diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index d0f2beb74..83ca72add 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -435,6 +435,40 @@ export namespace MessageV2 {
 
   export function toModelMessages(input: WithParts[], model: Provider.Model): ModelMessage[] {
     const result: UIMessage[] = []
+    const toolNames = new Set<string>()
+
+    const toModelOutput = (output: unknown) => {
+      if (typeof output === "string") {
+        return { type: "text", value: output }
+      }
+
+      if (typeof output === "object") {
+        const outputObject = output as {
+          text: string
+          attachments?: Array<{ mime: string; url: string }>
+        }
+        const attachments = (outputObject.attachments ?? []).filter((attachment) => {
+          return attachment.url.startsWith("data:") && attachment.url.includes(",")
+        })
+
+        return {
+          type: "content",
+          value: [
+            { type: "text", text: outputObject.text },
+            ...attachments.map((attachment) => ({
+              type: "media",
+              mediaType: attachment.mime,
+              data: iife(() => {
+                const commaIndex = attachment.url.indexOf(",")
+                return commaIndex === -1 ? attachment.url : attachment.url.slice(commaIndex + 1)
+              }),
+            })),
+          ],
+        }
+      }
+
+      return { type: "json", value: output as never }
+    }
 
     for (const msg of input) {
       if (msg.parts.length === 0) continue
@@ -505,31 +539,24 @@ export namespace MessageV2 {
               type: "step-start",
             })
           if (part.type === "tool") {
+            toolNames.add(part.tool)
             if (part.state.status === "completed") {
-              if (part.state.attachments?.length) {
-                result.push({
-                  id: Identifier.ascending("message"),
-                  role: "user",
-                  parts: [
-                    {
-                      type: "text",
-                      text: `The tool ${part.tool} returned the following attachments:`,
-                    },
-                    ...part.state.attachments.map((attachment) => ({
-                      type: "file" as const,
-                      url: attachment.url,
-                      mediaType: attachment.mime,
-                      filename: attachment.filename,
-                    })),
-                  ],
-                })
-              }
+              const outputText = part.state.time.compacted ? "[Old tool result content cleared]" : part.state.output
+              const attachments = part.state.time.compacted ? [] : (part.state.attachments ?? [])
+              const output =
+                attachments.length > 0
+                  ? {
+                      text: outputText,
+                      attachments,
+                    }
+                  : outputText
+
               assistantMessage.parts.push({
                 type: ("tool-" + part.tool) as `tool-${string}`,
                 state: "output-available",
                 toolCallId: part.callID,
                 input: part.state.input,
-                output: part.state.time.compacted ? "[Old tool result content cleared]" : part.state.output,
+                output,
                 ...(differentModel ? {} : { callProviderMetadata: part.metadata }),
               })
             }
@@ -568,7 +595,15 @@ export namespace MessageV2 {
       }
     }
 
-    return convertToModelMessages(result.filter((msg) => msg.parts.some((part) => part.type !== "step-start")))
+    const tools = Object.fromEntries(Array.from(toolNames).map((toolName) => [toolName, { toModelOutput }]))
+
+    return convertToModelMessages(
+      result.filter((msg) => msg.parts.some((part) => part.type !== "step-start")),
+      {
+        //@ts-expect-error (convertToModelMessages expects a ToolSet but only actually needs tools[name]?.toModelOutput)
+        tools,
+      },
+    )
   }
 
   export const stream = fn(Identifier.schema("session"), async function* (sessionID) {
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 185c97a75..de6278820 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -722,12 +722,6 @@ export namespace SessionPrompt {
           )
           return result
         },
-        toModelOutput(result) {
-          return {
-            type: "text",
-            value: result.output,
-          }
-        },
       })
     }
 
@@ -819,12 +813,6 @@ export namespace SessionPrompt {
           content: result.content, // directly return content to preserve ordering when outputting to model
         }
       }
-      item.toModelOutput = (result) => {
-        return {
-          type: "text",
-          value: result.output,
-        }
-      }
       tools[key] = item
     }
 
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index b8d056433..2f632ad1c 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -262,7 +262,7 @@ describe("session.message-v2.toModelMessage", () => {
     ])
   })
 
-  test("converts assistant tool completion into tool-call + tool-result messages and emits attachment message", () => {
+  test("converts assistant tool completion into tool-call + tool-result messages with attachments", () => {
     const userID = "m-user"
     const assistantID = "m-assistant"
 
@@ -304,7 +304,7 @@ describe("session.message-v2.toModelMessage", () => {
                   type: "file",
                   mime: "image/png",
                   filename: "attachment.png",
-                  url: "https://example.com/attachment.png",
+                  url: "data:image/png;base64,Zm9v",
                 },
               ],
             },
@@ -319,18 +319,6 @@ describe("session.message-v2.toModelMessage", () => {
         role: "user",
         content: [{ type: "text", text: "run tool" }],
       },
-      {
-        role: "user",
-        content: [
-          { type: "text", text: "The tool bash returned the following attachments:" },
-          {
-            type: "file",
-            mediaType: "image/png",
-            filename: "attachment.png",
-            data: "https://example.com/attachment.png",
-          },
-        ],
-      },
       {
         role: "assistant",
         content: [
@@ -352,7 +340,13 @@ describe("session.message-v2.toModelMessage", () => {
             type: "tool-result",
             toolCallId: "call-1",
             toolName: "bash",
-            output: { type: "text", value: "ok" },
+            output: {
+              type: "content",
+              value: [
+                { type: "text", text: "ok" },
+                { type: "media", mediaType: "image/png", data: "Zm9v" },
+              ],
+            },
             providerOptions: { openai: { tool: "meta" } },
           },
         ],