feat(gsd): ADR-004 — derived-graph reactive task execution (#1546)

Add reactive (graph-derived parallel) task execution within slices.
When enabled via preferences, the dispatch table derives a task dependency
graph from IO annotations in task plans and dispatches multiple ready,
non-conflicting tasks in parallel via subagent.

Architecture:
- Graph derivation happens at dispatch time (auto-dispatch.ts)
- A new reactive-execute prompt instructs the agent to use subagent
  parallel mode to dispatch all currently-ready tasks
- The auto-loop treats reactive-execute as a single unit type
- After agent_end, the orchestrator checks which tasks completed and loops

New files:
- reactive-graph.ts: pure graph derivation, ready-set resolution,
  conflict detection, deadlock detection, IO loader, state persistence
- prompts/reactive-execute.md: prompt template for parallel dispatch
- tests/reactive-graph.test.ts: 22 unit tests for graph functions
- tests/reactive-executor.test.ts: 11 integration tests for dispatch
  rules, preferences validation, state persistence, re-entry

Modified files:
- types.ts: TaskIO, DerivedTaskNode, ReactiveExecutionConfig,
  ReactiveExecutionState interfaces
- files.ts: parseTaskPlanIO() extracts IO from task plan sections
- preferences-types.ts: reactive_execution config + known keys
- preferences-validation.ts: validation with range checks
- auto-dispatch.ts: new reactive-execute dispatch rule
- auto-prompts.ts: buildReactiveExecutePrompt()
- auto-recovery.ts: artifact verification for reactive-execute
- auto-post-unit.ts: reactive state cleanup on slice completion

Backward compatible: disabled by default, falls through to sequential
execution when disabled, ambiguous, or only 1 task is ready.

src/resources/extensions/gsd/auto-dispatch.ts

@@ -38,6 +38,7 @@ import {
   buildRunUatPrompt,
   buildReassessRoadmapPrompt,
   buildRewriteDocsPrompt,
+  buildReactiveExecutePrompt,
   checkNeedsReassessment,
   checkNeedsRunUat,
 } from "./auto-prompts.js";
@@ -309,6 +310,83 @@ const DISPATCH_RULES: DispatchRule[] = [
       };
     },
   },
+  {
+    name: "executing → reactive-execute (parallel dispatch)",
+    match: async ({ state, mid, midTitle, basePath, prefs }) => {
+      if (state.phase !== "executing" || !state.activeTask) return null;
+      if (!state.activeSlice) return null; // fall through
+
+      // Only activate when reactive_execution is explicitly enabled
+      const reactiveConfig = prefs?.reactive_execution;
+      if (!reactiveConfig?.enabled) return null;
+
+      const sid = state.activeSlice.id;
+      const sTitle = state.activeSlice.title;
+      const maxParallel = reactiveConfig.max_parallel ?? 2;
+
+      // Dry-run mode: max_parallel=1 means graph is derived and logged but
+      // execution remains sequential
+      if (maxParallel <= 1) return null;
+
+      try {
+        const {
+          loadSliceTaskIO,
+          deriveTaskGraph,
+          isGraphAmbiguous,
+          getReadyTasks,
+          chooseNonConflictingSubset,
+          graphMetrics,
+        } = await import("./reactive-graph.js");
+
+        const taskIO = await loadSliceTaskIO(basePath, mid, sid);
+        if (taskIO.length < 2) return null; // single task, no point
+
+        const graph = deriveTaskGraph(taskIO);
+
+        // Ambiguous graph → fall through to sequential
+        if (isGraphAmbiguous(graph)) return null;
+
+        const completed = new Set(graph.filter((n) => n.done).map((n) => n.id));
+        const readyIds = getReadyTasks(graph, completed, new Set());
+
+        // Only activate reactive dispatch when >1 task is ready
+        if (readyIds.length <= 1) return null;
+
+        const selected = chooseNonConflictingSubset(
+          readyIds,
+          graph,
+          maxParallel,
+          new Set(),
+        );
+        if (selected.length <= 1) return null;
+
+        // Log graph metrics for observability
+        const metrics = graphMetrics(graph);
+        process.stderr.write(
+          `gsd-reactive: ${mid}/${sid} graph — tasks:${metrics.taskCount} edges:${metrics.edgeCount} ` +
+          `ready:${metrics.readySetSize} dispatching:${selected.length} ambiguous:${metrics.ambiguous}\n`,
+        );
+
+        return {
+          action: "dispatch",
+          unitType: "reactive-execute",
+          unitId: `${mid}/${sid}/reactive`,
+          prompt: await buildReactiveExecutePrompt(
+            mid,
+            midTitle,
+            sid,
+            sTitle,
+            selected,
+            basePath,
+          ),
+        };
+      } catch (err) {
+        // Non-fatal — fall through to sequential execution
+        process.stderr.write(`gsd-reactive: graph derivation failed: ${(err as Error).message}\n`);
+        return null;
+      }
+    },
+  },
   {
     name: "executing → execute-task (recover missing task plan → plan-slice)",
     match: async ({ state, mid, midTitle, basePath }) => {

src/resources/extensions/gsd/auto-post-unit.ts

@@ -217,6 +217,20 @@ export async function postUnitPreVerification(pctx: PostUnitContext): Promise<"d
       }
     }
 
+    // Reactive state cleanup on slice completion
+    if (s.currentUnit.type === "complete-slice") {
+      try {
+        const parts = s.currentUnit.id.split("/");
+        const [mid, sid] = parts;
+        if (mid && sid) {
+          const { clearReactiveState } = await import("./reactive-graph.js");
+          clearReactiveState(s.basePath, mid, sid);
+        }
+      } catch {
+        // Non-fatal
+      }
+    }
+
     // Post-triage: execute actionable resolutions
     if (s.currentUnit.type === "triage-captures") {
       try {

src/resources/extensions/gsd/auto-prompts.ts

@@ -1234,6 +1234,74 @@ export async function buildReassessRoadmapPrompt(
   });
 }
 
