docs(proxy): say how spend logs record which requests a client-forwarded OAuth token paid for

docs/proxy/forward_client_headers.md

@@ -112,7 +112,7 @@ general_settings:
   forward_llm_provider_auth_headers: true  # Enable BYOK
 ```
 
-For **Claude Code**, see [Claude Code BYOK](../tutorials/claude_code_byok.md). Use `ANTHROPIC_CUSTOM_HEADERS="x-litellm-api-key: $LITELLM_API_KEY"` to pass your LiteLLM key. A configured Anthropic API key is sent as `x-api-key` and needs `forward_llm_provider_auth_headers` above to be forwarded; `/login` instead sends an OAuth token as `Authorization: Bearer <token>`, which LiteLLM forwards regardless of this setting.
+For **Claude Code**, see [Claude Code BYOK](../tutorials/claude_code_byok.md). Use `ANTHROPIC_CUSTOM_HEADERS="x-litellm-api-key: $LITELLM_API_KEY"` to pass your LiteLLM key. A configured Anthropic API key is sent as `x-api-key` and needs `forward_llm_provider_auth_headers` above to be forwarded; `/login` instead sends an OAuth token as `Authorization: Bearer <token>`, which LiteLLM forwards regardless of this setting. Each spend log row records which credential the upstream call used in `metadata.used_client_oauth_token` (`true` for a forwarded OAuth token, `false` for the deployment's configured key, never the token itself), filterable on the Logs page **Credential** dropdown or with `GET /spend/logs/ui?used_client_oauth_token=true`.
 
 Client request:
 ```bash

docs/tutorials/claude_code_max_subscription.md

@@ -312,6 +312,21 @@ curl -X POST "http://localhost:4000/key/generate" \
   }'
 ```
 
+### Seeing Which Requests Were Billed to a Seat
+
+Every spend log row records which credential the upstream call used in `metadata.used_client_oauth_token`: `true` when the request went to Anthropic with the developer's forwarded OAuth token (the Max seat paid for it), `false` when it went out with the deployment's configured `api_key`. The token itself is never written to the log. Rows written by a LiteLLM version before this field existed have no value
+
+`spend` stays at the model's list price on both kinds of rows, so budgets and rate limits keep working across seat-billed and key-billed traffic. To get the real API bill, subtract the seat-billed rows
+
+Filter the Logs page at `http://localhost:4000/ui/?page=logs` with the **Credential** dropdown (**Client OAuth token** or **Configured key**); the row's detail drawer shows the same value under Request Details. The same filter is available on the spend logs API:
+
+```bash showLineNumbers title="List Seat-Billed Requests"
+curl "http://localhost:4000/spend/logs/ui?used_client_oauth_token=true" \
+  -H "Authorization: Bearer $LITELLM_MASTER_KEY"
+```
+
+Pass `used_client_oauth_token=false` for the requests the configured key paid for
+
 ## Troubleshooting
 
 ### OAuth Token Not Being Forwarded