diff --git a/.changeset/multimodal-tool-results.md b/.changeset/multimodal-tool-results.md
new file mode 100644
index 000000000..f4e414043
--- /dev/null
+++ b/.changeset/multimodal-tool-results.md
@@ -0,0 +1,17 @@
+---
+'@tanstack/ai': patch
+'@tanstack/ai-openai': patch
+'@tanstack/ai-anthropic': patch
+'@tanstack/ai-client': patch
+'@tanstack/ai-react-ui': patch
+'@tanstack/ai-solid-ui': patch
+'@tanstack/ai-vue-ui': patch
+---
+
+Preserve multimodal tool results across chat history and provider adapters.
+
+This fixes tool result handling so `ContentPart[]` outputs are preserved instead
+of being stringified in core chat flows, OpenAI Responses tool outputs, and
+Anthropic tool results. It also updates the client-side tool result type and
+the default React, Solid, and Vue chat message renderers to handle text and
+image tool result content.
diff --git a/packages/typescript/ai-anthropic/src/adapters/text.ts b/packages/typescript/ai-anthropic/src/adapters/text.ts
index 235d9f5b5..e87e305d9 100644
--- a/packages/typescript/ai-anthropic/src/adapters/text.ts
+++ b/packages/typescript/ai-anthropic/src/adapters/text.ts
@@ -379,7 +379,13 @@ export class AnthropicTextAdapter<
               type: 'tool_result',
               tool_use_id: message.toolCallId,
               content:
-                typeof message.content === 'string' ? message.content : '',
+                typeof message.content === 'string'
+                  ? message.content
+                  : Array.isArray(message.content)
+                    ? message.content.map((part) =>
+                        this.convertContentPartToAnthropic(part),
+                      )
+                    : '',
             },
           ],
         })
diff --git a/packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts b/packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts
index 76c50ccf2..dc28c8d9f 100644
--- a/packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts
+++ b/packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts
@@ -184,6 +184,95 @@ describe('Anthropic adapter option mapping', () => {
     })
   })
 
+  it('preserves multimodal tool result content for tool_result blocks', async () => {
+    const mockStream = (async function* () {
+      yield {
+        type: 'message_start',
+        message: {
+          id: 'msg_123',
+          model: 'claude-3-7-sonnet-20250219',
+          role: 'assistant',
+          type: 'message',
+          content: [],
+          stop_reason: null,
+          stop_sequence: null,
+          usage: { input_tokens: 10, output_tokens: 0 },
+        },
+      }
+      yield {
+        type: 'message_delta',
+        delta: { stop_reason: 'end_turn' },
+        usage: { output_tokens: 1 },
+      }
+      yield {
+        type: 'message_stop',
+      }
+    })()
+
+    mocks.betaMessagesCreate.mockResolvedValueOnce(mockStream as any)
+
+    const adapter = createAdapter('claude-3-7-sonnet-20250219')
+    const toolResult = [
+      {
+        type: 'text' as const,
+        content: 'Screenshot of the current state',
+      },
+      {
+        type: 'image' as const,
+        source: {
+          type: 'url' as const,
+          value: 'https://example.com/screenshot.png',
+        },
+      },
+    ]
+
+    for await (const _chunk of chat({
+      adapter,
+      messages: [
+        { role: 'user', content: 'What changed?' },
+        {
+          role: 'assistant',
+          content: 'Checking',
+          toolCalls: [
+            {
+              id: 'call_canvas',
+              type: 'function',
+              function: { name: 'view_canvas', arguments: '{}' },
+            },
+          ],
+        },
+        { role: 'tool', toolCallId: 'call_canvas', content: toolResult },
+      ],
+      tools: [weatherTool],
+    })) {
+      // drain
+    }
+
+    const [payload] = mocks.betaMessagesCreate.mock.calls.at(-1) as [any]
+    expect(payload.messages[2]).toEqual({
+      role: 'user',
+      content: [
+        {
+          type: 'tool_result',
+          tool_use_id: 'call_canvas',
+          content: [
+            {
+              type: 'text',
+              text: 'Screenshot of the current state',
+            },
+            {
+              type: 'image',
+              source: {
+                type: 'url',
+                url: 'https://example.com/screenshot.png',
+              },
+            },
+          ],
+        },
+      ],
+    })
+  })
+
   it('merges consecutive user messages when tool results precede a follow-up user message', async () => {
     // This is the core multi-turn bug: after a tool call + result, the next user message
     // creates consecutive role:'user' messages (tool_result as user + new user message).
diff --git a/packages/typescript/ai-client/src/types.ts b/packages/typescript/ai-client/src/types.ts
index b705ebbdf..9b1a55a52 100644
--- a/packages/typescript/ai-client/src/types.ts
+++ b/packages/typescript/ai-client/src/types.ts
@@ -13,6 +13,8 @@ import type {
 } from '@tanstack/ai'
 import type { ConnectionAdapter } from './connection-adapters'
 
