feat(server): let one route forward a gateway key to the gateway's OpenAI and Anthropic APIs

[elyasmnvidian]

What

Developers who reach models through their organization's LLM gateway hold one gateway key that works for every model on it, GPT and Claude alike. When such a developer points their agent at Switchyard, a route that forwards their key to the gateway's GPT endpoint and its Claude endpoint fails to load, even though both endpoints are on the same gateway and accept the same key. This PR lets that route load when all of its forwarding clients point at the same host.

This only matters when Switchyard forwards the developer's own key (forward_auth = true). When Switchyard holds the key itself (api_key_env), a route can already mix OpenAI and Anthropic endpoints, even across two providers, and this PR does not change that.

Who uses gateway keys

Gateway keys belong to developers at organizations that run an LLM gateway, such as a LiteLLM proxy, so that developers never hold OpenAI or Anthropic keys themselves. Anthropic's Claude Code gateway docs give the reasons:

• The provider key stays on the server.
• Usage is attributed to each developer or team.
• Budgets and rate limits apply in one place.
• Offboarding someone revokes one credential.

How a gateway key is set up

Admin, once:  puts the real credentials (OpenAI or Azure, Anthropic, AWS Bedrock, ...) in the gateway config
              issues each developer a gateway key with its allowed models, budget, and rate limits

Developer:    agent ──(gateway key)──▶ gateway ──(gateway's own credentials)──▶ GPT, Claude, ...

• Admin: in LiteLLM, the admin lists each model in the gateway config with its provider credential. Then they create a key per developer or team with POST /key/generate, which sets the key's allowed models, budget, and rate limits (LiteLLM virtual keys).
• Developer: uses the same key in every agent. Codex and pi take it as an OpenAI API key. Claude Code takes it as ANTHROPIC_AUTH_TOKEN, with ANTHROPIC_BASE_URL set to the gateway. Anthropic's connect page says that variable is sent "in Authorization: Bearer". So all three agents send the same Authorization: Bearer <gateway key> header.

The gateway key is neither an OpenAI key nor an Anthropic key. Only the gateway accepts it.

Where Switchyard fits

The developer points the agent at Switchyard instead of the gateway. Switchyard does not check the key. It makes its own calls to the gateway, and each [llm_clients] entry sets which key those calls carry:

Without Switchyard:        agent ──(gateway key)──────────▶ gateway
Switchyard + api_key_env:  agent ──(gateway key, dropped)─▶ Switchyard ──(server's key)──────▶ gateway
Switchyard + forward_auth: agent ──(gateway key)──────────▶ Switchyard ──(same key, copied)──▶ gateway

• api_key_env = "VAR": the Switchyard server holds one gateway key in an environment variable and sends it on every call. The gateway sees one user for everyone: all usage counts against that key, and each developer's budget and model list stop applying. Because Switchyard does not check callers, anyone who can reach the server can use that key. Mixing OpenAI and Anthropic endpoints in one route already works in this mode; the cost is that the gateway can't tell developers apart.
• forward_auth = true: the server holds no key. Switchyard copies the developer's own credential headers onto every upstream call, judge calls included. The gateway still tracks usage per developer and applies that person's budget and rate limits, and a 429 stays with that developer instead of putting the model in cooldown for everyone. Forwarding clients do not follow redirects, so the key reaches only the configured URL.

Setting both on one client is a config error. The shipped Codex configs use forward_auth the same way for ChatGPT logins, which also belong to one person.

