diff --git a/CHANGELOG.md b/CHANGELOG.md
index 71b4785..81047ed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@ All notable changes to this project are documented in this file. The format is b
 
 - **Optional custom User-Agent for LLM requests**: an LLM profile can set a `User-Agent` value that overrides the header on every outbound LLM request; left empty, the default User-Agent is unchanged. The value is trimmed and rejected if it contains control characters or exceeds 256 bytes. Part of #154 (the fixed-full-URL mode is not included).
 
+### Docs
+
+- **LLM base URL note in the README**: for the Anthropic format, enter the host without a `/v1` suffix (the provider appends `/v1/messages`, so a `/v1` base becomes `/v1/v1/messages`); OpenAI-format providers append `/chat/completions`, so their base does end in `/v1`. Follow-up to #154.
+
 ## [0.5.1] - 2026-11-05
 
 Login rate limiting per client IP with trusted-proxy support, a 12-character minimum for new passwords, a loopback-only default for the binary, and the JWT signing key in its own Docker volume.
diff --git a/README.md b/README.md
index da3f6c2..ae481dd 100644
--- a/README.md
+++ b/README.md
@@ -256,6 +256,8 @@ CGO_ENABLED=0 go build -tags embedui -o artex ./cmd/artex
 **LLM**: `export ANTHROPIC_API_KEY=sk-...` (or `OPENAI_API_KEY`); you can also fill it in on the UI's "LLM Configuration" page — which can add a local Ollama server as a keyless provider (format "Ollama", base URL default `http://127.0.0.1:11434/v1`, models loaded from the server).
 Optional: `ARTEX_LLM_PROVIDER` (anthropic | openai | openai-responses | ollama) / `ARTEX_LLM_MODEL` / `ARTEX_LLM_BASE_URL` / `ARTEX_LLM_PROXY`.
 
+**Base URL**: for the Anthropic format, enter the host **without** a `/v1` suffix (e.g. `https://api.anthropic.com`) — the provider appends `/v1/messages` itself, so a base ending in `/v1` becomes `https://api.anthropic.com/v1/v1/messages`. OpenAI-format providers append `/chat/completions`, so their base **does** end in `/v1` (e.g. `https://api.openai.com/v1`).
+
 **Concurrency**: the number of work agents per task is configured in "System Settings" (default 3).
 
 **Common flags**: `./start.sh -addr :8787 -proxy :8788` (`-addr` frontend+API, `-proxy` traffic recording proxy). The startup script passes arguments through to `artex` verbatim.