MCP tool errors (`isError: true`) reach the model as a bare `Error executing tool <name>`, discarding the full error message

Description

When an MCP tool call fails with isError: true, the model only receives a bare Error executing tool <name> --
the real error text the server put in content[].text is dropped. A successful call (isError: false) forwards
its full content fine.

Proof: the same MCP server, tool, and input, called two ways -- via the MCP Inspector (raw MCP protocol, without
OpenCode) and via a live OpenCode session. The Inspector always returns the full error; OpenCode always truncates
it to the bare tool name, across every failure mode tested on two unrelated tools of the same server.

#	Tool call	Failure mode	Inspector	OpenCode
1	get_req(id="./non-existing-file.txt")	path-like id, isError:true	full message	Error executing tool get_req
2	set_status(id=<uuid>, type=req, status="bogus-status")	invalid status, isError:false (control)	full JSON	full JSON (not truncated)
3	set_status(id="deadbeef-dead-dead-dead-deadbeefdead", type=req, status=draft)	unknown id, isError:true	full message	Error executing tool set_status
4	set_status(id="./non-existing-file.txt", type=req, status=draft)	path-like id, isError:true	full message	Error executing tool set_status
5	set_status(id=<uuid>, type=req, status=draft, superseded_by="x")	superseded_by misuse, isError:true	full message	Error executing tool set_status

Row 2 is a control: the one failure mode that doesn't raise comes through OpenCode intact, so the bug is
specifically about isError: true, not the tool or server.

Worked example (row 1):

npx @modelcontextprotocol/inspector --cli specmgr mcp \
  --method tools/call --tool-name get_req --tool-arg id="./non-existing-file.txt" --format json

{"result":{"content":[{"type":"text","text":"Error executing tool get_req: id './non-existing-file.txt' contains a path separator or a '..' traversal sequence; a bare id is expected"}],"isError":true}}

Same call, live in OpenCode, returns only:

Error executing tool get_req

Where that message comes from, in the MCP server:

# src/biz/dfch/specmgr/req/tools/get_req.py:109 (inside the @mcp.tool()-decorated get_req)
validate_id("req", id)

# src/biz/dfch/specmgr/general/tools/_path_safety.py:106-111 (assert_no_traversal, called by validate_id)
if not id_.strip():
    raise ValueError(f"id {id_!r} is empty; a non-empty id is required")
if _TRAVERSAL_SEQUENCE in id_ or any(separator in id_ for separator in _PATH_SEPARATORS):
    raise ValueError(f"id {id_!r} contains a path separator or a '..' traversal sequence; a bare id is expected")

Plain raise ValueError, nothing MCP-specific -- the mcp Python SDK (FastMCP) is what turns an uncaught
exception from inside a @mcp.tool() function into
CallToolResult(isError=True, content=[TextContent(text=f"Error executing tool {name}: {exc}")]) automatically;
the server itself never constructs that envelope.

• get_req.py#L109
• _path_safety.py#L106-L111

Plugins

None -- this is an MCP server (biz-dfch-specmgr), not a plugin.

OpenCode version

1.18.29

Steps to reproduce

1. Add an MCP server whose tool raises a plain exception on invalid input (default FastMCP behavior):

   {
     "mcp": {
       "specmgr": {
         "type": "local",
         "enabled": true,
         "command": ["uvx", "--from", "biz-dfch-specmgr[mcp]==0.23.0", "specmgr", "mcp"]
       }
     }
   }
   

2. Call a tool with input that triggers the failure, e.g. get_req(id="./non-existing-file.txt")
   -- any str containing a path separator is rejected, the file doesn't need to exist.
3. Compare against calling the same tool/args through the MCP Inspector:

   npx @modelcontextprotocol/inspector --cli <server-command> \
     --method tools/call --tool-name <tool> --tool-arg <name>=<value> --format json
   

biz-dfch-specmgr[mcp]==0.23.0 = tag v0.23.0 (commit 89484c4) of github.com/dfch/biz.dfch.SpecMgr, main
branch. Tool versions: uv/uvx 0.12.1, npx 9.2.0 (Node v22.22.1), Python 3.13.13 (main, May 10 2026, 19:26:54) [Clang 22.1.3].

Screenshot and/or share link

None. This is only about tool-result content.

Operating System

Ubuntu 26.04 LTS

Terminal

OpenCode TUI with bash.