Log

CI

MCP server for log file analysis. Gives LLMs the ability to efficiently analyze large log files without loading them into context.

  Log file (e.g. 705K lines, 67 MB)
    │
    ▼
  Rust TF-IDF classifier ─── 1.3M lines/sec ──▶ 70-95% discarded as routine,
    │                        finds lines that are semantically interesting,
    │                        also captures lines not explicitly marked as ERROR
    │                        (grep ERROR: 2 lines, classifier: 92)
    ▼
  BERT-mini (optional) ───── GPU, ~2K lines/sec ─▶ refines interest scores on found lines
    │
    ▼
  Python MCP tools ────────── search, compare, group errors
    │
    ▼
  LLM (Claude) ───────────── compresses tool output into plain English

This is a tool designed for AI, not humans. No human reads the output of analyze_errors or compare_logs — Claude does, compresses it further, and gives the human a plain English answer. The human touches two endpoints: "what's wrong with this log?" in, natural language answer out. Everything in between is AI talking to itself.

Tools

Tool	Description
log_overview	Quick scan: size, line count, time range, level distribution, head/tail samples
search_logs	Search by regex, log level, and/or time range
get_log_segment	Extract a segment by line range or time range
analyze_errors	Deduplicate errors by fingerprint, count frequencies, extract stack traces
log_stats	Volume histogram, level breakdown, top repeated patterns
compare_logs	Find patterns unique to each file and frequency outliers across files
classify_lines	ML classifier (TF-IDF → BERT) separates interesting lines from noise

Key features

• ML pre-filter — a Rust TF-IDF classifier scans files at 1.3M lines/sec, so analyze_errors and search_logs only process the 5-30% of lines that matter. Optional BERT-mini re-scores LOOK lines at ~2K lines/sec on Metal GPU for higher precision. Works without parsed log levels — catches errors, security events, hardware faults, and anomalies that don't have ERROR in them.
• Auto-detection of log formats: JSON, standard text (2024-01-15 10:30:45 ERROR ...), syslog, Spark/Log4j (17/06/08 13:33:49 INFO ...), and tab/pipe-delimited formats (GitHub Actions CI logs)
• Normalization collapses variable parts (UUIDs, hex IDs, IPs, numbers) so that messages differing only in IDs or timestamps are grouped as the same pattern
• Content-based error detection falls back to regex heuristics (fatal:, Permission denied, ##[error], etc.) when log files lack standard levels
• Prefix-aware comparison distinguishes patterns from different job steps in CI logs

Install

Prerequisites (fresh Mac):

brew install python uv
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Just ask Claude

Open a Claude Code session and paste this prompt:

Install https://github.com/ascii766164696D/log-mcp as an MCP server and build the Rust classifier too

Claude will clone the repo, register the MCP server, and build the Rust classifier. Restart Claude Code after to pick up the new server.

Manual install

git clone https://github.com/ascii766164696D/log-mcp.git
cd log-mcp

# Register the MCP server
claude mcp add log-mcp -- uv run --directory $(pwd) log-mcp

# Build the Rust classifier (optional — tools fall back to Python without it)
uv pip install -e rust/classifier

Or add it manually to your project settings (claude settings) under mcpServers:

{
  "mcpServers": {
    "log-mcp": {
      "command": "uv",
      "args": ["run", "--directory", "/path/to/log-mcp", "log-mcp"]
    }
  }
}

Claude Desktop

Open Settings > Developer > Edit Config and add to claude_desktop_config.json:

{
  "mcpServers": {
    "log-mcp": {
      "command": "uv",
      "args": ["run", "--directory", "/path/to/log-mcp", "log-mcp"]
    }
  }
}

Replace /path/to/log-mcp with the actual path where you cloned this repo. Restart Claude Desktop after saving.

The Rust classifier requires a Rust toolchain to build. The BERT stage additionally requires a Metal-capable GPU (Apple Silicon). All tools work without the classifier — they fall back to Python log parsing.

