diff --git a/pkg/behaviourtest/steps/base_dispatch.go b/pkg/behaviourtest/steps/base_dispatch.go
index b867acd9ae..598fff95ee 100644
--- a/pkg/behaviourtest/steps/base_dispatch.go
+++ b/pkg/behaviourtest/steps/base_dispatch.go
@@ -222,6 +222,11 @@ func givenCustomHarnessWithURLBase(w *world.World, name, baseName, doc string) e
 // update boilerplate shared by givenCustomHarnessWithLocalBase and
 // givenCustomHarnessWithURLBase.
 func registerLocalAgentConfig(ctx context.Context, w *world.World, name, commitMsg string) error {
+	// Snapshot agents before modification so CleanupScenario can restore.
+	if err := snapshotAgents(w); err != nil {
+		return fmt.Errorf("snapshotting agents: %w", err)
+	}
+
 	cfgPath := path.Join(".fullsend", "config.yaml")
 	cfgData, err := w.SCM.GetFileContent(ctx, w.Org, w.RepoName, cfgPath)
 	if err != nil {
diff --git a/pkg/behaviourtest/steps/cleanup.go b/pkg/behaviourtest/steps/cleanup.go
index 0747ce6a40..198d542574 100644
--- a/pkg/behaviourtest/steps/cleanup.go
+++ b/pkg/behaviourtest/steps/cleanup.go
@@ -231,6 +231,19 @@ func CleanupScenario(w *world.World) {
 		}
 	}
 
+	// --- Agents cleanup ---
+	// Restore the pre-scenario agents list so a later scenario on this
+	// slot does not inherit custom agent entries (local or URL-sourced)
+	// from the previous lessee. Without this, harness registrations
+	// accumulate on the config overlay for the rest of the run.
+	if w.AgentsOverridden {
+		if err := cleanupRetry(w.Logf, "restore agents", func() error {
+			return RestoreAgents(w)
+		}); err != nil {
+			worldLogf(w, "behaviour cleanup: restore agents: %v", err)
+		}
+	}
+
 	// --- Dummy script cleanup ---
 	if len(w.DummyOps) > 0 {
 		if w.Org == "" || w.RepoName == "" {
diff --git a/pkg/behaviourtest/steps/cleanup_test.go b/pkg/behaviourtest/steps/cleanup_test.go
index 44f1905dad..bd18e2518c 100644
--- a/pkg/behaviourtest/steps/cleanup_test.go
+++ b/pkg/behaviourtest/steps/cleanup_test.go
@@ -775,6 +775,62 @@ func TestCleanupScenario_RestoreAllowedResources_Error(t *testing.T) {
 	assert.Contains(t, logged[0], "restore allowed_remote_resources")
 }
 
+// --- Agents cleanup tests ---
+
+func TestCleanupScenario_RestoresAgents(t *testing.T) {
+	t.Parallel()
+
+	scmDriver := &fakeCleanupSCM{
+		fileContent: []byte("version: \"1\"\nagents:\n  - name: custom\n    source: harness/custom.yaml\nroles:\n  - triage\n"),
+	}
+	w := &world.World{
+		Org:              "org",
+		RepoOwner:        "org",
+		RepoName:         "repo",
+		AgentsOverridden: true,
+		AgentsOriginal:   nil, // restore to empty (install-time default)
+		SCM:              scmDriver,
+	}
+	CleanupScenario(w)
+	assert.True(t, scmDriver.commitFileCalled, "should commit config to restore agents")
+}
+
+func TestCleanupScenario_SkipsAgentsWhenNotOverridden(t *testing.T) {
+	t.Parallel()
+
+	scmDriver := &fakeCleanupSCM{}
+	w := &world.World{
+		RepoOwner:        "org",
+		RepoName:         "repo",
+		AgentsOverridden: false,
+		SCM:              scmDriver,
+	}
+	CleanupScenario(w)
+	assert.False(t, scmDriver.commitFileCalled, "should not commit when agents were not overridden")
+}
+
+func TestCleanupScenario_RestoreAgents_Error(t *testing.T) {
+	t.Parallel()
+
+	var logged []string
+	scmDriver := &fakeCleanupSCM{
+		fileContent:   []byte("version: \"1\"\nagents:\n  - name: custom\n    source: harness/custom.yaml\nroles:\n  - triage\n"),
+		commitFileErr: fmt.Errorf("commit failed"),
+	}
+	w := &world.World{
+		Org:              "org",
+		RepoOwner:        "org",
+		RepoName:         "repo",
+		AgentsOverridden: true,
+		AgentsOriginal:   nil,
+		SCM:              scmDriver,
+		Logf:             func(format string, args ...any) { logged = append(logged, fmt.Sprintf(format, args...)) },
+	}
+	CleanupScenario(w)
+	require.Len(t, logged, 1)
+	assert.Contains(t, logged[0], "restore agents")
+}
+
 func TestCleanupScenario_BranchScenarioSweep(t *testing.T) {
 	t.Parallel()
 
