The non-native (prompted) tool-call path fed tool output back to the model as a plain "[Tool execution results]" user message, bypassing the untrusted_context_message wrapper that THREAT_MODEL.md requires for tool output. That path is what models without native tool-calling (many smaller local models) use, so prompt-injection inside a tool result (fetched page, file read, MCP/email output) could be read as instructions there.
Wrap it via untrusted_context_message("tool execution results", ...), the same hardening already applied to skills (#788) and escalation traces (#275). Also update _recent_context_for_retrieval, which used the old "[Tool execution results]" prefix as a sentinel to keep tool envelopes out of the retrieval query, to recognise the wrapped envelope via metadata.trusted.
The native path keeps returning tool-role messages (a user-role wrapper would break the native tool-call contract); it is covered by UNTRUSTED_CONTEXT_POLICY. Adds tests/test_tool_output_prompt_injection.py.
Fixes#1627.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Rudy Wolf