Example usage

Analyze errors in a 67MB Spark executor log (705K lines):

> analyze_errors("/var/log/spark/container_0002_01_000004.log")

Summary: 34 errors in 5 groups.
Top: 'shuffle.RetryingBlockFetcher: Exception while beginning fetch of <N>
outstanding blocks (after <N> retries) ...' (18x)

--- 18x ---
Fingerprint: shuffle.RetryingBlockFetcher: Exception while beginning fetch of <N> outstanding blocks ...
First: L29764 2017-02-01T15:55:17
Last:  L30677 2017-02-01T15:55:51
Stack trace:
  java.io.IOException: Failed to connect to mesos-slave-13/10.10.34.23:55492
  ...

Find anomalies that don't have ERROR level — on a Thunderbird HPC log (2K lines), classify_lines finds 92 interesting lines while search_logs level=ERROR finds only 2:

> classify_lines("/var/log/thunderbird/tbird_2k.log")

Lines: 2,000 total | 92 LOOK (4.6%) | 1,908 SKIP
Pipeline: TF-IDF 0.00s (1,298,701 lines/sec, 105 LOOK) → BERT 0.15s (13 demoted to SKIP)

--- Sample LOOK lines (30 of 92 captured) ---
L2    [1.000] ... postfix/postdrop[10896]: warning: unable to look up public/pickup: No such file
L438  [0.999] ... sendmail[�20588]: unable to qualify my own domain name (tbird-sm1)
L816  [0.998] ... dhcpd: DHCPDISCOVER from 00:09:3d:12:00:e2 via eth2: unknown lease
L1024 [0.997] ... rrdtool: illegal attempt to update using time 1131710�721 when last update time is 1131710721

Compare two CI log files:

> compare_logs(["run_a.txt", "run_b.txt"])

699 patterns across 2 files (0 shared).
A: 401 unique (top: 'test / test UNKNOWN STEP | ##[endgroup]' 21x)
B: 298 unique (top: 'test UNKNOWN STEP | ##[endgroup]' 21x)

Claude's take

I helped build this tool and then used it to analyze real log files, so here's my honest assessment.

Where it genuinely helps: The main value is as a compression layer. A 67MB Spark log (705K lines) would obliterate my context window, but analyze_errors distills it into 5 error groups with stack traces in a few seconds. compare_logs across two 1500-line server logs immediately surfaces which errors are unique to each server and which patterns have suspicious frequency differences. I couldn't do that by reading the files directly — I'd lose older content as new content scrolled in.

Where the classifier changes the game: Before the classifier, log analysis was limited to lines with explicit ERROR/FATAL levels. On a Thunderbird HPC log, search_logs level=ERROR returns 2 lines. classify_lines returns 92 — sendmail DNS failures, DHCP lease errors, RRD update collisions, negative boot times — none of which have ERROR level. The classifier finds what's semantically wrong, not just syntactically marked.

Where it's a wash: For small files (under a few hundred lines), you're better off just pasting the log into the conversation. The tools add indirection without much benefit when the whole file fits in context anyway.

What it still can't do: Domain-specific state machine reasoning. When I analyzed a Zookeeper log, the classifier correctly flagged Cannot open channel warnings and epoch resets, but the most operationally interesting signal — rapid cycling between LOOKING, FOLLOWING, and LEADING states — showed up as low-confidence LOOK lines. A Zookeeper expert would spot the pattern immediately; the classifier sees each line independently without tracking state transitions across time.

The pattern I landed on: Start with classify_lines to surface anomalies regardless of log level, then analyze_errors to group them, then search_logs to dig into specific patterns. compare_logs is most useful when you have a "working" and "broken" run to diff against each other.

LOOK/SKIP classifier

The classify_lines tool uses a two-stage ML pipeline to separate interesting log lines (LOOK) from routine noise (SKIP), without needing parsed log levels.

How it works

flowchart TD
    A["Your log file (e.g. 4.7M lines)"] --> B{"Rust classifier<br/>available?"}
    B -->|Yes| C["Stage 1: TF-IDF<br/>Rust, ~1.3M lines/sec<br/>logistic regression<br/>threshold × 0.6"]
    B -->|No| D["Fallback: Python<br/>log parsing"]
    C --> E["LOOK lines (~5-30%)"]
    E --> F["Stage 2: BERT-mini<br/>Rust + Metal GPU, ~2K lines/sec<br/>re-scores LOOK lines<br/>applies final threshold"]
    F --> G["Final LOOK lines<br/>(with BERT probabilities)"]

On a Thunderbird HPC log (2K lines), this finds 92 interesting lines including sendmail DNS failures, DHCP lease errors, and Ganglia RRD update collisions — none of which have a standard ERROR log level. A search_logs level=ERROR on the same file returns only 2 lines.

