diff --git a/.changeset/server-tool-results-complete.md b/.changeset/server-tool-results-complete.md
new file mode 100644
index 000000000..0b97fbd43
--- /dev/null
+++ b/.changeset/server-tool-results-complete.md
@@ -0,0 +1,7 @@
+---
+'@tanstack/ai': patch
+'@tanstack/ai-client': patch
+'@tanstack/ai-event-client': patch
+---
+
+Populate server-executed tool results on the matching `tool-call` part and mark successful tool calls as `complete`.
diff --git a/examples/ts-react-chat/src/routeTree.gen.ts b/examples/ts-react-chat/src/routeTree.gen.ts
index 45a3107ea..d0966d88a 100644
--- a/examples/ts-react-chat/src/routeTree.gen.ts
+++ b/examples/ts-react-chat/src/routeTree.gen.ts
@@ -10,6 +10,7 @@
 
 import { Route as rootRouteImport } from './routes/__root'
 import { Route as RealtimeRouteImport } from './routes/realtime'
+import { Route as Issue176ToolResultRouteImport } from './routes/issue-176-tool-result'
 import { Route as ImageGenRouteImport } from './routes/image-gen'
 import { Route as IndexRouteImport } from './routes/index'
 import { Route as GenerationsVideoRouteImport } from './routes/generations.video'
@@ -38,6 +39,11 @@ const RealtimeRoute = RealtimeRouteImport.update({
   path: '/realtime',
   getParentRoute: () => rootRouteImport,
 } as any)
+const Issue176ToolResultRoute = Issue176ToolResultRouteImport.update({
+  id: '/issue-176-tool-result',
+  path: '/issue-176-tool-result',
+  getParentRoute: () => rootRouteImport,
+} as any)
 const ImageGenRoute = ImageGenRouteImport.update({
   id: '/image-gen',
   path: '/image-gen',
@@ -155,6 +161,7 @@ const ApiGenerateAudioRoute = ApiGenerateAudioRouteImport.update({
 export interface FileRoutesByFullPath {
   '/': typeof IndexRoute
   '/image-gen': typeof ImageGenRoute
+  '/issue-176-tool-result': typeof Issue176ToolResultRoute
   '/realtime': typeof RealtimeRoute
   '/api/image-gen': typeof ApiImageGenRoute
   '/api/structured-chat': typeof ApiStructuredChatRoute
@@ -180,6 +187,7 @@ export interface FileRoutesByFullPath {
 export interface FileRoutesByTo {
   '/': typeof IndexRoute
   '/image-gen': typeof ImageGenRoute
+  '/issue-176-tool-result': typeof Issue176ToolResultRoute
   '/realtime': typeof RealtimeRoute
   '/api/image-gen': typeof ApiImageGenRoute
   '/api/structured-chat': typeof ApiStructuredChatRoute
@@ -206,6 +214,7 @@ export interface FileRoutesById {
   __root__: typeof rootRouteImport
   '/': typeof IndexRoute
   '/image-gen': typeof ImageGenRoute
+  '/issue-176-tool-result': typeof Issue176ToolResultRoute
   '/realtime': typeof RealtimeRoute
   '/api/image-gen': typeof ApiImageGenRoute
   '/api/structured-chat': typeof ApiStructuredChatRoute
@@ -233,6 +242,7 @@ export interface FileRouteTypes {
   fullPaths:
     | '/'
     | '/image-gen'
+    | '/issue-176-tool-result'
     | '/realtime'
     | '/api/image-gen'
     | '/api/structured-chat'
@@ -258,6 +268,7 @@ export interface FileRouteTypes {
   to:
     | '/'
     | '/image-gen'
+    | '/issue-176-tool-result'
     | '/realtime'
     | '/api/image-gen'
     | '/api/structured-chat'
@@ -283,6 +294,7 @@ export interface FileRouteTypes {
     | '__root__'
     | '/'
     | '/image-gen'
+    | '/issue-176-tool-result'
     | '/realtime'
     | '/api/image-gen'
     | '/api/structured-chat'
@@ -309,6 +321,7 @@ export interface FileRouteTypes {
 export interface RootRouteChildren {
   IndexRoute: typeof IndexRoute
   ImageGenRoute: typeof ImageGenRoute
+  Issue176ToolResultRoute: typeof Issue176ToolResultRoute
   RealtimeRoute: typeof RealtimeRoute
   ApiImageGenRoute: typeof ApiImageGenRoute
   ApiStructuredChatRoute: typeof ApiStructuredChatRoute
@@ -341,6 +354,13 @@ declare module '@tanstack/react-router' {
       preLoaderRoute: typeof RealtimeRouteImport
       parentRoute: typeof rootRouteImport
     }
+    '/issue-176-tool-result': {
+      id: '/issue-176-tool-result'
+      path: '/issue-176-tool-result'
+      fullPath: '/issue-176-tool-result'
+      preLoaderRoute: typeof Issue176ToolResultRouteImport
+      parentRoute: typeof rootRouteImport
