feat: add autodiscover enabled feature (#3895)

* add flag to allow the user disable the autodiscover * add global config and doc * feat: Implement autodiscover.mode * fix: Minor doc fixes * fix: Small fixes to docs/indent/tests * fix: Line length, quoting, function comments * fix: Add a few more tests and remove newlines * fix: Always camel case never snake --------- Co-authored-by: Marcelo Medeiros <m.medeiros@carepay.com> Co-authored-by: nitrocode <7775707+nitrocode@users.noreply.github.com> Co-authored-by: PePe Amengual <jose.amengual@gmail.com>
2026-07-29 05:08:22 +00:00 · 2023-11-29 18:43:53 -05:00
parent 6ff0e2f90e
commit 00aae565b2
22 changed files with 822 additions and 35 deletions
--- a/cmd/server.go
+++ b/cmd/server.go
@@ -54,6 +54,7 @@ const (
 	AllowForkPRsFlag                 = "allow-fork-prs"
 	AllowRepoConfigFlag              = "allow-repo-config"
 	AtlantisURLFlag                  = "atlantis-url"
+	AutoDiscoverModeFlag             = "autodiscover-mode"
 	AutomergeFlag                    = "automerge"
 	ParallelPlanFlag                 = "parallel-plan"
 	ParallelApplyFlag                = "parallel-apply"
@@ -148,6 +149,7 @@ const (
 	DefaultADBasicUser                  = ""
 	DefaultADBasicPassword              = ""
 	DefaultADHostname                   = "dev.azure.com"
+	DefaultAutoDiscoverMode             = "auto"
 	DefaultAutoplanFileList             = "**/*.tf,**/*.tfvars,**/*.tfvars.json,**/terragrunt.hcl,**/.terraform.lock.hcl"
 	DefaultAllowCommands                = "version,plan,apply,unlock,approve_policies"
 	DefaultCheckoutStrategy             = CheckoutStrategyBranch
@@ -207,6 +209,12 @@ var stringFlags = map[string]stringFlag{
 	AtlantisURLFlag: {
 		description: "URL that Atlantis can be reached at. Defaults to http://$(hostname):$port where $port is from --" + PortFlag + ". Supports a base path ex. https://example.com/basepath.",
 	},
+	AutoDiscoverModeFlag: {
+		description: "Auto discover mode controls whether projects in a repo are discovered by Atlantis. Defaults to 'auto' which " +
+			"means projects will be discovered when no explicit projects are defined in repo config. Also supports 'enabled' (always " +
+			"discover projects) and 'disabled' (never discover projects).",
+		defaultValue: DefaultAutoDiscoverMode,
+	},
 	AutoplanModulesFromProjects: {
 		description: "Comma separated list of file patterns to select projects Atlantis will index for module dependencies." +
 			" Indexed projects will automatically be planned if a module they depend on is modified." +
@@ -871,6 +879,9 @@ func (s *ServerCmd) setDefaults(c *server.UserConfig) {
 	if c.WebPassword == "" {
 		c.WebPassword = DefaultWebPassword
 	}
+	if c.AutoDiscoverModeFlag == "" {
+		c.AutoDiscoverModeFlag = DefaultAutoDiscoverMode
+	}
 }

 func (s *ServerCmd) validate(userConfig server.UserConfig) error {
--- a/cmd/server_test.go
+++ b/cmd/server_test.go
@@ -60,6 +60,7 @@ var testFlags = map[string]interface{}{
 	AllowCommandsFlag:                "version,plan,unlock,import,approve_policies", // apply is disabled by DisableApply
 	AllowForkPRsFlag:                 true,
 	AllowRepoConfigFlag:              true,
+	AutoDiscoverModeFlag:             "auto",
 	AutomergeFlag:                    true,
 	AutoplanFileListFlag:             "**/*.tf,**/*.yml",
 	BitbucketBaseURLFlag:             "https://bitbucket-base-url.com",
--- a/runatlantis.io/docs/repo-level-atlantis-yaml.md
+++ b/runatlantis.io/docs/repo-level-atlantis-yaml.md
@@ -34,12 +34,18 @@ By default, this is not allowed.
 :::

 ::: warning
-Once an `atlantis.yaml` file exists in a repo, Atlantis won't try to determine
-where to run plan automatically. Instead it will just follow the project configuration.
-This means that you'll need to define each project in your repo.
+Once an `atlantis.yaml` file exists in a repo and one or more `projects` are configured,
+Atlantis won't try to determine where to run plan automatically. Instead it will just
+follow the project configuration. This means that you'll need to define each project
+in your repo.

 If you have many directories with Terraform configuration, each directory will
 need to be defined.
+
+This behavior can be overriden by setting `autodiscover.mode` to
+`enabled` in which case Atlantis will still try to discover projects which were not
+explicitly configured. If the directory of any discovered project conflicts with a
+manually configured project, the manually configured project will take precedence.
 :::

 ## Example Using All Keys
@@ -47,6 +53,8 @@ need to be defined.
 ```yaml
 version: 3
 automerge: true
+autodiscover:
+  mode: auto
 delete_source_branch_on_merge: true
 parallel_plan: true
 parallel_apply: true
@@ -281,6 +289,34 @@ in each group one by one.
 If any plan/apply fails and `abort_on_execution_order_fail` is set to true on a repo level, all the 
 following groups will be aborted. For this example, if project2 fails then project1 will not run.

+### Autodiscovery Config
+```yaml
+autodiscover:
+  mode: "auto"
+```
+The above is the default configuration for `autodiscover.mode`. When `autodiscover.mode` is auto,
+projects will be discovered only if the repo has no `projects` configured.
+
+```yaml
+autodiscover:
+  mode: "disabled"
+```
+With the config above, Atlantis will never try to discover projects, even when there are no
+`projects` configured. This is useful if dynamically generating Atlantis config in pre_workflow hooks.
+See [Dynamic Repo Config Generation](pre-workflow-hooks.html#dynamic-repo-config-generation).
+
+```yaml
+autodiscover:
+  mode: "enabled"
+```
+With the config above, Atlantis will unconditionally try to discover projects based on modified_files,
+even when the directory of the project is missing from the configured `projects` in the repo configuration.
+If a discovered project has the same directory as a project which was manually configured in `projects`,
+the manual configuration will take precedence.
+
+Use this feature when some projects require specific configuration in a repo with many projects yet
+it's still desirable for Atlantis to plan/apply for projects not enumerated in the config.
+
 ### Custom Backend Config
 See [Custom Workflow Use Cases: Custom Backend Config](custom-workflows.html#custom-backend-config)

--- a/runatlantis.io/docs/server-configuration.md
+++ b/runatlantis.io/docs/server-configuration.md
@@ -106,6 +106,22 @@ Values are chosen in this order:
  * If a load balancer with a non http/https port (not the one defined in the `--port` flag) is used, update the URL to include the port like in the example above.
   * This URL is used as the `details` link next to each atlantis job to view the job's logs.

+### `--autodiscover-mode`
+  ```bash
+  atlantis server --autodiscover-mode="<auto|enabled|disabled>"
+  # or
+  ATLANTIS_AUTODISCOVER_MODE="<auto|enabled|disabled>"
+  ```
+  Sets auto discover mode, default is `auto`. When set to `auto`, projects in a repo will be discovered by
+  Atlantis when there are no projects configured in the repo config. If one or more projects are defined
+  in the repo config then auto discovery will be completely disabled.
+
+  When set to `enabled` projects will be discovered unconditionally. If an auto discovered project is already
+  defined in the projects section of the repo config, the project from the repo config will take precedence over
+  the auto discovered project.
+
+  When set to `disabled` projects will never be discovered, even if there are no projects configured in the repo config.
+
 ### `--automerge`
  ```bash
  atlantis server --automerge
--- a/runatlantis.io/docs/server-side-repo-config.md
+++ b/runatlantis.io/docs/server-side-repo-config.md
@@ -88,6 +88,10 @@ repos:
  # policy_check defines if policy checking should be enable on this repository.
  policy_check: false

