feat(coder/mux): add restart retries for mux exits (#800)

## Summary
- add optional mux auto-restarts with delay, lock cleanup, and
restart-attempt caps
- restart mux after any exit when enabled, including intentional exits
and signals
- require `max_restart_attempts` to be a non-negative whole number and
update docs/tests for the new restart semantics

## Validation
- `bash -n registry/coder/modules/mux/run.sh`
- `cd registry/coder/modules/mux && terraform validate`
- `cd registry/coder/modules/mux && terraform test -verbose`
- `cd registry/coder/modules/mux && bun test main.test.ts`

Generated with OpenAI using Mux

registry/coder/modules/mux/README.md

@@ -8,13 +8,13 @@ tags: [ai, agents, development, multiplexer]
 
 # Mux
 
-Automatically install and run [Mux](https://github.com/coder/mux) in a Coder workspace. By default, the module auto-detects an available package manager (`npm`, `pnpm`, or `bun`) to install `mux@next` (with a fallback to downloading the npm tarball if none is found). You can also force a specific package manager via `package_manager` and point to a custom registry with `registry_url`. The launcher now keeps watching the mux process after startup and appends signal/exit-code diagnostics to the mux log when the server is killed outside the Node runtime. Mux is a desktop application for parallel agentic development that enables developers to run multiple AI agents simultaneously across isolated workspaces.
+Automatically install and run [Mux](https://github.com/coder/mux) in a Coder workspace. By default, the module auto-detects an available package manager (`npm`, `pnpm`, or `bun`) to install `mux@next` (with a fallback to downloading the npm tarball if none is found). You can also force a specific package manager via `package_manager` and point to a custom registry with `registry_url`. The launcher keeps watching the mux process after startup, appends signal/exit-code diagnostics to the mux log when the server is killed outside the Node runtime, and can optionally wait a few seconds, remove the stale server lock, and restart Mux after any exit until an optional restart-attempt cap is reached. Mux is a desktop application for parallel agentic development that enables developers to run multiple AI agents simultaneously across isolated workspaces.
 
 ```tf
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.4.0"
+  version  = "1.4.3"
   agent_id = coder_agent.main.id
 }
 ```
@@ -37,7 +37,7 @@ module "mux" {
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.4.0"
+  version  = "1.4.3"
   agent_id = coder_agent.main.id
 }
 ```
@@ -48,7 +48,7 @@ module "mux" {
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.4.0"
+  version  = "1.4.3"
   agent_id = coder_agent.main.id
   # Default is "latest"; set to a specific version to pin
   install_version = "0.4.0"
@@ -63,7 +63,7 @@ Start Mux with `mux server --add-project /path/to/project`:
 module "mux" {
   count       = data.coder_workspace.me.start_count
   source      = "registry.coder.com/coder/mux/coder"
-  version     = "1.4.0"
+  version     = "1.4.3"
   agent_id    = coder_agent.main.id
   add_project = "/path/to/project"
 }
@@ -78,19 +78,35 @@ The module parses quoted values, so grouped arguments remain intact.
 module "mux" {
   count                = data.coder_workspace.me.start_count
   source               = "registry.coder.com/coder/mux/coder"
-  version              = "1.4.0"
+  version              = "1.4.3"
   agent_id             = coder_agent.main.id
   additional_arguments = "--open-mode pinned --add-project '/workspaces/my repo'"
 }
 ```
 
+### Restart After Mux Exits
+
+Enable automatic restarts after Mux exits, including clean exits and intentional shutdown signals such as `SIGTERM`. The launcher waits for `restart_delay_seconds`, removes `~/.mux/server.lock`, and starts Mux again. Set `max_restart_attempts` to a whole number to stop retrying after a fixed number of restarts, or leave it at `0` for unlimited retries.
