diff --git a/pkg/agent/cost_test.go b/pkg/agent/cost_test.go
new file mode 100644
index 0000000..a3c3540
--- /dev/null
+++ b/pkg/agent/cost_test.go
@@ -0,0 +1,100 @@
+package agent
+
+import (
+	"math"
+	"testing"
+
+	"ollie/pkg/backend"
+)
+
+func approxEqual(a, b, tol float64) bool {
+	return math.Abs(a-b) <= tol
+}
+
+// TestComputeCostUSD_BaseTokens verifies input+output pricing with no cache fields.
+func TestComputeCostUSD_BaseTokens(t *testing.T) {
+	// claude-sonnet-4: $3/M input, $15/M output
+	got := computeCostUSD("claude-sonnet-4-5", backend.Usage{
+		InputTokens:  1_000_000,
+		OutputTokens: 1_000_000,
+	})
+	want := 3.00 + 15.00
+	if !approxEqual(got, want, 0.001) {
+		t.Errorf("got %.4f; want %.4f", got, want)
+	}
+}
+
+// TestComputeCostUSD_ClaudeCacheReadDiscount verifies that cached input tokens
+// are charged at 10% of the normal input rate for Claude models.
+func TestComputeCostUSD_ClaudeCacheReadDiscount(t *testing.T) {
+	// claude-sonnet-4: $3/M input → cache read = $0.30/M
+	got := computeCostUSD("claude-sonnet-4-5", backend.Usage{
+		InputTokens:       0,
+		CachedInputTokens: 1_000_000,
+		OutputTokens:      0,
+	})
+	want := 0.30 // 10% of $3.00
+	if !approxEqual(got, want, 0.001) {
+		t.Errorf("claude cache read: got %.4f; want %.4f", got, want)
+	}
+}
+
+// TestComputeCostUSD_NonClaudeCacheReadDiscount verifies that cached input tokens
+// are charged at 50% of the normal input rate for non-Claude models (OpenAI).
+func TestComputeCostUSD_NonClaudeCacheReadDiscount(t *testing.T) {
+	// gpt-4o: $2.50/M input → cache read = $1.25/M
+	got := computeCostUSD("gpt-4o", backend.Usage{
+		InputTokens:       0,
+		CachedInputTokens: 1_000_000,
+		OutputTokens:      0,
+	})
+	want := 1.25 // 50% of $2.50
+	if !approxEqual(got, want, 0.001) {
+		t.Errorf("gpt-4o cache read: got %.4f; want %.4f", got, want)
+	}
+}
+
+// TestComputeCostUSD_CacheCreationSurcharge verifies cache creation tokens are
+// charged at 125% of the normal input rate.
+func TestComputeCostUSD_CacheCreationSurcharge(t *testing.T) {
+	// claude-sonnet-4: $3/M input → cache creation = $3.75/M
+	got := computeCostUSD("claude-sonnet-4-5", backend.Usage{
+		InputTokens:         0,
+		CacheCreationTokens: 1_000_000,
+		OutputTokens:        0,
+	})
+	want := 3.75 // 125% of $3.00
+	if !approxEqual(got, want, 0.001) {
+		t.Errorf("cache creation surcharge: got %.4f; want %.4f", got, want)
+	}
+}
+
+// TestComputeCostUSD_AllFieldsCombined verifies that all four token categories
+// are summed correctly in a single call.
+func TestComputeCostUSD_AllFieldsCombined(t *testing.T) {
+	// claude-sonnet-4: $3/M in, $15/M out
+	//   100k normal input  = $0.30
+	//   200k cached input  = $0.06   (10% of $3/M)
+	//   50k  cache create  = $0.1875 (125% of $3/M)
+	//   100k output        = $1.50
+	got := computeCostUSD("claude-sonnet-4-5", backend.Usage{
+		InputTokens:         100_000,
+		CachedInputTokens:   200_000,
+		CacheCreationTokens: 50_000,
+		OutputTokens:        100_000,
+	})
+	want := 0.30 + 0.06 + 0.1875 + 1.50
+	if !approxEqual(got, want, 0.0001) {
+		t.Errorf("combined: got %.6f; want %.6f", got, want)
+	}
+}
+
+// TestComputeCostUSD_UnknownModelZero verifies that unknown/local models return 0.
