fix(filter): enforce CEL syntax semantics

Reject non-standard truthy numeric expressions in filters and document the parser as a supported subset of standard CEL syntax.

- remove legacy filter rewrites
- support standard equality in tag exists predicates
- add regression coverage for accepted and rejected expressions

plugin/filter/README.md

 # Memo Filter Engine
 
-This package houses the memo-only filter engine that turns CEL expressions into
-SQL fragments. The engine follows a three phase pipeline inspired by systems
+This package houses the memo-only filter engine that turns standard CEL syntax
+into SQL fragments for the subset of expressions supported by the memo schema.
+The engine follows a three phase pipeline inspired by systems
 such as Calcite or Prisma:
 
 1. **Parsing** – CEL expressions are parsed with `cel-go` and validated against
    the memo-specific environment declared in `schema.go`. Only fields that
-   exist in the schema can surface in the filter.
+   exist in the schema can surface in the filter, and non-standard legacy
+   coercions are rejected.
 2. **Normalization** – the raw CEL AST is converted into an intermediate
    representation (IR) defined in `ir.go`. The IR is a dialect-agnostic tree of
    conditions (logical operators, comparisons, list membership, etc.). This

plugin/filter/engine.go

@@ -2,7 +2,6 @@ package filter
 
 import (
 	"context"
-	"fmt"
 	"strings"
 	"sync"
 
@@ -45,8 +44,6 @@ func (e *Engine) Compile(_ context.Context, filter string) (*Program, error) {
 		return nil, errors.New("filter expression is empty")
 	}
 
-	filter = normalizeLegacyFilter(filter)
-
 	ast, issues := e.env.Compile(filter)
 	if issues != nil && issues.Err() != nil {
 		return nil, errors.Wrap(issues.Err(), "failed to compile filter")
@@ -119,73 +116,3 @@ func DefaultAttachmentEngine() (*Engine, error) {
 	})
 	return defaultAttachmentInst, defaultAttachmentErr
 }
-
-func normalizeLegacyFilter(expr string) string {
-	expr = rewriteNumericLogicalOperand(expr, "&&")
-	expr = rewriteNumericLogicalOperand(expr, "||")
-	return expr
-}
-
-func rewriteNumericLogicalOperand(expr, op string) string {
-	var builder strings.Builder
-	n := len(expr)
-	i := 0
-	var inQuote rune
-
-	for i < n {
-		ch := expr[i]
-
-		if inQuote != 0 {
-			builder.WriteByte(ch)
-			if ch == '\\' && i+1 < n {
-				builder.WriteByte(expr[i+1])
-				i += 2
-				continue
-			}
-			if ch == byte(inQuote) {
-				inQuote = 0
-			}
-			i++
-			continue
-		}
-
-		if ch == '\'' || ch == '"' {
-			inQuote = rune(ch)
-			builder.WriteByte(ch)
-			i++
-			continue
-		}
-
-		if strings.HasPrefix(expr[i:], op) {
-			builder.WriteString(op)
-			i += len(op)
-
-			// Preserve whitespace following the operator.
-			wsStart := i
-			for i < n && (expr[i] == ' ' || expr[i] == '\t') {
-				i++
-			}
-			builder.WriteString(expr[wsStart:i])
-
-			signStart := i
-			if i < n && (expr[i] == '+' || expr[i] == '-') {
-				i++
-			}
-			for i < n && expr[i] >= '0' && expr[i] <= '9' {
-				i++
-			}
-			if i > signStart {
-				numLiteral := expr[signStart:i]
-				fmt.Fprintf(&builder, "(%s != 0)", numLiteral)
-			} else {
-				builder.WriteString(expr[signStart:i])
-			}
-			continue
-		}
-
-		builder.WriteByte(ch)
-		i++
-	}
-
-	return builder.String()
-}

plugin/filter/engine_test.go

+package filter
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCompileAcceptsStandardTagEqualityPredicate(t *testing.T) {
+	t.Parallel()
+
+	engine, err := NewEngine(NewSchema())
+	require.NoError(t, err)
+
+	_, err = engine.Compile(context.Background(), `tags.exists(t, t == "1231")`)
+	require.NoError(t, err)
+}
+
+func TestCompileRejectsLegacyNumericLogicalOperand(t *testing.T) {
+	t.Parallel()
+
+	engine, err := NewEngine(NewSchema())
+	require.NoError(t, err)
+
+	_, err = engine.Compile(context.Background(), `pinned && 1`)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "failed to compile filter")
+}
+
+func TestCompileRejectsNonBooleanTopLevelConstant(t *testing.T) {
+	t.Parallel()
+
+	engine, err := NewEngine(NewSchema())
+	require.NoError(t, err)
+
+	_, err = engine.Compile(context.Background(), `1`)
+	require.EqualError(t, err, "filter must evaluate to a boolean value")
+}

