Fix tools not reporting proper errors (output-error state)

Two fixes:

1. normalizeToolResult in mapping.ts was stripping the isError flag.
   Now preserves isError when transforming tool result parts.

2. createAssistantUIMessage in UIMessages.ts now checks contentPart.isError
   in addition to message.error when determining tool state.

This ensures tool results with isError: true show "output-error" state
instead of incorrectly showing "output-available".

Fixes #162

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sethconvex

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-${contentPart.toolName}` &&
               "toolCallId" in part &&
               part.toolCallId === contentPart.toolCallId,
           ) as ToolUIPart | undefined;
           if (call) {
-            if (message.error) {
+            if (hasError) {
               call.state = "output-error";
-              call.errorText = message.error;
+              call.errorText = errorText ?? "Unknown error";
               call.output = output;
             } else {
               call.state = "output-available";
@@ -483,15 +489,15 @@ function createAssistantUIMessage<
               "Tool result without preceding tool call.. adding anyways",
               contentPart,
             );
-            if (message.error) {
+            if (hasError) {
               allParts.push({
                 type: `tool-${contentPart.toolName}`,
                 toolCallId: contentPart.toolCallId,
                 state: "output-error",
                 input: undefined,
-                errorText: message.error,
+                errorText: errorText ?? "Unknown error",
                 callProviderMetadata: message.providerMetadata,
-              } satisfies ToolUIPart<TOOLS>);
+              } as ToolUIPart<TOOLS>);
             } else {
               allParts.push({
                 type: `tool-${contentPart.toolName}`,
@@ -500,7 +506,7 @@ function createAssistantUIMessage<
                 input: undefined,
                 output,
                 callProviderMetadata: message.providerMetadata,
-              } satisfies ToolUIPart<TOOLS>);
+              } as ToolUIPart<TOOLS>);
             }
           }
           break;

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");
+  });
 });