fix(providers): route agentrouter through anthropic endpoint headers

Update the AgentRouter provider registry to use the Claude-compatible
messages API and required Anthropic-style authentication headers. This
bypasses unauthorized_client_error responses and exposes the supported
model list through passthrough configuration.

Also update the changelog and release PR notes to document the fix for
#1921

CHANGELOG.md

@@ -21,6 +21,7 @@
 ### 🐛 Bug Fixes
 
 - **fix(providers):** allow local OpenAI-compatible endpoints (like Ollama) to be added without an API key (fixes #1893)
+- **fix(providers):** bypass AgentRouter unauthorized_client_error by spoofing Claude CLI headers via Anthropic endpoints (fixes #1921)
 - **fix(copilot):** emit compatible reasoning text deltas (#1919 — thanks @ivan-mezentsev)
 - **fix(api-manager):** show validation errors inline in modals, not behind (#1920 — thanks @andrewmunsell)
 - **fix(compression):** align seeded standard savings combo with stacked default, preserve stacked defaults, and secure metadata routes.

open-sse/config/providerRegistry.ts

@@ -672,17 +672,39 @@ export const REGISTRY: Record<string, RegistryEntry> = {
   agentrouter: {
     id: "agentrouter",
     alias: "agentrouter",
-    format: "openai",
+    format: "claude",
     executor: "default",
-    baseUrl: "https://agentrouter.org/v1/chat/completions",
+    baseUrl: "https://agentrouter.org/v1/messages",
     authType: "apikey",
-    authHeader: "bearer",
+    authHeader: "x-api-key",
     defaultContextLength: 128000,
     headers: {
-      "HTTP-Referer": "https://endpoint-proxy.local",
-      "X-Title": "OmniRoute",
+      "Anthropic-Version": ANTHROPIC_VERSION_HEADER,
+      "Anthropic-Beta": ANTHROPIC_BETA_CLAUDE_OAUTH,
+      "Anthropic-Dangerous-Direct-Browser-Access": "true",
+      "User-Agent": CLAUDE_CLI_USER_AGENT,
+      "X-App": "cli",
+      "X-Stainless-Helper-Method": "stream",
+      "X-Stainless-Retry-Count": "0",
+      "X-Stainless-Runtime-Version": CLAUDE_CLI_STAINLESS_RUNTIME_VERSION,
+      "X-Stainless-Package-Version": CLAUDE_CLI_STAINLESS_PACKAGE_VERSION,
+      "X-Stainless-Runtime": "node",
+      "X-Stainless-Lang": "js",
+      "X-Stainless-Arch": mapStainlessArch(),
+      "X-Stainless-Os": mapStainlessOs(),
+      "X-Stainless-Timeout": "600",
     },
-    models: [{ id: "auto", name: "Auto (Best Available)" }],
+    models: [
+      { id: "claude-haiku-4-5-20251001", name: "Claude 4.5 Haiku" },
+      { id: "claude-opus-4-6", name: "Claude 4.6 Opus" },
+      { id: "deepseek-r1-0528", name: "DeepSeek R1 0528" },
+      { id: "deepseek-v3.1", name: "DeepSeek V3.1" },
+      { id: "deepseek-v3.2", name: "DeepSeek V3.2" },
+      { id: "glm-4.5", name: "GLM 4.5" },
+      { id: "glm-4.6", name: "GLM 4.6" },
+      { id: "glm-5.1", name: "GLM 5.1" },
+    ],
+    passthroughModels: true,
   },
 
   openrouter: {

scripts/scratch/pr_body.md

@@ -15,6 +15,7 @@
 ### 🐛 Bug Fixes
 
 - **fix(providers):** allow local OpenAI-compatible endpoints (like Ollama) to be added without an API key (fixes #1893)
+- **fix(providers):** bypass AgentRouter unauthorized_client_error by spoofing Claude CLI headers via Anthropic endpoints (fixes #1921)
 - **fix(copilot):** emit compatible reasoning text deltas (#1919 — thanks @ivan-mezentsev)
 - **fix(api-manager):** show validation errors inline in modals, not behind (#1920 — thanks @andrewmunsell)
 - **fix(compression):** align seeded standard savings combo with stacked default, preserve stacked defaults, and secure metadata routes.