diff --git a/src/acp.test.ts b/src/acp.test.ts
index 1bf4def..43eaca1 100644
--- a/src/acp.test.ts
+++ b/src/acp.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach } from "vitest";
+import { describe, it, expect, beforeEach, vi } from "vitest";
 import {
   Agent,
   ClientSideConnection,
@@ -24,6 +24,8 @@ import {
   SessionNotification,
   PROTOCOL_VERSION,
   ndJsonStream,
+  RequestError,
+  ErrorCode,
 } from "./acp.js";
 
 describe("Connection", () => {
@@ -1105,4 +1107,52 @@ describe("Connection", () => {
     });
     expect(loadResponse).toEqual({});
   });
+
+  it("logs RESOURCE_NOT_FOUND at debug level", async () => {
+    const debugSpy = vi.spyOn(console, "debug").mockImplementation(() => {});
+    const errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+
+    const client: Client = {
+      readTextFile: () => Promise.reject(RequestError.resourceNotFound()),
+      writeTextFile: () => Promise.resolve({}),
+      requestPermission: () =>
+        Promise.resolve({ outcome: { outcome: "selected", optionId: "allow" } }),
+      sessionUpdate: () => Promise.resolve(),
+    };
+
+    const agent: Agent = {
+      initialize: () =>
+        Promise.resolve({
+          protocolVersion: PROTOCOL_VERSION,
+          agentCapabilities: { loadSession: false },
+        }),
+      newSession: () => Promise.resolve({ sessionId: "s1" }),
+      authenticate: () => Promise.resolve(),
+      prompt: () => Promise.resolve({ stopReason: "end_turn" }),
+      cancel: () => Promise.resolve(),
+    };
+
+    new ClientSideConnection(
+      () => client,
+      ndJsonStream(clientToAgent.writable, agentToClient.readable),
+    );
+    const conn = new AgentSideConnection(
+      () => agent,
+      ndJsonStream(agentToClient.writable, clientToAgent.readable),
+    );
+
+    await expect(
+      conn.readTextFile({ path: "/test.txt", sessionId: "s1" }),
+    ).rejects.toThrow();
+
+    expect(debugSpy).toHaveBeenCalledWith(
+      "Error handling request",
+      expect.anything(),
+      expect.objectContaining({ code: ErrorCode.RESOURCE_NOT_FOUND }),
+    );
+    expect(errorSpy).not.toHaveBeenCalled();
+
+    debugSpy.mockRestore();
+    errorSpy.mockRestore();
+  });
 });
diff --git a/src/acp.ts b/src/acp.ts
index 7abacb9..aacb0a0 100644
--- a/src/acp.ts
+++ b/src/acp.ts
@@ -882,6 +882,16 @@ class Connection {
     }
   }
 
+  #logError(context: string, message: AnyMessage, error: ErrorResponse) {
+    // RESOURCE_NOT_FOUND is expected when checking file existence
+    // before write, since ACP has no stat API
+    if (error.code === ErrorCode.RESOURCE_NOT_FOUND) {
+      console.debug(context, message, error);
+    } else {
+      console.error(context, message, error);
+    }
+  }
+
   async #processMessage(message: AnyMessage) {
     if ("method" in message && "id" in message) {
       // It's a request
@@ -890,7 +900,7 @@ class Connection {
         message.params,
       );
       if ("error" in response) {
-        console.error("Error handling request", message, response.error);
+        this.#logError("Error handling request", message, response.error);
       }
 
       await this.#sendMessage({
@@ -905,7 +915,7 @@ class Connection {
         message.params,
       );
       if ("error" in response) {
-        console.error("Error handling notification", message, response.error);
+        this.#logError("Error handling notification", message, response.error);
       }
     } else if ("id" in message) {
       // It's a response
@@ -1038,6 +1048,19 @@ class Connection {
   }
 }
 
+/**
+ * JSON-RPC error codes used by the ACP protocol.
+ */
+export const ErrorCode = {
+  PARSE_ERROR: -32700,
+  INVALID_REQUEST: -32600,
+  METHOD_NOT_FOUND: -32601,
+  INVALID_PARAMS: -32602,
+  INTERNAL_ERROR: -32603,
+  AUTH_REQUIRED: -32000,
+  RESOURCE_NOT_FOUND: -32002,
+} as const;
+
 /**
  * JSON-RPC error object.
  *