diff --git a/.changeset/2099-eos-copilot.md b/.changeset/2099-eos-copilot.md
new file mode 100644
index 000000000..a08e5dcba
--- /dev/null
+++ b/.changeset/2099-eos-copilot.md
@@ -0,0 +1,6 @@
+---
+type: Changed
+pr: 2172
+---
+
+**GitHub Copilot now wires GSD's full lifecycle hook bus and is driven by its capability descriptor** — installing GSD into Copilot registers `preToolUse`, `postToolUse`, `userPromptSubmitted`, and `sessionEnd` handlers in its `hooks/gsd-session.json` (beyond today's `sessionStart`-only advisory), and Copilot's residual hardcoded runtime branches are folded onto descriptor-driven `hostBehaviors`. (#2099)
diff --git a/bin/install.js b/bin/install.js
index 649bddb22..19cf0ea19 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -6851,7 +6851,9 @@ function uninstall(isGlobal, runtime = DEFAULT_RUNTIME) {
   // consumer, so its former `&& !isKimi` uninstall guards were removed.
   // #2096: isAntigravity dropped — unused in this function.
   // #2098: isCodebuddy dropped — unused in this function.
-  const { isOpencode, isCodex, isCopilot, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
+  // #2099: isCopilot dropped — both Copilot side-effect branches below are now
+  // gated on resolveInstallPlan(runtime).installSurface === 'copilot-instructions'.
+  const { isOpencode, isCodex, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
   const dirName = getDirName(runtime);
 
   // Get the target directory based on runtime and install type. Cline local
@@ -6880,7 +6882,13 @@ function uninstall(isGlobal, runtime = DEFAULT_RUNTIME) {
   // #786: AGENTS.md lives at the repo root (outside targetDir) for local Copilot
   // installs, so its cleanup must run even when .github (targetDir) was already
   // removed — i.e. BEFORE the "target directory missing" early-return below.
-  if (isCopilot && !isGlobal) {
+  // #2099: descriptor-driven via resolveInstallPlan(runtime).installSurface ===
+  // 'copilot-instructions' (was hardcoded `isCopilot`). Mirrors the install-time
+  // gate at the 'copilot-instructions' branch below (~line 10471 equivalent),
+  // which writes this same repo-root AGENTS.md only for local ('!isGlobal')
+  // installs — 'copilot-instructions' is unique to copilot's descriptor, so
+  // this is byte-parity.
+  if (resolveInstallPlan(runtime).installSurface === 'copilot-instructions' && !isGlobal) {
     const agentsMdPath = path.join(process.cwd(), 'AGENTS.md');
     if (fs.existsSync(agentsMdPath)) {
       const content = fs.readFileSync(agentsMdPath, 'utf8');
@@ -7064,7 +7072,10 @@ function uninstall(isGlobal, runtime = DEFAULT_RUNTIME) {
   }
 
   // 1b. Non-layout Copilot side-effect: copilot-instructions.md cleanup
-  if (isCopilot) {
+  // #2099: descriptor-driven via resolveInstallPlan(runtime).installSurface ===
+  // 'copilot-instructions' (was hardcoded `isCopilot`), mirroring the same
+  // gate used at the install-time 'copilot-instructions' branch.
+  if (resolveInstallPlan(runtime).installSurface === 'copilot-instructions') {
     const instructionsPath = path.join(targetDir, 'copilot-instructions.md');
     if (fs.existsSync(instructionsPath)) {
       const content = fs.readFileSync(instructionsPath, 'utf8');
@@ -8223,7 +8234,9 @@ function writeManifest(configDir, runtime = DEFAULT_RUNTIME, options = {}) {
   // settings-json-adjacent runtime, so the `&& !isKimi` term below was removed.
   // #2096: isAntigravity dropped — unused in this function.
   // #2098: isCodebuddy dropped — unused in this function.
-  const { isOpencode, isCodex, isCopilot, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
+  // #2099: isCopilot dropped — was only used in the hooks-tracking conditional
+  // above, now covered by hostBehaviors.skipSharedHooksInstall.
+  const { isOpencode, isCodex, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
   const gsdDir = path.join(configDir, 'gsd-core');
   // #1367: Claude local now writes flat gsd-*.md files at commands/ (not commands/gsd/).
   // Claude local uses flatCommandsDir instead for manifest recording.
@@ -8335,7 +8348,9 @@ function writeManifest(configDir, runtime = DEFAULT_RUNTIME, options = {}) {
   // skipSharedHooksInstall:true) — the redundant `&& !isTrae` was removed.
   // #2095: kimi is now a hooks/ consumer (native config.toml [[hooks]] bus) —
   // the redundant `&& !isKimi` was removed so its hook files are tracked too.
-  if (!isCodex && !isCopilot && _hostBehaviors(runtime).skipSharedHooksInstall !== true && !isWindsurf) {
+  // #2099: Copilot's exclusion is likewise descriptor-driven (copilot declares
+  // skipSharedHooksInstall:true) — the redundant `&& !isCopilot` was removed.
+  if (!isCodex && _hostBehaviors(runtime).skipSharedHooksInstall !== true && !isWindsurf) {
     const hooksDir = path.join(configDir, 'hooks');
     if (fs.existsSync(hooksDir)) {
       // Drive from INSTALLED_HOOK_FILES (the canonical HOOKS_TO_COPY set from
@@ -8738,7 +8753,14 @@ function install(isGlobal, runtime = DEFAULT_RUNTIME, options = {}) {
   // _DESCRIPTOR_AGENTS_RUNTIMES below, so its legacy converter-dispatch branch
   // (the `isCodebuddy` arm calling convertClaudeAgentToCodebuddyAgent) was
   // unreachable dead code and was removed rather than re-gated.
-  const { isOpencode, isZcode, isCodex, isCopilot, isCursor, isWindsurf, isAugment, isTrae, isQwen, isHermes, isCline } = runtimeFlags(runtime);
+  // #2099: isCopilot dropped — copilot is also in _DESCRIPTOR_AGENTS_RUNTIMES
+  // below, so its three legacy-agent-loop branches (the path-rewrite skip,
+  // the converter dispatch, and the .agent.md destName ternary) were
+  // unreachable dead code and were removed rather than re-gated; the
+  // .agent.md suffix now lives on hostBehaviors.agentFileExtension in
+  // src/install-engine.cts, and the skipSharedHooksInstall check above no
+  // longer needs `&& !isCopilot`.
+  const { isOpencode, isZcode, isCodex, isCursor, isWindsurf, isAugment, isTrae, isQwen, isHermes, isCline } = runtimeFlags(runtime);
   const plan = resolveInstallPlan(runtime);
   const dirName = getDirName(runtime);
   const src = path.join(__dirname, '..');
@@ -9608,12 +9630,14 @@ function install(isGlobal, runtime = DEFAULT_RUNTIME, options = {}) {
         // _DESCRIPTOR_AGENTS_RUNTIMES above, so this whole branch is already
         // unreachable for it; the path-rewrite skip for antigravity now lives
         // in the descriptor-driven `applyAgentPathRewrites` (hostBehaviors.noPathRewrite).
-        if (!isCopilot) {
-          content = content.replace(dirRegex, pathPrefix);
-          content = content.replace(homeDirRegex, pathPrefix);
-          content = content.replace(bareDirRegex, normalizedPathPrefix);
-          content = content.replace(bareHomeDirRegex, normalizedPathPrefix);
-        }
+        // #2099: `if (!isCopilot)` guard dropped — copilot is ALSO in
+        // _DESCRIPTOR_AGENTS_RUNTIMES (line ~9564 above), so this whole
+        // `else if (fs.existsSync(agentsSrc))` branch is unreachable for it;
+        // isCopilot was therefore always false here, making the guard a no-op.
+        content = content.replace(dirRegex, pathPrefix);
+        content = content.replace(homeDirRegex, pathPrefix);
+        content = content.replace(bareDirRegex, normalizedPathPrefix);
+        content = content.replace(bareHomeDirRegex, normalizedPathPrefix);
         content = processAttribution(content, getCommitAttribution(runtime));
         // Convert frontmatter for runtime compatibility (agents need different handling)
         if (_hostBehaviors(runtime).frontmatterDialect === 'opencode') {
@@ -9657,8 +9681,11 @@ function install(isGlobal, runtime = DEFAULT_RUNTIME, options = {}) {
           content = convertClaudeToKiloFrontmatter(content, { isAgent: true, modelOverride: _kiloModelOverride });
         } else if (_hostBehaviors(runtime).frontmatterDialect === 'codex') {
           content = convertClaudeAgentToCodexAgent(content);
-        } else if (isCopilot) {
-          content = convertClaudeAgentToCopilotAgent(content, isGlobal);
+        // #2099: `else if (isCopilot)` arm dropped — copilot is unreachable
+        // here (see the isCopilot-guard-drop comment above); its content
+        // conversion is applied pre-staging via the descriptor's
+        // artifactLayout.converter (runtime-artifact-layout.cts), independent
+        // of this legacy loop.
         } else if (isWindsurf) {
           content = convertClaudeAgentToWindsurfAgent(content);
         } else if (_hostBehaviors(runtime).frontmatterDialect === 'cline') {
@@ -9698,7 +9725,12 @@ function install(isGlobal, runtime = DEFAULT_RUNTIME, options = {}) {
         // shouldNormalizeHyphenNamespaceInAgentBody above. Mirrors the
         // SKILL.md-body fix shipped via #3629.
         content = normalizeAgentBodyForRuntime(content, runtime, readGsdCommandNames());
-        const destName = isCopilot ? entry.name.replace('.md', '.agent.md') : entry.name;
+        // #2099: `isCopilot ? ... : entry.name` ternary dropped — copilot is
+        // unreachable here (see the isCopilot-guard-drop comment above), so
+        // the ternary always evaluated to entry.name in practice; its
+        // .agent.md suffix is applied by the descriptor-driven fold in
+        // src/install-engine.cts (hostBehaviors.agentFileExtension).
+        const destName = entry.name;
         fs.writeFileSync(path.join(agentsDest, destName), content);
       }
     }
@@ -9886,7 +9918,9 @@ function install(isGlobal, runtime = DEFAULT_RUNTIME, options = {}) {
   // resolveKimiHooksTomlDir) via installSharedHooksBundle, at the
   // kimi-hooks-toml branch further below — never under the generic
   // Agent-Skills configDir GSD installs skills/agents into for kimi.
-  if (!isCodex && !isCopilot && _hostBehaviors(runtime).skipSharedHooksInstall !== true && !isWindsurf && !isZcode) {
+  // #2099: Copilot's exclusion is likewise descriptor-driven (copilot declares
+  // skipSharedHooksInstall:true) — the redundant `&& !isCopilot` was removed.
+  if (!isCodex && _hostBehaviors(runtime).skipSharedHooksInstall !== true && !isWindsurf && !isZcode) {
     if (!installSharedHooksBundle(targetDir)) {
       failures.push('hooks');
     }
@@ -10869,7 +10903,8 @@ function finishInstall(settingsPath, settings, statuslineCommand, shouldInstallS
   // _hostBehaviors(runtime).doneBannerStyle === 'kimi-agent-file' (descriptor-driven), not this flag.
   // #2096: isAntigravity dropped — unused in this function.
   // #2098: isCodebuddy dropped — unused in this function.
-  const { isOpencode, isCodex, isCopilot, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
+  // #2099: isCopilot dropped — unused in this function.
+  const { isOpencode, isCodex, isCursor, isWindsurf, isAugment, isQwen, isHermes, isCline } = runtimeFlags(runtime);
   const plan = resolveInstallPlan(runtime);
 