+  # autodiscover defines how atlantis should automatically discover projects in this repository.
+  autodiscover:
+    mode: auto
+
  # id can also be an exact match.
 - id: github.com/myorg/specific-repo

@@ -496,6 +500,7 @@ If you set a workflow with the key `default`, it will override this.
 | repo_locking                  | bool     | false   | no       | Whether or not to get a lock.                                                                                                                                                                                                                                                                             |
 | policy_check                  | bool     | false   | no       | Whether or not to run policy checks on this repository.                                                                                                                                                                                                                                                   |
 | custom_policy_check                  | bool     | false   | no       | Whether or not to enable custom policy check tools outside of Conftest on this repository.                                                                                                                                                                                                       |
+| autodiscover                  | AutoDiscover     | none   | no       | Auto discover settings for this repo


 :::tip Notes
--- a/server/controllers/events/events_controller_e2e_test.go
+++ b/server/controllers/events/events_controller_e2e_test.go
@@ -1352,6 +1352,7 @@ func setupE2E(t *testing.T, repoDir string, opt setupOption) (events_controllers
 		false,
 		false,
 		false,
+		"auto",
 		statsScope,
 		logger,
 		terraformClient,
--- a/server/core/config/parser_validator_test.go
+++ b/server/core/config/parser_validator_test.go
@@ -1329,6 +1329,22 @@ func TestParseGlobalCfg(t *testing.T) {
  import_requirements: [invalid]`,
 			expErr: "repos: (0: (import_requirements: \"invalid\" is not a valid import_requirement, only \"approved\", \"mergeable\" and \"undiverged\" are supported.).).",
 		},
+		"disable autodiscover": {
+			input: `repos: 
+- id: /.*/
+  autodiscover:
+    mode: disabled`,
+			exp: valid.GlobalCfg{
+				Repos: []valid.Repo{
+					defaultCfg.Repos[0],
+					{
+						IDRegex:      regexp.MustCompile(".*"),
+						AutoDiscover: &valid.AutoDiscover{Mode: valid.AutoDiscoverDisabledMode},
+					},
+				},
+				Workflows: defaultCfg.Workflows,
+			},
+		},
 		"no workflows key": {
 			input: `repos: []`,
 			exp:   defaultCfg,
@@ -1404,6 +1420,8 @@ repos:
  allowed_overrides: [plan_requirements, apply_requirements, import_requirements, workflow, delete_source_branch_on_merge]
  allow_custom_workflows: true
  policy_check: true
+  autodiscover:
+    mode: enabled
 - id: /.*/
  branch: /(master|main)/
  pre_workflow_hooks:
@@ -1411,6 +1429,8 @@ repos:
  post_workflow_hooks:
    - run: custom workflow command
  policy_check: false
+  autodiscover:
+    mode: disabled
 workflows:
  custom1:
    plan:
@@ -1457,6 +1477,7 @@ policies:
 						AllowedOverrides:     []string{"plan_requirements", "apply_requirements", "import_requirements", "workflow", "delete_source_branch_on_merge"},
 						AllowCustomWorkflows: Bool(true),
 						PolicyCheck:          Bool(true),
+						AutoDiscover:         &valid.AutoDiscover{Mode: valid.AutoDiscoverEnabledMode},
 					},
 					{
 						IDRegex:           regexp.MustCompile(".*"),
@@ -1464,6 +1485,7 @@ policies:
 						PreWorkflowHooks:  preWorkflowHooks,
 						PostWorkflowHooks: postWorkflowHooks,
 						PolicyCheck:       Bool(false),
+						AutoDiscover:      &valid.AutoDiscover{Mode: valid.AutoDiscoverDisabledMode},
 					},
 				},
 				Workflows: map[string]valid.Workflow{
@@ -1574,6 +1596,7 @@ workflows:
 						RepoLocking:               Bool(true),
 						PolicyCheck:               Bool(false),
 						CustomPolicyCheck:         Bool(false),
+						AutoDiscover:              raw.DefaultAutoDiscover(),
 					},
 				},
 				Workflows: map[string]valid.Workflow{
@@ -1727,7 +1750,10 @@ func TestParserValidator_ParseGlobalCfgJSON(t *testing.T) {
      "allowed_workflows": ["custom"],
      "apply_requirements": ["mergeable", "approved"],
      "allowed_overrides": ["workflow", "apply_requirements"],
-      "allow_custom_workflows": true
+      "allow_custom_workflows": true,
+      "autodiscover": {
+        "mode": "enabled"
+      }
    },
    {
      "id": "github.com/owner/repo"
@@ -1792,6 +1818,7 @@ func TestParserValidator_ParseGlobalCfgJSON(t *testing.T) {
 						AllowedWorkflows:     []string{"custom"},
 						AllowedOverrides:     []string{"workflow", "apply_requirements"},
 						AllowCustomWorkflows: Bool(true),
+						AutoDiscover:         &valid.AutoDiscover{Mode: valid.AutoDiscoverEnabledMode},
 					},
 					{
 						ID:                   "github.com/owner/repo",
@@ -1799,6 +1826,7 @@ func TestParserValidator_ParseGlobalCfgJSON(t *testing.T) {
 						ApplyRequirements:    nil,
 						AllowedOverrides:     nil,
 						AllowCustomWorkflows: nil,
+						AutoDiscover:         nil,
 					},
 				},
 				Workflows: map[string]valid.Workflow{
--- a/server/core/config/raw/autodiscover.go
+++ b/server/core/config/raw/autodiscover.go
@@ -0,0 +1,38 @@
+package raw
+
+import (
+	validation "github.com/go-ozzo/ozzo-validation"
+	"github.com/runatlantis/atlantis/server/core/config/valid"
+)
+
+var DefaultAutoDiscoverMode = valid.AutoDiscoverAutoMode
+
+type AutoDiscover struct {
+	Mode *valid.AutoDiscoverMode `yaml:"mode,omitempty"`
+}
+
+func (a AutoDiscover) ToValid() *valid.AutoDiscover {
+	var v valid.AutoDiscover
+
+	if a.Mode != nil {
+		v.Mode = *a.Mode
+	} else {
+		v.Mode = DefaultAutoDiscoverMode
+	}
+
+	return &v
+}
+
+func (a AutoDiscover) Validate() error {
+	res := validation.ValidateStruct(&a,
+		// If a.Mode is nil, this should still pass validation.
+		validation.Field(&a.Mode, validation.In(valid.AutoDiscoverAutoMode, valid.AutoDiscoverDisabledMode, valid.AutoDiscoverEnabledMode)),
+	)
+	return res
+}
+
+func DefaultAutoDiscover() *valid.AutoDiscover {
+	return &valid.AutoDiscover{
+		Mode: DefaultAutoDiscoverMode,
+	}
+}
--- a/server/core/config/raw/autodiscover_test.go
+++ b/server/core/config/raw/autodiscover_test.go
@@ -0,0 +1,131 @@
+package raw_test
+
+import (
+	"testing"
+
+	"github.com/runatlantis/atlantis/server/core/config/raw"
+	"github.com/runatlantis/atlantis/server/core/config/valid"
+	. "github.com/runatlantis/atlantis/testing"
+	yaml "gopkg.in/yaml.v2"
+)
+
+func TestAutoDiscover_UnmarshalYAML(t *testing.T) {
+	autoDiscoverEnabled := valid.AutoDiscoverEnabledMode
+	cases := []struct {
+		description string
+		input       string
+		exp         raw.AutoDiscover
+	}{
+		{
+			description: "omit unset fields",
+			input:       "",
+			exp: raw.AutoDiscover{
+				Mode: nil,
+			},
+		},
+		{
+			description: "all fields set",
+			input: `
+mode: enabled
+`,
+			exp: raw.AutoDiscover{
+				Mode: &autoDiscoverEnabled,
+			},
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.description, func(t *testing.T) {
+			var a raw.AutoDiscover
+			err := yaml.UnmarshalStrict([]byte(c.input), &a)
+			Ok(t, err)
+			Equals(t, c.exp, a)
+		})
+	}
+}
+
+func TestAutoDiscover_Validate(t *testing.T) {
+	autoDiscoverAuto := valid.AutoDiscoverAutoMode
+	autoDiscoverEnabled := valid.AutoDiscoverEnabledMode
+	autoDiscoverDisabled := valid.AutoDiscoverDisabledMode
+	randomString := valid.AutoDiscoverMode("random_string")
+	cases := []struct {
+		description string
+		input       raw.AutoDiscover
+		errContains *string
+	}{
+		{
+			description: "nothing set",
+			input:       raw.AutoDiscover{},
+			errContains: nil,
+		},
+		{
+			description: "mode set to auto",
+			input: raw.AutoDiscover{
+				Mode: &autoDiscoverAuto,
+			},
+			errContains: nil,
+		},
+		{
+			description: "mode set to disabled",
+			input: raw.AutoDiscover{
+				Mode: &autoDiscoverDisabled,
+			},
+			errContains: nil,
+		},
+		{
+			description: "mode set to enabled",
+			input: raw.AutoDiscover{
+				Mode: &autoDiscoverEnabled,
+			},
+			errContains: nil,
+		},
+		{
+			description: "mode set to random string",
+			input: raw.AutoDiscover{
+				Mode: &randomString,
+			},
+			errContains: String("valid value"),
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.description, func(t *testing.T) {
+			if c.errContains == nil {
+				Ok(t, c.input.Validate())
+			} else {
+				ErrContains(t, *c.errContains, c.input.Validate())
+			}
+		})
+	}
+}
+
+func TestAutoDiscover_ToValid(t *testing.T) {
+	autoDiscoverEnabled := valid.AutoDiscoverEnabledMode
+	cases := []struct {
+		description string
+		input       raw.AutoDiscover
+		exp         *valid.AutoDiscover
+	}{
+		{
+			description: "nothing set",
+			input:       raw.AutoDiscover{},
+			exp: &valid.AutoDiscover{
+				Mode: valid.AutoDiscoverAutoMode,
+			},
+		},
+		{
+			description: "value set",
+			input: raw.AutoDiscover{
+				Mode: &autoDiscoverEnabled,
+			},
+			exp: &valid.AutoDiscover{
+				Mode: valid.AutoDiscoverEnabledMode,
+			},
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.description, func(t *testing.T) {
+			Equals(t, c.exp, c.input.ToValid())
+		})
+	}
+}
--- a/server/core/config/raw/global_cfg.go
+++ b/server/core/config/raw/global_cfg.go
@@ -36,6 +36,7 @@ type Repo struct {
 	RepoLocking               *bool          `yaml:"repo_locking,omitempty" json:"repo_locking,omitempty"`
 	PolicyCheck               *bool          `yaml:"policy_check,omitempty" json:"policy_check,omitempty"`
 	CustomPolicyCheck         *bool          `yaml:"custom_policy_check,omitempty" json:"custom_policy_check,omitempty"`
+	AutoDiscover              *AutoDiscover  `yaml:"autodiscover,omitempty" json:"autodiscover,omitempty"`
 }