diff --git a/pkg/behaviourtest/steps/dispatch.go b/pkg/behaviourtest/steps/dispatch.go
index 06911f605e..cf09034305 100644
--- a/pkg/behaviourtest/steps/dispatch.go
+++ b/pkg/behaviourtest/steps/dispatch.go
@@ -117,6 +117,11 @@ func givenDisabledCustomHarness(w *world.World, name, doc string) error {
 		return err
 	}
 
+	// Snapshot agents before modification so CleanupScenario can restore.
+	if err := snapshotAgents(w); err != nil {
+		return fmt.Errorf("snapshotting agents: %w", err)
+	}
+
 	cfgPath := filepath.Join(".fullsend", "config.yaml")
 	cfgData, err := w.SCM.GetFileContent(context.Background(), w.Org, w.RepoName, cfgPath)
 	if err != nil {
@@ -171,6 +176,11 @@ func givenCustomHarness(w *world.World, name, doc string) error {
 		return err
 	}
 
+	// Snapshot agents before modification so CleanupScenario can restore.
+	if err := snapshotAgents(w); err != nil {
+		return fmt.Errorf("snapshotting agents: %w", err)
+	}
+
 	cfgPath := filepath.Join(".fullsend", "config.yaml")
 	cfgData, err := w.SCM.GetFileContent(context.Background(), w.Org, w.RepoName, cfgPath)
 	if err != nil {
diff --git a/pkg/behaviourtest/steps/url_dispatch.go b/pkg/behaviourtest/steps/url_dispatch.go
index aed6ac2c48..1126823c5c 100644
--- a/pkg/behaviourtest/steps/url_dispatch.go
+++ b/pkg/behaviourtest/steps/url_dispatch.go
@@ -68,6 +68,35 @@ func snapshotAllowedResources(w *world.World) error {
 	return nil
 }
 
+// snapshotAgents captures the current agents list from config.yaml into
+// w.AgentsOriginal — but only on the first call per scenario, so that
+// multiple harness steps in the same scenario do not overwrite the
+// original value with an already-modified copy.
+//
+// Called before any cfg.SetAgents() call (dispatch.go, url_dispatch.go,
+// base_dispatch.go). CleanupScenario uses the snapshot to restore the
+// original agents when the scenario is done.
+func snapshotAgents(w *world.World) error {
+	if w.AgentsOverridden {
+		return nil // already snapshotted this scenario
+	}
+	cfgPath := path.Join(".fullsend", "config.yaml")
+	cfgData, err := w.SCM.GetFileContent(context.Background(), w.Org, w.RepoName, cfgPath)
+	if err != nil {
+		return fmt.Errorf("reading config for agents snapshot: %w", err)
+	}
+	cfg, err := config.ParsePerRepoConfigWriter(cfgData)
+	if err != nil {
+		return fmt.Errorf("parsing config for agents snapshot: %w", err)
+	}
+	orig := cfg.AgentEntries()
+	// Store a copy so later mutations do not alias the snapshot.
+	w.AgentsOriginal = make([]config.AgentEntry, len(orig))
+	copy(w.AgentsOriginal, orig)
+	w.AgentsOverridden = true
+	return nil
+}
+
 // RestoreAllowedResources sets allowed_remote_resources back to the
 // pre-scenario value captured by snapshotAllowedResources. Exported so
 // CleanupScenario can call it during scenario teardown.
@@ -95,6 +124,33 @@ func RestoreAllowedResources(w *world.World) error {
 	return nil
 }
 
