From decc3490ab75362fd20e60f50b5def2e5a50625f Mon Sep 17 00:00:00 2001
From: Robel Estifanos <robel@estifanos.com>
Date: Fri, 30 Oct 2026 16:20:03 -0400
Subject: [PATCH] fix(ui): a tool result with an error output renders as
 output-error

---
 CHANGELOG.md                    |  2 +
 src/vercel/UIMessages.ts        | 13 +++++-
 src/vercel/toUIMessages.test.ts | 76 +++++++++++++++++++++++++++++++++
 3 files changed, 89 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 83a5f6e8d..535ff284c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,8 @@
 - Keep the whole order being continued in context, past the recent window (#433,
   #436)
 - `useStreamingText` decodes UTF-8 characters split across chunks (#440)
+- Render a tool result with an `error-text` or `error-json` output as
+  `output-error` (#162, #441)
 
 ## 0.7.9
 
diff --git a/src/vercel/UIMessages.ts b/src/vercel/UIMessages.ts
index 527885ada..78c9204a2 100644
--- a/src/vercel/UIMessages.ts
+++ b/src/vercel/UIMessages.ts
@@ -588,10 +588,19 @@ function createAssistantUIMessage<
               : typedPart.output;
           // Check for error at both the content part level (isError) and message level
           // isError may exist on stored tool results but isn't in ToolResultPart type
+          const outputType = typedPart.output?.type;
           const hasError =
-            (contentPart as { isError?: boolean }).isError || message.error;
+            (contentPart as { isError?: boolean }).isError ||
+            message.error ||
+            outputType === "error-text" ||
+            outputType === "error-json";
           const errorText =
-            message.error || (hasError ? String(output) : undefined);
+            message.error ||
+            (hasError
+              ? outputType === "error-json"
+                ? JSON.stringify(output)
+                : String(output)
+              : undefined);
           const call = allParts.find(
             (part) =>
               part.type === `tool-${contentPart.toolName}` &&
diff --git a/src/vercel/toUIMessages.test.ts b/src/vercel/toUIMessages.test.ts
index f01fa1dd0..14bc73e1e 100644
--- a/src/vercel/toUIMessages.test.ts
+++ b/src/vercel/toUIMessages.test.ts
@@ -886,6 +886,82 @@ describe("toUIMessages", () => {
     expect(toolPart.state).toBe("output-error");
   });
 
+  describe("tool result with an error output (issue #162)", () => {
+    const toolCall = baseMessageDoc({
+      _id: "msg1",
+      order: 1,
+      stepOrder: 1,
+      tool: true,
+      message: {
+        role: "assistant",
+        content: [
+          {
+            type: "tool-call",
+            toolName: "explode",
+            toolCallId: "call1",
+            input: {},
+            args: {},
+          },
+        ],
+      },
+    });
+    const toolResult = (
+      output:
+        | { type: "error-text"; value: string }
+        | { type: "error-json"; value: { code: number } },
+    ) =>
+      baseMessageDoc({
+        _id: "msg2",
+        order: 1,
+        stepOrder: 2,
+        tool: true,
+        message: {
+          role: "tool",
+          content: [
+            {
+              type: "tool-result",
+              toolCallId: "call1",
+              toolName: "explode",
+              output,
+            },
+          ],
+        },
+      });
+    const toolPart = (messages: MessageDoc[]) => {
+      const parts = toUIMessages(messages)[0].parts.filter(
+        (p) => p.type === "tool-explode",
+      );
+      expect(parts).toHaveLength(1);
+      return parts[0] as any;
+    };
+
+    it("renders error-text with its call as output-error", () => {
+      const part = toolPart([
+        toolCall,
+        toolResult({ type: "error-text", value: "Error: kaboom" }),
+      ]);
+      expect(part.state).toBe("output-error");
+      expect(part.errorText).toBe("Error: kaboom");
+    });
+
+    it("renders an orphan error-text result as output-error", () => {
+      const part = toolPart([
+        toolResult({ type: "error-text", value: "Error: kaboom" }),
+      ]);
+      expect(part.state).toBe("output-error");
+      expect(part.errorText).toBe("Error: kaboom");
+    });
+
+    it("renders error-json as output-error with the JSON as errorText", () => {
+      const part = toolPart([
+        toolCall,
+        toolResult({ type: "error-json", value: { code: 42 } }),
+      ]);
+      expect(part.state).toBe("output-error");
+      expect(part.errorText).toBe('{"code":42}');
+    });
+  });
+
   describe("userId preservation", () => {
     it("preserves userId in user messages", () => {
       const messages = [