diff --git a/app/components/llm_connections/default_models_component.html.erb b/app/components/llm_connections/default_models_component.html.erb
index 7293172f74a0..42a22a12dc26 100644
--- a/app/components/llm_connections/default_models_component.html.erb
+++ b/app/components/llm_connections/default_models_component.html.erb
@@ -35,6 +35,10 @@ See COPYRIGHT and LICENSE files for more details.
     end
   %>
 
+  <% if connection.configured_from_env? %>
+    <%= render(Primer::Alpha::Banner.new(mb: 3, icon: :info)) { t("admin.banners.environment_configured_readonly") } %>
+  <% end %>
+
   <%= settings_primer_form_with(**form_options) do |f| %>
     <%= render(LlmConnections::DefaultModelsForm.new(f)) %>
   <% end %>
diff --git a/app/components/llm_connections/form_component.html.erb b/app/components/llm_connections/form_component.html.erb
index 35a5273aa4aa..401606ee473a 100644
--- a/app/components/llm_connections/form_component.html.erb
+++ b/app/components/llm_connections/form_component.html.erb
@@ -1,5 +1,13 @@
 <%=
   component_wrapper(tag: "turbo-frame", **wrapper_options) do
+    if connection.configured_from_env?
+      concat(
+        render(Primer::Alpha::Banner.new(mb: 3, icon: :info)) do
+          t("admin.banners.environment_configured_readonly")
+        end
+      )
+    end
+
     concat(
       settings_primer_form_with(**form_options) do |f|
         render(LlmConnections::ConnectionForm.new(f))
diff --git a/app/contracts/llm_connections/base_contract.rb b/app/contracts/llm_connections/base_contract.rb
index f4c12e335379..9865a6b8eb39 100644
--- a/app/contracts/llm_connections/base_contract.rb
+++ b/app/contracts/llm_connections/base_contract.rb
@@ -54,6 +54,13 @@ class BaseContract < ModelContract
     validate :features_require_connection
     validate :default_models_offered_by_server
     validate :default_chat_model_can_chat
+    validate :not_configured_from_env
+
+    def not_configured_from_env
+      return unless model.configured_from_env?
+
+      errors.add :base, :configured_via_env
+    end
 
     private
 
diff --git a/app/contracts/llm_connections/environment_update_contract.rb b/app/contracts/llm_connections/environment_update_contract.rb
new file mode 100644
index 000000000000..72529132bef7
--- /dev/null
+++ b/app/contracts/llm_connections/environment_update_contract.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module LlmConnections
+  # Used when the connection is provisioned from the environment.
+  #
+  # Inherits from BaseContract, not UpdateContract: seeding must never reach out
+  # to the LLM server, because the container it runs in may well start before the
+  # server does. It also lifts the "configured from environment is read-only"
+  # guard, since this is the code path that legitimately writes those values.
+  class EnvironmentUpdateContract < BaseContract
+    def not_configured_from_env = nil
+
+    # On a fresh installation the seed runs before any model synchronisation, so
+    # there is no catalogue to validate a default model against. A wrong id is
+    # surfaced afterwards, the same way as a model that vanished: the binding
+    # shows as no longer offered.
+    def default_models_offered_by_server = nil
+  end
+end
diff --git a/app/controllers/admin/llm_connections_controller.rb b/app/controllers/admin/llm_connections_controller.rb
index 50061aebde98..a9eb6659bddd 100644
--- a/app/controllers/admin/llm_connections_controller.rb
+++ b/app/controllers/admin/llm_connections_controller.rb
@@ -57,6 +57,8 @@ def disconnect_dialog
     # Clears the credential and switches the AI features off, keeping the endpoint
     # and the catalogue. Deliberately not a destroy.
     def disconnect
+      return redirect_with_error(t(".configured_from_env")) if @connection.configured_from_env?
+
       ApplicationRecord.transaction do
         @connection.update!(api_key: nil)
         Setting.llm_features_enabled = false
@@ -69,7 +71,12 @@ def delete_api_key_dialog
       respond_with_dialog LlmConnections::DeleteApiKeyDialogComponent.new(@connection)
     end
 