plugin/filter/ir.go

@@ -157,3 +157,10 @@ type ContainsPredicate struct {
 }
 
 func (*ContainsPredicate) isPredicateExpr() {}
+
+// EqualsPredicate represents t == "value".
+type EqualsPredicate struct {
+	Value string
+}
+
+func (*EqualsPredicate) isPredicateExpr() {}

plugin/filter/parser.go

@@ -16,16 +16,10 @@ func buildCondition(expr *exprv1.Expr, schema Schema) (Condition, error) {
 		if err != nil {
 			return nil, err
 		}
-		switch v := val.(type) {
-		case bool:
+		if v, ok := val.(bool); ok {
 			return &ConstantCondition{Value: v}, nil
-		case int64:
-			return &ConstantCondition{Value: v != 0}, nil
-		case float64:
-			return &ConstantCondition{Value: v != 0}, nil
-		default:
-			return nil, errors.New("filter must evaluate to a boolean value")
 		}
+		return nil, errors.New("filter must evaluate to a boolean value")
 	case *exprv1.Expr_IdentExpr:
 		name := v.IdentExpr.GetName()
 		field, ok := schema.Field(name)
@@ -504,6 +498,8 @@ func extractPredicate(comp *exprv1.Expr_Comprehension, _ Schema) (PredicateExpr,
 
 	// Handle different predicate functions
 	switch predicateCall.Function {
+	case "_==_":
+		return buildEqualsPredicate(predicateCall, comp.IterVar)
 	case "startsWith":
 		return buildStartsWithPredicate(predicateCall, comp.IterVar)
 	case "endsWith":
@@ -511,8 +507,42 @@ func extractPredicate(comp *exprv1.Expr_Comprehension, _ Schema) (PredicateExpr,
 	case "contains":
 		return buildContainsPredicate(predicateCall, comp.IterVar)
 	default:
-		return nil, errors.Errorf("unsupported predicate function %q in comprehension (supported: startsWith, endsWith, contains)", predicateCall.Function)
+		return nil, errors.Errorf(`unsupported predicate function %q in comprehension (supported: ==, startsWith, endsWith, contains)`, predicateCall.Function)
+	}
+}
+
+// buildEqualsPredicate extracts the value from t == "value".
+func buildEqualsPredicate(call *exprv1.Expr_Call, iterVar string) (PredicateExpr, error) {
+	if len(call.Args) != 2 {
+		return nil, errors.New("equality predicate expects exactly two arguments")
+	}
+
+	var constExpr *exprv1.Expr
+	switch {
+	case isIterVarExpr(call.Args[0], iterVar):
+		constExpr = call.Args[1]
+	case isIterVarExpr(call.Args[1], iterVar):
+		constExpr = call.Args[0]
+	default:
+		return nil, errors.Errorf("equality predicate must compare against the iteration variable %q", iterVar)
+	}
+
+	value, err := getConstValue(constExpr)
+	if err != nil {
+		return nil, errors.Wrap(err, "equality argument must be a constant string")
+	}
+
+	valueStr, ok := value.(string)
+	if !ok {
+		return nil, errors.New("equality argument must be a string")
 	}
+
+	return &EqualsPredicate{Value: valueStr}, nil
+}
+
+func isIterVarExpr(expr *exprv1.Expr, iterVar string) bool {
+	target := expr.GetIdentExpr()
+	return target != nil && target.GetName() == iterVar
 }
 
 // buildStartsWithPredicate extracts the pattern from t.startsWith("prefix").

plugin/filter/render.go

@@ -480,6 +480,8 @@ func (r *renderer) renderListComprehension(cond *ListComprehensionCondition) (re
 
 	// Render based on predicate type
 	switch pred := cond.Predicate.(type) {
+	case *EqualsPredicate:
+		return r.renderTagEquals(field, pred.Value, cond.Kind)
 	case *StartsWithPredicate:
 		return r.renderTagStartsWith(field, pred.Prefix, cond.Kind)
 	case *EndsWithPredicate:
@@ -491,6 +493,22 @@ func (r *renderer) renderListComprehension(cond *ListComprehensionCondition) (re
 	}
 }
 
+// renderTagEquals generates SQL for tags.exists(t, t == "value").
+func (r *renderer) renderTagEquals(field Field, value string, _ ComprehensionKind) (renderResult, error) {
+	arrayExpr := jsonArrayExpr(r.dialect, field)
+
+	switch r.dialect {
+	case DialectSQLite, DialectMySQL:
+		exactMatch := r.buildJSONArrayLike(arrayExpr, fmt.Sprintf(`%%"%s"%%`, value))
+		return renderResult{sql: r.wrapWithNullCheck(arrayExpr, exactMatch)}, nil
+	case DialectPostgres:
+		exactMatch := fmt.Sprintf("%s @> jsonb_build_array(%s::json)", arrayExpr, r.addArg(fmt.Sprintf(`"%s"`, value)))
+		return renderResult{sql: r.wrapWithNullCheck(arrayExpr, exactMatch)}, nil
+	default:
+		return renderResult{}, errors.Errorf("unsupported dialect %s", r.dialect)
+	}
+}
+
 // renderTagStartsWith generates SQL for tags.exists(t, t.startsWith("prefix")).
 func (r *renderer) renderTagStartsWith(field Field, prefix string, _ ComprehensionKind) (renderResult, error) {
 	arrayExpr := jsonArrayExpr(r.dialect, field)