+
+```tf
+module "mux" {
+  count                 = data.coder_workspace.me.start_count
+  source                = "registry.coder.com/coder/mux/coder"
+  version               = "1.4.3"
+  agent_id              = coder_agent.main.id
+  restart_on_kill       = true
+  restart_delay_seconds = 3
+  max_restart_attempts  = 5
+}
+```
+
 ### Custom Port
 
 ```tf
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.4.0"
+  version  = "1.4.3"
   agent_id = coder_agent.main.id
   port     = 8080
 }
@@ -104,7 +120,7 @@ Force a specific package manager instead of auto-detection:
 module "mux" {
   count           = data.coder_workspace.me.start_count
   source          = "registry.coder.com/coder/mux/coder"
-  version         = "1.4.0"
+  version         = "1.4.3"
   agent_id        = coder_agent.main.id
   package_manager = "pnpm" # or "npm", "bun"
 }
@@ -118,7 +134,7 @@ Use a private or mirrored npm registry:
 module "mux" {
   count        = data.coder_workspace.me.start_count
   source       = "registry.coder.com/coder/mux/coder"
-  version      = "1.4.0"
+  version      = "1.4.3"
   agent_id     = coder_agent.main.id
   registry_url = "https://npm.pkg.github.com"
 }
@@ -132,7 +148,7 @@ Run an existing copy of Mux if found, otherwise install from npm:
 module "mux" {
   count      = data.coder_workspace.me.start_count
   source     = "registry.coder.com/coder/mux/coder"
-  version    = "1.4.0"
+  version    = "1.4.3"
   agent_id   = coder_agent.main.id
   use_cached = true
 }
@@ -146,7 +162,7 @@ Run without installing from the network (requires Mux to be pre-installed):
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.4.0"
+  version  = "1.4.3"
   agent_id = coder_agent.main.id
   install  = false
 }
@@ -164,3 +180,5 @@ module "mux" {
 - Installs `mux@next` from the npm registry by default; set `registry_url` to use a private or mirrored registry
 - Falls back to a direct tarball download when no package manager is found
 - Appends best-effort signal and external-kill diagnostics to `log_path` if the mux process dies after startup
+- Set `restart_on_kill = true` to wait `restart_delay_seconds`, remove `~/.mux/server.lock`, and restart Mux after it exits
+- Set `max_restart_attempts` to a whole-number cap on restart attempts, or leave it at `0` for unlimited retries

registry/coder/modules/mux/main.test.ts

@@ -145,6 +145,143 @@ chmod +x /tmp/mux/mux`,
     }
   }, 60000);
 
+  it("restarts after a clean exit when enabled", async () => {
+    const state = await runTerraformApply(import.meta.dir, {
+      agent_id: "foo",
+      install: false,
+      log_path: "/tmp/mux.log",
+      restart_on_kill: true,
+      restart_delay_seconds: 1,
+      max_restart_attempts: 1,
+    });
+
+    const instance = findResourceInstance(state, "coder_script");
+    const id = await runContainer("alpine/curl");
+
+    try {
+      const setup = await execContainer(id, [
+        "sh",
+        "-c",
+        `apk add --no-cache bash >/dev/null
+mkdir -p /tmp/mux
+cat <<'EOF' > /tmp/mux/mux
+#!/usr/bin/env sh
+run_count_file="/tmp/mux-run-count"
+run_count=0
+if [ -f "$run_count_file" ]; then
+  run_count=$(cat "$run_count_file")
+fi
+run_count=$((run_count + 1))
+printf '%s' "$run_count" > "$run_count_file"
+echo "run=$run_count"
+if [ "$run_count" -eq 1 ]; then
+  mkdir -p "$HOME/.mux"
+  touch "$HOME/.mux/server.lock"
+  exit 0
+fi
+if [ -f "$HOME/.mux/server.lock" ]; then
+  echo "lock=present"
+else
+  echo "lock=cleaned"
+fi
+exit 0
+EOF
+chmod +x /tmp/mux/mux`,
+      ]);
+      expect(setup.exitCode).toBe(0);
+
+      const output = await execContainer(id, ["sh", "-c", instance.script]);
+      if (output.exitCode !== 0) {
+        console.log("STDOUT:\n" + output.stdout);
+        console.log("STDERR:\n" + output.stderr);
+      }
+      expect(output.exitCode).toBe(0);
+
+      await execContainer(id, ["sh", "-c", "sleep 4"]);
+      const log = await readFileContainer(id, "/tmp/mux.log");
+      const runCount = await readFileContainer(id, "/tmp/mux-run-count");
