diff --git a/betamessageutil.go b/betamessageutil.go
index d3b53f49..ec36a03b 100644
--- a/betamessageutil.go
+++ b/betamessageutil.go
@@ -452,9 +452,19 @@ func (r BetaToolSearchToolResultBlock) ToParam() BetaToolSearchToolResultBlockPa
 	p.Type = r.Type
 	p.ToolUseID = r.ToolUseID
 	if r.Content.JSON.ErrorCode.Valid() {
-		p.Content.OfRequestToolSearchToolResultError = &BetaToolSearchToolResultErrorParam{
+		errParam := &BetaToolSearchToolResultErrorParam{
 			ErrorCode: BetaToolSearchToolResultErrorParamErrorCode(r.Content.ErrorCode),
 		}
+		// error_code is required by the API but is tagged `omitzero`, so an
+		// empty string (e.g. an enum value the SDK does not recognize yet)
+		// would otherwise be dropped during marshaling. Force the field to be
+		// emitted using the raw value received from the server.
+		if string(errParam.ErrorCode) == "" {
+			errParam.SetExtraFields(map[string]any{
+				"error_code": string(r.Content.ErrorCode),
+			})
+		}
+		p.Content.OfRequestToolSearchToolResultError = errParam
 	} else {
 		p.Content.OfRequestToolSearchToolSearchResultBlock = &BetaToolSearchToolSearchResultBlockParam{}
 		for _, block := range r.Content.ToolReferences {
diff --git a/messageutil.go b/messageutil.go
index e735e479..e16ac8e9 100644
--- a/messageutil.go
+++ b/messageutil.go
@@ -390,9 +390,19 @@ func (r ToolSearchToolResultBlock) ToParam() ToolSearchToolResultBlockParam {
 	p.Type = r.Type
 	p.ToolUseID = r.ToolUseID
 	if r.Content.JSON.ErrorCode.Valid() {
-		p.Content.OfRequestToolSearchToolResultError = &ToolSearchToolResultErrorParam{
+		errParam := &ToolSearchToolResultErrorParam{
 			ErrorCode: ToolSearchToolResultErrorCode(r.Content.ErrorCode),
 		}
+		// error_code is required by the API but is tagged `omitzero`, so an
+		// empty string (e.g. an enum value the SDK does not recognize yet)
+		// would otherwise be dropped during marshaling. Force the field to be
+		// emitted using the raw value received from the server.
+		if string(errParam.ErrorCode) == "" {
+			errParam.SetExtraFields(map[string]any{
+				"error_code": string(r.Content.ErrorCode),
+			})
+		}
+		p.Content.OfRequestToolSearchToolResultError = errParam
 	} else {
 		p.Content.OfRequestToolSearchToolSearchResultBlock = &ToolSearchToolSearchResultBlockParam{}
 		for _, block := range r.Content.ToolReferences {
diff --git a/messageutil_test.go b/messageutil_test.go
index cf1aedcb..ce729931 100644
--- a/messageutil_test.go
+++ b/messageutil_test.go
@@ -53,4 +53,81 @@ func TestContentBlockUnionToParam(t *testing.T) {
 			t.Errorf("Expected 1 search result in param, got %d", len(result.OfWebSearchToolResult.Content.OfWebSearchToolResultBlockItem))
 		}
 	})
+
+	// Regression test for https://github.com/anthropics/anthropic-sdk-go/issues/317.
+	// The API requires `error_code` on a tool_search_tool_result_error block, but
+	// the param field is tagged `omitzero`, so a zero-value enum (e.g. an enum value
+	// the SDK does not yet recognize) would be elided and the API would 400 on the
+	// next request.
+	t.Run("ToolSearchToolResultBlock error preserves known error_code", func(t *testing.T) {
+		result := unmarshalContentBlockParam(t, `{"type":"tool_search_tool_result","tool_use_id":"tsu_1","content":{"type":"tool_search_tool_result_error","error_code":"unavailable"}}`)
+		if result.OfToolSearchToolResult == nil {
+			t.Fatal("Expected OfToolSearchToolResult to be non-nil")
+		}
+		if result.OfToolSearchToolResult.Content.OfRequestToolSearchToolResultError == nil {
+			t.Fatal("Expected OfRequestToolSearchToolResultError to be non-nil")
+		}
+
+		data, err := json.Marshal(result)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var decoded struct {
+			Content struct {
+				ErrorCode *string `json:"error_code"`
+				Type      string  `json:"type"`
+			} `json:"content"`
+		}
+		if err := json.Unmarshal(data, &decoded); err != nil {
+			t.Fatalf("Failed to unmarshal round-trip: %v", err)
+		}
+		if decoded.Content.Type != "tool_search_tool_result_error" {
+			t.Errorf("Expected content.type 'tool_search_tool_result_error', got %q", decoded.Content.Type)
+		}
+		if decoded.Content.ErrorCode == nil {
+			t.Fatalf("Expected content.error_code to be present, got missing field; raw: %s", string(data))
+		}
+		if *decoded.Content.ErrorCode != "unavailable" {
+			t.Errorf("Expected content.error_code 'unavailable', got %q", *decoded.Content.ErrorCode)
+		}
+	})
+
+	t.Run("ToolSearchToolResultBlock error preserves empty error_code", func(t *testing.T) {
+		// Regression: when the response's error_code resolves to the zero value
+		// of ToolSearchToolResultErrorCode (an empty string), the param field
+		// is tagged `omitzero` and the field gets silently dropped. The API
+		// then rejects the next request with
+		// "tool_search_tool_result.content.RequestToolSearchToolResultError.error_code: Field required".
+		// ToParam() must still emit the field with the original value.
+		result := unmarshalContentBlockParam(t, `{"type":"tool_search_tool_result","tool_use_id":"tsu_1","content":{"type":"tool_search_tool_result_error","error_code":""}}`)
+		if result.OfToolSearchToolResult == nil {
+			t.Fatal("Expected OfToolSearchToolResult to be non-nil")
+		}
+		if result.OfToolSearchToolResult.Content.OfRequestToolSearchToolResultError == nil {
+			t.Fatal("Expected OfRequestToolSearchToolResultError to be non-nil")
+		}
+
+		data, err := json.Marshal(result)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var decoded struct {
+			Content struct {
+				ErrorCode *string `json:"error_code"`
+				Type      string  `json:"type"`
+			} `json:"content"`
+		}
+		if err := json.Unmarshal(data, &decoded); err != nil {
+			t.Fatalf("Failed to unmarshal round-trip: %v", err)
+		}
+		if decoded.Content.Type != "tool_search_tool_result_error" {
+			t.Errorf("Expected content.type 'tool_search_tool_result_error', got %q", decoded.Content.Type)
+		}
+		if decoded.Content.ErrorCode == nil {
+			t.Fatalf("Expected content.error_code to be present even when empty, got missing field; raw: %s", string(data))
+		}
+		if *decoded.Content.ErrorCode != "" {
+			t.Errorf("Expected content.error_code '', got %q", *decoded.Content.ErrorCode)
+		}
+	})
 }