 func (g GlobalCfg) Validate() error {
@@ -211,6 +212,14 @@ func (r Repo) Validate() error {
 		return nil
 	}

+	autoDiscoverValid := func(value interface{}) error {
+		autoDiscover := value.(*AutoDiscover)
+		if autoDiscover != nil {
+			return autoDiscover.Validate()
+		}
+		return nil
+	}
+
 	return validation.ValidateStruct(&r,
 		validation.Field(&r.ID, validation.Required, validation.By(idValid)),
 		validation.Field(&r.Branch, validation.By(branchValid)),
@@ -221,6 +230,7 @@ func (r Repo) Validate() error {
 		validation.Field(&r.ImportRequirements, validation.By(validImportReq)),
 		validation.Field(&r.Workflow, validation.By(workflowExists)),
 		validation.Field(&r.DeleteSourceBranchOnMerge, validation.By(deleteSourceBranchOnMergeValid)),
+		validation.Field(&r.AutoDiscover, validation.By(autoDiscoverValid)),
 	)
 }

@@ -281,7 +291,7 @@ OuterGlobalPlanReqs:
 		}

 		// dont add policy_check step if repo have it explicitly disabled
-		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && *r.PolicyCheck == false {
+		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && !*r.PolicyCheck {
 			continue
 		}
 		mergedPlanReqs = append(mergedPlanReqs, globalReq)
@@ -295,7 +305,7 @@ OuterGlobalApplyReqs:
 		}

 		// dont add policy_check step if repo have it explicitly disabled
-		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && *r.PolicyCheck == false {
+		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && !*r.PolicyCheck {
 			continue
 		}
 		mergedApplyReqs = append(mergedApplyReqs, globalReq)
@@ -309,12 +319,17 @@ OuterGlobalImportReqs:
 		}

 		// dont add policy_check step if repo have it explicitly disabled
-		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && *r.PolicyCheck == false {
+		if globalReq == valid.PoliciesPassedCommandReq && r.PolicyCheck != nil && !*r.PolicyCheck {
 			continue
 		}
 		mergedImportReqs = append(mergedImportReqs, globalReq)
 	}

+	var autoDiscover *valid.AutoDiscover
+	if r.AutoDiscover != nil {
+		autoDiscover = r.AutoDiscover.ToValid()
+	}
+
 	return valid.Repo{
 		ID:                        id,
 		IDRegex:                   idRegex,
@@ -333,5 +348,6 @@ OuterGlobalImportReqs:
 		RepoLocking:               r.RepoLocking,
 		PolicyCheck:               r.PolicyCheck,
 		CustomPolicyCheck:         r.CustomPolicyCheck,
+		AutoDiscover:              autoDiscover,
 	}
 }
--- a/server/core/config/raw/repo_cfg.go
+++ b/server/core/config/raw/repo_cfg.go
@@ -19,6 +19,7 @@ type RepoCfg struct {
 	Projects                   []Project           `yaml:"projects,omitempty"`
 	Workflows                  map[string]Workflow `yaml:"workflows,omitempty"`
 	PolicySets                 PolicySets          `yaml:"policies,omitempty"`
+	AutoDiscover               *AutoDiscover       `yaml:"autodiscover,omitempty"`
 	Automerge                  *bool               `yaml:"automerge,omitempty"`
 	ParallelApply              *bool               `yaml:"parallel_apply,omitempty"`
 	ParallelPlan               *bool               `yaml:"parallel_plan,omitempty"`
@@ -71,10 +72,16 @@ func (r RepoCfg) ToValid() valid.RepoCfg {
 		abortOnExcecutionOrderFail = *r.AbortOnExcecutionOrderFail
 	}

+	var autoDiscover *valid.AutoDiscover
+	if r.AutoDiscover != nil {
+		autoDiscover = r.AutoDiscover.ToValid()
+	}
+
 	return valid.RepoCfg{
 		Version:                    *r.Version,
 		Projects:                   validProjects,
 		Workflows:                  validWorkflows,
+		AutoDiscover:               autoDiscover,
 		Automerge:                  automerge,
 		ParallelApply:              parallelApply,
 		ParallelPlan:               parallelPlan,
--- a/server/core/config/raw/repo_cfg_test.go
+++ b/server/core/config/raw/repo_cfg_test.go
@@ -11,6 +11,7 @@ import (
 )

 func TestConfig_UnmarshalYAML(t *testing.T) {
+	autoDiscoverEnabled := valid.AutoDiscoverEnabledMode
 	cases := []struct {
 		description string
 		input       string
@@ -126,6 +127,8 @@ func TestConfig_UnmarshalYAML(t *testing.T) {
 			input: `
 version: 3
 automerge: true
+autodiscover:
+  mode: enabled
 parallel_apply: true
 parallel_plan: false
 projects:
@@ -150,6 +153,7 @@ allowed_regexp_prefixes:
 - staging/`,
 			exp: raw.RepoCfg{
 				Version:       Int(3),
+				AutoDiscover:  &raw.AutoDiscover{Mode: &autoDiscoverEnabled},
 				Automerge:     Bool(true),
 				ParallelApply: Bool(true),
 				ParallelPlan:  Bool(false),
@@ -232,6 +236,7 @@ func TestConfig_Validate(t *testing.T) {
 }