   if (shouldInstallStatusline && plan.writesSharedSettings && !_hostBehaviors(runtime).skipSettingsUi) {
diff --git a/capabilities/copilot/capability.json b/capabilities/copilot/capability.json
index ad6f58435..2a1360752 100644
--- a/capabilities/copilot/capability.json
+++ b/capabilities/copilot/capability.json
@@ -84,7 +84,10 @@
       "runtime": "undocumented"
     },
     "hostBehaviors": {
-      "reapplyCommand": "/gsd-update --reapply"
+      "reapplyCommand": "/gsd-update --reapply",
+      "agentFileExtension": ".agent.md",
+      "skipSharedHooksInstall": true,
+      "noPathRewrite": true
     }
   }
 }
diff --git a/docs/how-to/add-or-update-a-host-integration.md b/docs/how-to/add-or-update-a-host-integration.md
index 0c279e70d..5acf1e3e1 100644
--- a/docs/how-to/add-or-update-a-host-integration.md
+++ b/docs/how-to/add-or-update-a-host-integration.md
@@ -147,6 +147,19 @@ yields the same truth value**, so behavior is unchanged and only the brittle cou
    the host correctly, negotiation fails closed on a corrupted descriptor, and — with a source-grep behind
    an `// allow-test-rule:` exemption — that **no `runtime === '<id>'` branch remains** in `bin/install.js`.
 
+**Another completed worked example: `copilot` (#2099).** Copilot was already installing through the
+declarative artifactLayout (not the direct `installRuntimeArtifacts` calls step 4 describes), so its
+migration folded the *residual* hardcoded branches rather than the whole install path: the `.agent.md`
+destination-suffix rename in `src/install-engine.cts` (→ `hostBehaviors.agentFileExtension`), two
+uninstall side-effect branches in `bin/install.js` (→
+`resolveInstallPlan(runtime).installSurface === 'copilot-instructions'`, already a live descriptor field
+elsewhere in the same file), and two `skipSharedHooksInstall` gates (→
+`hostBehaviors.skipSharedHooksInstall: true`). A dead legacy agent-converter dispatch arm — unreachable
+because copilot is a member of `_DESCRIPTOR_AGENTS_RUNTIMES` — was deleted outright rather than re-gated,
+mirroring step 6's guard: `tests/declarative-reference-copilot.test.cjs` source-greps both files for the
+retired `isCopilot` reads. See the `copilot` section of the reference matrix for the full EoS migration
+note, including the two upgrades (multi-event hook bus; negotiated `dispatch.background`) this PR adds.
+
 ---
 
 ## Related
diff --git a/docs/reference/host-integration-capability-matrix.md b/docs/reference/host-integration-capability-matrix.md
index 375c7e00f..38775fec1 100644
--- a/docs/reference/host-integration-capability-matrix.md
+++ b/docs/reference/host-integration-capability-matrix.md
@@ -461,6 +461,8 @@ Documentation gaps:
 - runtime — docs describe the CLI binary and the SDK (Node.js/Go/Python/Rust) but do not state what runtime the CLI host itself or its plugin/extension loader executes in.
 - dispatch.nested exact authoritative source is awesome-copilot.github.com (community docs) not docs.github.com.
 
+**EoS migration status (#2099):** Migrated onto the declarative adapter (dogfooded in `tests/declarative-reference-copilot.test.cjs`). The residual `isCopilot` branches were folded onto descriptor-driven `runtime.hostBehaviors`: the `.agent.md` destination-suffix rename in `src/install-engine.cts` now reads `hostBehaviors.agentFileExtension`; `bin/install.js`'s two uninstall side-effect branches (repo-root `AGENTS.md` cleanup, `copilot-instructions.md`/hook cleanup) now gate on `resolveInstallPlan(runtime).installSurface === 'copilot-instructions'` (unique to copilot, so byte-identical); and the two `skipSharedHooksInstall` checks now read `hostBehaviors.skipSharedHooksInstall:true` (copilot's golden has only `hooks/gsd-session.json`, no shared `gsd-*.js` scripts). A dead legacy agent-converter dispatch arm in the inline agent-copy loop — unreachable since copilot is a member of `_DESCRIPTOR_AGENTS_RUNTIMES` — was removed outright; `isCopilot` no longer appears as a live read anywhere in `bin/install.js` or `src/install-engine.cts`. Two upgrades land: (1) **multi-event hook bus** — `buildCopilotHookConfig()` previously emitted only `sessionStart`; this PR wires four additional events — `preToolUse`/`postToolUse`/`userPromptSubmitted`/`sessionEnd` — each a static, deterministic advisory command (no node-runner invocation), so an install's `hooks/gsd-session.json` now registers all five events. (2) **`dispatch.background`** — the descriptor already declared `true`, exceeding the `declarative-cli` profile baseline of `false`; the negotiation contract (`negotiateHostCapabilities`) surfaces that value with no downgrade warning, documenting the legitimate deviation. Note: Copilot's `.agent.md` frontmatter has no background-dispatch field (fields are `description`/`infer`/`mcp-servers`/`model`/`name`/`tools`) — background dispatch remains a negotiated-contract-only axis, not a field GSD's agent artifacts emit. MCP companion tooling is out of scope for this migration (AC4 names only the two upgrades above).
+
 ---
 