+func TestComputeCostUSD_UnknownModelZero(t *testing.T) {
+	got := computeCostUSD("llama-3-local", backend.Usage{
+		InputTokens: 1_000_000, OutputTokens: 1_000_000,
+	})
+	if got != 0 {
+		t.Errorf("unknown model: got %.4f; want 0", got)
+	}
+}
diff --git a/pkg/agent/loop_cache_test.go b/pkg/agent/loop_cache_test.go
index 4262fab..0ff83e9 100644
--- a/pkg/agent/loop_cache_test.go
+++ b/pkg/agent/loop_cache_test.go
@@ -143,6 +143,43 @@ func TestResultCache_ErrorNotCached(t *testing.T) {
 	}
 }
 
+// TestResultCache_IdenticalParallelReads verifies that two identical read-safe
+// tool calls issued in the same parallel batch (both goroutines miss the cache
+// simultaneously) both complete without error and return consistent results.
+func TestResultCache_IdenticalParallelReads(t *testing.T) {
+	var execCount int32
+	be := defaultBE()
+	be.respond = toolsStream([]backend.ToolCall{
+		{ID: "1", Name: "file_read", Arguments: json.RawMessage(`{"path":"a.txt"}`)},
+		{ID: "2", Name: "file_read", Arguments: json.RawMessage(`{"path":"a.txt"}`)},
+	})
+
+	c := newCore(t, be, nil)
+	c.cfg.ClassifyTool = func(string) bool { return true } // both parallel-safe → same batch
+	c.cfg.Exec = func(_ context.Context, _ string, _ json.RawMessage) (string, error) {
+		atomic.AddInt32(&execCount, 1)
+		return "file contents", nil
+	}
+
+	evs := collectEvents(context.Background(), c, "identical parallel")
+
+	toolEvs := byRole(evs, "tool")
+	if len(toolEvs) != 2 {
+		t.Fatalf("tool events = %d; want 2", len(toolEvs))
+	}
+	for i, ev := range toolEvs {
+		if ev != "file contents" {
+			t.Errorf("tool event[%d] = %q; want %q", i, ev, "file contents")
+		}
+	}
+	// Both may have executed (cache miss race) or one may have hit cache —
+	// either is correct. What must not happen: panic, empty result, or wrong value.
+	n := atomic.LoadInt32(&execCount)
+	if n < 1 || n > 2 {
+		t.Errorf("Exec called %d times; want 1 or 2", n)
+	}
+}
+
 type mockErr struct{ msg string }
 
 func (e *mockErr) Error() string { return e.msg }
diff --git a/pkg/agent/loop_prune_test.go b/pkg/agent/loop_prune_test.go
index 9b184d1..e9a7f2f 100644
--- a/pkg/agent/loop_prune_test.go
+++ b/pkg/agent/loop_prune_test.go
@@ -168,3 +168,86 @@ func TestExtractExecCodePaths_Basic(t *testing.T) {
 		t.Errorf("writes = %v; want [/b.go /c.go]", writes)
 	}
 }
+
+// TestPruneStaleReads_ReadWriteReadCycle is the primary correctness invariant:
+// read before a write is pruned; read after the same write is kept.
+func TestPruneStaleReads_ReadWriteReadCycle(t *testing.T) {
+	history := []backend.Message{
+		{Role: "user", Content: "update foo"},
+		makeExecCodeMsg("c1", []map[string]any{{"tool": "file_read", "args": []string{"/foo.go"}}}),
+		makeToolResult("c1", "old content"),
+		makeExecCodeMsg("c2", []map[string]any{{"tool": "file_write", "args": []string{"/foo.go", "new content"}}}),
+		makeToolResult("c2", "ok"),
+		makeExecCodeMsg("c3", []map[string]any{{"tool": "file_read", "args": []string{"/foo.go"}}}),
+		makeToolResult("c3", "new content"),
+	}
+
+	out := pruneStaleReads(history)
+
+	// c1 (pre-write read) must be gone; c3 (post-write read) must remain.
+	for _, m := range out {
+		if m.Role == "tool" && m.ToolCallID == "c1" {
+			t.Error("pre-write read result c1 should have been pruned")
+		}
+	}
+	found := false
+	for _, m := range out {
+		if m.Role == "tool" && m.ToolCallID == "c3" {
+			found = true
+		}
+	}
+	if !found {
+		t.Error("post-write read result c3 should be kept")
+	}
+	if len(out) != len(history)-1 {
+		t.Errorf("len(out) = %d; want %d", len(out), len(history)-1)
+	}
+}
+
+// TestPruneStaleReads_AssistantMessagesUnaffected verifies that the assistant
+// messages that issued the tool calls are never removed — only the tool result
+// messages (role:"tool") for stale reads are dropped.
