Preserve isError flag for tool error reporting in UIMessages

src/UIMessages.ts

@@ -463,16 +463,22 @@ function createAssistantUIMessage<
             typeof contentPart.output?.type === "string"
               ? contentPart.output.value
               : contentPart.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 hasError =
+            (contentPart as { isError?: boolean }).isError || message.error;
+          const errorText =
+            message.error || (hasError ? String(output) : undefined);
           const call = allParts.find(
             (part) =>
               part.type === `tool-${contentPart.toolName}` &&
               "toolCallId" in part &&
               part.toolCallId === contentPart.toolCallId,
           ) as ToolUIPart | undefined;
           if (call) {
-            if (message.error) {
+            if (hasError) {
               call.state = "output-error";
-              call.errorText = message.error;
+              call.errorText = errorText ?? "Unknown error";
               call.output = output;
             } else {
               call.state = "output-available";
@@ -483,13 +489,13 @@ function createAssistantUIMessage<
               "Tool result without preceding tool call.. adding anyways",
               contentPart,
             );
-            if (message.error) {
+            if (hasError) {
               allParts.push({
                 type: `tool-${contentPart.toolName}`,
                 toolCallId: contentPart.toolCallId,
                 state: "output-error",
                 input: undefined,
-                errorText: message.error,
+                errorText: errorText ?? "Unknown error",
                 callProviderMetadata: message.providerMetadata,
               } satisfies ToolUIPart<TOOLS>);
             } else {

src/mapping.ts

@@ -549,6 +549,8 @@ function normalizeToolResult(
       normalizeToolOutput("result" in part ? part.result : undefined),
     toolCallId: part.toolCallId,
     toolName: part.toolName,
+    // Preserve isError flag for error reporting
+    ...("isError" in part && part.isError ? { isError: true } : {}),
     ...metadata,
   } satisfies ToolResultPart;
 }

src/toUIMessages.test.ts

@@ -728,4 +728,106 @@ describe("toUIMessages", () => {
     expect(textParts).toHaveLength(1);
     expect(textParts[0].text).toBe("The result is 42.");
   });
+
+  it("shows output-error state when tool result has isError: true (issue #162)", () => {
+    const messages = [
+      // Tool call
+      baseMessageDoc({
+        _id: "msg1",
+        order: 1,
+        stepOrder: 1,
+        tool: true,
+        message: {
+          role: "assistant",
+          content: [
+            {
+              type: "tool-call",
+              toolName: "generateImage",
+              toolCallId: "call1",
+              args: { id: "invalid-id" },
+            },
+          ],
+        },
+      }),
+      // Tool result with error
+      baseMessageDoc({
+        _id: "msg2",
+        order: 1,
+        stepOrder: 2,
+        tool: true,
+        message: {
+          role: "tool",
+          content: [
+            {
+              type: "tool-result",
+              toolCallId: "call1",
+              toolName: "generateImage",
+              output: {
+                type: "text",
+                value:
+                  'ArgumentValidationError: Value does not match validator.\nPath: .id\nValue: "invalid-id"\nValidator: v.id("images")',
+              },
+              isError: true,
+            },
+          ],
+        },
+      }),
+    ];
+
+    const uiMessages = toUIMessages(messages);
+
+    expect(uiMessages).toHaveLength(1);
+    expect(uiMessages[0].role).toBe("assistant");
+
+    const toolParts = uiMessages[0].parts.filter(
+      (p) => p.type === "tool-generateImage",
+    );
+    expect(toolParts).toHaveLength(1);
+
+    const toolPart = toolParts[0] as any;
+    expect(toolPart.toolCallId).toBe("call1");
+    // Should show output-error, not output-available
+    expect(toolPart.state).toBe("output-error");
+    expect(toolPart.output).toContain("ArgumentValidationError");
+  });
+
+  it("shows output-error when tool result has isError: true without tool call present (issue #162)", () => {
+    // This simulates the case where the tool-call message wasn't saved
+    const messages = [
+      baseMessageDoc({
+        _id: "msg1",
+        order: 1,
+        stepOrder: 1,
+        tool: true,
+        message: {
+          role: "tool",
+          content: [
+            {
+              type: "tool-result",
+              toolCallId: "call1",
+              toolName: "generateImage",
+              output: {
+                type: "text",
+                value: "Error: Something went wrong",
+              },
+              isError: true,
+            },
+          ],
+        },
+      }),
+    ];
+
+    const uiMessages = toUIMessages(messages);
+
+    expect(uiMessages).toHaveLength(1);
+    expect(uiMessages[0].role).toBe("assistant");
+
+    const toolParts = uiMessages[0].parts.filter(
+      (p) => p.type === "tool-generateImage",
+    );
+    expect(toolParts).toHaveLength(1);
+
+    const toolPart = toolParts[0] as any;
+    expect(toolPart.state).toBe("output-error");
+  });
 });