[BUG] VSCode: Fork conversation while model is generating spawns with resume: undefined, immediately exits

Description

Forking a conversation while the model is actively generating fails silently. The fork request succeeds (file history is hard-linked), but the new Claude process spawns with resume: undefined and exits immediately (~200ms after start).

Environment

• OS: macOS (Darwin 25.1.0, Apple Silicon)
• VSCode Extension: v2.1.71
• Auth: OAuth (Anthropic)

Steps to Reproduce

1. Start a conversation in the Claude Code VSCode extension
2. Send a message and wait for the model to begin generating a response
3. While the response is still streaming, click "Fork conversation from here" on a previous message
4. A new tab opens but the forked session immediately dies

Expected Behavior

The forked conversation should open in a new tab with the conversation history up to the fork point, ready for a new prompt.

Actual Behavior

The new tab opens briefly, then the Claude process exits. The error Failed to fetch Claude settings: Error: Query closed before response received appears in the extension logs.

Log Analysis

The fork request itself succeeds — file history hard-links are created:

Received message from webview: {"type":"request",...,"request":{"type":"fork_conversation","forkedFromSession":"67c9473d-...","resumeSessionAt":"8a66cc2c-..."}}
FileHistory: Hard linked backup 16a17a605bc47e89@v2 from 67c9473d-... to b9850d99-...
FileHistory: Hard linked backup 43954cdbd0d12487@v4 from 67c9473d-... to b9850d99-...
[... more hard links succeed ...]

But the new process spawns without the session ID:

Spawning Claude with SDK query function - cwd: /Users/.../fyp, permission mode: default, version: 2.1.71, ..., resume: undefined

The process then immediately shuts down:

[DEBUG] Getting matching hook commands for SessionStart with query: startup
[DEBUG] Found 0 hook matchers in settings
[... ~200ms later ...]
[DEBUG] Getting matching hook commands for SessionEnd with query: other
[DEBUG] LSP server manager shut down successfully

Followed by:

[error] Failed to fetch Claude settings: Error: Query closed before response received

Root Cause Hypothesis

Race condition: when forking mid-generation, the extension tears down the active streaming session to create the fork, but the new session's spawn doesn't receive the sessionId/resume parameter (it's undefined). The forked process starts as a blank session and immediately exits.

Related Issues

• #22016 — Fork doesn't work (Windows, ENOENT on .claude/projects/ dir — different root cause)
• #23692 — Forked sessions not in /resume list, unexpected termination (index not updated — different root cause)

Workaround

Wait for the model to finish generating before forking.