diff --git a/internal/llm/tool.go b/internal/llm/tool.go
index b5bd8f5..5c90b06 100644
--- a/internal/llm/tool.go
+++ b/internal/llm/tool.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net/http"
 	"os"
 	"os/exec"
 	"strings"
@@ -175,33 +176,50 @@ func (r *Registry) ToAPI() []APITool {
 	return tools
 }
 
+// binaryOutputPrefix is the message shown when tool output is detected as binary.
+const binaryOutputPrefix = "error: binary output detected: "
+
 // ToolResult holds the structured output from a tool execution.
 type ToolResult struct {
-	Stdout       string
-	Stderr       string
+	Stdout       []byte
+	Stderr       []byte
 	ProcessState *os.ProcessState
 }
 
 // FormatResult formats the tool result using the structured label-based
 // format with stdout, stderr, and exit code sections.
-// FormatResult formats the tool result using the structured label-based
-// format with stdout, stderr, and exit code sections.
+//
+// http.DetectContentType implements the WHATWG MIME-sniffing spec and
+// returns text/plain for any content consisting entirely of printable
+// characters and whitespace. Text-based formats like JSON, JavaScript,
+// and XML will be detected as text/plain; charset=utf-8. We assume
+// checking for the "text/" prefix is sufficient.
 func (r *ToolResult) FormatResult() string {
 	var buf strings.Builder
 
 	fmt.Fprintln(&buf, "▸ stdout:")
-	if r.Stdout != "" {
-		fmt.Fprint(&buf, r.Stdout)
-		if !strings.HasSuffix(r.Stdout, "\n") {
-			fmt.Fprint(&buf, "\n")
+	if len(r.Stdout) > 0 {
+		ctype := http.DetectContentType(r.Stdout)
+		if !strings.HasPrefix(ctype, "text/") {
+			fmt.Fprintf(&buf, binaryOutputPrefix+"%s\n", ctype)
+		} else {
+			buf.Write(r.Stdout)
+			if r.Stdout[len(r.Stdout)-1] != '\n' {
+				fmt.Fprint(&buf, "\n")
+			}
 		}
 	}
 
 	fmt.Fprintln(&buf, "▸ stderr:")
-	if r.Stderr != "" {
-		fmt.Fprint(&buf, r.Stderr)
-		if !strings.HasSuffix(r.Stderr, "\n") {
-			fmt.Fprint(&buf, "\n")
+	if len(r.Stderr) > 0 {
+		ctype := http.DetectContentType(r.Stderr)
+		if !strings.HasPrefix(ctype, "text/") {
+			fmt.Fprintf(&buf, binaryOutputPrefix+"%s\n", ctype)
+		} else {
+			buf.Write(r.Stderr)
+			if r.Stderr[len(r.Stderr)-1] != '\n' {
+				fmt.Fprint(&buf, "\n")
+			}
 		}
 	}
 
@@ -301,8 +319,8 @@ func (r *Registry) Execute(
 		// ProcessState is set; return it along with captured output.
 		default:
 			return &ToolResult{
-				Stdout:       stdout.String(),
-				Stderr:       stderr.String(),
+				Stdout:       stdout.Bytes(),
+				Stderr:       stderr.Bytes(),
 				ProcessState: cmd.ProcessState,
 			}, nil
 		}
@@ -310,8 +328,8 @@ func (r *Registry) Execute(
 
 	// No error. Process exited with status 0.
 	return &ToolResult{
-		Stdout:       stdout.String(),
-		Stderr:       stderr.String(),
+		Stdout:       stdout.Bytes(),
+		Stderr:       stderr.Bytes(),
 		ProcessState: cmd.ProcessState,
 	}, nil
 }
diff --git a/internal/llm/tool_test.go b/internal/llm/tool_test.go
index a5d6a72..a998fe1 100644
--- a/internal/llm/tool_test.go
+++ b/internal/llm/tool_test.go
@@ -1,6 +1,7 @@
 package llm
 
