diff --git a/.changeset/fix-anthropic-compatible-custom-provider.md b/.changeset/fix-anthropic-compatible-custom-provider.md
new file mode 100644
index 0000000000..227f5ac008
--- /dev/null
+++ b/.changeset/fix-anthropic-compatible-custom-provider.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix anthropic-compatible custom providers failing with "No API provider registered for api: anthropic".
+
+`resolveCustomProviderApiType` mapped the `anthropic-compatible` provider type to the api key `"anthropic"`, but pi-ai registers the Anthropic Messages API under `"anthropic-messages"`. Any custom provider configured as `anthropic-compatible` (self-hosted Claude proxy, gateway, etc.) therefore selected a model whose `api` did not match a registered provider and threw at stream time. Mapped it to `"anthropic-messages"` and added a regression assertion alongside the existing openai-compatible / openai-responses coverage.
diff --git a/packages/engine/src/__tests__/pi-create-fn-agent.test.ts b/packages/engine/src/__tests__/pi-create-fn-agent.test.ts
index 321358c385..05c8c5e68d 100644
--- a/packages/engine/src/__tests__/pi-create-fn-agent.test.ts
+++ b/packages/engine/src/__tests__/pi-create-fn-agent.test.ts
@@ -1363,6 +1363,14 @@ describe("createFnAgent", () => {
         apiKey: "RESPONSES_API_KEY",
         models: [{ id: "responses-model", name: "Responses Model" }],
       },
+      {
+        id: "770e8400-e29b-41d4-a716-446655440002",
+        name: "Custom Anthropic",
+        apiType: "anthropic-compatible",
+        baseUrl: "https://anthropic.example",
+        apiKey: "ANTHROPIC_API_KEY",
+        models: [{ id: "anthropic-model", name: "Anthropic Model" }],
+      },
     ] as any);
 
     const { createFnAgent } = await import("../pi.js");
@@ -1387,6 +1395,15 @@ describe("createFnAgent", () => {
       apiKey: "RESPONSES_API_KEY",
       models: [expect.objectContaining({ id: "responses-model", name: "Responses Model" })],
     }));
+    // anthropic-compatible must map to pi-ai's registered "anthropic-messages"
+    // api key, NOT bare "anthropic" (which throws "No API provider registered
+    // for api: anthropic" at stream time). Regression guard for that bug.
+    expect(registerProviderMock).toHaveBeenCalledWith("custom-anthropic", expect.objectContaining({
+      baseUrl: "https://anthropic.example",
+      api: "anthropic-messages",
+      apiKey: "ANTHROPIC_API_KEY",
+      models: [expect.objectContaining({ id: "anthropic-model", name: "Anthropic Model" })],
+    }));
   });
 
   it("avoids lock-based SettingsManager.create when loading extension providers", async () => {
diff --git a/packages/engine/src/pi.ts b/packages/engine/src/pi.ts
index 84ee5cc048..84dd590f0f 100644
--- a/packages/engine/src/pi.ts
+++ b/packages/engine/src/pi.ts
@@ -1006,9 +1006,13 @@ export interface AgentOptions {
   permanentAgentGating?: PermanentAgentGatingContext;
 }
 
-function resolveCustomProviderApiType(apiType: string): "anthropic" | "openai-responses" | "openai-completions" {
+function resolveCustomProviderApiType(apiType: string): "anthropic-messages" | "openai-responses" | "openai-completions" {
   if (apiType === "anthropic-compatible") {
-    return "anthropic";
+    // pi-ai registers the Anthropic Messages API under the key
+    // "anthropic-messages" (see @earendil-works/pi-ai register-builtins).
+    // Returning bare "anthropic" throws "No API provider registered for
+    // api: anthropic" at stream time, so map to the real registry key.
+    return "anthropic-messages";
   }
   if (apiType === "openai-responses") {
     return "openai-responses";