diff --git a/README.md b/README.md
index e57f723..9032c21 100644
--- a/README.md
+++ b/README.md
@@ -82,6 +82,38 @@ The plugin checks these in order:
 1. macOS Keychain (all `Claude Code-credentials*` entries — multiple accounts are detected automatically)
 2. `~/.claude/.credentials.json` (fallback, works on all platforms)
 
+## Custom base URL / Proxy support
+
+You can route Anthropic API requests through a proxy or custom endpoint by configuring the `baseURL` option in your `opencode.json`:
+
+```json
+{
+  "provider": {
+    "anthropic": {
+      "options": {
+        "baseURL": "https://your-proxy.example.com/v1",
+        "headers": {
+          "X-Custom-Header": "value"
+        }
+      }
+    }
+  },
+  "plugin": ["opencode-claude-auth@latest"]
+}
+```
+
+The plugin respects the following priority (highest to lowest):
+
+1. `provider.anthropic.options.baseURL` from `opencode.json`
+2. `ANTHROPIC_BASE_URL` environment variable
+3. Default: `https://api.anthropic.com/v1`
+
+Any custom headers you define in `provider.anthropic.options.headers` are automatically forwarded to all API requests. This is useful for:
+
+- Using an HTTP proxy or API gateway
+- Adding authentication headers (e.g., `X-Gateway-Key`)
+- Routing through a load balancer or monitoring proxy
+
 ## Multiple accounts (macOS)
 
 If you have [multiple Claude Code accounts](https://gist.github.com/KMJ-007/0979814968722051620461ab2aa01bf2) authenticated on macOS, the plugin detects all of them from the Keychain automatically. Each account is labeled by its subscription tier (Claude Pro, Claude Max, etc.).
@@ -182,6 +214,7 @@ All configurable parameters can be overridden via environment variables. If Anth
 
 | Variable                            | Description                                                                                                                                                                            | Default                                                                                                 |
 | ----------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
+| `ANTHROPIC_BASE_URL`                | Custom base URL for Anthropic API requests (e.g., proxy or custom endpoint). Takes precedence over config file.                                                                        | `https://api.anthropic.com/v1`                                                                          |
 | `ANTHROPIC_CLI_VERSION`             | Claude CLI version for user-agent and billing headers                                                                                                                                  | `2.1.80`                                                                                                |
 | `ANTHROPIC_USER_AGENT`              | Full User-Agent string (overrides CLI version)                                                                                                                                         | `claude-cli/{version} (external, cli)`                                                                  |
 | `ANTHROPIC_BETA_FLAGS`              | Comma-separated beta feature flags                                                                                                                                                     | `claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,prompt-caching-scope-2026-01-05` |
diff --git a/src/index.test.ts b/src/index.test.ts
index 91a2ab9..15c5a6c 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -1201,3 +1201,285 @@ describe("refreshIfNeeded — token expiry", () => {
     )
   })
 })
+
+describe("custom baseURL support via provider.options.baseURL and ANTHROPIC_BASE_URL env", () => {
+  it("uses default baseURL when no config or env var is set", async () => {
+    const originalSetInterval = globalThis.setInterval
+    const originalHome = process.env.HOME
+    const originalFetch = globalThis.fetch
+    const originalEnv = process.env.ANTHROPIC_BASE_URL
+
+    delete process.env.ANTHROPIC_BASE_URL
+    const tempHome = await mkdtemp(join(tmpdir(), "opencode-claude-auth-baseurl-"))
+    process.env.HOME = tempHome
+    Date.now = () => 1_700_000_000_000
+    globalThis.setInterval = (() => ({ unref() {} })) as unknown as typeof setInterval
+
+    let forwardedInput: RequestInfo | URL | undefined
+
+    try {
+      const { helpersModule } = await loadHelpersWithCountingKeychain(
+        Date.now() + 10 * 60_000,
+      )
+      globalThis.fetch = (async (input: RequestInfo | URL) => {
+        forwardedInput = input
+        return new Response("ok")
+      }) as typeof fetch
+
+      const plugin = await helpersModule.default({} as never)
+      const typedPlugin = plugin as { auth?: { loader?: TestAuthLoader } }
+      const authConfig = await typedPlugin.auth!.loader!(
+        async () => ({
