diff --git a/internal/config/config_stdin.go b/internal/config/config_stdin.go
index b93c37c59..ebf6ab226 100644
--- a/internal/config/config_stdin.go
+++ b/internal/config/config_stdin.go
@@ -3,6 +3,7 @@
 package config
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -10,6 +11,7 @@ import (
 	"os"
 
 	"github.com/github/gh-aw-mcpg/internal/logger"
+	"github.com/santhosh-tekuri/jsonschema/v6"
 )
 
 var logStdin = logger.New("config:config_stdin")
@@ -205,10 +207,18 @@ func (s *StdinServerConfig) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
-	// Now unmarshal into a map to capture all fields
-	var allFields map[string]interface{}
-	if err := json.Unmarshal(data, &allFields); err != nil {
-		return err
+	// Now unmarshal into a map to capture all fields.
+	// Use jsonschema.UnmarshalJSON (which calls decoder.UseNumber()) so that
+	// numbers are stored as json.Number rather than float64.  This preserves
+	// precision for large integers such as 9007199254740993 that cannot be
+	// represented exactly as float64.
+	allFieldsObj, parseErr := jsonschema.UnmarshalJSON(bytes.NewReader(data))
+	if parseErr != nil {
+		return parseErr
+	}
+	allFields, ok := allFieldsObj.(map[string]interface{})
+	if !ok {
+		return fmt.Errorf("expected JSON object for server config, got %T", allFieldsObj)
 	}
 
 	// Known fields in the struct
diff --git a/internal/config/config_stdin_unmarshal_coverage_test.go b/internal/config/config_stdin_unmarshal_coverage_test.go
index 7c791542b..eb7315abd 100644
--- a/internal/config/config_stdin_unmarshal_coverage_test.go
+++ b/internal/config/config_stdin_unmarshal_coverage_test.go
@@ -1,6 +1,7 @@
 package config
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -154,7 +155,9 @@ func TestStdinServerConfig_UnmarshalJSON_ErrorPaths(t *testing.T) {
 		err := server.UnmarshalJSON(data)
 		require.NoError(t, err)
 		assert.Equal(t, "customValue", server.AdditionalProperties["customField"])
-		assert.Equal(t, float64(42), server.AdditionalProperties["anotherExtra"])
+		// Numbers are stored as json.Number (not float64) to preserve precision for
+		// large integers such as 9007199254740993 that cannot be represented by float64.
+		assert.Equal(t, json.Number("42"), server.AdditionalProperties["anotherExtra"])
 		// Known fields must not appear in AdditionalProperties.
 		_, typeExists := server.AdditionalProperties["type"]
 		assert.False(t, typeExists, "known field 'type' should not appear in AdditionalProperties")
diff --git a/internal/config/validate_server_against_schema_test.go b/internal/config/validate_server_against_schema_test.go
index 236a182b6..dba7224e7 100644
--- a/internal/config/validate_server_against_schema_test.go
+++ b/internal/config/validate_server_against_schema_test.go
@@ -239,3 +239,43 @@ func TestValidateServerAgainstSchema_TypeMismatchInAdditionalProperties(t *testi
 		"wrong type for an additional property should cause a schema validation error")
 	assert.Contains(t, err.Error(), "does not match custom schema")
 }
+
+// TestValidateServerAgainstSchema_LargeIntegerPrecision is a regression test that
+// verifies large integers (beyond float64 safe range) in additional properties are
+// preserved with full precision throughout the validation path.
+//
+// 9007199254740993 == 2^53+1 cannot be represented exactly as float64
+// (it rounds to 9007199254740992).  If UnmarshalJSON uses encoding/json the
+// const constraint below would fail even though the original JSON is correct.
+func TestValidateServerAgainstSchema_LargeIntegerPrecision(t *testing.T) {
+	const largeInt = `9007199254740993`
+
+	// Schema constrains "seq-id" to the exact large-integer value.
+	schema := compileSchemaForTest(t, `{
+		"type": "object",
+		"required": ["seq-id"],
+		"properties": {
+			"seq-id": {"const": `+largeInt+`}
+		}
+	}`)
+
+	// Parse the server config through the real UnmarshalJSON path so that the
+	// large integer travels through StdinServerConfig.UnmarshalJSON and lands
+	// in AdditionalProperties.
+	serverJSON := []byte(`{
+		"type": "stdio",
+		"container": "ghcr.io/example/server:latest",
+		"seq-id": ` + largeInt + `
+	}`)
+
+	var server StdinServerConfig
+	require.NoError(t, server.UnmarshalJSON(serverJSON))
+
+	err := validateServerAgainstSchema(
+		"test-server", &server, schema,
+		"https://test.example.com/schema.json",
+		"mcpServers.test-server",
+	)
+	assert.NoError(t, err,
+		"large integer 9007199254740993 must be preserved without float64 rounding")
+}
diff --git a/internal/config/validation_schema.go b/internal/config/validation_schema.go
index 2e6ef3bd1..735a2ab7f 100644
--- a/internal/config/validation_schema.go
+++ b/internal/config/validation_schema.go
@@ -249,7 +249,7 @@ func formatSchemaError(err error) error {
 
 		AppendConfigDocsFooter(&sb)
 
-		return fmt.Errorf("%s", sb.String())
+		return errors.New(sb.String())
 	}
 
 	return fmt.Errorf("configuration validation error (version: %s): %s", version.Get(), err.Error())
diff --git a/internal/config/validation_server.go b/internal/config/validation_server.go
index d9c54bc06..d55802826 100644
--- a/internal/config/validation_server.go
+++ b/internal/config/validation_server.go
@@ -302,7 +302,7 @@ func validateAgainstCustomSchema(name string, server *StdinServerConfig, schemaU
 	if err != nil {
 		return schemaErr(
 			fmt.Sprintf("failed to compile custom schema: %v", err),
-			fmt.Sprintf("The schema at '%s' must be a valid JSON Schema Draft 7 document", schemaURL))
+			fmt.Sprintf("The schema at '%s' must be a valid JSON Schema document (Draft 2020-12 or earlier)", schemaURL))
 	}
 
 	logValidation.Printf("Custom schema compiled successfully: name=%s", name)
@@ -330,12 +330,18 @@ func validateServerAgainstSchema(name string, server *StdinServerConfig, schema
 			"Internal error - please report this issue")
 	}
 
-	// Unmarshal to map to get struct fields
-	if err := json.Unmarshal(serverJSON, &serverMap); err != nil {
+	// Parse using jsonschema.UnmarshalJSON for number-precision consistency with embedded schema path
+	serverObj, parseErr := jsonschema.UnmarshalJSON(bytes.NewReader(serverJSON))
+	if parseErr != nil {
 		return schemaErr(
-			fmt.Sprintf("failed to unmarshal server config for validation: %v", err),
+			fmt.Sprintf("failed to parse server config for validation: %v", parseErr),
 			"Internal error - please report this issue")
 	}
+	if obj, ok := serverObj.(map[string]interface{}); ok {
+		serverMap = obj
+	} else {
+		logValidation.Printf("unexpected: server config parsed to non-object type, using empty map for validation: name=%s", name)
+	}
 
 	// Merge additional properties (custom fields) into the map
 	for key, value := range server.AdditionalProperties {