diff --git a/pkg/agent/context.go b/pkg/agent/context.go
index 631974337..54d21d775 100644
--- a/pkg/agent/context.go
+++ b/pkg/agent/context.go
@@ -738,7 +738,54 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 		final = append(final, msg)
 	}
 
-	return final
+	// Third pass: ensure valid message alternation.
+	// Dropping messages in earlier passes can create consecutive same-role
+	// messages (e.g. two user messages with a dropped assistant between them).
+	// LLM APIs (Anthropic, DeepSeek, etc.) require strict user/assistant
+	// alternation and reject requests with consecutive same-role messages.
+	merged := make([]providers.Message, 0, len(final))
+	for _, msg := range final {
+		// Skip empty user messages (no text content, no tool result)
+		if msg.Role == "user" && strings.TrimSpace(msg.Content) == "" && msg.ToolCallID == "" {
+			logger.DebugCF("agent", "Dropping empty user message", nil)
+			continue
+		}
+
+		if len(merged) == 0 {
+			// First message must be a user message (after system is stripped).
+			// Drop leading assistant/tool messages.
+			if msg.Role != "user" {
+				logger.DebugCF("agent", "Dropping leading non-user message", map[string]any{
+					"role": msg.Role,
+				})
+				continue
+			}
+			merged = append(merged, msg)
+			continue
+		}
+
+		prev := &merged[len(merged)-1]
+
+		// Merge consecutive user text messages (not tool results).
+		if msg.Role == "user" && msg.ToolCallID == "" &&
+			prev.Role == "user" && prev.ToolCallID == "" {
+			if prev.Content != "" && msg.Content != "" {
+				prev.Content = prev.Content + "\n" + msg.Content
+			} else if msg.Content != "" {
+				prev.Content = msg.Content
+			}
+			logger.DebugCF("agent", "Merged consecutive user messages", nil)
+			continue
+		}
+
+		// Tool results (role "tool" or "user" with ToolCallID) count as user
+		// messages in the Anthropic API. If the previous message is also a
+		// tool/user, that's fine — but if the previous is a plain user text
+		// message and this is also a plain user text message, we merged above.
+		merged = append(merged, msg)
+	}
+
+	return merged
 }
 
 func (cb *ContextBuilder) AddToolResult(
diff --git a/pkg/agent/context_test.go b/pkg/agent/context_test.go
index f307a637b..a84783eca 100644
--- a/pkg/agent/context_test.go
+++ b/pkg/agent/context_test.go
@@ -233,12 +233,13 @@ func TestSanitizeHistoryForProvider_IncompleteToolResults(t *testing.T) {
 
 	result := sanitizeHistoryForProvider(history)
 	// The assistant message with incomplete tool results should be dropped,
-	// along with its partial tool result. The remaining messages are:
-	// user ("do two things"), user ("next question"), assistant ("answer")
-	if len(result) != 3 {
-		t.Fatalf("expected 3 messages, got %d: %+v", len(result), roles(result))
+	// along with its partial tool result. The two consecutive user messages
+	// are then merged by the third pass.
+	// Remaining: user ("do two things\nnext question"), assistant ("answer")
+	if len(result) != 2 {
+		t.Fatalf("expected 2 messages, got %d: %+v", len(result), roles(result))
 	}
-	assertRoles(t, result, "user", "user", "assistant")
+	assertRoles(t, result, "user", "assistant")
 }
 
 // TestSanitizeHistoryForProvider_MissingAllToolResults tests the case where
@@ -254,11 +255,12 @@ func TestSanitizeHistoryForProvider_MissingAllToolResults(t *testing.T) {
 
 	result := sanitizeHistoryForProvider(history)
 	// The assistant message with no tool results should be dropped.
-	// Remaining: user ("do something"), user ("hello"), assistant ("hi")
-	if len(result) != 3 {
-		t.Fatalf("expected 3 messages, got %d: %+v", len(result), roles(result))
+	// The two consecutive user messages are merged by the third pass.
+	// Remaining: user ("do something\nhello"), assistant ("hi")
+	if len(result) != 2 {
+		t.Fatalf("expected 2 messages, got %d: %+v", len(result), roles(result))
 	}
-	assertRoles(t, result, "user", "user", "assistant")
+	assertRoles(t, result, "user", "assistant")
 }
 
 // TestSanitizeHistoryForProvider_PartialToolResultsInMiddle tests that
@@ -283,11 +285,12 @@ func TestSanitizeHistoryForProvider_PartialToolResultsInMiddle(t *testing.T) {
 	// First round is complete (user, assistant+tools, tool, assistant),
 	// second round is incomplete and dropped (assistant+tools, partial tool),
 	// third round is complete (user, assistant+tools, tool, assistant).
-	// Remaining: user, assistant, tool, assistant, user, user, assistant, tool, assistant
-	if len(result) != 9 {
-		t.Fatalf("expected 9 messages, got %d: %+v", len(result), roles(result))
+	// The two consecutive user messages ("second", "third") are merged.
+	// Remaining: user, assistant, tool, assistant, user, assistant, tool, assistant
+	if len(result) != 8 {
+		t.Fatalf("expected 8 messages, got %d: %+v", len(result), roles(result))
 	}
-	assertRoles(t, result, "user", "assistant", "tool", "assistant", "user", "user", "assistant", "tool", "assistant")
+	assertRoles(t, result, "user", "assistant", "tool", "assistant", "user", "assistant", "tool", "assistant")
 }
 
