feat: context compaction — summarize-and-truncate per the merged design

cmd/harness/main.go

@@ -382,18 +382,21 @@ func runCmd(args []string) error {
 	lateAPI.Bind(newLazyRunClientAPI(func() *engine.Session { return sess }))
 
 	s, err := resolveSession(engine.Config{
-		Providers:    registry(cfg),
-		Model:        model,
-		System:       systemPrompt(workDir, opts.system),
-		MaxTokens:    opts.maxTokens,
-		WorkDir:      workDir,
-		SessionDir:   sesDir,
-		OnEvent:      onEvent,
-		Instructions: instructionsConfig(cfg, opts.noInstructions),
-		SkillsDirs:   skillsDirs(cfg, opts.skillsDirs, workDir),
-		Hooks:        pluginHooks(host),
-		MCP:          mcpRegistry(mcpMgr),
-		Processes:    processRegistry(procMgr),
+		Providers:           registry(cfg),
+		Model:               model,
+		System:              systemPrompt(workDir, opts.system),
+		MaxTokens:           opts.maxTokens,
+		WorkDir:             workDir,
+		SessionDir:          sesDir,
+		OnEvent:             onEvent,
+		Instructions:        instructionsConfig(cfg, opts.noInstructions),
+		SkillsDirs:          skillsDirs(cfg, opts.skillsDirs, workDir),
+		Hooks:               pluginHooks(host),
+		MCP:                 mcpRegistry(mcpMgr),
+		Processes:           processRegistry(procMgr),
+		ContextWindowTokens: cfg.ContextWindowTokens,
+		CompactionThreshold: cfg.CompactionThreshold,
+		CompactionKeepTurns: cfg.CompactionKeepTurns,
 	}, opts.resume, opts.cont, modelSet)
 	if err != nil {
 		return err
@@ -693,17 +696,20 @@ func serveCmd(args []string) error {
 	var srv *server.Server
 	mkCfg := func(model message.ModelRef) engine.Config {
 		return engine.Config{
-			Providers:    reg,
-			Model:        model,
-			System:       systemPrompt(workDir, ""),
-			WorkDir:      workDir,
-			SessionDir:   sesDir,
-			OnEvent:      func(ev engine.Event) { srv.Publish(ev) },
-			Instructions: instructionsConfig(cfg, noInstructions),
-			SkillsDirs:   skillsDirs(cfg, skillDirs, workDir),
-			Hooks:        pluginHooks(pluginHost),
-			MCP:          mcpRegistry(mcpMgr),
-			Processes:    processRegistry(procMgr),
+			Providers:           reg,
+			Model:               model,
+			System:              systemPrompt(workDir, ""),
+			WorkDir:             workDir,
+			SessionDir:          sesDir,
+			OnEvent:             func(ev engine.Event) { srv.Publish(ev) },
+			Instructions:        instructionsConfig(cfg, noInstructions),
+			SkillsDirs:          skillsDirs(cfg, skillDirs, workDir),
+			Hooks:               pluginHooks(pluginHost),
+			MCP:                 mcpRegistry(mcpMgr),
+			Processes:           processRegistry(procMgr),
+			ContextWindowTokens: cfg.ContextWindowTokens,
+			CompactionThreshold: cfg.CompactionThreshold,
+			CompactionKeepTurns: cfg.CompactionKeepTurns,
 		}
 	}
 	srv, err = server.New(server.Options{

config/config.go

@@ -79,6 +79,21 @@ type Config struct {
 	// configures no processes. Merge rules mirror MCPServers: keys merge,
 	// but a same-name project entry replaces the user entry wholesale.
 	Processes map[string]ProcessSpec `json:"processes,omitempty"`
+	// ContextWindowTokens sets engine.Config.ContextWindowTokens for every
+	// session this process creates: the model's context window size, in
+	// tokens. Zero (omitted, the default) disables automatic compaction
+	// entirely — see docs/design/context-compaction.md and issue #62 layer
+	// 3. Opt-in: the engine has no built-in per-model table.
+	ContextWindowTokens int `json:"context_window_tokens,omitempty"`
+	// CompactionThreshold sets engine.Config.CompactionThreshold: the
+	// fraction of ContextWindowTokens at which automatic compaction
+	// triggers. Zero (omitted) defaults to 0.8 (see the engine).
+	CompactionThreshold float64 `json:"compaction_threshold,omitempty"`
+	// CompactionKeepTurns sets engine.Config.CompactionKeepTurns: how many
+	// of the most recent turns automatic compaction always keeps verbatim.
+	// Zero (omitted) defaults to 2 (see the engine); the effective value
+	// can never go below 1.
+	CompactionKeepTurns int `json:"compaction_keep_turns,omitempty"`
 }
 
 // ProcessSpec configures one managed process (package engine's
@@ -520,6 +535,15 @@ func merge(base, over *Config) *Config {
 	if over.GoalEvaluatorModel != "" {
 		out.GoalEvaluatorModel = over.GoalEvaluatorModel
 	}
+	if over.ContextWindowTokens != 0 {
+		out.ContextWindowTokens = over.ContextWindowTokens
+	}
+	if over.CompactionThreshold != 0 {
+		out.CompactionThreshold = over.CompactionThreshold
+	}
+	if over.CompactionKeepTurns != 0 {
+		out.CompactionKeepTurns = over.CompactionKeepTurns
+	}
 	// Arrays override wholesale: a non-empty project value replaces the user
 	// value entirely; otherwise inherit. Copy so the merged config never
 	// aliases either input's slice.

config/config_test.go

@@ -483,6 +483,37 @@ func TestLoadSkillsDirs(t *testing.T) {
 	})
 }
 
+// TestMergeCompactionFields is the red-first test for docs/design/context-
+// compaction.md's config fields: project non-zero values override the user
+// layer, same scalar-override rule as GoalEvaluatorModel.
+func TestMergeCompactionFields(t *testing.T) {
+	base := &Config{ContextWindowTokens: 100000, CompactionThreshold: 0.9, CompactionKeepTurns: 3}
+	t.Run("zero project values inherit the user layer", func(t *testing.T) {
+		got := merge(base, &Config{})
+		if got.ContextWindowTokens != 100000 {
+			t.Errorf("ContextWindowTokens = %d, want inherited 100000", got.ContextWindowTokens)
+		}
+		if got.CompactionThreshold != 0.9 {
+			t.Errorf("CompactionThreshold = %v, want inherited 0.9", got.CompactionThreshold)
+		}
+		if got.CompactionKeepTurns != 3 {
+			t.Errorf("CompactionKeepTurns = %d, want inherited 3", got.CompactionKeepTurns)
+		}
+	})
+	t.Run("non-zero project values override", func(t *testing.T) {
+		got := merge(base, &Config{ContextWindowTokens: 50000, CompactionThreshold: 0.7, CompactionKeepTurns: 1})
+		if got.ContextWindowTokens != 50000 {
+			t.Errorf("ContextWindowTokens = %d, want project override 50000", got.ContextWindowTokens)
+		}
+		if got.CompactionThreshold != 0.7 {
+			t.Errorf("CompactionThreshold = %v, want project override 0.7", got.CompactionThreshold)
+		}
+		if got.CompactionKeepTurns != 1 {
+			t.Errorf("CompactionKeepTurns = %d, want project override 1", got.CompactionKeepTurns)
+		}
+	})
+}
+
 func TestMergeSkillsDirs(t *testing.T) {
 	t.Run("non-empty project overrides user entirely", func(t *testing.T) {
 		base := &Config{SkillsDirs: []string{"user/a", "user/b"}}