refactor: update skill catalog injection logic and add corresponding tests

pkg/agent/context.go

@@ -690,12 +690,16 @@ func (cb *ContextBuilder) BuildMessagesFromPrompt(req PromptBuildRequest) []prov
 		}, &providers.CacheControl{Type: "ephemeral"}),
 	}
 
-	// Skip the skill catalog on tool-call continuations: the LLM already saw
-	// it in the initial turn request and doesn't need it re-sent for every
-	// intermediate tool round-trip. This saves significant tokens on providers
-	// without prompt caching (OpenAI-compat).
+	// Inject the skill catalog only when the LLM needs to (re)discover available skills:
+	//   - Turn 1: no history yet, LLM hasn't seen the catalog.
+	//   - After compaction: history was summarized; early turns (including the original
+	//     catalog injection) are gone, so the LLM must see it again.
+	// Skip it on tool-call continuations (mid-turn round-trips) and on ordinary
+	// subsequent turns where the catalog is already in the LLM's context window.
 	isToolContinuation := len(req.History) > 0 && req.History[len(req.History)-1].Role == "tool"
-	if !isToolContinuation {
+	isFirstTurn := len(req.History) == 0
+	isAfterCompaction := req.Summary != ""
+	if !isToolContinuation && (isFirstTurn || isAfterCompaction) {
 		if skillsSummary := cb.skillsLoader.BuildSkillsSummary(); skillsSummary != "" {
 			catalogPart := PromptPart{
 				ID:    "capability.skill_catalog",

pkg/agent/context_cache_test.go

@@ -614,6 +614,51 @@ description: delete-me-v1
 	}
 }
 
+// TestSkillCatalogInjectionPolicy verifies that the catalog is included only
+// when the LLM needs to (re)discover skills: turn 1 and after compaction.
+func TestSkillCatalogInjectionPolicy(t *testing.T) {
+	tmpDir := setupWorkspace(t, map[string]string{
+		"skills/demo/SKILL.md": "---\nname: demo\ndescription: \"demo skill\"\n---\n# Demo",
+	})
+	defer os.RemoveAll(tmpDir)
+
+	cb := NewContextBuilder(tmpDir)
+	userMsg := providers.Message{Role: "user", Content: "hello"}
+	assistantMsg := providers.Message{Role: "assistant", Content: "hi"}
+	toolMsg := providers.Message{Role: "tool", Content: "result", ToolCallID: "tc1"}
+
+	contains := func(msgs []providers.Message) bool {
+		return strings.Contains(systemPromptFromMessages(msgs), "demo skill")
+	}
+
+	// Turn 1: no history — catalog must appear.
+	if !contains(cb.BuildMessagesFromPrompt(PromptBuildRequest{})) {
+		t.Error("turn 1 (no history): catalog should be included")
+	}
+
+	// Tool continuation: last message is a tool result — catalog must be skipped.
+	if contains(cb.BuildMessagesFromPrompt(PromptBuildRequest{
+		History: []providers.Message{userMsg, assistantMsg, toolMsg},
+	})) {
+		t.Error("tool continuation: catalog should be skipped")
+	}
+
+	// Turn > 1, no compaction: catalog must be skipped.
+	if contains(cb.BuildMessagesFromPrompt(PromptBuildRequest{
+		History: []providers.Message{userMsg, assistantMsg},
+	})) {
+		t.Error("turn > 1, no summary: catalog should be skipped")
+	}
+
+	// After compaction (summary present): catalog must be re-injected.
+	if !contains(cb.BuildMessagesFromPrompt(PromptBuildRequest{
+		History: []providers.Message{userMsg, assistantMsg},
+		Summary: "prior conversation summary",
+	})) {
+		t.Error("after compaction (summary present): catalog should be re-injected")
+	}
+}
+
 // TestConcurrentBuildSystemPromptWithCache verifies that multiple goroutines
 // can safely call BuildSystemPromptWithCache concurrently without producing
 // empty results, panics, or data races.