diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index d326976f1ae..e4cd17a6890 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -527,7 +527,7 @@ export namespace MessageV2 {
                 toolCallId: part.callID,
                 input: part.state.input,
                 output: part.state.time.compacted ? "[Old tool result content cleared]" : part.state.output,
-                callProviderMetadata: part.metadata,
+                callProviderMetadata: part.metadata ?? {},
               })
             }
             if (part.state.status === "error")
diff --git a/packages/opencode/test/session/issue-8862.test.ts b/packages/opencode/test/session/issue-8862.test.ts
new file mode 100644
index 00000000000..aa19d77e07d
--- /dev/null
+++ b/packages/opencode/test/session/issue-8862.test.ts
@@ -0,0 +1,96 @@
+import { describe, expect, test } from "bun:test"
+import { MessageV2 } from "../../src/session/message-v2"
+
+const sessionID = "session"
+
+function assistantInfo(id: string, parentID: string): MessageV2.Assistant {
+  return {
+    id,
+    sessionID,
+    role: "assistant",
+    time: { created: 0 },
+    parentID,
+    modelID: "model",
+    providerID: "provider",
+    mode: "",
+    agent: "agent",
+    path: { cwd: "/", root: "/" },
+    cost: 0,
+    tokens: {
+      input: 0,
+      output: 0,
+      reasoning: 0,
+      cache: { read: 0, write: 0 },
+    },
+  } as unknown as MessageV2.Assistant
+}
+
+function basePart(messageID: string, id: string) {
+  return {
+    id,
+    sessionID,
+    messageID,
+  }
+}
+
+describe("issue-8862 regression test", () => {
+  test("successfully converts tool results with undefined metadata/attachments", () => {
+    const userID = "m-user"
+    const assistantID = "m-assistant"
+
+    const input: MessageV2.WithParts[] = [
+      {
+        info: assistantInfo(assistantID, userID),
+        parts: [
+          {
+            ...basePart(assistantID, "a2"),
+            type: "tool",
+            callID: "call-1",
+            tool: "bash",
+            state: {
+              status: "completed",
+              input: { cmd: "ls" },
+              output: "ok",
+              title: "Bash",
+              metadata: undefined as any,
+              time: { start: 0, end: 1 },
+              attachments: undefined as any,
+            } as any,
+            metadata: undefined as any,
+          },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    expect(MessageV2.toModelMessage(input)).toStrictEqual([
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "tool-call",
+            toolCallId: "call-1",
+            toolName: "bash",
+            input: { cmd: "ls" },
+            providerExecuted: undefined,
+            providerOptions: {},
+          },
+        ],
+      },
+      {
+        role: "tool",
+        content: [
+          {
+            type: "tool-result",
+            toolCallId: "call-1",
+            toolName: "bash",
+            output: {
+              type: "text",
+              value: "ok",
+            },
+            providerOptions: {},
+          },
+        ],
+      },
+    ])
+  })
+})