+    }
     '/image-gen': {
       id: '/image-gen'
       path: '/image-gen'
@@ -501,6 +521,7 @@ declare module '@tanstack/react-router' {
 const rootRouteChildren: RootRouteChildren = {
   IndexRoute: IndexRoute,
   ImageGenRoute: ImageGenRoute,
+  Issue176ToolResultRoute: Issue176ToolResultRoute,
   RealtimeRoute: RealtimeRoute,
   ApiImageGenRoute: ApiImageGenRoute,
   ApiStructuredChatRoute: ApiStructuredChatRoute,
diff --git a/examples/ts-react-chat/src/routes/issue-176-tool-result.tsx b/examples/ts-react-chat/src/routes/issue-176-tool-result.tsx
new file mode 100644
index 000000000..0eac5c48d
--- /dev/null
+++ b/examples/ts-react-chat/src/routes/issue-176-tool-result.tsx
@@ -0,0 +1,292 @@
+import { useMemo, useState } from 'react'
+import { createFileRoute } from '@tanstack/react-router'
+import { fetchServerSentEvents, useChat } from '@tanstack/ai-react'
+import { clientTools } from '@tanstack/ai-client'
+import { modelMessagesToUIMessages, type ModelMessage } from '@tanstack/ai'
+import { recommendGuitarToolDef } from '@/lib/guitar-tools'
+
+const modelMessages: Array<ModelMessage> = [
+  {
+    role: 'assistant',
+    content: 'Let me check the weather.',
+    toolCalls: [
+      {
+        id: 'issue-176-tool-call',
+        type: 'function',
+        function: {
+          name: 'getWeather',
+          arguments: '{"city":"NYC"}',
+        },
+      },
+    ],
+  },
+  {
+    role: 'tool',
+    content: '{"temp":72,"condition":"sunny"}',
+    toolCallId: 'issue-176-tool-call',
+  },
+]
+
+function Issue176ToolResultRepro() {
+  const [prompt, setPrompt] = useState(
+    'I want an acoustic guitar recommendation. Use the required tools.',
+  )
+  const initialMessages = useMemo(
+    () => modelMessagesToUIMessages(modelMessages),
+    [],
+  )
+  const liveTools = useMemo(
+    () =>
+      clientTools(
+        recommendGuitarToolDef.client(({ id }) => ({
+          id: Number(id),
+        })),
+      ),
+    [],
+  )
+
+  const { messages: fixtureMessages } = useChat({
+    id: 'issue-176-tool-result-repro',
+    connection: fetchServerSentEvents('/api/tanchat'),
+    initialMessages,
+  })
+  const {
+    messages: liveMessages,
+    sendMessage,
+    isLoading,
+    error,
+  } = useChat({
+    id: 'issue-176-live-tool-result-repro',
+    connection: fetchServerSentEvents('/api/tanchat'),
+    tools: liveTools,
+    body: {
+      provider: 'openai',
+      model: 'gpt-4o',
+    },
+  })
+
+  const toolCall = fixtureMessages
+    .flatMap((message) => message.parts)
+    .find(
+      (part) => part.type === 'tool-call' && part.id === 'issue-176-tool-call',
+    )
+  const toolResult = fixtureMessages
+    .flatMap((message) => message.parts)
+    .find(
+      (part) =>
+        part.type === 'tool-result' &&
+        part.toolCallId === 'issue-176-tool-call',
+    )
+  const isFixed =
+    toolCall?.type === 'tool-call' &&
+    toolCall.state === 'complete' &&
+    toolCall.output !== undefined
+  const liveServerToolCall = liveMessages
+    .flatMap((message) => message.parts)
+    .find((part) => part.type === 'tool-call' && part.name === 'getGuitars')
+  const liveServerToolResult = liveMessages
+    .flatMap((message) => message.parts)
+    .find(
+      (part) =>
+        part.type === 'tool-result' &&
+        liveServerToolCall?.type === 'tool-call' &&
+        part.toolCallId === liveServerToolCall.id,
+    )
+  const isLiveFixed =
+    liveServerToolCall?.type === 'tool-call' &&
+    liveServerToolCall.state === 'complete' &&
+    liveServerToolCall.output !== undefined
+
+  return (
+    <main className="min-h-screen bg-gray-950 text-gray-100">
+      <div className="mx-auto flex max-w-6xl flex-col gap-6 px-6 py-8">
+        <div>
+          <p className="text-sm font-medium uppercase tracking-wider text-orange-300">
+            Issue #176 manual repro
+          </p>
+          <h1 className="mt-2 text-3xl font-semibold">
+            Server tool result hydration
+          </h1>
+          <p className="mt-3 max-w-3xl text-sm leading-6 text-gray-300">