+      expect(log).toContain("run=1");
+      expect(log).toContain("mux server exited cleanly.");
+      expect(log).toContain(
+        "Waiting 1 seconds before restarting mux after it exited.",
+      );
+      expect(log).toContain(
+        "Removing /root/.mux/server.lock before restarting mux.",
+      );
+      expect(log).toContain("run=2");
+      expect(log).toContain("lock=cleaned");
+      expect(log).toContain(
+        "Reached the max restart attempts limit (1); not restarting mux again.",
+      );
+      expect(runCount.trim()).toBe("2");
+    } finally {
+      await removeContainer(id);
+    }
+  }, 60000);
+
+  it("restarts after SIGTERM when enabled", async () => {
+    const state = await runTerraformApply(import.meta.dir, {
+      agent_id: "foo",
+      install: false,
+      log_path: "/tmp/mux.log",
+      restart_on_kill: true,
+      restart_delay_seconds: 1,
+      max_restart_attempts: 1,
+    });
+
+    const instance = findResourceInstance(state, "coder_script");
+    const id = await runContainer("alpine/curl");
+
+    try {
+      const setup = await execContainer(id, [
+        "sh",
+        "-c",
+        `apk add --no-cache bash >/dev/null
+mkdir -p /tmp/mux
+cat <<'EOF' > /tmp/mux/mux
+#!/usr/bin/env sh
+run_count_file="/tmp/mux-run-count"
+run_count=0
+if [ -f "$run_count_file" ]; then
+  run_count=$(cat "$run_count_file")
+fi
+run_count=$((run_count + 1))
+printf '%s' "$run_count" > "$run_count_file"
+echo "run=$run_count"
+if [ "$run_count" -eq 1 ]; then
+  kill -TERM $$
+fi
+exit 0
+EOF
+chmod +x /tmp/mux/mux`,
+      ]);
+      expect(setup.exitCode).toBe(0);
+
+      const output = await execContainer(id, ["sh", "-c", instance.script]);
+      if (output.exitCode !== 0) {
+        console.log("STDOUT:\n" + output.stdout);
+        console.log("STDERR:\n" + output.stderr);
+      }
+      expect(output.exitCode).toBe(0);
+
+      await execContainer(id, ["sh", "-c", "sleep 4"]);
+      const log = await readFileContainer(id, "/tmp/mux.log");
+      const runCount = await readFileContainer(id, "/tmp/mux-run-count");
+      expect(log).toContain("run=1");
+      expect(log).toContain("signal TERM (15); shell exit code 143.");
+      expect(log).toContain(
+        "Waiting 1 seconds before restarting mux after it exited.",
+      );
+      expect(log).toContain("run=2");
+      expect(log).toContain(
+        "Reached the max restart attempts limit (1); not restarting mux again.",
+      );
+      expect(runCount.trim()).toBe("2");
+    } finally {
+      await removeContainer(id);
+    }
+  }, 60000);
+
   it("runs with npm present", async () => {
     const state = await runTerraformApply(import.meta.dir, {
       agent_id: "foo",

registry/coder/modules/mux/main.tf

@@ -49,6 +49,34 @@ variable "log_path" {
   default     = "/tmp/mux.log"
 }
 
+variable "restart_on_kill" {
+  type        = bool
+  description = "Restart Mux after it exits by waiting briefly, removing the server lock, and launching it again."
+  default     = false
+}
+
+variable "restart_delay_seconds" {
+  type        = number
+  description = "How long to wait before restarting Mux after it exits when restart_on_kill is enabled."
+  default     = 5
+
+  validation {
+    condition     = var.restart_delay_seconds >= 0
+    error_message = "The 'restart_delay_seconds' variable must be greater than or equal to 0."
+  }
+}
+
+variable "max_restart_attempts" {
+  type        = number
+  description = "Maximum whole-number restart attempts before giving up. Set to 0 for unlimited restarts when restart_on_kill is enabled."
+  default     = 0
+
+  validation {
+    condition     = var.max_restart_attempts >= 0 && floor(var.max_restart_attempts) == var.max_restart_attempts
+    error_message = "The 'max_restart_attempts' variable must be a whole number greater than or equal to 0."
+  }
+}
+
 variable "add_project" {
   type        = string
   description = "Optional path to add/open as a project in Mux on startup."
