fix: full-scan history sanitization for interleaved/out-of-order tool calls

The previous tail-only trimming didn't catch mid-history corruption from
session collisions (e.g. a user message interleaved between an assistant
tool call and its result). APIs like MiniMax reject this with "tool call
result does not follow tool call".

Rewrote SanitizeHistory to walk the full history, keeping only well-formed
groups where tool results immediately follow their assistant message.

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

pkg/session/manager.go

@@ -265,75 +265,78 @@ func (sm *SessionManager) loadSessions() error {
 	return nil
 }
 
-// SanitizeHistory removes orphaned tool calls from session history.
-// An orphaned tool call is an assistant message containing ToolCalls where
-// one or more call IDs have no matching tool-result message (role="tool")
-// following it. This can happen if the process crashed mid-execution.
-// The function trims incomplete assistant+tool-result groups from the tail.
+// SanitizeHistory rebuilds session history to ensure valid tool-call ordering.
+// LLM APIs require that every assistant message with ToolCalls is immediately
+// followed by exactly the matching tool-result messages (role="tool"), with no
+// other messages in between. Violations can happen from session collisions or
+// mid-execution crashes.
+//
+// The function walks the full history and copies only well-formed groups:
+//   - user/system messages are always kept
+//   - assistant messages without tool calls are always kept
+//   - assistant messages WITH tool calls are kept only if the immediately
+//     following messages are the complete set of matching tool results
+//
 // Returns the sanitized history and the number of messages removed.
 func SanitizeHistory(history []providers.Message) ([]providers.Message, int) {
 	if len(history) == 0 {
 		return history, 0
 	}
 
-	original := len(history)
+	result := make([]providers.Message, 0, len(history))
+	i := 0
 
-	// Walk backwards from the tail, trimming incomplete tool-call groups.
-	for len(history) > 0 {
-		last := history[len(history)-1]
+	for i < len(history) {
+		msg := history[i]
 
-		// If tail is a tool result, find its parent assistant message and check completeness
-		if last.Role == "tool" {
-			// Find the nearest preceding assistant message with tool calls
-			assistantIdx := -1
-			for i := len(history) - 2; i >= 0; i-- {
-				if history[i].Role == "assistant" && len(history[i].ToolCalls) > 0 {
-					assistantIdx = i
-					break
-				}
-			}
-
-			if assistantIdx < 0 {
-				// Orphaned tool result with no assistant — remove it
-				history = history[:len(history)-1]
+		// Non-assistant messages or assistant without tool calls: keep
+		if msg.Role != "assistant" || len(msg.ToolCalls) == 0 {
+			// Skip stray tool results not preceded by their assistant
+			if msg.Role == "tool" {
+				i++
 				continue
 			}
-
-			// Collect all expected tool call IDs from the assistant message
-			expected := make(map[string]bool)
-			for _, tc := range history[assistantIdx].ToolCalls {
-				expected[tc.ID] = true
-			}
-
-			// Check how many results exist between assistant and end of history
-			for i := assistantIdx + 1; i < len(history); i++ {
-				if history[i].Role == "tool" && expected[history[i].ToolCallID] {
-					delete(expected, history[i].ToolCallID)
-				}
-			}
-
-			if len(expected) > 0 {
-				// Incomplete group — remove everything from assistantIdx onward
-				history = history[:assistantIdx]
-				continue
-			}
-
-			// Group is complete, we're done
-			break
-		}
-
-		// If tail is an assistant with tool calls, check if ALL results follow
-		if last.Role == "assistant" && len(last.ToolCalls) > 0 {
-			// No tool results follow at all — orphaned
-			history = history[:len(history)-1]
+			result = append(result, msg)
+			i++
 			continue
 		}
 