+// RestoreAgents sets the agents list back to the pre-scenario value
+// captured by snapshotAgents. Exported so CleanupScenario can call it
+// during scenario teardown.
+func RestoreAgents(w *world.World) error {
+	if w.Org == "" || w.RepoName == "" {
+		return fmt.Errorf("no repo configured; call 'Given the enrolled test repository' before agent operations")
+	}
+	cfgPath := path.Join(".fullsend", "config.yaml")
+	cfgData, err := w.SCM.GetFileContent(context.Background(), w.Org, w.RepoName, cfgPath)
+	if err != nil {
+		return fmt.Errorf("reading config: %w", err)
+	}
+	cfg, err := config.ParsePerRepoConfigWriter(cfgData)
+	if err != nil {
+		return fmt.Errorf("parsing config: %w", err)
+	}
+	cfg.SetAgents(w.AgentsOriginal)
+	merged, err := cfg.Marshal()
+	if err != nil {
+		return err
+	}
+	if err := w.SCM.CommitFile(context.Background(), w.Org, w.RepoName, cfgPath, "behaviour: restore agents", merged); err != nil {
+		return fmt.Errorf("updating config: %w", err)
+	}
+	return nil
+}
+
 // givenHarnessHostingRepo creates a public repository to host URL-sourced
 // harness YAML files. The repo is created in the same org as the test
 // repository. It is idempotent — if the repo already exists, it returns
@@ -248,11 +304,15 @@ func givenURLSourcedCustomHarness(w *world.World, name, doc string, opts urlHarn
 	// Build the URL prefix for the allowlist.
 	urlPrefix := fmt.Sprintf("https://raw.githubusercontent.com/%s/%s/", hostOwner, hostRepo)
 
-	// Snapshot the current allowed_remote_resources before any modification
-	// so CleanupScenario can restore it when the slot is reused.
+	// Snapshot the current allowed_remote_resources and agents before any
+	// modification so CleanupScenario can restore them when the slot is
+	// reused.
 	if err := snapshotAllowedResources(w); err != nil {
 		return fmt.Errorf("snapshotting allowed_remote_resources: %w", err)
 	}
+	if err := snapshotAgents(w); err != nil {
+		return fmt.Errorf("snapshotting agents: %w", err)
+	}
 
 	// Update config.yaml on the enrolled test repo: register agent with URL
 	// source and update allowlist.
diff --git a/pkg/behaviourtest/steps/url_dispatch_test.go b/pkg/behaviourtest/steps/url_dispatch_test.go
index e14f028858..009bfed12f 100644
--- a/pkg/behaviourtest/steps/url_dispatch_test.go
+++ b/pkg/behaviourtest/steps/url_dispatch_test.go
@@ -899,6 +899,122 @@ func TestGivenURLSourcedCustomHarness_SnapshotsAllowlist(t *testing.T) {
 		"snapshot should not contain the newly added prefix")
 }
 
