diff --git a/api/openapi.yaml b/api/openapi.yaml
index 5610aab1..2b6c3508 100644
--- a/api/openapi.yaml
+++ b/api/openapi.yaml
@@ -1309,6 +1309,8 @@ components:
     HealthResponseBody:
       additionalProperties: false
       properties:
+        api_schema_version:
+          type: string
         db_path:
           type: string
         ok:
diff --git a/docs/reference/http-api.md b/docs/reference/http-api.md
new file mode 100644
index 00000000..4d65c60a
--- /dev/null
+++ b/docs/reference/http-api.md
@@ -0,0 +1,83 @@
+# HTTP API schema
+
+The daemon exposes an HTTP API (used by the CLI, TUI, and remote clients). Its
+shape is published as an OpenAPI 3.1 document so out-of-process clients can
+generate typed clients instead of hand-copying wire structs.
+
+## Getting the schema
+
+The schema is committed at [`api/openapi.yaml`](https://github.com/kenn-io/kata/blob/main/api/openapi.yaml)
+and regenerated by the `kata` binary:
+
+```sh
+kata openapi > openapi.yaml
+```
+
+`kata openapi` builds the document in-process from the daemon's route
+definitions, so it needs neither a running daemon nor a database. The runtime
+`/openapi.json` route stays disabled; the committed artifact and this command
+are the supported way to obtain the schema.
+
+## What the schema is
+
+A **per-release snapshot** of the daemon's current HTTP API — a faithful
+description of the routes and wire types as they exist at that commit. It is
+useful for client generation and review.
+
+It is **not** a promise of a forever-stable API. Treat it as the contract for
+the daemon version you generated it against, not a guarantee that a future
+daemon will accept the same calls.
+
+## Detecting the API version
+
+The schema carries a version in its `info.version` field
+(`APISchemaVersion`). The same value is reported at runtime by
+[`GET /api/v1/health`](#) as `api_schema_version`:
+
+```json
+{
+  "ok": true,
+  "schema_version": 7,
+  "api_schema_version": "0.1.0",
+  "version": "1.4.2",
+  "uptime": "5m0s",
+  "db_path": "/path/to/kata.db"
+}
+```
+
+Three distinct version fields appear here; they answer different questions:
+
+| Field | Meaning |
+| --- | --- |
+| `api_schema_version` | The HTTP API contract version — match this against the schema you generated your client from. |
+| `schema_version` | The database/storage schema version (`meta.schema_version`), an internal storage concern. |
+| `version` | The daemon build version. |
+
+A client can read `api_schema_version` once at startup and decide whether it
+recognizes the daemon's API before issuing further calls.
+
+The field is **optional in the schema** even though current daemons always send
+it. That is deliberate: a version-detection field has to survive version skew,
+so a client generated from a schema that includes it can still parse the
+response of an older daemon that predates it. Treat an **absent or empty**
+`api_schema_version` as "a daemon older than this field," not a parse error.
+
+## Compatibility expectations
+
+These are the current intentions, not a contractual guarantee:
+
+- **Additive changes are not signalled.** New endpoints, and new optional
+  response fields, can appear without an `api_schema_version` change. Clients
+  should ignore unknown fields rather than fail on them.
+- **Breaking changes bump `api_schema_version`.** Removing or renaming a field,
+  changing a field's type, or removing an endpoint is a breaking change and is
+  signalled by a change to `api_schema_version`. A client that pins or checks
+  the value it was generated against can detect the mismatch instead of failing
+  at an arbitrary call site.
+- **Regeneration stays honest.** A committed golden test fails if
+  `api/openapi.yaml` drifts from the routes, so the published schema cannot
+  silently fall out of date with the daemon it describes.
+
+If you build against this schema and hit a gap, please open an issue — the
+contract is meant to be useful to external clients, and feedback shapes how far
+the compatibility guarantees are taken.
diff --git a/docs/zensical.toml b/docs/zensical.toml
index fe36b42f..e1bf21d1 100644
--- a/docs/zensical.toml
+++ b/docs/zensical.toml
@@ -26,6 +26,7 @@ nav = [
     {"CLI" = "reference/cli.md"},
     {"Configuration" = "reference/configuration.md"},
     {"Agent output format" = "reference/agent-output.md"},
+    {"HTTP API schema" = "reference/http-api.md"},
   ]},
   {"Workflows" = [
     {"Agent workflows" = "workflows/agents.md"},
diff --git a/internal/api/types.go b/internal/api/types.go
index 8d944109..7dae689e 100644
--- a/internal/api/types.go
+++ b/internal/api/types.go
@@ -19,14 +19,25 @@ type PingResponse struct {
 }
 
