Fix #46: Port file written to $TMPDIR (/var/folders/...) instead of /...

[JiwaniZakir]

Closes #46

Fixes port discovery on macOS by writing port advertisement files to /tmp instead of os.tmpdir(), which resolves to /var/folders/... on some macOS systems.

Changes

src/core/port-discovery.ts

• Replaced const PORT_FILE_DIR = tmpdir() with a platform conditional: process.platform === 'win32' ? tmpdir() : '/tmp'. The Figma Desktop Bridge plugin hardcodes /tmp when scanning for port files, so the server must write there regardless of what os.tmpdir() returns on the host system.

tests/port-discovery.test.ts

• Updated the existing getPortFilePath test to assert the expected directory based on platform (/tmp on non-Windows, tmpdir() on Windows).
• Added a dedicated test asserting path.startsWith('/tmp/') on non-Windows platforms, explicitly covering the macOS os.tmpdir() divergence scenario.

Motivation

On macOS, os.tmpdir() can return /var/folders/l2/<hash>/T/ rather than /tmp. The Figma Desktop Bridge plugin scans only /tmp for figma-console-mcp-*.json port files. When these two paths diverge, the server successfully advertises its port (e.g. figma-console-mcp-9223.json) but writes it to /var/folders/..., which the plugin never checks. The result is the plugin stalling on "MCP scanning..." indefinitely and all write tools failing with "Cannot connect to Figma Desktop."

Workarounds like setting TMPDIR=/tmp in the MCP env config had no effect because the server reads os.tmpdir() at module load time. Hardcoding /tmp on non-Windows systems aligns the server's write location with the plugin's scan location without affecting Windows, where C:\Users\...\AppData\Local\Temp is the correct path.

Testing

Automated: The updated getPortFilePath tests in tests/port-discovery.test.ts verify the correct directory is selected per platform. The new explicit /tmp prefix assertion will catch any regression that reintroduces os.tmpdir() on macOS/Linux.

Manual verification: On a macOS host where os.tmpdir() returns /var/folders/..., after this change the server log should show:

"filePath":"/tmp/figma-console-mcp-9223.json","msg":"Port advertised"

and the Figma Desktop Bridge plugin should transition from "MCP scanning..." to a connected state. Confirm ls /tmp/figma-console-mcp-*.json shows the file immediately after the server starts.

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This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.