+func TestPruneStaleReads_AssistantMessagesUnaffected(t *testing.T) {
+	history := []backend.Message{
+		{Role: "user", Content: "go"},
+		makeExecCodeMsg("c1", []map[string]any{{"tool": "file_read", "args": []string{"/foo.go"}}}),
+		makeToolResult("c1", "old"),
+		makeExecCodeMsg("c2", []map[string]any{{"tool": "file_write", "args": []string{"/foo.go", "new"}}}),
+		makeToolResult("c2", "ok"),
+	}
+
+	out := pruneStaleReads(history)
+
+	var assistantCount int
+	for _, m := range out {
+		if m.Role == "assistant" {
+			assistantCount++
+		}
+	}
+	if assistantCount != 2 {
+		t.Errorf("assistant messages = %d; want 2 (pruning must not remove assistant messages)", assistantCount)
+	}
+}
+
+// TestPruneStaleReads_NonExecuteCodeUnaffected verifies that tool calls with
+// names other than "execute_code" are ignored by the path tracker and never
+// cause unrelated tool results to be pruned.
+func TestPruneStaleReads_NonExecuteCodeUnaffected(t *testing.T) {
+	history := []backend.Message{
+		{Role: "user", Content: "go"},
+		// A non-execute_code call that happens to reference a path.
+		{Role: "assistant", ToolCalls: []backend.ToolCall{
+			{ID: "c1", Name: "some_other_tool", Arguments: json.RawMessage(`{"path":"/foo.go"}`)},
+		}},
+		makeToolResult("c1", "other tool result"),
+		// execute_code write — should not prune the unrelated result above.
+		makeExecCodeMsg("c2", []map[string]any{{"tool": "file_write", "args": []string{"/foo.go", "new"}}}),
+		makeToolResult("c2", "ok"),
+	}
+
+	out := pruneStaleReads(history)
+
+	if len(out) != len(history) {
+		t.Errorf("len(out) = %d; want %d (non-execute_code results must not be pruned)", len(out), len(history))
+	}
+}
diff --git a/pkg/backend/openai_encode_test.go b/pkg/backend/openai_encode_test.go
index 2e2e9ff..5db196a 100644
--- a/pkg/backend/openai_encode_test.go
+++ b/pkg/backend/openai_encode_test.go
@@ -1,7 +1,12 @@
 package backend
 
 import (
+	"context"
 	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 )
@@ -228,6 +233,135 @@ func TestEncodeMessages_OpenRouterClaudeCacheControl(t *testing.T) {
 
 func strPtr(s string) *string { return &s }
 
+// --- OpenRouter cache injection: positive and negative cases ---
+
+// captureRawRequest spins up an httptest server that captures the raw request
+// body JSON, calls the backend, and drains the stream.
+func captureRawRequest(t *testing.T, name, model string, msgs []Message, tools []Tool) map[string]json.RawMessage {
+	t.Helper()
+	var rawBody []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rawBody, _ = io.ReadAll(r.Body)
+		w.Header().Set("Content-Type", "text/event-stream")
+		fmt.Fprint(w, "data: [DONE]\n\n")
+	}))
+	defer srv.Close()
+
+	b, err := NewOpenAI(name, srv.URL, "key")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b.SetModel(model)
+	ch, err := b.ChatStream(context.Background(), msgs, tools, GenerationParams{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for range ch {
+	}
+
+	var top map[string]json.RawMessage
+	json.Unmarshal(rawBody, &top) //nolint:errcheck
+	return top
+}
+
+// TestOpenRouterClaude_SystemCacheControl verifies that when the backend is
+// "openrouter" and the model contains "claude-", the system message content is
+// encoded as a JSON array with a cache_control:ephemeral block.
+func TestOpenRouterClaude_SystemCacheControl(t *testing.T) {
+	top := captureRawRequest(t, "openrouter", "claude-sonnet-4-5",
+		[]Message{{Role: "system", Content: "be helpful"}, {Role: "user", Content: "hi"}},
+		nil,
+	)
+
+	var messages []map[string]json.RawMessage
+	json.Unmarshal(top["messages"], &messages) //nolint:errcheck
+	if len(messages) == 0 {
+		t.Fatal("no messages in captured request")
+	}
+	sys := messages[0]
+
+	// content must be an array (first byte '['), not a plain string.
