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Agent skill

Cost Session

by ruvnet in ruvnet/ruflo

Per-message cost breakdown within a single session. An agent skill from ruvnet/ruflo.

MITAuto-check: notes

Install Cost Session

skills CLI
$ npx skills add ruvnet/ruflo --skill cost-session -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install ruvnet/ruflo cost-session --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/ruvnet/ruflo.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/ruflo-cost-tracker/skills/cost-session .claude/skills/cost-session && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
cost-session
GitHub stars
74k
Used in
1 other repo
Token cost
~781 tokens
SKILL.md length
288 words
Files
1
Skills in repo
264
Repo updated
First seen
Licence
MIT

At a glance

Per-message cost breakdown within a single session. An agent skill from ruvnet/ruflo.

  • Works in 6 steps: Resolve session jsonl: --session-id… → Parse all assistant messages with usage… → Cost each message via shared PRICING… → …
  • SKILL.md covers Algorithm, Why this matters: cache writes…, Drill-down workflow and Percentile context, plus 2 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Cost Session is an agent skill from ruvnet/ruflo. Per-message cost breakdown within a single session. The drill-down companion to cost-anomaly — when an outlier session is flagged, this surfaces the specific expensive messages so operators can see whether the cost came from output tokens, cache writes, or model escalations.

Its SKILL.md is about 780 tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

The repository describes itself as: 🌊 The original agent harness. Deploy intelligent multi-player swarms, coordinate autonomous workflows, and build conversational AI systems. Features adaptive memory…. The licence is MIT.

Example prompts

  • “/cost-session”

Requirements

  • Pre-approved tools (allowed-tools): Bash

Workflow steps

6 steps, taken from the first numbered list in SKILL.md.

  1. Resolve session jsonl: --session-id (scans ~/.claude/projects/*/)
  2. Parse all assistant messages with usage blocks.
  3. Cost each message via shared PRICING (_prices.mjs).
  4. Sort descending by cost_usd, surface top-N (default 20).
  5. Compute p50/p90/p99 of message costs for in-session percentile context.
  6. Flag the top message if it's >2× the p99 — that's an in-session outlier.

What it can do on your machine

Read from SKILL.md and the folder at commit 6051f67. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves these tools, so the agent can use them without asking each time:

    • Bash

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md (its code samples are bash).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Cost Session loads about 781 tokens when it runs. Until then it costs about 72 tokens; SKILL.md has 288 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~72
When it runs · the whole SKILL.md, loaded when a task matches
~781

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check: notes

The automated check noted patterns worth knowing about, such as sudo or a known installer.

  • NotePre-approves every shell command (allowed-tools: Bash)SKILL.md
    allowed-tools: Bash

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from ruvnet/ruflo at commit 6051f67, republished under its MIT licence (© ruvnet). 288 words, ~781 tokens.

Download SKILL.mdSave it as .claude/skills/cost-session/SKILL.md (or your agent's skills folder).
name
cost-session
description
Per-message cost breakdown within a single session. The drill-down companion to cost-anomaly — when an outlier session is flagged, this surfaces the specific expensive messages so operators can see whether the cost came from output tokens, cache writes, or model escalations.
allowed-tools
Bash
argument-hint
[--session-id <id>] [--top 20] [--since <iso-ts>] [--format table|json]

When cost-anomaly flags a session as a >3.5σ outlier, the next question is "which MESSAGES were expensive?". cost-session answers that.

QuestionSkill
"Which sessions cost the most?"cost-conversation
"Which sessions are outliers?"cost-anomaly
"Which messages in THIS session were expensive?"cost-session ← this

Algorithm

Implementation: scripts/session.mjs.

  1. Resolve session jsonl: --session-id <id> (scans ~/.claude/projects/*/) or --latest (default; picks most-recently-modified jsonl).
  2. Parse all assistant messages with usage blocks.
  3. Cost each message via shared PRICING (_prices.mjs).
  4. Sort descending by cost_usd, surface top-N (default 20).
  5. Compute p50/p90/p99 of message costs for in-session percentile context.
  6. Flag the top message if it's >2× the p99 — that's an in-session outlier.

Why this matters: cache writes are the silent cost

Example real session, top message:

| # | Model    | In | Out | Cache W | Cache R | Cost      |
| 1 | opus-4-7 | 6  | 569 | 881898  | 0       | $16.58    |

Without the Cache W column it looks like "569 output tokens cost $16" — that's wrong by 380×. The actual cost is ephemeral 1h cache write at opus pricing: 881,898 tokens × $18.75/1M = $16.54.

