diff --git a/pkg/tools/builtin/sql_agent.go b/pkg/tools/builtin/sql_agent.go
index f9c6821..a35af6d 100644
--- a/pkg/tools/builtin/sql_agent.go
+++ b/pkg/tools/builtin/sql_agent.go
@@ -89,6 +89,7 @@ type CallSQLAgentTool struct {
 	allowSelectStar             bool
 	execSQLRequiresConfirmation bool
 	providerHint                string
+	cellRedactor                CellRedactor
 }
 
 // NewCallSQLAgentTool initializes a tool capable of querying databases. The
@@ -171,6 +172,30 @@ func (t *CallSQLAgentTool) WithLLMPreviewRows(n int) *CallSQLAgentTool {
 	return t
 }
 
+// CellRedactor transforms a single result-cell value before it is serialized
+// into the text the sub-agent LLM reads — a privacy guard for sending SQL
+// results to a third-party provider (e.g. mask an email to "j***@***.com"). It
+// receives the column name and the raw cell value and returns the value to
+// expose to the model. It runs ONLY on the model-facing preview: the
+// host-facing rows (OnSQL / SQLQueryEvent and tools.Result.Structured) and the
+// local grid keep full-fidelity values. Return the value unchanged to leave a
+// cell as-is. Returning a mutable value (slice/map) aliases it into the
+// model-facing copy — return a fresh value if the host mutates it later.
+type CellRedactor func(column string, value any) any
+
+// WithCellRedactor installs a per-cell transform applied to result values
+// before they are serialized into the model's context — a privacy guard for
+// sending SQL results to a third-party LLM. The redactor runs on a deep copy of
+// the (already row-capped) preview only: the OnSQL / SQLQueryEvent hook, the
+// tools.Result.Structured payload, and the host's grid all keep the real
+// values, so the user still sees unmasked data locally while the model sees
+// masked values. Pairs naturally with WithLLMPreviewRows, which bounds how many
+// rows are copied and masked. nil (default) disables redaction with no copying.
+func (t *CallSQLAgentTool) WithCellRedactor(fn CellRedactor) *CallSQLAgentTool {
+	t.cellRedactor = fn
+	return t
+}
+
 // WithQueryTimeout caps the wall-clock time of each underlying QueryContext
 // call. d <= 0 disables the timeout (default). Separate from the agent's
 // overall request context, which may be much longer.
@@ -413,6 +438,7 @@ func (t *CallSQLAgentTool) runOnce(ctx context.Context, query string, idx int) s
 		sessionKey:           subSessionKey,
 		maxRows:              t.maxRows,
 		llmPreviewRows:       t.llmPreviewRows,
+		cellRedactor:         t.cellRedactor,
 		queryTimeout:         t.queryTimeout,
 		allowMutations:       t.allowMutations,
 		allowDDL:             t.allowDDL,
@@ -632,6 +658,7 @@ type executeSQLTool struct {
 	onSQL                func(context.Context, SQLQueryEvent)
 	maxRows              int
 	llmPreviewRows       int
+	cellRedactor         CellRedactor
 	queryTimeout         time.Duration
 	allowMutations       bool
 	allowDDL             bool
@@ -735,7 +762,10 @@ func (t *executeSQLTool) makeEmitFunc(ctx context.Context) func(SQLResult) (tool
 				Truncated:   res.Truncated,
 			})
 		}
-		b, err := json.Marshal(previewForLLM(res, t.llmPreviewRows))
+		// Mask the model-facing copy only; res (OnSQL, Structured, the host
+		// grid) keeps full-fidelity values. redactRows deep-copies, so the
+		// shared row maps under res are never mutated.
+		b, err := json.Marshal(redactRows(previewForLLM(res, t.llmPreviewRows), t.cellRedactor))
 		if err != nil {
 			return tools.Result{}, fmt.Errorf("tools: marshal result: %w", err)
 		}
@@ -758,6 +788,30 @@ func previewForLLM(res SQLResult, n int) SQLResult {
 	return preview
 }
 