+	content := sys["content"]
+	if len(content) == 0 || content[0] != '[' {
+		t.Errorf("system content should be array; got %s", content)
+	}
+	if !strings.Contains(string(content), "cache_control") {
+		t.Errorf("system content missing cache_control: %s", content)
+	}
+	if !strings.Contains(string(content), "ephemeral") {
+		t.Errorf("system content missing ephemeral: %s", content)
+	}
+}
+
+// TestOpenRouterClaude_LastToolCacheControl verifies that the last tool in the
+// array gets cache_control:ephemeral on openrouter+claude requests.
+func TestOpenRouterClaude_LastToolCacheControl(t *testing.T) {
+	top := captureRawRequest(t, "openrouter", "claude-sonnet-4-5",
+		[]Message{{Role: "user", Content: "hi"}},
+		[]Tool{
+			{Name: "alpha", Parameters: json.RawMessage(`{}`)},
+			{Name: "beta", Parameters: json.RawMessage(`{}`)},
+		},
+	)
+
+	var tools []map[string]json.RawMessage
+	json.Unmarshal(top["tools"], &tools) //nolint:errcheck
+	if len(tools) != 2 {
+		t.Fatalf("tools len = %d; want 2", len(tools))
+	}
+	if strings.Contains(string(tools[0]["cache_control"]), "ephemeral") {
+		t.Errorf("first tool should not have cache_control:ephemeral")
+	}
+	if !strings.Contains(string(tools[1]["cache_control"]), "ephemeral") {
+		t.Errorf("last tool missing cache_control:ephemeral; got %s", tools[1]["cache_control"])
+	}
+}
+
+// TestOpenRouterNonClaude_NoCacheControl verifies that a non-Claude model on
+// openrouter does NOT get cache_control injected into system messages or tools.
+func TestOpenRouterNonClaude_NoCacheControl(t *testing.T) {
+	top := captureRawRequest(t, "openrouter", "mistral-7b",
+		[]Message{{Role: "system", Content: "be helpful"}, {Role: "user", Content: "hi"}},
+		[]Tool{{Name: "run", Parameters: json.RawMessage(`{}`)}},
+	)
+
+	var messages []map[string]json.RawMessage
+	json.Unmarshal(top["messages"], &messages) //nolint:errcheck
+	content := messages[0]["content"]
+	// Must be a plain JSON string, not an array.
+	if len(content) == 0 || content[0] != '"' {
+		t.Errorf("non-claude system content should be plain string; got %s", content)
+	}
+	if strings.Contains(string(content), "cache_control") {
+		t.Errorf("non-claude system content must not contain cache_control: %s", content)
+	}
+
+	var tools []map[string]json.RawMessage
+	json.Unmarshal(top["tools"], &tools) //nolint:errcheck
+	if len(tools) > 0 && strings.Contains(string(tools[0]["cache_control"]), "ephemeral") {
+		t.Errorf("non-claude tool should not have cache_control:ephemeral")
+	}
+}
+
+// TestOpenAI_NoCacheControl verifies that a plain "openai" backend (not
+// openrouter) never injects cache_control, even for a claude-named model.
+func TestOpenAI_NoCacheControl(t *testing.T) {
+	top := captureRawRequest(t, "openai", "claude-sonnet-4-5",
+		[]Message{{Role: "system", Content: "sys"}, {Role: "user", Content: "hi"}},
+		[]Tool{{Name: "run", Parameters: json.RawMessage(`{}`)}},
+	)
+
+	var messages []map[string]json.RawMessage
+	json.Unmarshal(top["messages"], &messages) //nolint:errcheck
+	content := messages[0]["content"]
+	if len(content) == 0 || content[0] != '"' {
+		t.Errorf("openai system content should be plain string; got %s", content)
+	}
+	if strings.Contains(string(content), "cache_control") {
+		t.Errorf("openai backend must not inject cache_control: %s", content)
+	}
+}
+
 func TestEncodeRequest_ToolsOmittedWhenEmpty(t *testing.T) {
 	req := openAIChatRequest{
 		Model:    "m",