+          type: "oauth",
+          refresh: "refresh",
+          access: "access",
+          expires: Date.now() + 60_000,
+        }),
+        { models: {} },
+      )
+
+      await authConfig.fetch("https://api.anthropic.com/v1/messages", {
+        method: "POST",
+        body: JSON.stringify({ model: "claude-haiku-4-5", messages: [] }),
+      })
+
+      assert.equal(
+        String(forwardedInput),
+        "https://api.anthropic.com/v1/messages?beta=true",
+        "Should use default baseURL when no override is set",
+      )
+    } finally {
+      rmSync(tempHome, { recursive: true, force: true })
+      globalThis.setInterval = originalSetInterval
+      globalThis.fetch = originalFetch
+      if (typeof originalHome === "string") {
+        process.env.HOME = originalHome
+      } else {
+        delete process.env.HOME
+      }
+      if (typeof originalEnv === "string") {
+        process.env.ANTHROPIC_BASE_URL = originalEnv
+      } else {
+        delete process.env.ANTHROPIC_BASE_URL
+      }
+    }
+  })
+
+  it("uses provider.options.baseURL when provided", async () => {
+    const originalSetInterval = globalThis.setInterval
+    const originalHome = process.env.HOME
+    const originalFetch = globalThis.fetch
+
+    const tempHome = await mkdtemp(join(tmpdir(), "opencode-claude-auth-baseurl-"))
+    process.env.HOME = tempHome
+    Date.now = () => 1_700_000_000_000
+    globalThis.setInterval = (() => ({ unref() {} })) as unknown as typeof setInterval
+
+    try {
+      const { helpersModule } = await loadHelpersWithCountingKeychain(
+        Date.now() + 10 * 60_000,
+      )
+      globalThis.fetch = (async () => new Response("ok")) as typeof fetch
+
+      const plugin = await helpersModule.default({} as never)
+      const typedPlugin = plugin as { auth?: { loader?: TestAuthLoader } }
+
+      const customProvider = {
+        models: { "claude-haiku-4-5": { cost: 0 } },
+        options: { baseURL: "https://proxy.example.com/v1" },
+      } as unknown as { models: Record<string, { cost?: unknown }> }
+
+      const authConfigWithBaseURL = await typedPlugin.auth!.loader!(
+        async () => ({
+          type: "oauth",
+          refresh: "refresh",
+          access: "access",
+          expires: Date.now() + 60_000,
+        }),
+        customProvider,
+      )
+
+      assert.equal(
+        (authConfigWithBaseURL as { baseURL?: string }).baseURL,
+        "https://proxy.example.com/v1",
+        "Should use provider.options.baseURL when provided",
+      )
+    } finally {
+      rmSync(tempHome, { recursive: true, force: true })
+      globalThis.setInterval = originalSetInterval
+      globalThis.fetch = originalFetch
+      if (typeof originalHome === "string") {
+        process.env.HOME = originalHome
+      } else {
+        delete process.env.HOME
+      }
+    }
+  })
+
+  it("uses ANTHROPIC_BASE_URL env var when set and no provider.options.baseURL", async () => {
+    const originalSetInterval = globalThis.setInterval
+    const originalHome = process.env.HOME
+    const originalFetch = globalThis.fetch
+    const originalEnv = process.env.ANTHROPIC_BASE_URL
+
+    process.env.ANTHROPIC_BASE_URL = "https://gateway.example.com/anthropic/v1"
+    const tempHome = await mkdtemp(join(tmpdir(), "opencode-claude-auth-baseurl-"))
+    process.env.HOME = tempHome
+    Date.now = () => 1_700_000_000_000
+    globalThis.setInterval = (() => ({ unref() {} })) as unknown as typeof setInterval
+
+    try {
+      const { helpersModule } = await loadHelpersWithCountingKeychain(
+        Date.now() + 10 * 60_000,
+      )
+      globalThis.fetch = (async () => new Response("ok")) as typeof fetch
+
+      const plugin = await helpersModule.default({} as never)
+      const typedPlugin = plugin as { auth?: { loader?: TestAuthLoader } }
+      const authConfig = await typedPlugin.auth!.loader!(
+        async () => ({
+          type: "oauth",
+          refresh: "refresh",
+          access: "access",
+          expires: Date.now() + 60_000,
+        }),
+        { models: {} },
+      )
+
+      assert.equal(
+        (authConfig as { baseURL?: string }).baseURL,
+        "https://gateway.example.com/anthropic/v1",
+        "Should use ANTHROPIC_BASE_URL when env var is set",
+      )
+    } finally {
+      rmSync(tempHome, { recursive: true, force: true })
+      globalThis.setInterval = originalSetInterval
+      globalThis.fetch = originalFetch
+      if (typeof originalHome === "string") {
+        process.env.HOME = originalHome
+      } else {
+        delete process.env.HOME
+      }
+      if (typeof originalEnv === "string") {
+        process.env.ANTHROPIC_BASE_URL = originalEnv
+      } else {
+        delete process.env.ANTHROPIC_BASE_URL
+      }
+    }
+  })
+
+  it("prioritizes provider.options.baseURL over ANTHROPIC_BASE_URL env var", async () => {
