diff --git a/docs/event-ui-conformance-harness.md b/docs/event-ui-conformance-harness.md
new file mode 100644
index 0000000..8ebddf3
--- /dev/null
+++ b/docs/event-ui-conformance-harness.md
@@ -0,0 +1,52 @@
+# Event UI conformance harness
+
+Shared regression contract for mapping public Traverse embedder events to UI
+presentation state (Specs
+[`001`](specs/001-ui-event-state-conformance/spec.md) /
+[`002`](specs/002-capability-event-ui-subscription/spec.md), ADR
+[`0001`](adr/0001-event-fixture-harness-first.md)).
+
+## Fixtures (language-agnostic)
+
+Path: [`fixtures/event-ui-conformance/`](../fixtures/event-ui-conformance/)
+
+| Case | Expected state |
+|---|---|
+| `happy-path.json` | `loaded` |
+| `error-path.json` | `error` |
+| `blocked-path.json` | `blocked` |
+| `ended-path.json` | `ended` |
+| `multi-capability.json` | `loaded` (+ ordered capability progress) |
+| `replay-late-subscriber.json` | `loaded` (prefix replay ≡ full map) |
+
+Catalog index: `fixtures/event-ui-conformance/catalog.json`.
+
+Fixtures are static JSON — no network calls.
+
+## TypeScript package
+
+Workspace: `packages/event-ui-conformance` (`event-ui-conformance`).
+
+```ts
+import {
+  loadFixtureCase,
+  mapCapabilityProgress,
+  mapPresentationState,
+} from 'event-ui-conformance'
+
+const fixture = loadFixtureCase('happy-path.json')
+const ui = mapPresentationState(fixture.events)
+// ui.state === 'loaded'; ui.output from capability_result only
+
+const progress = mapCapabilityProgress(fixture.events)
+```
+
+Primary apps should import these helpers (or re-run the same fixtures with a
+platform-native mapper) rather than inventing per-app event→state tables.
+
+## Architecture boundary
+
+- Subscribe / consume only public embedder envelopes (`state_changed`,
+  `capability_invoked`, `capability_result`, `error`).
+- Do **not** emit synthetic embedder events from production app code.
+- Do **not** invent business fields in the UI; render runtime-provided `output`.
diff --git a/fixtures/event-ui-conformance/README.md b/fixtures/event-ui-conformance/README.md
new file mode 100644
index 0000000..368b7a2
--- /dev/null
+++ b/fixtures/event-ui-conformance/README.md
@@ -0,0 +1,20 @@
+# Event UI conformance fixtures
+
+Language-agnostic embedder event streams for Specs
+[`001-ui-event-state-conformance`](../../docs/specs/001-ui-event-state-conformance/spec.md) and
+[`002-capability-event-ui-subscription`](../../docs/specs/002-capability-event-ui-subscription/spec.md).
+
+| Case | File | Expected presentation state |
+|---|---|---|
+| Happy path | `happy-path.json` | `loaded` |
+| Error path | `error-path.json` | `error` |
+| Blocked path | `blocked-path.json` | `blocked` |
+| Ended path | `ended-path.json` | `ended` |
+| Multi-capability | `multi-capability.json` | `loaded` |
+| Replay / late subscriber | `replay-late-subscriber.json` | `loaded` |
+
+Index: [`catalog.json`](./catalog.json).
+
+TypeScript mappers and Vitest consumers live in
+[`packages/event-ui-conformance`](../../packages/event-ui-conformance/).
+See [`docs/event-ui-conformance-harness.md`](../../docs/event-ui-conformance-harness.md).