+type ToolResultContent = string | Array<ContentPart>
+
 /**
  * Tool call states - track the lifecycle of a tool call
  */
@@ -152,7 +154,7 @@ export type ToolCallPart<TTools extends ReadonlyArray<AnyClientTool> = any> =
 export interface ToolResultPart {
   type: 'tool-result'
   toolCallId: string
-  content: string
+  content: ToolResultContent
   state: ToolResultState
   error?: string // Error message if state is "error"
 }
diff --git a/packages/typescript/ai-code-mode/models-eval/metrics.ts b/packages/typescript/ai-code-mode/models-eval/metrics.ts
index 97a9d2944..3468f612c 100644
--- a/packages/typescript/ai-code-mode/models-eval/metrics.ts
+++ b/packages/typescript/ai-code-mode/models-eval/metrics.ts
@@ -1,4 +1,4 @@
-import type { UIMessage } from '@tanstack/ai'
+import type { ToolResultPart, UIMessage } from '@tanstack/ai'
 
 export interface TypeScriptAttempt {
   toolCallId: string
@@ -44,11 +44,33 @@ function safeJsonParse(value: string): unknown {
   }
 }
 
+function parseToolResultContent(
+  content: ToolResultPart['content'],
+):
+  | {
+      success?: boolean
+      error?: { name?: string; message?: string }
+    }
+  | undefined {
+  return typeof content === 'string'
+    ? (safeJsonParse(content) as
+        | {
+            success?: boolean
+            error?: { name?: string; message?: string }
+          }
+        | undefined)
+    : undefined
+}
+
 export function computeMetrics(messages: Array<UIMessage>): ComputedMetrics {
   const toolCallLookup = new Map<string, { name: string; arguments: string }>()
   const toolResultLookup = new Map<
     string,
-    { content: string; state?: string; error?: string }
+    {
+      content: ToolResultPart['content']
+      state?: string
+      error?: string
+    }
   >()
 
   let totalToolCalls = 0
@@ -95,13 +117,8 @@ export function computeMetrics(messages: Array<UIMessage>): ComputedMetrics {
       | { typescriptCode?: string }
       | undefined
     const result = toolResultLookup.get(toolCallId)
-    const parsedResult = result?.content
-      ? (safeJsonParse(result.content) as
-          | {
-              success?: boolean
-              error?: { name?: string; message?: string }
-            }
-          | undefined)
+    const parsedResult = result
+      ? parseToolResultContent(result.content)
       : undefined
 
