The mid-run steer marker was '[USER STEER (injected mid-run, not tool
output): <text>]'. Replaced with a plain two-newline-prefixed
'User guidance: <text>' suffix.
Rationale: the marker lives inside the tool result's content string
regardless of whether the tool returned JSON, plain text, an MCP
result, or a plugin result. The bracketed tag read like structured
metadata that some tools (terminal, execute_code) could confuse with
their own output formatting. A plain labelled suffix works uniformly
across every content shape we produce.
Behavior unchanged:
- Still injected into the last tool-role message's content.
- Still preserves multimodal (Anthropic) content-block lists by
appending a text block.
- Still drained at both sites added in #12959 and #13205 — per-tool
drain between individual calls, and pre-API-call drain at the top
of each main-loop iteration.
Checked Codex's equivalent (pending_input / inject_user_message_without_turn
in codex-rs/core): they record mid-turn user input as a real role:user
message via record_user_prompt_and_emit_turn_item(). That's cleaner for
their Responses-API model but not portable to Chat Completions where
role alternation after tool_calls is strict. Embedding the guidance in
the last tool result remains the correct placement for us.
Validation: all 21 tests in tests/run_agent/test_steer.py pass.
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