diff --git a/fixtures/event-ui-conformance/blocked-path.json b/fixtures/event-ui-conformance/blocked-path.json
new file mode 100644
index 0000000..e5294c7
--- /dev/null
+++ b/fixtures/event-ui-conformance/blocked-path.json
@@ -0,0 +1,40 @@
+{
+  "id": "blocked-path",
+  "description": "Runtime state_changed with waiting-for-human → blocked",
+  "expected_presentation_state": "blocked",
+  "expected_error_message": null,
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-blocked",
+      "data": {
+        "execution_id": "exec_blocked",
+        "capability_id": "fixture.approve",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "state_changed",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-blocked",
+      "data": {
+        "state": "waiting_for_human",
+        "previous_state": "running",
+        "capability_id": "fixture.approve"
+      }
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/catalog.json b/fixtures/event-ui-conformance/catalog.json
new file mode 100644
index 0000000..f1b1b71
--- /dev/null
+++ b/fixtures/event-ui-conformance/catalog.json
@@ -0,0 +1,37 @@
+{
+  "schema_version": "1.0.0",
+  "description": "Shared embedder event streams for UI presentation-state + capability-progress conformance (Specs 001/002).",
+  "cases": [
+    {
+      "id": "happy-path",
+      "file": "happy-path.json",
+      "expected_presentation_state": "loaded"
+    },
+    {
+      "id": "error-path",
+      "file": "error-path.json",
+      "expected_presentation_state": "error"
+    },
+    {
+      "id": "blocked-path",
+      "file": "blocked-path.json",
+      "expected_presentation_state": "blocked"
+    },
+    {
+      "id": "ended-path",
+      "file": "ended-path.json",
+      "expected_presentation_state": "ended"
+    },
+    {
+      "id": "multi-capability",
+      "file": "multi-capability.json",
+      "expected_presentation_state": "loaded"
+    },
+    {
+      "id": "replay-late-subscriber",
+      "file": "replay-late-subscriber.json",
+      "expected_presentation_state": "loaded",
+      "notes": "Same ordered stream is mapped fully and via prefix replay; both must agree."
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/ended-path.json b/fixtures/event-ui-conformance/ended-path.json
new file mode 100644
index 0000000..57008f2
--- /dev/null
+++ b/fixtures/event-ui-conformance/ended-path.json
@@ -0,0 +1,41 @@
+{
+  "id": "ended-path",
+  "description": "Terminal capability_result with no renderable output → ended",
+  "expected_presentation_state": "ended",
+  "expected_error_message": null,
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-ended",
+      "data": {
+        "execution_id": "exec_ended",
+        "capability_id": "fixture.cancel",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "capability_result",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-ended",
+      "data": {
+        "execution_id": "exec_ended",
+        "capability_id": "fixture.cancel",
+        "status": "completed",
+        "output": null
+      }
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/error-path.json b/fixtures/event-ui-conformance/error-path.json
new file mode 100644
index 0000000..38a80d5
--- /dev/null
+++ b/fixtures/event-ui-conformance/error-path.json
@@ -0,0 +1,45 @@
+{
+  "id": "error-path",
+  "description": "capability_invoked then embedder error → error with payload message",
+  "expected_presentation_state": "error",
+  "expected_error_message": "capability failed under runtime governance",
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-error",
+      "data": {
+        "execution_id": "exec_error",
+        "capability_id": "fixture.process",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "error",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-error",
+      "data": {
+        "execution_id": "exec_error",
+        "capability_id": "fixture.process",
+        "status": "error",
+        "error": {
+          "code": "execution_failed",
+          "message": "capability failed under runtime governance",
+          "details": {}
+        }
+      }
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/happy-path.json b/fixtures/event-ui-conformance/happy-path.json
new file mode 100644
index 0000000..b4a905a