+// --- snapshotAgents / RestoreAgents tests ---
+
+func TestSnapshotAgents_CapturesOriginal(t *testing.T) {
+	t.Parallel()
+	scm := &fakeURLSCM{files: map[string][]byte{
+		"org/repo/.fullsend/config.yaml": []byte("version: \"1\"\nagents:\n  - name: triage\n    source: harness/triage.yaml\n"),
+	}}
+	w := &world.World{
+		Org:      "org",
+		RepoName: "repo",
+		SCM:      scm,
+	}
+	err := snapshotAgents(w)
+	require.NoError(t, err)
+	assert.True(t, w.AgentsOverridden)
+	require.Len(t, w.AgentsOriginal, 1)
+	assert.Equal(t, "triage", w.AgentsOriginal[0].Name)
+	assert.Equal(t, "harness/triage.yaml", w.AgentsOriginal[0].Source)
+}
+
+func TestSnapshotAgents_OnlySnapshotsOnce(t *testing.T) {
+	t.Parallel()
+	scm := &fakeURLSCM{files: map[string][]byte{
+		"org/repo/.fullsend/config.yaml": []byte("version: \"1\"\nagents:\n  - name: triage\n    source: harness/triage.yaml\n"),
+	}}
+	w := &world.World{
+		Org:      "org",
+		RepoName: "repo",
+		SCM:      scm,
+	}
+	err := snapshotAgents(w)
+	require.NoError(t, err)
+	require.Len(t, w.AgentsOriginal, 1)
+
+	// Mutate the stored snapshot to verify it isn't overwritten on second call.
+	w.AgentsOriginal = nil
+	err = snapshotAgents(w)
+	require.NoError(t, err)
+	assert.Nil(t, w.AgentsOriginal,
+		"second call should not re-snapshot")
+}
+
+func TestSnapshotAgents_EmptyAgentsList(t *testing.T) {
+	t.Parallel()
+	scm := &fakeURLSCM{files: map[string][]byte{
+		"org/repo/.fullsend/config.yaml": []byte("version: \"1\"\nagents: []\n"),
+	}}
+	w := &world.World{
+		Org:      "org",
+		RepoName: "repo",
+		SCM:      scm,
+	}
+	err := snapshotAgents(w)
+	require.NoError(t, err)
+	assert.True(t, w.AgentsOverridden)
+	assert.Empty(t, w.AgentsOriginal)
+}
+
+func TestRestoreAgents_RestoresOriginal(t *testing.T) {
+	t.Parallel()
+	scm := &fakeURLSCM{files: map[string][]byte{
+		"org/repo/.fullsend/config.yaml": []byte("version: \"1\"\nagents:\n  - name: custom\n    source: harness/custom.yaml\n"),
+	}}
+	original := []config.AgentEntry{{Name: "triage", Source: "harness/triage.yaml"}}
+	w := &world.World{
+		Org:            "org",
+		RepoName:       "repo",
+		SCM:            scm,
+		AgentsOriginal: original,
+	}
+	err := RestoreAgents(w)
+	require.NoError(t, err)
+
+	// Parse the committed config and verify agents were restored.
+	cfgData := scm.files["org/repo/.fullsend/config.yaml"]
+	require.NotNil(t, cfgData)
+	assert.Contains(t, string(cfgData), "triage")
+	assert.NotContains(t, string(cfgData), "custom")
+}
+
+func TestRestoreAgents_EmptyOrg(t *testing.T) {
+	t.Parallel()
+	w := &world.World{
+		RepoName: "repo",
+		SCM:      &fakeURLSCM{files: map[string][]byte{}},
+	}
+	err := RestoreAgents(w)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "no repo configured")
+}
+
+func TestGivenURLSourcedCustomHarness_SnapshotsAgents(t *testing.T) {
+	stubRawHTTPClient(t)
+	scm := &fakeURLSCM{files: map[string][]byte{
+		"my-org/my-repo/.fullsend/config.yaml": []byte(
+			"version: \"1\"\nagents:\n  - name: existing\n    source: harness/existing.yaml\nallowed_remote_resources:\n  - \"https://example.com/\"\n"),
+	}}
+	w := &world.World{
+		Org:                 "my-org",
+		RepoName:            "my-repo",
+		SCM:                 scm,
+		URLHarnessRepoOwner: "my-org",
+		URLHarnessRepoName:  "harness-host",
+	}
+
+	err := givenURLSourcedCustomHarness(w, "url-test",
+		"agent: agents/triage.md\nrole: triage\nslug: url-test", urlHarnessOpts{})
+	require.NoError(t, err)
+
+	assert.True(t, w.AgentsOverridden,
+		"AgentsOverridden should be set after URL harness step")
+	require.Len(t, w.AgentsOriginal, 1)
+	assert.Equal(t, "existing", w.AgentsOriginal[0].Name,
+		"original agents should be preserved in snapshot")
+}
+
 // --- fakes ---
 
