diff --git a/docs/commands/agents.md b/docs/commands/agents.md
index f54133a36f0..694ca08c49b 100644
--- a/docs/commands/agents.md
+++ b/docs/commands/agents.md
@@ -31,6 +31,7 @@ netlify agents
 | [`agents:commit`](/commands/agents#agentscommit) | Commit an agent run’s changes directly to a branch  |
 | [`agents:create`](/commands/agents#agentscreate) | Create and start a new agent run on your site  |
 | [`agents:diff`](/commands/agents#agentsdiff) | Print the code changes produced by an agent run  |
+| [`agents:follow-up`](/commands/agents#agentsfollow-up) | Send a follow-up prompt to an existing agent run  |
 | [`agents:list`](/commands/agents#agentslist) | List agent runs for the current site  |
 | [`agents:open`](/commands/agents#agentsopen) | Open the agent run preview, dashboard, or pull request in a browser  |
 | [`agents:pr`](/commands/agents#agentspr) | Open a pull request for an agent run  |
@@ -49,6 +50,7 @@ netlify agents
 netlify agents:create --prompt "Add a contact form"
 netlify agents:list --status running
 netlify agents:show 60c7c3b3e7b4a0001f5e4b3a --watch
+netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a "Also add tests"
 netlify agents:diff 60c7c3b3e7b4a0001f5e4b3a
 netlify agents:open 60c7c3b3e7b4a0001f5e4b3a
 ```
@@ -191,6 +193,41 @@ netlify agents:diff 60c7c3b3e7b4a0001f5e4b3a --session 70d8... --cumulative
 netlify agents:diff 60c7c3b3e7b4a0001f5e4b3a --no-color | less
 ```
 
+---
+## `agents:follow-up`
+
+Send a follow-up prompt to an existing agent run
+
+**Usage**
+
+```bash
+netlify agents:follow-up
+```
+
+**Arguments**
+
+- id - agent run ID to follow up on
+- prompt - the follow-up prompt
+
+**Flags**
+
+- `agent` (*string*) - override agent type for this session
+- `attach` (*string*) - attach a file or image (repeatable)
+- `filter` (*string*) - For monorepos, specify the name of the application to run the command in
+- `json` (*boolean*) - output result as JSON
+- `model` (*string*) - override model for this session
+- `project` (*string*) - project ID or name (if not in a linked directory)
+- `prompt` (*string*) - follow-up prompt
+- `debug` (*boolean*) - Print debugging information
+- `auth` (*string*) - Netlify auth token - can be used to run this command without logging in
+
+**Examples**
+
+```bash
+netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a "Also add tests"
+netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a -p "Fix the lint error"
+```
+
 ---
 ## `agents:list`
 
diff --git a/docs/index.md b/docs/index.md
index 0abb807eaf6..63bb9c2fc32 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -28,6 +28,7 @@ Manage Netlify AI agent runs
 | [`agents:commit`](/commands/agents#agentscommit) | Commit an agent run’s changes directly to a branch  |
 | [`agents:create`](/commands/agents#agentscreate) | Create and start a new agent run on your site  |
 | [`agents:diff`](/commands/agents#agentsdiff) | Print the code changes produced by an agent run  |
+| [`agents:follow-up`](/commands/agents#agentsfollow-up) | Send a follow-up prompt to an existing agent run  |
 | [`agents:list`](/commands/agents#agentslist) | List agent runs for the current site  |
 | [`agents:open`](/commands/agents#agentsopen) | Open the agent run preview, dashboard, or pull request in a browser  |
 | [`agents:pr`](/commands/agents#agentspr) | Open a pull request for an agent run  |
diff --git a/src/commands/agents/agents-follow-up.ts b/src/commands/agents/agents-follow-up.ts
new file mode 100644
index 00000000000..b19108f66d8
--- /dev/null
+++ b/src/commands/agents/agents-follow-up.ts
@@ -0,0 +1,155 @@
+import type { OptionValues } from 'commander'
+import inquirer from 'inquirer'
+
+import { chalk, log, logAndThrowError, logJson } from '../../utils/command-helpers.js'
+import { startSpinner, stopSpinner } from '../../lib/spinner.js'
+import type BaseCommand from '../base-command.js'
+import { createAgentsApi } from './api.js'
+import { uploadAttachments, type UploadedAttachment } from './attachments.js'
+import { AVAILABLE_AGENTS, TERMINAL_SESSION_STATES, type AvailableAgent } from './constants.js'
+import type { CreateAgentRunnerSessionPayload } from './types.js'
+import {
+  checkModelAvailability,
+  formatBytes,
+  formatStatus,
+  getAgentName,
+  sanitizePromptText,
+  validateAgent,
+  validatePrompt,
+} from './utils.js'
+
+interface AgentFollowUpOptions extends OptionValues {
+  prompt?: string
+  agent?: string
+  model?: string
+  attach?: string[]
+  json?: boolean
+}
+
+export const agentsFollowUp = async (
+  id: string,
+  promptArg: string,
+  options: AgentFollowUpOptions,
+  command: BaseCommand,
+) => {
+  if (!id) return logAndThrowError('Agent run ID is required')
+  await command.authenticate()
+  const { siteInfo } = command.netlify
+  const api = createAgentsApi(command.netlify)
+
+  if (options.attach && options.attach.length > 0 && !siteInfo.account_id) {
+    return logAndThrowError('Cannot attach files: no account ID is available for this site')
+  }
+
+  let finalPrompt = promptArg || options.prompt
+  if (!finalPrompt) {
+    if (options.json) {
+      return logAndThrowError('A prompt is required. Pass it as the positional argument or via --prompt.')