 func TestConfig_ToValid(t *testing.T) {
+	autoDiscoverEnabled := valid.AutoDiscoverEnabledMode
 	cases := []struct {
 		description string
 		input       raw.RepoCfg
@@ -248,18 +253,20 @@ func TestConfig_ToValid(t *testing.T) {
 		{
 			description: "set to empty",
 			input: raw.RepoCfg{
-				Version:   Int(2),
-				Workflows: map[string]raw.Workflow{},
-				Projects:  []raw.Project{},
+				Version:      Int(2),
+				AutoDiscover: &raw.AutoDiscover{},
+				Workflows:    map[string]raw.Workflow{},
+				Projects:     []raw.Project{},
 			},
 			exp: valid.RepoCfg{
-				Version:   2,
-				Workflows: map[string]valid.Workflow{},
-				Projects:  nil,
+				Version:      2,
+				AutoDiscover: raw.DefaultAutoDiscover(),
+				Workflows:    map[string]valid.Workflow{},
+				Projects:     nil,
 			},
 		},
 		{
-			description: "automerge, parallel_apply and abort_on_execution_order_fail omitted",
+			description: "automerge, parallel_apply, abort_on_execution_order_fail omitted",
 			input: raw.RepoCfg{
 				Version: Int(2),
 			},
@@ -272,7 +279,7 @@ func TestConfig_ToValid(t *testing.T) {
 			},
 		},
 		{
-			description: "automerge, parallel_apply and abort_on_execution_order_fail true",
+			description: "automerge, parallel_apply, abort_on_execution_order_fail true",
 			input: raw.RepoCfg{
 				Version:                    Int(2),
 				Automerge:                  Bool(true),
@@ -288,7 +295,7 @@ func TestConfig_ToValid(t *testing.T) {
 			},
 		},
 		{
-			description: "automerge, parallel_apply and abort_on_execution_order_fail false",
+			description: "automerge, parallel_apply, abort_on_execution_order_fail false",
 			input: raw.RepoCfg{
 				Version:                    Int(2),
 				Automerge:                  Bool(false),
@@ -303,6 +310,30 @@ func TestConfig_ToValid(t *testing.T) {
 				Workflows:                  map[string]valid.Workflow{},
 			},
 		},
+		{
+			description: "autodiscover omitted",
+			input: raw.RepoCfg{
+				Version: Int(2),
+			},
+			exp: valid.RepoCfg{
+				Version:   2,
+				Workflows: map[string]valid.Workflow{},
+			},
+		},
+		{
+			description: "autodiscover included",
+			input: raw.RepoCfg{
+				Version:      Int(2),
+				AutoDiscover: &raw.AutoDiscover{Mode: &autoDiscoverEnabled},
+			},
+			exp: valid.RepoCfg{
+				Version: 2,
+				AutoDiscover: &valid.AutoDiscover{
+					Mode: valid.AutoDiscoverEnabledMode,
+				},
+				Workflows: map[string]valid.Workflow{},
+			},
+		},
 		{
 			description: "only plan stage set",
 			input: raw.RepoCfg{
@@ -339,6 +370,9 @@ func TestConfig_ToValid(t *testing.T) {
 				Version:       Int(2),
 				Automerge:     Bool(true),
 				ParallelApply: Bool(true),
+				AutoDiscover: &raw.AutoDiscover{
+					Mode: &autoDiscoverEnabled,
+				},
 				Workflows: map[string]raw.Workflow{
 					"myworkflow": {
 						Apply: &raw.Stage{
@@ -388,6 +422,9 @@ func TestConfig_ToValid(t *testing.T) {
 				Version:       2,
 				Automerge:     Bool(true),
 				ParallelApply: Bool(true),
+				AutoDiscover: &valid.AutoDiscover{
+					Mode: valid.AutoDiscoverEnabledMode,
+				},
 				Workflows: map[string]valid.Workflow{
 					"myworkflow": {
 						Name: "myworkflow",
--- a/server/core/config/valid/autodiscover.go
+++ b/server/core/config/valid/autodiscover.go
@@ -0,0 +1,14 @@
+package valid
+
+// AutoDiscoverMode enum
+type AutoDiscoverMode string
+
+const (
+	AutoDiscoverEnabledMode  AutoDiscoverMode = "enabled"
+	AutoDiscoverDisabledMode AutoDiscoverMode = "disabled"
+	AutoDiscoverAutoMode     AutoDiscoverMode = "auto"
+)
+
+type AutoDiscover struct {
+	Mode AutoDiscoverMode
+}
--- a/server/core/config/valid/global_cfg.go
+++ b/server/core/config/valid/global_cfg.go
@@ -28,6 +28,7 @@ const DeleteSourceBranchOnMergeKey = "delete_source_branch_on_merge"
 const RepoLockingKey = "repo_locking"
 const PolicyCheckKey = "policy_check"
 const CustomPolicyCheckKey = "custom_policy_check"
+const AutoDiscoverKey = "autodiscover"

 // DefaultAtlantisFile is the default name of the config file for each repo.
 const DefaultAtlantisFile = "atlantis.yaml"
@@ -84,6 +85,7 @@ type Repo struct {
 	RepoLocking               *bool
 	PolicyCheck               *bool
 	CustomPolicyCheck         *bool
+	AutoDiscover              *AutoDiscover
 }

 type MergedProjectCfg struct {
@@ -245,6 +247,7 @@ func NewGlobalCfgFromArgs(args GlobalCfgArgs) GlobalCfg {
 	deleteSourceBranchOnMerge := false
 	repoLockingKey := true
 	customPolicyCheck := false
+	autoDiscover := AutoDiscover{Mode: AutoDiscoverAutoMode}
 	if args.AllowRepoCfg {
 		allowedOverrides = []string{PlanRequirementsKey, ApplyRequirementsKey, ImportRequirementsKey, WorkflowKey, DeleteSourceBranchOnMergeKey, RepoLockingKey, PolicyCheckKey}
 		allowCustomWorkflows = true
@@ -269,6 +272,7 @@ func NewGlobalCfgFromArgs(args GlobalCfgArgs) GlobalCfg {
 				RepoLocking:               &repoLockingKey,
 				PolicyCheck:               &policyCheck,
 				CustomPolicyCheck:         &customPolicyCheck,
+				AutoDiscover:              &autoDiscover,
 			},
 		},
 		Workflows: map[string]Workflow{
@@ -305,7 +309,7 @@ func (r Repo) IDString() string {
 // final config. It assumes that all configs have been validated.
 func (g GlobalCfg) MergeProjectCfg(log logging.SimpleLogging, repoID string, proj Project, rCfg RepoCfg) MergedProjectCfg {
 	log.Debug("MergeProjectCfg started")
-	planReqs, applyReqs, importReqs, workflow, allowedOverrides, allowCustomWorkflows, deleteSourceBranchOnMerge, repoLocking, policyCheck, customPolicyCheck := g.getMatchingCfg(log, repoID)
+	planReqs, applyReqs, importReqs, workflow, allowedOverrides, allowCustomWorkflows, deleteSourceBranchOnMerge, repoLocking, policyCheck, customPolicyCheck, _ := g.getMatchingCfg(log, repoID)

 	// If repos are allowed to override certain keys then override them.
 	for _, key := range allowedOverrides {
@@ -407,7 +411,7 @@ func (g GlobalCfg) MergeProjectCfg(log logging.SimpleLogging, repoID string, pro
 // repo with id repoID. It is used when there is no repo config.
 func (g GlobalCfg) DefaultProjCfg(log logging.SimpleLogging, repoID string, repoRelDir string, workspace string) MergedProjectCfg {
 	log.Debug("building config based on server-side config")
-	planReqs, applyReqs, importReqs, workflow, _, _, deleteSourceBranchOnMerge, repoLocking, policyCheck, customPolicyCheck := g.getMatchingCfg(log, repoID)
+	planReqs, applyReqs, importReqs, workflow, _, _, deleteSourceBranchOnMerge, repoLocking, policyCheck, customPolicyCheck, _ := g.getMatchingCfg(log, repoID)
 	return MergedProjectCfg{
 		PlanRequirements:          planReqs,
 		ApplyRequirements:         applyReqs,
@@ -426,6 +430,17 @@ func (g GlobalCfg) DefaultProjCfg(log logging.SimpleLogging, repoID string, repo
 	}
 }