     const success = parsedResult?.success
diff --git a/packages/typescript/ai-openai/src/adapters/text.ts b/packages/typescript/ai-openai/src/adapters/text.ts
index 1747ce4ec..9ca00d0f5 100644
--- a/packages/typescript/ai-openai/src/adapters/text.ts
+++ b/packages/typescript/ai-openai/src/adapters/text.ts
@@ -703,13 +703,24 @@ export class OpenAITextAdapter<
     for (const message of messages) {
       // Handle tool messages - convert to FunctionToolCallOutput
       if (message.role === 'tool') {
+        const output =
+          typeof message.content === 'string'
+            ? message.content
+            : this.normalizeContent(message.content).map((part) =>
+                this.convertContentPartToOpenAI(
+                  part as ContentPart<
+                    unknown,
+                    OpenAIImageMetadata,
+                    OpenAIAudioMetadata,
+                    unknown,
+                    unknown
+                  >,
+                ),
+              )
         result.push({
           type: 'function_call_output',
           call_id: message.toolCallId || '',
-          output:
-            typeof message.content === 'string'
-              ? message.content
-              : JSON.stringify(message.content),
+          output,
         })
         continue
       }
diff --git a/packages/typescript/ai-openai/tests/openai-adapter.test.ts b/packages/typescript/ai-openai/tests/openai-adapter.test.ts
index 552793a2e..ab883292d 100644
--- a/packages/typescript/ai-openai/tests/openai-adapter.test.ts
+++ b/packages/typescript/ai-openai/tests/openai-adapter.test.ts
@@ -129,4 +129,85 @@ describe('OpenAI adapter option mapping', () => {
     expect(Array.isArray(payload.tools)).toBe(true)
     expect(payload.tools.length).toBeGreaterThan(0)
   })
+
+  it('preserves multimodal tool result content for function_call_output', async () => {
+    const mockStream = createMockChatCompletionsStream([
+      {
+        type: 'response.done',
+        response: {
+          id: 'resp-123',
+          model: 'gpt-4o-mini',
+          status: 'completed',
+          created_at: 1234567891,
+          usage: {
+            input_tokens: 12,
+            output_tokens: 0,
+          },
+        },
+      },
+    ])
+
+    const responsesCreate = vi.fn().mockResolvedValueOnce(mockStream)
+
+    const adapter = createAdapter('gpt-4o-mini')
+    ;(adapter as any).client = {
+      responses: {
+        create: responsesCreate,
+      },
+    }
+
+    const toolResult = [
+      {
+        type: 'text' as const,
+        content: 'Screenshot of the current state',
+      },
+      {
+        type: 'image' as const,
+        source: {
+          type: 'url' as const,
+          value: 'https://example.com/screenshot.png',
+        },
+      },
+    ]
+
+    for await (const _chunk of chat({
+      adapter,
+      messages: [
+        { role: 'user', content: 'What changed?' },
+        {
+          role: 'assistant',
+          content: 'Checking',
+          toolCalls: [
+            {
+              id: 'call_canvas',
+              type: 'function',
+              function: { name: 'view_canvas', arguments: '{}' },
+            },
+          ],
+        },
+        { role: 'tool', toolCallId: 'call_canvas', content: toolResult },
+      ],
+      tools: [weatherTool],
+    })) {
+      // drain
+    }
+
+    const [payload] = responsesCreate.mock.calls[0]
+    const toolOutput = payload.input.find(
+      (item: any) =>
+        item.type === 'function_call_output' && item.call_id === 'call_canvas',
+    )
+
+    expect(toolOutput.output).toEqual([
+      {
+        type: 'input_text',
+        text: 'Screenshot of the current state',
+      },
+      {
+        type: 'input_image',
+        image_url: 'https://example.com/screenshot.png',
+        detail: 'auto',
+      },
+    ])
+  })
 })
diff --git a/packages/typescript/ai-react-ui/src/chat-message.tsx b/packages/typescript/ai-react-ui/src/chat-message.tsx
index beacdfae3..18c5d22da 100644
--- a/packages/typescript/ai-react-ui/src/chat-message.tsx
+++ b/packages/typescript/ai-react-ui/src/chat-message.tsx
@@ -2,6 +2,22 @@ import { ThinkingPart } from './thinking-part'
 import type { ReactNode } from 'react'
 import type { UIMessage } from '@tanstack/ai-react'
 
+type ToolResultContent = Extract<
+  UIMessage['parts'][number],
+  { type: 'tool-result' }
+>['content']
+type ToolResultContentSource =
+  | {
+      type: 'url'
+      value: string
+      mimeType?: string
+    }
+  | {
+      type: 'data'
+      value: string
+      mimeType: string
+    }
+
 export interface ToolCallRenderProps {
   id: string
   name: string
@@ -34,11 +50,55 @@ export interface ChatMessageProps {
   /** Custom renderer for tool result parts */
   toolResultRenderer?: (props: {
     toolCallId: string
-    content: string
+    content: ToolResultContent
     state: string
   }) => ReactNode
 }
 