Operators reading the table see immediately: "the model wrote 881K tokens to ephemeral cache". From there the question becomes "why did we cache 881K tokens of context for a 6-input request?" — that's a real engineering signal.

Drill-down workflow

bash
# Step 1: find outliers across all sessions
cost anomaly --alert-on-outliers 1 || cost anomaly  # see which session-ids

# Step 2: drill into the flagged session
cost session --session-id <flagged-id> --top 10

# Step 3: open that jsonl at the timestamp the top message reports,
# inspect the prompt + tool calls

Percentile context

Top of output:

| p50 (median) message | $0.85 |
| p90 message          | $1.45 |
| p99 message          | $1.74 |

Lets operators ask "is this top message a 2× outlier or a 380× one?" without having to compute it themselves. The "top is >2× p99" footer fires when the answer is "yes, this is an in-session outlier worth investigating".

--since filter

Useful for drilling into a specific time range within a long session:

bash
cost session --since 2026-06-16T13:00:00Z --top 5

Only messages with timestamp >= --since are considered.

Edge cases

  • No costed assistant messages → "No costed assistant messages" + exit 0.
  • --session-id not found in any project's jsonls → exit 2 with error.
  • --top must be a positive integer → exit 2.

© ruvnet, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in plugins/ruflo-cost-tracker/skills/cost-session of ruvnet/ruflo.

Open the folder on GitHubat commit 6051f67

Used in 1 other repository

We found 1 copy of this SKILL.md (exact, near-identical or edited) in other folders, from 1 other GitHub owner. This page covers the copy in ruvnet/ruflo, which our catalogue first saw on October 7, 2026.

Compare with similar skills

Cost Session next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Cost Session compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Cost Session this skillruvnet/ruflo74k1 repos~781Automated safety check: NotesMIT
Messages Opsaffaan-m/ECC275k1 repos~724Automated safety check: PassMIT
Cost Trackingaffaan-m/ECC275k1 repos~1.3kAutomated safety check: PassMIT
Cost Breakdownhoangsonww/Claude-Code-Agent-Monitor1.1k1 repos~845Automated safety check: PassMIT
Cost Trackingaffaan-m/ECC275k—~829Automated safety check: PassMIT
Exploring LLM CostsPostHog/posthog40k—~2.9kAutomated safety check: PassCustom licence

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Questions about Cost Session

What does Cost Session do?

Per-message cost breakdown within a single session. An agent skill from ruvnet/ruflo. Cost Session is an agent skill from ruvnet/ruflo. Per-message cost breakdown within a single session.

How do I install Cost Session in Claude Code?

Run `npx skills add ruvnet/ruflo --skill cost-session -a claude-code`. Or copy the skill folder (plugins/ruflo-cost-tracker/skills/cost-session in ruvnet/ruflo) into .claude/skills/cost-session in your project. Claude Code loads it when a task matches its description.

How do I install Cost Session in Codex?

Run `npx skills add ruvnet/ruflo --skill cost-session -a codex`. Or copy the skill folder (plugins/ruflo-cost-tracker/skills/cost-session in ruvnet/ruflo) into .agents/skills/cost-session in your project. Codex loads it when a task matches its description.

Can I use Cost Session in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add ruvnet/ruflo --skill cost-session -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/cost-session, .gemini/skills/cost-session, .github/skills/cost-session and .opencode/skills/cost-session in your project.

What does Cost Session need to run?

SKILL.md names no scripts, command-line tools or credentials: Cost Session is instructions for the agent only. Its frontmatter pre-approves these tools: Bash.

Does Cost Session access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Cost Session safe to install?

Our automated static check of SKILL.md found notes only (pre-approves every shell command (allowed-tools: bash)), nothing it rates as a warning. It is not a guarantee. Review the folder before installing.

What licence does Cost Session use?

Cost Session is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Cost Session use?

About 781 tokens (SKILL.md is roughly 3.1k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Cost Session?

Skills that share tags, products or a category with Cost Session: Messages Ops (affaan-m/ECC, 275k stars), Cost Tracking (affaan-m/ECC, 275k stars), Cost Breakdown (hoangsonww/Claude-Code-Agent-Monitor, 1.1k stars) and Cost Tracking (affaan-m/ECC, 275k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Cost Session?

ruvnet (a GitHub user) maintains it in ruvnet/ruflo, which has 74,089 GitHub stars. The repository holds 264 skills in this directory. The repository was last updated on October 8, 2026.

Source: ruvnet/ruflo on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.