fix: passthrough tool call when catalog fails to load

When a package's catalog status is 'error' (e.g., the upstream server
doesn't respond to tools/list), attempt to forward the callTool request
directly instead of hard-failing with PACKAGE_UNAVAILABLE. This handles
MCP servers where tools/list is broken but tools/call works (e.g., Figma
Desktop MCP). Schema validation is skipped in this path — the upstream
server validates arguments itself.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: harryblam

src/handlers/useTool.ts

@@ -101,13 +101,64 @@ export async function handleUseTool(
       data: { package_id, status: packageStatus },
     };
   }
-  if (packageStatus === "error") {
+  // When the catalog failed to load tools (e.g., server doesn't respond to tools/list),
+  // attempt a passthrough call rather than hard-failing. Some MCP servers support
+  // tools/call without supporting tools/list (e.g., Figma Desktop MCP).
+  // Skip schema validation since we don't have the schema.
+  const catalogDegraded = packageStatus === "error";
+  if (catalogDegraded) {
     const reason = catalog.getPackageError(package_id) || "See logs for details";
-    throw {
-      code: ERROR_CODES.PACKAGE_UNAVAILABLE,
-      message: `Package '${package_id}' is unavailable: ${reason}`,
-      data: { package_id, status: packageStatus },
-    };
+    logger.warn("Catalog unavailable, attempting passthrough tool call", {
+      package_id,
+      tool_id,
+      catalog_error: reason,
+    });
+
+    if (dry_run) {
+      throw {
+        code: ERROR_CODES.PACKAGE_UNAVAILABLE,
+        message: `Package '${package_id}' catalog is unavailable (${reason}). Cannot dry-run without schema.`,
+        data: { package_id, status: packageStatus },
+      };
+    }
+
+    const startTime = Date.now();
+    try {
+      const client = await registry.getClient(package_id);
+      const toolResult = await client.callTool(tool_id, args);
+      registry.notifyActivity(package_id);
+      const duration = Date.now() - startTime;
+
+      logger.info("Passthrough tool call succeeded despite catalog error", {
+        package_id,
+        tool_id,
+        duration_ms: duration,
+      });
+
+      const result: UseToolOutput = {
+        package_id,
+        tool_id,
+        args_used: args,
+        result: toolResult,
+        telemetry: { duration_ms: duration, status: "ok", passthrough: true },
+      };
+
+      let outputJson = JSON.stringify(result, null, 2);
+      if (max_output_chars && outputJson.length > max_output_chars) {
+        outputJson = outputJson.slice(0, max_output_chars) +
+          `\n\n[OUTPUT TRUNCATED: ${max_output_chars.toLocaleString()} of ${outputJson.length.toLocaleString()} chars]`;
+      }
+      return { content: [{ type: "text", text: outputJson }], isError: false };
+    } catch (error) {
+      registry.notifyActivity(package_id);
+      const duration = Date.now() - startTime;
+      const errorMessage = error instanceof Error ? error.message : String(error);
+      throw {
+        code: ERROR_CODES.PACKAGE_UNAVAILABLE,
+        message: `Package '${package_id}' is unavailable: catalog error (${reason}), passthrough also failed (${errorMessage})`,
+        data: { package_id, tool_id, status: packageStatus, duration_ms: duration },
+      };
+    }
   }
 
   const schema = await catalog.getToolSchema(package_id, tool_id);

src/types.ts

@@ -173,6 +173,7 @@ export interface UseToolOutput {
     output_chars?: number;
     output_truncated?: boolean;
     original_output_chars?: number;
+    passthrough?: boolean;
   };
 }