@@ -171,6 +199,9 @@ resource "coder_script" "mux" {
     OFFLINE : !var.install,
     USE_CACHED : var.use_cached,
     AUTH_TOKEN : local.mux_auth_token,
+    RESTART_ON_KILL : var.restart_on_kill,
+    RESTART_DELAY_SECONDS : var.restart_delay_seconds,
+    MAX_RESTART_ATTEMPTS : var.max_restart_attempts,
     PACKAGE_MANAGER : var.package_manager,
     REGISTRY_URL : local.registry_url,
   })

registry/coder/modules/mux/mux.tftest.hcl

@@ -111,6 +111,111 @@ run "launcher_logs_external_kills" {
   }
 }
 
+run "restart_on_kill_enabled" {
+  command = plan
+
+  variables {
+    agent_id              = "foo"
+    restart_on_kill       = true
+    restart_delay_seconds = 7
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "restart_on_kill_value=\"true\"")
+    error_message = "mux launcher must receive the restart_on_kill setting"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "restart_delay_seconds_value=\"7\"")
+    error_message = "mux launcher must receive the configured restart delay"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "Waiting $${RESTART_DELAY_SECONDS_VALUE} seconds before restarting mux after it exited.")
+    error_message = "mux launcher must log the restart delay before relaunching"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "Removing $HOME/.mux/server.lock before restarting mux.")
+    error_message = "mux launcher must clean up the server lock before relaunching"
+  }
+
+  assert {
+    condition     = !strcontains(resource.coder_script.mux.script, "\"$exit_code\" -le 128")
+    error_message = "mux launcher must no longer exclude non-signal exits from restart handling"
+  }
+
+  assert {
+    condition     = !strcontains(resource.coder_script.mux.script, "1|2|15)")
+    error_message = "mux launcher must no longer exclude intentional signals from restart handling"
+  }
+}
+
+run "restart_on_kill_with_restart_cap" {
+  command = plan
+
+  variables {
+    agent_id              = "foo"
+    restart_on_kill       = true
+    restart_delay_seconds = 7
+    max_restart_attempts  = 2
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "max_restart_attempts_value=\"2\"")
+    error_message = "mux launcher must receive the configured restart cap"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "Mux will stop restarting after $${max_restart_attempts_value} restart attempts.")
+    error_message = "mux launcher must describe the configured restart cap"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "Reached the max restart attempts limit ($MAX_RESTART_ATTEMPTS_VALUE); not restarting mux again.")
+    error_message = "mux launcher must log when it hits the restart cap"
+  }
+}
+
+run "invalid_max_restart_attempts" {
+  command = plan
+
+  variables {
+    agent_id             = "foo"
+    max_restart_attempts = -1
+  }
+
+  expect_failures = [
+    var.max_restart_attempts
+  ]
+}
+
+run "fractional_max_restart_attempts" {
+  command = plan
+
+  variables {
+    agent_id             = "foo"
+    max_restart_attempts = 0.5
+  }
+
+  expect_failures = [
+    var.max_restart_attempts
+  ]
+}
+
+run "invalid_restart_delay_seconds" {
+  command = plan
+
+  variables {
+    agent_id              = "foo"
+    restart_delay_seconds = -1
+  }
+
+  expect_failures = [
+    var.restart_delay_seconds
+  ]
+}
+
 run "custom_version" {
   command = plan
 

registry/coder/modules/mux/run.sh

@@ -5,17 +5,30 @@ RESET='\033[0m'
 MUX_BINARY="${INSTALL_PREFIX}/mux"
 
 function run_mux() {
-  # Remove stale server lock if present
-  rm -f "$HOME/.mux/server.lock"
-
   local port_value
   local auth_token_value
+  local restart_on_kill_value
+  local restart_delay_seconds_value
+  local max_restart_attempts_value
+
   port_value="${PORT}"
   auth_token_value="${AUTH_TOKEN}"
+  restart_on_kill_value="${RESTART_ON_KILL}"
+  restart_delay_seconds_value="${RESTART_DELAY_SECONDS}"
+  max_restart_attempts_value="${MAX_RESTART_ATTEMPTS}"
+
   if [ -z "$port_value" ]; then
     port_value="4000"
   fi
 
+  if [ -z "$restart_delay_seconds_value" ]; then
+    restart_delay_seconds_value="5"
+  fi
+
+  if [ -z "$max_restart_attempts_value" ]; then
+    max_restart_attempts_value="0"
+  fi
+
   mkdir -p "$(dirname "${LOG_PATH}")"
 
   # Build args for mux (POSIX-compatible, avoid bash arrays)
@@ -41,13 +54,24 @@ EOF_ARGS
 
   echo "🚀 Starting mux server on port $port_value..."
   echo "Check logs at ${LOG_PATH}!"