+function getContentPartSourceUrl(
+  source: ToolResultContentSource,
+): string {
+  if (source.type === 'url') {
+    return source.value
+  }
+
+  return source.value.startsWith('data:')
+    ? source.value
+    : `data:${source.mimeType};base64,${source.value}`
+}
+
+function renderToolResultContent(content: ToolResultContent): ReactNode {
+  if (typeof content === 'string') {
+    return content
+  }
+
+  return content.map((part, index) => {
+    switch (part.type) {
+      case 'text':
+        return (
+          <div key={index} data-tool-result-part-type="text">
+            {part.content}
+          </div>
+        )
+      case 'image':
+        return (
+          <img
+            key={index}
+            data-tool-result-part-type="image"
+            src={getContentPartSourceUrl(part.source)}
+            alt="Tool result"
+          />
+        )
+      default:
+        return (
+          <div key={index} data-tool-result-part-type={part.type}>
+            Unsupported tool result content: {part.type}
+          </div>
+        )
+    }
+  })
+}
+
 /**
  * Message component - renders a single message with all its parts
  *
@@ -258,7 +318,7 @@ function MessagePart({
         data-tool-call-id={part.toolCallId}
         data-tool-result-state={part.state}
       >
-        <div data-tool-result-content>{part.content}</div>
+        <div data-tool-result-content>{renderToolResultContent(part.content)}</div>
       </div>
     )
   }
diff --git a/packages/typescript/ai-solid-ui/src/chat-message.tsx b/packages/typescript/ai-solid-ui/src/chat-message.tsx
index 10b3c5715..8ac782080 100644
--- a/packages/typescript/ai-solid-ui/src/chat-message.tsx
+++ b/packages/typescript/ai-solid-ui/src/chat-message.tsx
@@ -3,6 +3,22 @@ import { ThinkingPart } from './thinking-part'
 import type { JSX } from 'solid-js'
 import type { UIMessage } from '@tanstack/ai-solid'
 
+type ToolResultContent = Extract<
+  UIMessage['parts'][number],
+  { type: 'tool-result' }
+>['content']
+type ToolResultContentSource =
+  | {
+      type: 'url'
+      value: string
+      mimeType?: string
+    }
+  | {
+      type: 'data'
+      value: string
+      mimeType: string
+    }
+
 export interface ToolCallRenderProps {
   id: string
   name: string
@@ -35,11 +51,50 @@ export interface ChatMessageProps {
   /** Custom renderer for tool result parts */
   toolResultRenderer?: (props: {
     toolCallId: string
-    content: string
+    content: ToolResultContent
     state: string
   }) => JSX.Element
 }
 
+function getContentPartSourceUrl(
+  source: ToolResultContentSource,
+): string {
+  if (source.type === 'url') {
+    return source.value
+  }
+
+  return source.value.startsWith('data:')
+    ? source.value
+    : `data:${source.mimeType};base64,${source.value}`
+}
+
+function renderToolResultContent(content: ToolResultContent): JSX.Element {
+  if (typeof content === 'string') {
+    return <>{content}</>
+  }
+
+  return (
+    <For each={content}>
+      {(part) => {
+        switch (part.type) {
+          case 'text':
+            return <div data-tool-result-part-type="text">{part.content}</div>
+          case 'image':
+            return (
+              <img
+                data-tool-result-part-type="image"
+                src={getContentPartSourceUrl(part.source)}
+                alt="Tool result"
+              />
+            )
+          default:
+            return null
+        }
+      }}
+    </For>
+  )
+}
+
 /**
  * Message component - renders a single message with all its parts
  *
@@ -249,7 +304,9 @@ function MessagePart(props: {
         data-tool-call-id={props.part.toolCallId}
         data-tool-result-state={props.part.state}
       >
-        <div data-tool-result-content>{props.part.content}</div>
+        <div data-tool-result-content>
+          {renderToolResultContent(props.part.content)}
+        </div>
       </div>
     )
   }
diff --git a/packages/typescript/ai-vue-ui/src/chat-message.vue b/packages/typescript/ai-vue-ui/src/chat-message.vue
index deeaeb03a..0a9cc59ad 100644
--- a/packages/typescript/ai-vue-ui/src/chat-message.vue
+++ b/packages/typescript/ai-vue-ui/src/chat-message.vue
@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { computed } from 'vue'
 import MessagePart from './message-part.vue'
-import type { ChatMessageProps } from './types'
+import type { ChatMessageProps, ToolResultContent } from './types'
 
 const props = defineProps<ChatMessageProps>()
 
