diff --git a/src/resources/extensions/gsd/activity-log.ts b/src/resources/extensions/gsd/activity-log.ts
index 53f792937..932f28e2e 100644
--- a/src/resources/extensions/gsd/activity-log.ts
+++ b/src/resources/extensions/gsd/activity-log.ts
@@ -11,6 +11,7 @@
 import { writeFileSync, writeSync, mkdirSync, readdirSync, unlinkSync, statSync, openSync, closeSync, constants } from "node:fs";
 import { createHash } from "node:crypto";
 import { join } from "node:path";
+import { GSDError, GSD_IO_ERROR } from "./errors.js";
 
 const SEQ_PREFIX_RE = /^(\d+)-/;
 import type { ExtensionContext } from "@gsd/pi-coding-agent";
@@ -95,7 +96,7 @@ function nextActivityFilePath(
     }
   }
   // Fallback: should never reach here in practice
-  throw new Error(`Failed to find available activity log sequence in ${activityDir}`);
+  throw new GSDError(GSD_IO_ERROR, `Failed to find available activity log sequence in ${activityDir}`);
 }
 
 export function saveActivityLog(
diff --git a/src/resources/extensions/gsd/auto-worktree.ts b/src/resources/extensions/gsd/auto-worktree.ts
index 874a9e699..3edd40475 100644
--- a/src/resources/extensions/gsd/auto-worktree.ts
+++ b/src/resources/extensions/gsd/auto-worktree.ts
@@ -8,6 +8,7 @@
 
 import { existsSync, cpSync, readFileSync, writeFileSync, readdirSync, mkdirSync, realpathSync, utimesSync, unlinkSync } from "node:fs";
 import { isAbsolute, join, resolve } from "node:path";
+import { GSDError, GSD_IO_ERROR, GSD_GIT_ERROR } from "./errors.js";
 import { copyWorktreeDb, reconcileWorktreeDb, isDbAvailable } from "./gsd-db.js";
 import { execSync, execFileSync } from "node:child_process";
 import {
@@ -301,7 +302,8 @@ export function createAutoWorktree(basePath: string, milestoneId: string): strin
   } catch (err) {
     // If chdir fails, the worktree was created but we couldn't enter it.
     // Don't store originalBase -- caller can retry or clean up.
-    throw new Error(
+    throw new GSDError(
+      GSD_IO_ERROR,
       `Auto-worktree created at ${info.path} but chdir failed: ${err instanceof Error ? err.message : String(err)}`,
     );
   }
@@ -367,7 +369,8 @@ export function teardownAutoWorktree(
     process.chdir(originalBasePath);
     originalBase = null;
   } catch (err) {
-    throw new Error(
+    throw new GSDError(
+      GSD_IO_ERROR,
       `Failed to chdir back to ${originalBasePath} during teardown: ${err instanceof Error ? err.message : String(err)}`,
     );
   }
@@ -425,22 +428,22 @@ export function getAutoWorktreePath(basePath: string, milestoneId: string): stri
 export function enterAutoWorktree(basePath: string, milestoneId: string): string {
   const p = worktreePath(basePath, milestoneId);
   if (!existsSync(p)) {
-    throw new Error(`Auto-worktree for ${milestoneId} does not exist at ${p}`);
+    throw new GSDError(GSD_IO_ERROR, `Auto-worktree for ${milestoneId} does not exist at ${p}`);
   }
 
   // Validate this is a real git worktree, not a stray directory (#695)
   const gitPath = join(p, ".git");
   if (!existsSync(gitPath)) {
-    throw new Error(`Auto-worktree path ${p} exists but is not a git worktree (no .git)`);
+    throw new GSDError(GSD_GIT_ERROR, `Auto-worktree path ${p} exists but is not a git worktree (no .git)`);
   }
   try {
     const content = readFileSync(gitPath, "utf8").trim();
     if (!content.startsWith("gitdir: ")) {
-      throw new Error(`Auto-worktree path ${p} has a .git but it is not a worktree gitdir pointer`);
+      throw new GSDError(GSD_GIT_ERROR, `Auto-worktree path ${p} has a .git but it is not a worktree gitdir pointer`);
     }
   } catch (err) {
     if (err instanceof Error && err.message.includes("worktree")) throw err;
-    throw new Error(`Auto-worktree path ${p} exists but .git is unreadable`);
+    throw new GSDError(GSD_IO_ERROR, `Auto-worktree path ${p} exists but .git is unreadable`);
   }
 