+            This page initializes a chat from model-message history containing
+            an assistant server tool call followed by a matching tool result.
+            The original tool-call part should be complete and include output.
+          </p>
+        </div>
+
+        <section className="rounded-lg border border-gray-800 bg-gray-900">
+          <div className="border-b border-gray-800 px-4 py-3">
+            <h2 className="text-lg font-semibold">Live LLM repro</h2>
+          </div>
+          <div className="grid gap-4 p-4 lg:grid-cols-[minmax(0,1fr)_220px_220px]">
+            <div className="flex flex-col gap-3">
+              <textarea
+                value={prompt}
+                onChange={(event) => setPrompt(event.target.value)}
+                className="min-h-28 rounded-lg border border-gray-700 bg-gray-950 p-3 text-sm text-gray-100 outline-none focus:border-orange-400"
+              />
+              <div className="flex items-center gap-3">
+                <button
+                  type="button"
+                  disabled={isLoading || !prompt.trim()}
+                  onClick={() => sendMessage(prompt)}
+                  className="rounded-lg bg-orange-500 px-4 py-2 text-sm font-semibold text-gray-950 transition hover:bg-orange-400 disabled:cursor-not-allowed disabled:opacity-60"
+                >
+                  Send to LLM
+                </button>
+                {error ? (
+                  <span className="text-sm text-red-300">{error.message}</span>
+                ) : (
+                  <span className="text-sm text-gray-400">
+                    {isLoading ? 'Running...' : 'Ready'}
+                  </span>
+                )}
+              </div>
+            </div>
+
+            <div className="rounded-lg border border-gray-800 bg-gray-950 p-4">
+              <div className="text-xs uppercase tracking-wider text-gray-400">
+                Live getGuitars state
+              </div>
+              <div
+                id="issue-176-live-tool-state"
+                className={`mt-2 text-2xl font-semibold ${
+                  isLiveFixed ? 'text-emerald-300' : 'text-amber-300'
+                }`}
+              >
+                {liveServerToolCall?.type === 'tool-call'
+                  ? liveServerToolCall.state
+                  : 'not run'}
+              </div>
+            </div>
+
+            <div className="rounded-lg border border-gray-800 bg-gray-950 p-4">
+              <div className="text-xs uppercase tracking-wider text-gray-400">
+                Live getGuitars output
+              </div>
+              <div
+                id="issue-176-live-tool-output"
+                className={`mt-2 text-2xl font-semibold ${
+                  liveServerToolCall?.type === 'tool-call' &&
+                  liveServerToolCall.output !== undefined
+                    ? 'text-emerald-300'
+                    : 'text-amber-300'
+                }`}
+              >
+                {liveServerToolCall?.type === 'tool-call' &&
+                liveServerToolCall.output !== undefined
+                  ? 'present'
+                  : 'missing'}
+              </div>
+            </div>
+          </div>
+          <div className="grid gap-4 border-t border-gray-800 p-4 lg:grid-cols-2">
+            <div>
+              <div className="mb-2 text-sm font-medium text-gray-200">
+                Live server tool-call part
+              </div>
+              <pre className="max-h-80 overflow-auto rounded-lg bg-gray-950 p-4 text-xs leading-5 text-gray-300">
+                {JSON.stringify(liveServerToolCall ?? null, null, 2)}
+              </pre>
+            </div>
+            <div>
+              <div className="mb-2 text-sm font-medium text-gray-200">
+                Live matching tool-result part
+              </div>
+              <pre className="max-h-80 overflow-auto rounded-lg bg-gray-950 p-4 text-xs leading-5 text-gray-300">
