feat: harden changelog-synth prompts

internal/orchestrator/orchestrator.go

@@ -713,6 +713,13 @@ func (o *Orchestrator) PlanPrompt(task *tasks.Task) string {
 
 func taskSpecificPlanGuidance(task *tasks.Task) string {
 	switch task.Type {
+	case tasks.TaskChangelogSynth:
+		return "\n\n## Task-Specific Guidance\n" +
+			"- Inspect the repo's current changelog and release signals before deciding the scope: CHANGELOG.md, release workflow files if present (for example .github/workflows/release.yml), and recent git tags/commits.\n" +
+			"- Compare the current branch to `main`; derive the commit range from `git merge-base main HEAD` through `HEAD`, and exclude merge commits so the draft is deterministic.\n" +
+			"- If the release boundary or target changelog section is unclear, state the assumptions you made.\n" +
+			"- Prefer updating the existing changelog artifact and structure instead of inventing a new format, and preserve prior changelog history.\n" +
+			"- Plan for stable Markdown sections such as Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other. Omit empty sections when appropriate.\n"
 	case tasks.TaskReleaseNotes:
 		return "\n\n## Task-Specific Guidance\n" +
 			"- Inspect the repo's existing release signals before deciding the scope: CHANGELOG.md, .github/workflows/release.yml, and recent git tags/commits.\n" +
@@ -726,6 +733,14 @@ func taskSpecificPlanGuidance(task *tasks.Task) string {
 
 func taskSpecificImplementGuidance(task *tasks.Task) string {
 	switch task.Type {
+	case tasks.TaskChangelogSynth:
+		return "\n\n## Task-Specific Guidance\n" +
+			"- Inspect the repo's current changelog and release signals before drafting: CHANGELOG.md, release workflow files if present (for example .github/workflows/release.yml), and recent git tags/commits.\n" +
+			"- Compare the current branch to `main`; derive the commit range from `git merge-base main HEAD` through `HEAD`, and exclude merge commits so the draft is deterministic.\n" +
+			"- Synthesize only the newest changelog section from supported commits, note assumptions if the release boundary is unclear, and do not invent entries.\n" +
+			"- Preserve prior changelog sections and structure; prefer updating the existing changelog artifact over creating a new format.\n" +
+			"- Organize the update into stable Markdown sections such as Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other. Omit empty sections when appropriate.\n" +
+			"- Emit Markdown-ready changelog content only.\n"
 	case tasks.TaskReleaseNotes:
 		return "\n\n## Task-Specific Guidance\n" +
 			"- Inspect the repo's existing release signals before drafting: CHANGELOG.md, .github/workflows/release.yml, and recent git tags/commits.\n" +

internal/orchestrator/orchestrator_test.go

@@ -809,6 +809,78 @@ func TestBuildImplementPrompt_WithoutBranch(t *testing.T) {
 	}
 }
 
+func TestBuildPlanPrompt_ChangelogSynthIncludesRepoAwareGuidance(t *testing.T) {
+	o := New()
+
+	task := &tasks.Task{
+		ID:          "changelog-synth:/tmp/nightshift",
+		Title:       "Changelog Synthesizer",
+		Description: "Draft the next changelog update from branch commits",
+		Type:        tasks.TaskChangelogSynth,
+	}
+
+	prompt := o.buildPlanPrompt(task)
+
+	expected := []string{
+		"CHANGELOG.md",
+		"release workflow files if present",
+		".github/workflows/release.yml",
+		"recent git tags/commits",
+		"current branch to `main`",
+		"`git merge-base main HEAD`",
+		"exclude merge commits",
+		"target changelog section is unclear",
+		"existing changelog artifact and structure",
+		"preserve prior changelog history",
+		"Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other",
+	}
+
+	for _, want := range expected {
+		if !strings.Contains(prompt, want) {
+			t.Errorf("plan prompt missing %q\nGot:\n%s", want, prompt)
+		}
+	}
+}
+
+func TestBuildImplementPrompt_ChangelogSynthIncludesRepoAwareGuidance(t *testing.T) {
+	o := New()
+
+	task := &tasks.Task{
+		ID:          "changelog-synth:/tmp/nightshift",
+		Title:       "Changelog Synthesizer",
+		Description: "Draft the next changelog update from branch commits",
+		Type:        tasks.TaskChangelogSynth,
+	}
+	plan := &PlanOutput{
+		Steps:       []string{"Inspect CHANGELOG.md", "Draft the next section"},
+		Description: "Use the current branch diff against main to update the changelog.",
+	}
+
+	prompt := o.buildImplementPrompt(task, plan, 1)
+
+	expected := []string{
+		"CHANGELOG.md",
+		"release workflow files if present",
+		".github/workflows/release.yml",
+		"recent git tags/commits",
+		"current branch to `main`",
+		"`git merge-base main HEAD`",
+		"exclude merge commits",
+		"newest changelog section",
+		"do not invent entries",
+		"existing changelog artifact",
+		"prior changelog sections and structure",
+		"Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other",
+		"Emit Markdown-ready changelog content only",
+	}
+
+	for _, want := range expected {
+		if !strings.Contains(prompt, want) {
+			t.Errorf("implement prompt missing %q\nGot:\n%s", want, prompt)
+		}
+	}
+}
+
 func TestBuildPlanPrompt_ReleaseNotesIncludesRepoAwareGuidance(t *testing.T) {
 	o := New()
 