   const previousCwd = process.cwd();
@@ -449,7 +452,8 @@ export function enterAutoWorktree(basePath: string, milestoneId: string): string
     process.chdir(p);
     originalBase = basePath;
   } catch (err) {
-    throw new Error(
+    throw new GSDError(
+      GSD_IO_ERROR,
       `Failed to enter auto-worktree at ${p}: ${err instanceof Error ? err.message : String(err)}`,
     );
   }
diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index dc687c4e7..0904a4d3e 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -101,6 +101,7 @@ import {
   getProjectTotals, formatCost, formatTokenCount,
 } from "./metrics.js";
 import { computeBudgets, resolveExecutorContextWindow } from "./context-budget.js";
+import { GSDError, GSD_ARTIFACT_MISSING } from "./errors.js";
 import { join } from "node:path";
 import { sep as pathSep } from "node:path";
 import { readdirSync, readFileSync, existsSync, mkdirSync, writeFileSync, renameSync, unlinkSync, statSync } from "node:fs";
@@ -2301,7 +2302,7 @@ async function dispatchNextUnit(
     if (s.currentMilestoneId && isInAutoWorktree(s.basePath) && s.originalBasePath) {
       try {
         const roadmapPath = resolveMilestoneFile(s.originalBasePath, s.currentMilestoneId, "ROADMAP");
-        if (!roadmapPath) throw new Error(`Cannot resolve ROADMAP file for milestone ${ s.currentMilestoneId }`);
+        if (!roadmapPath) throw new GSDError(GSD_ARTIFACT_MISSING, `Cannot resolve ROADMAP file for milestone ${ s.currentMilestoneId }`);
         const roadmapContent = readFileSync(roadmapPath, "utf-8");
         const mergeResult = mergeMilestoneToMain(s.originalBasePath, s.currentMilestoneId, roadmapContent);
         s.basePath = s.originalBasePath;
diff --git a/src/resources/extensions/gsd/db-writer.ts b/src/resources/extensions/gsd/db-writer.ts
index c62fe0140..6671a47b5 100644
--- a/src/resources/extensions/gsd/db-writer.ts
+++ b/src/resources/extensions/gsd/db-writer.ts
@@ -12,6 +12,7 @@ import { join, resolve } from 'node:path';
 import type { Decision, Requirement } from './types.js';
 import { resolveGsdRootFile } from './paths.js';
 import { saveFile } from './files.js';
+import { GSDError, GSD_STALE_STATE, GSD_IO_ERROR } from './errors.js';
 
 // ─── Markdown Generators ──────────────────────────────────────────────────
 
@@ -249,7 +250,7 @@ export async function updateRequirementInDb(
 
     const existing = db.getRequirementById(id);
     if (!existing) {
-      throw new Error(`Requirement ${id} not found`);
+      throw new GSDError(GSD_STALE_STATE, `Requirement ${id} not found`);
     }
 
     // Merge updates into existing
@@ -331,7 +332,7 @@ export async function saveArtifactToDb(
     const gsdDir = resolve(basePath, '.gsd');
     const fullPath = resolve(basePath, '.gsd', opts.path);
     if (!fullPath.startsWith(gsdDir)) {
-      throw new Error(`saveArtifactToDb: path escapes .gsd/ directory: ${opts.path}`);
+      throw new GSDError(GSD_IO_ERROR, `saveArtifactToDb: path escapes .gsd/ directory: ${opts.path}`);
     }
     await saveFile(fullPath, opts.content);
   } catch (err) {
diff --git a/src/resources/extensions/gsd/diff-context.ts b/src/resources/extensions/gsd/diff-context.ts
index c0431348e..637679c94 100644
--- a/src/resources/extensions/gsd/diff-context.ts
+++ b/src/resources/extensions/gsd/diff-context.ts
@@ -8,6 +8,7 @@
 
 import { execFileSync, execFile } from "node:child_process";
 import { resolve } from "node:path";
+import { GSDError, GSD_PARSE_ERROR } from "./errors.js";
 
 // ─── Types ──────────────────────────────────────────────────────────────────
 
@@ -75,7 +76,7 @@ export async function getRecentlyChangedFiles(
 
   try {
     const days = Math.max(1, Math.floor(Number(sinceDays)));
-    if (!Number.isFinite(days)) throw new Error("invalid sinceDays");
+    if (!Number.isFinite(days)) throw new GSDError(GSD_PARSE_ERROR, "invalid sinceDays");
 