+// ─── Reactive Execute Prompt ──────────────────────────────────────────────
+
+export async function buildReactiveExecutePrompt(
+  mid: string, midTitle: string, sid: string, sTitle: string,
+  readyTaskIds: string[], base: string,
+): Promise<string> {
+  const { loadSliceTaskIO, deriveTaskGraph, graphMetrics } = await import("./reactive-graph.js");
+
+  // Build graph for context
+  const taskIO = await loadSliceTaskIO(base, mid, sid);
+  const graph = deriveTaskGraph(taskIO);
+  const metrics = graphMetrics(graph);
+
+  // Build graph context section
+  const graphLines: string[] = [];
+  for (const node of graph) {
+    const status = node.done ? "✅ done" : readyTaskIds.includes(node.id) ? "🟢 ready" : "⏳ waiting";
+    const deps = node.dependsOn.length > 0 ? ` (depends on: ${node.dependsOn.join(", ")})` : "";
+    graphLines.push(`- **${node.id}: ${node.title}** — ${status}${deps}`);
+    if (node.outputFiles.length > 0) {
+      graphLines.push(`  - Outputs: ${node.outputFiles.map(f => `\`${f}\``).join(", ")}`);
+    }
+  }
+  const graphContext = [
+    `Tasks: ${metrics.taskCount}, Edges: ${metrics.edgeCount}, Ready: ${metrics.readySetSize}`,
+    "",
+    ...graphLines,
+  ].join("\n");
+
+  // Build individual subagent prompts for each ready task
+  const subagentSections: string[] = [];
+  const readyTaskListLines: string[] = [];
+
+  for (const tid of readyTaskIds) {
+    const node = graph.find((n) => n.id === tid);
+    const tTitle = node?.title ?? tid;
+    readyTaskListLines.push(`- **${tid}: ${tTitle}**`);
+
+    // Build a full execute-task prompt for this task (reuse existing builder)
+    const taskPrompt = await buildExecuteTaskPrompt(mid, sid, sTitle, tid, tTitle, base);
+
+    subagentSections.push([
+      `### ${tid}: ${tTitle}`,
+      "",
+      "Use this as the prompt for a `subagent` call:",
+      "",
+      "```",
+      taskPrompt,
+      "```",
+    ].join("\n"));
+  }
+
+  const inlinedTemplates = inlineTemplate("task-summary", "Task Summary");
+
+  return loadPrompt("reactive-execute", {
+    workingDirectory: base,
+    milestoneId: mid,
+    milestoneTitle: midTitle,
+    sliceId: sid,
+    sliceTitle: sTitle,
+    graphContext,
+    readyTaskCount: String(readyTaskIds.length),
+    readyTaskList: readyTaskListLines.join("\n"),
+    subagentPrompts: subagentSections.join("\n\n---\n\n"),
+    inlinedTemplates,
+  });
+}
+
 export async function buildRewriteDocsPrompt(
   mid: string, midTitle: string,
   activeSlice: { id: string; title: string } | null,

src/resources/extensions/gsd/auto-recovery.ts

@@ -26,6 +26,7 @@ import {
   resolveSlicePath,
   resolveSliceFile,
   resolveTasksDir,
+  resolveTaskFiles,
   relMilestoneFile,
   relSliceFile,
   relSlicePath,
@@ -110,6 +111,9 @@ export function resolveExpectedArtifactPath(
     }
     case "rewrite-docs":
       return null;
+    case "reactive-execute":
+      // Reactive execute produces multiple task summaries — verified separately
+      return null;
     default:
       return null;
   }
@@ -148,6 +152,20 @@ export function verifyExpectedArtifact(
     return !content.includes("**Scope:** active");
   }
 
+  // Reactive-execute: verify that at least one new task summary was written.
+  // The unitId is "{mid}/{sid}/reactive" — extract mid and sid to check.
+  if (unitType === "reactive-execute") {
+    const parts = unitId.split("/");
+    const mid = parts[0];
+    const sid = parts[1];
+    if (!mid || !sid) return false;
+    const tDir = resolveTasksDir(base, mid, sid);
+    if (!tDir) return false;
+    const summaryFiles = resolveTaskFiles(tDir, "SUMMARY");
+    // At least one summary file should exist
+    return summaryFiles.length > 0;
+  }
+
   const absPath = resolveExpectedArtifactPath(unitType, unitId, base);
   // For unit types with no verifiable artifact (null path), the parent directory
   // is missing on disk — treat as stale completion state so the key gets evicted (#313).

src/resources/extensions/gsd/files.ts

@@ -15,6 +15,7 @@ import type {
   Summary, SummaryFrontmatter, SummaryRequires, FileModified,
   Continue, ContinueFrontmatter, ContinueStatus,
   RequirementCounts,
+  TaskIO,
   SecretsManifest, SecretsManifestEntry, SecretsManifestEntryStatus,
   ManifestStatus,
 } from './types.js';
@@ -724,6 +725,50 @@ export function countMustHavesMentionedInSummary(
   return count;
 }
 
+// ─── Task Plan IO Extractor ────────────────────────────────────────────────
+
+/**
+ * Extract input and output file paths from a task plan's `## Inputs` and
+ * `## Expected Output` sections. Looks for backtick-wrapped file paths on
+ * each line (e.g. `` `src/foo.ts` ``).
+ *
+ * Returns empty arrays for missing/empty sections — callers should treat
+ * tasks with no IO as ambiguous (sequential fallback trigger).
+ */
+export function parseTaskPlanIO(content: string): { inputFiles: string[]; outputFiles: string[] } {
+  const backtickPathRegex = /`([^`]+)`/g;
+
+  function extractPaths(sectionText: string | null): string[] {
+    if (!sectionText) return [];
+    const paths: string[] = [];
+    for (const line of sectionText.split("\n")) {
+      const trimmed = line.trim();
+      if (!trimmed || trimmed.startsWith("#")) continue;
+      let match: RegExpExecArray | null;
+      backtickPathRegex.lastIndex = 0;
+      while ((match = backtickPathRegex.exec(trimmed)) !== null) {
+        const candidate = match[1];
+        // Filter out things that look like code tokens rather than file paths
+        // (e.g. `true`, `false`, `npm run test`). A file path has at least one
+        // dot or slash.
+        if (candidate.includes("/") || candidate.includes(".")) {
+          paths.push(candidate);
+        }
+      }
+    }
+    return paths;
+  }
+
+  const [, body] = splitFrontmatter(content);
+  const inputSection = extractSection(body, "Inputs");
+  const outputSection = extractSection(body, "Expected Output");
+
+  return {
+    inputFiles: extractPaths(inputSection),
+    outputFiles: extractPaths(outputSection),
+  };
+}
+
 // ─── UAT Type Extractor ────────────────────────────────────────────────────
 
 /**

src/resources/extensions/gsd/preferences-types.ts

@@ -18,6 +18,7 @@ import type {
   ParallelConfig,
   CompressionStrategy,
   ContextSelectionMode,
+  ReactiveExecutionConfig,
 } from "./types.js";
 import type { DynamicRoutingConfig } from "./model-router.js";
 