+// redactRows returns a copy of res whose every cell value is passed through fn,
+// for masking sensitive values before they reach the LLM. It deep-copies the
+// row maps, so res — shared with the OnSQL hook, tools.Result.Structured, and
+// the host grid — keeps its full-fidelity values (the maps in res.Rows are
+// never mutated). fn == nil or no rows is a no-op returning res unchanged, so
+// the default path allocates nothing. Only the rows already selected for the
+// preview are copied, so the cost scales with the LLM preview size.
+func redactRows(res SQLResult, fn CellRedactor) SQLResult {
+	if fn == nil || len(res.Rows) == 0 {
+		return res
+	}
+	rows := make([]map[string]any, len(res.Rows))
+	for i, row := range res.Rows {
+		masked := make(map[string]any, len(row))
+		for col, val := range row {
+			masked[col] = fn(col, val)
+		}
+		rows[i] = masked
+	}
+	out := res
+	out.Rows = rows
+	return out
+}
+
 // executeRead runs the read-only path: optional LIMIT injection, then
 // QueryContext + row iteration into a structured SQLResult.
 func (t *executeSQLTool) executeRead(ctx context.Context, sqlStr string, emit func(SQLResult) (tools.Result, error)) (tools.Result, error) {
diff --git a/pkg/tools/builtin/sql_agent_redact_test.go b/pkg/tools/builtin/sql_agent_redact_test.go
new file mode 100644
index 0000000..7d3a429
--- /dev/null
+++ b/pkg/tools/builtin/sql_agent_redact_test.go
@@ -0,0 +1,124 @@
+package builtin
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+)
+
+func maskEmail(col string, val any) any {
+	if col == "email" {
+		return "***"
+	}
+	return val
+}
+
+// TestEmit_CellRedactor_MasksTextNotHookOrStructured asserts WithCellRedactor
+// masks ONLY the LLM-visible result text; the OnSQL hook (host grid), the
+// tools.Result.Structured payload, and the caller's source row maps all keep
+// full-fidelity values — i.e. the redaction deep-copies and never mutates the
+// shared row maps.
+func TestEmit_CellRedactor_MasksTextNotHookOrStructured(t *testing.T) {
+	var hookEv SQLQueryEvent
+	exec := &executeSQLTool{
+		sessionKey:   "s",
+		onSQL:        func(_ context.Context, ev SQLQueryEvent) { hookEv = ev },
+		cellRedactor: maskEmail,
+	}
+	// "phone": nil exercises fn(col, nil) and the marshal of a redacted nil.
+	row := map[string]any{"id": 1, "email": "alice@example.com", "phone": nil}
+	res := SQLResult{SQL: "SELECT id, email, phone FROM users", Columns: []string{"id", "email", "phone"}, Rows: []map[string]any{row}, RowCount: 1}
+
+	out, err := exec.makeEmitFunc(context.Background())(res)
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+
+	// Model-facing Text is masked.
+	var seen SQLResult
+	if err := json.Unmarshal([]byte(out.Text), &seen); err != nil {
+		t.Fatalf("unmarshal text: %v", err)
+	}
+	if got := seen.Rows[0]["email"]; got != "***" {
+		t.Fatalf("model-facing email should be masked, got %v", got)
+	}
+	if _, ok := seen.Rows[0]["id"]; !ok {
+		t.Fatalf("non-sensitive column dropped from preview: %v", seen.Rows[0])
+	}
+	// A nil cell flows through the redactor and marshals without panicking.
+	if got, ok := seen.Rows[0]["phone"]; !ok || got != nil {
+		t.Fatalf("nil cell should pass through redactor as null, got %v (present=%v)", got, ok)
+	}
+
+	// Host hook keeps the real value (aliasing guard).
+	if got := hookEv.Rows[0]["email"]; got != "alice@example.com" {
+		t.Fatalf("OnSQL hook email must stay unmasked, got %v", got)
+	}
+	// Structured (host via OnToolResult) keeps the real value.
+	if sr, ok := out.Structured.(SQLResult); !ok || sr.Rows[0]["email"] != "alice@example.com" {
+		t.Fatalf("Structured email must stay unmasked, got %#v", out.Structured)
+	}
+	// The caller's source row map must not be mutated in place.
+	if got := row["email"]; got != "alice@example.com" {
+		t.Fatalf("source row map mutated by redactor: %v", got)
+	}
+}
+
+// TestEmit_CellRedactor_ComposesWithPreview asserts redaction runs on the
+// row-capped preview and still leaves the shared source maps untouched even on
+// the truncating branch (where the preview shares the underlying maps).
+func TestEmit_CellRedactor_ComposesWithPreview(t *testing.T) {
+	r0 := map[string]any{"id": 0, "email": "a@x.com"}
+	r1 := map[string]any{"id": 1, "email": "b@x.com"}
+	res := SQLResult{Columns: []string{"id", "email"}, Rows: []map[string]any{r0, r1}, RowCount: 2}
+	exec := &executeSQLTool{sessionKey: "s", llmPreviewRows: 1, cellRedactor: maskEmail}
+
+	out, err := exec.makeEmitFunc(context.Background())(res)
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+	var seen SQLResult
+	if err := json.Unmarshal([]byte(out.Text), &seen); err != nil {
+		t.Fatalf("unmarshal text: %v", err)
+	}
+	if len(seen.Rows) != 1 {
+		t.Fatalf("preview should cap to 1 row, got %d", len(seen.Rows))
+	}
+	if seen.Rows[0]["email"] != "***" {
+		t.Fatalf("preview email not masked: %v", seen.Rows[0]["email"])
+	}
+	if !seen.Truncated {
+		t.Fatal("preview should be flagged Truncated")
+	}
+	// Source maps (shared with the host grid / Structured) untouched.
+	if r0["email"] != "a@x.com" || r1["email"] != "b@x.com" {
+		t.Fatalf("source rows mutated: r0=%v r1=%v", r0["email"], r1["email"])
+	}
+}
+
+// TestEmit_CellRedactor_NilIsNoOp asserts the default (no redactor) passes
+// values through unchanged, byte-identical to pre-feature behavior.
+func TestEmit_CellRedactor_NilIsNoOp(t *testing.T) {
+	exec := &executeSQLTool{sessionKey: "s"} // cellRedactor nil
+	res := SQLResult{Columns: []string{"email"}, Rows: []map[string]any{{"email": "real@x.com"}}, RowCount: 1}
+
+	out, err := exec.makeEmitFunc(context.Background())(res)
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+	var seen SQLResult
+	if err := json.Unmarshal([]byte(out.Text), &seen); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if seen.Rows[0]["email"] != "real@x.com" {
+		t.Fatalf("nil redactor must be a no-op, got %v", seen.Rows[0]["email"])
+	}
+}
+
+// TestWithCellRedactor_SetsField asserts the builder wires the redactor through.
+func TestWithCellRedactor_SetsField(t *testing.T) {
+	tool := NewCallSQLAgentTool(nil, "", nil, nil).WithCellRedactor(maskEmail)
+	if tool.cellRedactor == nil {
+		t.Fatal("WithCellRedactor did not set the redactor")
+	}
+}