fix(agent): drop content-empty assistant messages after tool call normalization

When all tool calls in an assistant history message are dropped (because
both Name and Function.Name are empty after normalization), the message
ends up with no tool calls and no text content. Sending it to Anthropic
produces either an empty content array ("content": []) or an empty text
block — both are rejected with a generic 400 "Error" response.

Fix: after the tool-call normalization loop, if the assistant message has
no remaining tool calls and no text content, drop the entire message. Any
tool results that referenced the dropped calls are already handled as
orphans by the existing second-pass logic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/context.go

@@ -634,6 +634,16 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 				}
 				msg.ToolCalls = cleaned
 			}
+			// If all tool calls were dropped and the message has no text
+			// content either, the message would produce an empty content
+			// array ("content": []) or an empty text block — both rejected
+			// by the Anthropic API with a generic 400 "Error". Drop it
+			// entirely; the second pass will orphan any associated tool
+			// results, which are also harmless to drop.
+			if len(msg.ToolCalls) == 0 && strings.TrimSpace(msg.Content) == "" {
+				logger.DebugCF("agent", "Dropping assistant message with no content and no tool calls", map[string]any{})
+				continue
+			}
 			if len(msg.ToolCalls) > 0 {
 				if len(sanitized) == 0 {
 					logger.DebugCF("agent", "Dropping assistant tool-call turn at history start", map[string]any{})

pkg/agent/context_test.go

@@ -340,7 +340,8 @@ func TestSanitizeHistoryForProvider_EmptyToolCallName(t *testing.T) {
 }
 
 // TestSanitizeHistoryForProvider_EmptyToolCallID tests that tool calls with an
-// empty ID are dropped (an empty ID also causes Anthropic API errors).
+// empty ID are dropped, and that the resulting content-less assistant message
+// is also dropped (avoiding an empty content array sent to the API).
 func TestSanitizeHistoryForProvider_EmptyToolCallID(t *testing.T) {
 	history := []providers.Message{
 		msg("user", "hello"),
@@ -351,14 +352,36 @@ func TestSanitizeHistoryForProvider_EmptyToolCallID(t *testing.T) {
 	}
 
 	result := sanitizeHistoryForProvider(history)
-	// The tool call with empty ID is dropped; the assistant message has no tool
-	// calls after filtering, so it passes through as a plain assistant message.
-	// user + assistant (plain) = 2
+	// The tool call with empty ID is dropped; the assistant message then has no
+	// tool calls and no text content → it is also dropped to avoid sending an
+	// empty content array to the API.
+	// Remaining: user ("hello") only.
+	if len(result) != 1 {
+		t.Fatalf("expected 1 message, got %d: %+v", len(result), roles(result))
+	}
+	assertRoles(t, result, "user")
+}
+
+// TestSanitizeHistoryForProvider_AllToolCallsDroppedEmptyContent tests that an
+// assistant message whose every tool call is dropped AND whose text content is
+// empty is itself dropped. Sending such a message would produce "content": []
+// or an empty text block — both cause a generic Anthropic 400 "Error".
+func TestSanitizeHistoryForProvider_AllToolCallsDroppedEmptyContent(t *testing.T) {
+	history := []providers.Message{
+		msg("user", "hello"),
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "A", Type: "function", Function: &providers.FunctionCall{Name: "", Arguments: "{}"}},
+		}},
+		toolResult("A"),
+		msg("user", "follow up"),
+	}
+
+	result := sanitizeHistoryForProvider(history)
+	// The assistant message has no valid tool calls and no content → dropped.
+	// Its tool result is orphaned and also dropped.
+	// Remaining: user ("hello"), user ("follow up")
 	if len(result) != 2 {
 		t.Fatalf("expected 2 messages, got %d: %+v", len(result), roles(result))
 	}
-	assertRoles(t, result, "user", "assistant")
-	if len(result[1].ToolCalls) != 0 {
-		t.Fatalf("expected 0 tool calls in assistant, got %d", len(result[1].ToolCalls))
-	}
+	assertRoles(t, result, "user", "user")
 }