A route that uses both GPT and Claude sends the developer's key to two endpoints of the same gateway, for example a GPT judge and a Claude answer:

agent ──(gateway key)──▶ Switchyard ─┬─(same key)─▶ gateway /v1/responses ──▶ GPT judge
                                     └─(same key)─▶ gateway /v1/messages  ──▶ Claude answer

The problem this PR fixes

Switchyard's config check sees an OpenAI-format client (openai_responses) and an Anthropic-format client (anthropic_messages) in one forwarding route and treats them as two providers, so the config fails to load:

route claude_route cannot forward both Anthropic and OpenAI caller credentials

The check exists for a real case. If one route forwarded a ChatGPT login to both OpenAI and Anthropic, Anthropic would see the token:

agent ──(ChatGPT token)──▶ Switchyard ─┬─▶ chatgpt.com        accepts it
                                       └─▶ api.anthropic.com  would return 401, but would have seen the token

To prevent that, Switchyard puts each forwarding client into one of two credential families by its format: OpenAI (openai_chat, openai_responses) or Anthropic (anthropic_messages). It refuses a route that has both. Clients with api_key_env never count, because they send the server's key, not the caller's. The family also sets which callers a forwarding route serves: an OpenAI route serves /v1/chat/completions and /v1/responses callers, and an Anthropic route serves /v1/messages callers.

The check treats the format as the provider. On a gateway that is wrong: both clients point at the same gateway, which accepts the same key on both endpoints. The key goes to one service, yet the route fails to load.

What changes

A forwarding route may now mix the two families when all of its forwarding clients use the same scheme, host, and port in base_url. The path does not count, so https://gateway.example.com and https://gateway.example.com/v1 match. This config now loads:

[llm_clients.gateway_responses]
format = "openai_responses"
base_url = "https://gateway.example.com/v1"
forward_auth = true

[llm_clients.gateway_messages]
format = "anthropic_messages"
base_url = "https://gateway.example.com"
forward_auth = true

A mixed route serves Chat Completions and Responses callers. It forwards their Authorization: Bearer key unchanged to every forwarding client, including the anthropic_messages clients. A /v1/messages caller gets 400 before any upstream call. Claude Code may send its key as x-api-key, and an OpenAI-format client does not send x-api-key as the credential; it passes it along as an ordinary header.

Mixing the families across different hosts, ports, or schemes still fails. The error now lists the origins and both ways to fix it: point the clients at one origin, or give one provider's clients a server key:

route claude_route cannot forward both Anthropic and OpenAI caller credentials to different origins (https://api.anthropic.com, https://gateway.example.com); point all of its forwarding clients at one origin (same scheme, host, and port), such as an LLM gateway, or set api_key_env instead of forward_auth on one provider's clients

Which routes this PR changes, from switchyard-server --dry-run on main and on this branch. Each route has a GPT judge on openai_responses and Claude on anthropic_messages:

Key	Hosts	main	This PR
Server key (api_key_env)	One gateway	Loads	Loads
Server keys (api_key_env)	api.openai.com and api.anthropic.com	Loads	Loads
Forwarded key (forward_auth)	One gateway	Fails	Loads
Forwarded key (forward_auth)	api.openai.com and api.anthropic.com	Fails	Fails, with the new error
Forwarded OpenAI key, server key for Anthropic	api.openai.com and api.anthropic.com	Loads	Loads

What stays the same: there is no new config key, and every config that loads today loads the same way. Routes that use server keys were never checked. A route that does not mix families behaves as before, so a passthrough route on the anthropic_messages client above still serves /v1/messages callers such as Claude Code.

Why

A gateway route needs both formats when Claude answers, because Claude gets the caller's reasoning effort only through /v1/messages. On the gateway I tested, Claude returns 400 when Switchyard sends the effort in OpenAI form (reasoning_effort on /v1/chat/completions, reasoning.effort on /v1/responses): "thinking.type.enabled" is not supported for this model. On /v1/messages, Switchyard sends thinking: {"type": "adaptive"} plus output_config.effort, and the gateway accepts it. Caching is not the reason: /v1/chat/completions caches Claude prompts too.

So today a gateway user can forward their key or send Claude the effort, not both. Claude on openai_chat with omit_body_fields = ["reasoning_effort"] uses the developer's key but never gets the effort. Claude on anthropic_messages with api_key_env gets the effort, but then the server holds one gateway key that every caller shares, and the gateway can no longer track usage or apply limits per developer.

The new rule still keeps the developer's key on one host. When every forwarding client of a route uses the same scheme, host, and port, the key can reach only that host, which the operator configured.