+    # The environment guard is checked explicitly because this write bypasses
+    # the contract: removing a credential must always be possible, even against
+    # a server that would reject the resulting unauthenticated probe.
     def delete_api_key
+      return redirect_with_error(t(".configured_from_env")) if @connection.configured_from_env?
+
       @connection.update!(api_key: nil)
 
       redirect_with_notice(t(".success"))
diff --git a/app/forms/llm_connections/connection_form.rb b/app/forms/llm_connections/connection_form.rb
index 8669f08b9968..29f905ee14c1 100644
--- a/app/forms/llm_connections/connection_form.rb
+++ b/app/forms/llm_connections/connection_form.rb
@@ -35,6 +35,7 @@ class ConnectionForm < ApplicationForm
         name: :llm_features_enabled,
         label: LlmConnection.human_attribute_name(:llm_features_enabled),
         caption: I18n.t("admin.llm_connections.form.llm_features_enabled_caption"),
+        disabled: read_only?,
         data: { target_name: "llm_features_enabled", show_when_checked_target: "cause" }
       )
 
@@ -55,6 +56,7 @@ class ConnectionForm < ApplicationForm
           caption: I18n.t("admin.llm_connections.form.api_format_caption"),
           include_blank: false,
           input_width: :medium,
+          disabled: read_only?,
           data: { target_name: "llm_connection_api_format", show_when_value_selected_target: "cause" }
         ) do |select|
           supported_formats.each do |format|
@@ -69,7 +71,8 @@ class ConnectionForm < ApplicationForm
           placeholder: "https://example.com/v1",
           required: true,
           type: :url,
-          input_width: :large
+          input_width: :large,
+          disabled: read_only?
         )
 
         fg.group(layout: :horizontal) do |row|
@@ -85,10 +88,11 @@ class ConnectionForm < ApplicationForm
             type: :password,
             autocomplete: "off",
             input_width: :large,
+            disabled: read_only?,
             data: { "admin--llm-connection-form-target": "secretInput" }
           )
 
-          if model.api_key_stored?
+          if model.api_key_stored? && !read_only?
             row.button(
               name: :remove_api_key,
               tag: :a,
@@ -103,16 +107,22 @@ class ConnectionForm < ApplicationForm
         end
       end
 
-      f.submit(
-        name: :submit,
-        label: submit_label,
-        scheme: :primary,
-        data: { "admin--llm-connection-form-target": "submitButton" }
-      )
+      unless read_only?
+        f.submit(
+          name: :submit,
+          label: submit_label,
+          scheme: :primary,
+          data: { "admin--llm-connection-form-target": "submitButton" }
+        )
+      end
     end
 
     private
 
+    def read_only?
+      model.configured_from_env?
+    end
+
     # Only formats a request can actually be sent in. The contract rejects the
     # rest as a backstop, but they should not be offered in the first place.
     def supported_formats
@@ -147,6 +157,8 @@ def submit_label
     end
 
     def api_key_caption
+      return I18n.t("admin.llm_connections.form.api_key_caption_env") if read_only?
+
       I18n.t("admin.llm_connections.form.api_key_caption#{'_stored' if model.api_key_stored?}")
     end
 
diff --git a/app/forms/llm_connections/default_models_form.rb b/app/forms/llm_connections/default_models_form.rb
index 56e1c692785f..a8a049c54ed4 100644
--- a/app/forms/llm_connections/default_models_form.rb
+++ b/app/forms/llm_connections/default_models_form.rb
@@ -41,6 +41,7 @@ class DefaultModelsForm < ApplicationForm
         caption: I18n.t("admin.llm_models.defaults.chat_caption"),
         autocomplete_options: {
           decorated: true,
+          disabled: read_only?,
           inputValue: model.default_chat_model_id,
           placeholder: I18n.t("label_none_parentheses")
         }
@@ -54,11 +55,15 @@ class DefaultModelsForm < ApplicationForm
         end
       end
 
-      f.submit(name: :submit, label: I18n.t(:button_save), scheme: :primary)
+      f.submit(name: :submit, label: I18n.t(:button_save), scheme: :primary) unless read_only?
     end
 
     private
 
+    def read_only?
+      model.configured_from_env?
+    end
+
     # The one already chosen is kept regardless of what the server offers today:
     # dropping it would silently blank the field on the next save.
     def default_chat_model_options
diff --git a/app/models/llm_connection.rb b/app/models/llm_connection.rb
index dc76772937f7..37e5d0c7a0f3 100644
--- a/app/models/llm_connection.rb
+++ b/app/models/llm_connection.rb
@@ -81,6 +81,10 @@ def configured?
     base_url.present?
   end
 