@@ -894,6 +966,12 @@ func TestBuildPrompts_GenericTasksDoNotReceiveReleaseNotesGuidance(t *testing.T)
 		"release boundary",
 		"current release-notes artifact and format",
 		"Features, Fixes, Breaking Changes, and Other",
+		"release workflow files if present",
+		"`git merge-base main HEAD`",
+		"exclude merge commits",
+		"newest changelog section",
+		"Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other",
+		"Emit Markdown-ready changelog content only",
 	}
 
 	for _, notWant := range unexpected {

internal/tasks/tasks.go

@@ -338,10 +338,14 @@ Apply safe updates directly, and leave concise follow-ups for anything uncertain
 		DefaultInterval: 24 * time.Hour,
 	},
 	TaskChangelogSynth: {
-		Type:            TaskChangelogSynth,
-		Category:        CategoryPR,
-		Name:            "Changelog Synthesizer",
-		Description:     "Generate changelog from commits",
+		Type:     TaskChangelogSynth,
+		Category: CategoryPR,
+		Name:     "Changelog Synthesizer",
+		Description: `Inspect the current branch against main and synthesize the next changelog update for this repository.
+Determine the commit range from git merge-base main HEAD through HEAD, excluding merge commits so the output stays deterministic.
+Review the existing changelog artifact and preserve prior history and structure, updating only the newest relevant section instead of rewriting older entries.
+Group entries into stable Markdown sections such as Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other, omitting empty sections when appropriate.
+Base every bullet on actual commits, do not invent changes, and emit Markdown-ready changelog content only.`,
 		CostTier:        CostLow,
 		RiskLevel:       RiskLow,
 		DefaultInterval: 168 * time.Hour,

internal/tasks/tasks_test.go

@@ -243,6 +243,23 @@ func TestTaskDefinitionEstimatedTokens(t *testing.T) {
 	}
 }
 
+func TestTaskChangelogSynthDescriptionIsDeterministicAndRepoAware(t *testing.T) {
+	def, err := GetDefinition(TaskChangelogSynth)
+	if err != nil {
+		t.Fatalf("GetDefinition(TaskChangelogSynth) returned error: %v", err)
+	}
+
+	want := `Inspect the current branch against main and synthesize the next changelog update for this repository.
+Determine the commit range from git merge-base main HEAD through HEAD, excluding merge commits so the output stays deterministic.
+Review the existing changelog artifact and preserve prior history and structure, updating only the newest relevant section instead of rewriting older entries.
+Group entries into stable Markdown sections such as Added, Changed, Fixed, Docs, Refactor, Tests, Chore, and Other, omitting empty sections when appropriate.
+Base every bullet on actual commits, do not invent changes, and emit Markdown-ready changelog content only.`
+
+	if def.Description != want {
+		t.Errorf("TaskChangelogSynth description mismatch\nGot:\n%s\n\nWant:\n%s", def.Description, want)
+	}
+}
+
 func TestRegistryCompleteness(t *testing.T) {
 	// All task type constants should be in registry
 	taskTypes := []TaskType{

website/docs/task-reference.md

@@ -24,7 +24,7 @@ Fully formed, review-ready artifacts. These tasks create branches and open pull
 | `build-optimize` | Build Time Optimization | Optimize build configuration for faster builds | High | Medium | 7d |
 | `docs-backfill` | Documentation Backfiller | Generate missing documentation | Low | Low | 7d |
 | `commit-normalize` | Commit Message Normalizer | Standardize commit message format | Low | Low | 24h |
-| `changelog-synth` | Changelog Synthesizer | Generate changelog from commits | Low | Low | 7d |
+| `changelog-synth` | Changelog Synthesizer | Draft a deterministic changelog update from current-branch commits since `git merge-base main HEAD`, preserving existing `CHANGELOG.md` structure | Low | Low | 7d |
 | `release-notes` | Release Note Drafter | Draft release-ready notes for the next version | Low | Low | 7d |
 | `adr-draft` | ADR Drafter | Draft Architecture Decision Records | Medium | Low | 7d |
 | `td-review` | TD Review Session | Review open td reviews, fix obvious bugs, create tasks for bigger issues | High | Medium | 72h |