Notes for reviewers

Start with build_route_clients in crates/switchyard-runner/src/config.rs. The server's per-request check reads the credential family that this function returns, so no server code changed. As with any forward_auth client today, the check cannot tell whether that one host should receive the caller's login; the operator decides that.

The second commit brings the docs, doc comments, and the error for forwarding clients on different origins in line with this description. Several of them said that every backend reachable through a forwarding route must use one family. Only forwarding backends must, and each doc that states the rule now names the gateway case and says server keys are not limited.

#874 says in the pi and Oh My Pi guides that a forwarding route cannot use both formats. Whichever PR merges second should add the one-host exception there.

Tests

• route_on_one_host_forwards_the_bearer_token_to_responses_and_messages (crates/switchyard-server/tests/server.rs): a stub gateway serves /v1/responses and /v1/messages on one host. A composite route has its judge on an openai_responses client (base URL with /v1) and its tiers on an anthropic_messages client (no path), both forwarding. One Responses call with Authorization: Bearer gateway-key reaches both endpoints, and each call carries that header exactly once, unchanged.
• forwarding_route_mixes_formats_only_on_one_host (crates/switchyard-runner/src/config.rs): on one host, the mixed route gets the OpenAI credential family, and a passthrough route on the Messages client keeps the Anthropic family. A different host, port, or scheme fails, and the error names it. The test fails if the rule refuses every mix, as main does, or if it drops the host check.

anthropic_client_forwards_oauth_when_configured already covers a Messages-only forwarding route and the 400 for a caller on the wrong API.

Live run

Host names are replaced with gateway.example.com, and the gateway's model IDs with public model IDs. The config held no key. Both clients pointed at a local logging proxy in front of the gateway. The proxy recorded header names and SHA-256 prefixes of credential headers, never their values. Each caller sent Authorization: Bearer <gateway key>.

[llm_clients.gateway_responses]
format = "openai_responses"
base_url = "http://127.0.0.1:18731/v1"
forward_auth = true
max_retries = 0

[llm_clients.gateway_messages]
format = "anthropic_messages"
base_url = "http://127.0.0.1:18731"
forward_auth = true
max_retries = 0

[targets]
capable = { id = "claude-opus-5-5", llm_client = "gateway_messages" }
efficient = { id = "claude-sonnet-5", llm_client = "gateway_messages" }
judge = { id = "gpt-5.6-terra", llm_client = "gateway_responses" }

[routes.claude_route]
id = "claude-route"
type = "composite"
classifier = { target = "judge", base_threshold = 0.5, classify_trigger = "user_turn", message_hash_fallback = true }
stage = { capable_target = "capable", efficient_target = "efficient", confidence_threshold = 0.5 }

[routes.sonnet_route]
id = "sonnet-route"
type = "passthrough"
target = "efficient"

--dry-run with both clients on https://gateway.example.com, then with the Messages client moved to https://api.anthropic.com. I reran these after the error message changed:

$ switchyard-server --config one-host.toml --dry-run
server OK: claude-route, sonnet-route
$ switchyard-server --config second-host.toml --dry-run
invalid server config second-host.toml: route claude_route cannot forward both Anthropic and OpenAI caller credentials to different origins (https://api.anthropic.com, https://gateway.example.com); point all of its forwarding clients at one origin (same scheme, host, and port), such as an LLM gateway, or set api_key_env instead of forward_auth on one provider's clients: route claude_route cannot forward both ...

The error appears twice because of the existing error-chain printing. The same config with both clients on /v1 also loads, and two loopback ports fail with the same error.