server/router/mcp/README.md

@@ -48,7 +48,7 @@ For Streamable HTTP safety, requests with an `Origin` header must be same-origin
 
 | Tool | Description | Required params | Optional params |
 |---|---|---|---|
-| `list_memos` | List memos | — | `page_size`, `page`, `state`, `order_by_pinned`, `filter` (CEL) |
+| `list_memos` | List memos | — | `page_size`, `page`, `state`, `order_by_pinned`, `filter` (supported subset of standard CEL syntax) |
 | `get_memo` | Get a single memo | `name` | — |
 | `search_memos` | Full-text search | `query` | — |
 | `create_memo` | Create a memo | `content` | `visibility` |

server/router/mcp/access.go

@@ -41,6 +41,10 @@ func checkMemoOwnership(memo *store.Memo, userID int32) error {
 	return nil
 }
 
+func hasMemoOwnership(memo *store.Memo, userID int32) bool {
+	return memo.CreatorID == userID
+}
+
 // applyVisibilityFilter restricts find to memos the caller may see.
 func applyVisibilityFilter(find *store.FindMemo, userID int32, rowStatus *store.RowStatus) {
 	if rowStatus != nil && *rowStatus == store.Archived {

server/router/mcp/tools_memo.go

@@ -223,7 +223,7 @@ func (s *MCPService) registerMemoTools(mcpSrv *mcpserver.MCPServer) {
 			mcp.Description("Filter by state: NORMAL (default) or ARCHIVED"),
 		),
 		mcp.WithBoolean("order_by_pinned", mcp.Description("When true, pinned memos appear first (default false)")),
-		mcp.WithString("filter", mcp.Description(`Optional CEL filter, e.g. content.contains("keyword") or tags.exists(t, t == "work")`)),
+		mcp.WithString("filter", mcp.Description(`Optional CEL filter (supported subset of standard CEL syntax), e.g. content.contains("keyword") or tags.exists(t, t == "work")`)),
 	), s.handleListMemos)
 
 	mcpSrv.AddTool(mcp.NewTool("get_memo",

server/router/mcp/tools_relation.go

@@ -142,7 +142,7 @@ func (s *MCPService) handleCreateMemoRelation(ctx context.Context, req mcp.CallT
 	if srcMemo == nil {
 		return mcp.NewToolResultError("source memo not found"), nil
 	}
-	if err := checkMemoOwnership(srcMemo, userID); err != nil {
+	if !hasMemoOwnership(srcMemo, userID) {
 		return mcp.NewToolResultError("permission denied: must own the source memo"), nil
 	}
 
@@ -199,7 +199,7 @@ func (s *MCPService) handleDeleteMemoRelation(ctx context.Context, req mcp.CallT
 	if srcMemo == nil {
 		return mcp.NewToolResultError("source memo not found"), nil
 	}
-	if err := checkMemoOwnership(srcMemo, userID); err != nil {
+	if !hasMemoOwnership(srcMemo, userID) {
 		return mcp.NewToolResultError("permission denied: must own the source memo"), nil
 	}
 

store/test/memo_filter_test.go

@@ -730,6 +730,31 @@ func TestMemoFilterTagsExistsContains(t *testing.T) {
 	require.Len(t, memos, 1, "Should find 1 non-todo memo")
 }
 
+func TestMemoFilterTagsExistsEquals(t *testing.T) {
+	t.Parallel()
+	tc := NewMemoFilterTestContext(t)
+	defer tc.Close()
+
+	tc.CreateMemo(NewMemoBuilder("memo-1231", tc.User.ID).
+		Content("Memo with exact numeric tag").
+		Tags("1231", "project"))
+
+	tc.CreateMemo(NewMemoBuilder("memo-1231-suffix", tc.User.ID).
+		Content("Memo with related tag").
+		Tags("tag/1231", "other"))
+
+	tc.CreateMemo(NewMemoBuilder("memo-other", tc.User.ID).
+		Content("Memo with different tag").
+		Tags("9999"))
+
+	memos := tc.ListWithFilter(`tags.exists(t, t == "1231")`)
+	require.Len(t, memos, 1, "Should find only the memo with exact matching tag")
+	require.Equal(t, "memo-1231", memos[0].UID)
+
+	memos = tc.ListWithFilter(`!tags.exists(t, t == "1231")`)
+	require.Len(t, memos, 2, "Should exclude only the memo with exact matching tag")
+}
+
 func TestMemoFilterTagsExistsEndsWith(t *testing.T) {
 	t.Parallel()
 	tc := NewMemoFilterTestContext(t)