--- /dev/null
+++ b/fixtures/event-ui-conformance/happy-path.json
@@ -0,0 +1,59 @@
+{
+  "id": "happy-path",
+  "description": "Non-terminal progress then capability_result with renderable output → loaded",
+  "expected_presentation_state": "loaded",
+  "expected_error_message": null,
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "state_changed",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-happy",
+      "data": {
+        "state": "running",
+        "previous_state": "idle"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-happy",
+      "data": {
+        "execution_id": "exec_happy",
+        "capability_id": "fixture.process",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0003",
+      "sequence": 3,
+      "event_type": "capability_result",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-happy",
+      "data": {
+        "execution_id": "exec_happy",
+        "capability_id": "fixture.process",
+        "status": "completed",
+        "output": {
+          "title": "runtime-owned title",
+          "summary": "fields come only from the fixture output"
+        }
+      }
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/multi-capability.json b/fixtures/event-ui-conformance/multi-capability.json
new file mode 100644
index 0000000..1ae19cd
--- /dev/null
+++ b/fixtures/event-ui-conformance/multi-capability.json
@@ -0,0 +1,84 @@
+{
+  "id": "multi-capability",
+  "description": "Two capabilities invoke→result in order; terminal output from B only",
+  "expected_presentation_state": "loaded",
+  "expected_error_message": null,
+  "expected_capability_order": [
+    { "capability_id": "fixture.analyze", "phase": "invoked" },
+    { "capability_id": "fixture.analyze", "phase": "result" },
+    { "capability_id": "fixture.recommend", "phase": "invoked" },
+    { "capability_id": "fixture.recommend", "phase": "result" }
+  ],
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-multi",
+      "data": {
+        "execution_id": "exec_a",
+        "capability_id": "fixture.analyze",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "capability_result",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-multi",
+      "data": {
+        "execution_id": "exec_a",
+        "capability_id": "fixture.analyze",
+        "status": "completed",
+        "output": {
+          "analysis": "runtime-owned analysis"
+        }
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0003",
+      "sequence": 3,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-multi",
+      "data": {
+        "execution_id": "exec_b",
+        "capability_id": "fixture.recommend",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0004",
+      "sequence": 4,
+      "event_type": "capability_result",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-multi",
+      "data": {
+        "execution_id": "exec_b",
+        "capability_id": "fixture.recommend",
+        "status": "completed",
+        "output": {
+          "recommendation": "runtime-owned recommendation"
+        }
+      }
+    }
+  ]
+}
diff --git a/fixtures/event-ui-conformance/replay-late-subscriber.json b/fixtures/event-ui-conformance/replay-late-subscriber.json
new file mode 100644
index 0000000..c0af1b6
--- /dev/null
+++ b/fixtures/event-ui-conformance/replay-late-subscriber.json
@@ -0,0 +1,58 @@
+{
+  "id": "replay-late-subscriber",
+  "description": "Full recorded stream; late subscriber mapping prefixes must match full map",
+  "expected_presentation_state": "loaded",
+  "expected_error_message": null,
+  "events": [
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0001",
+      "sequence": 1,
+      "event_type": "state_changed",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-replay",
+      "data": {
+        "state": "running",
+        "previous_state": "idle"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0002",
+      "sequence": 2,
+      "event_type": "capability_invoked",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-replay",
+      "data": {
+        "execution_id": "exec_replay",
+        "capability_id": "fixture.process",
+        "capability_version": "1.0.0"
+      }
+    },
+    {
+      "kind": "embedder_event",
+      "schema_version": "1.0.0",