-		// Tail is a normal message (user, assistant without tools) — we're done
-		break
+		// Assistant with tool calls: validate the immediately following messages
+		expectedIDs := make(map[string]bool, len(msg.ToolCalls))
+		for _, tc := range msg.ToolCalls {
+			expectedIDs[tc.ID] = true
+		}
+		needed := len(expectedIDs)
+
+		// Peek ahead: the next `needed` messages must all be tool results with matching IDs
+		groupOK := true
+		if i+needed >= len(history) {
+			groupOK = false
+		} else {
+			for j := 0; j < needed; j++ {
+				next := history[i+1+j]
+				if next.Role != "tool" || !expectedIDs[next.ToolCallID] {
+					groupOK = false
+					break
+				}
+			}
+		}
+
+		if groupOK {
+			// Copy assistant + all tool results
+			result = append(result, msg)
+			for j := 0; j < needed; j++ {
+				result = append(result, history[i+1+j])
+			}
+			i += 1 + needed
+		} else {
+			// Skip the broken assistant message; tool results will be skipped
+			// individually when encountered (the "stray tool result" check above)
+			i++
+		}
 	}
 
-	return history, original - len(history)
+	return result, len(history) - len(result)
 }
 
 // SetHistory updates the messages of a session.

pkg/session/manager_test.go

@@ -74,14 +74,39 @@ func TestSanitizeHistory_OrphanedToolCall(t *testing.T) {
 	}
 
 	sanitized, removed := SanitizeHistory(history)
-	// The orphaned assistant msg (with call_2 missing) and the trailing tool result
-	// should both be removed, leaving just the user message
 	if removed == 0 {
 		t.Fatal("expected orphaned messages to be removed")
 	}
 	// After sanitization, only the user message should remain
 	if len(sanitized) != 1 || sanitized[0].Role != "user" {
-		t.Errorf("expected [user], got %d messages: %v", len(sanitized), sanitized)
+		t.Errorf("expected [user], got %d messages", len(sanitized))
+	}
+}
+
+func TestSanitizeHistory_InterleavedMessages(t *testing.T) {
+	// Simulates session collision: a user message got interleaved between
+	// an assistant tool call and its tool result
+	history := []providers.Message{
+		{Role: "user", Content: "first"},
+		{Role: "assistant", Content: "ok", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "exec"},
+		}},
+		{Role: "user", Content: "collision!"},  // ← interleaved from other session
+		{Role: "tool", Content: "ok", ToolCallID: "call_1"}, // ← out of order
+		{Role: "assistant", Content: "done"},
+	}
+
+	sanitized, removed := SanitizeHistory(history)
+	if removed == 0 {
+		t.Fatal("expected interleaved messages to be removed")
+	}
+	// Should keep: user("first"), user("collision!"), assistant("done")
+	// Should remove: assistant(call_1), tool(call_1)
+	if len(sanitized) != 3 {
+		t.Errorf("expected 3 messages, got %d", len(sanitized))
+		for i, m := range sanitized {
+			t.Logf("  [%d] role=%s content=%q", i, m.Role, m.Content)
+		}
 	}
 }
 
@@ -104,6 +129,27 @@ func TestSanitizeHistory_CleanHistory(t *testing.T) {
 	}
 }
 
+func TestSanitizeHistory_MultipleToolCalls(t *testing.T) {
+	history := []providers.Message{
+		{Role: "user", Content: "hello"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "exec"},
+			{ID: "call_2", Name: "read_file"},
+		}},
+		{Role: "tool", Content: "ok", ToolCallID: "call_1"},
+		{Role: "tool", Content: "content", ToolCallID: "call_2"},
+		{Role: "assistant", Content: "all done"},
+	}
+
+	sanitized, removed := SanitizeHistory(history)
+	if removed != 0 {
+		t.Errorf("expected 0 removed, got %d", removed)
+	}
+	if len(sanitized) != 5 {
+		t.Errorf("expected 5 messages, got %d", len(sanitized))
+	}
+}
+
 func TestSanitizeHistory_Empty(t *testing.T) {
 	sanitized, removed := SanitizeHistory(nil)
 	if removed != 0 || sanitized != nil {