 // fakeURLSCM keys files by "owner/repo/path" so multi-repo tests
diff --git a/pkg/behaviourtest/suite/init.go b/pkg/behaviourtest/suite/init.go
index ee71ab201e..a5be4cb9a2 100644
--- a/pkg/behaviourtest/suite/init.go
+++ b/pkg/behaviourtest/suite/init.go
@@ -100,6 +100,8 @@ func resetScenarioWorld(w *world.World) {
 	w.RuntimeOriginal = ""
 	w.AllowedResourcesOverridden = false
 	w.AllowedResourcesOriginal = nil
+	w.AgentsOverridden = false
+	w.AgentsOriginal = nil
 	w.JiraMockServer = nil
 	w.JiraMockState = nil
 	w.JiraConfigDir = ""
diff --git a/pkg/behaviourtest/suite/init_test.go b/pkg/behaviourtest/suite/init_test.go
index 630eb09e18..903a49d5d8 100644
--- a/pkg/behaviourtest/suite/init_test.go
+++ b/pkg/behaviourtest/suite/init_test.go
@@ -12,6 +12,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/fullsend-ai/fullsend/internal/config"
 	"github.com/fullsend-ai/fullsend/internal/forge"
 	"github.com/fullsend-ai/fullsend/pkg/behaviourtest/drivers/env"
 	"github.com/fullsend-ai/fullsend/pkg/behaviourtest/drivers/install"
@@ -149,6 +150,8 @@ func TestResetScenarioWorld_ClearsSharedState(t *testing.T) {
 		URLHarnessRepoName:         "harness-host",
 		AllowedResourcesOverridden: true,
 		AllowedResourcesOriginal:   []string{"https://example.com/"},
+		AgentsOverridden:           true,
+		AgentsOriginal:             []config.AgentEntry{{Name: "test", Source: "harness/test.yaml"}},
 	}
 	resetScenarioWorld(w)
 	assert.Equal(t, 0, w.PRNumber)
@@ -164,6 +167,8 @@ func TestResetScenarioWorld_ClearsSharedState(t *testing.T) {
 	assert.Equal(t, "", w.URLHarnessRepoName)
 	assert.False(t, w.AllowedResourcesOverridden)
 	assert.Nil(t, w.AllowedResourcesOriginal)
+	assert.False(t, w.AgentsOverridden)
+	assert.Nil(t, w.AgentsOriginal)
 }
 
 func TestSkipErrorForTagNames(t *testing.T) {
diff --git a/pkg/behaviourtest/world/world.go b/pkg/behaviourtest/world/world.go
index 8850080a23..3d351f8473 100644
--- a/pkg/behaviourtest/world/world.go
+++ b/pkg/behaviourtest/world/world.go
@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/fullsend-ai/fullsend/internal/config"
 	"github.com/fullsend-ai/fullsend/internal/forge"
 	"github.com/fullsend-ai/fullsend/internal/runtime"
 	"github.com/fullsend-ai/fullsend/pkg/behaviourtest/drivers/ci"
@@ -105,6 +106,13 @@ type World struct {
 	AllowedResourcesOverridden bool
 	AllowedResourcesOriginal   []string
 
+	// AgentsOverridden records that this scenario modified the agents
+	// list in config.yaml; AgentsOriginal holds the pre-scenario value.
+	// CleanupScenario restores it so the next scenario on this slot
+	// does not inherit custom agent entries from the previous lessee.
+	AgentsOverridden bool
+	AgentsOriginal   []config.AgentEntry
+
 	// Jira mock state — set by the "Given a mock Jira server" step.
 	JiraMockServer *httptest.Server
 	JiraMockState  *jiramock.State