 ## kilo
diff --git a/gsd-core/bin/lib/capability-registry.cjs b/gsd-core/bin/lib/capability-registry.cjs
index 439e4e193..eb3df8545 100644
--- a/gsd-core/bin/lib/capability-registry.cjs
+++ b/gsd-core/bin/lib/capability-registry.cjs
@@ -987,7 +987,10 @@ const capabilities = {
         "runtime": "undocumented"
       },
       "hostBehaviors": {
-        "reapplyCommand": "/gsd-update --reapply"
+        "reapplyCommand": "/gsd-update --reapply",
+        "agentFileExtension": ".agent.md",
+        "skipSharedHooksInstall": true,
+        "noPathRewrite": true
       }
     }
   },
@@ -4444,7 +4447,10 @@ const runtimes = {
         "runtime": "undocumented"
       },
       "hostBehaviors": {
-        "reapplyCommand": "/gsd-update --reapply"
+        "reapplyCommand": "/gsd-update --reapply",
+        "agentFileExtension": ".agent.md",
+        "skipSharedHooksInstall": true,
+        "noPathRewrite": true
       }
     }
   },
diff --git a/src/install-engine.cts b/src/install-engine.cts
index b9c779d24..d991f5997 100644
--- a/src/install-engine.cts
+++ b/src/install-engine.cts
@@ -342,9 +342,13 @@ function _copyStaged(stagedDir: string, destDir: string, kind: any, configDir: s
     let destName: string;
     if (kind.kind === 'agents') {
       // Agent files already carry the gsd- prefix in the source dir.
-      // #1575: copilot agents get .agent.md suffix (mirrors inline loop line ~9118).
-      destName = runtime === 'copilot'
-        ? entry.name.replace(/\.md$/, '.agent.md')
+      // #2099: descriptor-driven via hostBehaviors.agentFileExtension (was
+      // hardcoded `runtime === 'copilot'`). copilot declares '.agent.md';
+      // every other runtime's descriptor leaves this unset, so destName falls
+      // back to entry.name unchanged (byte-parity, #1575 origin comment).
+      const _agentExt = runtime ? _hostBehaviors(runtime).agentFileExtension : undefined;
+      destName = _agentExt
+        ? entry.name.replace(/\.md$/, _agentExt)
         : entry.name;
     } else if (namespacedByDir) {
       // Directory is the namespace; don't double-prefix the filename
diff --git a/src/runtime-artifact-conversion.cts b/src/runtime-artifact-conversion.cts
index 0a235a09b..79454dba8 100644
--- a/src/runtime-artifact-conversion.cts
+++ b/src/runtime-artifact-conversion.cts
@@ -83,6 +83,21 @@ function _hostBehaviors(runtime: string): Record<string, unknown> {
   );
 }
 
+/**
+ * Public accessor for the `hostBehaviors.agentFileExtension` descriptor field
+ * (ADR-1239 / #2099 / #2103). Returns the runtime's declared agent-file
+ * destination-suffix rename target (e.g. copilot's `.agent.md`), or
+ * `undefined` when the runtime declares none (the generic no-rename
+ * default). Exported so callers outside this module (surface.cts's
+ * `_syncGsdDir`) can derive the SAME rename decision as
+ * install-engine.cts's staged-copy loop from ONE descriptor read, instead of
+ * duplicating a hardcoded `runtime === 'copilot'` check (#2103 fold).
+ */
+function agentFileExtensionFor(runtime: string): string | undefined {
+  const ext = _hostBehaviors(runtime).agentFileExtension;
+  return typeof ext === 'string' ? ext : undefined;
+}
+
 
 const colorNameToHex = {
   cyan: '#00FFFF',
@@ -2756,10 +2771,12 @@ function normalizeAgentBodyForRuntime(content: string, runtime: string, cmdNames
  *   ~/\.claude\b → normalizedPathPrefix
  *   $HOME/\.claude\b → normalizedPathPrefix
  *
- * Skipped for copilot (hardcoded — #2099 will fold it) and for any runtime
- * that declares `hostBehaviors.noPathRewrite` (descriptor-driven, ADR-1239 /
- * #2096 — folds the prior hardcoded `runtime === 'antigravity'` literal;
- * Antigravity does NOT do path rewrites in the inline loop). NO stamp
+ * Skipped for any runtime that declares `hostBehaviors.noPathRewrite`
+ * (descriptor-driven, ADR-1239 / #2096 — folds the prior hardcoded
+ * `runtime === 'antigravity'` literal; Antigravity does NOT do path rewrites
+ * in the inline loop / #2103 — folds the prior hardcoded
+ * `runtime === 'copilot'` literal onto the same descriptor field, since
+ * copilot also skips these rewrites). NO stamp
  * (_stampNonClaudeRuntimeDefaults) — agents are NOT stamped in the inline loop.
  *
  * ADR-1235 §1: pre-converter cross-cutting for descriptor-driven agent pipeline.
@@ -2769,10 +2786,10 @@ function normalizeAgentBodyForRuntime(content: string, runtime: string, cmdNames
  * @param content     raw agent file content
  * @param runtime     canonical runtime ID
  * @param pathPrefix  trailing-slash path prefix (e.g. '$HOME/.cursor/')
- * @returns content with path-prefix rewrites applied (or unchanged for copilot / noPathRewrite runtimes)
+ * @returns content with path-prefix rewrites applied (or unchanged for noPathRewrite runtimes, e.g. copilot)
  */
 function applyAgentPathRewrites(content: string, runtime: string, pathPrefix: string): string {
-  if (runtime === 'copilot' || _hostBehaviors(runtime).noPathRewrite === true) return content;
+  if (_hostBehaviors(runtime).noPathRewrite === true) return content;
   const normalizedPathPrefix = pathPrefix.replace(/\/$/, '');
   content = content.replace(/~\/\.claude\//g, pathPrefix);
   content = content.replace(/\$HOME\/\.claude\//g, pathPrefix);
@@ -2813,6 +2830,11 @@ function processAttribution(
 
 export = {
   processAttribution,
+  // #2103: public accessor for hostBehaviors.agentFileExtension, exported so
+  // surface.cts's _syncGsdDir can derive the .agent.md rename from the SAME
+  // descriptor read as install-engine.cts (folds a duplicated hardcoded
+  // `runtime === 'copilot'` literal).
+  agentFileExtensionFor,
   yamlIdentifier,
   yamlQuote,
   toSingleLine,
diff --git a/src/runtime-hooks-surface.cts b/src/runtime-hooks-surface.cts
index 88d92f2f0..1338c3fcd 100644
--- a/src/runtime-hooks-surface.cts
+++ b/src/runtime-hooks-surface.cts
@@ -84,6 +84,39 @@ const GSD_COPILOT_SESSION_HOOK_PWSH =
   `{ '{"additionalContext":"${GSD_COPILOT_SESSION_MSG_PRESENT}"}' } ` +
   `else { '{"additionalContext":"${GSD_COPILOT_SESSION_MSG_ABSENT}"}' }`;
 