+      "embedder_api_version": "embedder-api/1.0.0",
+      "event_id": "evt-0003",
+      "sequence": 3,
+      "event_type": "capability_result",
+      "workspace_id": "local-default",
+      "app_id": "fixture-app",
+      "session_id": "sess-replay",
+      "data": {
+        "execution_id": "exec_replay",
+        "capability_id": "fixture.process",
+        "status": "completed",
+        "output": {
+          "title": "replay-safe title"
+        }
+      }
+    }
+  ]
+}
diff --git a/package-lock.json b/package-lock.json
index 271decf..15b71b1 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,7 +9,8 @@
         "apps/traverse-starter/web-react",
         "apps/trace-explorer/web-react",
         "apps/doc-approval/web-react",
-        "apps/meeting-notes/web-react"
+        "apps/meeting-notes/web-react",
+        "packages/event-ui-conformance"
       ]
     },
     "apps/doc-approval/web-react": {
@@ -2853,6 +2854,10 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/event-ui-conformance": {
+      "resolved": "packages/event-ui-conformance",
+      "link": true
+    },
     "node_modules/expect-type": {
       "version": "1.4.0",
       "resolved": "https://registry.npmjs.org/expect-type/-/expect-type-1.4.0.tgz",
@@ -4695,6 +4700,54 @@
         "zod": "^3.25.0 || ^4.0.0"
       }
     },
+    "packages/event-ui-conformance": {
+      "version": "0.0.0",
+      "devDependencies": {
+        "@eslint/js": "^10.0.1",
+        "@types/node": "^26.1.0",
+        "@vitest/coverage-v8": "^4.1.10",
+        "eslint": "^10.8.1",
+        "globals": "^17.9.0",
+        "typescript": "~7.0.2",
+        "typescript-eslint": "^8.66.0",
+        "vitest": "^4.1.10"
+      }
+    },
+    "packages/event-ui-conformance/node_modules/typescript": {
+      "version": "7.0.2",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-7.0.2.tgz",
+      "integrity": "sha512-8FYau96o3NKOhbjKi/qNvG/W5jhzxkbdm5sj9AbZ/5T5sWqn3hJgLfGx27sRKZWTvyzCP8dLRBTf5tBTSRVUNA==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc"
+      },
+      "engines": {
+        "node": ">=16.20.0"
+      },
+      "optionalDependencies": {
+        "@typescript/typescript-aix-ppc64": "7.0.2",
+        "@typescript/typescript-darwin-arm64": "7.0.2",
+        "@typescript/typescript-darwin-x64": "7.0.2",
+        "@typescript/typescript-freebsd-arm64": "7.0.2",
+        "@typescript/typescript-freebsd-x64": "7.0.2",
+        "@typescript/typescript-linux-arm": "7.0.2",
+        "@typescript/typescript-linux-arm64": "7.0.2",
+        "@typescript/typescript-linux-loong64": "7.0.2",
+        "@typescript/typescript-linux-mips64el": "7.0.2",
+        "@typescript/typescript-linux-ppc64": "7.0.2",
+        "@typescript/typescript-linux-riscv64": "7.0.2",
+        "@typescript/typescript-linux-s390x": "7.0.2",
+        "@typescript/typescript-linux-x64": "7.0.2",
+        "@typescript/typescript-netbsd-arm64": "7.0.2",
+        "@typescript/typescript-netbsd-x64": "7.0.2",
+        "@typescript/typescript-openbsd-arm64": "7.0.2",
+        "@typescript/typescript-openbsd-x64": "7.0.2",
+        "@typescript/typescript-sunos-x64": "7.0.2",
+        "@typescript/typescript-win32-arm64": "7.0.2",
+        "@typescript/typescript-win32-x64": "7.0.2"
+      }
+    },
     "vendor/traverse-embedder-web": {
       "version": "0.7.0",
       "license": "Apache-2.0",
diff --git a/package.json b/package.json
index 73b17b8..02dedb4 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,8 @@
     "apps/traverse-starter/web-react",
     "apps/trace-explorer/web-react",
     "apps/doc-approval/web-react",
-    "apps/meeting-notes/web-react"
+    "apps/meeting-notes/web-react",
+    "packages/event-ui-conformance"
   ],
   "scripts": {
     "dev": "npm run dev -w apps/traverse-starter/web-react",
diff --git a/packages/event-ui-conformance/README.md b/packages/event-ui-conformance/README.md
new file mode 100644
index 0000000..c09c28c
--- /dev/null
+++ b/packages/event-ui-conformance/README.md
@@ -0,0 +1,14 @@
+# event-ui-conformance
+
+Shared TypeScript helpers for Specs 001/002:
+
+- Pure `mapPresentationState` → `idle|loading|loaded|blocked|ended|error`
+- `mapCapabilityProgress` / `activeCapabilityId` from public embedder events