+    }
+    const { promptInput } = await inquirer.prompt<{ promptInput: string }>([
+      {
+        type: 'input',
+        name: 'promptInput',
+        message: 'What would you like the agent to do next?',
+        validate: validatePrompt,
+      },
+    ])
+    finalPrompt = promptInput
+  }
+  const promptValid = validatePrompt(finalPrompt)
+  if (promptValid !== true) return logAndThrowError(promptValid)
+
+  let lastSessionAgent: AvailableAgent | undefined
+  let lastSessionModel: string | undefined
+  let recent: import('./types.js').AgentRunnerSession | undefined
+  try {
+    const recentSessions = await api.listAgentRunnerSessions(id, { page: 1, per_page: 1, order_by: 'desc' })
+    if (recentSessions.length > 0) recent = recentSessions[0]
+  } catch {
+    // If lookup fails, fall through and let the create call surface the real error.
+  }
+
+  if (recent && !TERMINAL_SESSION_STATES.includes(recent.state as (typeof TERMINAL_SESSION_STATES)[number])) {
+    log(chalk.yellow('A session is already running on this run. Wait for it to finish or stop it first:'))
+    log(`  ${chalk.cyan(`netlify agents:stop ${id}`)}`)
+    return logAndThrowError('Cannot create a follow-up while a session is still active')
+  }
+
+  if (recent) {
+    const previousAgent = recent.agent_config?.agent
+    if (previousAgent && AVAILABLE_AGENTS.some((entry) => entry.value === previousAgent)) {
+      lastSessionAgent = previousAgent
+    }
+    if (recent.agent_config?.model) lastSessionModel = recent.agent_config.model
+  }
+
+  let agent: AvailableAgent | undefined = lastSessionAgent
+  if (options.agent) {
+    const valid = validateAgent(options.agent)
+    if (valid !== true) return logAndThrowError(valid)
+    agent = options.agent as AvailableAgent
+  }
+  const model = options.model ?? lastSessionModel
+  if (model && agent) {
+    const valid = await checkModelAvailability(api, agent, model)
+    if (valid !== true) log(chalk.yellow(`⚠ ${valid}`))
+  }
+
+  let attachments: UploadedAttachment[] = []
+  if (options.attach && options.attach.length > 0 && siteInfo.account_id) {
+    const uploadSpinner = startSpinner({ text: `Uploading ${options.attach.length.toString()} attachment(s)...` })
+    try {
+      attachments = await uploadAttachments(api, siteInfo.account_id, options.attach)
+      stopSpinner({ spinner: uploadSpinner })
+      for (const file of attachments) {
+        log(`  ${chalk.green('✓')} ${file.filename} ${chalk.dim(`(${formatBytes(file.size)})`)}`)
+      }
+    } catch (error_) {
+      stopSpinner({ spinner: uploadSpinner, error: true })
+      const error = error_ as Error
+      return logAndThrowError(error.message)
+    }
+  }
+
+  const payload: CreateAgentRunnerSessionPayload = {
+    prompt: sanitizePromptText(finalPrompt),
+    agent,
+    model,
+    file_keys: attachments.length > 0 ? attachments.map((entry) => entry.fileKey) : undefined,
+  }
+
+  const spinner = startSpinner({ text: 'Sending follow-up prompt...' })
+  try {
+    const session = await api.createAgentRunnerSession(id, payload)
+    stopSpinner({ spinner })
+
+    if (options.json) {
+      logJson(session)
+      return session
+    }
+
+    log(`${chalk.green('✓')} Follow-up session created!`)
+    log()
+    log(chalk.bold('Details:'))
+    log(`  Run ID: ${chalk.cyan(id)}`)
+    log(`  Session ID: ${chalk.cyan(session.id)}`)
+    log(`  Prompt: ${chalk.dim(finalPrompt)}`)
+    if (agent) log(`  Agent: ${chalk.cyan(getAgentName(agent))}${model ? ` (${model})` : ''}`)
+    log(`  Status: ${formatStatus(session.state)}`)
+    log()
+    log(chalk.bold('Monitor progress:'))
+    log(`  Watch: ${chalk.cyan(`netlify agents:show ${id} --watch`)}`)
+    log(`  Show:  ${chalk.cyan(`netlify agents:show ${id}`)}`)
+    return session
+  } catch (error_) {
+    stopSpinner({ spinner, error: true })
+    const error = error_ as Error & { status?: number }
+    if (error.status === 404) return logAndThrowError(`Agent run not found: ${id}`)
