diff --git a/packages/ai/src/tool-history.ts b/packages/ai/src/tool-history.ts
index 09a3999559c..5d09660d1f5 100644
--- a/packages/ai/src/tool-history.ts
+++ b/packages/ai/src/tool-history.ts
@@ -6,10 +6,17 @@ const MISSING_TOOL_RESULT = "Tool result missing"
 export function normalizeToolHistory(messages: ReadonlyArray<Message>) {
   const normalized: Message[] = []
   const pending = new Map<string, ToolCallPart>()
+  // System updates cannot sit between a tool call and its results, so they wait until every pending call is answered.
+  const held: Message[] = []
+  const releaseHeld = () => {
+    if (pending.size > 0) return
+    normalized.push(...held)
+    held.length = 0
+  }
   const appendMissingResults = () => {
-    if (pending.size === 0) return
-    normalized.push(missingToolResults(pending.values()))
+    if (pending.size > 0) normalized.push(missingToolResults(pending.values()))
     pending.clear()
+    releaseHeld()
   }
 
   for (const message of messages) {
@@ -18,6 +25,12 @@ export function normalizeToolHistory(messages: ReadonlyArray<Message>) {
     if (message.role === "tool") {
       const tool = normalizeToolMessage(message, pending)
       if (tool) normalized.push(tool)
+      releaseHeld()
+      continue
+    }
+
+    if (message.role === "system" && pending.size > 0) {
+      held.push(message)
       continue
     }
 
diff --git a/packages/ai/test/effort-updates.test.ts b/packages/ai/test/effort-updates.test.ts
index f6d3f27aab5..c7095c538c7 100644
--- a/packages/ai/test/effort-updates.test.ts
+++ b/packages/ai/test/effort-updates.test.ts
@@ -147,7 +147,7 @@ describe("Anthropic Messages effort updates", () => {
     }),
   )
 
-  it.effect("accepts a marker between a tool call and its result", () =>
+  it.effect("moves a marker between a tool call and its result after the result", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(
         LLM.request({
@@ -166,8 +166,8 @@ describe("Anthropic Messages effort updates", () => {
       expect(prepared.body.messages).toEqual([
         { role: "user", content: [{ type: "text", text: "Weather?" }] },
         { role: "assistant", content: [{ type: "tool_use", id: "call_1", name: "lookup", input: {} }] },
-        { role: "system", content: [], output_config: { effort: "low" } },
         { role: "user", content: [{ type: "tool_result", tool_use_id: "call_1", content: '{"temp":72}' }] },
+        { role: "system", content: [], output_config: { effort: "low" } },
       ])
     }),
   )
diff --git a/packages/ai/test/provider/anthropic-messages.test.ts b/packages/ai/test/provider/anthropic-messages.test.ts
index 173a8c7b8f9..20d5b6bcfeb 100644
--- a/packages/ai/test/provider/anthropic-messages.test.ts
+++ b/packages/ai/test/provider/anthropic-messages.test.ts
@@ -588,9 +588,9 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
-  it.effect("rejects a system update between a local tool call and its result", () =>
+  it.effect("moves a system update between a local tool call and its result after the result", () =>
     Effect.gen(function* () {
-      const error = yield* compileRequest(
+      const prepared = yield* compileRequest(
         LLM.request({
           model: opus48,
           messages: [
@@ -601,9 +601,13 @@ describe("Anthropic Messages route", () => {
           ],
           cache: "none",
         }),
-      ).pipe(Effect.flip)
+      )
 
-      expect(error.message).toContain("system updates cannot split a local tool call from its tool result")
+      expect(prepared.body.messages.slice(1)).toEqual([
+        { role: "assistant", content: [{ type: "tool_use", id: "call_1", name: "lookup", input: {} }] },
+        { role: "user", content: [{ type: "tool_result", tool_use_id: "call_1", content: '"Done."' }] },
+        { role: "system", content: [{ type: "text", text: "Too early.", cache_control: undefined }] },
+      ])
     }),
   )
 
diff --git a/packages/ai/test/tool-history.test.ts b/packages/ai/test/tool-history.test.ts
index 6ec5552ef39..80eac7a3694 100644
--- a/packages/ai/test/tool-history.test.ts
+++ b/packages/ai/test/tool-history.test.ts
@@ -112,4 +112,51 @@ describe("tool history normalization", () => {
 
     expect(normalized[1]?.content[0]).toMatchObject({ name: "lookup", namespace: undefined })
   })
+
+  test("moves system updates after the results of pending calls", () => {
+    const calls = Message.assistant([toolCall("first"), toolCall("second")])
+    const first = toolResult("first", "one", "first", "text")
+    const second = toolResult("second", "two", "second", "text")
+    const update = Message.system("First update.")
+    const later = Message.system("Second update.")
+    const user = Message.user("Continue.")
+    const missing = (id: string) =>
+      Message.tool(ToolResultPart.make({ id, name: id, result: "Tool result missing", resultType: "error" }))
+
+    expect(normalizeToolHistory([calls, first, update, later, second, user])).toEqual([
+      calls,
+      first,
+      second,
+      update,
+      later,
+      user,
+    ])
+    expect(normalizeToolHistory([Message.assistant(toolCall("first")), update, user])).toEqual([
+      Message.assistant(toolCall("first")),
+      missing("first"),
+      update,
+      user,
+    ])
+    expect(normalizeToolHistory([Message.assistant(toolCall("first")), update])).toEqual([
+      Message.assistant(toolCall("first")),
+      missing("first"),
+      update,
+    ])
+  })
+
+  test("moves effort updates after the results of pending calls", () => {
+    const call = Message.assistant(toolCall("first"))
+    const result = toolResult("first", "one", "first", "text")
+    const effort = Message.effort({ effort: "low", previous: "high" })
+
+    expect(normalizeToolHistory([call, effort, result])).toEqual([call, result, effort])
+  })
+
+  test("keeps system updates in place when no call is pending", () => {
+    const history = [Message.assistant(toolCall("first")), toolResult("first", "one", "first", "text")]
+    const update = Message.system("Update.")
+    const input = [Message.user("Start."), update, ...history, update, Message.user("Continue.")]
+
+    expect(normalizeToolHistory(input)).toBe(input)
+  })
 })