feat(langchain): add respond decision to HITL middleware

[Sydney Runkle (sydney-runkle)]

Extends HumanInTheLoopMiddleware with a new respond decision type for "ask user" style tools — tools whose real implementation is the human's response. The interrupt is raised with the tool call as usual; the resume payload becomes the body of a synthetic ToolMessage with status="success", and the tool itself is not executed.

This complements reject (which produces a synthetic ToolMessage with status="error") by enabling the symmetric success path: a reviewer can answer on the tool's behalf without invoking it.

Changes

• New RespondDecision TypedDict with a required message: str field; added to the Decision union.
• "respond" added to the DecisionType literal.
• _process_decision handles "respond" by emitting a ToolMessage with status="success" and preserving the original tool call on the AIMessage so provider-required tool-call/tool-message pairing is maintained.
• The True shortcut in interrupt_on now expands to ["approve", "edit", "reject", "respond"], so existing callers that opted into "all decisions" pick up the new capability without code changes. The reject decision already permits a reviewer to inject arbitrary ToolMessage content, so respond extends the same trust model — not a new capability class.

Example

from langchain.agents.middleware import HumanInTheLoopMiddleware

middleware = HumanInTheLoopMiddleware(
    interrupt_on={"ask_user": {"allowed_decisions": ["respond"]}}
)
# Resume payload: {"decisions": [{"type": "respond", "message": "blue"}]}
# → synthetic ToolMessage(content="blue", status="success") for `ask_user`.

---

Implementation drafted with AI-agent assistance.

[codspeed-hq]

Merging this PR will degrade performance by 99.6%

❌ 1 regressed benchmark
✅ 1 untouched benchmark
⏩ 13 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	test_create_agent_instantiation_with_middleware	15.1 ms	3,760.5 ms	-99.6%

---

_{Comparing sr/hitl-middleware-extension (23aaa1d) with master (38553c3)}

Footnotes

1. 13 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