+                {JSON.stringify(liveServerToolResult ?? null, null, 2)}
+              </pre>
+            </div>
+          </div>
+        </section>
+
+        <section className="grid gap-4 md:grid-cols-3">
+          <div className="rounded-lg border border-gray-800 bg-gray-900 p-4">
+            <div className="text-xs uppercase tracking-wider text-gray-400">
+              Tool-call state
+            </div>
+            <div
+              id="issue-176-tool-state"
+              className={`mt-2 text-2xl font-semibold ${
+                isFixed ? 'text-emerald-300' : 'text-amber-300'
+              }`}
+            >
+              {toolCall?.type === 'tool-call' ? toolCall.state : 'missing'}
+            </div>
+          </div>
+
+          <div className="rounded-lg border border-gray-800 bg-gray-900 p-4">
+            <div className="text-xs uppercase tracking-wider text-gray-400">
+              Tool-call output
+            </div>
+            <div
+              id="issue-176-tool-output"
+              className={`mt-2 text-2xl font-semibold ${
+                toolCall?.type === 'tool-call' && toolCall.output !== undefined
+                  ? 'text-emerald-300'
+                  : 'text-amber-300'
+              }`}
+            >
+              {toolCall?.type === 'tool-call' && toolCall.output !== undefined
+                ? 'present'
+                : 'missing'}
+            </div>
+          </div>
+
+          <div className="rounded-lg border border-gray-800 bg-gray-900 p-4">
+            <div className="text-xs uppercase tracking-wider text-gray-400">
+              Tool-result part
+            </div>
+            <div
+              id="issue-176-tool-result"
+              className={`mt-2 text-2xl font-semibold ${
+                toolResult ? 'text-emerald-300' : 'text-amber-300'
+              }`}
+            >
+              {toolResult ? 'present' : 'missing'}
+            </div>
+          </div>
+        </section>
+
+        <section className="grid gap-6 lg:grid-cols-2">
+          <div className="rounded-lg border border-gray-800 bg-gray-900">
+            <div className="border-b border-gray-800 px-4 py-3 text-sm font-medium text-gray-200">
+              ModelMessage history fixture
+            </div>
+            <pre className="overflow-auto p-4 text-xs leading-5 text-gray-300">
+              {JSON.stringify(modelMessages, null, 2)}
+            </pre>
+          </div>
+
+          <div className="rounded-lg border border-gray-800 bg-gray-900">
+            <div className="border-b border-gray-800 px-4 py-3 text-sm font-medium text-gray-200">
+              Hydrated UIMessage.parts
+            </div>
+            <pre
+              id="issue-176-messages-json"
+              className="overflow-auto p-4 text-xs leading-5 text-gray-300"
+            >
+              {JSON.stringify(fixtureMessages, null, 2)}
+            </pre>
+          </div>
+        </section>
+
+        <section className="rounded-lg border border-gray-800 bg-gray-900">
+          <div className="border-b border-gray-800 px-4 py-3 text-sm font-medium text-gray-200">
+            Full live messages
+          </div>
+          <pre
+            id="issue-176-live-messages-json"
+            className="max-h-[520px] overflow-auto p-4 text-xs leading-5 text-gray-300"
+          >
+            {JSON.stringify(liveMessages, null, 2)}
+          </pre>
+        </section>
+      </div>
+    </main>
+  )
+}
+
+export const Route = createFileRoute('/issue-176-tool-result')({
+  component: Issue176ToolResultRepro,
+})
diff --git a/packages/typescript/ai-client/src/types.ts b/packages/typescript/ai-client/src/types.ts
index 815d6a838..1159150c1 100644
--- a/packages/typescript/ai-client/src/types.ts
+++ b/packages/typescript/ai-client/src/types.ts
@@ -25,6 +25,7 @@ export type ToolCallState =
   | 'input-complete' // All arguments received
   | 'approval-requested' // Waiting for user approval
   | 'approval-responded' // User has approved/denied
+  | 'complete' // Result is complete
 
