feat(acp): add resumeSessionId to sessions_spawn for ACP session resume (#41847)

* feat(acp): add resumeSessionId to sessions_spawn for ACP session resume

Thread resumeSessionId through the ACP session spawn pipeline so agents
can resume existing sessions (e.g. a prior Codex conversation) instead
of starting fresh.

Flow: sessions_spawn tool → spawnAcpDirect → initializeSession →
ensureSession → acpx --resume-session flag → agent session/load

- Add resumeSessionId param to sessions-spawn-tool schema with
  description so agents can discover and use it
- Thread through SpawnAcpParams → AcpInitializeSessionInput →
  AcpRuntimeEnsureInput → acpx extension runtime
- Pass as --resume-session flag to acpx CLI
- Error hard (exit 4) on non-existent session, no silent fallback
- All new fields optional for backward compatibility

Depends on acpx >= 0.1.16 (openclaw/acpx#85, merged, pending release).

Tests: 26/26 pass (runtime + tool schema)
Verified e2e: Discord → sessions_spawn(resumeSessionId) → Codex
resumed session and recalled stored secret.

🤖 AI-assisted

* fix: guard resumeSessionId against non-ACP runtime

Add early-return error when resumeSessionId is passed without
runtime="acp" (mirrors existing streamTo guard). Without this,
the parameter is silently ignored and the agent gets a fresh
session instead of resuming.

Also update schema description to note the runtime=acp requirement.

Addresses Greptile review feedback.

* ACP: add changelog entry for session resume (#41847) (thanks @pejmanjohn)

---------

Co-authored-by: Pejman Pour-Moezzi <481729+pejmanjohn@users.noreply.github.com>
Co-authored-by: Onur <onur@textcortex.com>

src/acp/control-plane/manager.core.ts

@@ -234,6 +234,7 @@ export class AcpSessionManager {
             sessionKey,
             agent,
             mode: input.mode,
+            resumeSessionId: input.resumeSessionId,
             cwd: requestedCwd,
           }),
         fallbackCode: "ACP_SESSION_INIT_FAILED",

src/acp/control-plane/manager.types.ts

@@ -43,6 +43,7 @@ export type AcpInitializeSessionInput = {
   sessionKey: string;
   agent: string;
   mode: AcpRuntimeSessionMode;
+  resumeSessionId?: string;
   cwd?: string;
   backendId?: string;
 };

src/acp/runtime/types.ts

@@ -35,6 +35,7 @@ export type AcpRuntimeEnsureInput = {
   sessionKey: string;
   agent: string;
   mode: AcpRuntimeSessionMode;
+  resumeSessionId?: string;
   cwd?: string;
   env?: Record<string, string>;
 };

src/agents/acp-spawn.ts

@@ -56,6 +56,7 @@ export type SpawnAcpParams = {
   task: string;
   label?: string;
   agentId?: string;
+  resumeSessionId?: string;
   cwd?: string;
   mode?: SpawnAcpMode;
   thread?: boolean;
@@ -426,6 +427,7 @@ export async function spawnAcpDirect(
       sessionKey,
       agent: targetAgentId,
       mode: runtimeMode,
+      resumeSessionId: params.resumeSessionId,
       cwd: params.cwd,
       backendId: cfg.acp?.backend,
     });

src/agents/tools/sessions-spawn-tool.test.ts

@@ -163,6 +163,43 @@ describe("sessions_spawn tool", () => {
     );
   });
 
+  it("passes resumeSessionId through to ACP spawns", async () => {
+    const tool = createSessionsSpawnTool({
+      agentSessionKey: "agent:main:main",
+    });
+
+    await tool.execute("call-2c", {
+      runtime: "acp",
+      task: "resume prior work",
+      agentId: "codex",
+      resumeSessionId: "7f4a78e0-f6be-43fe-855c-c1c4fd229bc4",
+    });
+
+    expect(hoisted.spawnAcpDirectMock).toHaveBeenCalledWith(
+      expect.objectContaining({
+        task: "resume prior work",
+        agentId: "codex",
+        resumeSessionId: "7f4a78e0-f6be-43fe-855c-c1c4fd229bc4",
+      }),
+      expect.any(Object),
+    );
+  });
+
+  it("rejects resumeSessionId without runtime=acp", async () => {
+    const tool = createSessionsSpawnTool({
+      agentSessionKey: "agent:main:main",
+    });
+
+    const result = await tool.execute("call-guard", {
+      task: "resume prior work",
+      resumeSessionId: "7f4a78e0-f6be-43fe-855c-c1c4fd229bc4",
+    });
+
+    expect(JSON.stringify(result)).toContain("resumeSessionId is only supported for runtime=acp");
+    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+    expect(hoisted.spawnAcpDirectMock).not.toHaveBeenCalled();
+  });
+
   it("rejects attachments for ACP runtime", async () => {
     const tool = createSessionsSpawnTool({
       agentSessionKey: "agent:main:main",

src/agents/tools/sessions-spawn-tool.ts

@@ -25,6 +25,12 @@ const SessionsSpawnToolSchema = Type.Object({
   label: Type.Optional(Type.String()),
   runtime: optionalStringEnum(SESSIONS_SPAWN_RUNTIMES),
   agentId: Type.Optional(Type.String()),
+  resumeSessionId: Type.Optional(
+    Type.String({
+      description:
+        'Resume an existing agent session by its ID (e.g. a Codex session UUID from ~/.codex/sessions/). Requires runtime="acp". The agent replays conversation history via session/load instead of starting fresh.',
+    }),
+  ),
   model: Type.Optional(Type.String()),
   thinking: Type.Optional(Type.String()),
   cwd: Type.Optional(Type.String()),
@@ -91,6 +97,7 @@ export function createSessionsSpawnTool(
       const label = typeof params.label === "string" ? params.label.trim() : "";
       const runtime = params.runtime === "acp" ? "acp" : "subagent";
       const requestedAgentId = readStringParam(params, "agentId");
+      const resumeSessionId = readStringParam(params, "resumeSessionId");
       const modelOverride = readStringParam(params, "model");
       const thinkingOverrideRaw = readStringParam(params, "thinking");
       const cwd = readStringParam(params, "cwd");
@@ -127,6 +134,13 @@ export function createSessionsSpawnTool(
         });
       }
 
+      if (resumeSessionId && runtime !== "acp") {
+        return jsonResult({
+          status: "error",
+          error: `resumeSessionId is only supported for runtime=acp; got runtime=${runtime}`,
+        });
+      }
+
       if (runtime === "acp") {
         if (Array.isArray(attachments) && attachments.length > 0) {
           return jsonResult({
@@ -140,6 +154,7 @@ export function createSessionsSpawnTool(
             task,
             label: label || undefined,
             agentId: requestedAgentId,
+            resumeSessionId,
             cwd,
             mode: mode && ACP_SPAWN_MODES.includes(mode) ? mode : undefined,
             thread,