fix(cache): invalidate cached responses when .intentrc changes

internal/cache/cache.go

@@ -26,6 +26,10 @@ type KeyInputs struct {
 	BinariesFingerprint   string
 	BackendIdentity       string
 	PromptTemplateVersion string
+	// ProjectRCFingerprint captures .intentrc directives that are
+	// rendered into the system prompt. Whitespace-only content hashes
+	// to empty so users without a real project config keep today's key.
+	ProjectRCFingerprint string
 	// UserContextFingerprint captures any --context values that are
 	// injected into the system prompt. Order is preserved because the
 	// model sees them in order; see UserContextFingerprint.
@@ -41,12 +45,26 @@ func Key(in KeyInputs) string {
 		in.BinariesFingerprint,
 		in.BackendIdentity,
 		in.PromptTemplateVersion,
+		in.ProjectRCFingerprint,
 		in.UserContextFingerprint,
 	}
 	h := sha256.Sum256([]byte(strings.Join(parts, "\x1f")))
 	return hex.EncodeToString(h[:])
 }
 
+// ProjectRCFingerprint returns a stable fingerprint of .intentrc
+// directives that are injected into the system prompt. Blank or
+// whitespace-only content yields "" so the baseline cache key shape
+// stays unchanged for repos without project directives.
+func ProjectRCFingerprint(contents string) string {
+	trimmed := strings.TrimSpace(contents)
+	if trimmed == "" {
+		return ""
+	}
+	h := sha256.Sum256([]byte(trimmed))
+	return hex.EncodeToString(h[:8])
+}
+
 // UserContextFingerprint returns a stable fingerprint of the ordered
 // --context values forwarded into the system prompt. Order is preserved
 // because the model sees the entries in order, and two orderings may

internal/engine/engine.go

@@ -81,6 +81,7 @@ func (e *Engine) Run(ctx context.Context, prompt string, opts Options) (*Result,
 		BinariesFingerprint:    cache.BinariesFingerprint(pack.AvailableBins),
 		BackendIdentity:        backendCacheIdentity(opts.Backend),
 		PromptTemplateVersion:  model.PromptTemplateVersion,
+		ProjectRCFingerprint:   cache.ProjectRCFingerprint(opts.ProjectRC),
 		UserContextFingerprint: cache.UserContextFingerprint(opts.UserContext),
 	})
 	res.CacheKey = key

internal/engine/engine_test.go

@@ -151,3 +151,64 @@ func TestRunCacheKeyIncludesUserContext(t *testing.T) {
 		t.Fatalf("cache key ignored --context order: %q", forward.CacheKey)
 	}
 }
+
+func TestRunCacheKeyIncludesProjectRC(t *testing.T) {
+	eng := New(nil)
+	ctx := context.Background()
+	backend := &cacheIdentityBackend{
+		name:          "llamafile",
+		cacheIdentity: "llamafile|http://127.0.0.1:8080|qwen2.5-coder-3b",
+	}
+
+	base, err := eng.Run(ctx, "list files", Options{Backend: backend})
+	if err != nil {
+		t.Fatalf("run base: %v", err)
+	}
+
+	withProjectRC, err := eng.Run(ctx, "list files", Options{
+		Backend:   backend,
+		ProjectRC: "prefer rg over grep",
+	})
+	if err != nil {
+		t.Fatalf("run with project rc: %v", err)
+	}
+	if base.CacheKey == withProjectRC.CacheKey {
+		t.Fatalf("cache key collision when .intentrc directives were added: %q", base.CacheKey)
+	}
+
+	otherProjectRC, err := eng.Run(ctx, "list files", Options{
+		Backend:   backend,
+		ProjectRC: "prefer fd over find",
+	})
+	if err != nil {
+		t.Fatalf("run with other project rc: %v", err)
+	}
+	if withProjectRC.CacheKey == otherProjectRC.CacheKey {
+		t.Fatalf("cache key collision across distinct .intentrc directives: %q", withProjectRC.CacheKey)
+	}
+}
+
+func TestRunCacheKeyTreatsBlankProjectRCAsAbsent(t *testing.T) {
+	eng := New(nil)
+	ctx := context.Background()
+	backend := &cacheIdentityBackend{
+		name:          "llamafile",
+		cacheIdentity: "llamafile|http://127.0.0.1:8080|qwen2.5-coder-3b",
+	}
+
+	base, err := eng.Run(ctx, "list files", Options{Backend: backend})
+	if err != nil {
+		t.Fatalf("run base: %v", err)
+	}
+
+	blank, err := eng.Run(ctx, "list files", Options{
+		Backend:   backend,
+		ProjectRC: "  \n\t  ",
+	})
+	if err != nil {
+		t.Fatalf("run blank project rc: %v", err)
+	}
+	if base.CacheKey != blank.CacheKey {
+		t.Fatalf("blank .intentrc content should preserve the baseline cache key: base=%q blank=%q", base.CacheKey, blank.CacheKey)
+	}
+}