+// #2099 UPGRADE 1: multi-event hook bus. Each additional event is a static,
+// deterministic advisory (no branching/no-op-style, matching sessionStart's
+// tone) so the emitted hooks/gsd-session.json stays golden-trackable — no
+// node-runner invocation, no filesystem probing beyond what sessionStart
+// already does.
+const GSD_COPILOT_PRE_TOOL_MSG =
+  'GSD: confirm this tool use is in scope for the active phase before proceeding.';
+const GSD_COPILOT_PRE_TOOL_HOOK_BASH =
+  `printf '%s' '{"additionalContext":"${GSD_COPILOT_PRE_TOOL_MSG}"}'`;
+const GSD_COPILOT_PRE_TOOL_HOOK_PWSH =
+  `'{"additionalContext":"${GSD_COPILOT_PRE_TOOL_MSG}"}'`;
+
+const GSD_COPILOT_POST_TOOL_MSG =
+  'GSD: review the tool result against the active phase before continuing.';
+const GSD_COPILOT_POST_TOOL_HOOK_BASH =
+  `printf '%s' '{"additionalContext":"${GSD_COPILOT_POST_TOOL_MSG}"}'`;
+const GSD_COPILOT_POST_TOOL_HOOK_PWSH =
+  `'{"additionalContext":"${GSD_COPILOT_POST_TOOL_MSG}"}'`;
+
+const GSD_COPILOT_PROMPT_SUBMIT_MSG =
+  'GSD: check this request against .planning/STATE.md scope before acting.';
+const GSD_COPILOT_PROMPT_SUBMIT_HOOK_BASH =
+  `printf '%s' '{"additionalContext":"${GSD_COPILOT_PROMPT_SUBMIT_MSG}"}'`;
+const GSD_COPILOT_PROMPT_SUBMIT_HOOK_PWSH =
+  `'{"additionalContext":"${GSD_COPILOT_PROMPT_SUBMIT_MSG}"}'`;
+
+const GSD_COPILOT_SESSION_END_MSG =
+  'GSD: update .planning/STATE.md with the session outcome before ending.';
+const GSD_COPILOT_SESSION_END_HOOK_BASH =
+  `printf '%s' '{"additionalContext":"${GSD_COPILOT_SESSION_END_MSG}"}'`;
+const GSD_COPILOT_SESSION_END_HOOK_PWSH =
+  `'{"additionalContext":"${GSD_COPILOT_SESSION_END_MSG}"}'`;
+
 // ---------------------------------------------------------------------------
 // Cursor hook constants
 // ---------------------------------------------------------------------------
@@ -1147,6 +1180,41 @@ function buildCopilotHookConfig(): Record<string, unknown> {
           timeoutSec: 10,
         },
       ],
+      // #2099 UPGRADE 1: multi-event hook bus — preToolUse (worktree/read-safety
+      // advisory), postToolUse (context-monitor advisory), userPromptSubmitted
+      // (prompt-guard advisory), sessionEnd (session-finalize advisory).
+      preToolUse: [
+        {
+          type: 'command',
+          bash: GSD_COPILOT_PRE_TOOL_HOOK_BASH,
+          powershell: GSD_COPILOT_PRE_TOOL_HOOK_PWSH,
+          timeoutSec: 10,
+        },
+      ],
+      postToolUse: [
+        {
+          type: 'command',
+          bash: GSD_COPILOT_POST_TOOL_HOOK_BASH,
+          powershell: GSD_COPILOT_POST_TOOL_HOOK_PWSH,
+          timeoutSec: 10,
+        },
+      ],
+      userPromptSubmitted: [
+        {
+          type: 'command',
+          bash: GSD_COPILOT_PROMPT_SUBMIT_HOOK_BASH,
+          powershell: GSD_COPILOT_PROMPT_SUBMIT_HOOK_PWSH,
+          timeoutSec: 10,
+        },
+      ],
+      sessionEnd: [
+        {
+          type: 'command',
+          bash: GSD_COPILOT_SESSION_END_HOOK_BASH,
+          powershell: GSD_COPILOT_SESSION_END_HOOK_PWSH,
+          timeoutSec: 10,
+        },
+      ],
     },
   };
 }
diff --git a/src/surface.cts b/src/surface.cts
index 263b773b8..63f536b52 100644
--- a/src/surface.cts
+++ b/src/surface.cts
@@ -521,10 +521,14 @@ function _syncGsdDir(stagedDir: string, destDir: string, kind: ArtifactKind | st
   const kindName = (typeof kind === 'string') ? kind : kind.kind;
   const kindPrefix = (typeof kind === 'object' && kind !== null) ? kind.prefix : 'gsd-';
 
-  // #1575: copilot agents are renamed .md -> .agent.md at copy time, mirroring
-  // the inline agent loop in bin/install.js (line ~9118). Other runtimes keep
-  // the staged filename verbatim.
-  const isCopilotAgents = runtime === 'copilot' && kindName === 'agents';
+  // #1575 / #2103: agent files are renamed .md -> <agentFileExtension> at copy
+  // time when the runtime's descriptor declares hostBehaviors.agentFileExtension
+  // (e.g. copilot's '.agent.md'), mirroring install-engine.cts's staged-copy
+  // loop (`_copyStaged`) — ONE descriptor read shared by both surfaces instead
+  // of a duplicated hardcoded `runtime === 'copilot'` literal. Other runtimes
+  // (no agentFileExtension declared) keep the staged filename verbatim.
+  const _agentExt = runtime ? runtimeArtifactConversion.agentFileExtensionFor(runtime) : undefined;
+  const isRenamedAgents = !!_agentExt && kindName === 'agents';
 
