Skills produce no output under Claude Opus 4.8 in the Desktop App when many MCP servers are enabled (works with GPT-5.x)

### Short summary

Invoking any skill under Claude Opus 4.8 shows "Loaded skill …" and then produces no output at all — no text, no tool calls — for that turn and all following turns. The same skill and config work under GPT-5.5/5.6, and work under Opus 4.8 once most MCP servers are disabled.

### Affected version or release

1.0.25

### Installation context

Desktop App on Windows. 15 MCP servers enabled (~580 tools total; heaviest: ecs ~135, azure-devops ~90, geneva ~85, azure ~65, icm ~45). Global custom instructions ~152 KB (~26K tokens). Affects all skills/agents whenever the selected model is Claude Opus 4.8.

### What happened?

_No response_

### Steps to reproduce

_No response_

### Expected behavior

### Observed behavior
Selecting Claude Opus 4.8 and invoking a skill (e.g. /s360-sla-status) shows `Loaded skill …` and then nothing — no assistant text, no tool calls. Follow-up messages ("continue", plain text) also return no output. The turn appears to fail silently with no error.

### Steps to reproduce
1. Enable ~15 MCP servers (~580 tools total).
2. Select **Claude Opus 4.8**.
3. Invoke a skill (e.g. `/s360-sla-status`).
4. Result: `Loaded skill …`, then no response; subsequent messages also produce nothing.
5. Switch to **GPT-5.5 / GPT-5.6** → works. **Or** disable all but the 2 MCP servers the skill needs, under Opus 4.8 → the skill runs end-to-end successfully.

### Expected
Opus 4.8 executes the skill — or, if a context/tool limit is exceeded, surfaces an **explicit error** instead of empty output.

### Actual
Silent no-op: zero output across multiple turns, no error shown.

### Likely root cause
The Desktop App appears to load **all** enabled MCP tool schemas into the model request up front. ~580 tool schemas (~120–175K tokens) plus ~26K tokens of instructions likely exceed Opus 4.8's context budget (or a per-request tool-count limit), causing a silent request failure. GPT-5.x tolerates the larger payload.

**Key signal:** the **GitHub Copilot CLI does not reproduce this** — it defers MCP tools (surfaces them on demand via tool search), keeping requests small, and Opus 4.8 works there with all 15 servers enabled.

### Suggested fix
1. Add **on-demand tool loading / tool deferral** in the Desktop App (parity with the CLI).
2. When a request would exceed context/tool limits, **fail loudly** with an error rather than producing empty output.
3. Optional: show a per-model tool/context-budget warning in the MCP settings UI.

### Workaround
Enable only the MCP servers a task needs when using Opus 4.8.

### Additional context

_No response_

Bắt đầu bằng cách tái hiện lỗi trong Desktop App với Claude Opus 4.8, khoảng 15 máy chủ MCP và kích thước chỉ dẫn được báo cáo; so sánh hành vi với GPT-5.x và GitHub Copilot CLI. Theo dõi quá trình tải công cụ MCP và MCP settings UI, sau đó xác minh rằng skills tạo ra đầu ra hoặc xuất hiện lỗi giới hạn context/tool rõ ràng thay vì một no-op im lặng.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.