feat: handle large diffs with a size budget

README.md

@@ -74,6 +74,7 @@ Shareable, safe for dotfiles:
 ```yaml
 language: English
 num_suggestions: 10
+# max_diff_chars: 40000              # size budget for the diff sent to the model
 # system_message: ...
 # commit_message_template: "... %s"   # %s is replaced by the diff
 # pr_title_template: "... %s"
@@ -171,6 +172,7 @@ Variant that lets you edit the message before committing:
 - `No staged changes to commit.` — run `git add` first.
 - `has no model configured` — run `lazycommit config set`.
 - `environment variable X is not set` — your config references `$X`; export it or store the key directly.
+- Large change sets: when the diff is longer than `max_diff_chars`, lazycommit sends a list of the changed files with line counts plus as many file diffs as fit; a very long file list is itself cut short with a count of the remaining files. Lock files, minified files, and binary files are listed without their diffs. Lower the value if your model still rejects the request as too large.
 
 ## License
 

internal/app/ports.go

@@ -28,6 +28,7 @@ type PromptSettings struct {
 	PRTitleTemplate domain.PromptTemplate
 	Language        domain.Language
 	SuggestionCount int
+	MaxDiffChars    int
 }
 
 // ConfigRepository yields the effective settings the use cases need.

internal/app/suggest.go

@@ -55,7 +55,7 @@ func (uc *GeneratePRTitles) Execute(ctx context.Context, target string) (Suggest
 }
 
 // suggestionPipeline is the shared flow: read diff, short-circuit when
-// empty, build prompt, generate, parse. Commit and PR generation differ
+// empty, compact to the size budget, build prompt, generate, parse. Commit and PR generation differ
 // only in diff source and template.
 type suggestionPipeline struct {
 	gen   Generator
@@ -86,6 +86,12 @@ func (p suggestionPipeline) run(
 		return SuggestionsResult{}, fmt.Errorf("loading configuration: %w", err)
 	}
 
+	maxDiff := settings.MaxDiffChars
+	if maxDiff <= 0 {
+		maxDiff = domain.DefaultMaxDiffChars
+	}
+	diff = diff.Compact(maxDiff)
+
 	prompt := domain.NewPromptBuilder().
 		WithSystemMessage(settings.SystemMessage).
 		WithTemplate(pickTemplate(settings)).

internal/app/suggest_test.go

@@ -96,6 +96,27 @@ func TestCommitSuggestionsHappyPath(t *testing.T) {
 	}
 }
 
+func TestCommitSuggestionsCompactsLargeDiff(t *testing.T) {
+	raw := "diff --git a/big.txt b/big.txt\n--- a/big.txt\n+++ b/big.txt\n@@ -0,0 +1,500 @@\n" +
+		strings.Repeat("+some added content\n", 500)
+	settings := testSettings(t)
+	settings.MaxDiffChars = 1000
+	gen := &fakeGenerator{output: "feat: one"}
+	uc := NewGenerateCommitSuggestions(gen,
+		&fakeDiffSource{staged: raw},
+		&fakeConfig{settings: settings})
+
+	if _, err := uc.Execute(context.Background()); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if strings.Contains(gen.lastPrompt.User, raw) {
+		t.Fatal("full diff must not be sent when it exceeds the budget")
+	}
+	if !strings.Contains(gen.lastPrompt.User, "big.txt +500 -0") {
+		t.Fatalf("prompt misses the file summary: %q", gen.lastPrompt.User)
+	}
+}
+
 func TestCommitSuggestionsEmptyDiffShortCircuits(t *testing.T) {
 	gen := &fakeGenerator{output: "unused"}
 	uc := NewGenerateCommitSuggestions(gen,

internal/config/config.go

@@ -50,6 +50,7 @@ type Prompts struct {
 	CommitMessageTemplate string `yaml:"commit_message_template,omitempty"`
 	PRTitleTemplate       string `yaml:"pr_title_template,omitempty"`
 	NumSuggestions        int    `yaml:"num_suggestions,omitempty"`
+	MaxDiffChars          int    `yaml:"max_diff_chars,omitempty"`
 }
 
 // DefaultBackends is the effective configuration when no file exists; the
@@ -198,13 +199,18 @@ func (r *Repository) PromptSettings() (app.PromptSettings, error) {
 	if count <= 0 {
 		count = domain.DefaultSuggestionCount
 	}
+	maxDiff := p.MaxDiffChars
+	if maxDiff <= 0 {
+		maxDiff = domain.DefaultMaxDiffChars
+	}
 
 	return app.PromptSettings{
 		SystemMessage:   system,
 		CommitTemplate:  commit,
 		PRTitleTemplate: pr,
 		Language:        domain.NewLanguage(p.Language),
 		SuggestionCount: count,
+		MaxDiffChars:    maxDiff,
 	}, nil
 }
 
@@ -238,6 +244,9 @@ func mergePrompts(top, bottom Prompts) Prompts {
 	if top.NumSuggestions > 0 {
 		out.NumSuggestions = top.NumSuggestions
 	}
+	if top.MaxDiffChars > 0 {
+		out.MaxDiffChars = top.MaxDiffChars
+	}
 	return out
 }
 

internal/config/config_test.go

@@ -125,6 +125,9 @@ func TestPromptSettingsDefaultsWhenNoFiles(t *testing.T) {
 	if s.SystemMessage != domain.DefaultSystemMessage {
 		t.Fatalf("system = %q", s.SystemMessage)
 	}
+	if s.MaxDiffChars != domain.DefaultMaxDiffChars {
+		t.Fatalf("max diff chars = %d", s.MaxDiffChars)
+	}
 }
 
 func TestPromptSettingsLayering(t *testing.T) {
@@ -134,10 +137,12 @@ func TestPromptSettingsLayering(t *testing.T) {
 language: Spanish
 system_message: global system
 num_suggestions: 7
+max_diff_chars: 5000
 `)
 	writeFile(t, filepath.Join(repoRoot, "lazycommit.prompts.yaml"), `
 language: Korean
 commit_message_template: "REPO %s"
+max_diff_chars: 8000
 `)
 
 	s, err := NewRepository(globalDir, repoRoot).PromptSettings()
@@ -153,6 +158,9 @@ commit_message_template: "REPO %s"
 	if s.SuggestionCount != 7 {
 		t.Fatalf("global count should fall through, got %d", s.SuggestionCount)
 	}
+	if s.MaxDiffChars != 8000 {
+		t.Fatalf("repo-local max diff chars should win, got %d", s.MaxDiffChars)
+	}
 	if s.CommitTemplate.String() != "REPO %s" {
 		t.Fatalf("repo-local template should win, got %q", s.CommitTemplate)
 	}