diff --git a/src/acp-agent.ts b/src/acp-agent.ts
index bf362a69..adead543 100644
--- a/src/acp-agent.ts
+++ b/src/acp-agent.ts
@@ -1554,7 +1554,8 @@ export class ClaudeAcpAgent implements Agent {
       if (
         creationOpts.resume &&
         error instanceof Error &&
-        error.message === "Query closed before response received"
+        (error.message === "Query closed before response received" ||
+          error.message.includes("No conversation found with session ID"))
       ) {
         throw RequestError.resourceNotFound(sessionId);
       }
diff --git a/src/tests/session-load.test.ts b/src/tests/session-load.test.ts
new file mode 100644
index 00000000..33b91c8a
--- /dev/null
+++ b/src/tests/session-load.test.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect } from "vitest";
+import { randomUUID } from "crypto";
+import { AgentSideConnection, RequestError, SessionNotification } from "@agentclientprotocol/sdk";
+import { query, getSessionMessages } from "@anthropic-ai/claude-agent-sdk";
+import { ClaudeAcpAgent } from "../acp-agent.js";
+import { Pushable } from "../utils.js";
+import type { SDKUserMessage } from "@anthropic-ai/claude-agent-sdk";
+
+function createMockClient(): AgentSideConnection {
+  return {
+    sessionUpdate: async (_notification: SessionNotification) => {},
+    requestPermission: async () => ({ outcome: { outcome: "cancelled" } }),
+    readTextFile: async () => ({ content: "" }),
+    writeTextFile: async () => ({}),
+  } as unknown as AgentSideConnection;
+}
+
+describe.skipIf(!process.env.RUN_INTEGRATION_TESTS)("session load/resume lifecycle", () => {
+  it("SDK: session created but never prompted has no messages and is not resumable", async () => {
+    // Create a session via the SDK, initialize it, but never send a prompt
+    const sessionId = randomUUID();
+    const input = new Pushable<SDKUserMessage>();
+
+    const q = query({
+      prompt: input,
+      options: {
+        systemPrompt: { type: "preset", preset: "claude_code" },
+        sessionId,
+        settingSources: ["user", "project", "local"],
+        includePartialMessages: true,
+      },
+    });
+
+    // initializationResult() works without needing a prompt pushed
+    const initResult = await q.initializationResult();
+    expect(initResult).toBeDefined();
+
+    // Close without ever prompting
+    input.end();
+    q.return(undefined);
+
+    // Verify no messages were stored
+    const messages = await getSessionMessages(sessionId);
+    expect(messages).toEqual([]);
+
+    // Verify the session is not resumable
+    const input2 = new Pushable<SDKUserMessage>();
+    const q2 = query({
+      prompt: input2,
+      options: {
+        systemPrompt: { type: "preset", preset: "claude_code" },
+        resume: sessionId,
+        settingSources: ["user", "project", "local"],
+        includePartialMessages: true,
+      },
+    });
+
+    await expect(q2.initializationResult()).rejects.toThrow(
+      /No conversation found with session ID/,
+    );
+
+    input2.end();
+    q2.return(undefined);
+  }, 30000);
+
+  it("ACP: loadSession throws resourceNotFound for a non-existent session", async () => {
+    const agent = new ClaudeAcpAgent(createMockClient());
+    const bogusSessionId = randomUUID();
+
+    try {
+      await expect(
+        agent.loadSession({
+          sessionId: bogusSessionId,
+          cwd: process.cwd(),
+          mcpServers: [],
+        }),
+      ).rejects.toThrow(RequestError);
+    } finally {
+      await agent.dispose();
+    }
+  }, 30000);
+
+  it("ACP: resumeSession throws resourceNotFound for a non-existent session", async () => {
+    const agent = new ClaudeAcpAgent(createMockClient());
+    const bogusSessionId = randomUUID();
+
+    try {
+      await expect(
+        agent.unstable_resumeSession({
+          sessionId: bogusSessionId,
+          cwd: process.cwd(),
+          mcpServers: [],
+        }),
+      ).rejects.toThrow(RequestError);
+    } finally {
+      await agent.dispose();
+    }
+  }, 30000);
+
+  it("ACP: newSession without prompt, then loadSession on fresh agent throws resourceNotFound", async () => {
+    // Step 1: Create a real session via ACP, never prompt, dispose
+    const agentA = new ClaudeAcpAgent(createMockClient());
+    const { sessionId } = await agentA.newSession({
+      cwd: process.cwd(),
+      mcpServers: [],
+    });
+    expect(sessionId).toBeDefined();
+    await agentA.dispose();
+
+    // Step 2: Fresh agent tries to load that session
+    const agentB = new ClaudeAcpAgent(createMockClient());
+
+    try {
+      await expect(
+        agentB.loadSession({
+          sessionId,
+          cwd: process.cwd(),
+          mcpServers: [],
+        }),
+      ).rejects.toThrow(RequestError);
+    } finally {
+      await agentB.dispose();
+    }
+  }, 30000);
+
+  it("ACP: newSession without prompt, then resumeSession on fresh agent throws resourceNotFound", async () => {
+    const agentA = new ClaudeAcpAgent(createMockClient());
+    const { sessionId } = await agentA.newSession({
+      cwd: process.cwd(),
+      mcpServers: [],
+    });
+    await agentA.dispose();
+
+    const agentB = new ClaudeAcpAgent(createMockClient());
+
+    try {
+      await expect(
+        agentB.unstable_resumeSession({
+          sessionId,
+          cwd: process.cwd(),
+          mcpServers: [],
+        }),
+      ).rejects.toThrow(RequestError);
+    } finally {
+      await agentB.dispose();
+    }
+  }, 30000);
+});