+// RepoAutoDiscoverCfg returns the AutoDiscover config from the global config
+// for the repo with id repoID. If no matching repo is found or there is no
+// AutoDiscover config then this function returns nil.
+func (g GlobalCfg) RepoAutoDiscoverCfg(repoID string) *AutoDiscover {
+	repo := g.MatchingRepo(repoID)
+	if repo != nil {
+		return repo.AutoDiscover
+	}
+	return nil
+}
+
 // ValidateRepoCfg validates that rCfg for repo with id repoID is valid based
 // on our global config.
 func (g GlobalCfg) ValidateRepoCfg(rCfg RepoCfg, repoID string) error {
@@ -528,7 +543,7 @@ func (g GlobalCfg) ValidateRepoCfg(rCfg RepoCfg, repoID string) error {
 }

 // getMatchingCfg returns the key settings for repoID.
-func (g GlobalCfg) getMatchingCfg(log logging.SimpleLogging, repoID string) (planReqs []string, applyReqs []string, importReqs []string, workflow Workflow, allowedOverrides []string, allowCustomWorkflows bool, deleteSourceBranchOnMerge bool, repoLocking bool, policyCheck bool, customPolicyCheck bool) {
+func (g GlobalCfg) getMatchingCfg(log logging.SimpleLogging, repoID string) (planReqs []string, applyReqs []string, importReqs []string, workflow Workflow, allowedOverrides []string, allowCustomWorkflows bool, deleteSourceBranchOnMerge bool, repoLocking bool, policyCheck bool, customPolicyCheck bool, autoDiscover AutoDiscover) {
 	toLog := make(map[string]string)
 	traceF := func(repoIdx int, repoID string, key string, val interface{}) string {
 		from := "default server config"
@@ -550,6 +565,9 @@ func (g GlobalCfg) getMatchingCfg(log logging.SimpleLogging, repoID string) (pla
 		return fmt.Sprintf("setting %s: %s from %s", key, valStr, from)
 	}

+	// Can't use raw.DefaultAutoDiscoverMode() because of an import cycle. Should refactor to avoid that.
+	autoDiscover = AutoDiscover{Mode: AutoDiscoverAutoMode}
+
 	for _, key := range []string{PlanRequirementsKey, ApplyRequirementsKey, ImportRequirementsKey, WorkflowKey, AllowedOverridesKey, AllowCustomWorkflowsKey, DeleteSourceBranchOnMergeKey, RepoLockingKey, PolicyCheckKey, CustomPolicyCheckKey} {
 		for i, repo := range g.Repos {
 			if repo.IDMatches(repoID) {
@@ -604,6 +622,11 @@ func (g GlobalCfg) getMatchingCfg(log logging.SimpleLogging, repoID string) (pla
 						toLog[CustomPolicyCheckKey] = traceF(i, repo.IDString(), CustomPolicyCheckKey, *repo.CustomPolicyCheck)
 						customPolicyCheck = *repo.CustomPolicyCheck
 					}
+				case AutoDiscoverKey:
+					if repo.AutoDiscover != nil {
+						toLog[AutoDiscoverKey] = traceF(i, repo.IDString(), AutoDiscoverKey, repo.AutoDiscover.Mode)
+						autoDiscover = *repo.AutoDiscover
+					}
 				}
 			}
 		}
--- a/server/core/config/valid/global_cfg_test.go
+++ b/server/core/config/valid/global_cfg_test.go
@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/go-version"
 	"github.com/mohae/deepcopy"
 	"github.com/runatlantis/atlantis/server/core/config"
+	"github.com/runatlantis/atlantis/server/core/config/raw"
 	"github.com/runatlantis/atlantis/server/core/config/valid"
 	"github.com/runatlantis/atlantis/server/logging"
 	. "github.com/runatlantis/atlantis/testing"
@@ -82,6 +83,7 @@ func TestNewGlobalCfg(t *testing.T) {
 				RepoLocking:               Bool(true),
 				PolicyCheck:               Bool(false),
 				CustomPolicyCheck:         Bool(false),
+				AutoDiscover:              raw.DefaultAutoDiscover(),
 			},
 		},
 		Workflows: map[string]valid.Workflow{
--- a/server/core/config/valid/repo_cfg.go
+++ b/server/core/config/valid/repo_cfg.go
@@ -19,6 +19,7 @@ type RepoCfg struct {
 	Workflows                  map[string]Workflow
 	PolicySets                 PolicySets
 	Automerge                  *bool
+	AutoDiscover               *AutoDiscover
 	ParallelApply              *bool
 	ParallelPlan               *bool
 	ParallelPolicyCheck        *bool
@@ -91,6 +92,24 @@ func isRegexAllowed(name string, allowedRegexpPrefixes []string) bool {
 	return false
 }

+// This function returns a final true/false decision for whether AutoDiscover is enabled
+// for a repo. It takes into account the defaultAutoDiscoverMode when there is no explicit
+// repo config. The defaultAutoDiscoverMode param should be understood as the default
+// AutoDiscover mode as may be set via CLI params or server side repo config.
+func (r RepoCfg) AutoDiscoverEnabled(defaultAutoDiscoverMode AutoDiscoverMode) bool {
+	autoDiscoverMode := defaultAutoDiscoverMode
+	if r.AutoDiscover != nil {
+		autoDiscoverMode = r.AutoDiscover.Mode
+	}
+
+	if autoDiscoverMode == AutoDiscoverAutoMode {
+		// AutoDiscover is enabled by default when no projects are defined
+		return len(r.Projects) == 0
+	}
+
+	return autoDiscoverMode == AutoDiscoverEnabledMode
+}
+
 // validateWorkspaceAllowed returns an error if repoCfg defines projects in
 // repoRelDir but none of them use workspace. We want this to be an error
 // because if users have gone to the trouble of defining projects in repoRelDir
--- a/server/core/config/valid/repo_cfg_test.go
+++ b/server/core/config/valid/repo_cfg_test.go
@@ -216,3 +216,110 @@ func TestConfig_FindProjectsByDir(t *testing.T) {
 		})
 	}
 }
+
+func TestConfig_AutoDiscoverEnabled(t *testing.T) {
+	cases := []struct {
+		description         string
+		repoAutoDiscover    valid.AutoDiscoverMode
+		defaultAutoDiscover valid.AutoDiscoverMode
+		projects            []valid.Project
+		expEnabled          bool
+	}{
+		{
+			description:         "repo disabled autodiscover default enabled",
+			repoAutoDiscover:    valid.AutoDiscoverDisabledMode,
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			expEnabled:          false,
+		},
+		{
+			description:         "repo disabled autodiscover default disabled",
+			repoAutoDiscover:    valid.AutoDiscoverDisabledMode,
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			expEnabled:          false,
+		},
+		{
+			description:         "repo enabled autodiscover default enabled",
+			repoAutoDiscover:    valid.AutoDiscoverEnabledMode,
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo enabled autodiscover default disabled",
+			repoAutoDiscover:    valid.AutoDiscoverEnabledMode,
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo set auto autodiscover with no projects default enabled",
+			repoAutoDiscover:    valid.AutoDiscoverAutoMode,
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo set auto autodiscover with no projects default disabled",
+			repoAutoDiscover:    valid.AutoDiscoverAutoMode,
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo set auto autodiscover with a project default enabled",
+			repoAutoDiscover:    valid.AutoDiscoverAutoMode,
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			projects:            []valid.Project{{}},
+			expEnabled:          false,
+		},
+		{
+			description:         "repo set auto autodiscover with a project default disabled",
+			repoAutoDiscover:    valid.AutoDiscoverAutoMode,
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			projects:            []valid.Project{{}},
+			expEnabled:          false,
+		},
+		{
+			description:         "repo unset autodiscover with no projects default enabled",
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo unset autodiscover with no projects default disabled",
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			expEnabled:          false,
+		},
+		{
+			description:         "repo unset autodiscover with no projects default auto",
+			defaultAutoDiscover: valid.AutoDiscoverAutoMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo unset autodiscover with a project default enabled",
+			projects:            []valid.Project{{}},
+			defaultAutoDiscover: valid.AutoDiscoverEnabledMode,
+			expEnabled:          true,
+		},
+		{
+			description:         "repo unset autodiscover with a project default disabled",
+			projects:            []valid.Project{{}},
+			defaultAutoDiscover: valid.AutoDiscoverDisabledMode,
+			expEnabled:          false,
+		},
+		{
+			description:         "repo unset autodiscover with a project default auto",
+			projects:            []valid.Project{{}},
+			defaultAutoDiscover: valid.AutoDiscoverAutoMode,
+			expEnabled:          false,
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.description, func(t *testing.T) {
+			r := valid.RepoCfg{
+				Projects:     c.projects,
+				AutoDiscover: nil,
+			}
+			if c.repoAutoDiscover != "" {
+				r.AutoDiscover = &valid.AutoDiscover{c.repoAutoDiscover}
+			}
+			enabled := r.AutoDiscoverEnabled(c.defaultAutoDiscover)
+			Equals(t, c.expEnabled, enabled)
+		})
+	}
+}
--- a/server/events/project_command_builder.go
+++ b/server/events/project_command_builder.go
@@ -18,6 +18,7 @@ import (