@@ -11,7 +11,7 @@ type ChatMessageSlots = {
   'tool-default'?: (props: any) => any
   'tool-result'?: (props: {
     toolCallId: string
-    content: string
+    content: ToolResultContent
     state: string
   }) => any
 } & Partial<Record<`tool-${string}`, (props: any) => any>>
diff --git a/packages/typescript/ai-vue-ui/src/message-part.vue b/packages/typescript/ai-vue-ui/src/message-part.vue
index 855fe7722..8aae7e674 100644
--- a/packages/typescript/ai-vue-ui/src/message-part.vue
+++ b/packages/typescript/ai-vue-ui/src/message-part.vue
@@ -1,7 +1,19 @@
 <script setup lang="ts">
 import { computed } from 'vue'
 import ThinkingPart from './thinking-part.vue'
-import type { ToolCallRenderProps } from './types'
+import type { ToolCallRenderProps, ToolResultContent } from './types'
+
+type ToolResultContentSource =
+  | {
+      type: 'url'
+      value: string
+      mimeType?: string
+    }
+  | {
+      type: 'data'
+      value: string
+      mimeType: string
+    }
 
 interface MessagePartProps {
   part: any
@@ -16,7 +28,7 @@ type MessagePartSlots = {
   'tool-default'?: (props: ToolCallRenderProps) => any
   'tool-result'?: (props: {
     toolCallId: string
-    content: string
+    content: ToolResultContent
     state: string
   }) => any
 } & Partial<Record<`tool-${string}`, (props: ToolCallRenderProps) => any>>
@@ -36,6 +48,24 @@ const toolProps = computed<ToolCallRenderProps | null>(() => {
   }
   return null
 })
+
+function isStringToolResultContent(
+  content: ToolResultContent,
+): content is string {
+  return typeof content === 'string'
+}
+
+function getContentPartSourceUrl(
+  source: ToolResultContentSource,
+): string {
+  if (source.type === 'url') {
+    return source.value
+  }
+
+  return source.value.startsWith('data:')
+    ? source.value
+    : `data:${source.mimeType};base64,${source.value}`
+}
 </script>
 
 <template>
@@ -120,6 +150,29 @@ const toolProps = computed<ToolCallRenderProps | null>(() => {
       :content="part.content"
       :state="part.state"
     />
-    <div v-else data-tool-result-content>{{ part.content }}</div>
+    <div v-else data-tool-result-content>
+      <template v-if="isStringToolResultContent(part.content)">
+        {{ part.content }}
+      </template>
+      <template v-else>
+        <template v-for="(contentPart, index) in part.content" :key="index">
+          <div
+            v-if="contentPart.type === 'text'"
+            data-tool-result-part-type="text"
+          >
+            {{ contentPart.content }}
+          </div>
+          <img
+            v-else-if="contentPart.type === 'image'"
+            data-tool-result-part-type="image"
+            :src="getContentPartSourceUrl(contentPart.source)"
+            alt="Tool result"
+          />
+          <div v-else :data-tool-result-part-type="contentPart.type">
+            Unsupported tool result content: {{ contentPart.type }}
+          </div>
+        </template>
+      </template>
+    </div>
   </div>
 </template>
diff --git a/packages/typescript/ai-vue-ui/src/types.ts b/packages/typescript/ai-vue-ui/src/types.ts
index d4e07419c..1caada595 100644
--- a/packages/typescript/ai-vue-ui/src/types.ts
+++ b/packages/typescript/ai-vue-ui/src/types.ts
@@ -1,5 +1,10 @@
 import type { ConnectionAdapter, UIMessage } from '@tanstack/ai-vue'
 
+export type ToolResultContent = Extract<
+  UIMessage['parts'][number],
+  { type: 'tool-result' }
+>['content']
+
 export interface ChatProps {
   /** CSS class name for the root element */
   class?: string
diff --git a/packages/typescript/ai/src/activities/chat/index.ts b/packages/typescript/ai/src/activities/chat/index.ts
index c7ec866d6..bfe6da197 100644
--- a/packages/typescript/ai/src/activities/chat/index.ts
+++ b/packages/typescript/ai/src/activities/chat/index.ts
@@ -19,7 +19,11 @@ import {
   parseWithStandardSchema,
 } from './tools/schema-converter'
 import { maxIterations as maxIterationsStrategy } from './agent-loop-strategies'
-import { convertMessagesToModelMessages } from './messages'
+import {
+  convertMessagesToModelMessages,
+  normalizeToolResultContent,
+  parseToolResultValue,
+} from './messages'
 import { MiddlewareRunner } from './middleware/compose'
 import type {
   ApprovalRequest,
@@ -1002,12 +1006,7 @@ class TextEngine<
       // This handles results sent back from the client after executing client-side tools
       if (message.role === 'tool' && message.toolCallId) {
         // Parse content back to original output (was stringified by uiMessageToModelMessages)
-        let output: unknown
-        try {
-          output = JSON.parse(message.content as string)
-        } catch {
-          output = message.content
-        }
+        const output = parseToolResultValue(message.content)
         // Skip approval response messages (they have pendingExecution marker)
         // These are NOT real client tool results — they are synthetic tool messages
         // created by uiMessageToModelMessages for approved-but-not-yet-executed tools.
@@ -1084,7 +1083,7 @@ class TextEngine<
     const chunks: Array<StreamChunk> = []
 
