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153 lines
7.4 KiB
Markdown
153 lines
7.4 KiB
Markdown
---
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issue: 33
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issue_title: "sanitizeAvailableToolsSection silently removes content after the last recognised section header"
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---
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# Fix `findSection` greedy end boundary
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## Problem Statement
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`findSection` in `src/system-prompt-sanitizer.ts` defaults `end` to `lines.length` when no subsequent top-level section header is found.
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This means any content after the last recognised section — plain prose, custom instructions, trailing notes — is silently included in the section range and deleted by `removeLineSection`.
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The bug is masked in production because the real Pi system prompt always places `Guidelines:` after `Available tools:`, so `end` is always updated before EOF.
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Unit tests from #21 exposed the bug with a minimal prompt where `"Other content"` follows the tools section.
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## Goals
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- Make `findSection` stop at the end of the section's own content (header + bullet/indented lines), not at EOF.
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- Preserve all content that follows the section being removed.
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- Flip the existing `test.fails` test for bug #33 to a passing assertion.
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## Non-Goals
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- Refactoring `sanitizeGuidelinesSection` — it uses the same `findSection` but is always followed by another section in practice; any latent issue there is covered by the same fix.
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- Changing the `isTopLevelSectionHeader` heuristic beyond what's needed to fix the boundary.
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- Adding new config fields or schema changes.
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## Background
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### Relevant modules
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| File | Role |
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| --------------------------------------- | --------------------------------------------------------------------------------------------------------- |
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| `src/system-prompt-sanitizer.ts` | Contains `findSection`, `removeLineSection`, `sanitizeAvailableToolsSection`, `sanitizeGuidelinesSection` |
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| `tests/system-prompt-sanitizer.test.ts` | Existing tests including the `test.fails` for #33 |
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### Permission surface
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None — this is a system-prompt sanitisation bug, not a permission-policy change.
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However, the impact is security-adjacent: silently deleting post-section content could remove user-authored safety instructions from the system prompt.
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## Design Overview
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### Current behaviour
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```typescript
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let end = lines.length; // greedy: eat to EOF
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for (let index = start + 1; ...) {
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if (isTopLevelSectionHeader(lines[index])) {
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end = index;
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break;
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}
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}
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```
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### Proposed change
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Treat the section as the header line plus all contiguous "section body" lines that follow it.
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A line is part of the section body if it is:
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- blank, or
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- a bullet (`- …`), or
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- indented (starts with whitespace).
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The first line that is non-blank, non-bullet, non-indented, and not a recognised section header marks the end of the section.
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A recognised section header also ends the section (preserving existing behaviour).
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```typescript
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function isSectionBodyLine(line: string): boolean {
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const trimmed = line.trim();
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if (trimmed.length === 0) return true; // blank
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if (trimmed.startsWith("- ")) return true; // bullet
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if (line !== line.trimStart()) return true; // indented
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return false;
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}
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```
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Then `findSection` becomes:
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```typescript
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let end = start + 1;
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for (let index = start + 1; index < lines.length; index += 1) {
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if (isTopLevelSectionHeader(lines[index])) {
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end = index;
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break;
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}
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if (!isSectionBodyLine(lines[index])) {
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end = index;
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break;
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}
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end = index + 1;
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}
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```
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This ensures trailing blank lines between the section and the next non-section content are consumed by the section (avoiding stray blank lines after removal), while non-section prose is preserved.
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### Edge cases
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1. **Section at EOF with trailing blanks only** — `end` reaches `lines.length`, same as today; no content is lost because only blank lines follow.
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2. **Section immediately followed by another header** — `isTopLevelSectionHeader` fires first, same as today.
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3. **Section followed by non-bullet, non-indented prose** — new `isSectionBodyLine` check fires, `end` stops before the prose.
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This is the fix.
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4. **Guidelines section** — same `findSection` is used, same fix applies.
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## Module-Level Changes
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### `src/system-prompt-sanitizer.ts`
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- Add `isSectionBodyLine(line: string): boolean` helper.
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- Update `findSection` loop to stop at non-body lines (see Design Overview).
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### `tests/system-prompt-sanitizer.test.ts`
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- Change `test.fails` for bug #33 to a regular `test`.
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- Add new cases:
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- Content after `Guidelines:` section is preserved when Guidelines is the last section.
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- Content after both sections removed; trailing prose survives.
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- Section at EOF (no trailing content) still works.
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- Section followed by blank lines then prose — prose survives, extra blanks collapsed.
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## TDD Order
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1. **Red → green: flip the existing `test.fails` to `test` and verify it fails before the fix.**
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- Surface: `tests/system-prompt-sanitizer.test.ts` — the `test.fails` for bug #33.
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- Commit: `test: expect content after Available tools section to be preserved (#33)`
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2. **Green: implement `isSectionBodyLine` and update `findSection`.**
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- Surface: `src/system-prompt-sanitizer.ts`.
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- The flipped test should now pass.
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- Commit: `fix: stop findSection at first non-body line instead of EOF (#33)`
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3. **Add edge-case tests.**
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- Content after `Guidelines:` preserved.
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- Both sections removed, trailing prose survives.
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- Section at EOF with only trailing blanks.
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- Commit: `test: add edge cases for findSection boundary (#33)`
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4. **Verify all existing tests still pass.**
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- `npm test` — no regressions.
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## Risks and Mitigations
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| Risk | Mitigation |
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| ----------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| Could this silently weaken a permission? | No policy logic changes. The fix only affects system-prompt text manipulation. However, the bug itself weakens safety by silently removing user content; fixing it restores the intended behaviour. |
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| `isSectionBodyLine` is too conservative and leaves bullet lines outside the section | The heuristic mirrors the existing section format (header + bullets). Tests explicitly cover bullet-only and mixed content. |
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| `isSectionBodyLine` is too liberal and still eats non-section content | Non-blank, non-bullet, non-indented lines stop the scan. The reproducer from the issue (`"Other content"`) is the direct test. |
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| Guidelines section has the same latent bug | Same `findSection` is used — the fix applies to both. Added edge-case test confirms. |
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## Open Questions
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None — the fix is well-scoped and the issue's proposed approach aligns with the design above.
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