+- Loaders for language-agnostic fixtures under `fixtures/event-ui-conformance/`
+
+Consumer guide: [`docs/event-ui-conformance-harness.md`](../../docs/event-ui-conformance-harness.md).
+
+```bash
+npm run test -w event-ui-conformance
+npm run typecheck -w event-ui-conformance
+```
diff --git a/packages/event-ui-conformance/eslint.config.js b/packages/event-ui-conformance/eslint.config.js
new file mode 100644
index 0000000..f89bc63
--- /dev/null
+++ b/packages/event-ui-conformance/eslint.config.js
@@ -0,0 +1,15 @@
+import js from '@eslint/js'
+import globals from 'globals'
+import tseslint from 'typescript-eslint'
+import { defineConfig, globalIgnores } from 'eslint/config'
+
+export default defineConfig([
+  globalIgnores(['dist', 'coverage']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [js.configs.recommended, tseslint.configs.recommended],
+    languageOptions: {
+      globals: globals.node,
+    },
+  },
+])
diff --git a/packages/event-ui-conformance/package.json b/packages/event-ui-conformance/package.json
new file mode 100644
index 0000000..6ad44c2
--- /dev/null
+++ b/packages/event-ui-conformance/package.json
@@ -0,0 +1,25 @@
+{
+  "name": "event-ui-conformance",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "scripts": {
+    "typecheck": "tsc -p tsconfig.json --noEmit",
+    "lint": "eslint .",
+    "test": "vitest run",
+    "test:coverage": "vitest run --coverage"
+  },
+  "devDependencies": {
+    "@eslint/js": "^10.0.1",
+    "@types/node": "^26.1.0",
+    "@vitest/coverage-v8": "^4.1.10",
+    "eslint": "^10.8.1",
+    "globals": "^17.9.0",
+    "typescript": "~7.0.2",
+    "typescript-eslint": "^8.66.0",
+    "vitest": "^4.1.10"
+  }
+}
diff --git a/packages/event-ui-conformance/src/capabilityProgress.test.ts b/packages/event-ui-conformance/src/capabilityProgress.test.ts
new file mode 100644
index 0000000..f67f113
--- /dev/null
+++ b/packages/event-ui-conformance/src/capabilityProgress.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest'
+import { activeCapabilityId, mapCapabilityProgress } from './capabilityProgress.ts'
+import { loadFixtureCase } from './loadFixture.ts'
+import { mapPresentationState } from './mapPresentationState.ts'
+import type { EmbedderEventLike } from './types.ts'
+
+describe('mapCapabilityProgress', () => {
+  it('reflects invoke/result order for multi-capability fixtures', () => {
+    const fixture = loadFixtureCase('multi-capability.json')
+    const progress = mapCapabilityProgress(fixture.events)
+    expect(progress.map((step) => ({ capability_id: step.capabilityId, phase: step.phase }))).toEqual(
+      fixture.expected_capability_order,
+    )
+
+    const snapshot = mapPresentationState(fixture.events)
+    expect(snapshot.state).toBe('loaded')
+    expect(snapshot.output).toEqual({ recommendation: 'runtime-owned recommendation' })
+    expect(activeCapabilityId(fixture.events)).toBeNull()
+  })
+
+  it('keeps loading and active capability when invoke has no result yet', () => {
+    const events: EmbedderEventLike[] = [
+      {
+        event_type: 'capability_invoked',
+        sequence: 1,
+        data: { capability_id: 'fixture.process', capability_version: '1.0.0' },
+      },
+    ]
+    expect(mapPresentationState(events).state).toBe('loading')
+    expect(activeCapabilityId(events)).toBe('fixture.process')
+    expect(mapCapabilityProgress(events)).toEqual([
+      {
+        capabilityId: 'fixture.process',
+        phase: 'invoked',
+        sequence: 1,
+        status: null,
+        output: null,
+      },
+    ])
+  })
+})
diff --git a/packages/event-ui-conformance/src/capabilityProgress.ts b/packages/event-ui-conformance/src/capabilityProgress.ts
new file mode 100644
index 0000000..5b5859b
--- /dev/null
+++ b/packages/event-ui-conformance/src/capabilityProgress.ts
@@ -0,0 +1,72 @@
+import { asString, isRecord } from './json.ts'
+import type { CapabilityProgressStep, EmbedderEventLike, JsonValue } from './types.ts'
+
+function capabilityIdFromData(data: JsonValue): string | null {
+  if (!isRecord(data)) return null
+  return asString(data.capability_id)
+}
+
+/**
+ * Ordered capability progress from `capability_invoked` / `capability_result`.
+ * Identity and status/output come only from event payloads.