 /**
  * Tool result states - track the lifecycle of a tool result
diff --git a/packages/typescript/ai-event-client/src/index.ts b/packages/typescript/ai-event-client/src/index.ts
index 17fa1d6e0..a4169c13c 100644
--- a/packages/typescript/ai-event-client/src/index.ts
+++ b/packages/typescript/ai-event-client/src/index.ts
@@ -112,6 +112,7 @@ export type ToolCallState =
   | 'input-complete' // All arguments received
   | 'approval-requested' // Waiting for user approval
   | 'approval-responded' // User has approved/denied
+  | 'complete' // Result is complete
 
 /**
  * Tool result states - track the lifecycle of a tool result
diff --git a/packages/typescript/ai/src/activities/chat/messages.ts b/packages/typescript/ai/src/activities/chat/messages.ts
index 6bfef628f..cf9892627 100644
--- a/packages/typescript/ai/src/activities/chat/messages.ts
+++ b/packages/typescript/ai/src/activities/chat/messages.ts
@@ -32,6 +32,14 @@ function safeJsonStringify(value: unknown): string {
   }
 }
 
+function parseToolResultContent(content: string): unknown {
+  try {
+    return JSON.parse(content)
+  } catch {
+    return content
+  }
+}
+
 /**
  * Collapse an array of ContentParts into the most compact ModelMessage content:
  * - Empty array → null
@@ -200,6 +208,7 @@ function createSegment(): AssistantSegment {
 function isToolCallIncluded(part: ToolCallPart): boolean {
   return (
     part.state === 'input-complete' ||
+    part.state === 'complete' ||
     part.state === 'approval-responded' ||
     part.output !== undefined
   )
@@ -467,10 +476,21 @@ export function modelMessagesToUIMessages(
         currentAssistantMessage &&
         currentAssistantMessage.role === 'assistant'
       ) {
+        const content = getTextContent(msg.content)
+        const toolCallPart = currentAssistantMessage.parts.find(
+          (part): part is ToolCallPart =>
+            part.type === 'tool-call' && part.id === msg.toolCallId,
+        )
+
+        if (toolCallPart) {
+          toolCallPart.output = parseToolResultContent(content)
+          toolCallPart.state = 'complete'
+        }
+
         currentAssistantMessage.parts.push({
           type: 'tool-result',
           toolCallId: msg.toolCallId,
-          content: getTextContent(msg.content),
+          content,
           state: 'complete',
         })
       } else {
diff --git a/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts b/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
index e6d18f0da..3fd8895f3 100644
--- a/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
+++ b/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
@@ -217,11 +217,7 @@ export function updateToolCallWithOutput(
 
     if (toolCallPart) {
       toolCallPart.output = errorText ? { error: errorText } : output
-      if (state) {
-        toolCallPart.state = state
-      } else {
-        toolCallPart.state = 'input-complete'
-      }
+      toolCallPart.state = state ?? (errorText ? 'input-complete' : 'complete')
     }
 
     return { ...msg, parts }
diff --git a/packages/typescript/ai/src/activities/chat/stream/processor.ts b/packages/typescript/ai/src/activities/chat/stream/processor.ts
index 1b1f25e6a..9a6bc6740 100644
--- a/packages/typescript/ai/src/activities/chat/stream/processor.ts
+++ b/packages/typescript/ai/src/activities/chat/stream/processor.ts
@@ -378,6 +378,7 @@ export class StreamProcessor {
     // 3. It has a corresponding tool-result part (server tool completed)
     return toolParts.every(
       (part) =>
+        part.state === 'complete' ||
         part.state === 'approval-responded' ||
         (part.output !== undefined && !part.approval) ||
         toolResultIds.has(part.id),
diff --git a/packages/typescript/ai/src/types.ts b/packages/typescript/ai/src/types.ts
index a12964981..9566cb32f 100644
--- a/packages/typescript/ai/src/types.ts
+++ b/packages/typescript/ai/src/types.ts
@@ -40,6 +40,7 @@ export type ToolCallState =
   | 'input-complete' // All arguments received
   | 'approval-requested' // Waiting for user approval
   | 'approval-responded' // User has approved/denied
+  | 'complete' // Result is complete
 
 /**
  * Tool result states - track the lifecycle of a tool result
diff --git a/packages/typescript/ai/tests/message-converters.test.ts b/packages/typescript/ai/tests/message-converters.test.ts
index 9264d96b7..073868fd3 100644
--- a/packages/typescript/ai/tests/message-converters.test.ts
+++ b/packages/typescript/ai/tests/message-converters.test.ts
@@ -919,7 +919,8 @@ describe('Message Converters', () => {
           id: 'tc-1',
           name: 'getWeather',
           arguments: '{"city":"NYC"}',
-          state: 'input-complete',
+          state: 'complete',
+          output: { temp: 72 },
         },
         {
           type: 'tool-result',
@@ -978,7 +979,8 @@ describe('Message Converters', () => {
           id: 'tc-1',
           name: 'getGuitars',
           arguments: '',
-          state: 'input-complete',
+          state: 'complete',
+          output: [{ id: 7 }],
         },
         {
           type: 'tool-result',
@@ -995,7 +997,8 @@ describe('Message Converters', () => {
           id: 'tc-2',
           name: 'recommend',
           arguments: '{"id":7}',
-          state: 'input-complete',
+          state: 'complete',
+          output: { recommended: true },
         },
         {
           type: 'tool-result',
@@ -1083,7 +1086,8 @@ describe('Message Converters', () => {
         id: 'tc-1',
         name: 'getWeather',
         arguments: '{"city":"NYC"}',
-        state: 'input-complete',
+        state: 'complete',
+        output: { temp: 72 },
       })
       expect(assistantParts).toContainEqual({
         type: 'tool-result',
@@ -1222,7 +1226,8 @@ describe('Message Converters', () => {
             id: 'tc-1',
             name: 'getWeather',
             arguments: '{"city":"NYC"}',
-            state: 'input-complete',
+            state: 'complete',
+            output: { temp: 72 },
           },
           {
             type: 'tool-result',
@@ -1314,7 +1319,8 @@ describe('Message Converters', () => {
           id: 'tc-1',
           name: 'getGuitars',
           arguments: '',
-          state: 'input-complete',
+          state: 'complete',
+          output: [{ id: 7 }],
         },
         {
           type: 'tool-result',
@@ -1332,7 +1338,8 @@ describe('Message Converters', () => {
           id: 'tc-2',
           name: 'recommend',
           arguments: '{"id":7}',
-          state: 'input-complete',
+          state: 'complete',
+          output: { recommended: true },
         },
         {
           type: 'tool-result',
diff --git a/packages/typescript/ai/tests/message-updaters.test.ts b/packages/typescript/ai/tests/message-updaters.test.ts
index efae2cf3a..358cb2b3d 100644
--- a/packages/typescript/ai/tests/message-updaters.test.ts
+++ b/packages/typescript/ai/tests/message-updaters.test.ts
@@ -538,7 +538,7 @@ describe('message-updaters', () => {
   })
 