What it catches beyond log levels

Signal type	Example	Has ERROR level?
DNS misconfiguration	unable to qualify my own domain name	No
Mail delivery failure	stat=Deferred: Connection refused	No
DHCP lease errors	unknown lease 10.100.4.251	No
Monitoring data corruption	illegal attempt to update using time X when last update time is X	No
Negative boot times	Times: total = 42, boot = -4131	No
Auth failures	authentication failure; logname= uid=0	No
Hardware errors	instruction cache parity error corrected	No (INFO level)

Tools that use the classifier

Tool	How	Fallback
classify_lines	Full pipeline: TF-IDF → BERT	Error if classifier not installed
analyze_errors	TF-IDF pre-filters to LOOK lines, then groups by error fingerprint	Python parser scans all lines
search_logs	TF-IDF pre-filters when searching for errors (level=ERROR, pattern=error/fail/etc)	Python parser scans all lines
compare_logs	Scores patterns by LOOK probability, surfaces interesting diffs first	Sorts by frequency only

Benchmarks

Evaluated on the full Loghub dataset — 450 million lines across 16 log sources — on an Apple M3 Ultra (32 cores).

TF-IDF classifier (Rust)

Dataset	Lines	Time	Speed	LOOK %	Error capture
Thunderbird	211M	139s	1.52M/s	30%	100%
Windows	115M	90s	1.27M/s	1%	100%
HDFS_v2	71M	59s	1.21M/s	20%	100%
Spark	33M	23s	1.44M/s	12%	100%
HDFS	11M	7s	1.62M/s	3%	100%
BGL	4.7M	3.3s	1.46M/s	42%	98.6%
Android	1.6M	1.1s	1.37M/s	12%	100%
SSH	655K	0.5s	1.47M/s	90%	100%
Total	450M	325s	1.38M/s		99.95%

Error capture = percentage of lines with ERROR/FATAL level that the classifier marks as LOOK. At threshold 0.3, the classifier catches 99.95% of all error lines (35.9M of 35.9M) and 100% of warning lines (14.3M of 14.3M). The 0.05% "missed" errors are lines like BGL's repetitive instruction cache parity error corrected — routine hardware telemetry that happens to carry ERROR level but isn't operationally interesting. The classifier learned to skip these.

Model accuracy

Model	Metric	Score
TF-IDF + LogReg	Best CV LOOK F1	0.792
TF-IDF + LogReg	LOOK precision	0.892
BERT-mini	Overall accuracy	0.849
BERT-mini	LOOK F1	0.887

TF-IDF model evaluated with GroupKFold cross-validation (holdout: BGL, Thunderbird). BERT-mini fine-tuned on 4 layers, 256 hidden, 11M params.

End-to-end reduction

flowchart TD
    A["450M log lines<br/>16 datasets, ~67 GB"] --> B["Rust TF-IDF classifier<br/>1.38M lines/sec, 325s"]
    B --> C["~87M LOOK lines (19%)<br/>81% of lines eliminated"]
    C --> D["BERT re-scoring (optional)<br/>demotes 20-40% of TF-IDF LOOK"]
    D --> E["~50-70M final LOOK lines"]
    E --> F["Python tool logic<br/>group errors, search, etc."]
    F --> G["5-50 error groups / search results<br/>fits in LLM context"]

Retraining with your own logs

The classifier ships pre-trained, but you can retrain it on your own logs. The short version:

# 1. Add your logs
head -2000 /var/log/myapp/app.log > data/loghub/MyApp_2k.log

# 2. Label with Claude (Batch API, ~$0.10-0.50 per file)
export ANTHROPIC_API_KEY=sk-ant-...
uv sync --group labeling --group training
uv run python -m scripts.labeling.label_new

# 3. Train and export
uv run --group training python -m scripts.labeling.train_model
uv run --group training python -m scripts.labeling.export_model

# 4. Rebuild Rust classifier
uv pip install -e rust/classifier

See scripts/labeling/RETRAINING.md for the full guide — how labeling works, what features the model uses, how to customize the prompt, and how to train the optional BERT model.