diff --git a/pkg/agentic/prep.go b/pkg/agentic/prep.go
index 33a64cf..16e15c9 100644
--- a/pkg/agentic/prep.go
+++ b/pkg/agentic/prep.go
@@ -206,6 +206,7 @@ func (s *PrepSubsystem) OnStartup(ctx context.Context) core.Result {
 	c.Action("state.set", s.handleStateSet).Description = "Store shared plan state for later sessions"
 	c.Action("state.get", s.handleStateGet).Description = "Read shared plan state by key"
 	c.Action("state.list", s.handleStateList).Description = "List shared plan state for a plan"
+	c.Action("state.delete", s.handleStateDelete).Description = "Delete shared plan state by key"
 	c.Action("template.list", s.handleTemplateList).Description = "List available YAML plan templates"
 	c.Action("template.preview", s.handleTemplatePreview).Description = "Preview a YAML plan template with variable substitution"
 	c.Action("template.create_plan", s.handleTemplateCreatePlan).Description = "Create a stored plan from a YAML template"
diff --git a/pkg/agentic/prep_test.go b/pkg/agentic/prep_test.go
index 08d4d46..1390a35 100644
--- a/pkg/agentic/prep_test.go
+++ b/pkg/agentic/prep_test.go
@@ -478,6 +478,7 @@ func TestPrep_OnStartup_Good_RegistersSessionActions(t *testing.T) {
 	assert.True(t, c.Action("state.set").Exists())
 	assert.True(t, c.Action("state.get").Exists())
 	assert.True(t, c.Action("state.list").Exists())
+	assert.True(t, c.Action("state.delete").Exists())
 	assert.True(t, c.Action("issue.create").Exists())
 	assert.True(t, c.Action("issue.get").Exists())
 	assert.True(t, c.Action("issue.list").Exists())
diff --git a/pkg/agentic/state.go b/pkg/agentic/state.go
index f264991..b473044 100644
--- a/pkg/agentic/state.go
+++ b/pkg/agentic/state.go
@@ -46,6 +46,12 @@ type StateListInput struct {
 	Category string `json:"category,omitempty"`
 }
 
+// input := agentic.StateDeleteInput{PlanSlug: "ax-follow-up", Key: "pattern"}
+type StateDeleteInput struct {
+	PlanSlug string `json:"plan_slug"`
+	Key      string `json:"key"`
+}
+
 // out := agentic.StateOutput{Success: true, State: agentic.WorkspaceState{Key: "pattern"}}
 type StateOutput struct {
 	Success bool           `json:"success"`
@@ -59,6 +65,12 @@ type StateListOutput struct {
 	States  []WorkspaceState `json:"states"`
 }
 
+// out := agentic.StateDeleteOutput{Success: true, Deleted: agentic.WorkspaceState{Key: "pattern"}}
+type StateDeleteOutput struct {
+	Success bool           `json:"success"`
+	Deleted WorkspaceState `json:"deleted"`
+}
+
 // result := c.Action("state.set").Run(ctx, core.NewOptions(
 //
 //	core.Option{Key: "plan_slug", Value: "ax-follow-up"},
@@ -111,6 +123,23 @@ func (s *PrepSubsystem) handleStateList(ctx context.Context, options core.Option
 	return core.Result{Value: output, OK: true}
 }
 
+// result := c.Action("state.delete").Run(ctx, core.NewOptions(
+//
+//	core.Option{Key: "plan_slug", Value: "ax-follow-up"},
+//	core.Option{Key: "key", Value: "pattern"},
+//
+// ))
+func (s *PrepSubsystem) handleStateDelete(ctx context.Context, options core.Options) core.Result {
+	_, output, err := s.stateDelete(ctx, nil, StateDeleteInput{
+		PlanSlug: optionStringValue(options, "plan_slug", "plan"),
+		Key:      optionStringValue(options, "key"),
+	})
+	if err != nil {
+		return core.Result{Value: err, OK: false}
+	}
+	return core.Result{Value: output, OK: true}
+}
+
 func (s *PrepSubsystem) registerStateTools(server *mcp.Server) {
 	mcp.AddTool(server, &mcp.Tool{
 		Name:        "state_set",
@@ -126,6 +155,11 @@ func (s *PrepSubsystem) registerStateTools(server *mcp.Server) {
 		Name:        "state_list",
 		Description: "List all stored workspace state values for a plan, with optional type or category filtering.",
 	}, s.stateList)
+
+	mcp.AddTool(server, &mcp.Tool{
+		Name:        "state_delete",
+		Description: "Delete a stored workspace state value for a plan by key.",
+	}, s.stateDelete)
 }
 
 func (s *PrepSubsystem) stateSet(_ context.Context, _ *mcp.CallToolRequest, input StateSetInput) (*mcp.CallToolResult, StateOutput, error) {
@@ -238,6 +272,51 @@ func (s *PrepSubsystem) stateList(_ context.Context, _ *mcp.CallToolRequest, inp
 	}, nil
 }
 
