortParts([...existing, part])
+          const existingPart = existing[idx]
+          if (!existingPart.id || isNewer(existingPart, part)) return existing
+          return existing.map((p, i) => (i === idx ? part : p))
+        }
+
+        const noIdParts = existing.filter((p) => !p.id)
+        const withIdParts = existing.filter((p) => !!p.id)
+        return sortParts([...withIdParts, ...noIdParts, part])
       })
 
       // Update parts in existing messages only - don't synthesize messages from parts
@@ -269,9 +379,16 @@ export function SyncProvider(props: ParentProps) {
         // Update existing message parts
         return msgs.map((m, i) => {
           if (i !== msgIdx) return m
-          const partIdx = m.parts.findIndex((p) => p.id === part.id)
-          const newParts = partIdx === -1 ? [...m.parts, part] : m.parts.map((p, pi) => (pi === partIdx ? part : p))
-          return { ...m, parts: newParts }
+          if (hasId) {
+            const partIdx = m.parts.findIndex((p) => p.id === part.id)
+            if (partIdx !== -1) {
+              const existingPart = m.parts[partIdx]
+              if (!existingPart.id || isNewer(existingPart, part)) return m
+            }
+            const newParts = partIdx === -1 ? [...m.parts, part] : m.parts.map((p, pi) => (pi === partIdx ? part : p))
+            return { ...m, parts: newParts }
+          }
+          return { ...m, parts: [...m.parts, part] }
         })
       })
     }
@@ -306,9 +423,17 @@ export function SyncProvider(props: ParentProps) {
         if (!existing || existing.length === 0) return existing
         return existing.map((m) => {
           if (m.info.id !== info.id) return m
-          // Merge info and optionally update parts if provided
+          // Prefer info with newer time.completed
+          const mergedInfo =
+            m.info.time?.completed && info.time?.completed
+              ? m.info.time.completed > info.time.completed
+                ? m.info
+                : info
+              : info.time?.completed
+                ? info
+                : m.info
           const updatedParts = parts ? sortParts(parts) : m.parts
-          return { info, parts: updatedParts }
+          return { info: mergedInfo, parts: updatedParts }
         })
       })
 
@@ -405,19 +530,18 @@ export function SyncProvider(props: ParentProps) {
             setStore("message", sessionID, (existing: MessageWithParts[]) => {
               if (!existing || existing.length === 0) return synced
 
-              // Merge: use existing message if it has more recent parts
-              const merged = synced.map((s) => {
-                const e = existing.find((m) => m.info.id === s.info.id)
-                if (!e) return s
-                // Keep existing if it has more parts (SSE updates arrived)
-                return e.parts.length >= s.parts.length ? e : s
+              const map = new Map(existing.map((msg) => [msg.info.id, msg]))
+              const merged = synced.map((msg) => {
+                const current = map.get(msg.info.id)
+                if (!current) return msg
+                return mergeMessage(current, msg)
               })
+              const ids = new Set(merged.map((msg) => msg.info.id))
 
               // Add any messages from existing that aren't in synced (new SSE messages)
-              for (const e of existing) {
-                if (!merged.find((m) => m.info.id === e.info.id)) {
-                  merged.push(e)
-                }
+              for (const msg of existing) {
+                if (ids.has(msg.info.id)) continue
+                merged.push(msg)
               }
 
               return merged.sort((a, b) => cmp(a.info.id, b.info.id))