 	"github.com/runatlantis/atlantis/server/core/config"
 	"github.com/runatlantis/atlantis/server/events/command"
+	"github.com/runatlantis/atlantis/server/events/models"
 	"github.com/runatlantis/atlantis/server/events/vcs"
 )

@@ -54,6 +55,7 @@ func NewInstrumentedProjectCommandBuilder(
 	RestrictFileList bool,
 	SilenceNoProjects bool,
 	IncludeGitUntrackedFiles bool,
+	AutoDiscoverMode string,
 	scope tally.Scope,
 	logger logging.SimpleLogging,
 	terraformClient terraform.Client,
@@ -85,6 +87,7 @@ func NewInstrumentedProjectCommandBuilder(
 			RestrictFileList,
 			SilenceNoProjects,
 			IncludeGitUntrackedFiles,
+			AutoDiscoverMode,
 			scope,
 			logger,
 			terraformClient,
@@ -114,6 +117,7 @@ func NewProjectCommandBuilder(
 	RestrictFileList bool,
 	SilenceNoProjects bool,
 	IncludeGitUntrackedFiles bool,
+	AutoDiscoverMode string,
 	scope tally.Scope,
 	logger logging.SimpleLogging,
 	terraformClient terraform.Client,
@@ -136,6 +140,7 @@ func NewProjectCommandBuilder(
 		RestrictFileList:         RestrictFileList,
 		SilenceNoProjects:        SilenceNoProjects,
 		IncludeGitUntrackedFiles: IncludeGitUntrackedFiles,
+		AutoDiscoverMode:         AutoDiscoverMode,
 		ProjectCommandContextBuilder: NewProjectCommandContextBuilder(
 			policyChecksSupported,
 			commentBuilder,
@@ -242,6 +247,8 @@ type DefaultProjectCommandBuilder struct {
 	SilenceNoProjects bool
 	// User config option: Include git untracked files in the modified file list.
 	IncludeGitUntrackedFiles bool
+	// User config option: Controls auto-discovery of projects in a repository.
+	AutoDiscoverMode string
 	// Handles the actual running of Terraform commands.
 	TerraformExecutor terraform.Client
 }
@@ -336,6 +343,13 @@ func (p *DefaultProjectCommandBuilder) buildAllCommandsByCfg(ctx *command.Contex

 	ctx.Log.Debug("%d files were modified in this pull request. Modified files: %v", len(modifiedFiles), modifiedFiles)

+	// Get default AutoDiscoverMode from userConfig/globalConfig
+	defaultAutoDiscoverMode := valid.AutoDiscoverMode(p.AutoDiscoverMode)
+	globalAutoDiscover := p.GlobalCfg.RepoAutoDiscoverCfg(ctx.Pull.BaseRepo.ID())
+	if globalAutoDiscover != nil {
+		defaultAutoDiscoverMode = globalAutoDiscover.Mode
+	}
+
 	if p.SkipCloneNoChanges && p.VCSClient.SupportsSingleFileDownload(ctx.Pull.BaseRepo) {
 		repoCfgFile := p.GlobalCfg.RepoConfigFile(ctx.Pull.BaseRepo.ID())
 		hasRepoCfg, repoCfgData, err := p.VCSClient.GetFileContent(ctx.Pull, repoCfgFile)
@@ -349,18 +363,27 @@ func (p *DefaultProjectCommandBuilder) buildAllCommandsByCfg(ctx *command.Contex
 				return nil, errors.Wrapf(err, "parsing %s", repoCfgFile)
 			}
 			ctx.Log.Info("successfully parsed remote %s file", repoCfgFile)
-			if len(repoCfg.Projects) > 0 {
-				matchingProjects, err := p.ProjectFinder.DetermineProjectsViaConfig(ctx.Log, modifiedFiles, repoCfg, "", nil)
-				if err != nil {
-					return nil, err
-				}
-				ctx.Log.Info("%d projects are changed on MR %q based on their when_modified config", len(matchingProjects), ctx.Pull.Num)
-				if len(matchingProjects) == 0 {
-					ctx.Log.Info("skipping repo clone since no project was modified")
-					return []command.ProjectContext{}, nil
+
+			if repoCfg.AutoDiscover != nil {
+				defaultAutoDiscoverMode = repoCfg.AutoDiscover.Mode
+			}
+			// If auto discover is enabled, we never want to skip cloning
+			if !repoCfg.AutoDiscoverEnabled(defaultAutoDiscoverMode) {
+				if len(repoCfg.Projects) > 0 {
+					matchingProjects, err := p.ProjectFinder.DetermineProjectsViaConfig(ctx.Log, modifiedFiles, repoCfg, "", nil)
+					if err != nil {
+						return nil, err
+					}
+					ctx.Log.Info("%d projects are changed on MR %q based on their when_modified config", len(matchingProjects), ctx.Pull.Num)
+					if len(matchingProjects) == 0 {
+						ctx.Log.Info("skipping repo clone since no project was modified")
+						return []command.ProjectContext{}, nil
+					}
+				} else {
+					ctx.Log.Info("no projects are defined in %s. Will resume automatic detection", repoCfgFile)
 				}
 			} else {
-				ctx.Log.Info("No projects are defined in %s. Will resume automatic detection", repoCfgFile)
+				ctx.Log.Info("automatic project discovery enabled. Will resume automatic detection")
 			}
 			// NOTE: We discard this work here and end up doing it again after
 			// cloning to ensure all the return values are set properly with
@@ -402,6 +425,15 @@ func (p *DefaultProjectCommandBuilder) buildAllCommandsByCfg(ctx *command.Contex
 			return nil, errors.Wrapf(err, "parsing %s", repoCfgFile)
 		}
 		ctx.Log.Info("successfully parsed %s file", repoCfgFile)
+		// It's possible we've already set defaultAutoDiscoverMode
+		// from the config file while checking whether we can skip
+		// cloning. We still need to set it here in the case that
+		// we were not able to check whether we can skip cloning
+		// and thus were not able to previously fetch the repo
+		// config.
+		if repoCfg.AutoDiscover != nil {
+			defaultAutoDiscoverMode = repoCfg.AutoDiscover.Mode
+		}
 	}