+  def configured_from_env?
+    Setting.llm_connection.present?
+  end
+
   # Every model that can be addressed today: discovered and still offered, plus
   # anything an administrator entered by hand.
   #
diff --git a/app/seeders/env_data/llm_connection_seeder.rb b/app/seeders/env_data/llm_connection_seeder.rb
new file mode 100644
index 000000000000..912471599a91
--- /dev/null
+++ b/app/seeders/env_data/llm_connection_seeder.rb
@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module EnvData
+  # Provisions the LLM connection from OPENPROJECT_LLM__CONNECTION_* variables so
+  # a container comes up connected without anyone opening the administration UI.
+  #
+  # Never contacts the LLM server: the catalogue refresh is enqueued, so seeding
+  # succeeds even when the server starts after OpenProject does. It is enqueued
+  # only while nothing is stored: a re-seed against another server must not
+  # discard a list an administrator has curated.
+  class LlmConnectionSeeder < Seeder
+    KNOWN_KEYS = %w[base_url api_key default_chat_model default_embedding_model enabled].freeze
+
+    def seed_data!
+      print_status "    ↳ Creating LLM connection" do
+        validate_options!(config)
+
+        result = LlmConnections::EnvSyncService.new(config).call
+        raise result.errors.full_messages.join(", ") if result.failure?
+
+        Llm::SyncModelsJob.perform_later if result.result.catalogue_fetched_at.nil?
+      end
+    end
+
+    def applicable?
+      config.present?
+    end
+
+    def not_applicable_message
+      "No LLM connection configured through environment variables."
+    end
+
+    private
+
+    def config
+      Setting.llm_connection
+    end
+
+    def validate_options!(options)
+      check_unknown_keys!(options, KNOWN_KEYS)
+      return if options["base_url"].present?
+
+      raise "LLM connection: #{env_form('base_url')} is required."
+    end
+
+    def check_unknown_keys!(options, known_keys)
+      unknown = options.keys - known_keys
+      return if unknown.empty?
+
+      raise <<~MSG.strip
+        LLM connection: unknown configuration key(s): #{unknown.map { |k| env_form(k) }.join(', ')}.
+        Accepted keys: #{known_keys.map { |k| env_form(k) }.join(', ')}.
+        Note: in environment variable names, single underscores split path segments and double underscores encode a literal underscore (e.g. BASE__URL, not BASE_URL).
+      MSG
+    end
+
+    def env_form(key)
+      key.gsub("_", "__").upcase
+    end
+  end
+end
diff --git a/app/seeders/env_data_seeder.rb b/app/seeders/env_data_seeder.rb
index 48a5959cced8..91300692b014 100644
--- a/app/seeders/env_data_seeder.rb
+++ b/app/seeders/env_data_seeder.rb
@@ -31,6 +31,7 @@ def data_seeder_classes
     [
       EnvData::CustomDesignSeeder,
       EnvData::LdapSeeder,
+      EnvData::LlmConnectionSeeder,
       EnvData::ScimClientSeeder,
       EnvData::TokenSeeder
     ]
diff --git a/app/services/llm_connections/env_sync_service.rb b/app/services/llm_connections/env_sync_service.rb
new file mode 100644
index 000000000000..b7291502f756
--- /dev/null
+++ b/app/services/llm_connections/env_sync_service.rb
@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module LlmConnections
+  # Applies an environment-provided configuration to the connection record.
+  #
+  # Uses EnvironmentUpdateContract, which lifts the "configured from environment
+  # is read-only" guard and, crucially, does not probe the LLM server: the
+  # container running the seed may well start before the server does.
+  class EnvSyncService
+    def initialize(env_config)
+      @config = env_config.deep_symbolize_keys
+    end
+
+    def call
+      ApplicationRecord.transaction do
+        result = write(attributes)
+        break result if result.failure?
+
+        write(default_model_references(result.result), model: result.result)
+      end
+    end
+
+    private
+
+    attr_reader :config
+
+    # Absent keys are written as nil on purpose: the environment is the source
+    # of truth here, and the form is read-only while it is. Keeping a stored
+    # value that was removed from the environment would leave, for example, an
+    # obsolete API key in use with no supported way to clear it.
+    def attributes
+      {
+        base_url: config.fetch(:base_url),
+        api_key: config[:api_key],
+        llm_features_enabled: ActiveRecord::Type::Boolean.new.deserialize(config.fetch(:enabled, true))
+      }
+    end
+
+    def write(attributes, model: LlmConnection.active_connection)
+      UpdateService
+        .new(user: User.system,
+             model:,
+             contract_class: EnvironmentUpdateContract,
+             sync_models: false)
+        .call(**attributes)
+    end
+
+    # The environment names a model, and on a fresh installation nothing has
+    # asked the server for a catalogue yet, so the row it must reference is
+    # entered here the way an administrator would enter it by hand.
+    def default_model_references(connection)
+      { default_chat_model_id: model_row_id(connection, config[:default_chat_model]),
+        default_embedding_model_id: model_row_id(connection, config[:default_embedding_model]) }
+    end
+
+    def model_row_id(connection, external_id)
+      return if external_id.blank?
+
+      connection.models.create_with(manual: true).find_or_create_by!(external_id:).id
+    end
+  end
+end
diff --git a/app/services/llm_connections/update_service.rb b/app/services/llm_connections/update_service.rb
index 2b77bcb2f979..59087f74818b 100644
--- a/app/services/llm_connections/update_service.rb
+++ b/app/services/llm_connections/update_service.rb
@@ -30,6 +30,15 @@
 
