diff --git a/workspace-server/internal/handlers/a2a_proxy.go b/workspace-server/internal/handlers/a2a_proxy.go
index 4a7c8026..8520f564 100644
--- a/workspace-server/internal/handlers/a2a_proxy.go
+++ b/workspace-server/internal/handlers/a2a_proxy.go
@@ -486,11 +486,54 @@ func normalizeA2APayload(body []byte) ([]byte, string, *proxyA2AError) {
 	}
 
 	// Ensure params.message.messageId exists (required by a2a-sdk)
+	// AND v0.2→v0.3 compat (#2345): when sender supplies
+	// params.message.content (v0.2) instead of params.message.parts
+	// (v0.3), wrap the content as a single text Part so the downstream
+	// a2a-sdk's v0.3 Pydantic validator accepts the message.
+	//
+	// Pre-fix: Design Director silently dropped briefs whose sender
+	// used v0.2 shape — Pydantic rejected at parse time, the rejection
+	// went only to logs, and the sender saw a happy 200/202.
+	//
+	// Reject loud (HTTP 400) when neither content nor parts is present;
+	// previously the SDK's own rejection happened post-handler-dispatch
+	// and was invisible to the original sender.
 	if params, ok := payload["params"].(map[string]interface{}); ok {
 		if msg, ok := params["message"].(map[string]interface{}); ok {
 			if _, hasID := msg["messageId"]; !hasID {
 				msg["messageId"] = uuid.New().String()
 			}
+			_, hasParts := msg["parts"]
+			rawContent, hasContent := msg["content"]
+			if !hasParts {
+				if hasContent {
+					switch v := rawContent.(type) {
+					case string:
+						msg["parts"] = []interface{}{
+							map[string]interface{}{"kind": "text", "text": v},
+						}
+					case []interface{}:
+						msg["parts"] = v
+					default:
+						return nil, "", &proxyA2AError{
+							Status: http.StatusBadRequest,
+							Response: gin.H{
+								"error": "invalid params.message.content type",
+								"hint":  "content must be a string (v0.2 compat) or omitted in favour of parts (v0.3)",
+							},
+						}
+					}
+					delete(msg, "content")
+				} else {
+					return nil, "", &proxyA2AError{
+						Status: http.StatusBadRequest,
+						Response: gin.H{
+							"error": "params.message must contain either 'parts' (v0.3) or 'content' (v0.2 compat)",
+							"hint":  "v0.3 example: {\"parts\":[{\"kind\":\"text\",\"text\":\"...\"}]}",
+						},
+					}
+				}
+			}
 		}
 	}
 
diff --git a/workspace-server/internal/handlers/a2a_proxy_test.go b/workspace-server/internal/handlers/a2a_proxy_test.go
index 1a33a866..1b43c402 100644
--- a/workspace-server/internal/handlers/a2a_proxy_test.go
+++ b/workspace-server/internal/handlers/a2a_proxy_test.go
@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -1137,7 +1138,10 @@ func TestNormalizeA2APayload_PreservesExistingMessageId(t *testing.T) {
 }
 
 func TestNormalizeA2APayload_MissingMethodReturnsEmpty(t *testing.T) {
-	raw := []byte(`{"params":{"message":{"role":"user"}}}`)
+	// Method extraction returns empty string when method is absent,
+	// regardless of message validity. Include parts: [] so the v0.2→v0.3
+	// compat check (#2345) doesn't reject before method extraction.
+	raw := []byte(`{"params":{"message":{"role":"user","parts":[]}}}`)
 	_, method, perr := normalizeA2APayload(raw)
 	if perr != nil {
 		t.Fatalf("unexpected error: %+v", perr)
@@ -1147,6 +1151,102 @@ func TestNormalizeA2APayload_MissingMethodReturnsEmpty(t *testing.T) {
 	}
 }
 