@@ -86,12 +87,13 @@ export const KNOWN_PREFERENCE_KEYS = new Set<string>([
   "compression_strategy",
   "context_selection",
   "widget_mode",
+  "reactive_execution",
 ]);
 
 /** Canonical list of all dispatch unit types. */
 export const KNOWN_UNIT_TYPES = [
   "research-milestone", "plan-milestone", "research-slice", "plan-slice",
-  "execute-task", "complete-slice", "replan-slice", "reassess-roadmap",
+  "execute-task", "reactive-execute", "complete-slice", "replan-slice", "reassess-roadmap",
   "run-uat", "complete-milestone",
 ] as const;
 export type UnitType = (typeof KNOWN_UNIT_TYPES)[number];
@@ -215,6 +217,8 @@ export interface GSDPreferences {
   context_selection?: ContextSelectionMode;
   /** Default widget display mode for auto-mode dashboard. "full" | "small" | "min" | "off". Default: "full". */
   widget_mode?: "full" | "small" | "min" | "off";
+  /** Reactive (graph-derived parallel) task execution within slices. Disabled by default. */
+  reactive_execution?: ReactiveExecutionConfig;
 }
 
 export interface LoadedGSDPreferences {

src/resources/extensions/gsd/preferences-validation.ts

@@ -496,6 +496,47 @@ export function validatePreferences(preferences: GSDPreferences): {
     }
   }
 
+  // ─── Reactive Execution ─────────────────────────────────────────────────
+  if (preferences.reactive_execution !== undefined) {
+    if (typeof preferences.reactive_execution === "object" && preferences.reactive_execution !== null) {
+      const re = preferences.reactive_execution as unknown as Record<string, unknown>;
+      const validRe: Record<string, unknown> = {};
+
+      if (re.enabled !== undefined) {
+        if (typeof re.enabled === "boolean") validRe.enabled = re.enabled;
+        else errors.push("reactive_execution.enabled must be a boolean");
+      }
+      if (re.max_parallel !== undefined) {
+        const mp = typeof re.max_parallel === "number" ? re.max_parallel : Number(re.max_parallel);
+        if (Number.isFinite(mp) && mp >= 1 && mp <= 8) {
+          validRe.max_parallel = Math.floor(mp);
+        } else {
+          errors.push("reactive_execution.max_parallel must be a number between 1 and 8");
+        }
+      }
+      if (re.isolation_mode !== undefined) {
+        if (re.isolation_mode === "same-tree") {
+          validRe.isolation_mode = "same-tree";
+        } else {
+          errors.push('reactive_execution.isolation_mode must be "same-tree"');
+        }
+      }
+
+      const knownReKeys = new Set(["enabled", "max_parallel", "isolation_mode"]);
+      for (const key of Object.keys(re)) {
+        if (!knownReKeys.has(key)) {
+          warnings.push(`unknown reactive_execution key "${key}" — ignored`);
+        }
+      }
+
+      if (Object.keys(validRe).length > 0) {
+        validated.reactive_execution = validRe as unknown as import("./types.js").ReactiveExecutionConfig;
+      }
+    } else {
+      errors.push("reactive_execution must be an object");
+    }
+  }
+
   // ─── Verification Preferences ───────────────────────────────────────────
   if (preferences.verification_commands !== undefined) {
     if (Array.isArray(preferences.verification_commands)) {

src/resources/extensions/gsd/prompts/reactive-execute.md

@@ -0,0 +1,41 @@
+# Reactive Task Execution — Parallel Dispatch
+
+**Working directory:** `{{workingDirectory}}`
+**Milestone:** {{milestoneId}} — {{milestoneTitle}}
+**Slice:** {{sliceId}} — {{sliceTitle}}
+
+## Mission
+
+You are executing **multiple tasks in parallel** for this slice. The task graph below shows which tasks are ready for simultaneous execution based on their input/output dependencies.
+
+**Critical rule:** Use the `subagent` tool in **parallel mode** to dispatch all ready tasks simultaneously. Each subagent gets a self-contained execute-task prompt. After all subagents return, verify each task's outputs and write summaries.
+
+## Task Dependency Graph
+
+{{graphContext}}
+
+## Ready Tasks for Parallel Dispatch
+
+{{readyTaskCount}} tasks are ready for parallel execution:
+
+{{readyTaskList}}
+
+## Execution Protocol
+
+1. **Dispatch all ready tasks** using `subagent` in parallel mode. Each subagent prompt is provided below.
+2. **Wait for all subagents** to complete.
+3. **Verify each task's outputs** — check that expected files were created/modified and that verification commands pass.
+4. **Write task summaries** for each completed task using the task-summary template.
+5. **Mark completed tasks** as done in the slice plan (checkbox `[x]`).
+6. **Commit** all changes with a clear message covering the parallel batch.
+
+If any subagent fails:
+- Write a summary for the failed task with `blocker_discovered: true`
+- Continue marking the successful tasks as done
+- The orchestrator will handle re-dispatch on the next iteration
+
+## Subagent Prompts
+
+{{subagentPrompts}}
+
+{{inlinedTemplates}}