 module LlmConnections
   class UpdateService < BaseServices::Update
+    # @param sync_models [Boolean] whether to refresh the model catalogue inline
+    #   after a successful save. Provisioning from the environment passes false:
+    #   seeding must not block on an LLM server that has not started yet, and
+    #   enqueues Llm::SyncModelsJob instead.
+    def initialize(*, sync_models: true, **)
+      super(*, **)
+      @sync_models = sync_models
+    end
+
     private
 
     # The contract has already proven the server reachable when the credentials
@@ -40,6 +49,7 @@ def after_perform(service_call)
         next unless service_call.success?
 
         Setting.llm_features_enabled = model.llm_features_enabled
+        next unless @sync_models
         next unless initial_fill?(service_call.result)
 
         SyncModelsService.new(service_call.result).call
diff --git a/app/views/admin/llm_connections/show.html.erb b/app/views/admin/llm_connections/show.html.erb
index 3cda79136b33..0734814891be 100644
--- a/app/views/admin/llm_connections/show.html.erb
+++ b/app/views/admin/llm_connections/show.html.erb
@@ -50,7 +50,9 @@ See COPYRIGHT and LICENSE files for more details.
     )
     render_tab_header_nav(header, llm_settings_tabs(@connection), test_selector: "llm-settings--tabs")
 
-    if @connection.persisted?
+    # Nothing here applies to a connection provisioned from the environment:
+    # those settings belong to the deployment, not to the administrator.
+    if @connection.persisted? && !@connection.configured_from_env?
       header.with_action_menu(
         menu_arguments: { anchor_align: :end },
         button_arguments: {
diff --git a/app/workers/llm/sync_models_job.rb b/app/workers/llm/sync_models_job.rb
index 9cb300e24624..3cc043c8ebf2 100644
--- a/app/workers/llm/sync_models_job.rb
+++ b/app/workers/llm/sync_models_job.rb
@@ -34,9 +34,17 @@ module Llm
   # Used by the environment seeder, which must not block on -- or fail because of
   # -- an LLM server that has not finished starting.
   class SyncModelsJob < ApplicationJob
+    class SyncFailed < StandardError; end
+
+    # The usual reason for a failure here is the startup race with the LLM
+    # sidecar this job exists for, so a failed sync retries with backoff rather
+    # than leaving the provisioned connection without its catalogue.
+    retry_on SyncFailed, wait: :polynomially_longer, attempts: 10
+
     def perform
       LlmConnection.find_each do |connection|
-        LlmConnections::SyncModelsService.new(connection).call
+        result = LlmConnections::SyncModelsService.new(connection).call
+        raise SyncFailed, result.errors.to_s unless result.success?
       end
     end
   end
diff --git a/config/constants/settings/definition.rb b/config/constants/settings/definition.rb
index a64ff34394c0..3e85326d7c47 100644
--- a/config/constants/settings/definition.rb
+++ b/config/constants/settings/definition.rb
@@ -753,6 +753,13 @@ class Definition
         format: :boolean,
         default: false
       },
+      llm_connection: {
+        description: "Configure the connection to an OpenAI-API-compatible LLM server through environment variables",
+        writable: false,
+        default: {},
+        format: :hash,
+        string_values: true
+      },
       llm_features_enabled: {
         description: "Enable the AI features backed by the configured LLM connection",
         format: :boolean,
diff --git a/config/locales/en.yml b/config/locales/en.yml
index ff34b418ade2..1a1196bb4b2b 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1778,6 +1778,7 @@ en:
 