+    const originalSetInterval = globalThis.setInterval
+    const originalHome = process.env.HOME
+    const originalFetch = globalThis.fetch
+    const originalEnv = process.env.ANTHROPIC_BASE_URL
+
+    process.env.ANTHROPIC_BASE_URL = "https://gateway.example.com/anthropic/v1"
+    const tempHome = await mkdtemp(join(tmpdir(), "opencode-claude-auth-baseurl-"))
+    process.env.HOME = tempHome
+    Date.now = () => 1_700_000_000_000
+    globalThis.setInterval = (() => ({ unref() {} })) as unknown as typeof setInterval
+
+    try {
+      const { helpersModule } = await loadHelpersWithCountingKeychain(
+        Date.now() + 10 * 60_000,
+      )
+      globalThis.fetch = (async () => new Response("ok")) as typeof fetch
+
+      const plugin = await helpersModule.default({} as never)
+      const typedPlugin = plugin as { auth?: { loader?: TestAuthLoader } }
+
+      const customProvider = {
+        models: { "claude-haiku-4-5": { cost: 0 } },
+        options: { baseURL: "https://config-priority.example.com/v1" },
+      } as unknown as { models: Record<string, { cost?: unknown }> }
+
+      const authConfig = await typedPlugin.auth!.loader!(
+        async () => ({
+          type: "oauth",
+          refresh: "refresh",
+          access: "access",
+          expires: Date.now() + 60_000,
+        }),
+        customProvider,
+      )
+
+      assert.equal(
+        (authConfig as { baseURL?: string }).baseURL,
+        "https://config-priority.example.com/v1",
+        "Should prioritize provider.options.baseURL over ANTHROPIC_BASE_URL",
+      )
+    } finally {
+      rmSync(tempHome, { recursive: true, force: true })
+      globalThis.setInterval = originalSetInterval
+      globalThis.fetch = originalFetch
+      if (typeof originalHome === "string") {
+        process.env.HOME = originalHome
+      } else {
+        delete process.env.HOME
+      }
+      if (typeof originalEnv === "string") {
+        process.env.ANTHROPIC_BASE_URL = originalEnv
+      } else {
+        delete process.env.ANTHROPIC_BASE_URL
+      }
+    }
+  })
+
+  it("does not double the /v1 path segment", async () => {
+    const originalSetInterval = globalThis.setInterval
+    const originalHome = process.env.HOME
+    const originalFetch = globalThis.fetch
+    const originalEnv = process.env.ANTHROPIC_BASE_URL
+
+    process.env.ANTHROPIC_BASE_URL = "https://proxy.example.com/v1"
+    const tempHome = await mkdtemp(join(tmpdir(), "opencode-claude-auth-baseurl-"))
+    process.env.HOME = tempHome
+    Date.now = () => 1_700_000_000_000
+    globalThis.setInterval = (() => ({ unref() {} })) as unknown as typeof setInterval
+
+    try {
+      const { helpersModule } = await loadHelpersWithCountingKeychain(
+        Date.now() + 10 * 60_000,
+      )
+      globalThis.fetch = (async () => new Response("ok")) as typeof fetch
+
+      const plugin = await helpersModule.default({} as never)
+      const typedPlugin = plugin as { auth?: { loader?: TestAuthLoader } }
+      const authConfig = await typedPlugin.auth!.loader!(
+        async () => ({
+          type: "oauth",
+          refresh: "refresh",
+          access: "access",
+          expires: Date.now() + 60_000,
+        }),
+        { models: {} },
+      )
+
+      const baseURL = (authConfig as { baseURL?: string }).baseURL
+      assert.ok(
+        baseURL && !baseURL.includes("/v1/v1"),
+        `baseURL should not contain doubled /v1 path: ${baseURL}`,
+      )
+      assert.equal(
+        baseURL,
+        "https://proxy.example.com/v1",
+        "baseURL should be correctly set from ANTHROPIC_BASE_URL env var",
+      )
+    } finally {
+      rmSync(tempHome, { recursive: true, force: true })
+      globalThis.setInterval = originalSetInterval
+      globalThis.fetch = originalFetch
+      if (typeof originalHome === "string") {
+        process.env.HOME = originalHome
+      } else {
+        delete process.env.HOME
+      }
+      if (typeof originalEnv === "string") {
+        process.env.ANTHROPIC_BASE_URL = originalEnv
+      } else {
+        delete process.env.ANTHROPIC_BASE_URL
+      }
+    }
+  })
+})
diff --git a/src/index.ts b/src/index.ts
index d60cfb1..0bb1d61 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -318,9 +318,14 @@ const plugin: Plugin = async () => {
           modelCount: Object.keys(provider.models).length,
         })
 
+        const effectiveBaseURL =
+          provider.options?.baseURL ||
+          process.env.ANTHROPIC_BASE_URL ||
+          "https://api.anthropic.com/v1"
+
         return {
           apiKey: "",
-          baseURL: "https://api.anthropic.com/v1",
+          baseURL: effectiveBaseURL,
           async fetch(input: RequestInfo | URL, init?: RequestInit) {
             const latest = getCachedCredentials()
             if (!latest) {