+ */
+export function mapCapabilityProgress(
+  events: readonly EmbedderEventLike[],
+): CapabilityProgressStep[] {
+  const steps: CapabilityProgressStep[] = []
+
+  for (const event of events) {
+    if (event.event_type !== 'capability_invoked' && event.event_type !== 'capability_result') {
+      continue
+    }
+    const capabilityId = capabilityIdFromData(event.data)
+    if (!capabilityId) continue
+
+    if (event.event_type === 'capability_invoked') {
+      steps.push({
+        capabilityId,
+        phase: 'invoked',
+        sequence: event.sequence,
+        status: null,
+        output: null,
+      })
+      continue
+    }
+
+    const status = isRecord(event.data) ? asString(event.data.status) : null
+    const output = isRecord(event.data) && 'output' in event.data ? event.data.output : null
+    steps.push({
+      capabilityId,
+      phase: 'result',
+      sequence: event.sequence,
+      status,
+      output: output ?? null,
+    })
+  }
+
+  return steps
+}
+
+/** Active capability for UI chrome: last invoked without a later result for that id. */
+export function activeCapabilityId(events: readonly EmbedderEventLike[]): string | null {
+  const progress = mapCapabilityProgress(events)
+  const open = new Map<string, number>()
+
+  for (const step of progress) {
+    if (step.phase === 'invoked') {
+      open.set(step.capabilityId, (open.get(step.capabilityId) ?? 0) + 1)
+    } else {
+      const count = open.get(step.capabilityId) ?? 0
+      if (count <= 1) open.delete(step.capabilityId)
+      else open.set(step.capabilityId, count - 1)
+    }
+  }
+
+  for (let i = progress.length - 1; i >= 0; i -= 1) {
+    const step = progress[i]
+    if (step.phase === 'invoked' && open.has(step.capabilityId)) {
+      return step.capabilityId
+    }
+  }
+  return null
+}
diff --git a/packages/event-ui-conformance/src/index.ts b/packages/event-ui-conformance/src/index.ts
new file mode 100644
index 0000000..0d07b5f
--- /dev/null
+++ b/packages/event-ui-conformance/src/index.ts
@@ -0,0 +1,15 @@
+export type {
+  CapabilityPhase,
+  CapabilityProgressStep,
+  EmbedderEventLike,
+  FixtureCase,
+  FixtureCatalog,
+  FixtureCatalogEntry,
+  JsonValue,
+  PresentationSnapshot,
+  PresentationState,
+} from './types.ts'
+
+export { mapPresentationState } from './mapPresentationState.ts'
+export { activeCapabilityId, mapCapabilityProgress } from './capabilityProgress.ts'
+export { FIXTURES_DIR, loadAllFixtureCases, loadCatalog, loadFixtureCase } from './loadFixture.ts'
diff --git a/packages/event-ui-conformance/src/json.ts b/packages/event-ui-conformance/src/json.ts
new file mode 100644
index 0000000..3b830cc
--- /dev/null
+++ b/packages/event-ui-conformance/src/json.ts
@@ -0,0 +1,9 @@
+import type { JsonValue } from './types.ts'
+
+export function isRecord(value: JsonValue): value is { [key: string]: JsonValue } {
+  return value !== null && typeof value === 'object' && !Array.isArray(value)
+}
+
+export function asString(value: JsonValue | undefined): string | null {
+  return typeof value === 'string' ? value : null
+}
diff --git a/packages/event-ui-conformance/src/loadFixture.ts b/packages/event-ui-conformance/src/loadFixture.ts
new file mode 100644
index 0000000..a454b3c
--- /dev/null
+++ b/packages/event-ui-conformance/src/loadFixture.ts
@@ -0,0 +1,21 @@
+import { readFileSync } from 'node:fs'
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import type { FixtureCase, FixtureCatalog } from './types.ts'
+
+const PACKAGE_DIR = dirname(fileURLToPath(import.meta.url))
+export const FIXTURES_DIR = join(PACKAGE_DIR, '../../../fixtures/event-ui-conformance')
+
+export function loadCatalog(): FixtureCatalog {
+  const raw = readFileSync(join(FIXTURES_DIR, 'catalog.json'), 'utf8')
+  return JSON.parse(raw) as FixtureCatalog
+}
+
+export function loadFixtureCase(fileName: string): FixtureCase {
+  const raw = readFileSync(join(FIXTURES_DIR, fileName), 'utf8')
+  return JSON.parse(raw) as FixtureCase
+}
+
+export function loadAllFixtureCases(): FixtureCase[] {
+  return loadCatalog().cases.map((entry) => loadFixtureCase(entry.file))
+}
diff --git a/packages/event-ui-conformance/src/mapPresentationState.test.ts b/packages/event-ui-conformance/src/mapPresentationState.test.ts
new file mode 100644
index 0000000..df90adc
--- /dev/null
+++ b/packages/event-ui-conformance/src/mapPresentationState.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it } from 'vitest'