src/resources/extensions/gsd/reactive-graph.ts

@@ -0,0 +1,289 @@
+/**
+ * Reactive Task Graph — derives dependency edges from task plan IO signatures.
+ *
+ * Pure functions that build a DAG from task IO intersections and resolve
+ * which tasks are currently ready for parallel dispatch. Used by the
+ * reactive-execute dispatch path (ADR-004).
+ *
+ * Graph derivation and resolution functions are pure (no filesystem access).
+ * The `loadSliceTaskIO` loader at the bottom is the only async/IO function.
+ */
+
+import type { TaskIO, DerivedTaskNode, ReactiveExecutionState } from "./types.js";
+import { loadFile, parsePlan, parseTaskPlanIO } from "./files.js";
+import { resolveTasksDir, resolveTaskFiles } from "./paths.js";
+import { join } from "node:path";
+import { loadJsonFileOrNull, saveJsonFile } from "./json-persistence.js";
+import { existsSync, unlinkSync } from "node:fs";
+
+// ─── Graph Construction ───────────────────────────────────────────────────
+
+/**
+ * Build a dependency graph from task IO signatures.
+ *
+ * A task T_b depends on T_a when any of T_b's inputFiles appear in T_a's
+ * outputFiles. Self-references are excluded.
+ *
+ * Tasks are returned in the same order as the input array.
+ */
+export function deriveTaskGraph(tasks: TaskIO[]): DerivedTaskNode[] {
+  // Build output → producer lookup
+  const outputToProducer = new Map<string, string[]>();
+  for (const task of tasks) {
+    for (const outFile of task.outputFiles) {
+      const existing = outputToProducer.get(outFile);
+      if (existing) {
+        existing.push(task.id);
+      } else {
+        outputToProducer.set(outFile, [task.id]);
+      }
+    }
+  }
+
+  return tasks.map((task) => {
+    const deps = new Set<string>();
+    for (const inFile of task.inputFiles) {
+      const producers = outputToProducer.get(inFile);
+      if (producers) {
+        for (const pid of producers) {
+          if (pid !== task.id) deps.add(pid);
+        }
+      }
+    }
+    return {
+      ...task,
+      dependsOn: [...deps].sort(),
+    };
+  });
+}
+
+// ─── Ready Set Resolution ─────────────────────────────────────────────────
+
+/**
+ * Return task IDs whose dependencies are all in `completed`.
+ * Excludes tasks that are already done or in-flight.
+ */
+export function getReadyTasks(
+  graph: DerivedTaskNode[],
+  completed: Set<string>,
+  inFlight: Set<string>,
+): string[] {
+  return graph
+    .filter((node) => {
+      if (node.done || completed.has(node.id) || inFlight.has(node.id)) return false;
+      return node.dependsOn.every((dep) => completed.has(dep));
+    })
+    .map((node) => node.id);
+}
+
+// ─── Conflict-Free Subset Selection ──────────────────────────────────────
+
+/**
+ * Greedy selection of non-conflicting tasks up to `maxParallel`.
+ *
+ * Two tasks conflict if they share any outputFile. We also exclude tasks
+ * whose outputs overlap with `inFlightOutputs` (files being written by
+ * tasks currently in progress).
+ */
+export function chooseNonConflictingSubset(
+  readyIds: string[],
+  graph: DerivedTaskNode[],
+  maxParallel: number,
+  inFlightOutputs: Set<string>,
+): string[] {
+  const nodeMap = new Map(graph.map((n) => [n.id, n]));
+  const claimed = new Set(inFlightOutputs);
+  const selected: string[] = [];
+
+  for (const id of readyIds) {
+    if (selected.length >= maxParallel) break;
+    const node = nodeMap.get(id);
+    if (!node) continue;
+
+    // Check for output overlap with already-selected or in-flight
+    const conflicts = node.outputFiles.some((f) => claimed.has(f));
+    if (conflicts) continue;
+
+    // Claim this task's outputs
+    for (const f of node.outputFiles) claimed.add(f);
+    selected.push(id);
+  }
+
+  return selected;
+}
+
+// ─── Graph Quality Checks ─────────────────────────────────────────────────
+
+/**
+ * Returns true if any incomplete task has 0 inputFiles AND 0 outputFiles.
+ *
+ * An ambiguous graph means IO annotations are too sparse to derive reliable
+ * edges — the dispatcher should fall back to sequential execution.
+ */
+export function isGraphAmbiguous(graph: DerivedTaskNode[]): boolean {
+  return graph.some(
+    (node) =>
+      !node.done &&
+      node.inputFiles.length === 0 &&
+      node.outputFiles.length === 0,
+  );
+}
+
+/**
+ * Detect deadlock: no tasks are ready and none are in-flight, yet incomplete
+ * tasks remain. This indicates a circular dependency or impossible state.
+ */
+export function detectDeadlock(
+  graph: DerivedTaskNode[],
+  completed: Set<string>,
+  inFlight: Set<string>,
+): boolean {
+  const incomplete = graph.filter(
+    (n) => !n.done && !completed.has(n.id) && !inFlight.has(n.id),
+  );
+  if (incomplete.length === 0) return false; // all done
+  if (inFlight.size > 0) return false; // something is running, wait for it
+
+  // Nothing in flight, but incomplete tasks remain — check if any are ready
+  const ready = getReadyTasks(graph, completed, inFlight);
+  return ready.length === 0;
+}
+
+// ─── Graph Metrics ────────────────────────────────────────────────────────
+
+/** Compute summary metrics for logging. */
+export function graphMetrics(graph: DerivedTaskNode[]): {
+  taskCount: number;
+  edgeCount: number;
+  readySetSize: number;
+  ambiguous: boolean;
+} {
+  const completed = new Set(graph.filter((n) => n.done).map((n) => n.id));
+  const ready = getReadyTasks(graph, completed, new Set());
+  const edgeCount = graph.reduce((sum, n) => sum + n.dependsOn.length, 0);
+
+  return {
+    taskCount: graph.length,
+    edgeCount,
+    readySetSize: ready.length,
+    ambiguous: isGraphAmbiguous(graph),
+  };
+}
+
+// ─── IO Loader (async, filesystem) ────────────────────────────────────────
+
+/**
+ * Load TaskIO for all tasks in a slice by reading the slice plan (for done
+ * status and task IDs) and individual task plan files (for IO sections).
+ *
+ * Returns [] when the slice plan or tasks directory doesn't exist.
+ */
+export async function loadSliceTaskIO(
+  basePath: string,
+  mid: string,
+  sid: string,
+): Promise<TaskIO[]> {
+  const { resolveSliceFile } = await import("./paths.js");
+  const slicePlanPath = resolveSliceFile(basePath, mid, sid, "PLAN");
+  const planContent = slicePlanPath ? await loadFile(slicePlanPath) : null;
+  if (!planContent) return [];
+
+  const plan = parsePlan(planContent);
+  const tDir = resolveTasksDir(basePath, mid, sid);
+  if (!tDir) return [];
+
+  const results: TaskIO[] = [];
+
+  for (const taskEntry of plan.tasks) {
+    const planFiles = resolveTaskFiles(tDir, "PLAN");
+    const taskFileName = planFiles.find((f) =>
+      f.toUpperCase().startsWith(taskEntry.id.toUpperCase() + "-"),
+    );
+    if (!taskFileName) {
+      // Task plan file missing — include with empty IO (will trigger ambiguous)
+      results.push({
+        id: taskEntry.id,
+        title: taskEntry.title,
+        inputFiles: [],
+        outputFiles: [],
+        done: taskEntry.done,
+      });
+      continue;
+    }
+
+    const taskContent = await loadFile(join(tDir, taskFileName));
+    if (!taskContent) {
+      results.push({
+        id: taskEntry.id,
+        title: taskEntry.title,
+        inputFiles: [],
+        outputFiles: [],
+        done: taskEntry.done,
+      });
+      continue;
+    }
+
+    const io = parseTaskPlanIO(taskContent);
+    results.push({
+      id: taskEntry.id,
+      title: taskEntry.title,
+      inputFiles: io.inputFiles,
+      outputFiles: io.outputFiles,
+      done: taskEntry.done,
+    });
+  }
+
+  return results;
+}
+
+// ─── State Persistence ────────────────────────────────────────────────────
+
+function reactiveStatePath(basePath: string, mid: string, sid: string): string {
+  return join(basePath, ".gsd", "runtime", `${mid}-${sid}-reactive.json`);
+}
+
+function isReactiveState(data: unknown): data is ReactiveExecutionState {
+  if (!data || typeof data !== "object") return false;
+  const d = data as Record<string, unknown>;
+  return typeof d.sliceId === "string" && Array.isArray(d.completed);
+}
+
+/**
+ * Load persisted reactive execution state for a slice.
+ * Returns null when no state file exists or the file is invalid.
+ */
+export function loadReactiveState(
+  basePath: string,
+  mid: string,
+  sid: string,
+): ReactiveExecutionState | null {
+  return loadJsonFileOrNull(reactiveStatePath(basePath, mid, sid), isReactiveState);
+}
+
+/**
+ * Save reactive execution state to disk.
+ */
+export function saveReactiveState(
+  basePath: string,
+  mid: string,
+  sid: string,
+  state: ReactiveExecutionState,
+): void {
+  saveJsonFile(reactiveStatePath(basePath, mid, sid), state);
+}
+
+/**
+ * Remove the reactive state file when a slice completes.
+ */
+export function clearReactiveState(
+  basePath: string,
+  mid: string,
+  sid: string,
+): void {
+  const path = reactiveStatePath(basePath, mid, sid);
+  try {
+    if (existsSync(path)) unlinkSync(path);
+  } catch {
+    // Non-fatal
+  }
+}