-  echo "ℹ️ Unexpected exits will be appended to ${LOG_PATH} by the launcher."
+  echo "ℹ️ Mux exit details will be appended to ${LOG_PATH} by the launcher."
+  if [ "$restart_on_kill_value" = true ]; then
+    echo "ℹ️ Auto-restart after mux exits is enabled with a $${restart_delay_seconds_value}-second delay."
+    if [ "$max_restart_attempts_value" = "0" ]; then
+      echo "ℹ️ Automatic restarts are unlimited for every mux exit."
+    else
+      echo "ℹ️ Mux will stop restarting after $${max_restart_attempts_value} restart attempts."
+    fi
+  fi
 
   nohup env \
     LOG_PATH="${LOG_PATH}" \
     MUX_BINARY="$MUX_BINARY" \
     AUTH_TOKEN="$auth_token_value" \
     PORT_VALUE="$port_value" \
+    RESTART_ON_KILL_VALUE="$restart_on_kill_value" \
+    RESTART_DELAY_SECONDS_VALUE="$restart_delay_seconds_value" \
+    MAX_RESTART_ATTEMPTS_VALUE="$max_restart_attempts_value" \
     bash -s -- "$@" > /dev/null 2>&1 << 'EOF_LAUNCHER' &
 signal_name() {
   local signal_number="$1"
@@ -82,6 +106,14 @@ append_kernel_kill_context() {
   fi
 }
 
+cleanup_mux_lock() {
+  rm -f "$HOME/.mux/server.lock"
+}
+
+should_restart_mux() {
+  [ "$RESTART_ON_KILL_VALUE" = "true" ]
+}
+
 log_mux_exit() {
   local mux_pid="$1"
   local exit_code="$2"
@@ -114,11 +146,52 @@ log_mux_exit() {
   echo "[$timestamp] Check the earlier mux log lines for any in-process crash breadcrumbs from mux itself."
 }
 
+log_mux_restart_wait() {
+  local timestamp
+
+  timestamp="$(date -Iseconds 2> /dev/null || date)"
+  echo "[$timestamp] Waiting $${RESTART_DELAY_SECONDS_VALUE} seconds before restarting mux after it exited."
+}
+
+log_mux_restart_cleanup() {
+  local timestamp
+
+  timestamp="$(date -Iseconds 2> /dev/null || date)"
+  echo "[$timestamp] Removing $HOME/.mux/server.lock before restarting mux."
+}
+
+log_mux_restart_cap_reached() {
+  local timestamp
+
+  timestamp="$(date -Iseconds 2> /dev/null || date)"
+  echo "[$timestamp] Reached the max restart attempts limit ($MAX_RESTART_ATTEMPTS_VALUE); not restarting mux again."
+}
+
+restart_attempt_count=0
+while true; do
+  cleanup_mux_lock
   MUX_SERVER_AUTH_TOKEN="$AUTH_TOKEN" PORT="$PORT_VALUE" "$MUX_BINARY" "$@" >> "$LOG_PATH" 2>&1 &
   mux_pid=$!
   wait "$mux_pid"
   exit_code=$?
   log_mux_exit "$mux_pid" "$exit_code" >> "$LOG_PATH" 2>&1
+
+  if should_restart_mux; then
+    if [ "$MAX_RESTART_ATTEMPTS_VALUE" -gt 0 ] && [ "$restart_attempt_count" -ge "$MAX_RESTART_ATTEMPTS_VALUE" ]; then
+      log_mux_restart_cap_reached >> "$LOG_PATH" 2>&1
+      break
+    fi
+
+    restart_attempt_count=$((restart_attempt_count + 1))
+    log_mux_restart_wait >> "$LOG_PATH" 2>&1
+    sleep "$RESTART_DELAY_SECONDS_VALUE"
+    cleanup_mux_lock
+    log_mux_restart_cleanup >> "$LOG_PATH" 2>&1
+    continue
+  fi
+
+  break
+done
 EOF_LAUNCHER
 }
 # Check if mux is already installed for offline mode