+    if (error.message.toLowerCase().includes('active session')) {
+      log()
+      log(chalk.yellow('A session is already running on this run. Wait for it to finish or stop it first:'))
+      log(`  ${chalk.cyan(`netlify agents:stop ${id}`)}`)
+    }
+    return logAndThrowError(`Failed to send follow-up: ${error.message}`)
+  }
+}
diff --git a/src/commands/agents/agents.ts b/src/commands/agents/agents.ts
index d0fdbcc4c79..59069ef6562 100644
--- a/src/commands/agents/agents.ts
+++ b/src/commands/agents/agents.ts
@@ -38,6 +38,27 @@ export const createAgentsCommand = (program: BaseCommand) => {
       await agentsCreate(prompt, options, command)
     })
 
+  program
+    .command('agents:follow-up')
+    .argument('<id>', 'agent run ID to follow up on')
+    .argument('[prompt]', 'the follow-up prompt')
+    .description('Send a follow-up prompt to an existing agent run')
+    .option('-p, --prompt <prompt>', 'follow-up prompt')
+    .option('-a, --agent <agent>', 'override agent type for this session')
+    .option('-m, --model <model>', 'override model for this session')
+    .option('--attach <path>', 'attach a file or image (repeatable)', collect, [])
+    .option('--project <project>', 'project ID or name (if not in a linked directory)')
+    .option('--json', 'output result as JSON')
+    .hook('preAction', requiresSiteInfoWithProject)
+    .addExamples([
+      'netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a "Also add tests"',
+      'netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a -p "Fix the lint error"',
+    ])
+    .action(async (id: string, prompt: string, options: OptionValues, command: BaseCommand) => {
+      const { agentsFollowUp } = await import('./agents-follow-up.js')
+      await agentsFollowUp(id, prompt, options, command)
+    })
+
   program
     .command('agents:list')
     .description('List agent runs for the current site')
@@ -277,6 +298,7 @@ Note: Agent runs execute remotely on Netlify infrastructure, not locally.`,
       'netlify agents:create --prompt "Add a contact form"',
       'netlify agents:list --status running',
       'netlify agents:show 60c7c3b3e7b4a0001f5e4b3a --watch',
+      'netlify agents:follow-up 60c7c3b3e7b4a0001f5e4b3a "Also add tests"',
       'netlify agents:diff 60c7c3b3e7b4a0001f5e4b3a',
       'netlify agents:open 60c7c3b3e7b4a0001f5e4b3a',
     ])
diff --git a/tests/integration/commands/agents/agents-follow-up.test.ts b/tests/integration/commands/agents/agents-follow-up.test.ts
new file mode 100644
index 00000000000..749959281fe
--- /dev/null
+++ b/tests/integration/commands/agents/agents-follow-up.test.ts
@@ -0,0 +1,243 @@
+import { beforeEach, describe, expect, test, vi } from 'vitest'
+
+import { callCli } from '../../utils/call-cli.js'
+import { getCLIOptions, withMockApi } from '../../utils/mock-api.js'
+import { withSiteBuilder } from '../../utils/site-builder.js'
+import { mockSiteInfo, mockAgentSession } from './fixtures.js'
+
+vi.mock('../../../../src/lib/spinner.js', () => ({
+  startSpinner: vi.fn(() => ({ text: 'test' })),
+  stopSpinner: vi.fn(),
+}))
+
+describe('agents:follow-up command', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  const baseRoutes = [
+    { path: 'sites/site_id', response: mockSiteInfo },
+    { path: 'sites/site_id/service-instances', response: [] },
+    { path: 'user', response: { name: 'test user' } },
+    { path: 'accounts', response: [{ slug: 'test-account' }] },
+  ]
+
+  test('should send a follow-up prompt and create a session', async (t) => {
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        response: mockAgentSession,
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl, requests }) => {
+        const cliResponse = (await callCli(
+          ['agents:follow-up', 'test_id', 'Also add tests'],
+          getCLIOptions({ apiUrl, builder }),
+        )) as string
+
+        expect(cliResponse).toContain('Follow-up session created!')