 // HealthResponse mirrors /api/v1/health.
+//
+// SchemaVersion is the database/storage schema version (meta.schema_version);
+// APISchemaVersion is the version stamped into the daemon's OpenAPI document,
+// letting an external client detect the HTTP API contract it is talking to.
+//
+// APISchemaVersion is schema-optional (omitempty) on purpose: the field exists
+// to detect version skew, so it must itself survive it. A client generated from
+// a schema that carries this field still needs to parse the response of an older
+// daemon that predates it — an absent value means "older than this field" rather
+// than a parse failure. Current daemons always populate it.
 type HealthResponse struct {
 	Body struct {
-		OK            bool      `json:"ok"`
-		DBPath        string    `json:"db_path"`
-		SchemaVersion int       `json:"schema_version"`
-		Version       string    `json:"version"`
-		Uptime        string    `json:"uptime"`
-		StartedAt     time.Time `json:"started_at"`
+		OK               bool      `json:"ok"`
+		DBPath           string    `json:"db_path"`
+		SchemaVersion    int       `json:"schema_version"`
+		APISchemaVersion string    `json:"api_schema_version,omitempty"`
+		Version          string    `json:"version"`
+		Uptime           string    `json:"uptime"`
+		StartedAt        time.Time `json:"started_at"`
 	}
 }
 
diff --git a/internal/daemon/handlers_health.go b/internal/daemon/handlers_health.go
index 821b5e66..f7b5f22c 100644
--- a/internal/daemon/handlers_health.go
+++ b/internal/daemon/handlers_health.go
@@ -42,6 +42,7 @@ func registerHealthHandlers(humaAPI huma.API, cfg ServerConfig) {
 		out.Body.OK = true
 		out.Body.DBPath = cfg.DB.Path()
 		out.Body.SchemaVersion = schema
+		out.Body.APISchemaVersion = APISchemaVersion
 		out.Body.Version = version.Version
 		out.Body.StartedAt = cfg.StartedAt
 		out.Body.Uptime = time.Since(cfg.StartedAt).Round(time.Second).String()
diff --git a/internal/daemon/handlers_health_test.go b/internal/daemon/handlers_health_test.go
index 977bec8f..3c8b0c5f 100644
--- a/internal/daemon/handlers_health_test.go
+++ b/internal/daemon/handlers_health_test.go
@@ -6,6 +6,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 
+	"go.kenn.io/kata/internal/daemon"
 	"go.kenn.io/kata/internal/db"
 )
 
@@ -13,14 +14,17 @@ func TestHealth_ReportsSchemaAndUptime(t *testing.T) {
 	ts, _ := startDefaultTestServer(t)
 
 	var body struct {
-		OK            bool   `json:"ok"`
-		SchemaVersion int    `json:"schema_version"`
-		Uptime        string `json:"uptime"`
-		DBPath        string `json:"db_path"`
+		OK               bool   `json:"ok"`
+		SchemaVersion    int    `json:"schema_version"`
+		APISchemaVersion string `json:"api_schema_version"`
+		Uptime           string `json:"uptime"`
+		DBPath           string `json:"db_path"`
 	}
 	getAndUnmarshal(t, ts, "/api/v1/health", http.StatusOK, &body)
 	assert.True(t, body.OK)
 	assert.Equal(t, db.CurrentSchemaVersion(), body.SchemaVersion)
+	assert.Equal(t, daemon.APISchemaVersion, body.APISchemaVersion)
+	assert.NotEmpty(t, body.APISchemaVersion)
 	assert.NotEmpty(t, body.Uptime)
 	assert.NotEmpty(t, body.DBPath)
 }