+func (s *PrepSubsystem) stateDelete(_ context.Context, _ *mcp.CallToolRequest, input StateDeleteInput) (*mcp.CallToolResult, StateDeleteOutput, error) {
+	if input.PlanSlug == "" {
+		return nil, StateDeleteOutput{}, core.E("stateDelete", "plan_slug is required", nil)
+	}
+	if input.Key == "" {
+		return nil, StateDeleteOutput{}, core.E("stateDelete", "key is required", nil)
+	}
+
+	states, err := readPlanStates(input.PlanSlug)
+	if err != nil {
+		return nil, StateDeleteOutput{}, err
+	}
+
+	filtered := make([]WorkspaceState, 0, len(states))
+	deleted := WorkspaceState{}
+	found := false
+	for _, state := range states {
+		if state.Key == input.Key {
+			deleted = normaliseWorkspaceState(state)
+			found = true
+			continue
+		}
+		filtered = append(filtered, state)
+	}
+
+	if !found {
+		return nil, StateDeleteOutput{}, core.E("stateDelete", core.Concat("state not found: ", input.Key), nil)
+	}
+
+	path := statePath(input.PlanSlug)
+	if len(filtered) == 0 {
+		if deleteResult := fs.Delete(path); !deleteResult.OK {
+			err, _ := deleteResult.Value.(error)
+			return nil, StateDeleteOutput{}, core.E("stateDelete", "failed to delete empty state file", err)
+		}
+	} else if err := writePlanStates(input.PlanSlug, filtered); err != nil {
+		return nil, StateDeleteOutput{}, err
+	}
+
+	return nil, StateDeleteOutput{
+		Success: true,
+		Deleted: deleted,
+	}, nil
+}
+
 func stateRoot() string {
 	return core.JoinPath(CoreRoot(), "state")
 }
diff --git a/pkg/agentic/state_test.go b/pkg/agentic/state_test.go
index 837b2df..568e82d 100644
--- a/pkg/agentic/state_test.go
+++ b/pkg/agentic/state_test.go
@@ -145,3 +145,54 @@ func TestState_HandleStateList_Ugly_CorruptStateFile(t *testing.T) {
 	))
 	assert.False(t, result.OK)
 }
+
+func TestState_HandleStateDelete_Good(t *testing.T) {
+	subsystem := testPrepWithPlatformServer(t, nil, "")
+	require.NoError(t, writePlanStates("ax-follow-up", []WorkspaceState{
+		{Key: "pattern", Value: "observer", Type: "general", Description: "Shared across sessions"},
+		{Key: "risk", Value: "auth", Type: "security"},
+	}))
+
+	result := subsystem.handleStateDelete(context.Background(), core.NewOptions(
+		core.Option{Key: "plan_slug", Value: "ax-follow-up"},
+		core.Option{Key: "key", Value: "pattern"},
+	))
+	require.True(t, result.OK)
+
+	output, ok := result.Value.(StateDeleteOutput)
+	require.True(t, ok)
+	assert.Equal(t, "pattern", output.Deleted.Key)
+	assert.Equal(t, "general", output.Deleted.Type)
+	assert.Equal(t, "Shared across sessions", output.Deleted.Description)
+
+	states, err := readPlanStates("ax-follow-up")
+	require.NoError(t, err)
+	require.Len(t, states, 1)
+	assert.Equal(t, "risk", states[0].Key)
+
+	require.True(t, subsystem.handleStateDelete(context.Background(), core.NewOptions(
+		core.Option{Key: "plan_slug", Value: "ax-follow-up"},
+		core.Option{Key: "key", Value: "risk"},
+	)).OK)
+	assert.False(t, fs.Exists(statePath("ax-follow-up")))
+}
+
+func TestState_HandleStateDelete_Bad(t *testing.T) {
+	subsystem := testPrepWithPlatformServer(t, nil, "")
+	result := subsystem.handleStateDelete(context.Background(), core.NewOptions(
+		core.Option{Key: "plan_slug", Value: "ax-follow-up"},
+	))
+	assert.False(t, result.OK)
+}
+
+func TestState_HandleStateDelete_Ugly_CorruptStateFile(t *testing.T) {
+	subsystem := testPrepWithPlatformServer(t, nil, "")
+	require.True(t, fs.EnsureDir(stateRoot()).OK)
+	require.True(t, fs.Write(statePath("ax-follow-up"), `{broken`).OK)
+
+	result := subsystem.handleStateDelete(context.Background(), core.NewOptions(
+		core.Option{Key: "plan_slug", Value: "ax-follow-up"},
+		core.Option{Key: "key", Value: "pattern"},
+	))
+	assert.False(t, result.OK)
+}