SpanHound

Inspect recorded agent-run failures and approvals from local trace files. SpanHound (formerly mcp-trace-inspector) is a read-only stdio MCP server for inspecting local agent-run traces, including tool failures, approval decisions, and changes between runs. It reads JSONL spans and OTLP JSON exports as described in the user guide, including traces from agent-harness.

Demo: Watch the full 48-second video · Read the reproducible story. This is a scripted regression trace, not a live-model benchmark.

The project's concrete origin was a shell-boundary regression: the harness blocked cat ../outside.txt, but the old trace recorded only that run_shell was called, not the returned diagnostic. SpanHound and the harness now preserve and query that bounded error.

Try the included trace

git clone https://github.com/abhishekash/spanhound.git
cd spanhound
uvx --from . spanhound --trace-dir examples --check

--check prints JSON with the trace directory, span count, and recent run summaries, then exits. It also reports a missing directory or malformed complete JSONL or OTLP JSON record. To ask questions through MCP, connect a client using the configuration below.

To try an OTLP JSON export, run the same command with --trace-dir tests/fixtures. The sample export includes a model call, token usage, a tool call, and an exception.

Where it fits

Use this when your agent already writes the supported JSONL span format or OTLP JSON export and you want an MCP client to inspect those files without setting up a trace backend. Its focused queries include recorded approval decisions, bounded failure diagnostics, and comparisons between selected runs.

MCP Agent Trace Inspector records steps into a local SQLite database and provides a dashboard and exports. Use it when you need trace capture. This project reads trace files written by an existing producer, including agent-harness; it does not record the agent's actions itself.

For traces stored in Jaeger, Tempo, or Traceloop, use a backend-connected tool such as OpenTelemetry MCP Server. For standard OTLP traces with a local receiver, see otel-mcp. For a trace UI, instrumentation, and evaluations, see Phoenix. This project reads saved OTLP/JSON trace exports. It does not receive live OTLP traffic or read backend-specific JSON exports.

AI-native use cases

Question during an agent run	MCP tool	Evidence returned
"Why did yesterday's run stall?"	list_runs → slowest_spans	Run IDs and the longest model or tool spans.
"Did the agent act after I denied the write?"	approval_log → span_tree	The recorded decision and subsequent execution path.
"How many model tokens did this run use?"	token_usage → span_tree	Run-level usage totals and the span tree for context.
"Which tool is failing or timing out?"	failure_report / tool_stats	Redacted diagnostics, error rates, denials, and p95 latency.
"Who/what was allowed to act?"	security_audit	Caller, tenant, server provenance, risk, and missing-evidence findings.
"What is happening in the run right now?"	recent_activity	Cursor-based polling of newly written spans.
"Did this model/version regress?"	compare_runs	Cost, latency, tokens, tool calls, and failure deltas.

The tools read stored traces and can poll files being written, but they do not push events or infer intent from the final answer. Start with the installation and user guide, then see the real trace fixture and the tool descriptions for the query contract.

Install & run

uvx spanhound --trace-dir ./traces
# or, for local development:
git clone https://github.com/abhishekash/spanhound
cd spanhound
uvx --from . spanhound --trace-dir ./traces

Client configuration

Claude Desktop (claude_desktop_config.json):

{
  "mcpServers": {
    "agent-traces": {
      "command": "uvx",
      "args": ["spanhound", "--trace-dir", "/path/to/traces"]
    }
  }
}

pi (~/.pi/agent/settings.json):

{
  "mcpServers": {
    "agent-traces": {
      "command": "uvx",
      "args": ["spanhound", "--trace-dir", "/path/to/traces"]
    }
  }
}

agent-harness (mounted as gated tools):

harness run "Why was my last run slow?" --mcp "uvx spanhound --trace-dir ./traces"

Tools

Tool	Use it when
list_runs	Starting out — recent runs with task, model, duration, cost, decision counts
run_summary	One run at a glance (accepts trace-id prefix)
span_tree	"What did the agent actually do?" — nested shape of the run
slowest_spans	"Why was it slow?" — top-k spans by duration
approval_log	HITL audit — every approve/deny/edit, who decided, and the rationale
token_usage	Cost questions — aggregated across runs or per-run
search_spans	Find spans by tool name, file path, "denied", …
failure_report	Find actionable, bounded, redacted errors and failed tool calls.
tool_stats	Rank tools by volume, error rate, denials, and latency.
security_audit	Audit caller identity, tenant, server provenance, risk, and approval evidence.
recent_activity	Poll new spans with a cursor while a run is active.
compare_runs	Compare selected traces for regressions across models or versions.

Tool descriptions are written as prompts (when-to-use, not just what-it-does) — descriptions are the interface for agent-called tools.

Example session (real fixture trace)

> list_runs
[{ "trace_id": "f920798dd2558fcb84f9977736432a8b", "task": "Summarize the workspace's notes…",
   "tool_calls": 4, "human_decisions": 2, "stopped_reason": "completed" }]

> approval_log
[{ "tool": "write_file", "decision": "approve", "approver": "auto", … },
 { "tool": "run_shell",  "decision": "approve", "approver": "auto", … }]

Design

traces/*.jsonl ──┐
traces/*.json  ──┴─▶ mcp_trace.core (local query functions)
                          │
                   mcp_trace.server (MCP adapter)
                          │
                    stdio (JSON-RPC)

• core/server split: all logic is pure functions over parsed spans; the MCP layer only parses args and JSON-encodes results. Tests hit both layers.
• trace_id prefixes: agents fumble full 32-char hex ids; every tool accepts prefixes.
• bounded output: diagnostics are truncated and obvious credentials are redacted before query results leave the server.
• honest cost totals: token_usage returns cost_known: false and a null cost when any model span lacks a cost or marks pricing as unavailable; unknown pricing is never presented as $0.
• cursor polling: recent_activity makes the snapshot reader useful while a run is still writing spans.
• The demo fixture (examples/example_trace.jsonl) is a real agent-harness run, not hand-written.

Honest limitations

• stdio transport only (Streamable HTTP plus authenticated remote access is the next transport boundary)
• live polling re-reads snapshots; it is not a push subscription
• security audit can only report identity/provenance that the trace producer records
• read-only tools; trace mutation (annotations) is roadmap
• The Python query and server modules are available as mcp_trace; older import and command aliases remain for existing clients.

License

MIT