`_maybe_wrap_untrusted` is the architectural defense against indirect
prompt injection. It wraps attacker-controllable tool output
(web_extract, web_search, browser_*, mcp_*) in
`<untrusted_tool_result>...</untrusted_tool_result>` so the model treats
it as data. The content was interpolated verbatim, so the boundary was
forgeable.
Two holes. A poisoned page that embeds `</untrusted_tool_result>` closes
the block early — everything after it reads as trusted instructions. And
the `startswith("<untrusted_tool_result")` re-entrancy guard returned
content that merely started with the opening tag completely unwrapped, so
an attacker just prefixed the tag to drop all data framing.
Fix neutralizes any embedded delimiter token (case-insensitive) before
interpolation and drops the forgeable fast-path, so content is always
sealed in exactly one well-formed block. Re-wrapping an already-wrapped
forward is harmless — it stays framed as data.
## What does this PR do?
Closes an indirect prompt-injection bypass in the untrusted-tool-result
wrapper. Attacker content can no longer break out of, or forge, the
trust boundary.
## Related Issue
N/A
## Type of Change
- [x] 🔒 Security fix
## Changes Made
- `agent/tool_dispatch_helpers.py`: add `_neutralize_delimiters` (case-insensitive defang of the `untrusted_tool_result` token); `_maybe_wrap_untrusted` now always neutralizes then wraps, and the forgeable `startswith` re-entrancy guard is removed.
- `tests/agent/test_tool_dispatch_helpers.py`: replace the double-wrap test (it encoded the bypass) with regression tests for embedded closing tag, leading opening tag, and a cased closing tag.
## How to Test
1. `scripts/run_tests.sh tests/agent/test_tool_dispatch_helpers.py` — 29 pass.
2. Embedded `</untrusted_tool_result>` mid-content: real closing delimiter appears once, at the end; payload trapped inside.
3. Content starting with the opening tag: data framing is applied, not skipped.
## Checklist
### Code
- [x] I've read the Contributing Guide
- [x] My commit messages follow Conventional Commits
- [x] I searched for existing PRs to make sure this isn't a duplicate
- [x] My PR contains only changes related to this fix
- [x] I've run the affected tests and they pass
- [x] I've added tests for my changes
- [x] I've tested on my platform: macOS 15 (Darwin 25.5)
### Documentation & Housekeeping
- [x] I've updated relevant documentation (docstrings) — or N/A
- [x] cli-config.yaml.example — N/A
- [x] CONTRIBUTING.md / AGENTS.md — N/A
- [x] Cross-platform impact — N/A (pure-Python, stdlib `re`)
- [x] Tool descriptions/schemas — N/A
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