src/resources/extensions/gsd/tests/reactive-executor.test.ts

@@ -0,0 +1,367 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync, existsSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import {
+  loadSliceTaskIO,
+  deriveTaskGraph,
+  isGraphAmbiguous,
+  getReadyTasks,
+  chooseNonConflictingSubset,
+  loadReactiveState,
+  saveReactiveState,
+  clearReactiveState,
+} from "../reactive-graph.ts";
+import { validatePreferences } from "../preferences-validation.ts";
+import type { ReactiveExecutionState } from "../types.ts";
+
+// ─── Preference Validation ────────────────────────────────────────────────
+
+test("reactive_execution validation accepts valid config", () => {
+  const result = validatePreferences({
+    reactive_execution: {
+      enabled: true,
+      max_parallel: 4,
+      isolation_mode: "same-tree",
+    },
+  });
+  assert.equal(result.errors.length, 0);
+  assert.deepEqual(result.preferences.reactive_execution, {
+    enabled: true,
+    max_parallel: 4,
+    isolation_mode: "same-tree",
+  });
+});
+
+test("reactive_execution validation rejects max_parallel out of range", () => {
+  const result = validatePreferences({
+    reactive_execution: {
+      enabled: true,
+      max_parallel: 10,
+      isolation_mode: "same-tree",
+    } as any,
+  });
+  assert.ok(result.errors.some((e) => e.includes("max_parallel")));
+});
+
+test("reactive_execution validation rejects invalid isolation_mode", () => {
+  const result = validatePreferences({
+    reactive_execution: {
+      enabled: true,
+      max_parallel: 2,
+      isolation_mode: "separate-branch",
+    } as any,
+  });
+  assert.ok(result.errors.some((e) => e.includes("isolation_mode")));
+});
+
+test("reactive_execution validation warns on unknown keys", () => {
+  const result = validatePreferences({
+    reactive_execution: {
+      enabled: true,
+      max_parallel: 2,
+      isolation_mode: "same-tree",
+      unknown_thing: true,
+    } as any,
+  });
+  assert.equal(result.errors.length, 0);
+  assert.ok(result.warnings.some((w) => w.includes("unknown_thing")));
+});
+
+// ─── Dispatch Rule Matching Logic ─────────────────────────────────────────
+
+test("reactive dispatch requires enabled config and multiple ready tasks", async () => {
+  // Build a minimal filesystem with a slice plan and task plans
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-dispatch-"));
+  try {
+    const gsd = join(repo, ".gsd", "milestones", "M001", "slices", "S01");
+    mkdirSync(join(gsd, "tasks"), { recursive: true });
+
+    // Slice plan with 3 tasks
+    writeFileSync(
+      join(gsd, "S01-PLAN.md"),
+      [
+        "# S01: Test Slice",
+        "",
+        "**Goal:** Test reactive execution",
+        "**Demo:** All three tasks run in parallel",
+        "",
+        "## Tasks",
+        "",
+        "- [ ] **T01: First** `est:15m`",
+        "  Create initial types",
+        "- [ ] **T02: Second** `est:15m`",
+        "  Create models",
+        "- [ ] **T03: Third** `est:15m`",
+        "  Create service layer",
+        "",
+      ].join("\n"),
+    );
+
+    // Task plans with non-overlapping IO (all independent)
+    writeFileSync(
+      join(gsd, "tasks", "T01-PLAN.md"),
+      [
+        "# T01: First",
+        "",
+        "## Description",
+        "Create types.",
+        "",
+        "## Inputs",
+        "",
+        "- `src/config.json` — Config schema",
+        "",
+        "## Expected Output",
+        "",
+        "- `src/types.ts` — Type definitions",
+      ].join("\n"),
+    );
+
+    writeFileSync(
+      join(gsd, "tasks", "T02-PLAN.md"),
+      [
+        "# T02: Second",
+        "",
+        "## Description",
+        "Create models.",
+        "",
+        "## Inputs",
+        "",
+        "- `src/schema.json` — Schema file",
+        "",
+        "## Expected Output",
+        "",
+        "- `src/models.ts` — Model definitions",
+      ].join("\n"),
+    );
+
+    writeFileSync(
+      join(gsd, "tasks", "T03-PLAN.md"),
+      [
+        "# T03: Third",
+        "",
+        "## Description",
+        "Create service.",
+        "",
+        "## Inputs",
+        "",
+        "- `src/api.json` — API spec",
+        "",
+        "## Expected Output",
+        "",
+        "- `src/service.ts` — Service layer",
+      ].join("\n"),
+    );
+
+    // Load IO and build graph
+    const basePath = repo;
+    const taskIO = await loadSliceTaskIO(basePath, "M001", "S01");
+    assert.equal(taskIO.length, 3);
+
+    const graph = deriveTaskGraph(taskIO);
+    assert.equal(isGraphAmbiguous(graph), false, "Graph should not be ambiguous");
+
+    // All independent → all should be ready
+    const ready = getReadyTasks(graph, new Set(), new Set());
+    assert.equal(ready.length, 3);
+
+    // Choose subset with max_parallel=2
+    const selected = chooseNonConflictingSubset(ready, graph, 2, new Set());
+    assert.equal(selected.length, 2);
+    assert.deepEqual(selected, ["T01", "T02"]);
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+test("reactive dispatch falls back when graph is ambiguous (task without IO)", async () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-ambiguous-"));
+  try {
+    const gsd = join(repo, ".gsd", "milestones", "M001", "slices", "S01");
+    mkdirSync(join(gsd, "tasks"), { recursive: true });
+
+    writeFileSync(
+      join(gsd, "S01-PLAN.md"),
+      [
+        "# S01: Test",
+        "",
+        "**Goal:** Test",
+        "**Demo:** Test",
+        "",
+        "## Tasks",
+        "",
+        "- [ ] **T01: A** `est:15m`",
+        "- [ ] **T02: B** `est:15m`",
+        "",
+      ].join("\n"),