+import { mapPresentationState } from './mapPresentationState.ts'
+import type { EmbedderEventLike } from './types.ts'
+import { loadAllFixtureCases, loadFixtureCase } from './loadFixture.ts'
+
+function event(
+  partial: Pick<EmbedderEventLike, 'event_type' | 'sequence' | 'data'>,
+): EmbedderEventLike {
+  return partial
+}
+
+describe('mapPresentationState', () => {
+  it('returns idle for an empty stream', () => {
+    expect(mapPresentationState([])).toEqual({
+      state: 'idle',
+      errorMessage: null,
+      output: null,
+    })
+  })
+
+  it('maps mid-stream invoke/state to loading', () => {
+    const events = [
+      event({
+        event_type: 'state_changed',
+        sequence: 1,
+        data: { state: 'running' },
+      }),
+      event({
+        event_type: 'capability_invoked',
+        sequence: 2,
+        data: { capability_id: 'fixture.process' },
+      }),
+    ]
+    expect(mapPresentationState(events).state).toBe('loading')
+  })
+
+  it('matches every catalog fixture expected presentation state', () => {
+    for (const fixture of loadAllFixtureCases()) {
+      const snapshot = mapPresentationState(fixture.events)
+      expect(snapshot.state, fixture.id).toBe(fixture.expected_presentation_state)
+      if (fixture.expected_error_message !== undefined) {
+        expect(snapshot.errorMessage, fixture.id).toBe(fixture.expected_error_message)
+      }
+    }
+  })
+
+  it('copies output only from capability_result payloads', () => {
+    const happy = loadFixtureCase('happy-path.json')
+    const snapshot = mapPresentationState(happy.events)
+    expect(snapshot.state).toBe('loaded')
+    expect(snapshot.output).toEqual({
+      title: 'runtime-owned title',
+      summary: 'fields come only from the fixture output',
+    })
+  })
+
+  it('treats late-subscriber prefix replay as equivalent to full map', () => {
+    const fixture = loadFixtureCase('replay-late-subscriber.json')
+    const full = mapPresentationState(fixture.events)
+    let late = mapPresentationState([])
+    for (let i = 1; i <= fixture.events.length; i += 1) {
+      late = mapPresentationState(fixture.events.slice(0, i))
+    }
+    expect(late).toEqual(full)
+    expect(full.state).toBe('loaded')
+  })
+})
diff --git a/packages/event-ui-conformance/src/mapPresentationState.ts b/packages/event-ui-conformance/src/mapPresentationState.ts
new file mode 100644
index 0000000..bea6620
--- /dev/null
+++ b/packages/event-ui-conformance/src/mapPresentationState.ts
@@ -0,0 +1,112 @@
+import { asString, isRecord } from './json.ts'
+import type { EmbedderEventLike, JsonValue, PresentationSnapshot, PresentationState } from './types.ts'
+
+const BLOCKED_STATES = new Set([
+  'blocked',
+  'waiting',
+  'waiting_for_human',
+  'awaiting_human',
+  'awaiting_input',
+])
+
+const ENDED_STATES = new Set(['cancelled', 'canceled', 'closed', 'ended'])
+
+function errorMessageFromData(data: JsonValue): string | null {
+  if (!isRecord(data)) return null
+  const err = data.error
+  if (typeof err === 'string') return err
+  if (isRecord(err)) {
+    const message = err.message
+    if (typeof message === 'string') return message
+  }
+  return null
+}
+
+function runtimeStateToken(data: JsonValue): string | null {
+  if (!isRecord(data)) return null
+  return (
+    asString(data.state) ??
+    asString(data.status) ??
+    asString(data.runtime_state) ??
+    null
+  )
+}
+
+function isBlockedPayload(data: JsonValue): boolean {
+  if (!isRecord(data)) return false
+  if (data.blocked === true || data.waiting_for_human === true) return true
+  const token = runtimeStateToken(data)
+  return token !== null && BLOCKED_STATES.has(token.toLowerCase())
+}
+
+function isEndedStatePayload(data: JsonValue): boolean {
+  const token = runtimeStateToken(data)
+  return token !== null && ENDED_STATES.has(token.toLowerCase())
+}
+
+function hasRenderableOutput(data: JsonValue): boolean {
+  if (!isRecord(data)) return false
+  if (!('output' in data)) return false
+  const output = data.output
+  if (output === null || output === undefined) return false
+  if (typeof output === 'object' && !Array.isArray(output) && Object.keys(output).length === 0) {
+    return false
+  }
+  return true
+}
+
+/**
+ * Pure mapper: ordered public embedder events → one presentation snapshot.
+ * Does not invent business fields; output is copied from capability_result only.