   if (kindName === 'skills') {
     // Skills kind: work with directories, not files.
@@ -564,8 +568,8 @@ function _syncGsdDir(stagedDir: string, destDir: string, kind: ArtifactKind | st
     const stagedFiles = fs.readdirSync(stagedDir).filter(f => f.endsWith('.md'));
     const stagedDestNames = new Set<string>();
     for (const file of stagedFiles) {
-      const destName = isCopilotAgents
-        ? file.replace(/\.md$/, '.agent.md')
+      const destName = isRenamedAgents
+        ? file.replace(/\.md$/, _agentExt)
         : (kindName === 'agents' || namespacedByDir)
           ? file
           : `${kindPrefix}${file.slice(0, -3)}.md`;
diff --git a/tests/copilot-upgrades.test.cjs b/tests/copilot-upgrades.test.cjs
new file mode 100644
index 000000000..065f8a3eb
--- /dev/null
+++ b/tests/copilot-upgrades.test.cjs
@@ -0,0 +1,149 @@
+'use strict';
+
+/**
+ * Copilot capability UPGRADES — ADR-1239 / #2099 (EoS/copilot).
+ *
+ * Drives the user-reachable surface (spawned `bin/install.js` via
+ * `runMinimalInstall`) plus a direct negotiation-contract check to prove the
+ * two real upgrades Copilot contributes as part of the EoS migration:
+ *
+ *   UPGRADE 1 — multi-event hook bus: buildCopilotHookConfig() previously
+ *   emitted only `sessionStart`. This PR wires four additional events —
+ *   `preToolUse`, `postToolUse`, `userPromptSubmitted`, `sessionEnd` — each a
+ *   static, deterministic advisory command (no node-runner invocation), so a
+ *   live install's hooks/gsd-session.json registers all five events.
+ *
+ *   UPGRADE 2 — dispatch.background: Copilot's capability.json already
+ *   declares `dispatch.background: true`, which legitimately EXCEEDS the
+ *   `declarative-cli` profile baseline (`false`, per
+ *   `PROFILE_BASELINES['declarative-cli']` in src/host-integration.cts).
+ *   NO agent-file/frontmatter change is involved — Copilot's .agent.md
+ *   frontmatter has no background-dispatch field (fields are
+ *   description/infer/mcp-servers/model/name/tools). This is surfaced
+ *   purely through the negotiated contract: a documented `true` value must
+ *   survive negotiation without a downgrade warning.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const { runMinimalInstall } = require('./helpers/install-shared.cjs');
+const { cleanup } = require('./helpers.cjs');
+const {
+  profileOf,
+  negotiateHostCapabilities,
+  PROFILE_BASELINES,
+} = require('../gsd-core/bin/lib/host-integration.cjs');
+
+const COPILOT_CAP = JSON.parse(
+  fs.readFileSync(path.join(__dirname, '..', 'capabilities', 'copilot', 'capability.json'), 'utf8'),
+);
+const COPILOT_AXES = COPILOT_CAP.runtime.hostIntegration;
+
+// ---------------------------------------------------------------------------
+// UPGRADE 1: multi-event hook bus — all 4 newly-wired events' live-install
+// coverage, on top of the pre-existing sessionStart.
+// ---------------------------------------------------------------------------
+
+const NEWLY_WIRED_EVENTS = ['preToolUse', 'postToolUse', 'userPromptSubmitted', 'sessionEnd'];
+const ALL_EXPECTED_EVENTS = ['sessionStart', ...NEWLY_WIRED_EVENTS];
+
+test('copilot --global: hooks/gsd-session.json wires sessionStart plus all 4 newly-added events (UPGRADE 1)', (t) => {
+  const { configDir, root } = runMinimalInstall({ runtime: 'copilot', scope: 'global' });
+  t.after(() => cleanup(root));
+
+  const hookPath = path.join(configDir, 'hooks', 'gsd-session.json');
+  assert.ok(fs.existsSync(hookPath), `${hookPath} must exist`);
+  const parsed = JSON.parse(fs.readFileSync(hookPath, 'utf8'));
+
+  assert.strictEqual(parsed.version, 1, 'hook config version must be 1');
+  assert.ok(parsed.hooks && typeof parsed.hooks === 'object', 'has hooks object');
+
+  for (const eventName of ALL_EXPECTED_EVENTS) {
+    const eventHooks = parsed.hooks[eventName];
+    assert.ok(Array.isArray(eventHooks) && eventHooks.length > 0,
+      `hooks.${eventName} must exist and be non-empty`);
+    const entry = eventHooks[0];
+    assert.strictEqual(entry.type, 'command', `${eventName} entry type must be 'command'`);
+    assert.ok(typeof entry.bash === 'string' && entry.bash.length > 0,
+      `${eventName} entry must have a non-empty inline bash body`);
+    assert.ok(typeof entry.powershell === 'string' && entry.powershell.length > 0,
+      `${eventName} entry must have a non-empty inline powershell body`);
+    assert.strictEqual(entry.timeoutSec, 10, `${eventName} entry must use timeoutSec 10`);
+  }
+});
+
+test('each newly-wired event emits a distinct static advisory (no shared boilerplate, no node-runner invocation)', (t) => {
+  const { configDir, root } = runMinimalInstall({ runtime: 'copilot', scope: 'global' });
+  t.after(() => cleanup(root));
+
+  const hookPath = path.join(configDir, 'hooks', 'gsd-session.json');
+  const parsed = JSON.parse(fs.readFileSync(hookPath, 'utf8'));
+
+  const bashBodies = new Set();
+  for (const eventName of ALL_EXPECTED_EVENTS) {
+    const entry = parsed.hooks[eventName][0];
+    assert.ok(entry.bash.includes('"additionalContext"'),
+      `${eventName} bash body must emit the additionalContext JSON envelope`);
+    assert.ok(!/hooks\/gsd-[\w-]+\.(js|cjs|sh)/.test(entry.bash),
+      `${eventName} bash body must not reference an external hook script (cannot dangle)`);
+    bashBodies.add(entry.bash);
+  }
+  assert.strictEqual(bashBodies.size, ALL_EXPECTED_EVENTS.length,
+    'each event must emit its own distinct advisory command, not a copy-pasted duplicate');
+});
+
+test('a runtime whose hook config omits the new events does NOT get them (descriptor-gated, not a global default)', (t) => {
+  const { configDir, root } = runMinimalInstall({ runtime: 'claude', scope: 'global' });
+  t.after(() => cleanup(root));
+
+  // Claude uses settings.json, not hooks/gsd-session.json — the Copilot
+  // multi-event bus additions are self-contained to buildCopilotHookConfig
+  // and must not leak into another runtime's hook surface.
+  const copilotHookPath = path.join(configDir, 'hooks', 'gsd-session.json');
+  assert.ok(!fs.existsSync(copilotHookPath),
+    'claude must not have a Copilot-style hooks/gsd-session.json file');
+});
+
+// ---------------------------------------------------------------------------
+// UPGRADE 2: dispatch.background negotiation — documented true survives,
+// exceeding the declarative-cli profile baseline. No agent-file/frontmatter
+// change; negotiated-contract only (Copilot .agent.md has no background field).
+// ---------------------------------------------------------------------------
+
+test("copilot classifies as the 'declarative-cli' profile, whose baseline dispatch.background is false", () => {
+  assert.equal(profileOf(COPILOT_AXES), 'declarative-cli');
+  assert.equal(PROFILE_BASELINES['declarative-cli'].dispatch.background, false,
+    'sanity: the declarative-cli baseline is false — copilot legitimately exceeds it');
+});
+
+test('negotiateHostCapabilities surfaces copilot\'s documented dispatch.background:true with no downgrade warning (UPGRADE 2)', () => {
+  assert.equal(COPILOT_AXES.dispatch.background, true,
+    'sanity: the descriptor declares dispatch.background: true (documented, not undocumented)');
+
+  const { effective, warnings } = negotiateHostCapabilities(COPILOT_AXES);
+
+  assert.equal(effective.dispatch.background, true,
+    'a documented true value must survive negotiation, exceeding the declarative-cli baseline of false');
+  assert.ok(
+    !warnings.some((w) => w.includes('dispatch.background')),
+    `no warning may be raised for the documented dispatch.background axis, got: ${JSON.stringify(warnings)}`,
+  );
+});
+
+test('copilot .agent.md frontmatter has no background-dispatch field (negotiated contract only, sanity)', (t) => {
+  const { configDir, root } = runMinimalInstall({ runtime: 'copilot', scope: 'global' });
+  t.after(() => cleanup(root));
+
+  const agentsDir = path.join(configDir, 'agents');
+  const agentFiles = fs.readdirSync(agentsDir).filter((f) => f.endsWith('.agent.md'));
+  assert.ok(agentFiles.length > 0, 'at least one .agent.md must be installed');
+
+  const sample = fs.readFileSync(path.join(agentsDir, agentFiles[0]), 'utf8');
+  const frontmatterMatch = sample.match(/^---\r?\n([\s\S]*?)\r?\n---/);
+  assert.ok(frontmatterMatch, 'agent file must have a frontmatter block');
+  assert.ok(!/^background:/m.test(frontmatterMatch[1]),
+    'UPGRADE 2 must not add a background: frontmatter field — Copilot .agent.md has no such field');
+});
diff --git a/tests/declarative-reference-copilot.test.cjs b/tests/declarative-reference-copilot.test.cjs
new file mode 100644
index 000000000..91bc9ffc3
--- /dev/null
+++ b/tests/declarative-reference-copilot.test.cjs
@@ -0,0 +1,164 @@
+// allow-test-rule: structural-regression-guard — AC2: assert no `runtime === 'copilot'` string-equality branch, no live `isCopilot` read remains in bin/install.js, src/install-engine.cts, src/surface.cts, or src/runtime-artifact-conversion.cts — a source-text property, so source-grep is the faithful check (#2099, expanded #2103)
+'use strict';
+
+/**
+ * Declarative reference host — GitHub Copilot (#2099 / ADR-1239 EoS).
+ *
+ * Copilot already installs through the descriptor-driven artifactLayout
+ * (skills/agents, each with a named `converter`), and its capability.json
+ * already declares `hostIntegration` + `dispatch` axes. Issue #2099's
+ * literal premises about a "legacy exclusion list" / writesSharedSettings
+ * mismatch and a hardcoded RUNTIME_CONTENT_DISPATCH branch were WRONG (see
+ * the deep-research writeup on the issue) — this migration instead folds
+ * the real residual `isCopilot` branches:
+ *   1. src/install-engine.cts's `.agent.md` destination-suffix rename —
+ *      folded onto `hostBehaviors.agentFileExtension`.
+ *   2. bin/install.js uninstall's two Copilot side-effect branches
+ *      (repo-root AGENTS.md cleanup + copilot-instructions.md/hook
+ *      cleanup) — folded onto `resolveInstallPlan(runtime).installSurface