+    );
+
+    // T01 has IO, T02 has NO IO sections → ambiguous
+    writeFileSync(
+      join(gsd, "tasks", "T01-PLAN.md"),
+      "# T01: A\n\n## Inputs\n\n- `src/a.ts`\n\n## Expected Output\n\n- `src/b.ts`\n",
+    );
+    writeFileSync(
+      join(gsd, "tasks", "T02-PLAN.md"),
+      "# T02: B\n\n## Description\n\nNo IO sections.\n",
+    );
+
+    const taskIO = await loadSliceTaskIO(repo, "M001", "S01");
+    const graph = deriveTaskGraph(taskIO);
+    assert.equal(isGraphAmbiguous(graph), true, "Graph should be ambiguous");
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+test("single ready task falls through to sequential", async () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-single-"));
+  try {
+    const gsd = join(repo, ".gsd", "milestones", "M001", "slices", "S01");
+    mkdirSync(join(gsd, "tasks"), { recursive: true });
+
+    writeFileSync(
+      join(gsd, "S01-PLAN.md"),
+      [
+        "# S01: Linear",
+        "",
+        "**Goal:** Linear chain",
+        "**Demo:** Sequential",
+        "",
+        "## Tasks",
+        "",
+        "- [ ] **T01: First** `est:15m`",
+        "- [ ] **T02: Second** `est:15m`",
+        "",
+      ].join("\n"),
+    );
+
+    writeFileSync(
+      join(gsd, "tasks", "T01-PLAN.md"),
+      "# T01: First\n\n## Inputs\n\n- `src/config.json`\n\n## Expected Output\n\n- `src/a.ts`\n",
+    );
+    writeFileSync(
+      join(gsd, "tasks", "T02-PLAN.md"),
+      "# T02: Second\n\n## Inputs\n\n- `src/a.ts`\n\n## Expected Output\n\n- `src/b.ts`\n",
+    );
+
+    const taskIO = await loadSliceTaskIO(repo, "M001", "S01");
+    const graph = deriveTaskGraph(taskIO);
+    const ready = getReadyTasks(graph, new Set(), new Set());
+    // Only T01 is ready (T02 depends on T01)
+    assert.equal(ready.length, 1);
+    assert.deepEqual(ready, ["T01"]);
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+// ─── State Persistence ────────────────────────────────────────────────────
+
+test("saveReactiveState and loadReactiveState round-trip", () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-state-"));
+  mkdirSync(join(repo, ".gsd", "runtime"), { recursive: true });
+  try {
+    const state: ReactiveExecutionState = {
+      sliceId: "S01",
+      completed: ["T01", "T02"],
+      graphSnapshot: { taskCount: 4, edgeCount: 2, readySetSize: 1, ambiguous: false },
+      updatedAt: "2025-01-01T00:00:00Z",
+    };
+
+    saveReactiveState(repo, "M001", "S01", state);
+    const loaded = loadReactiveState(repo, "M001", "S01");
+    assert.deepEqual(loaded, state);
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+test("clearReactiveState removes the file", () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-clear-"));
+  mkdirSync(join(repo, ".gsd", "runtime"), { recursive: true });
+  try {
+    const state: ReactiveExecutionState = {
+      sliceId: "S01",
+      completed: [],
+      graphSnapshot: { taskCount: 2, edgeCount: 0, readySetSize: 2, ambiguous: false },
+      updatedAt: "2025-01-01T00:00:00Z",
+    };
+
+    saveReactiveState(repo, "M001", "S01", state);
+    assert.ok(existsSync(join(repo, ".gsd", "runtime", "M001-S01-reactive.json")));
+
+    clearReactiveState(repo, "M001", "S01");
+    assert.ok(!existsSync(join(repo, ".gsd", "runtime", "M001-S01-reactive.json")));
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+test("loadReactiveState returns null when no file exists", () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-nofile-"));
+  mkdirSync(join(repo, ".gsd", "runtime"), { recursive: true });
+  try {
+    const loaded = loadReactiveState(repo, "M001", "S01");
+    assert.equal(loaded, null);
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});
+
+test("completed tasks are not re-dispatched on next iteration", async () => {
+  const repo = mkdtempSync(join(tmpdir(), "gsd-reactive-reentry-"));
+  try {
+    const gsd = join(repo, ".gsd", "milestones", "M001", "slices", "S01");
+    mkdirSync(join(gsd, "tasks"), { recursive: true });
+    mkdirSync(join(repo, ".gsd", "runtime"), { recursive: true });
+
+    writeFileSync(
+      join(gsd, "S01-PLAN.md"),
+      [
+        "# S01: Reentry Test",
+        "",
+        "**Goal:** Test re-entry",
+        "**Demo:** Correct resumption",
+        "",
+        "## Tasks",
+        "",
+        "- [x] **T01: Done** `est:15m`",
+        "- [ ] **T02: Pending** `est:15m`",
+        "- [ ] **T03: Also Pending** `est:15m`",
+        "",
+      ].join("\n"),
+    );
+
+    writeFileSync(
+      join(gsd, "tasks", "T01-PLAN.md"),
+      "# T01: Done\n\n## Inputs\n\n- `src/config.json`\n\n## Expected Output\n\n- `src/a.ts`\n",
+    );
+    writeFileSync(
+      join(gsd, "tasks", "T02-PLAN.md"),
+      "# T02: Pending\n\n## Inputs\n\n- `src/a.ts`\n\n## Expected Output\n\n- `src/b.ts`\n",
+    );
+    writeFileSync(
+      join(gsd, "tasks", "T03-PLAN.md"),
+      "# T03: Also Pending\n\n## Inputs\n\n- `src/a.ts`\n\n## Expected Output\n\n- `src/c.ts`\n",
+    );
+
+    const taskIO = await loadSliceTaskIO(repo, "M001", "S01");
+    const graph = deriveTaskGraph(taskIO);
+
+    // T01 is done, T02 and T03 depend on T01
+    const completed = new Set(["T01"]);
+    const ready = getReadyTasks(graph, completed, new Set());
+    // Both T02 and T03 should be ready (T01 is complete)
+    assert.deepEqual(ready, ["T02", "T03"]);
+
+    // Simulate T02 completes, re-derive
+    completed.add("T02");
+    const ready2 = getReadyTasks(graph, completed, new Set());
+    // Only T03 should be ready
+    assert.deepEqual(ready2, ["T03"]);
+  } finally {
+    rmSync(repo, { recursive: true, force: true });
+  }
+});

