fix(memory): use empty content: [] with structuredContent to avoid double-serialization (#2689)

src/memory/__tests__/no-dual-content-structured.test.ts

@@ -0,0 +1,95 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+
+describe('memory server tool returns (regression for #2689)', () => {
+  it('every tool return with structuredContent only has content: [] (no JSON text)', () => {
+    // Issue #2689: memory server tools returned both `content` (text array)
+    // and `structuredContent` (object) simultaneously. The MCP SDK serializes
+    // both into the response payload, producing invalid JSON like
+    // `{"entities":[]}<text JSON>` that some clients (e.g. Gemini CLI)
+    // reject with -32603: "Unexpected non-whitespace character after JSON".
+    //
+    // The fix: when `structuredContent` is set, `content` must be an
+    // EMPTY array (per MCP spec, when a tool has outputSchema, the
+    // structuredContent is the canonical output; text content is optional).
+    //
+    // An empty array is sufficient for the SDK's TypeScript type check
+    // AND avoids the double-serialization.
+
+    const serverPath = path.resolve(__dirname, '..', 'index.ts');
+    const content = fs.readFileSync(serverPath, 'utf-8');
+
+    // Find every return block with structuredContent.
+    const returnBlockRegex = /return\s*\{([\s\S]*?)\};/g;
+    let match: RegExpExecArray | null;
+    let inspected = 0;
+    const violations: string[] = [];
+
+    while ((match = returnBlockRegex.exec(content)) !== null) {
+      const block = match[1];
+      if (!/structuredContent/.test(block)) continue;
+      inspected += 1;
+
+      // Find the content: line in this block.
+      const contentMatch = /^\s*content:\s*(\[[^\]]*\])/m.exec(block);
+      if (!contentMatch) continue;
+
+      const contentArray = contentMatch[1];
+      // The content array must be empty (no text representation of the
+      // structured data — that would double-serialize).
+      if (contentArray !== '[]') {
+        const lineNum = content.slice(0, match.index).split('\n').length;
+        violations.push(`line ${lineNum}: content=${contentArray}`);
+      }
+    }
+
+    if (violations.length > 0) {
+      throw new Error(
+        `${violations.length} tool return block(s) have non-empty ` +
+          `\`content\` alongside \`structuredContent\` (MCP spec violation, ` +
+          `see #2689):\n  ` +
+          violations.join('\n  ')
+      );
+    }
+
+    // Sanity: we did inspect at least 6 blocks (the 6 tools that had
+    // duplicated content in the original code).
+    expect(inspected).toBeGreaterThanOrEqual(6);
+  });
+
+  it('string messages do not leak into the content array', () => {
+    // The hardcoded success messages ("Entities deleted successfully"
+    // etc.) used to be duplicated in `content: [{ type: "text", text: msg }]`
+    // AND `structuredContent: { message: msg }`. After the fix, the
+    // success message lives only in `structuredContent`.
+    const serverPath = path.resolve(__dirname, '..', 'index.ts');
+    const content = fs.readFileSync(serverPath, 'utf-8');
+
+    const successMessages = [
+      'Entities deleted successfully',
+      'Observations deleted successfully',
+      'Relations deleted successfully',
+    ];
+
+    for (const msg of successMessages) {
+      // The message must appear in structuredContent.
+      const objectContext = new RegExp(
+        `structuredContent\\s*:\\s*\\{[^}]*${msg.replace(/[\[\]]/g, '\\$&')}[^}]*\\}`,
+      );
+      expect(
+        content.match(objectContext),
+        `${msg} should be in structuredContent.object.message`,
+      ).toBeTruthy();
+
+      // The message must NOT appear in any content array entry.
+      const contentContext = new RegExp(
+        `content:\\s*\\[[^\\]]*${msg.replace(/[\[\]]/g, '\\$&')}[^\\]]*\\]`,
+      );
+      expect(
+        content.match(contentContext),
+        `${msg} should NOT be in content (would double-serialize)`,
+      ).toBeNull();
+    }
+  });
+});

src/memory/index.ts

@@ -296,7 +296,7 @@ server.registerTool(
     const result = await knowledgeGraphManager.createEntities(entities);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(result, null, 2) }],
+      content: [],
       structuredContent: { entities: result }
     };
   }
@@ -325,7 +325,7 @@ server.registerTool(
     const result = await knowledgeGraphManager.createRelations(relations);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(result, null, 2) }],
+      content: [],
       structuredContent: { relations: result }
     };
   }
@@ -360,7 +360,7 @@ server.registerTool(
     const result = await knowledgeGraphManager.addObservations(observations);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(result, null, 2) }],
+      content: [],
       structuredContent: { results: result }
     };
   }
@@ -390,7 +390,7 @@ server.registerTool(
     await knowledgeGraphManager.deleteEntities(entityNames);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: "Entities deleted successfully" }],
+      content: [],
       structuredContent: { success: true, message: "Entities deleted successfully" }
     };
   }
@@ -423,7 +423,7 @@ server.registerTool(
     await knowledgeGraphManager.deleteObservations(deletions);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: "Observations deleted successfully" }],
+      content: [],
       structuredContent: { success: true, message: "Observations deleted successfully" }
     };
   }
@@ -453,7 +453,7 @@ server.registerTool(
     await knowledgeGraphManager.deleteRelations(relations);
     notifyGraphUpdated();
     return {
-      content: [{ type: "text" as const, text: "Relations deleted successfully" }],
+      content: [],
       structuredContent: { success: true, message: "Relations deleted successfully" }
     };
   }
@@ -480,7 +480,7 @@ server.registerTool(
   async () => {
     const graph = await knowledgeGraphManager.readGraph();
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(graph, null, 2) }],
+      content: [],
       structuredContent: { ...graph }
     };
   }
@@ -509,7 +509,7 @@ server.registerTool(
   async ({ query }) => {
     const graph = await knowledgeGraphManager.searchNodes(query);
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(graph, null, 2) }],
+      content: [],
       structuredContent: { ...graph }
     };
   }
@@ -538,7 +538,7 @@ server.registerTool(
   async ({ names }) => {
     const graph = await knowledgeGraphManager.openNodes(names);
     return {
-      content: [{ type: "text" as const, text: JSON.stringify(graph, null, 2) }],
+      content: [],
       structuredContent: { ...graph }
     };
   }