fix(memory): add trailing newline to JSONL output

[nickveenhof]

Summary

saveGraph writes lines.join("\n") without a trailing newline. The JSONL spec requires every record to be terminated by \n. This makes the output non-conformant and fragile.

The problem

saveGraph produces a file where the last line has no terminating \n:

{"type":"entity","name":"Alice","entityType":"person","observations":["test"]}
{"type":"relation","from":"Alice","to":"Bob","relationType":"knows"}  <-- no \n

A JSONL file should look like this:

{"type":"entity","name":"Alice","entityType":"person","observations":["test"]}\n
{"type":"relation","from":"Alice","to":"Bob","relationType":"knows"}\n

The missing terminator means any tool or process that appends to the file (log rotation, crash recovery, file concatenation) produces invalid JSONL:

{"type":"relation","from":"Alice","to":"Bob","relationType":"knows"}{"type":"entity","name":"External",...}

JSON.parse() then throws on that line:

Unexpected non-whitespace character after JSON at position 88 (line 1 column 89)

How I hit this

I run the memory server across multiple concurrent sessions pointing at the same memory.jsonl file. After several days I found a line with two JSON objects concatenated. Every tool call returned the parse error above until I manually edited the file.

The root cause is likely the race condition described in #2579: multiple MCP server instances calling saveGraph concurrently on the same file. #3286 addresses that with cross-process file locking and atomic writes, which is the proper fix.

This PR does not solve the race condition. It makes the output JSONL-spec-conformant, which is correct regardless of whether #3286 lands.

Fix

// Before
await fs.writeFile(this.memoryFilePath, lines.join("\n"));

// After
await fs.writeFile(this.memoryFilePath, lines.join("\n") + "\n");

loadGraph already filters empty lines (filter(line => line.trim() !== "")), so the trailing \n has no side effects on read.

Tests

Three new tests in knowledge-graph.test.ts:

1. Trailing newline check: verifies the file ends with \n.
2. Per-line validity: verifies every non-empty line parses as valid JSON individually.
3. External append simulation: writes entities via saveGraph, appends a line via fs.appendFile, then reads back via a fresh KnowledgeGraphManager instance. This test fails without the fix and passes with it.

All 48 tests pass. No existing tests affected.

Related

• Bug Report and Resolution: mcp-server-memory Failures due to Race Condition and Environment Misconfiguration #2579: the race condition that causes JSONL corruption with concurrent instances
• Added in-memory locks for the process, fix for Bug #2579 temporary measure. #3060: in-memory lock (closed, insufficient for multi-process)
• feat(memory): add file locking to support multi-instance #3286: cross-process file locking with proper-lockfile (open, the full fix)