From f644ec905ae57ec80d377f4c9a27cda85b63c1ea Mon Sep 17 00:00:00 2001
From: Belaidouni Badr Islam <139909210+badrbel@users.noreply.github.com>
Date: Fri, 6 Nov 2026 21:49:33 +0100
Subject: [PATCH 1/2] fix(otel): set ERROR status and error.type=tool_error on
 client span when tool result has isError=true (#3810)

---
 src/mcp/shared/jsonrpc_dispatcher.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/mcp/shared/jsonrpc_dispatcher.py b/src/mcp/shared/jsonrpc_dispatcher.py
index 223373da0f..9c6a1521e6 100644
--- a/src/mcp/shared/jsonrpc_dispatcher.py
+++ b/src/mcp/shared/jsonrpc_dispatcher.py
@@ -23,6 +23,7 @@
     INTERNAL_ERROR,
     INVALID_PARAMS,
     REQUEST_TIMEOUT,
+    CallToolResult,
     ErrorData,
     JSONRPCError,
     JSONRPCMessage,
@@ -418,6 +419,13 @@ async def send_raw_request(
                     code = str(outcome.code)
                     span.set_attributes({"error.type": code, "rpc.response.status_code": code})
                     span.set_status(StatusCode.ERROR, outcome.message)
+                elif method == "tools/call":
+                    match outcome:
+                        case CallToolResult(is_error=True) | {"isError": True}:
+                            span.set_attribute("error.type", "tool_error")
+                            span.set_status(StatusCode.ERROR)
+                        case _:
+                            pass
         except TimeoutError:
             if not timeout_armed:
                 # The clock starts only after the write, so this TimeoutError is the

From c70ab3adf039f2a6263ba1d2c83d6f5cd5298379 Mon Sep 17 00:00:00 2001
From: Belaidouni Badr Islam <139909210+badrbel@users.noreply.github.com>
Date: Fri, 6 Nov 2026 21:49:34 +0100
Subject: [PATCH 2/2] test(otel): verify client and server span agreement on
 tool_error, protocol error, and success (#3810)

---
 tests/shared/test_otel.py | 60 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/tests/shared/test_otel.py b/tests/shared/test_otel.py
index a26d2fd40c..c2895c0b56 100644
--- a/tests/shared/test_otel.py
+++ b/tests/shared/test_otel.py
@@ -91,3 +91,63 @@ async def list_prompts(
     assert failed.attributes is not None
     assert failed.attributes["error.type"] == str(types.INVALID_PARAMS)
     assert failed.attributes["rpc.response.status_code"] == str(types.INVALID_PARAMS)
+
+
+async def test_client_and_server_spans_agree_on_tool_error(capfire: CaptureLogfire):
+    """When tools/call returns a CallToolResult with isError=true, both the client and server spans
+    must record status ERROR and error.type='tool_error', while a successful tools/call leaves both
+    spans UNSET (#3810).
+    """
+    server = MCPServer("test")
+
+    @server.tool()
+    def ok_tool() -> str:
+        return "ok"
+
+    @server.tool()
+    def failing_tool() -> types.CallToolResult:
+        return types.CallToolResult(
+            content=[types.TextContent(type="text", text="tool failure")],
+            is_error=True,
+        )
+
+    @server.tool()
+    def protocol_error_tool() -> str:
+        raise MCPError(types.INTERNAL_ERROR, "upstream failure")
+
+    async with Client(server, mode="legacy") as client:
+        ok_res = await client.call_tool("ok_tool", {})
+        err_res = await client.call_tool("failing_tool", {})
+        with pytest.raises(MCPError):
+            await client.call_tool("protocol_error_tool", {})
+
+    assert not ok_res.is_error
+    assert err_res.is_error
+
+    spans = {
+        span.name: span
+        for span in capfire.exporter.exported_spans
+        if (span.attributes or {}).get("logfire.span_type") != "pending_span"
+    }
+
+    # 1. Successful tool call leaves both client and server spans UNSET
+    assert spans["MCP send tools/call ok_tool"].status.status_code == StatusCode.UNSET
+    assert "error.type" not in (spans["MCP send tools/call ok_tool"].attributes or {})
+    assert spans["tools/call ok_tool"].status.status_code == StatusCode.UNSET
+
+    # 2. Tool error (isError=true) sets ERROR and error.type="tool_error" on both spans
+    client_tool_err = spans["MCP send tools/call failing_tool"]
+    server_tool_err = spans["tools/call failing_tool"]
+    assert client_tool_err.status.status_code == StatusCode.ERROR
+    assert server_tool_err.status.status_code == StatusCode.ERROR
+    assert (client_tool_err.attributes or {}).get("error.type") == "tool_error"
+    assert (server_tool_err.attributes or {}).get("error.type") == "tool_error"
+
+    # 3. Protocol error sets ERROR and matching error.type code on both spans
+    client_proto_err = spans["MCP send tools/call protocol_error_tool"]
+    server_proto_err = spans["tools/call protocol_error_tool"]
+    assert client_proto_err.status.status_code == StatusCode.ERROR
+    assert server_proto_err.status.status_code == StatusCode.ERROR
+    assert (client_proto_err.attributes or {}).get("error.type") == str(types.INTERNAL_ERROR)
+    assert (server_proto_err.attributes or {}).get("error.type") == str(types.INTERNAL_ERROR)
+