   describe('updateToolCallWithOutput', () => {
-    it('should update tool call with output', () => {
+    it('should update tool call with output and complete state', () => {
       const messages = [
         createMessage('msg-1', 'assistant', [
           {
@@ -558,7 +558,7 @@ describe('message-updaters', () => {
         id: 'call-1',
         name: 'getWeather',
         arguments: '{}',
-        state: 'input-complete',
+        state: 'complete',
         output,
       })
     })
@@ -588,7 +588,7 @@ describe('message-updaters', () => {
       expect(part?.output).toEqual(output)
     })
 
-    it('should default to input-complete state when not provided', () => {
+    it('should default to complete state when not provided', () => {
       const messages = [
         createMessage('msg-1', 'assistant', [
           {
@@ -604,7 +604,7 @@ describe('message-updaters', () => {
       const result = updateToolCallWithOutput(messages, 'call-1', output)
 
       const part = result[0]?.parts[0] as ToolCallPart | undefined
-      expect(part?.state).toBe('input-complete')
+      expect(part?.state).toBe('complete')
     })
 
     it('should handle error output', () => {
@@ -629,6 +629,7 @@ describe('message-updaters', () => {
 
       const part = result[0]?.parts[0] as ToolCallPart | undefined
       expect(part?.output).toEqual({ error: 'Tool execution failed' })
+      expect(part?.state).toBe('input-complete')
     })
 
     it('should search across all messages', () => {
diff --git a/packages/typescript/ai/tests/stream-processor.test.ts b/packages/typescript/ai/tests/stream-processor.test.ts
index 7b4dbeaa1..2e1f613bd 100644
--- a/packages/typescript/ai/tests/stream-processor.test.ts
+++ b/packages/typescript/ai/tests/stream-processor.test.ts
@@ -917,6 +917,7 @@ describe('StreamProcessor', () => {
         (p) => p.type === 'tool-call',
       ) as ToolCallPart
       expect((toolCallPart as any).output).toEqual({ temp: 72 })
+      expect(toolCallPart.state).toBe('complete')
 
       const toolResultPart = messages[0]?.parts.find(
         (p) => p.type === 'tool-result',
@@ -981,6 +982,7 @@ describe('StreamProcessor', () => {
         (p) => p.type === 'tool-call',
       ) as ToolCallPart
       expect((toolCallPart as any).output).toEqual({ temp: 72 })
+      expect(toolCallPart.state).toBe('complete')
 
       const toolResultPart = messages[0]!.parts.find(
         (p) => p.type === 'tool-result',
@@ -1020,6 +1022,7 @@ describe('StreamProcessor', () => {
         .getMessages()[0]!
         .parts.find((p) => p.type === 'tool-call') as ToolCallPart
       expect((toolCallPart as any).output).toEqual({ error: 'Network error' })
+      expect(toolCallPart.state).toBe('input-complete')
 
       const toolResultPart = processor
         .getMessages()[0]!
@@ -3326,6 +3329,7 @@ describe('StreamProcessor', () => {
         (p) => p.type === 'tool-call',
       ) as ToolCallPart
       expect((toolCallPart as any).output).toEqual({ temp: 72 })
+      expect(toolCallPart.state).toBe('complete')
 
       const toolResultPart = messages[0]?.parts.find(
         (p) => p.type === 'tool-result',
diff --git a/testing/e2e/src/routes/tools-test.tsx b/testing/e2e/src/routes/tools-test.tsx
index 893c55168..30f3ad55e 100644
--- a/testing/e2e/src/routes/tools-test.tsx
+++ b/testing/e2e/src/routes/tools-test.tsx
@@ -1,7 +1,11 @@
-import { useState, useCallback, useRef, useEffect } from 'react'
+import { useState, useCallback, useRef, useEffect, useMemo } from 'react'
 import { createFileRoute } from '@tanstack/react-router'
 import { useChat, fetchServerSentEvents } from '@tanstack/ai-react'
-import { toolDefinition } from '@tanstack/ai'
+import {
+  modelMessagesToUIMessages,
+  toolDefinition,
+  type ModelMessage,
+} from '@tanstack/ai'
 import { z } from 'zod'
 import { SCENARIO_LIST } from '@/lib/tools-test-tools'
 
@@ -98,8 +102,38 @@ function createTrackedTools(
   return [showNotificationTool, displayChartTool]
 }
 