+ *      === 'copilot-instructions'` (unique to copilot, so byte-parity).
+ *   3. bin/install.js's two `skipSharedHooksInstall` checks — folded onto
+ *      `hostBehaviors.skipSharedHooksInstall:true` (copilot's golden has
+ *      only hooks/gsd-session.json, no shared gsd-*.js scripts).
+ *   4. bin/install.js's dead legacy agent-converter dispatch arm in the
+ *      inline agent-copy loop — unreachable because copilot is a member of
+ *      `_DESCRIPTOR_AGENTS_RUNTIMES` (installRuntimeArtifacts already wrote
+ *      the agents before that loop runs) — deleted outright.
+ *
+ * This test is the reference-host dogfood mirroring
+ * tests/declarative-reference-codebuddy.test.cjs: it (1) classifies
+ * Copilot's profile via profileOf, (2) confirms the public declarative
+ * adapter classifies it as declarative, (3) round-trips a real install
+ * proving a gsd agent/skill surface is emitted, (4) proves negotiation
+ * fails CLOSED on a corrupted descriptor, (5) proves the validator accepts
+ * the descriptor, and (6) source-greps the folded modules for the retired
+ * `isCopilot` branches (AC2).
+ *
+ * UPGRADE 1 (multi-event hook bus) + UPGRADE 2 (dispatch.background
+ * negotiation) live-install coverage is in tests/copilot-upgrades.test.cjs
+ * — not duplicated here.
+ */
+
+const { test, before } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const { execFileSync } = require('node:child_process');
+
+const {
+  profileOf,
+  negotiateHostCapabilities,
+  PROFILE_BASELINES,
+  UNDOCUMENTED,
+} = require('../gsd-core/bin/lib/host-integration.cjs');
+const { validateCapability } = require('../gsd-core/bin/lib/capability-validator.cjs');
+const { createDeclarativeAdapter } = require('../gsd-core/bin/lib/adapter-declarative.cjs');
+const { cleanup } = require('./helpers.cjs');
+const { walk, runMinimalInstall, BUILD_SCRIPT } = require('./helpers/install-shared.cjs');
+
+const DESC = path.join(__dirname, '..', 'capabilities', 'copilot', 'capability.json');
+const COPILOT_CAP = JSON.parse(fs.readFileSync(DESC, 'utf8'));
+const COPILOT_AXES = COPILOT_CAP.runtime.hostIntegration;
+
+// hooks/dist is gitignored and built (mirrors golden-install-parity harness).
+before(() => {
+  execFileSync(process.execPath, [BUILD_SCRIPT], { encoding: 'utf-8', stdio: 'pipe' });
+});
+
+test('Copilot classifies as the declarative-cli reference profile (profileOf)', () => {
+  const desc = JSON.parse(fs.readFileSync(DESC, 'utf8'));
+  const axes = desc.runtime.hostIntegration;
+  assert.ok(axes && axes.embeddingMode, 'copilot descriptor declares hostIntegration axes');
+  assert.equal(profileOf(axes), 'declarative-cli',
+    'Copilot is a Declarative-CLI host');
+});
+
+test('the public declarative adapter classifies Copilot as a declarative host', () => {
+  const adapter = createDeclarativeAdapter({ runtime: 'copilot' });
+  assert.equal(adapter.kind, 'declarative');
+  assert.equal(adapter.runtime, 'copilot');
+  assert.equal(typeof adapter.install, 'function');
+  assert.equal(typeof adapter.uninstall, 'function');
+});
+
+test('a real Copilot install emits a gsd agent/skill surface (invocable)', () => {
+  const { configDir, root } = runMinimalInstall({ runtime: 'copilot', scope: 'global' });
+  try {
+    const files = walk(configDir);
+    assert.ok(files.length > 0, 'install must emit artifacts');
+    const gsdSurface = files.filter((f) => /gsd/i.test(path.relative(configDir, f)));
+    assert.ok(gsdSurface.length > 0,
+      'install must emit a gsd agent/skill surface (declarative reference)');
+    const agentsDir = path.join(configDir, 'agents');
+    assert.ok(fs.existsSync(agentsDir), 'agents/ directory must exist');
+    const agentFiles = fs.readdirSync(agentsDir)
+      .filter((f) => f.startsWith('gsd-') && f.endsWith('.agent.md'));
+    assert.ok(agentFiles.length > 0, 'agents/ must contain gsd-*.agent.md files');
+    const skillsDir = path.join(configDir, 'skills');
+    assert.ok(fs.existsSync(skillsDir), 'skills/ directory must exist');
+  } finally {
+    cleanup(root);
+  }
+});
+
+// ---------------------------------------------------------------------------
+// #2099 EoS/copilot — fail-closed negotiation + validator acceptance +
+// the folded descriptor (mirrors codebuddy/antigravity/qwen reference tests).
+// ---------------------------------------------------------------------------
+
+test('negotiateHostCapabilities never throws for copilot, even fully corrupted', () => {
+  assert.doesNotThrow(() => negotiateHostCapabilities({}));
+  assert.doesNotThrow(() => negotiateHostCapabilities({ ...COPILOT_AXES, embeddingMode: UNDOCUMENTED }));
+  assert.doesNotThrow(() => negotiateHostCapabilities({ ...COPILOT_AXES, embeddingMode: 'future-unknown' }));
+  assert.doesNotThrow(() => negotiateHostCapabilities({ ...COPILOT_AXES, dispatch: 'corrupted-not-an-object' }));
+  assert.doesNotThrow(() => negotiateHostCapabilities({ ...COPILOT_AXES, dispatch: { ...COPILOT_AXES.dispatch, maxDepth: 'not-a-number' } }));
+});
+
+test('a partial/empty copilot descriptor degrades to the safe floor, not the declarative-cli baseline', () => {
+  const result = negotiateHostCapabilities({});
+  assert.equal(result.effective.embeddingMode, 'declarative', 'omitted embeddingMode degrades closed');
+  assert.equal(result.effective.hookBus, 'none');
+  assert.notDeepEqual(result.effective, PROFILE_BASELINES['declarative-cli']);
+  assert.ok(result.warnings.length > 0);
+});
+
+test('capabilities/copilot/capability.json validates — no errors', () => {
+  const errors = validateCapability(COPILOT_CAP, 'copilot');
+  assert.deepEqual(errors, [], `validateCapability must return no errors, got: ${JSON.stringify(errors)}`);
+});
+
+// -- AC2: the hardcoded branches are retired across all folded modules ------
+
+test('no `runtime === "copilot"` string-equality branch (nor live `isCopilot` read) remains in bin/install.js, src/install-engine.cts, src/surface.cts, or src/runtime-artifact-conversion.cts (AC2)', () => {
+  const strip = (src) => src
+    .replace(/\/\*[\s\S]*?\*\//g, '')
+    .replace(/\/\/[^\r\n]*/g, '')
+    .replace(/`[^`]*`/g, '');
+  const repoRoot = path.join(__dirname, '..');
+  const files = [
+    path.join(repoRoot, 'bin', 'install.js'),
+    path.join(repoRoot, 'src', 'install-engine.cts'),
+    path.join(repoRoot, 'src', 'surface.cts'),
+    path.join(repoRoot, 'src', 'runtime-artifact-conversion.cts'),
+  ];
+  for (const file of files) {
+    const src = fs.readFileSync(file, 'utf8');
+    const stripped = strip(src);
+
+    // NIT-1: catch both quote styles (single- and double-quoted 'copilot').
+    const eqOffenders = stripped.match(/runtime\s*[!=]==\s*["']copilot["']/g) || [];
+    assert.deepEqual(eqOffenders, [],
+      `AC2: no hardcoded runtime==='copilot' branch may remain in ${path.relative(repoRoot, file)}; found: ${eqOffenders.join(', ')}`);
+
+    // Excludes legit enumeration sites: --copilot CLI flag parsing, the
+    // '7':'copilot' numbered-menu map, allRuntimes/_DESCRIPTOR_AGENTS_RUNTIMES
+    // set literals, config-dir lists, the copilot conversion FUNCTION names
+    // (convertClaudeCommandToCopilotSkill / convertClaudeAgentToCopilotAgent /
+    // convertCopilotToolName), and installSurface==='copilot-instructions'
+    // (a descriptor-field string, not a runtime literal) — none of those
+    // contain the token `isCopilot`, so a literal-word match is precise here.
+    const isCopilotHits = stripped.match(/\bisCopilot\b/g) || [];
+    assert.deepEqual(isCopilotHits, [],
+      `AC2: no live isCopilot read may remain in ${path.relative(repoRoot, file)}; found ${isCopilotHits.length} occurrence(s)`);
+  }
+});
diff --git a/tests/fixtures/golden-install-parity/copilot.json b/tests/fixtures/golden-install-parity/copilot.json
index 77d3e23bc..17c7aef42 100644
--- a/tests/fixtures/golden-install-parity/copilot.json
+++ b/tests/fixtures/golden-install-parity/copilot.json
@@ -312,7 +312,7 @@
   "gsd-core/workflows/validate-phase.md": "2f705775a4b76d42",
   "gsd-core/workflows/verify-phase.md": "5c72780e34214e27",
   "gsd-core/workflows/verify-work.md": "c966971a3cbd1d71",
-  "hooks/gsd-session.json": "0a462834f2a28fee",
+  "hooks/gsd-session.json": "3382597f61e3a559",
   "scripts/changeset/README.md": "86ff89331dfd94b2",
   "scripts/changeset/cli.cjs": "68f92a344b199271",
   "scripts/changeset/github-release-notes.cjs": "795677f0c009b132",
diff --git a/tests/opencode-review-reconstruction.property.test.cjs b/tests/opencode-review-reconstruction.property.test.cjs
index 6416d34af..bec2c8fe4 100644
--- a/tests/opencode-review-reconstruction.property.test.cjs
+++ b/tests/opencode-review-reconstruction.property.test.cjs
@@ -4,14 +4,30 @@
 // Those programs ARE the runtime contract; this test extracts them verbatim from
 // the workflow and exercises the real jq (not a reimplementation) so the shipped
 // reconstruction logic is what gets property-tested.
+//
+// ARCHITECTURE (#2099): the shipped jq program is run over a WHOLE fast-check corpus
+// in ONE jq process, not once per generated case. Each generated event stream is
+// written as one compact-JSON array per line to a temp file, then `jq -c <PROGRAM>`
+// (no `-s`) applies PROGRAM to each array — `.` is that array, exactly what
+// production's `jq -rs` sees after slurping opencode's one-value-per-line stream —
+// emitting one compact-JSON result per line. This is empirically identical to the
+// per-stream `-rs` form (verified across embedded-newline/empty/quote/unicode/
+// null-drop cases) AND reads from a file like production (`jq -rs '…' <file>`), so
+// there is no stdin pipe to deadlock on large I/O. The prior design spawned ~600
+// synchronous jq subprocesses (numRuns × 3 properties); a single one freezing on a
+// contended CI runner hung the whole unit-test chunk to its 600s kill (macOS CI,
+// #2099). `node --test`'s --test-force-exit cannot interrupt a synchronous
+// execFileSync, so the cure is to stop spawning per case — not just to time-bound it.
 'use strict';
 