     // Run all three queries concurrently — they read independent git state
     const [logRaw, stagedRaw, statusRaw] = await Promise.all([
diff --git a/src/resources/extensions/gsd/errors.ts b/src/resources/extensions/gsd/errors.ts
index ac30bb714..f6ee5090f 100644
--- a/src/resources/extensions/gsd/errors.ts
+++ b/src/resources/extensions/gsd/errors.ts
@@ -10,8 +10,6 @@
 
 export const GSD_STALE_STATE = "GSD_STALE_STATE";
 export const GSD_LOCK_HELD = "GSD_LOCK_HELD";
-export const GSD_DISPATCH_FAILED = "GSD_DISPATCH_FAILED";
-export const GSD_TIMEOUT = "GSD_TIMEOUT";
 export const GSD_ARTIFACT_MISSING = "GSD_ARTIFACT_MISSING";
 export const GSD_GIT_ERROR = "GSD_GIT_ERROR";
 export const GSD_MERGE_CONFLICT = "GSD_MERGE_CONFLICT";
diff --git a/src/resources/extensions/gsd/git-service.ts b/src/resources/extensions/gsd/git-service.ts
index 00c65c4dd..cb0e6a5d1 100644
--- a/src/resources/extensions/gsd/git-service.ts
+++ b/src/resources/extensions/gsd/git-service.ts
@@ -29,7 +29,7 @@ import {
   nativeUpdateRef,
   nativeAddPaths,
 } from "./native-git-bridge.js";
-import { GSDError, GSD_MERGE_CONFLICT } from "./errors.js";
+import { GSDError, GSD_MERGE_CONFLICT, GSD_GIT_ERROR } from "./errors.js";
 
 // ─── Types ─────────────────────────────────────────────────────────────────
 
@@ -292,7 +292,7 @@ export function runGit(basePath: string, args: string[], options: { allowFailure
   } catch (error) {
     if (options.allowFailure) return "";
     const message = error instanceof Error ? error.message : String(error);
-    throw new Error(`git ${args.join(" ")} failed in ${basePath}: ${filterGitSvnNoise(message)}`);
+    throw new GSDError(GSD_GIT_ERROR, `git ${args.join(" ")} failed in ${basePath}: ${filterGitSvnNoise(message)}`);
   }
 }
 
diff --git a/src/resources/extensions/gsd/gsd-db.ts b/src/resources/extensions/gsd/gsd-db.ts
index 70c0bbeb1..518f6946f 100644
--- a/src/resources/extensions/gsd/gsd-db.ts
+++ b/src/resources/extensions/gsd/gsd-db.ts
@@ -9,6 +9,7 @@ import { createRequire } from 'node:module';
 import { copyFileSync, existsSync, mkdirSync } from 'node:fs';
 import { dirname } from 'node:path';
 import type { Decision, Requirement } from './types.js';
+import { GSDError, GSD_STALE_STATE } from './errors.js';
 