+function createHistoryFixtureMessages(historyFixture?: string) {
+  if (historyFixture !== 'server-tool-result') {
+    return []
+  }
+
+  const modelMessages: Array<ModelMessage> = [
+    {
+      role: 'assistant',
+      content: 'Let me check the weather.',
+      toolCalls: [
+        {
+          id: 'history-tc-1',
+          type: 'function',
+          function: {
+            name: 'getWeather',
+            arguments: '{"city":"NYC"}',
+          },
+        },
+      ],
+    },
+    {
+      role: 'tool',
+      content: '{"temp":72,"condition":"sunny"}',
+      toolCallId: 'history-tc-1',
+    },
+  ]
+
+  return modelMessagesToUIMessages(modelMessages)
+}
+
 function ToolsTestPage() {
-  const { testId, aimockPort } = Route.useSearch()
+  const { testId, aimockPort, historyFixture } = Route.useSearch()
   const [scenario, setScenario] = useState('text-only')
   const [toolEvents, setToolEvents] = useState<Array<ToolEvent>>([])
   const [testStartTime, setTestStartTime] = useState<number | null>(null)
@@ -115,6 +149,10 @@ function ToolsTestPage() {
 
   // Create tracked tools (memoized since addEvent is stable)
   const clientTools = useRef(createTrackedTools(addEvent)).current
+  const initialMessages = useMemo(
+    () => createHistoryFixtureMessages(historyFixture),
+    [historyFixture],
+  )
 
   const {
     messages,
@@ -125,8 +163,9 @@ function ToolsTestPage() {
     error,
   } = useChat({
     // Include scenario in ID so client is recreated when scenario changes
-    id: `tools-test-${scenario}`,
+    id: `tools-test-${scenario}-${historyFixture || 'empty'}`,
     connection: fetchServerSentEvents('/api/tools-test'),
+    initialMessages,
     body: { scenario, testId, aimockPort },
     tools: clientTools,
     onFinish: () => {
@@ -565,6 +604,7 @@ function ToolsTestPage() {
         id="test-metadata"
         style={{ display: 'none' }}
         data-scenario={scenario}
+        data-history-fixture={historyFixture || ''}
         data-is-loading={isLoading.toString()}
         data-test-complete={testComplete.toString()}
         data-tool-call-count={toolCalls.length}
@@ -633,6 +673,10 @@ export const Route = createFileRoute('/tools-test')({
     return {
       testId: typeof search.testId === 'string' ? search.testId : undefined,
       aimockPort: port != null && !isNaN(port) ? port : undefined,
+      historyFixture:
+        typeof search.historyFixture === 'string'
+          ? search.historyFixture
+          : undefined,
     }
   },
 })
diff --git a/testing/e2e/tests/tools-test/helpers.ts b/testing/e2e/tests/tools-test/helpers.ts
index a83853cd9..4388ca80b 100644
--- a/testing/e2e/tests/tools-test/helpers.ts
+++ b/testing/e2e/tests/tools-test/helpers.ts
@@ -180,6 +180,21 @@ export async function getToolCalls(
   })
 }
 
+/**
+ * Parse the full UIMessage array from #messages-json-content.
+ */
+export async function getMessages(page: Page): Promise<Array<any>> {
+  return page.evaluate(() => {
+    const el = document.getElementById('messages-json-content')
+    if (!el) return []
+    try {
+      return JSON.parse(el.textContent || '[]')
+    } catch {
+      return []
+    }
+  })
+}
+
 /**
  * Extract tool-call parts with parsed arguments from #messages-json-content.
  */
diff --git a/testing/e2e/tests/tools-test/server-client-sequence.spec.ts b/testing/e2e/tests/tools-test/server-client-sequence.spec.ts
index e0f136ed8..a7c64426c 100644
--- a/testing/e2e/tests/tools-test/server-client-sequence.spec.ts
+++ b/testing/e2e/tests/tools-test/server-client-sequence.spec.ts
@@ -6,6 +6,7 @@ import {
   getMetadata,
   getEventLog,
   getToolCalls,
+  getMessages,
 } from './helpers'
 
 /**
@@ -58,6 +59,73 @@ test.describe('Server-Client Sequence E2E Tests', () => {
     expect(chartExecution).toBeTruthy()
   })
 