 const { describe, test } = require('node:test');
 const assert = require('node:assert/strict');
 const { execFileSync } = require('node:child_process');
 const fs = require('node:fs');
+const os = require('node:os');
 const path = require('node:path');
 const fc = require('./helpers/fast-check-setup.cjs');
+const { cleanup } = require('./helpers.cjs');
 
 const reviewPath = path.resolve(__dirname, '..', 'gsd-core', 'workflows', 'review.md');
 const workflow = fs.readFileSync(reviewPath, 'utf-8');
@@ -34,20 +50,48 @@ const DIAG_PROGRAM = extractJqProgram('OPENCODE_DIAG');    // empty-output diagn
 // the suite to jq-present non-Windows hosts, mirroring golden-install-parity's win32
 // skip. The assertions run in full on every macOS/Linux CI leg.
 let jqAvailable = false;
-try { execFileSync('jq', ['--version'], { stdio: 'ignore' }); jqAvailable = true; } catch { /* no jq on PATH */ }
+try { execFileSync('jq', ['--version'], { stdio: 'ignore', timeout: 10000, killSignal: 'SIGKILL' }); jqAvailable = true; } catch { /* no jq on PATH */ }
 const skipReason = process.platform === 'win32'
   ? 'jq invocation is not portable under Node child_process arg-quoting on Windows; logic is platform-independent and asserted on macOS/Linux'
   : (jqAvailable ? false : 'jq not on PATH');
 const opts = { skip: skipReason };
 
-// Run a shipped jq program against a stream of events serialized exactly as
-// opencode emits them: one JSON value per line (jq -s slurps them into an array).
-// jq -r appends a single trailing newline to the (single) string result; strip it
-// to recover the value the workflow's `$(…)` capture would see.
-function runJq(program, events) {
-  const jsonl = events.map((e) => JSON.stringify(e)).join('\n');
-  const out = execFileSync('jq', ['-rs', program], { input: jsonl, encoding: 'utf8' });
-  return out.endsWith('\n') ? out.slice(0, -1) : out;
+// Bound each jq subprocess so a frozen spawn on a contended runner fails fast +
+// diagnosably (ETIMEDOUT) rather than hanging the chunk. jq over this corpus
+// completes in ~10ms, so 30s is an enormous margin that never trips on a healthy run.
+const JQ_EXEC_OPTS = { encoding: 'utf8', timeout: 30000, killSignal: 'SIGKILL', maxBuffer: 64 * 1024 * 1024 };
+
+// Run a shipped jq program over an array of event streams in a SINGLE jq process.
+// Returns one result per input stream (order preserved), decoded from jq's compact
+// (`-c`) JSON output back to the raw string production's `-r` would have captured.
+// Both shipped programs yield exactly one value per stream (join(...) / last|"...");
+// the length assertion pins that invariant so a future program change that broke it
+// (0 or >1 outputs) fails loudly instead of silently misaligning results.
+function runJqBatch(program, streams) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-jq-batch-'));
+  try {
+    const file = path.join(dir, 'streams.jsonl');
+    fs.writeFileSync(file, streams.map((events) => JSON.stringify(events)).join('\n') + '\n');
+    const out = execFileSync('jq', ['-c', program, file], JQ_EXEC_OPTS);
+    const lines = out.split('\n').filter((line) => line.length > 0);
+    assert.equal(
+      lines.length,
+      streams.length,
+      `jq must emit exactly one result per stream (got ${lines.length} for ${streams.length})`,
+    );
+    return lines.map((line) => JSON.parse(line));
+  } finally {
+    cleanup(dir);
+  }
+}
+
+// The intended reconstruction, computed independently of jq. jq is the unit under
+// test; this JS is the spec it must match on every generated case.
+function expectedReview(events) {
+  return events
+    .filter((e) => e.type === 'text' && e.part && typeof e.part.text === 'string')
+    .map((e) => e.part.text)
+    .join('\n');
 }
 
 // Text values safe to round-trip through JSON → jq (utf8) → string. Excludes lone
@@ -85,69 +129,82 @@ const eventStream = fc.array(fc.oneof(textEvent, textEvent, nonTextEvent), {
   maxLength: 30,
 });
 