     llm_connections:
       delete_api_key:
+        configured_from_env: "This connection is configured through the environment and cannot be changed here."
         description: "OpenProject will stop sending an API key with its requests. Features will keep working only if the server requires no authentication. You can enter a new key at any time."
         heading: "Remove the stored API key?"
         menu_label: "Remove API key"
@@ -1785,6 +1786,7 @@ en:
         title: "Remove API key"
       disabled_notice: "LLM features are switched off for this instance. Switch them on here first."
       disconnect:
+        configured_from_env: "This connection is configured through the environment and cannot be changed here."
         description: "OpenProject will stop using the LLM server. AI features will be unavailable until you connect again."
         heading: "Disconnect from the LLM server?"
         keeps_models: "The model list, including any models you added manually, is kept."
@@ -1795,6 +1797,7 @@ en:
       form:
         api_format_caption: "Which API the server speaks. Choose OpenAI for most gateways and self-hosted servers."
         api_key_caption: "The key OpenProject authenticates with. Leave blank if the server requires no authentication."
+        api_key_caption_env: "The key comes from the environment."
         api_key_caption_stored: "A key is stored. Leave blank to keep the current key, or enter a new one to replace it."
         api_key_placeholder_stored: "API key stored"
         api_key_remove: "Remove key"
diff --git a/docs/installation-and-operations/configuration/environment/README.md b/docs/installation-and-operations/configuration/environment/README.md
index ea5f31eade7f..78a5dbcb593b 100644
--- a/docs/installation-and-operations/configuration/environment/README.md
+++ b/docs/installation-and-operations/configuration/environment/README.md
@@ -289,6 +289,7 @@ OPENPROJECT_LDAP__FORCE__NO__PAGE (default=nil) Force LDAP to respond as a singl
 OPENPROJECT_LDAP__GROUPS__DISABLE__SYNC__JOB (default=false) Deactivate regular synchronization job for groups in case scheduled as a separate cronjob
 OPENPROJECT_LDAP__USERS__DISABLE__SYNC__JOB (default=false) Deactivate user attributes synchronization from LDAP
 OPENPROJECT_LDAP__USERS__SYNC__STATUS (default=false) Enable user status (locked/unlocked) synchronization from LDAP
+OPENPROJECT_LLM__CONNECTION (default={}) Configure the connection to an OpenAI-API-compatible LLM server through environment variables
 OPENPROJECT_LLM__FEATURES__ENABLED (default=false) Enable the AI features backed by the configured LLM connection
 OPENPROJECT_LOG__LEVEL (default="info") Set the OpenProject logger level
 OPENPROJECT_LOG__REQUESTING__USER (default=false) Log user login, name, and mail address for all requests
diff --git a/spec/requests/admin/llm_connections_spec.rb b/spec/requests/admin/llm_connections_spec.rb
index 52b4c3088fb6..3bc2dd3ae7b1 100644
--- a/spec/requests/admin/llm_connections_spec.rb
+++ b/spec/requests/admin/llm_connections_spec.rb
@@ -136,6 +136,35 @@
           expect(page).to have_css(remove_api_key, text: "Remove key", visible: :all)
         end
       end
+
+      context "when the connection comes from the environment" do
+        let!(:connection) { create(:llm_connection, base_url:, api_key: "sk-original") }
+
+        before do
+          # Provisioning from the environment switches the features on, and the
+          # server settings are only rendered once they are.
+          allow(Setting).to receive_messages(llm_connection: { "base_url" => base_url },
+                                             llm_features_enabled?: true)
+        end
+
+        it "renders the server settings read-only, with a banner saying why" do
+          get llm_connection_path
+
+          expect(response.body).to include("configured via environment variables")
+          expect(page).to have_field("Host URL", disabled: true)
+          expect(page).to have_field("API format", disabled: true)
+          expect(page).to have_field("Enable LLMs for this instance", disabled: true)
+          expect(page).to have_no_button("Save")
+        end
+
+        it "does not ask for a key that cannot be entered" do
+          get llm_connection_path
+
+          expect(response.body).to include("The key comes from the environment")
+          expect(response.body).not_to include("A key is stored")
+          expect(page).to have_no_css(remove_api_key, visible: :all)
+        end
+      end
     end
   end
 