+  test('server and client tool results populate tool-call output and complete state', async ({
+    page,
+    testId,
+    aimockPort,
+  }) => {
+    await selectScenario(page, 'sequence-server-client', testId, aimockPort)
+    await runTest(page)
+
+    await waitForTestComplete(page, 15000, 2)
+
+    await page.waitForFunction(
+      () => {
+        const messagesEl = document.getElementById('messages-json-content')
+        const messages = JSON.parse(messagesEl?.textContent || '[]')
+        const toolCalls = messages.flatMap((msg: any) =>
+          (msg.parts || []).filter((part: any) => part.type === 'tool-call'),
+        )
+
+        const fetchData = toolCalls.find(
+          (part: any) => part.name === 'fetch_data',
+        )
+        const displayChart = toolCalls.find(
+          (part: any) => part.name === 'display_chart',
+        )
+
+        return (
+          fetchData?.state === 'complete' &&
+          fetchData.output !== undefined &&
+          displayChart?.state === 'complete' &&
+          displayChart.output !== undefined
+        )
+      },
+      { timeout: 10000 },
+    )
+
+    const messages = await getMessages(page)
+    const toolCalls = messages.flatMap((msg) =>
+      (msg.parts || []).filter((part: any) => part.type === 'tool-call'),
+    )
+    const toolResults = messages.flatMap((msg) =>
+      (msg.parts || []).filter((part: any) => part.type === 'tool-result'),
+    )
+
+    const fetchData = toolCalls.find((part) => part.name === 'fetch_data')
+    expect(fetchData).toMatchObject({
+      state: 'complete',
+      output: {
+        source: 'api',
+        data: [1, 2, 3, 4, 5],
+      },
+    })
+
+    const displayChart = toolCalls.find((part) => part.name === 'display_chart')
+    expect(displayChart?.state).toBe('complete')
+    expect(displayChart?.output).toMatchObject({ rendered: true })
+    expect(typeof displayChart?.output?.chartId).toBe('string')
+
+    expect(
+      toolResults.some(
+        (part) =>
+          part.toolCallId === fetchData?.id &&
+          part.state === 'complete' &&
+          JSON.parse(part.content).source === 'api',
+      ),
+    ).toBe(true)
+  })
+
   test('server then two client tools in sequence', async ({
     page,
     testId,
@@ -162,10 +230,7 @@ test.describe('Server-Client Sequence E2E Tests', () => {
     const toolCalls = await getToolCalls(page)
     const weatherTool = toolCalls.find((tc) => tc.name === 'get_weather')
     expect(weatherTool).toBeTruthy()
-    // Server tools stay at 'input-complete' state but are tracked as complete via tool-result parts
-    expect(['complete', 'input-complete', 'output-available']).toContain(
-      weatherTool?.state,
-    )
+    expect(weatherTool?.state).toBe('complete')
   })
 
   test('text only scenario has no tool calls', async ({
diff --git a/testing/e2e/tests/tools-test/server-tool-history.spec.ts b/testing/e2e/tests/tools-test/server-tool-history.spec.ts
new file mode 100644
index 000000000..40e558eb6
--- /dev/null
+++ b/testing/e2e/tests/tools-test/server-tool-history.spec.ts
@@ -0,0 +1,64 @@
+import { test, expect } from '../fixtures'
+import { getMessages } from './helpers'
+
+test.describe('Server Tool History Hydration', () => {
+  test('hydrates server tool-call output from a matching tool result', async ({
+    page,
+    testId,
+    aimockPort,
+  }) => {
+    const params = new URLSearchParams({
+      historyFixture: 'server-tool-result',
+      testId,
+      aimockPort: String(aimockPort),
+    })
+
+    await page.goto(`/tools-test?${params.toString()}`)
+    await page.waitForSelector('#run-test-button')
+
+    await page.waitForFunction(
+      () => {
+        const messagesEl = document.getElementById('messages-json-content')
+        const messages = JSON.parse(messagesEl?.textContent || '[]')
+        const toolCall = messages
+          .flatMap((msg: any) => msg.parts || [])
+          .find(
+            (part: any) =>
+              part.type === 'tool-call' && part.id === 'history-tc-1',
+          )
+
+        return toolCall?.state === 'complete' && toolCall.output !== undefined
+      },
+      { timeout: 10000 },
+    )
+
+    const messages = await getMessages(page)
+    const assistant = messages.find((message) => message.role === 'assistant')
+    const toolCall = assistant?.parts.find(
+      (part: any) => part.type === 'tool-call' && part.id === 'history-tc-1',
+    )
+    const toolResult = assistant?.parts.find(
+      (part: any) =>
+        part.type === 'tool-result' && part.toolCallId === 'history-tc-1',
+    )
+
+    expect(toolCall).toMatchObject({
+      type: 'tool-call',
+      id: 'history-tc-1',
+      name: 'getWeather',
+      arguments: '{"city":"NYC"}',
+      state: 'complete',
+      output: {
+        temp: 72,
+        condition: 'sunny',
+      },
+    })
+
+    expect(toolResult).toMatchObject({
+      type: 'tool-result',
+      toolCallId: 'history-tc-1',
+      content: '{"temp":72,"condition":"sunny"}',
+      state: 'complete',
+    })
+  })
+})