 // Create a require function for loading native modules in ESM context
 const _require = createRequire(import.meta.url);
@@ -416,7 +417,7 @@ export function closeDatabase(): void {
  * Runs a function inside a transaction. Rolls back on error.
  */
 export function transaction<T>(fn: () => T): T {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.exec('BEGIN');
   try {
     const result = fn();
@@ -434,7 +435,7 @@ export function transaction<T>(fn: () => T): T {
  * Insert a decision. The `seq` field is auto-generated.
  */
 export function insertDecision(d: Omit<Decision, 'seq'>): void {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.prepare(
     `INSERT INTO decisions (id, when_context, scope, decision, choice, rationale, revisable, superseded_by)
      VALUES (:id, :when_context, :scope, :decision, :choice, :rationale, :revisable, :superseded_by)`,
@@ -495,7 +496,7 @@ export function getActiveDecisions(): Decision[] {
  * Insert a requirement.
  */
 export function insertRequirement(r: Requirement): void {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.prepare(
     `INSERT INTO requirements (id, class, status, description, why, source, primary_owner, supporting_slices, validation, notes, full_content, superseded_by)
      VALUES (:id, :class, :status, :description, :why, :source, :primary_owner, :supporting_slices, :validation, :notes, :full_content, :superseded_by)`,
@@ -747,7 +748,7 @@ export function _resetProvider(): void {
  * Insert or replace a decision. Uses the `id` UNIQUE constraint for idempotency.
  */
 export function upsertDecision(d: Omit<Decision, 'seq'>): void {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.prepare(
     `INSERT OR REPLACE INTO decisions (id, when_context, scope, decision, choice, rationale, revisable, superseded_by)
      VALUES (:id, :when_context, :scope, :decision, :choice, :rationale, :revisable, :superseded_by)`,
@@ -767,7 +768,7 @@ export function upsertDecision(d: Omit<Decision, 'seq'>): void {
  * Insert or replace a requirement. Uses the `id` PK for idempotency.
  */
 export function upsertRequirement(r: Requirement): void {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.prepare(
     `INSERT OR REPLACE INTO requirements (id, class, status, description, why, source, primary_owner, supporting_slices, validation, notes, full_content, superseded_by)
      VALUES (:id, :class, :status, :description, :why, :source, :primary_owner, :supporting_slices, :validation, :notes, :full_content, :superseded_by)`,
@@ -813,7 +814,7 @@ export function insertArtifact(a: {
   task_id: string | null;
   full_content: string;
 }): void {
-  if (!currentDb) throw new Error('gsd-db: No database open');
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, 'gsd-db: No database open');
   currentDb.prepare(
     `INSERT OR REPLACE INTO artifacts (path, artifact_type, milestone_id, slice_id, task_id, full_content, imported_at)
      VALUES (:path, :artifact_type, :milestone_id, :slice_id, :task_id, :full_content, :imported_at)`,
diff --git a/src/resources/extensions/gsd/native-git-bridge.ts b/src/resources/extensions/gsd/native-git-bridge.ts
index ee507650b..0ce89662b 100644
--- a/src/resources/extensions/gsd/native-git-bridge.ts
+++ b/src/resources/extensions/gsd/native-git-bridge.ts
@@ -8,6 +8,7 @@
 import { execSync, execFileSync } from "node:child_process";
 import { existsSync, readFileSync, unlinkSync, rmSync } from "node:fs";
 import { join } from "node:path";
+import { GSDError, GSD_GIT_ERROR } from "./errors.js";
 
 /** Env overlay that suppresses interactive git credential prompts and git-svn noise. */
 const GIT_NO_PROMPT_ENV = {
@@ -148,7 +149,7 @@ function gitExec(basePath: string, args: string[], allowFailure = false): string
     }).trim();
   } catch {
     if (allowFailure) return "";
-    throw new Error(`git ${args.join(" ")} failed in ${basePath}`);
+    throw new GSDError(GSD_GIT_ERROR, `git ${args.join(" ")} failed in ${basePath}`);
   }
 }
 
@@ -162,7 +163,7 @@ function gitFileExec(basePath: string, args: string[], allowFailure = false): st
     }).trim();
   } catch {
     if (allowFailure) return "";
-    throw new Error(`git ${args.join(" ")} failed in ${basePath}`);
+    throw new GSDError(GSD_GIT_ERROR, `git ${args.join(" ")} failed in ${basePath}`);
   }
 }
 
diff --git a/src/resources/extensions/gsd/plugin-importer.ts b/src/resources/extensions/gsd/plugin-importer.ts
index 863a936f1..7a8fbe29e 100644
--- a/src/resources/extensions/gsd/plugin-importer.ts
+++ b/src/resources/extensions/gsd/plugin-importer.ts
@@ -19,6 +19,7 @@ import {
 	type MarketplaceDiscoveryResult,
 	type DiscoveredPlugin,
 } from './marketplace-discovery.js';
+import { GSDError, GSD_STALE_STATE } from './errors.js';
 import {
 	NamespacedRegistry,
 	componentsFromDiscovery,
@@ -252,7 +253,7 @@ export class PluginImporter {
 		componentFilter: (component: NamespacedComponent) => boolean
 	): NamespacedComponent[] {
 		if (!this.registry) {
-			throw new Error('Must call discover() before selectComponents()');
+			throw new GSDError(GSD_STALE_STATE, 'Must call discover() before selectComponents()');
 		}
 
 		return this.registry.getAll().filter(componentFilter);
@@ -270,7 +271,7 @@ export class PluginImporter {
 	 */
 	validateImport(selected: NamespacedComponent[]): ValidationResult {
 		if (!this.registry) {
-			throw new Error('Must call discover() before validateImport()');
+			throw new GSDError(GSD_STALE_STATE, 'Must call discover() before validateImport()');
 		}
 
