feat(sessions): replace sendHandoff resume path with acpx native session continuation

backend/src/acpx/session-manager.test.ts

@@ -96,12 +96,13 @@ describe('AcpxSessionManager', () => {
   })
 
   describe('getEndpointSupport', () => {
-    it('reports session/new, session/prompt, session/update as implemented', () => {
+    it('reports session/new, session/prompt, session/update, session/resume as implemented', () => {
       const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
       const support = mgr.getEndpointSupport()
       expect(support.implemented).toContain('session/new')
       expect(support.implemented).toContain('session/prompt')
       expect(support.implemented).toContain('session/update')
+      expect(support.implemented).toContain('session/resume')
     })
   })
 
@@ -257,4 +258,97 @@ describe('AcpxSessionManager', () => {
       expect(details.messages[0].content).toContain('Assistant: Hi')
     })
   })
+
+  describe('continueSession', () => {
+    it('creates a new session using --from <acpxSessionId> and returns a UUID', async () => {
+      vi.mocked(spawn)
+        // continueSession spawn
+        .mockReturnValueOnce(
+          makeSpawnMock({ stdout: ['session-id: continued-456'] }) as ReturnType<typeof spawn>
+        )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      const sessionId = await mgr.continueSession('source-acpx-id', null)
+
+      expect(sessionId).toMatch(/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i)
+    })
+
+    it('registers the continued session so ownsSession returns true', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ stdout: ['session-id: continued-456'] }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      const sessionId = await mgr.continueSession('source-acpx-id', null)
+
+      expect(mgr.ownsSession(sessionId)).toBe(true)
+    })
+
+    it('passes --from flag to acpx when continuing', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ stdout: ['session-id: cont-999'] }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      await mgr.continueSession('my-source-session-id', null)
+
+      const spawnCall = vi.mocked(spawn).mock.calls[0]!
+      expect(spawnCall[1]).toContain('--from')
+      expect(spawnCall[1]).toContain('my-source-session-id')
+    })
+
+    it('throws when acpx fails to return a session id', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ exitCode: 1 }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      await expect(mgr.continueSession('source-id', null)).rejects.toThrow(
+        'Native continuation is unavailable'
+      )
+    })
+
+    it('uses the project path as cwd when a project is provided', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ stdout: ['session-id: cwd-test'] }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      const project = { id: 'p1', name: 'My Project', path: '/my/project' }
+      await mgr.continueSession('source-id', project)
+
+      const spawnCall = vi.mocked(spawn).mock.calls[0]!
+      expect(spawnCall[1]).toContain('--cwd')
+      expect(spawnCall[1]).toContain('/my/project')
+    })
+  })
+
+  describe('getAgentSessionId', () => {
+    it('returns the acpx session id for a known session', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ stdout: ['session-id: acpx-xyz'] }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      const sessionId = await mgr.newSession(null)
+
+      expect(mgr.getAgentSessionId(sessionId)).toBe('acpx-xyz')
+    })
+
+    it('returns null for an unknown session id', () => {
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      expect(mgr.getAgentSessionId('unknown-id')).toBeNull()
+    })
+
+    it('returns null when acpx session creation failed and no acpxSessionId was stored', async () => {
+      vi.mocked(spawn).mockReturnValueOnce(
+        makeSpawnMock({ exitCode: 1 }) as ReturnType<typeof spawn>
+      )
+
+      const mgr = new AcpxSessionManager('opencode', 'OpenCode', 'opencode')
+      const sessionId = await mgr.newSession(null)
+
+      expect(mgr.getAgentSessionId(sessionId)).toBeNull()
+    })
+  })
 })

backend/src/acpx/session-manager.ts

@@ -38,8 +38,8 @@ interface AcpxSessionState {
 
 const ENDPOINT_SUPPORT: BackendEndpointSupport = {
   source: 'connection',
-  implemented: ['session/new', 'session/prompt', 'session/update'],
-  unknown: ['session/load', 'session/resume', 'session/fork'],
+  implemented: ['session/new', 'session/prompt', 'session/update', 'session/resume'],
+  unknown: ['session/load', 'session/fork'],
 }
 
 /**
@@ -95,6 +95,40 @@ export class AcpxSessionManager implements SessionAdapter {
     return sessionId
   }
 
+  async continueSession(
+    fromAgentSessionId: string,
+    project: SessionProjectContext | null
+  ): Promise<string> {
+    const sessionId = randomUUID()
+    const cwd = project?.path ?? process.cwd()
+
+    // Create a new acpx session that inherits context from the source session:
+    // acpx <agentCommand> sessions new --from <acpx-session-id> --cwd <cwd>
+    const acpxSessionId = await this.runAcpxContinueSession(fromAgentSessionId, cwd)
+
+    if (!acpxSessionId) {
+      throw new Error(
+        `acpx failed to create a continued session from '${fromAgentSessionId}' — ` +
+          `no session id was returned. Native continuation is unavailable.`
+      )
+    }
+
+    this.sessions.set(sessionId, {
+      id: sessionId,
+      acpxSessionId,
+      agentCommand: this.agentCommand,
+      cwd,
+      title: 'New chat',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+      project,
+      messages: [],
+      proc: null,
+    })
+
+    return sessionId
+  }
+
   async sendMessage(sessionId: string, text: string): Promise<void> {
     const session = this.sessions.get(sessionId)
     if (!session) throw new Error(`Session not found: ${sessionId}`)
@@ -140,6 +174,11 @@ export class AcpxSessionManager implements SessionAdapter {
     await this.sendMessage(sessionId, handoffPrompt)
   }
 
+  getAgentSessionId(internalSessionId: string): string | null {
+    const session = this.sessions.get(internalSessionId)
+    return session?.acpxSessionId ?? null
+  }
+
   closeSession(sessionId: string): void {
     const session = this.sessions.get(sessionId)
     if (session?.proc && !session.proc.killed) {
@@ -197,6 +236,38 @@ export class AcpxSessionManager implements SessionAdapter {
     })
   }
 
+  /**
+   * Run `acpx <agentCommand> sessions new --from <fromAcpxSessionId> --cwd <cwd>` and parse
+   * the new session id from stdout. Returns null when acpx outputs no recognisable session id.
+   */
+  private async runAcpxContinueSession(
+    fromAgentSessionId: string,
+    cwd: string
+  ): Promise<string | null> {
+    return new Promise((resolve) => {
+      const proc = spawn(
+        'acpx',
+        [this.agentCommand, 'sessions', 'new', '--from', fromAgentSessionId, '--cwd', cwd],
+        {
+          stdio: ['ignore', 'pipe', 'pipe'],
+          env: { ...process.env },
+        }
+      )
+
+      let output = ''
+      proc.stdout?.on('data', (chunk: Buffer) => {
+        output += chunk.toString()
+      })
+
+      proc.on('close', () => {
+        const match = /session[- ]id:\s*(\S+)/i.exec(output) ?? /^(\S+)$/m.exec(output.trim())
+        resolve(match ? (match[1] ?? null) : null)
+      })
+
+      proc.on('error', () => resolve(null))
+    })
+  }
+
   /**
    * Stream a prompt through `acpx --format json <agentCommand> prompt "<text>"`
    * and call `onLine` for each parsed ACP NDJSON line.