@@ -304,6 +333,17 @@
       expect(response.body).not_to include("llm-connection--delete-api-key")
       expect(page).to have_no_css(remove_api_key, visible: :all)
     end
+
+    # update! bypasses the contract, so without the explicit guard a
+    # hand-crafted request could wipe a key the environment owns.
+    it "refuses when the connection comes from the environment" do
+      connection = create(:llm_connection, base_url:, api_key: "sk-original")
+      allow(Setting).to receive(:llm_connection).and_return({ "base_url" => base_url })
+
+      delete api_key_llm_connection_path
+
+      expect(connection.reload.api_key).to eq("sk-original")
+    end
   end
 
   describe "GET /admin/llm_connection/delete_api_key_dialog" do
@@ -352,6 +392,15 @@
       expect(connection.models.count).to eq(2)
     end
 
+    it "refuses when the connection comes from the environment" do
+      allow(Setting).to receive(:llm_connection).and_return({ "base_url" => "https://example.com/v1" })
+
+      post disconnect_llm_connection_path
+
+      expect(connection.reload.api_key).to eq("sk-test")
+      expect(Setting.llm_features_enabled?).to be(true)
+    end
+
     it "is refused to a non-admin" do
       login_as create(:user)
 
diff --git a/spec/requests/admin/llm_models_spec.rb b/spec/requests/admin/llm_models_spec.rb
index 3eb410121e05..b5ff48361fdc 100644
--- a/spec/requests/admin/llm_models_spec.rb
+++ b/spec/requests/admin/llm_models_spec.rb
@@ -159,6 +159,18 @@ def streamed_markup = Capybara.string(response.body.gsub(%r{</?template>}, ""))
         expect(response.body).not_to include("Default models")
       end
 
+      it "shows the default read-only when the environment owns the connection" do
+        connection = create(:llm_connection, :with_models, base_url:)
+        connection.update!(default_chat_model: connection.models.find_by(external_id: "qwen3.6-27b"))
+        allow(Setting).to receive(:llm_connection).and_return({ "base_url" => base_url })
+
+        get llm_models_path
+
+        expect(response.body).to include("configured via environment variables")
+        expect(page).to have_css("[data-test-selector='llm-connection--defaults-form'] opce-autocompleter[data-disabled='true']")
+        expect(page).to have_no_button("Save")
+      end
+
       it "keeps a stored default listed once its model is switched off" do
         connection = create(:llm_connection, :with_models, base_url:)
         chat_model = connection.models.find_by(external_id: "qwen3.6-27b")
@@ -660,6 +672,15 @@ def rendered_rows(body) = body.scan("llm-model--toggle-").size
       expect(connection.reload.base_url).to eq(base_url)
     end
 