 		// Create a temporary resolver for the selected components
diff --git a/src/resources/extensions/gsd/prompt-loader.ts b/src/resources/extensions/gsd/prompt-loader.ts
index ae3017826..3a4f67b6e 100644
--- a/src/resources/extensions/gsd/prompt-loader.ts
+++ b/src/resources/extensions/gsd/prompt-loader.ts
@@ -18,6 +18,7 @@
  */
 
 import { readFileSync, readdirSync } from "node:fs";
+import { GSDError, GSD_PARSE_ERROR } from "./errors.js";
 import { join, dirname } from "node:path";
 import { fileURLToPath } from "node:url";
 
@@ -87,10 +88,11 @@ export function loadPrompt(name: string, vars: Record<string, string> = {}): str
       .map(m => m.slice(2, -2))
       .filter(key => !(key in vars));
     if (missing.length > 0) {
-      throw new Error(
+      throw new GSDError(
+        GSD_PARSE_ERROR,
         `loadPrompt("${name}"): template declares {{${missing.join("}}, {{")}}}} but no value was provided. ` +
         `This usually means the extension code in memory is older than the template on disk. ` +
-        `Restart pi to reload the extension.`
+        `Restart pi to reload the extension.`,
       );
     }
   }
diff --git a/src/resources/extensions/gsd/worktree-manager.ts b/src/resources/extensions/gsd/worktree-manager.ts
index 1e894e623..191676ccf 100644
--- a/src/resources/extensions/gsd/worktree-manager.ts
+++ b/src/resources/extensions/gsd/worktree-manager.ts
@@ -17,6 +17,7 @@
 
 import { existsSync, mkdirSync, readFileSync, realpathSync } from "node:fs";
 import { join, resolve, sep } from "node:path";
+import { GSDError, GSD_PARSE_ERROR, GSD_STALE_STATE, GSD_LOCK_HELD, GSD_GIT_ERROR, GSD_MERGE_CONFLICT } from "./errors.js";
 import {
   nativeBranchDelete,
   nativeBranchExists,
@@ -121,14 +122,14 @@ export function worktreeBranchName(name: string): string {
 export function createWorktree(basePath: string, name: string, opts: { branch?: string; startPoint?: string; reuseExistingBranch?: boolean } = {}): WorktreeInfo {
   // Validate name: alphanumeric, hyphens, underscores only
   if (!/^[a-zA-Z0-9_-]+$/.test(name)) {
-    throw new Error(`Invalid worktree name "${name}". Use only letters, numbers, hyphens, and underscores.`);
+    throw new GSDError(GSD_PARSE_ERROR, `Invalid worktree name "${name}". Use only letters, numbers, hyphens, and underscores.`);
   }
 
   const wtPath = worktreePath(basePath, name);
   const branch = opts.branch ?? worktreeBranchName(name);
 
   if (existsSync(wtPath)) {
-    throw new Error(`Worktree "${name}" already exists at ${wtPath}`);
+    throw new GSDError(GSD_STALE_STATE, `Worktree "${name}" already exists at ${wtPath}`);
   }
 
   // Ensure the .gsd/worktrees/ directory exists
@@ -151,7 +152,8 @@ export function createWorktree(basePath: string, name: string, opts: { branch?:
     const branchInUse = worktreeEntries.some(entry => entry.branch === branch);
 
     if (branchInUse) {
-      throw new Error(
+      throw new GSDError(
+        GSD_LOCK_HELD,
         `Branch "${branch}" is already in use by another worktree. ` +
         `Remove the existing worktree first with /worktree remove ${name}.`,
       );
@@ -432,12 +434,12 @@ export function mergeWorktreeToMain(basePath: string, name: string, commitMessag
   const current = nativeGetCurrentBranch(basePath);
 
   if (current !== mainBranch) {
-    throw new Error(`Must be on ${mainBranch} to merge. Currently on ${current}.`);
+    throw new GSDError(GSD_GIT_ERROR, `Must be on ${mainBranch} to merge. Currently on ${current}.`);
   }
 
   const result = nativeMergeSquash(basePath, branch);
   if (!result.success) {
-    throw new Error(`Merge conflicts detected in: ${result.conflicts.join(", ")}`);
+    throw new GSDError(GSD_MERGE_CONFLICT, `Merge conflicts detected in: ${result.conflicts.join(", ")}`);
   }
 
   nativeCommit(basePath, commitMessage);