     for (const result of results) {
-      const content = JSON.stringify(result.result)
+      const content = normalizeToolResultContent(result.result)
 
       chunks.push({
         type: 'TOOL_CALL_END',
@@ -1116,17 +1115,11 @@ class TextEngine<
     for (const message of this.messages) {
       if (message.role === 'tool' && message.toolCallId) {
         // Check if this is an approval response with pendingExecution marker
-        let hasPendingExecution = false
-        if (typeof message.content === 'string') {
-          try {
-            const parsed = JSON.parse(message.content)
-            if (parsed.pendingExecution === true) {
-              hasPendingExecution = true
-            }
-          } catch {
-            // Not JSON, treat as regular tool result
-          }
-        }
+        const parsedContent = parseToolResultValue(message.content)
+        const hasPendingExecution =
+          parsedContent &&
+          typeof parsedContent === 'object' &&
+          (parsedContent as any).pendingExecution === true
 
         // Only mark as complete if NOT pending execution
         if (!hasPendingExecution) {
diff --git a/packages/typescript/ai/src/activities/chat/messages.ts b/packages/typescript/ai/src/activities/chat/messages.ts
index b7f97b880..687cefe18 100644
--- a/packages/typescript/ai/src/activities/chat/messages.ts
+++ b/packages/typescript/ai/src/activities/chat/messages.ts
@@ -1,9 +1,11 @@
 import type {
   ContentPart,
+  ContentPartSource,
   MessagePart,
   ModelMessage,
   TextPart,
   ToolCallPart,
+  ToolResultContent,
   UIMessage,
 } from '../../types'
 // ===========================
@@ -24,6 +26,92 @@ function isContentPart(part: MessagePart): part is ContentPart {
   )
 }
 
+function isContentPartSource(
+  source: unknown,
+): source is ContentPartSource {
+  if (!source || typeof source !== 'object') {
+    return false
+  }
+
+  const type = (source as { type?: unknown }).type
+  const value = (source as { value?: unknown }).value
+  const mimeType = (source as { mimeType?: unknown }).mimeType
+
+  if (type === 'url') {
+    return typeof value === 'string'
+  }
+
+  if (type === 'data') {
+    return typeof value === 'string' && typeof mimeType === 'string'
+  }
+
+  return false
+}
+
+function isUnknownContentPart(part: unknown): part is ContentPart {
+  if (!part || typeof part !== 'object') {
+    return false
+  }
+
+  const type = (part as { type?: unknown }).type
+  if (type === 'text') {
+    return typeof (part as { content?: unknown }).content === 'string'
+  }
+
+  if (
+    type === 'image' ||
+    type === 'audio' ||
+    type === 'video' ||
+    type === 'document'
+  ) {
+    return isContentPartSource((part as { source?: unknown }).source)
+  }
+
+  return false
+}
+
+export function isContentPartArray(
+  value: unknown,
+): value is Array<ContentPart> {
+  return Array.isArray(value) && value.every(isUnknownContentPart)
+}
+
+export function normalizeToolResultContent(
+  value: unknown,
+): ToolResultContent {
+  if (typeof value === 'string') {
+    return value
+  }
+
+  if (isContentPartArray(value)) {
+    return value
+  }
+
+  return JSON.stringify(value)
+}
+
+export function parseToolResultValue(value: unknown): unknown {
+  if (typeof value !== 'string') {
+    return value
+  }
+
+  try {
+    return JSON.parse(value)
+  } catch {
+    return value
+  }
+}
+
+export function decodeToolResultContent(value: unknown): ToolResultContent {
+  const parsed = parseToolResultValue(value)
+
+  if (isContentPartArray(parsed)) {
+    return parsed
+  }
+
+  return normalizeToolResultContent(value)
+}
+
 /**
  * Collapse an array of ContentParts into the most compact ModelMessage content:
  * - Empty array → null
@@ -57,6 +145,20 @@ function getTextContent(content: string | null | Array<ContentPart>): string {
     .join('')
 }
 