 import (
+	"bytes"
 	"context"
 	"os/exec"
 	"strings"
@@ -221,24 +222,54 @@ func TestToolResult_FormatResult(t *testing.T) {
 	}{
 		{
 			name:         "success with stdout",
-			result:       ToolResult{Stdout: "hello\n", Stderr: "", ProcessState: cmdOK.ProcessState},
+			result:       ToolResult{Stdout: []byte("hello\n"), Stderr: nil, ProcessState: cmdOK.ProcessState},
 			wantContains: []string{"▸ stdout:", "hello", "▸ stderr:", "┗ exit: 0"},
 		},
 		{
 			name:         "failure with stderr",
-			result:       ToolResult{Stdout: "", Stderr: "error occurred\n", ProcessState: cmdFail.ProcessState},
+			result:       ToolResult{Stdout: nil, Stderr: []byte("error occurred\n"), ProcessState: cmdFail.ProcessState},
 			wantContains: []string{"▸ stdout:", "▸ stderr:", "error occurred", "┗ exit: 3"},
 		},
 		{
 			name:         "both stdout and stderr",
-			result:       ToolResult{Stdout: "output", Stderr: "warning", ProcessState: cmdOK.ProcessState},
+			result:       ToolResult{Stdout: []byte("output"), Stderr: []byte("warning"), ProcessState: cmdOK.ProcessState},
 			wantContains: []string{"▸ stdout:", "output", "▸ stderr:", "warning", "┗ exit: 0"},
 		},
 		{
 			name:         "empty output",
-			result:       ToolResult{Stdout: "", Stderr: "", ProcessState: cmdOK.ProcessState},
+			result:       ToolResult{Stdout: nil, Stderr: nil, ProcessState: cmdOK.ProcessState},
 			wantContains: []string{"▸ stdout:", "▸ stderr:", "┗ exit: 0"},
 		},
+		{
+			name:         "binary stdout (PNG)",
+			result:       ToolResult{Stdout: []byte("\x89PNG\r\n\x1a\n"), Stderr: nil, ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", binaryOutputPrefix, "image/png", "▸ stderr:", "┗ exit: 0"},
+		},
+		{
+			name:         "binary stderr",
+			result:       ToolResult{Stdout: nil, Stderr: []byte("\x89PNG\r\n\x1a\n"), ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", "▸ stderr:", binaryOutputPrefix, "image/png", "┗ exit: 0"},
+		},
+		{
+			name:         "HTML",
+			result:       ToolResult{Stdout: []byte("<HTML><BODY>Hello</BODY></HTML>\n"), Stderr: nil, ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", "<HTML>", "▸ stderr:", "┗ exit: 0"},
+		},
+		{
+			name:         "XML",
+			result:       ToolResult{Stdout: []byte("<?xml version=\"1.0\"?><root/>\n"), Stderr: nil, ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", "<?xml", "▸ stderr:", "┗ exit: 0"},
+		},
+		{
+			name:         "Go source",
+			result:       ToolResult{Stdout: []byte("package main\n\nfunc main() {}\n"), Stderr: nil, ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", "package main", "▸ stderr:", "┗ exit: 0"},
+		},
+		{
+			name:         "JSON",
+			result:       ToolResult{Stdout: []byte(`{"key":"value"}`), Stderr: nil, ProcessState: cmdOK.ProcessState},
+			wantContains: []string{"▸ stdout:", `{"key":"value"}`, "▸ stderr:", "┗ exit: 0"},
+		},
 	}
 
 	for _, tt := range tests {
@@ -268,7 +299,7 @@ func TestRegistry_Execute_Success(t *testing.T) {
 	if result.ProcessState.ExitCode() != 0 {
 		t.Errorf("ExitCode = %d, want 0", result.ProcessState.ExitCode())
 	}
-	if !strings.Contains(result.Stdout, "hello") {
+	if !bytes.Contains(result.Stdout, []byte("hello")) {
 		t.Errorf("Stdout = %q, want to contain hello", result.Stdout)
 	}
 }