41 lines
1.9 KiB
Markdown
41 lines
1.9 KiB
Markdown
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# Copilot Plan Mode Prompt Parity
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> Discovered: 2026-03-06
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## Context
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The GitHub Copilot SDK pipe builds system prompts in two paths:
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- fresh session creation via `_build_session_config(...)`
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- resumed session injection via the `system_parts` rebuild branch
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Plan Mode guidance was duplicated across those branches.
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## Finding
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If Plan Mode instructions are edited in only one branch, resumed sessions silently lose planning behavior or capability hints that fresh sessions still have.
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This is especially easy to miss because both branches still work, but resumed chats receive a weaker or stale prompt.
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Session mode switching alone is also not enough. Even when `session.rpc.mode.set(Mode.PLAN)` succeeds, the SDK may still skip creating the expected `plan.md` if the runtime system prompt does not explicitly include the original Plan Mode persistence contract.
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## Solution / Pattern
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Extract the Plan Mode prompt into one shared helper and call it from both branches:
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```python
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def _build_plan_mode_context(plan_path: str) -> str:
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...
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```
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Then inject it in both places with the chat-specific `plan.md` path.
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For extra safety, when the pipe later reads `session.rpc.plan.read()`, mirror the returned content into the chat-specific `COPILOTSDK_CONFIG_DIR/session-state/<chat_id>/plan.md` path. This keeps the UI-visible file in sync even if the SDK persists plan state internally but does not materialize the file where the chat integration expects it.
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## Gotchas
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- Keep the helper dynamic: the `plan.md` path must still be resolved per chat/session.
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- Do not only update debug prompt artifacts; the effective runtime prompt lives in `plugins/pipes/github-copilot-sdk/github_copilot_sdk.py`.
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- Resume-session parity matters for capability guidance just as much as for session context.
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- If users report that Plan Mode is active but `plan.md` is missing, check both halves: prompt parity and the final `rpc.plan.read()` -> `plan.md` sync path.
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