diff --git a/docs/migration.md b/docs/migration.md
index 8b70885e8..f281f856a 100644
--- a/docs/migration.md
+++ b/docs/migration.md
@@ -1136,3 +1136,12 @@ If you encounter issues during migration:
 1. Check the [API Reference](api/mcp/index.md) for updated method signatures
 2. Review the [examples](https://github.com/modelcontextprotocol/python-sdk/tree/main/examples) for updated usage patterns
 3. Open an issue on [GitHub](https://github.com/modelcontextprotocol/python-sdk/issues) if you find a bug or need further assistance
+
+## Tool result: marking non-text results as errors
+Tools may now return a 3-tuple: (unstructured_content, structured_content, is_error).
+If the third element is True, the resulting CallToolResult sent to clients will have
+is_error=True. This allows returning non-text content (images, audio) while still
+indicating the tool execution failed or produced an error state.
+
+Alternatively, tools may return a full `CallToolResult` instance directly to control
+is_error and other fields explicitly.
diff --git a/src/mcp/server/mcpserver/server.py b/src/mcp/server/mcpserver/server.py
index b3471163b..6758a0e75 100644
--- a/src/mcp/server/mcpserver/server.py
+++ b/src/mcp/server/mcpserver/server.py
@@ -316,12 +316,24 @@ async def _handle_call_tool(
             return CallToolResult(content=[TextContent(type="text", text=str(e))], is_error=True)
         if isinstance(result, CallToolResult):
             return result
-        if isinstance(result, tuple) and len(result) == 2:
-            unstructured_content, structured_content = result
+        if isinstance(result, tuple):
+            # Support either (unstructured_content, structured_content) or
+            # (unstructured_content, structured_content, is_error). The third element,
+            # if present, controls the CallToolResult.is_error flag.
+            if len(result) == 2:
+                unstructured_content, structured_content = result
+                is_error = False
+            elif len(result) == 3:
+                unstructured_content, structured_content, is_error = result
+            else:
+                # Fallback: treat as a sequence of content blocks
+                return CallToolResult(content=list(result))
             return CallToolResult(
                 content=list(unstructured_content),  # type: ignore[arg-type]
                 structured_content=structured_content,  # type: ignore[arg-type]
+                is_error=bool(is_error),
             )
+
         if isinstance(result, dict):  # pragma: no cover
             # TODO: this code path is unreachable — convert_result never returns a raw dict.
             # The call_tool return type (Sequence[ContentBlock] | dict[str, Any]) is wrong