fix: discourage _noop tool call during LiteLLM compaction (#18539)

packages/opencode/src/session/compaction.ts

@@ -176,6 +176,7 @@ export namespace SessionCompaction {
         const defaultPrompt = `Provide a detailed prompt for continuing our conversation above.
 Focus on information that would be helpful for continuing the conversation, including what we did, what we're doing, which files we're working on, and what we're going to do next.
 The summary that you construct will be used so that another agent can read it and continue the work.
+Do not call any tools. Respond only with the summary text.
 
 When constructing the summary, try to stick to this template:
 ---

packages/opencode/src/session/llm.ts

@@ -199,11 +199,19 @@ export namespace LLM {
       input.model.providerID.toLowerCase().includes("litellm") ||
       input.model.api.id.toLowerCase().includes("litellm")
 
+    // LiteLLM/Bedrock rejects requests where the message history contains tool
+    // calls but no tools param is present. When there are no active tools (e.g.
+    // during compaction), inject a stub tool to satisfy the validation requirement.
+    // The stub description explicitly tells the model not to call it.
     if (isLiteLLMProxy && Object.keys(tools).length === 0 && hasToolCalls(input.messages)) {
       tools["_noop"] = tool({
-        description:
-          "Placeholder for LiteLLM/Anthropic proxy compatibility - required when message history contains tool calls but no active tools are needed",
-        inputSchema: jsonSchema({ type: "object", properties: {} }),
+        description: "Do not call this tool. It exists only for API compatibility and must never be invoked.",
+        inputSchema: jsonSchema({
+          type: "object",
+          properties: {
+            reason: { type: "string", description: "Unused" },
+          },
+        }),
         execute: async () => ({ output: "", title: "", metadata: {} }),
       })
     }