 	moduleInfo, err := FindModuleProjects(repoDir, p.AutoDetectModuleFiles)
@@ -450,21 +482,46 @@ func (p *DefaultProjectCommandBuilder) buildAllCommandsByCfg(ctx *command.Contex
 					parallelApply,
 					parallelPlan,
 					verbose,
-					repoCfg.AbortOnExcecutionOrderFail,
+					abortOnExcecutionOrderFail,
 					p.TerraformExecutor,
 				)...)
 		}
-	} else {
+	}
+
+	if repoCfg.AutoDiscoverEnabled(defaultAutoDiscoverMode) {
 		// If there is no config file or it specified no projects, then we'll plan each project that
 		// our algorithm determines was modified.
 		if hasRepoCfg {
-			ctx.Log.Info("No projects are defined in %s. Will resume automatic detection", repoCfgFile)
+			if len(repoCfg.Projects) == 0 {
+				ctx.Log.Info("no projects are defined in %s. Will resume automatic detection", repoCfgFile)
+			} else {
+				ctx.Log.Info("automatic project discovery enabled. Will resume automatic detection")
+			}
 		} else {
 			ctx.Log.Info("found no %s file", repoCfgFile)
 		}
 		// build a module index for projects that are explicitly included
-		modifiedProjects := p.ProjectFinder.DetermineProjects(ctx.Log, modifiedFiles, ctx.Pull.BaseRepo.FullName, repoDir, p.AutoplanFileList, moduleInfo)
-		ctx.Log.Info("automatically determined that there were %d projects modified in this pull request: %s", len(modifiedProjects), modifiedProjects)
+		allModifiedProjects := p.ProjectFinder.DetermineProjects(
+			ctx.Log, modifiedFiles, ctx.Pull.BaseRepo.FullName, repoDir, p.AutoplanFileList, moduleInfo)
+		// If a project is already manually configured with the same dir as a discovered project, the manually configured
+		// project should take precedence
+		modifiedProjects := make([]models.Project, 0)
+		configuredProjDirs := make(map[string]bool)
+		// We compare against all configured projects instead of projects which match the modified files in case a
+		// project is being specifically excluded (ex: when_modified doesn't match). We don't want to accidentally
+		// "discover" it again.
+		for _, configProj := range repoCfg.Projects {
+			// Clean the path to make sure ./rel_path is equivalent to rel_path, etc
+			configuredProjDirs[filepath.Clean(configProj.Dir)] = true
+		}
+		for _, mp := range allModifiedProjects {
+			_, dirExists := configuredProjDirs[filepath.Clean(mp.Path)]
+			if !dirExists {
+				modifiedProjects = append(modifiedProjects, mp)
+			}
+		}
+		ctx.Log.Info("automatically determined that there were %d additional projects modified in this pull request: %s",
+			len(modifiedProjects), modifiedProjects)
 		for _, mp := range modifiedProjects {
 			ctx.Log.Debug("determining config for project at dir: %q", mp.Path)
 			pWorkspace, err := p.ProjectFinder.DetermineWorkspaceFromHCL(ctx.Log, repoDir)
--- a/server/events/project_command_builder_internal_test.go
+++ b/server/events/project_command_builder_internal_test.go
@@ -673,6 +673,7 @@ projects:
 				false,
 				false,
 				false,
+				"auto",
 				statsScope,
 				logger,
 				terraformClient,
@@ -888,6 +889,7 @@ projects:
 				false,
 				false,
 				false,
+				"auto",
 				statsScope,
 				logger,
 				terraformClient,
@@ -1137,6 +1139,7 @@ workflows:
 				false,
 				false,
 				false,
+				"auto",
 				statsScope,
 				logger,
 				terraformClient,
@@ -1293,6 +1296,7 @@ projects:
 				false,
 				true,
 				false,
+				"auto",
 				statsScope,
 				logger,
 				terraformClient,
@@ -1318,6 +1322,149 @@ projects:
 	}
 }

+func TestBuildProjectCmdCtx_AutoDiscoverRespectsRepoConfig(t *testing.T) {
+	logger := logging.NewNoopLogger(t)
+	cases := map[string]struct {
+		globalCfg     string
+		repoCfg       string
+		modifiedFiles []string
+		expLen        int
+	}{
+		"autodiscover disabled": {
+			globalCfg: `
+repos:
+- id: /.*/
+  autodiscover:
+    mode: disabled
+`,
+			repoCfg: `
+version: 3
+automerge: true
+`,
+			modifiedFiles: []string{"project1/main.tf", "project2/main.tf", "project3/main.tf"},
+			expLen:        0,
+		},
+		"autodiscover auto": {
+			globalCfg: `
+repos:
+- id: /.*/
+  autodiscover:
+    mode: auto
+`,
+			repoCfg: `
+version: 3
+automerge: true
+projects:
+- dir: project1
+  workspace: myworkspace
+`,
+			modifiedFiles: []string{"project1/main.tf", "project2/main.tf", "project3/main.tf"},
+			expLen:        1,
+		},
+		"autodiscover enabled": {
+			globalCfg: `
+repos:
+- id: /.*/
+  autodiscover:
+    mode: enabled
+`,
+			repoCfg: `
+version: 3
+automerge: true
+projects:
+- dir: project1
+  workspace: myworkspace
+`,
+			modifiedFiles: []string{"project1/main.tf", "project2/main.tf", "project3/main.tf"},
+			expLen:        3,
+		},
+	}
+
+	for name, c := range cases {
+		t.Run(name, func(t *testing.T) {
+			tmp := DirStructure(t, map[string]interface{}{
+				"project1": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project2": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project3": map[string]interface{}{
+					"main.tf": nil,
+				},
+			})
+
+			workingDir := NewMockWorkingDir()
+			When(workingDir.Clone(Any[models.Repo](), Any[models.PullRequest](), Any[string]())).ThenReturn(tmp, false, nil)
+			vcsClient := vcsmocks.NewMockClient()
+			When(vcsClient.GetModifiedFiles(Any[models.Repo](), Any[models.PullRequest]())).ThenReturn(c.modifiedFiles, nil)
+
+			// Write and parse the global config file.
+			globalCfgPath := filepath.Join(tmp, "global.yaml")
+			Ok(t, os.WriteFile(globalCfgPath, []byte(c.globalCfg), 0600))
+			parser := &config.ParserValidator{}
+			globalCfgArgs := valid.GlobalCfgArgs{
+				AllowRepoCfg:  false,
+				MergeableReq:  false,
+				ApprovedReq:   false,
+				UnDivergedReq: false,
+			}
+
+			globalCfg, err := parser.ParseGlobalCfg(globalCfgPath, valid.NewGlobalCfgFromArgs(globalCfgArgs))
+			Ok(t, err)
+
+			if c.repoCfg != "" {
+				Ok(t, os.WriteFile(filepath.Join(tmp, "atlantis.yaml"), []byte(c.repoCfg), 0600))
+			}
+			statsScope, _, _ := metrics.NewLoggingScope(logging.NewNoopLogger(t), "atlantis")
+
+			terraformClient := mocks.NewMockClient()
+
+			builder := NewProjectCommandBuilder(
+				false,
+				parser,
+				&DefaultProjectFinder{},
+				vcsClient,
+				workingDir,
+				NewDefaultWorkingDirLocker(),
+				globalCfg,
+				&DefaultPendingPlanFinder{},
+				&CommentParser{ExecutableName: "atlantis"},
+				false,
+				false,
+				false,
+				false,
+				false,
+				"",
+				"**/*.tf,**/*.tfvars,**/*.tfvars.json,**/terragrunt.hcl,**/.terraform.lock.hcl",
+				false,
+				true,
+				false,
+				"auto",
+				statsScope,
+				logger,
+				terraformClient,
+			)
+
+			ctxs, err := builder.BuildPlanCommands(
+				&command.Context{
+					Log:   logger,
+					Scope: statsScope,
+				},
+				&CommentCommand{
+					RepoRelDir: "",
+					Flags:      nil,
+					Name:       command.Plan,
+					Verbose:    false,
+				},
+			)
+			Equals(t, c.expLen, len(ctxs))
+			Ok(t, err)
+
+		})
+	}
+}
+
 func mustVersion(v string) *version.Version {
 	vers, err := version.NewVersion(v)
 	if err != nil {
--- a/server/events/project_command_builder_test.go
+++ b/server/events/project_command_builder_test.go
@@ -33,6 +33,7 @@ var defaultUserConfig = struct {
 	RestrictFileList         bool
 	SilenceNoProjects        bool
 	IncludeGitUntrackedFiles bool
+	AutoDiscoverMode         string
 }{
 	SkipCloneNoChanges:       false,
 	EnableRegExpCmd:          false,
@@ -44,6 +45,7 @@ var defaultUserConfig = struct {
 	RestrictFileList:         false,
 	SilenceNoProjects:        false,
 	IncludeGitUntrackedFiles: true,
+	AutoDiscoverMode:         "auto",
 }