src/resources/extensions/gsd/tests/reactive-graph.test.ts

@@ -0,0 +1,299 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import {
+  deriveTaskGraph,
+  getReadyTasks,
+  chooseNonConflictingSubset,
+  isGraphAmbiguous,
+  detectDeadlock,
+  graphMetrics,
+} from "../reactive-graph.ts";
+import { parseTaskPlanIO } from "../files.ts";
+import type { TaskIO, DerivedTaskNode } from "../types.ts";
+
+// ─── parseTaskPlanIO ──────────────────────────────────────────────────────
+
+test("parseTaskPlanIO extracts backtick-wrapped file paths from Inputs and Expected Output", () => {
+  const content = `---
+estimated_steps: 3
+estimated_files: 2
+---
+
+# T01: Setup Models
+
+**Slice:** S01 — Core Setup
+**Milestone:** M001
+
+## Description
+
+Create the core data models.
+
+## Steps
+
+1. Create types file
+2. Create models file
+
+## Must-Haves
+
+- [ ] Type definitions complete
+
+## Verification
+
+- Run type checker
+
+## Inputs
+
+- \`src/types.ts\` — Existing type definitions from prior work
+- \`src/config.json\` — Configuration schema
+
+## Expected Output
+
+- \`src/models.ts\` — New data model definitions
+- \`src/models.test.ts\` — Unit tests for models
+`;
+
+  const io = parseTaskPlanIO(content);
+  assert.deepEqual(io.inputFiles, ["src/types.ts", "src/config.json"]);
+  assert.deepEqual(io.outputFiles, ["src/models.ts", "src/models.test.ts"]);
+});
+
+test("parseTaskPlanIO returns empty arrays for missing sections", () => {
+  const content = `# T01: Something\n\n## Description\n\nNo IO sections here.\n`;
+  const io = parseTaskPlanIO(content);
+  assert.deepEqual(io.inputFiles, []);
+  assert.deepEqual(io.outputFiles, []);
+});
+
+test("parseTaskPlanIO ignores non-file-path backtick tokens", () => {
+  const content = `# T01: Test
+
+## Inputs
+
+- \`true\` — a boolean flag
+- \`src/index.ts\` — main entry
+- \`npm run test\` — a command, not a file
+
+## Expected Output
+
+- \`dist/bundle.js\` — compiled output
+- \`false\` — not a file
+`;
+
+  const io = parseTaskPlanIO(content);
+  assert.deepEqual(io.inputFiles, ["src/index.ts"]);
+  assert.deepEqual(io.outputFiles, ["dist/bundle.js"]);
+});
+
+test("parseTaskPlanIO handles multiple backtick tokens on one line", () => {
+  const content = `# T01: Multi
+
+## Inputs
+
+- \`src/a.ts\` and \`src/b.ts\` — both needed
+
+## Expected Output
+
+- \`src/c.ts\` — output
+`;
+  const io = parseTaskPlanIO(content);
+  assert.deepEqual(io.inputFiles, ["src/a.ts", "src/b.ts"]);
+  assert.deepEqual(io.outputFiles, ["src/c.ts"]);
+});
+
+// ─── deriveTaskGraph ──────────────────────────────────────────────────────
+
+test("deriveTaskGraph: linear chain T01→T02→T03", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "First", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "Second", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false },
+    { id: "T03", title: "Third", inputFiles: ["src/b.ts"], outputFiles: ["src/c.ts"], done: false },
+  ];
+
+  const graph = deriveTaskGraph(tasks);
+  assert.deepEqual(graph[0].dependsOn, []);
+  assert.deepEqual(graph[1].dependsOn, ["T01"]);
+  assert.deepEqual(graph[2].dependsOn, ["T02"]);
+});
+
+test("deriveTaskGraph: diamond dependency", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "Base", inputFiles: [], outputFiles: ["src/base.ts"], done: false },
+    { id: "T02", title: "Left", inputFiles: ["src/base.ts"], outputFiles: ["src/left.ts"], done: false },
+    { id: "T03", title: "Right", inputFiles: ["src/base.ts"], outputFiles: ["src/right.ts"], done: false },
+    { id: "T04", title: "Merge", inputFiles: ["src/left.ts", "src/right.ts"], outputFiles: ["src/final.ts"], done: false },
+  ];
+
+  const graph = deriveTaskGraph(tasks);
+  assert.deepEqual(graph[0].dependsOn, []);
+  assert.deepEqual(graph[1].dependsOn, ["T01"]);
+  assert.deepEqual(graph[2].dependsOn, ["T01"]);
+  assert.deepEqual(graph[3].dependsOn, ["T02", "T03"]);
+});
+
+test("deriveTaskGraph: fully independent tasks", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/b.ts"], done: false },
+    { id: "T03", title: "C", inputFiles: [], outputFiles: ["src/c.ts"], done: false },
+  ];
+
+  const graph = deriveTaskGraph(tasks);
+  assert.deepEqual(graph[0].dependsOn, []);
+  assert.deepEqual(graph[1].dependsOn, []);
+  assert.deepEqual(graph[2].dependsOn, []);
+});
+
+test("deriveTaskGraph: self-referencing output→input is excluded", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "Self", inputFiles: ["src/a.ts"], outputFiles: ["src/a.ts"], done: false },
+  ];
+
+  const graph = deriveTaskGraph(tasks);
+  assert.deepEqual(graph[0].dependsOn, []);
+});
+
+// ─── getReadyTasks ────────────────────────────────────────────────────────
+
+test("getReadyTasks: partially completed graph", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "Base", inputFiles: [], outputFiles: ["src/a.ts"], done: true },
+    { id: "T02", title: "Dep", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false },
+    { id: "T03", title: "Blocked", inputFiles: ["src/b.ts"], outputFiles: ["src/c.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const ready = getReadyTasks(graph, new Set(["T01"]), new Set());
+  assert.deepEqual(ready, ["T02"]);
+});
+
+test("getReadyTasks: nothing complete → only root tasks ready", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "Root", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "Dep", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const ready = getReadyTasks(graph, new Set(), new Set());
+  assert.deepEqual(ready, ["T01"]);
+});
+
+test("getReadyTasks: all complete → empty", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "Done", inputFiles: [], outputFiles: ["src/a.ts"], done: true },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const ready = getReadyTasks(graph, new Set(["T01"]), new Set());
+  assert.deepEqual(ready, []);
+});
+
+test("getReadyTasks: in-flight tasks excluded", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/b.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const ready = getReadyTasks(graph, new Set(), new Set(["T01"]));
+  assert.deepEqual(ready, ["T02"]);
+});
+
+// ─── chooseNonConflictingSubset ───────────────────────────────────────────
+
+test("chooseNonConflictingSubset: output conflicts", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/shared.ts"], done: false },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/shared.ts"], done: false },
+    { id: "T03", title: "C", inputFiles: [], outputFiles: ["src/other.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const selected = chooseNonConflictingSubset(["T01", "T02", "T03"], graph, 3, new Set());
+  // T01 claims shared.ts, T02 conflicts, T03 is fine
+  assert.deepEqual(selected, ["T01", "T03"]);
+});
+
+test("chooseNonConflictingSubset: respects maxParallel", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/b.ts"], done: false },
+    { id: "T03", title: "C", inputFiles: [], outputFiles: ["src/c.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const selected = chooseNonConflictingSubset(["T01", "T02", "T03"], graph, 2, new Set());
+  assert.deepEqual(selected, ["T01", "T02"]);
+});
+
+test("chooseNonConflictingSubset: respects inFlightOutputs", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/b.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const selected = chooseNonConflictingSubset(["T01", "T02"], graph, 4, new Set(["src/a.ts"]));
+  assert.deepEqual(selected, ["T02"]);
+});
+
+// ─── isGraphAmbiguous ─────────────────────────────────────────────────────
+
+test("isGraphAmbiguous: task with no IO → ambiguous", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: [], done: false, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: [] },
+  ];
+  assert.equal(isGraphAmbiguous(graph), true);
+});
+
+test("isGraphAmbiguous: all tasks have IO → not ambiguous", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: ["T01"] },
+  ];
+  assert.equal(isGraphAmbiguous(graph), false);
+});
+
+test("isGraphAmbiguous: done tasks with no IO are ignored", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: [], done: true, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/b.ts"], done: false, dependsOn: [] },
+  ];
+  assert.equal(isGraphAmbiguous(graph), false);
+});
+
+// ─── detectDeadlock ───────────────────────────────────────────────────────
+
+test("detectDeadlock: circular dependency detected", () => {
+  // T01 depends on T02, T02 depends on T01 — deadlock
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: ["src/b.ts"], outputFiles: ["src/a.ts"], done: false, dependsOn: ["T02"] },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: ["T01"] },
+  ];
+  assert.equal(detectDeadlock(graph, new Set(), new Set()), true);
+});
+
+test("detectDeadlock: normal blocked-waiting-for-in-flight → not deadlock", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: false, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: ["T01"] },
+  ];
+  // T01 is in-flight, T02 is waiting → not deadlock
+  assert.equal(detectDeadlock(graph, new Set(), new Set(["T01"])), false);
+});
+
+test("detectDeadlock: all complete → not deadlock", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: true, dependsOn: [] },
+  ];
+  assert.equal(detectDeadlock(graph, new Set(["T01"]), new Set()), false);
+});
+
+// ─── graphMetrics ─────────────────────────────────────────────────────────
+
+test("graphMetrics computes correct values", () => {
+  const tasks: TaskIO[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: ["src/a.ts"], done: true },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false },
+    { id: "T03", title: "C", inputFiles: [], outputFiles: ["src/c.ts"], done: false },
+  ];
+  const graph = deriveTaskGraph(tasks);
+  const metrics = graphMetrics(graph);
+  assert.equal(metrics.taskCount, 3);
+  assert.equal(metrics.edgeCount, 1); // T02 depends on T01
+  assert.equal(metrics.readySetSize, 2); // T02 (T01 done) and T03 (no deps)
+  assert.equal(metrics.ambiguous, false);
+});

