Problem

WorkflowDesignerPanel._loadWorkflow accepted the result of an unchecked JSON.parse and wrote it straight into this._workflow:

if (uris && uris[0]) {
    const content = await vscode.workspace.fs.readFile(uris[0]);
    this._workflow = JSON.parse(content.toString());
    this._panel.webview.postMessage({ type: 'workflowLoaded', workflow: this._workflow });
}

Two failure modes silently passed through:

1. Invalid JSON — JSON.parse throws synchronously and the exception escapes _loadWorkflow into the unhandled-promise path. The webview is not told the load failed and the panel is left in a half-state.
2. Wrong shape — any well-formed JSON object successfully replaced this._workflow with arbitrary shape (a truncated file, a hand-edited file with nodes removed, a generic settings JSON the user picked by mistake). The next save / simulate / postMessage call dereferences .nodes.find(...), .edges.forEach(...), or .policies.includes(...) on whatever was parsed and surfaces an unhelpful TypeError from _simulate.

The Workflow / WorkflowNode / WorkflowEdge interfaces are declared in the same file, so the validator can mirror them directly without pulling in a schema-validation dependency.

Fix

• Catch JSON.parse failures and surface them via vscode.window.showErrorMessage with the parse error, then return without touching this._workflow.
• Add private static type guards _isWorkflow, _isWorkflowNode, _isWorkflowEdge that check each field declared on the local interfaces (id / name / description / nodes[] / edges[] / policies[] for the workflow itself; id / type / label / position.x / position.y / config for nodes; id / source / target for edges). Optional fields (policy on nodes, label on edges) are only type-checked when present.
• On a schema mismatch, surface a separate showErrorMessage describing the expected shape and return without mutating state. Only the success path replaces this._workflow and posts the workflowLoaded message to the webview.

The validator is intentionally shallow on node config (the existing Workflow interface declares it as Record<string, any>), which preserves backwards compatibility with workflow files containing tool-specific node configurations.

Test

The agent-os-vscode package's test script wraps the VS Code integration runner (node ./out/test/runTest.js) and the _loadWorkflow flow goes through vscode.window.showOpenDialog plus a webview postMessage; the existing test bed in src/test/ does not cover the workflow designer. The change is shape-only on the success path — a well-formed Workflow file still loads identically — and the new failure paths surface as showErrorMessage rather than unhandled promise rejections.

Surfaced during independent audit conducted by @finnoybu (Ken Tannenbaum, AEGIS Initiative); [LOW, TypeScript].