docs update

Author: vabruzzo

README.md

@@ -68,6 +68,10 @@ The harness uses the [Claude Agent SDK](https://pypi.org/project/claude-agent-sd
 
 You can also set `base_url` in your config to point at a custom Anthropic-compatible endpoint.
 
+If no API key is set, the SDK will use your Claude Code subscription credentials from `~/.claude/credentials.json` (requires Claude Pro/Max). Usage is covered by your subscription with rate limits rather than per-token billing.
+
+> **Cost reporting caveat:** Cost figures in `run_meta.json` and the web UI come from the SDK and are based on Anthropic's list pricing regardless of provider. They may not match your actual bill (especially on OpenRouter, Bedrock, or Vertex) and are purely informational when using a Claude Code subscription.
+
 Example configs:
 
 ```yaml

docs/guide/configuration.md

@@ -90,6 +90,17 @@ sessions:
 | Anthropic | `anthropic` | `ANTHROPIC_API_KEY` | Direct Anthropic API |
 | AWS Bedrock | `bedrock` | AWS credentials | Sets `CLAUDE_CODE_USE_BEDROCK=1` |
 | GCP Vertex AI | `vertex` | GCP credentials | Sets `CLAUDE_CODE_USE_VERTEX=1` |
+| Claude Code subscription | `anthropic` | *(none needed)* | If no `ANTHROPIC_API_KEY` is set, the SDK uses your Claude Code subscription credentials from `~/.claude/credentials.json`. Usage is covered by your subscription (Pro/Max) with rate limits rather than per-token billing. |
+
+### Cost reporting
+
+Cost figures shown in `run_meta.json`, `harness inspect`, and the web UI come from the Claude Agent SDK's `total_cost_usd` field, which is calculated using Anthropic's list pricing regardless of which provider you use. This means:
+
+- **OpenRouter** — reported cost reflects Anthropic list prices, not your actual OpenRouter bill (which may differ)
+- **Bedrock / Vertex** — reported cost may not match AWS or GCP billing
+- **Claude Code subscription** — cost is reported but you're not actually billed per-token
+
+Treat cost figures as rough estimates, not authoritative billing data.
 
 ## Automatic behaviors