src/resources/extensions/gsd/types.ts

@@ -436,3 +436,44 @@ export interface ParallelConfig {
   merge_strategy: MergeStrategy;
   auto_merge: AutoMergeMode;
 }
+
+// ─── Reactive Task Execution Types ───────────────────────────────────────
+
+/** IO signature extracted from a single task plan's Inputs/Expected Output sections. */
+export interface TaskIO {
+  id: string;        // e.g. "T01"
+  title: string;
+  inputFiles: string[];
+  outputFiles: string[];
+  done: boolean;
+}
+
+/** A task node with derived dependency edges from input/output intersection. */
+export interface DerivedTaskNode extends TaskIO {
+  /** IDs of tasks whose outputFiles overlap with this task's inputFiles. */
+  dependsOn: string[];
+}
+
+/** Configuration for reactive (graph-derived parallel) task execution within a slice. */
+export interface ReactiveExecutionConfig {
+  enabled: boolean;
+  /** Maximum number of tasks to dispatch in parallel. Clamped to 1–8. */
+  max_parallel: number;
+  /** Isolation mode for parallel tasks within a slice. Currently only "same-tree" is supported. */
+  isolation_mode: "same-tree";
+}
+
+/** Per-slice reactive execution runtime state, persisted to disk. */
+export interface ReactiveExecutionState {
+  sliceId: string;
+  /** Task IDs that have been verified as completed. */
+  completed: string[];
+  /** Snapshot of the graph at last dispatch. */
+  graphSnapshot: {
+    taskCount: number;
+    edgeCount: number;
+    readySetSize: number;
+    ambiguous: boolean;
+  };
+  updatedAt: string;
+}