+    it "refuses a default the environment owns" do
+      allow(Setting).to receive(:llm_connection).and_return({ "base_url" => base_url })
+
+      patch defaults_llm_models_path, params: { llm_connection: { default_chat_model_id: "qwen3.6-27b" } }
+
+      expect(connection.reload.default_chat_model_id).to be_nil
+      expect(flash[:error]).to be_present
+    end
+
     it "is refused to a non-admin" do
       login_as create(:user)
 
diff --git a/spec/seeders/env_data/llm_connection_seeder_spec.rb b/spec/seeders/env_data/llm_connection_seeder_spec.rb
new file mode 100644
index 000000000000..1495a8c5affa
--- /dev/null
+++ b/spec/seeders/env_data/llm_connection_seeder_spec.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe EnvData::LlmConnectionSeeder do
+  subject(:seed) { described_class.new(seed_data).seed! }
+
+  let(:seed_data) { Source::SeedData.new({}) }
+
+  it "does not seed a connection without configuration" do
+    expect { seed }.not_to change(LlmConnection, :count)
+  end
+
+  # On a fresh installation the seed runs before any model synchronisation, so
+  # the model the environment names has no row yet. Provisioning enters it the
+  # way an administrator would, and the refresh confirms it later.
+  context "with a default model configured on a fresh installation", with_settings: {
+    llm_connection: {
+      "base_url" => "https://example.com/v1",
+      "api_key" => "sk-from-env",
+      "default_chat_model" => "qwen3.6-35b-a3b"
+    }
+  } do
+    it "seeds the connection without contacting the server" do
+      expect { seed }.to change(LlmConnection, :count).from(0).to(1)
+
+      connection = LlmConnection.first
+      expect(connection.default_chat_model.external_id).to eq("qwen3.6-35b-a3b")
+      expect(connection.api_key).to eq("sk-from-env")
+    end
+
+    it "enqueues the initial catalogue fill" do
+      expect { seed }.to have_enqueued_job(Llm::SyncModelsJob)
+    end
+  end
+
+  # The seeder runs on every container start, so a refresh here would repeatedly
+  # overwrite a list an administrator has curated. The stale warning on the AI
+  # models page asks for the refresh instead.
+  context "when the environment moves a stored catalogue to another host", with_settings: {
+    llm_connection: { "base_url" => "https://other.example.com/v1", "api_key" => "sk-from-env" }
+  } do
+    let!(:connection) do
+      create(:llm_connection, :with_models, base_url: "https://example.com/v1", api_key: "sk-from-env")
+        .tap { |record| record.update!(connection_fingerprint: record.settings_fingerprint) }
+    end
+
+    it "keeps the stored models and flags them as stale" do
+      expect { seed }.not_to have_enqueued_job(Llm::SyncModelsJob)
+
+      expect(connection.reload.base_url).to eq("https://other.example.com/v1")
+      expect(connection.models.count).to eq(2)
+      expect(connection).to be_models_stale
+    end
+  end
+
+  # The environment is the source of truth while the form is read-only under it,
+  # so a value removed from the environment must not linger in the database.
+  context "when a previously set key is removed from the environment", with_settings: {
+    llm_connection: { "base_url" => "https://example.com/v1" }
+  } do
+    before do
+      connection = create(:llm_connection, base_url: "https://example.com/v1", api_key: "sk-stale")
+      connection.update!(default_chat_model: create(:llm_model, llm_connection: connection,
+                                                                external_id: "old-default"))
+    end
+
+    it "clears the values the environment no longer provides" do
+      expect { seed }.not_to change(LlmConnection, :count)
+
+      connection = LlmConnection.first
+      expect(connection.api_key).to be_nil
+      expect(connection.default_chat_model_id).to be_nil
+      expect(connection.base_url).to eq("https://example.com/v1")
+    end
+  end
+end
diff --git a/spec/workers/llm/sync_models_job_spec.rb b/spec/workers/llm/sync_models_job_spec.rb
index 90ed7c2ab257..e6b8ce96d098 100644
--- a/spec/workers/llm/sync_models_job_spec.rb
+++ b/spec/workers/llm/sync_models_job_spec.rb
@@ -46,4 +46,14 @@
   it "does nothing while no connection is stored" do
     expect { described_class.perform_now }.not_to raise_error
   end
+
+  # The job exists for the startup race with a provisioned LLM sidecar, so a
+  # failed fetch has to reach the retry rather than leave the connection without
+  # its catalogue.
+  it "tries again when the server is not up yet" do
+    create(:llm_connection, base_url:)
+    mock_llm_models_response(base_url, response_code: 404)
+
+    expect { described_class.perform_now }.to have_enqueued_job(described_class)
+  end
 end