+function getToolResultContent(
+  content: string | null | Array<ContentPart>,
+): ToolResultContent {
+  if (typeof content === 'string') {
+    return content
+  }
+
+  if (Array.isArray(content)) {
+    return content
+  }
+
+  return getTextContent(content)
+}
+
 /**
  * Convert UIMessages or ModelMessages to ModelMessages
  */
@@ -248,7 +350,7 @@ function buildAssistantMessages(uiMessage: UIMessage): Array<ModelMessage> {
     if (part.output !== undefined && !emittedToolResultIds.has(part.id)) {
       messageList.push({
         role: 'tool',
-        content: JSON.stringify(part.output),
+        content: normalizeToolResultContent(part.output),
         toolCallId: part.id,
       })
       emittedToolResultIds.add(part.id)
@@ -312,7 +414,7 @@ export function modelMessageToUIMessage(
     parts.push({
       type: 'tool-result',
       toolCallId: modelMessage.toolCallId,
-      content: getTextContent(modelMessage.content),
+      content: getToolResultContent(modelMessage.content),
       state: 'complete',
     })
   } else if (Array.isArray(modelMessage.content)) {
@@ -375,7 +477,7 @@ export function modelMessagesToUIMessages(
         currentAssistantMessage.parts.push({
           type: 'tool-result',
           toolCallId: msg.toolCallId!,
-          content: getTextContent(msg.content),
+          content: getToolResultContent(msg.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 80b94d59a..e16bfbe6c 100644
--- a/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
+++ b/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
@@ -8,6 +8,7 @@
 import type {
   ThinkingPart,
   ToolCallPart,
+  ToolResultContent,
   ToolResultPart,
   UIMessage,
 } from '../../../types'
@@ -97,7 +98,7 @@ export function updateToolResultPart(
   messages: Array<UIMessage>,
   messageId: string,
   toolCallId: string,
-  content: string,
+  content: ToolResultContent,
   state: ToolResultState,
   error?: string,
 ): Array<UIMessage> {
diff --git a/packages/typescript/ai/src/activities/chat/stream/processor.ts b/packages/typescript/ai/src/activities/chat/stream/processor.ts
index caed6a74d..6166a8850 100644
--- a/packages/typescript/ai/src/activities/chat/stream/processor.ts
+++ b/packages/typescript/ai/src/activities/chat/stream/processor.ts
@@ -17,7 +17,13 @@
  * @see docs/chat-architecture.md — Canonical reference for AG-UI chunk ordering,
  *   adapter contract, single-shot flows, and expected UIMessage output.
  */
-import { generateMessageId, uiMessageToModelMessages } from '../messages.js'
+import {
+  decodeToolResultContent,
+  generateMessageId,
+  normalizeToolResultContent,
+  parseToolResultValue,
+  uiMessageToModelMessages,
+} from '../messages.js'
 import { defaultJSONParser } from './json-parser'
 import {
   updateTextPart,
@@ -291,7 +297,7 @@ export class StreamProcessor {
     )
 
     // Step 2: Create a tool-result part (for LLM conversation history)
-    const content = typeof output === 'string' ? output : JSON.stringify(output)
+    const content = normalizeToolResultContent(output)
     const toolResultState: ToolResultState = error ? 'error' : 'complete'
 
     updatedMessages = updateToolResultPart(
@@ -984,15 +990,12 @@ export class StreamProcessor {
     }
 