 // TestSanitizeHistoryForProvider_EmptyToolCallName tests that tool calls with
@@ -379,9 +382,10 @@ func TestSanitizeHistoryForProvider_AllToolCallsDroppedEmptyContent(t *testing.T
 	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))
+	// The two consecutive user messages are merged by the third pass.
+	// Remaining: user ("hello\nfollow up")
+	if len(result) != 1 {
+		t.Fatalf("expected 1 message, got %d: %+v", len(result), roles(result))
 	}
-	assertRoles(t, result, "user", "user")
+	assertRoles(t, result, "user")
 }
diff --git a/pkg/providers/anthropic/provider.go b/pkg/providers/anthropic/provider.go
index 87beae99a..ac77f040a 100644
--- a/pkg/providers/anthropic/provider.go
+++ b/pkg/providers/anthropic/provider.go
@@ -146,7 +146,8 @@ func buildParams(
 	var system []anthropic.TextBlockParam
 	var anthropicMessages []anthropic.MessageParam
 
-	for _, msg := range messages {
+	for i := 0; i < len(messages); i++ {
+		msg := messages[i]
 		switch msg.Role {
 		case "system":
 			// Prefer structured SystemParts for per-block cache_control.
@@ -165,9 +166,16 @@ func buildParams(
 			}
 		case "user":
 			if msg.ToolCallID != "" {
-				anthropicMessages = append(anthropicMessages,
-					anthropic.NewUserMessage(anthropic.NewToolResultBlock(msg.ToolCallID, msg.Content, false)),
-				)
+				// Tool result — group with any following tool results into
+				// a single user message to avoid consecutive user messages.
+				var blocks []anthropic.ContentBlockParamUnion
+				blocks = append(blocks, anthropic.NewToolResultBlock(msg.ToolCallID, msg.Content, false))
+				for i+1 < len(messages) && (messages[i+1].Role == "tool" ||
+					(messages[i+1].Role == "user" && messages[i+1].ToolCallID != "")) {
+					i++
+					blocks = append(blocks, anthropic.NewToolResultBlock(messages[i].ToolCallID, messages[i].Content, false))
+				}
+				anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(blocks...))
 			} else {
 				anthropicMessages = append(anthropicMessages,
 					anthropic.NewUserMessage(anthropic.NewTextBlock(msg.Content)),
@@ -198,9 +206,16 @@ func buildParams(
 				)
 			}
 		case "tool":
-			anthropicMessages = append(anthropicMessages,
-				anthropic.NewUserMessage(anthropic.NewToolResultBlock(msg.ToolCallID, msg.Content, false)),
-			)
+			// Group consecutive tool results into a single user message.
+			// The Anthropic API requires alternating user/assistant messages;
+			// sending each tool result as a separate user message can violate this.
+			var blocks []anthropic.ContentBlockParamUnion
+			blocks = append(blocks, anthropic.NewToolResultBlock(msg.ToolCallID, msg.Content, false))
+			for i+1 < len(messages) && messages[i+1].Role == "tool" {
+				i++
+				blocks = append(blocks, anthropic.NewToolResultBlock(messages[i].ToolCallID, messages[i].Content, false))
+			}
+			anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(blocks...))
 		}
 	}
 
diff --git a/pkg/providers/anthropic_messages/provider.go b/pkg/providers/anthropic_messages/provider.go
index 8a83a7058..aeba3e11b 100644
--- a/pkg/providers/anthropic_messages/provider.go
+++ b/pkg/providers/anthropic_messages/provider.go
@@ -176,7 +176,8 @@ func buildRequestBody(
 	var systemPrompt string
 	var apiMessages []any
 
-	for _, msg := range messages {
+	for i := 0; i < len(messages); i++ {
+		msg := messages[i]
 		switch msg.Role {
 		case "system":
 			// Accumulate system messages
@@ -188,7 +189,8 @@ func buildRequestBody(
 
 		case "user":
 			if msg.ToolCallID != "" {
-				// Tool result message
+				// Tool result message — group consecutive tool results into
+				// a single user message to avoid consecutive user messages.
 				content := []map[string]any{
 					{
 						"type":        "tool_result",
@@ -196,6 +198,15 @@ func buildRequestBody(
 						"content":     msg.Content,
 					},
 				}
+				for i+1 < len(messages) && (messages[i+1].Role == "tool" ||
+					(messages[i+1].Role == "user" && messages[i+1].ToolCallID != "")) {
+					i++
+					content = append(content, map[string]any{
+						"type":        "tool_result",
+						"tool_use_id": messages[i].ToolCallID,
+						"content":     messages[i].Content,
+					})
+				}
 				apiMessages = append(apiMessages, map[string]any{
 					"role":    "user",
 					"content": content,
@@ -236,7 +247,7 @@ func buildRequestBody(
 			})
 
 		case "tool":
-			// Tool result (alternative format)
+			// Group consecutive tool results into a single user message.
 			content := []map[string]any{
 				{
 					"type":        "tool_result",
@@ -244,6 +255,14 @@ func buildRequestBody(
 					"content":     msg.Content,
 				},
 			}
+			for i+1 < len(messages) && messages[i+1].Role == "tool" {
+				i++
+				content = append(content, map[string]any{
+					"type":        "tool_result",
+					"tool_use_id": messages[i].ToolCallID,
+					"content":     messages[i].Content,
+				})
+			}
 			apiMessages = append(apiMessages, map[string]any{
 				"role":    "user",
 				"content": content,