Anthropic

The built-in anthropic provider is shunt’s default destination: any model with no matching route is forwarded to api.anthropic.com unchanged (configurable via server.default_provider). It is a kind = "anthropic" provider — the upstream already speaks the Anthropic Messages API, so shunt passes the request through without translation and only decides which credential goes upstream.

Quick start

Let a coding agent wire it up for you — shunt add prints an embedded setup blueprint (offline and read-only; the agent edits the config, the command never does):

shunt add upstream anthropic --print | claude

Or follow the manual steps below.

Configure the upstream

anthropic is built in — with no config at all, shunt already forwards unrouted models to it as passthrough. To declare it explicitly (for ordered failover or to change auth), use the preset, which supplies kind = "anthropic", base_url = "https://api.anthropic.com", and auth = "passthrough":

[[upstreams]]
name = "anthropic"
provider = "anthropic"

Explicit fields override preset defaults. The legacy [providers.anthropic] table form remains supported — but do not mix [[upstreams]] and [providers.*] in one file.

Passthrough (default)

Nothing else to configure: shunt forwards the client’s own x-api-key or Authorization header. No server-side secret belongs in the config. Which credential the client should send is covered in Connect Claude Code.

Server-side API key

Inject an operator-owned key instead of trusting callers to bring one:

[[upstreams]]
name = "anthropic"
provider = "anthropic"
auth = { mode = "api_key", env = "ANTHROPIC_API_KEY", header = "x_api_key" }

If you inject a server-side credential, gate the gateway with [server.auth] before exposing it beyond loopback.

Claude subscription accounts (claude_oauth)

Create a refreshable shunt-managed account once, then scope the upstream to it:

shunt login claude --name primary --mode oauth

[[upstreams]]
name = "anthropic"
provider = "anthropic"
auth = { mode = "claude_oauth", account = "primary" }

shunt login claude has three modes: --mode oauth runs shunt’s own refreshable OAuth flow (recommended), --mode import copies the current Claude Code login, and --mode setup-token creates a one-year inference-only token. Omit account (and accounts) to scan the whole shunt-managed account store — with several accounts in the store this becomes a load-balanced pool with session stickiness and quota-aware rotation. The full pooling behavior — explicit account entries, per-account thresholds, [server.pool] tuning, and failover — is documented in Anthropic Multi-Account.

Models & routing

As the default provider, anthropic needs no routes — everything you did not map elsewhere lands here. To pin a model to it explicitly inside an ordered failover chain:

[[models]]
id = "claude-sonnet-5"
display_name = "Claude Sonnet 5"

[models.upstream_model]
anthropic = "claude-sonnet-5"

The map key is the [[upstreams]].name, not the preset id. With auto_include_builtin_models = true (the default), GET /v1/models also advertises shunt’s builtin Claude catalog — see Model Discovery.

Verify

shunt check    # -> config ok
shunt run
curl -sS http://127.0.0.1:3001/v1/models

Then send one minimal request (with passthrough, include your own credential; with claude_oauth, omit it):

curl -sS http://127.0.0.1:3001/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H 'anthropic-version: 2023-06-01' \
  -H 'content-type: application/json' \
  -d '{"model":"claude-sonnet-5","max_tokens":16,"messages":[{"role":"user","content":"Reply with OK."}]}'

Related

• Anthropic Multi-Account — account pool, quota-aware selection, [server.pool]
• Admin & Remote Provisioning — provision accounts from a browser
• Configuration Reference — every account key