From 3819eb4feed637e5032411e1b4b97bd1a970aec6 Mon Sep 17 00:00:00 2001
From: "Hein (Warky)" <hein.puth@gmail.com>
Date: Tue, 31 Mar 2026 00:20:36 +0200
Subject: [PATCH] feat(files): update save_file tool description and enforce
 size limit for base64 payloads

---
 internal/mcpserver/schema.go | 16 +++++++++++++++-
 internal/mcpserver/server.go |  2 +-
 internal/tools/files.go      | 10 ++++++++++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/internal/mcpserver/schema.go b/internal/mcpserver/schema.go
index ee88a3c..c402b6f 100644
--- a/internal/mcpserver/schema.go
+++ b/internal/mcpserver/schema.go
@@ -2,6 +2,7 @@ package mcpserver
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"log/slog"
 	"reflect"
@@ -12,6 +13,8 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
+const maxLoggedArgBytes = 512
+
 var toolSchemaOptions = &jsonschema.ForOptions{
 	TypeSchemas: map[reflect.Type]*jsonschema.Schema{
 		reflect.TypeFor[uuid.UUID](): {
@@ -37,7 +40,7 @@ func logToolCall[In any, Out any](logger *slog.Logger, toolName string, handler
 		start := time.Now()
 		attrs := []any{slog.String("tool", toolName)}
 		if req != nil && req.Params != nil {
-			attrs = append(attrs, slog.Any("arguments", req.Params.Arguments))
+			attrs = append(attrs, slog.String("arguments", truncateArgs(req.Params.Arguments)))
 		}
 
 		logger.Info("mcp tool started", attrs...)
@@ -56,6 +59,17 @@ func logToolCall[In any, Out any](logger *slog.Logger, toolName string, handler
 	}
 }
 
+func truncateArgs(args any) string {
+	b, err := json.Marshal(args)
+	if err != nil {
+		return "<unserializable>"
+	}
+	if len(b) <= maxLoggedArgBytes {
+		return string(b)
+	}
+	return string(b[:maxLoggedArgBytes]) + fmt.Sprintf("… (%d bytes total)", len(b))
+}
+
 func setToolSchemas[In any, Out any](tool *mcp.Tool) error {
 	if tool.InputSchema == nil {
 		inputSchema, err := jsonschema.For[In](toolSchemaOptions)
diff --git a/internal/mcpserver/server.go b/internal/mcpserver/server.go
index aac65fd..ed7d3f7 100644
--- a/internal/mcpserver/server.go
+++ b/internal/mcpserver/server.go
@@ -136,7 +136,7 @@ func New(cfg config.MCPConfig, logger *slog.Logger, toolSet ToolSet) http.Handle
 
 	addTool(server, logger, &mcp.Tool{
 		Name:        "save_file",
-		Description: "Store a base64-encoded file such as an image, document, or audio clip, optionally linking it to a thought.",
+		Description: "Store a file and optionally link it to a thought. Supply either content_base64 (≤10 MB) or content_uri (amcs://files/{id} from a prior POST /files upload). For files larger than 10 MB, upload via POST /files first and pass the returned URI as content_uri.",
 	}, toolSet.Files.Save)
 
 	addTool(server, logger, &mcp.Tool{
diff --git a/internal/tools/files.go b/internal/tools/files.go
index 37b9655..181cda4 100644
--- a/internal/tools/files.go
+++ b/internal/tools/files.go
@@ -16,6 +16,11 @@ import (
 	thoughttypes "git.warky.dev/wdevs/amcs/internal/types"
 )
 
+// maxBase64ToolBytes is the maximum base64 payload accepted by save_file via
+// the MCP tool interface. For larger files use POST /files (binary) and pass
+// the returned amcs://files/{id} URI as content_uri instead.
+const maxBase64ToolBytes = 10 << 20 // 10 MB of base64 ≈ 7.5 MB decoded
+
 type FilesTool struct {
 	store    *store.DB
 	sessions *session.ActiveProjects
@@ -75,6 +80,11 @@ func (t *FilesTool) Save(ctx context.Context, req *mcp.CallToolRequest, in SaveF
 	if uri != "" && b64 != "" {
 		return nil, SaveFileOutput{}, errInvalidInput("provide content_uri or content_base64, not both")
 	}
+	if len(b64) > maxBase64ToolBytes {
+		return nil, SaveFileOutput{}, errInvalidInput(
+			"content_base64 exceeds the 10 MB MCP tool limit; upload the file via POST /files and pass the returned amcs://files/{id} URI as content_uri instead",
+		)
+	}
 
 	var content []byte
 	var mediaTypeFromSource string