+ */
+export function mapPresentationState(
+  events: readonly EmbedderEventLike[],
+): PresentationSnapshot {
+  if (events.length === 0) {
+    return { state: 'idle', errorMessage: null, output: null }
+  }
+
+  let state: PresentationState = 'idle'
+  let errorMessage: string | null = null
+  let output: JsonValue | null = null
+
+  for (const event of events) {
+    switch (event.event_type) {
+      case 'error': {
+        state = 'error'
+        errorMessage = errorMessageFromData(event.data) ?? 'execution failed'
+        break
+      }
+      case 'capability_invoked': {
+        if (state !== 'error') {
+          state = 'loading'
+        }
+        break
+      }
+      case 'state_changed': {
+        if (state === 'error') break
+        if (isBlockedPayload(event.data)) {
+          state = 'blocked'
+        } else if (isEndedStatePayload(event.data)) {
+          state = 'ended'
+        } else if (state !== 'loaded' && state !== 'ended') {
+          state = 'loading'
+        }
+        break
+      }
+      case 'capability_result': {
+        if (state === 'error') break
+        if (hasRenderableOutput(event.data) && isRecord(event.data)) {
+          state = 'loaded'
+          output = event.data.output ?? null
+        } else {
+          state = 'ended'
+          output = null
+        }
+        break
+      }
+    }
+  }
+
+  return { state, errorMessage, output }
+}
diff --git a/packages/event-ui-conformance/src/types.ts b/packages/event-ui-conformance/src/types.ts
new file mode 100644
index 0000000..9dc0db3
--- /dev/null
+++ b/packages/event-ui-conformance/src/types.ts
@@ -0,0 +1,70 @@
+/** Canonical UI presentation states (Spec 001). */
+export type PresentationState =
+  | 'idle'
+  | 'loading'
+  | 'loaded'
+  | 'blocked'
+  | 'ended'
+  | 'error'
+
+/** JSON wire value (matches public embedder JsonValue). */
+export type JsonValue =
+  | string
+  | number
+  | boolean
+  | null
+  | JsonValue[]
+  | { [key: string]: JsonValue }
+
+/**
+ * Public embedder event envelope fields required by the harness.
+ * Compatible with `traverse-embedder-web` `EmbedderEvent`.
+ */
+export interface EmbedderEventLike {
+  readonly event_type: 'state_changed' | 'capability_invoked' | 'capability_result' | 'error'
+  readonly sequence: number
+  readonly session_id?: string | null
+  readonly data: JsonValue
+}
+
+export interface PresentationSnapshot {
+  readonly state: PresentationState
+  readonly errorMessage: string | null
+  /** Last `capability_result.data.output` when present; never invented. */
+  readonly output: JsonValue | null
+}
+
+export type CapabilityPhase = 'invoked' | 'result'
+
+export interface CapabilityProgressStep {
+  readonly capabilityId: string
+  readonly phase: CapabilityPhase
+  readonly sequence: number
+  readonly status: string | null
+  readonly output: JsonValue | null
+}
+
+export interface FixtureCase {
+  readonly id: string
+  readonly description: string
+  readonly expected_presentation_state: PresentationState
+  readonly expected_error_message?: string | null
+  readonly expected_capability_order?: readonly {
+    readonly capability_id: string
+    readonly phase: CapabilityPhase
+  }[]
+  readonly events: readonly EmbedderEventLike[]
+}
+
+export interface FixtureCatalogEntry {
+  readonly id: string
+  readonly file: string
+  readonly expected_presentation_state: PresentationState
+  readonly notes?: string
+}
+
+export interface FixtureCatalog {
+  readonly schema_version: string
+  readonly description: string
+  readonly cases: readonly FixtureCatalogEntry[]
+}
diff --git a/packages/event-ui-conformance/tsconfig.json b/packages/event-ui-conformance/tsconfig.json
new file mode 100644
index 0000000..c628724
--- /dev/null
+++ b/packages/event-ui-conformance/tsconfig.json
@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "es2023",
+    "lib": ["ES2023"],
+    "module": "esnext",
+    "types": ["node", "vitest/globals"],
+    "skipLibCheck": true,
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true
+  },
+  "include": ["src", "vitest.config.ts"]
+}
diff --git a/packages/event-ui-conformance/vitest.config.ts b/packages/event-ui-conformance/vitest.config.ts
new file mode 100644
index 0000000..ecfe151
--- /dev/null
+++ b/packages/event-ui-conformance/vitest.config.ts
@@ -0,0 +1,13 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    globals: true,
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'json-summary', 'html'],
+      exclude: ['node_modules/', 'vitest.config.ts', 'eslint.config.js'],
+    },
+  },
+})