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..2d2b169ba6 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 +1395,40 @@ describe("createFnAgent", () => {
       apiKey: "RESPONSES_API_KEY",
       models: [expect.objectContaining({ id: "responses-model", name: "Responses Model" })],
     }));
+    /*
+    FNXC:CustomProviders 2026-06-21-13:45:
+    Invariant (FN-5893 surface = providers/execution paths): every custom-provider apiType must map to an api key pi-ai's registry actually registers. anthropic-compatible maps to "anthropic-messages", NOT bare "anthropic" — the latter registered fine but threw "No API provider registered for api: anthropic" the moment a task streamed. Assert the corrected value AND that the broken bare key is never used, so a future regression to "anthropic" fails here.
+    */
+    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" })],
+    }));
+    // Negative guard: the unregistered bare "anthropic" api key must never be emitted for any provider.
+    expect(registerProviderMock).not.toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({ api: "anthropic" }),
+    );
+    // Invariant: every api key handed to registerProvider must be one pi-ai registers
+    // (mirrors @earendil-works/pi-ai register-builtins). Catches a typo in any arm.
+    const PI_AI_REGISTERED_APIS = new Set([
+      "anthropic-messages",
+      "openai-completions",
+      "openai-responses",
+      "azure-openai-responses",
+      "openai-codex-responses",
+      "google-generative-ai",
+      "google-vertex",
+      "mistral-conversations",
+      "bedrock-converse-stream",
+    ]);
+    for (const [, config] of registerProviderMock.mock.calls) {
+      const api = (config as { api?: string } | undefined)?.api;
+      if (typeof api === "string") {
+        expect(PI_AI_REGISTERED_APIS.has(api)).toBe(true);
+      }
+    }
   });
 
   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..84c2bdac7e 100644
--- a/packages/engine/src/pi.ts
+++ b/packages/engine/src/pi.ts
@@ -1006,9 +1006,23 @@ export interface AgentOptions {
   permanentAgentGating?: PermanentAgentGatingContext;
 }
 
-function resolveCustomProviderApiType(apiType: string): "anthropic" | "openai-responses" | "openai-completions" {
+/**
+ * Map a user-facing custom-provider `apiType` to the pi-ai api-registry key.
+ *
+ * FNXC:CustomProviders 2026-06-21-13:45:
+ * Every arm must return a key that pi-ai's api-registry actually registers
+ * (see @earendil-works/pi-ai register-builtins). `anthropic-compatible` resolves
+ * to "anthropic-messages" — the key the Anthropic Messages API is registered
+ * under. The bare "anthropic" key is never registered, so returning it let a
+ * provider register but threw "No API provider registered for api: anthropic"
+ * the moment a task tried to stream.
+ *
+ * @param apiType - the custom provider's declared compatibility type.
+ * @returns the registered pi-ai api key to stream against.
+ */
+function resolveCustomProviderApiType(apiType: string): "anthropic-messages" | "openai-responses" | "openai-completions" {
   if (apiType === "anthropic-compatible") {
-    return "anthropic";
+    return "anthropic-messages";
   }
   if (apiType === "openai-responses") {
     return "openai-responses";