FCP 11.0 crashes frequently when MCP server triggers generate_captions (silent termination)

[lovexiaoyan]

Bug Description

Final Cut Pro 11.0 (build 430149) silently terminates (no crash dialog, no .crash file in DiagnosticReports) when the MCP server calls generate_captions() after set_caption_words() with ~110+ Chinese word-timed entries. The process appears to be SIGKILLed by the system (jetsam / memory pressure), as no exception handler fires.

The first call to generate_captions() in a session succeeds. The crash typically occurs after FCP is restarted and generate_captions() is called in a second session where persisted caption data gets restored, suggesting state accumulation across sessions.

Environment

• macOS: 26.5.1 (arm64)
• Hardware: Mac mini M4, 16GB RAM
• FCP Version: 11.0 (build 430149) — lifetime/perpetual license
• SpliceKit Version: 3.3.8
• Installation: DMG → patched copy at ~/Applications/SpliceKit/Final Cut Pro.app
• MCP Server: Python venv running /Applications/SpliceKit.app/Contents/Resources/mcp/server.py

Steps to Reproduce

1. Launch SpliceKit-patched FCP 11.0
2. Import a ~57s MP4 with Chinese voice-over
3. Delete leading silence (~12s), leaving ~43s of content on timeline
4. Call set_caption_words() with 113 Chinese words with precise word-level timestamps (from Whisper turbo model)
5. Call generate_captions(style="clean_minimal", animation="fade", max_words=3, word_highlight=True)
6. FCP silently terminates — no crash dialog, just disappears

Also reproduced with style="bold_pop", so it does not appear style-specific.

Logs

From ~/Library/Logs/SpliceKit/splicekit.log (~7500 lines in affected session):

1. FCP 11.0 compatibility issue at startup:

[00:06:57.545] [SpliceKit] [Favorites] Exception injecting favorites folder: 
-[FFBKEffectLibraryFolder isSubcategoryAll]: unrecognized selector sent to instance 0xbbbbcde90

This repeats 4× — isSubcategoryAll selector is missing from FFBKEffectLibraryFolder on FCP 11.0.

2. Caption generation writes back 113 titles word-by-word:

[00:07:12.599] [Captions][RuntimeTitle] Live effect template summary: runs=2 [0]{看看 下 } ...
[00:07:12.599] [Captions][RuntimeTitle] Applying attributed text summary: runs=2 ...
[00:07:12.599] [Captions][RuntimeTitle] FFMotionEffect setText:forField: text="看看 下 午" ...

These attributed-string operations repeat for every word in all 113 titles, totalling ~6800 log lines. Each title gets 3 write cycles (full → word-level → word-highlight pivot).

3. No crash handler report — Sentry crash handler is enabled at startup but termination is too abrupt to capture a report.

Suspected Cause

I suspect this is one of:

1. Jetsam / memory pressure — FCP 11.0 on 16GB M4 gets SIGKILLed by macOS when 113 Motion title clips with attributed strings and word-highlight channels consume too much memory.
2. FFBKEffectLibraryFolder compatibility — the startup exception (isSubcategoryAll unrecognized selector) may indicate broader FCP 11.0 API differences that manifest later during caption rendering.
3. Caption persistence state leak — the crash happens on the second launch after captions are persisted and restored, not the first.

Expected Behavior

• Caption generation should complete without crashing FCP 11.0, especially for 100+ word Chinese content
• If FCP 11.0 lacks the API surface needed, generate_captions() should at minimum return a clear error instead of crashing

Suggestion

Would it be possible to offer a lightweight caption mode for FCP 11.0 users that uses native FCP captions (FFAnchoredCaption / caption lane) instead of Motion title clips? The generate_native_captions() approach would likely avoid the Motion title rendering overhead entirely.