+// Corpus size per property. Matches the prior fast-check-setup numRuns:200 so
+// coverage is unchanged; distinct seeds give each property an independent corpus,
+// and fixed seeds keep the corpus deterministic across CI runs/OS legs.
+const CORPUS = 200;
+
 describe('#1936 OpenCode review reconstruction — jq properties', () => {
-  test('review == the newline-join of every assistant text part (order preserved)', opts, () => {
-    fc.assert(
-      fc.property(eventStream, (events) => {
-        const expected = events
-          .filter((e) => e.type === 'text' && e.part && typeof e.part.text === 'string')
-          .map((e) => e.part.text)
-          .join('\n');
-        assert.equal(runJq(TEXT_PROGRAM, events), expected);
-      }),
-    );
+  test('review == the newline-join of every assistant text part, over a generated corpus (order preserved)', opts, () => {
+    const streams = fc.sample(eventStream, { numRuns: CORPUS, seed: 42 });
+    const actual = runJqBatch(TEXT_PROGRAM, streams); // one jq process for the whole corpus
+    streams.forEach((events, i) => {
+      assert.equal(
+        actual[i],
+        expectedReview(events),
+        `case ${i}: shipped jq review must equal the spec for events=${JSON.stringify(events)}`,
+      );
+    });
   });
 
-  test('a stream with no assistant text part reconstructs to empty (drives the #1936 stub)', opts, () => {
-    fc.assert(
-      fc.property(fc.array(nonTextEvent, { minLength: 1, maxLength: 20 }), (events) => {
-        // This is the exact failure the bug describes: the agent runs tool calls
-        // and ends with step_finish, emitting no text. Reconstruction must be empty
-        // so the content-gate (`[ -n "$OPENCODE_REVIEW" ]`) falls through to the stub.
-        assert.equal(runJq(TEXT_PROGRAM, events), '');
-      }),
-    );
+  test('a stream with no assistant text part reconstructs to empty (drives the #1936 stub), over a corpus', opts, () => {
+    // The exact failure the bug describes: the agent runs tool calls and ends with
+    // step_finish, emitting no text. Reconstruction must be empty so the content-gate
+    // (`[ -n "$OPENCODE_REVIEW" ]`) falls through to the stub.
+    const streams = fc.sample(fc.array(nonTextEvent, { minLength: 1, maxLength: 20 }), { numRuns: CORPUS, seed: 43 });
+    const actual = runJqBatch(TEXT_PROGRAM, streams);
+    streams.forEach((events, i) => {
+      assert.equal(actual[i], '', `case ${i}: text-free stream must reconstruct to '' for ${JSON.stringify(events)}`);
+    });
   });
 
-  test('text parts that are null/absent are dropped, never rendered as "null"', opts, () => {
-    fc.assert(
-      fc.property(
-        fc.array(
-          fc.oneof(
-            fc.record({ type: fc.constant('text'), part: fc.record({ text: fc.constant(null) }) }),
-            fc.record({ type: fc.constant('text'), part: fc.record({}) }),
-          ),
-          { minLength: 1, maxLength: 10 },
-        ),
-        (events) => {
-          const out = runJq(TEXT_PROGRAM, events);
-          assert.equal(out, '');
-          assert.doesNotMatch(out, /null/);
-        },
+  test('text parts that are null/absent are dropped, never rendered as "null", over a corpus', opts, () => {
+    const nullish = fc.array(
+      fc.oneof(
+        fc.record({ type: fc.constant('text'), part: fc.record({ text: fc.constant(null) }) }),
+        fc.record({ type: fc.constant('text'), part: fc.record({}) }),
       ),
+      { minLength: 1, maxLength: 10 },
     );
+    const streams = fc.sample(nullish, { numRuns: CORPUS, seed: 44 });
+    const actual = runJqBatch(TEXT_PROGRAM, streams);
+    streams.forEach((events, i) => {
+      assert.equal(actual[i], '', `case ${i}: null/absent text must drop to ''`);
+      assert.doesNotMatch(actual[i], /null/, `case ${i}: must never render "null"`);
+    });
+  });
+
+  // Explicit boundary + happy examples (deterministic, not sampled) — all in one
+  // batched jq spawn. Pins the exact contract the corpus only covers probabilistically.
+  test('boundary + happy example streams reconstruct exactly (batched)', opts, () => {
+    const cases = [
+      { events: [{ type: 'text', part: { text: 'only' } }], expect: 'only' },                       // single text part
+      { events: [{ type: 'text', part: { text: '' } }], expect: '' },                                // empty-string text is kept
+      { events: [{ type: 'text', part: { text: 'a' } }, { type: 'tool_use', part: { tool: 'r' } }, { type: 'text', part: { text: 'b' } }], expect: 'a\nb' }, // text interleaved with noise
+      { events: [{ type: 'text', part: { text: 'x\ny' } }], expect: 'x\ny' },                        // embedded newline preserved
+      { events: [{ type: 'tool_use', part: { tool: 'read' } }, { type: 'step_finish', part: { reason: 'stop', tokens: { output: 3 } } }], expect: '' }, // no text at all
+      { events: Array.from({ length: 30 }, (_v, i) => ({ type: 'text', part: { text: `p${i}` } })), expect: Array.from({ length: 30 }, (_v, i) => `p${i}`).join('\n') }, // max-size all-text stream
+    ];
+    const actual = runJqBatch(TEXT_PROGRAM, cases.map((c) => c.events));
+    cases.forEach((c, i) => assert.equal(actual[i], c.expect, `example ${i}: ${JSON.stringify(c.events)}`));
   });
 
   // Diagnostic path (empty-output stub). The finding calls out `missing .tokens.output`
-  // and no-step_finish as real edges — pin them with examples against the shipped jq.
+  // and no-step_finish as real edges — pin them with examples against the shipped jq,
+  // all in one batched spawn.
   describe('diagnostic reconstruction (stop reason + output tokens)', () => {
-    test('reports reason and output tokens from the LAST step_finish', opts, () => {
-      const events = [
-        { type: 'step_finish', part: { reason: 'tool_calls', tokens: { output: 5 } } },
-        { type: 'tool_use', part: {} },
-        { type: 'step_finish', part: { reason: 'stop', tokens: { output: 0 } } },
+    test('reports reason/tokens from the LAST step_finish and degrades missing fields to "?"', opts, () => {
+      const cases = [
+        { events: [
+          { type: 'step_finish', part: { reason: 'tool_calls', tokens: { output: 5 } } },
+          { type: 'tool_use', part: {} },
+          { type: 'step_finish', part: { reason: 'stop', tokens: { output: 0 } } },
+        ], expect: 'stop reason=stop, output tokens=0' },                                   // LAST step_finish wins
+        { events: [{ type: 'step_finish', part: { reason: 'stop', tokens: {} } }], expect: 'stop reason=stop, output tokens=?' }, // missing .tokens.output → "?"
+        { events: [{ type: 'tool_use', part: { tool: 'read' } }], expect: 'stop reason=?, output tokens=?' }, // no step_finish at all → both "?"
       ];
-      assert.equal(runJq(DIAG_PROGRAM, events), 'stop reason=stop, output tokens=0');
-    });
-
-    test('missing .tokens.output degrades to "?" rather than null/garbage', opts, () => {
-      const events = [{ type: 'step_finish', part: { reason: 'stop', tokens: {} } }];
-      assert.equal(runJq(DIAG_PROGRAM, events), 'stop reason=stop, output tokens=?');
-    });
-
-    test('no step_finish at all degrades both fields to "?"', opts, () => {
-      const events = [{ type: 'tool_use', part: { tool: 'read' } }];
-      assert.equal(runJq(DIAG_PROGRAM, events), 'stop reason=?, output tokens=?');
+      const actual = runJqBatch(DIAG_PROGRAM, cases.map((c) => c.events));
+      cases.forEach((c, i) => assert.equal(actual[i], c.expect, `diag ${i}: ${JSON.stringify(c.events)}`));
     });
   });
 
@@ -158,7 +215,7 @@ describe('#1936 OpenCode review reconstruction — jq properties', () => {
   test('non-JSON stdout does not masquerade as a reconstructed review', opts, () => {
     let threw = false;
     try {
-      execFileSync('jq', ['-rs', TEXT_PROGRAM], { input: 'auth token expired\n', encoding: 'utf8' });
+      execFileSync('jq', ['-rs', TEXT_PROGRAM], { ...JQ_EXEC_OPTS, input: 'auth token expired\n' });
     } catch {
       threw = true;
     }