Borrowing it
Nothing to install: this file belongs to lewing/helix.mcp. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/lewing/helix.mcp/main/.squad/skills/mcp-error-surfacing/SKILL.mdgit clone --depth 1 https://github.com/lewing/helix.mcpWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/lewing/helix.mcp/mcp-error-surfacing)<a href="https://agentmods.dev/skills/lewing/helix.mcp/mcp-error-surfacing"><img src="https://agentmods.dev/badge/skills/lewing/helix.mcp/mcp-error-surfacing.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00000 | $0.02840 |
| Opus 5 | $0.00000 | $0.01420 |
| Sonnet 5 | $0.00000 | $0.00568 |
| Haiku 4.5 | $0.00000 | $0.00284 |
Grade A, and why
mcp-error-surfacing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 237 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MCP Error Surfacing via McpException
Updated: 2026-05-28 (CallToolFilters middleware implemented + schema audit findings captured)
Core Strategy
The MCP SDK deliberately suppresses inner exception messages as a security boundary. Our strategy is NOT to bypass this; it's to use McpException as the designated channel where we want clients to see detail. The MCP SDK's CallToolFilters middleware pipeline provides a clean interception point for SDK parameter binding failures.
Three Layers of MCP Error Handling
Layer 1: SDK Parameter Binding — FIXED via CallToolFilters (v0.7.5)
- Exception source:
Microsoft.Extensions.AI.AIFunctionFactoryduring reflection-based parameter marshalling - Observable: Exception logged to stderr, generic "An error occurred invoking 'X'." sent to client before the fix
- McpExceptionHandler reached? ❌ No (method not invoked)
- Our fix: ✅ CallToolFilters middleware — intercept
ArgumentExceptionwhereParamName == "arguments", convert toMcpException - Scope: All 25 tools automatically protected from both MCP startup paths
- Example: Agent uses
buildIdOrUrlon a Helix tool expectingjobId(parameter binding fails)
Layer 2: MCP Server Error Formatting (Infrastructure)
- Exception source: MCP Server's unhandled-exception handler
- Observable: Full exception logged to stderr, generic response to client
- Our lever: custom middleware only if the current CallToolFilters approach proves insufficient
Layer 3: Application Layer — COVERED via PR #64 (Runtime Exceptions)
- Exception source: Service layer, API client, or application code
- McpExceptionHandler reached? ✅ Yes (caught inside
RunServiceCallAsync) - Our fix: ✅ PR #64 (complete)
- Example: HttpRequestException, TaskCanceledException, ArgumentException from business logic
CallToolFilters Pattern (Class A Fix — v0.7.5)
Register cross-cutting CallToolFilters middleware through an extension method on McpServerOptions, shared by both startup paths. The ModelContextProtocol 1.3.0 API path is McpServerOptions.Filters.Request.CallToolFilters; this repo scopes the catch to ArgumentException with ParamName == "arguments" via a named constant so ordinary tool-body ArgumentExceptions are not mislabeled as binding failures.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 8d ago First seen · 237 lines · 0 tokens per session scan A 1f42340a445f
mcp-error-surfacing is a skill published in the GitHub repository lewing/helix.mcp (4 stars, last pushed today), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 2,840 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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