 func TestDefaultProjectCommandBuilder_BuildAutoplanCommands(t *testing.T) {
@@ -194,6 +196,7 @@ projects:
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -232,6 +235,7 @@ func TestDefaultProjectCommandBuilder_BuildSinglePlanApplyCommand(t *testing.T)
 		ExpErr                     string
 		ExpApplyReqs               []string
 		EnableAutoMergeUserCfg     bool
+		AutoDiscoverModeUserCfg    string
 		EnableParallelPlanUserCfg  bool
 		EnableParallelApplyUserCfg bool
 		ExpAutoMerge               bool
@@ -550,6 +554,7 @@ projects:
 					userConfig.RestrictFileList,
 					c.Silenced,
 					userConfig.IncludeGitUntrackedFiles,
+					c.AutoDiscoverModeUserCfg,
 					scope,
 					logger,
 					terraformClient,
@@ -739,6 +744,7 @@ projects:
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -770,6 +776,7 @@ func TestDefaultProjectCommandBuilder_BuildPlanCommands(t *testing.T) {
 		RepoRelDir       string
 		Workspace        string
 		Automerge        bool
+		AutoDiscover     valid.AutoDiscover
 		ExpParallelPlan  bool
 		ExpParallelApply bool
 	}
@@ -956,6 +963,67 @@ projects:
 				},
 			},
 		},
+		"follow autodiscover enabled config": {
+			DirStructure: map[string]interface{}{
+				"project1": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project2": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project3": map[string]interface{}{
+					"main.tf": nil,
+				},
+			},
+			AtlantisYAML: `version: 3
+autodiscover:
+  mode: enabled
+projects:
+- name: project1-custom-name
+  dir: project1`,
+			ModifiedFiles: []string{"project1/main.tf", "project2/main.tf"},
+			// project2 is autodiscovered, whereas project1 is not
+			Exp: []expCtxFields{
+				{
+					ProjectName: "project1-custom-name",
+					RepoRelDir:  "project1",
+					Workspace:   "default",
+				},
+				{
+					ProjectName: "",
+					RepoRelDir:  "project2",
+					Workspace:   "default",
+				},
+			},
+		},
+		"autodiscover enabled but project excluded by empty when_modified": {
+			DirStructure: map[string]interface{}{
+				"project1": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project2": map[string]interface{}{
+					"main.tf": nil,
+				},
+				"project3": map[string]interface{}{
+					"main.tf": nil,
+				},
+			},
+			AtlantisYAML: `version: 3
+autodiscover:
+  mode: enabled
+projects:
+- dir: project1
+  autoplan:
+    when_modified: []`,
+			ModifiedFiles: []string{"project1/main.tf", "project2/main.tf"},
+			Exp: []expCtxFields{
+				{
+					ProjectName: "",
+					RepoRelDir:  "project2",
+					Workspace:   "default",
+				},
+			},
+		},
 	}

 	logger := logging.NewNoopLogger(t)
@@ -1007,6 +1075,7 @@ projects:
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -1021,7 +1090,7 @@ projects:
 					RepoRelDir:  "",
 					Flags:       nil,
 					Name:        command.Plan,
-					Verbose:     false,
+					Verbose:     true,
 					Workspace:   "",
 					ProjectName: "",
 				})
@@ -1111,6 +1180,7 @@ func TestDefaultProjectCommandBuilder_BuildMultiApply(t *testing.T) {
 		userConfig.RestrictFileList,
 		userConfig.SilenceNoProjects,
 		userConfig.IncludeGitUntrackedFiles,
+		userConfig.AutoDiscoverMode,
 		scope,
 		logger,
 		terraformClient,
@@ -1206,6 +1276,7 @@ projects:
 		userConfig.RestrictFileList,
 		userConfig.SilenceNoProjects,
 		userConfig.IncludeGitUntrackedFiles,
+		userConfig.AutoDiscoverMode,
 		scope,
 		logger,
 		terraformClient,
@@ -1296,6 +1367,7 @@ func TestDefaultProjectCommandBuilder_EscapeArgs(t *testing.T) {
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -1467,6 +1539,7 @@ projects:
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -1523,6 +1596,17 @@ parallel_plan: true`,
 			ExpectedClones: Once(),
 			ModifiedFiles:  []string{"README.md"},
 		},
+		{
+			AtlantisYAML: `
+version: 3
+autodiscover:
+  mode: enabled
+projects:
+- dir: dir1`,
+			ExpectedCtxs:   0,
+			ExpectedClones: Once(),
+			ModifiedFiles:  []string{"dir2/main.tf"},
+		},
 	}

 	userConfig := defaultUserConfig
@@ -1568,6 +1652,7 @@ parallel_plan: true`,
 			userConfig.RestrictFileList,
 			userConfig.SilenceNoProjects,
 			userConfig.IncludeGitUntrackedFiles,
+			userConfig.AutoDiscoverMode,
 			scope,
 			logger,
 			terraformClient,
@@ -1638,6 +1723,7 @@ func TestDefaultProjectCommandBuilder_WithPolicyCheckEnabled_BuildAutoplanComman
 		userConfig.RestrictFileList,
 		userConfig.SilenceNoProjects,
 		userConfig.IncludeGitUntrackedFiles,
+		userConfig.AutoDiscoverMode,
 		scope,
 		logger,
 		terraformClient,
@@ -1730,6 +1816,7 @@ func TestDefaultProjectCommandBuilder_BuildVersionCommand(t *testing.T) {
 		userConfig.RestrictFileList,
 		userConfig.SilenceNoProjects,
 		userConfig.IncludeGitUntrackedFiles,
+		userConfig.AutoDiscoverMode,
 		scope,
 		logger,
 		terraformClient,
@@ -1861,6 +1948,7 @@ func TestDefaultProjectCommandBuilder_BuildPlanCommands_Single_With_RestrictFile
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
@@ -1972,6 +2060,7 @@ func TestDefaultProjectCommandBuilder_BuildPlanCommands_with_IncludeGitUntracked
 				userConfig.RestrictFileList,
 				userConfig.SilenceNoProjects,
 				userConfig.IncludeGitUntrackedFiles,
+				userConfig.AutoDiscoverMode,
 				scope,
 				logger,
 				terraformClient,
--- a/server/server.go
+++ b/server/server.go
@@ -597,6 +597,7 @@ func NewServer(userConfig UserConfig, config Config) (*Server, error) {
 		userConfig.RestrictFileList,
 		userConfig.SilenceNoProjects,
 		userConfig.IncludeGitUntrackedFiles,
+		userConfig.AutoDiscoverModeFlag,
 		statsScope,
 		logger,
 		terraformClient,
--- a/server/user_config.go
+++ b/server/user_config.go
@@ -15,6 +15,7 @@ type UserConfig struct {
 	AllowRepoConfig             bool   `mapstructure:"allow-repo-config"`
 	AllowCommands               string `mapstructure:"allow-commands"`
 	AtlantisURL                 string `mapstructure:"atlantis-url"`
+	AutoDiscoverModeFlag        string `mapstructure:"autodiscover-mode"`
 	Automerge                   bool   `mapstructure:"automerge"`
 	AutoplanFileList            string `mapstructure:"autoplan-file-list"`
 	AutoplanModules             bool   `mapstructure:"autoplan-modules"`