mcp: improve error handling in resources and tools

- Handle failures during HTML, Markdown, and link serialization.
- Return MCP internal errors when result serialization fails.
- Refactor resource reading logic for better clarity and consistency.

src/mcp/resources.zig

@@ -28,6 +28,7 @@ pub fn handleList(server: *Server, req: protocol.Request) !void {
 const ReadParams = struct {
     uri: []const u8,
 };
+const Format = enum { html, markdown };
 
 const ResourceStreamingResult = struct {
     contents: []const struct {
@@ -38,7 +39,7 @@ const ResourceStreamingResult = struct {
 
     const StreamingText = struct {
         page: *lp.Page,
-        format: enum { html, markdown },
+        format: Format,
 
         pub fn jsonStringify(self: @This(), jw: *std.json.Stringify) !void {
             try jw.beginWriteRaw();
@@ -47,9 +48,11 @@ const ResourceStreamingResult = struct {
             switch (self.format) {
                 .html => lp.dump.root(self.page.document, .{}, &escaped.writer, self.page) catch |err| {
                     log.err(.mcp, "html dump failed", .{ .err = err });
+                    return error.WriteFailed;
                 },
                 .markdown => lp.markdown.dump(self.page.document.asNode(), .{}, &escaped.writer, self.page) catch |err| {
                     log.err(.mcp, "markdown dump failed", .{ .err = err });
+                    return error.WriteFailed;
                 },
             }
             try jw.writer.writeByte('"');
@@ -86,28 +89,25 @@ pub fn handleRead(server: *Server, arena: std.mem.Allocator, req: protocol.Reque
         return server.sendError(req_id, .PageNotLoaded, "Page not loaded");
     };
 
-    switch (uri) {
+    const format: Format = switch (uri) {
-        .@"mcp://page/html" => {
+        .@"mcp://page/html" => .html,
+        .@"mcp://page/markdown" => .markdown,
+    };
+    const mime_type: []const u8 = switch (uri) {
+        .@"mcp://page/html" => "text/html",
+        .@"mcp://page/markdown" => "text/markdown",
+    };
+
     const result: ResourceStreamingResult = .{
         .contents = &.{.{
             .uri = params.uri,
-                    .mimeType = "text/html",
+            .mimeType = mime_type,
-                    .text = .{ .page = page, .format = .html },
+            .text = .{ .page = page, .format = format },
         }},
     };
-            try server.sendResult(req_id, result);
+    server.sendResult(req_id, result) catch {
-        },
+        return server.sendError(req_id, .InternalError, "Failed to serialize resource content");
-        .@"mcp://page/markdown" => {
-            const result: ResourceStreamingResult = .{
-                .contents = &.{.{
-                    .uri = params.uri,
-                    .mimeType = "text/markdown",
-                    .text = .{ .page = page, .format = .markdown },
-                }},
     };
-            try server.sendResult(req_id, result);
-        },
-    }
 }
 
 const testing = @import("../testing.zig");

src/mcp/tools.zig

@@ -205,18 +205,19 @@ const ToolStreamingText = struct {
         switch (self.action) {
             .markdown => lp.markdown.dump(self.page.document.asNode(), .{}, w, self.page) catch |err| {
                 log.err(.mcp, "markdown dump failed", .{ .err = err });
+                return error.WriteFailed;
             },
             .links => {
-                if (lp.links.collectLinks(self.page.call_arena, self.page.document.asNode(), self.page)) |links| {
+                const links = lp.links.collectLinks(self.page.call_arena, self.page.document.asNode(), self.page) catch |err| {
+                    log.err(.mcp, "query links failed", .{ .err = err });
+                    return error.WriteFailed;
+                };
                 var first = true;
                 for (links) |href| {
                     if (!first) try w.writeByte('\n');
                     try w.writeAll(href);
                     first = false;
                 }
-                } else |err| {
-                    log.err(.mcp, "query links failed", .{ .err = err });
-                }
             },
             .semantic_tree => {
                 var root_node = self.page.document.asNode();
@@ -241,6 +242,7 @@ const ToolStreamingText = struct {
 
                 st.textStringify(w) catch |err| {
                     log.err(.mcp, "semantic tree dump failed", .{ .err = err });
+                    return error.WriteFailed;
                 };
             },
         }
@@ -331,7 +333,9 @@ fn handleMarkdown(server: *Server, arena: std.mem.Allocator, id: std.json.Value,
     const content = [_]protocol.TextContent(ToolStreamingText){.{
         .text = .{ .page = page, .action = .markdown },
     }};
-    try server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content });
+    server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content }) catch {
+        return server.sendError(id, .InternalError, "Failed to serialize markdown content");
+    };
 }
 
 fn handleLinks(server: *Server, arena: std.mem.Allocator, id: std.json.Value, arguments: ?std.json.Value) !void {
@@ -341,7 +345,9 @@ fn handleLinks(server: *Server, arena: std.mem.Allocator, id: std.json.Value, ar
     const content = [_]protocol.TextContent(ToolStreamingText){.{
         .text = .{ .page = page, .action = .links },
     }};
-    try server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content });
+    server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content }) catch {
+        return server.sendError(id, .InternalError, "Failed to serialize links content");
+    };
 }
 
 fn handleSemanticTree(server: *Server, arena: std.mem.Allocator, id: std.json.Value, arguments: ?std.json.Value) !void {
@@ -363,7 +369,9 @@ fn handleSemanticTree(server: *Server, arena: std.mem.Allocator, id: std.json.Va
             .maxDepth = args.maxDepth,
         },
     }};
-    try server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content });
+    server.sendResult(id, protocol.CallToolResult(ToolStreamingText){ .content = &content }) catch {
+        return server.sendError(id, .InternalError, "Failed to serialize semantic tree content");
+    };
 }
 
 fn handleInteractiveElements(server: *Server, arena: std.mem.Allocator, id: std.json.Value, arguments: ?std.json.Value) !void {