#	Caller endpoint	Mode	Route	Served model	Result	Upstream calls
A	/v1/responses, reasoning.effort: high	stream	claude-route	claude-sonnet-5	200, answer 24, 0 reasoning tokens	judge /v1/responses, answer /v1/messages
A2	/v1/responses, reasoning.effort: high	stream	claude-route	claude-sonnet-5	200, reasoning item, 200 reasoning tokens, answer 105	judge /v1/responses, answer /v1/messages
B	/v1/chat/completions	buffered	claude-route	claude-sonnet-5	200, 1073, finish_reason stop	judge /v1/responses, answer /v1/messages
C	/v1/messages	buffered	claude-route	none	400 route claude-route forwards an OpenAI login; call it through /v1/chat/completions or /v1/responses	none
D	/v1/messages	buffered	sonnet-route	claude-sonnet-5	200, **Hallo!**	answer /v1/messages

Across the 7 upstream calls, each call carried exactly one authorization whose hash matched the caller's, and none carried x-api-key. Only the /v1/messages calls carried anthropic-version: 2023-06-01. For A and A2, the /v1/messages body had thinking: {"type": "adaptive"} and output_config: {"effort": "high"}. With adaptive thinking, Claude skipped thinking on A's easy question and thought on A2's.

Summary by CodeRabbit

• New Features
  • Forwarding routes can now combine OpenAI and Anthropic clients when all clients share the same scheme, host, and port.
  • These mixed routes serve Chat Completions and Responses requests, forwarding the caller’s bearer token to each client.
• Behavior Changes
  • Requests using an API that the route does not serve return HTTP 400 before being forwarded.
  • Mixed-family routes pointing to different origins are rejected during configuration.
• Documentation
  • Updated setup and integration guides to describe the routing and compatibility rules.

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Walkthrough

Forwarding routes can combine OpenAI and Anthropic credential families when forwarding clients share a scheme, host, and port. Configuration tests cover accepted and rejected origins. Server integration tests check bearer-token forwarding. Documentation describes supported APIs and configured-key behavior.

Changes

Credential forwarding

Layer / File(s)	Summary
Mixed-family route configuration crates/switchyard-runner/src/config.rs, crates/libsy-llm-client/src/backend.rs, crates/libsy-llm-client/src/client.rs, crates/libsy-llm-client/README.md, docs/reference/toml_schema.md	Configuration accepts mixed credential families when forwarding clients share a scheme, host, and port. Tests cover shared origins with different paths and reject different hosts, ports, or schemes. The documentation describes the credential-family rules and configured-key exception.
Request forwarding and API handling crates/switchyard-server/tests/server.rs, crates/switchyard-server/README.md, crates/switchyard-nemo-relay-plugin/README.md, docs/getting_started.md, docs/integrations/nemo_relay.md, docs/reference/toml_schema.md	Integration tests check that a caller’s bearer token reaches both endpoints on a mixed-format route. Documentation describes the supported APIs, forwarded token, and handling of requests through unsupported APIs.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 434d0

The change appears mergeable with bounded documentation and error-message corrections. Operators could otherwise be misled when configuring a route or determining which credential an upstream client sends.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 90.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 4 files. (6 skipped: 6 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: one route can forward a gateway key to both OpenAI and Anthropic APIs. It is specific and related to the changeset, although it omits the same-origin restr…

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---

_{A rabbit checks the routes at night
OpenAI and Anthropic share the site
One token travels, plain and true
Responses and Messages make it through
The moon approves the origins too}

_{Comment @coderabbitai help to get the list of available commands.}

[elyasmnvidian]

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @crates/switchyard-runner/src/config.rs:
- Line 410: Update the configuration error in the route validation assertion to
say the forwarding clients use different origins, and clarify that they must
share the same scheme, host, and port. Keep the existing guidance about using a
gateway or setting api_key_env.

Review comments at @docs/reference/toml_schema.md:
- Line 137: Update the bearer-token wording to specify forwarding clients, since
api_key_env clients use the server’s key. In docs/reference/toml_schema.md at
line 137, crates/libsy-llm-client/README.md at line 248, and
crates/switchyard-server/README.md at line 101, replace “every client” or “every
backend” with “every forwarding client” or “every forwarding backend,”
respectively; make the same wording change in docs/getting_started.md at lines
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• crates/switchyard-runner/src/config.rs
• crates/switchyard-server/README.md
• crates/switchyard-server/tests/server.rs
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