+// --- v0.2 → v0.3 compat shim (#2345) ---
+
+func TestNormalizeA2APayload_ConvertsV02StringContentToParts(t *testing.T) {
+	raw := []byte(`{"method":"message/send","params":{"message":{"role":"user","content":"hello world"}}}`)
+	out, _, perr := normalizeA2APayload(raw)
+	if perr != nil {
+		t.Fatalf("unexpected error: %+v", perr)
+	}
+	var parsed map[string]interface{}
+	if err := json.Unmarshal(out, &parsed); err != nil {
+		t.Fatalf("output not valid JSON: %v", err)
+	}
+	msg := parsed["params"].(map[string]interface{})["message"].(map[string]interface{})
+	if _, stillHasContent := msg["content"]; stillHasContent {
+		t.Error("v0.2 'content' field should be removed after conversion")
+	}
+	parts, ok := msg["parts"].([]interface{})
+	if !ok || len(parts) != 1 {
+		t.Fatalf("expected 1 part, got %v", msg["parts"])
+	}
+	part := parts[0].(map[string]interface{})
+	if part["kind"] != "text" || part["text"] != "hello world" {
+		t.Errorf("expected {kind:text, text:'hello world'}, got %v", part)
+	}
+}
+
+func TestNormalizeA2APayload_ConvertsV02ListContentToParts(t *testing.T) {
+	raw := []byte(`{"method":"message/send","params":{"message":{"role":"user","content":[{"kind":"text","text":"hi"}]}}}`)
+	out, _, perr := normalizeA2APayload(raw)
+	if perr != nil {
+		t.Fatalf("unexpected error: %+v", perr)
+	}
+	var parsed map[string]interface{}
+	_ = json.Unmarshal(out, &parsed)
+	msg := parsed["params"].(map[string]interface{})["message"].(map[string]interface{})
+	parts, ok := msg["parts"].([]interface{})
+	if !ok || len(parts) != 1 {
+		t.Fatalf("expected list preserved as parts, got %v", msg["parts"])
+	}
+}
+
+func TestNormalizeA2APayload_PreservesV03Parts(t *testing.T) {
+	raw := []byte(`{"method":"message/send","params":{"message":{"role":"user","parts":[{"kind":"text","text":"hi"}]}}}`)
+	out, _, perr := normalizeA2APayload(raw)
+	if perr != nil {
+		t.Fatalf("unexpected error: %+v", perr)
+	}
+	var parsed map[string]interface{}
+	_ = json.Unmarshal(out, &parsed)
+	msg := parsed["params"].(map[string]interface{})["message"].(map[string]interface{})
+	if _, hasContent := msg["content"]; hasContent {
+		t.Error("did not expect content field in v0.3-shaped payload output")
+	}
+	parts := msg["parts"].([]interface{})
+	if len(parts) != 1 {
+		t.Errorf("expected 1 part preserved, got %d", len(parts))
+	}
+}
+
+func TestNormalizeA2APayload_RejectsMessageWithNeitherContentNorParts(t *testing.T) {
+	raw := []byte(`{"method":"message/send","params":{"message":{"role":"user","metadata":{}}}}`)
+	_, _, perr := normalizeA2APayload(raw)
+	if perr == nil {
+		t.Fatal("expected error for message with neither content nor parts")
+	}
+	if perr.Status != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", perr.Status)
+	}
+	errMsg, _ := perr.Response["error"].(string)
+	if !strings.Contains(errMsg, "parts") || !strings.Contains(errMsg, "content") {
+		t.Errorf("error message should mention both 'parts' and 'content', got: %q", errMsg)
+	}
+}
+
+func TestNormalizeA2APayload_RejectsContentWithUnsupportedType(t *testing.T) {
+	raw := []byte(`{"method":"message/send","params":{"message":{"role":"user","content":42}}}`)
+	_, _, perr := normalizeA2APayload(raw)
+	if perr == nil {
+		t.Fatal("expected error for non-string non-list content")
+	}
+	if perr.Status != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", perr.Status)
+	}
+}
+
+func TestNormalizeA2APayload_NoMessageNoCheck(t *testing.T) {
+	raw := []byte(`{"method":"tasks/list","params":{}}`)
+	_, method, perr := normalizeA2APayload(raw)
+	if perr != nil {
+		t.Fatalf("unexpected error on params-message-absent payload: %+v", perr)
+	}
+	if method != "tasks/list" {
+		t.Errorf("expected method=tasks/list, got %q", method)
+	}
+}
+
 // --- resolveAgentURL direct unit tests ---
 
 func TestResolveAgentURL_CacheHit(t *testing.T) {