+        expect(cliResponse).toContain('Run ID: test_id')
+        expect(cliResponse).toContain('Session ID: session_id')
+        expect(cliResponse).toContain('Prompt: Also add tests')
+
+        const sessionRequest = requests.find((r) => r.path.endsWith('/sessions') && r.method === 'POST')
+        expect(sessionRequest).toBeDefined()
+        expect((sessionRequest?.body as { prompt: string }).prompt).toBe('Also add tests')
+      })
+    })
+  })
+
+  test('should accept prompt via --prompt flag', async (t) => {
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        response: mockAgentSession,
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl, requests }) => {
+        await callCli(
+          ['agents:follow-up', 'test_id', '--prompt', 'Fix the lint error'],
+          getCLIOptions({ apiUrl, builder }),
+        )
+
+        const sessionRequest = requests.find((r) => r.path.endsWith('/sessions') && r.method === 'POST')
+        expect((sessionRequest?.body as { prompt: string }).prompt).toBe('Fix the lint error')
+      })
+    })
+  })
+
+  test('should pass agent and model in the request body', async (t) => {
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        response: mockAgentSession,
+      },
+      {
+        path: 'ai-gateway/providers',
+        method: 'GET' as const,
+        response: { providers: {} },
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl, requests }) => {
+        await callCli(
+          ['agents:follow-up', 'test_id', 'Update README', '--agent', 'claude', '--model', 'claude-3-sonnet'],
+          getCLIOptions({ apiUrl, builder }),
+        )
+
+        const sessionRequest = requests.find((r) => r.path.endsWith('/sessions') && r.method === 'POST')
+        expect(sessionRequest?.body).toMatchObject({
+          prompt: 'Update README',
+          agent: 'claude',
+          model: 'claude-3-sonnet',
+        })
+      })
+    })
+  })
+
+  test('should return JSON when --json flag is used', async (t) => {
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        response: mockAgentSession,
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl }) => {
+        const cliResponse = (await callCli(
+          ['agents:follow-up', 'test_id', 'Add tests', '--json'],
+          getCLIOptions({ apiUrl, builder }),
+          true,
+        )) as typeof mockAgentSession
+
+        expect(cliResponse).toEqual(mockAgentSession)
+      })
+    })
+  })
+
+  test('should reject prompts shorter than 5 chars', async (t) => {
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(baseRoutes, async ({ apiUrl }) => {
+        await expect(
+          callCli(['agents:follow-up', 'test_id', 'no'], getCLIOptions({ apiUrl, builder })),
+        ).rejects.toThrow('more detailed prompt')
+      })
+    })
+  })
+
+  test('should surface "active session" hint on conflict', async (t) => {
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        status: 409,
+        response: { error: 'Cannot start: an active session is already running' },
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl }) => {
+        await expect(
+          callCli(['agents:follow-up', 'test_id', 'Add tests'], getCLIOptions({ apiUrl, builder })),
+        ).rejects.toThrow('Failed to send follow-up')
+      })
+    })
+  })
+
+  test('should carry over agent and model from the latest done session', async (t) => {
+    const lastSession = {
+      ...mockAgentSession,
+      id: 'prev_id',
+      state: 'done',
+      agent_config: { agent: 'codex', model: 'gpt-4.1' },
+    }
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'GET' as const,
+        response: [lastSession],
+      },
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'POST' as const,
+        response: { ...mockAgentSession, agent_config: { agent: 'codex', model: 'gpt-4.1' } },
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl, requests }) => {
+        await callCli(['agents:follow-up', 'test_id', 'Update README'], getCLIOptions({ apiUrl, builder }))
+        const sessionRequest = requests.find((r) => r.path.endsWith('/sessions') && r.method === 'POST')
+        expect(sessionRequest?.body).toMatchObject({
+          prompt: 'Update README',
+          agent: 'codex',
+          model: 'gpt-4.1',
+        })
+      })
+    })
+  })
+
+  test('should refuse to send a follow-up while the latest session is still running', async (t) => {
+    const activeSession = { ...mockAgentSession, state: 'running' }
+    const routes = [
+      ...baseRoutes,
+      {
+        path: 'agent_runners/test_id/sessions',
+        method: 'GET' as const,
+        response: [activeSession],
+      },
+    ]
+
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(routes, async ({ apiUrl }) => {
+        await expect(
+          callCli(['agents:follow-up', 'test_id', 'Add tests'], getCLIOptions({ apiUrl, builder })),
+        ).rejects.toThrow('Cannot create a follow-up while a session is still active')
+      })
+    })
+  })
+
+  test('should require agent ID argument', async (t) => {
+    await withSiteBuilder(t, async (builder) => {
+      await builder.build()
+
+      await withMockApi(baseRoutes, async ({ apiUrl }) => {
+        await expect(callCli(['agents:follow-up'], getCLIOptions({ apiUrl, builder }))).rejects.toThrow(
+          'missing required argument',
+        )
+      })
+    })
+  })
+})