diff --git a/registry/coder/modules/mux/README.md b/registry/coder/modules/mux/README.md
index b9cfafc0..69073926 100644
--- a/registry/coder/modules/mux/README.md
+++ b/registry/coder/modules/mux/README.md
@@ -14,7 +14,7 @@ Automatically install and run [Mux](https://github.com/coder/mux) in a Coder wor
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.1.0"
+  version  = "1.2.0"
   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  = "1.1.0"
+  version  = "1.2.0"
   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  = "1.1.0"
+  version  = "1.2.0"
   agent_id = coder_agent.main.id
   # Default is "latest"; set to a specific version to pin
   install_version = "0.4.0"
@@ -63,19 +63,34 @@ 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     = "1.1.0"
+  version     = "1.2.0"
   agent_id    = coder_agent.main.id
   add-project = "/path/to/project"
 }
 ```
 
+### Pass Arbitrary `mux server` Arguments
+
+Use `additional_arguments` to append additional arguments to `mux server`.
+The module parses quoted values, so grouped arguments remain intact.
+
+```tf
+module "mux" {
+  count                = data.coder_workspace.me.start_count
+  source               = "registry.coder.com/coder/mux/coder"
+  version              = "1.2.0"
+  agent_id             = coder_agent.main.id
+  additional_arguments = "--open-mode pinned --add-project '/workspaces/my repo'"
+}
+```
+
 ### Custom Port
 
 ```tf
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.1.0"
+  version  = "1.2.0"
   agent_id = coder_agent.main.id
   port     = 8080
 }
@@ -89,7 +104,7 @@ 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    = "1.1.0"
+  version    = "1.2.0"
   agent_id   = coder_agent.main.id
   use_cached = true
 }
@@ -103,7 +118,7 @@ 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  = "1.1.0"
+  version  = "1.2.0"
   agent_id = coder_agent.main.id
   install  = false
 }
diff --git a/registry/coder/modules/mux/main.test.ts b/registry/coder/modules/mux/main.test.ts
index 96fae5e4..c1b6e238 100644
--- a/registry/coder/modules/mux/main.test.ts
+++ b/registry/coder/modules/mux/main.test.ts
@@ -1,6 +1,11 @@
 import { describe, expect, it } from "bun:test";
 import {
   executeScriptInContainer,
+  execContainer,
+  findResourceInstance,
+  readFileContainer,
+  removeContainer,
+  runContainer,
   runTerraformApply,
   runTerraformInit,
   testRequiredVariables,
@@ -40,6 +45,57 @@ describe("mux", async () => {
     }
   }, 60000);
 
+  it("parses custom additional_arguments", async () => {
+    const state = await runTerraformApply(import.meta.dir, {
+      agent_id: "foo",
+      install: false,
+      log_path: "/tmp/mux.log",
+      additional_arguments:
+        "--open-mode pinned --add-project '/workspaces/my repo'",
+    });
+
+    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
+i=1
+for arg in "$@"; do
+  echo "arg$i=$arg"
+  i=$((i + 1))
+done
+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 1"]);
+      const log = await readFileContainer(id, "/tmp/mux.log");
+      expect(log).toContain("arg1=server");
+      expect(log).toContain("arg2=--port");
+      expect(log).toContain("arg3=4000");
+      expect(log).toContain("arg4=--open-mode");
+      expect(log).toContain("arg5=pinned");
+      expect(log).toContain("arg6=--add-project");
+      expect(log).toContain("arg7=/workspaces/my repo");
+    } finally {
+      await removeContainer(id);
+    }
+  }, 60000);
+
   it("runs with npm present", async () => {
     const state = await runTerraformApply(import.meta.dir, {
       agent_id: "foo",
diff --git a/registry/coder/modules/mux/main.tf b/registry/coder/modules/mux/main.tf
index 1eeddecf..dbc585d8 100644
--- a/registry/coder/modules/mux/main.tf
+++ b/registry/coder/modules/mux/main.tf
@@ -55,6 +55,12 @@ variable "add-project" {
   default     = null
 }
 
+variable "additional_arguments" {
+  type        = string
+  description = "Additional command-line arguments to pass to `mux server` (for example: `--add-project /path --open-mode pinned`)."
+  default     = ""
+}
+
 variable "install_version" {
   type        = string
   description = "The version or dist-tag of Mux to install."
@@ -142,6 +148,7 @@ resource "coder_script" "mux" {
     PORT : var.port,
     LOG_PATH : var.log_path,
     ADD_PROJECT : var.add-project == null ? "" : var.add-project,
+    ADDITIONAL_ARGUMENTS : var.additional_arguments,
     INSTALL_PREFIX : var.install_prefix,
     OFFLINE : !var.install,
     USE_CACHED : var.use_cached,
diff --git a/registry/coder/modules/mux/mux.tftest.hcl b/registry/coder/modules/mux/mux.tftest.hcl
index af103ae2..688ddedb 100644
--- a/registry/coder/modules/mux/mux.tftest.hcl
+++ b/registry/coder/modules/mux/mux.tftest.hcl
@@ -79,6 +79,20 @@ run "auth_token_in_url" {
   }
 }
 
+run "custom_additional_arguments" {
+  command = plan
+
+  variables {
+    agent_id             = "foo"
+    additional_arguments = "--open-mode pinned --add-project '/workspaces/my repo'"
+  }
+
+  assert {
+    condition     = strcontains(resource.coder_script.mux.script, "--open-mode pinned --add-project '/workspaces/my repo'")
+    error_message = "mux launch script must include the configured additional arguments"
+  }
+}
+
 run "custom_version" {
   command = plan
 
diff --git a/registry/coder/modules/mux/run.sh b/registry/coder/modules/mux/run.sh
index 0d0c6520..099448fb 100644
--- a/registry/coder/modules/mux/run.sh
+++ b/registry/coder/modules/mux/run.sh
@@ -20,6 +20,22 @@ function run_mux() {
   if [ -n "${ADD_PROJECT}" ]; then
     set -- "$@" --add-project "${ADD_PROJECT}"
   fi
+
+  # Parse additional user-supplied server arguments while preserving quoted groups.
+  if [ -n "${ADDITIONAL_ARGUMENTS}" ]; then
+    local parsed_additional_arguments
+    if ! parsed_additional_arguments="$(printf "%s\n" "${ADDITIONAL_ARGUMENTS}" | xargs -n1 printf "%s\n" 2> /dev/null)"; then
+      echo "❌ Failed to parse additional_arguments. Ensure quotes are balanced."
+      exit 1
+    fi
+    while IFS= read -r parsed_arg; do
+      [ -n "$parsed_arg" ] || continue
+      set -- "$@" "$parsed_arg"
+    done << EOF
+$${parsed_additional_arguments}
+EOF
+  fi
+
   echo "🚀 Starting mux server on port $port_value..."
   echo "Check logs at ${LOG_PATH}!"
   MUX_SERVER_AUTH_TOKEN="$auth_token_value" PORT="$port_value" "$MUX_BINARY" "$@" > "${LOG_PATH}" 2>&1 &