     // Update UIMessage if there's a result
-    if (chunk.result) {
+    if (chunk.result !== undefined) {
       // Step 1: Update the tool-call part's output field (for UI consistency
       // with client tools — see GitHub issue #176)
-      let output: unknown
-      try {
-        output = JSON.parse(chunk.result)
-      } catch {
-        output = chunk.result
-      }
+      const output = parseToolResultValue(chunk.result)
+      const content = decodeToolResultContent(chunk.result)
+
       this.messages = updateToolCallWithOutput(
         this.messages,
         chunk.toolCallId,
@@ -1005,7 +1008,7 @@ export class StreamProcessor {
         this.messages,
         messageId,
         chunk.toolCallId,
-        chunk.result,
+        content,
         resultState,
       )
       this.emitMessagesChange()
diff --git a/packages/typescript/ai/src/activities/chat/tools/tool-calls.ts b/packages/typescript/ai/src/activities/chat/tools/tool-calls.ts
index c05b759cf..57cdd73f5 100644
--- a/packages/typescript/ai/src/activities/chat/tools/tool-calls.ts
+++ b/packages/typescript/ai/src/activities/chat/tools/tool-calls.ts
@@ -1,3 +1,4 @@
+import { normalizeToolResultContent } from '../messages'
 import { isStandardSchema, parseWithStandardSchema } from './schema-converter'
 import type {
   CustomEvent,
@@ -9,6 +10,7 @@ import type {
   ToolCallEndEvent,
   ToolCallStartEvent,
   ToolExecutionContext,
+  ToolResultContent,
 } from '../../../types'
 import type {
   AfterToolCallInfo,
@@ -164,7 +166,7 @@ export class ToolCallManager {
     for (const toolCall of toolCallsArray) {
       const tool = this.tools.find((t) => t.name === toolCall.function.name)
 
-      let toolResultContent: string
+      let toolResultContent: ToolResultContent
       if (tool?.execute) {
         try {
           // Parse arguments (normalize "null" to "{}" for empty tool_use blocks)
@@ -216,8 +218,7 @@ export class ToolCallManager {
             }
           }
 
-          toolResultContent =
-            typeof result === 'string' ? result : JSON.stringify(result)
+          toolResultContent = normalizeToolResultContent(result)
         } catch (error: unknown) {
           // If tool execution fails, add error message
           const message =
diff --git a/packages/typescript/ai/src/types.ts b/packages/typescript/ai/src/types.ts
index 984e15125..d83ed59dd 100644
--- a/packages/typescript/ai/src/types.ts
+++ b/packages/typescript/ai/src/types.ts
@@ -259,6 +259,8 @@ export type ConstrainedContent<
   | null
   | Array<ContentPartForInputModalitiesTypes<TInputModalitiesTypes>>
 
+export type ToolResultContent = string | Array<ContentPart>
+
 export interface ModelMessage<
   TContent extends string | null | Array<ContentPart> =
     | string
@@ -300,7 +302,7 @@ export interface ToolCallPart {
 export interface ToolResultPart {
   type: 'tool-result'
   toolCallId: string
-  content: string
+  content: ToolResultContent
   state: ToolResultState
   error?: string // Error message if state is "error"
 }
@@ -880,7 +882,7 @@ export interface ToolCallEndEvent extends BaseAGUIEvent {
   /** Final parsed input arguments */
   input?: unknown
   /** Tool execution result (if executed) */
-  result?: string
+  result?: ToolResultContent
 }
 
 /**
diff --git a/packages/typescript/ai/tests/message-converters.test.ts b/packages/typescript/ai/tests/message-converters.test.ts
index 76f55270a..d7fe6275d 100644
--- a/packages/typescript/ai/tests/message-converters.test.ts
+++ b/packages/typescript/ai/tests/message-converters.test.ts
@@ -1066,6 +1066,48 @@ describe('Message Converters', () => {
         { type: 'text', content: 'The temperature is 72F.' },
       ])
     })
+
+    it('should preserve multimodal tool result content', () => {
+      const imageResult: Array<ContentPart> = [
+        {
+          type: 'text',
+          content: 'Screenshot of the current state',
+        },
+        {
+          type: 'image',
+          source: {
+            type: 'url',
+            value: 'https://example.com/screenshot.png',
+          },
+        },
+      ]
+
+      const modelMessages: Array<ModelMessage> = [
+        { role: 'user', content: 'Check the screenshot' },
+        {
+          role: 'assistant',
+          content: 'Inspecting',
+          toolCalls: [
+            {
+              id: 'tc-1',
+              type: 'function',
+              function: { name: 'view_canvas', arguments: '{}' },
+            },
+          ],
+        },
+        { role: 'tool', content: imageResult, toolCallId: 'tc-1' },
+      ]
+
+      const result = modelMessagesToUIMessages(modelMessages)
+      const assistantParts = result[1]?.parts || []
+
+      expect(assistantParts).toContainEqual({
+        type: 'tool-result',
+        toolCallId: 'tc-1',
+        content: imageResult,
+        state: 'complete',
+      })
+    })
   })
 
   describe('convertMessagesToModelMessages', () => {