fix(kosong): merge only same-kind consecutive user messages in anthropic adapter

Strict Anthropic-compatible backends reject consecutive user messages with
HTTP 400, so the adapter collapses them — but a plain-text user turn and an
adjacent tool-result user message carry different semantics and must stay
separate. Merge plain-text with plain-text (collapsing the post-compaction
run of kept prompts + user-role summary + reminders) and tool-result with
tool-result (parallel-tool-use spec), but not across the two kinds.

Author: 7Sageer

packages/kosong/src/providers/anthropic.ts

@@ -392,6 +392,18 @@ function injectCacheControlOnLastBlock(messages: MessageParam[]): void {
   }
 }
 
+/**
+ * Whether a user MessageParam consists solely of `tool_result` blocks. Used to
+ * keep tool results bundled with each other (parallel-tool-use spec) while
+ * not merging a tool-result user message into an adjacent plain-text user
+ * message — the two carry different semantics and must stay separate.
+ */
+function isToolResultOnly(message: MessageParam): boolean {
+  if (message.role !== 'user') return false;
+  const content = message.content;
+  if (!Array.isArray(content) || content.length === 0) return false;
+  return content.every((block) => block.type === 'tool_result');
+}
 interface AnthropicImageBlock {
   type: 'image';
   source: { type: 'base64'; data: string; media_type: string } | { type: 'url'; url: string };
@@ -983,15 +995,19 @@ export class AnthropicChatProvider implements ChatProvider {
         ]
       : undefined;
 
-    // Convert messages, merging consecutive user messages into one. Strict
-    // Anthropic-compatible backends reject consecutive user messages with HTTP
-    // 400 ("roles must alternate"), and api.anthropic.com concatenates them
-    // anyway — so merging is safe for native Anthropic and required for strict
-    // backends. This subsumes the parallel-tool-use requirement that all
-    // tool_result blocks answering parallel tool_use calls live in a single
-    // user message. Consecutive user messages arise naturally after compaction
-    // (kept user prompts + user-role summary + injected reminders) and from
-    // back-to-back system/tool messages converted to user role above.
+    // Convert messages, merging consecutive user messages of the same kind into
+    // one. Strict Anthropic-compatible backends reject consecutive user messages
+    // with HTTP 400 ("roles must alternate"), and api.anthropic.com concatenates
+    // them anyway — so merging is safe for native Anthropic and required for
+    // strict backends. Plain-text user messages merge with plain-text user
+    // messages; tool-result-only user messages merge with tool-result-only ones
+    // (the parallel-tool-use spec requires all tool_result blocks answering
+    // parallel tool_use calls to live in a single user message). A plain-text
+    // user message is intentionally NOT merged into an adjacent tool-result one:
+    // the two carry different semantics and must stay separate. Consecutive
+    // plain-text user messages arise naturally after compaction (kept user
+    // prompts + user-role summary + injected reminders) and from back-to-back
+    // system messages converted to user role above.
     const messages: MessageParam[] = [];
     const normalizedHistory = normalizeToolCallIdsForProvider(
       history,
@@ -1000,7 +1016,12 @@ export class AnthropicChatProvider implements ChatProvider {
     for (const msg of normalizedHistory) {
       const converted = convertMessage(msg, this._model);
       const last = messages.at(-1);
-      if (last !== undefined && last.role === 'user' && converted.role === 'user') {
+      if (
+        last !== undefined &&
+        last.role === 'user' &&
+        converted.role === 'user' &&
+        isToolResultOnly(last) === isToolResultOnly(converted)
+      ) {
         last.content = [
           ...(last.content as ContentBlockParam[]),
           ...(converted.content as ContentBlockParam[]),

packages/kosong/test/anthropic.test.ts

@@ -1024,6 +1024,28 @@ describe('AnthropicChatProvider', () => {
       expect(msgs[3]!.content[0]!.text).toBe('Now summarize');
     });
 
+    it('merges consecutive plain-text user messages into one', async () => {
+      const provider = createProvider();
+      const history: Message[] = [
+        { role: 'user', content: [{ type: 'text', text: 'First' }], toolCalls: [] },
+        { role: 'user', content: [{ type: 'text', text: 'Second' }], toolCalls: [] },
+        { role: 'user', content: [{ type: 'text', text: 'Third' }], toolCalls: [] },
+      ];
+      const body = await captureRequestBody(provider, '', [], history);
+
+      const msgs = body['messages'] as Array<{
+        role: string;
+        content: Array<{ type: string; text?: string }>;
+      }>;
+
+      // Strict Anthropic-compatible backends reject consecutive user messages,
+      // so back-to-back plain-text user turns (e.g. the post-compaction shape
+      // of kept prompts + user-role summary + reminders) must be collapsed.
+      expect(msgs).toHaveLength(1);
+      expect(msgs[0]!.role).toBe('user');
+      expect(msgs[0]!.content.map((block) => block.text)).toEqual(['First', 'Second', 'Third']);
+    });
+
     it('assistant with thinking (has encrypted -> ThinkingBlockParam)', async () => {
       const provider = createProvider();
       const history: Message[] = [