diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b5cb028..be2c338 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -44,6 +44,8 @@ jobs:
       - run: cargo clippy --all-targets -- -D warnings
       - name: Test (types regenerate from the schema on build)
         run: cargo test -p wavekat-flow
+      - name: Older readers must keep reading current documents
+        run: cargo test -p compat-old-readers
       - name: Fail on stale crate-local schema copy (synced by build.rs)
         run: |
           if ! git diff --exit-code -- crates/wavekat-flow/schema; then
diff --git a/Cargo.lock b/Cargo.lock
index bac6300..dbf0893 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -117,6 +117,14 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "compat-old-readers"
+version = "0.0.0"
+dependencies = [
+ "wavekat-flow 0.0.11",
+ "wavekat-flow 0.0.6",
+]
+
 [[package]]
 name = "deranged"
 version = "0.5.8"
@@ -1552,6 +1560,26 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "wavekat-flow"
+version = "0.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aa8de375720073be7590b3e3a0cb70ef88569bc483c3938d147d7a02c12408ca"
+dependencies = [
+ "anyhow",
+ "async-trait",
+ "prettyplease",
+ "schemars",
+ "serde",
+ "serde_json",
+ "serde_yaml_ng",
+ "syn 2.0.119",
+ "thiserror 2.0.19",
+ "time",
+ "time-tz",
+ "typify",
+]
+
 [[package]]
 name = "wavekat-flow"
 version = "0.0.11"
diff --git a/conformance/v2/valid/notes.expected.json b/conformance/v2/valid/notes.expected.json
new file mode 100644
index 0000000..d7529e6
--- /dev/null
+++ b/conformance/v2/valid/notes.expected.json
@@ -0,0 +1,5 @@
+{
+  "description": "The author's `note` on the flow and on every kind of step. A note is free text for people, never read by the engine, so it must not change any verdict. An older reader that predates `note` ignores it as an unknown field and must still read the document.",
+  "structurallyValid": true,
+  "semantic": { "ok": true, "errors": [] }
+}
diff --git a/conformance/v2/valid/notes.yaml b/conformance/v2/valid/notes.yaml
new file mode 100644
index 0000000..9b0a55e
--- /dev/null
+++ b/conformance/v2/valid/notes.yaml
@@ -0,0 +1,51 @@
+schema_version: 2
+id: flow_notes
+name: Notes on the flow and on every kind of step
+note: Written by the front desk; ring the owner first.
+entry: welcome
+nodes:
+  welcome:
+    kind: greeting
+    note: Keep the greeting short.
+    prompt: Thanks for calling.
+    exits: { next: check_hours }
+  check_hours:
+    kind: hours
+    note: Open Tuesday only.
+    timezone: America/New_York
+    schedule:
+      tue: [{ open: "09:00", close: "17:00" }]
+    exits: { open: front_desk, closed: menu }
+  front_desk:
+    kind: ring
+    note: Rings the shop phone.
+    timeout_secs: 20
+    exits: { no_answer: leave_message }
+  menu:
+    kind: menu
+    note: Press 1 to book, 2 for the owner.
+    prompt: Press 1 to book, or 2 to reach the owner.
+    options: { "1": Book, "2": Owner }
+    exits: { "1": book, "2": handoff, no_input: leave_message, invalid: leave_message }
+  book:
+    kind: book
+    note: Thirty-minute slots.
+    prompt: Here are the next available times.
+    confirm_prompt: You're booked for
+    timezone: America/New_York
+    schedule:
+      tue: [{ open: "09:00", close: "17:00" }]
+    duration_mins: 30
+    exits: { booked: bye, no_slots: leave_message, no_input: leave_message, unavailable: leave_message }
+  handoff:
+    kind: transfer
+    note: The owner's mobile.
+    target: "+15555550123"
+  leave_message:
+    kind: message
+    note: Goes to voicemail.
+    prompt: Leave your name and number.
+  bye:
+    kind: hangup
+    note: Always end politely.
+    prompt: Goodbye.
diff --git a/conformance/v2/valid/unknown-field.expected.json b/conformance/v2/valid/unknown-field.expected.json
new file mode 100644
index 0000000..6817228
--- /dev/null
+++ b/conformance/v2/valid/unknown-field.expected.json
@@ -0,0 +1,6 @@
+{
+  "description": "Version 2 counterpart of v1/valid/unknown-field: unknown fields (top-level `color`, node-level `volume`) are ACCEPTED. This is what lets an older reader keep reading a newer document that carries a field it has never heard of; tightening it would break every installed daemon still running an older build.",
+  "structurallyValid": true,
+  "semantic": { "ok": true, "errors": [] },
+  "tsWarnings": ["unknown_field"]
+}
diff --git a/conformance/v2/valid/unknown-field.yaml b/conformance/v2/valid/unknown-field.yaml
new file mode 100644
index 0000000..16e9948
--- /dev/null
+++ b/conformance/v2/valid/unknown-field.yaml
@@ -0,0 +1,14 @@
+schema_version: 2
+id: flow_unknown2
+name: Unknown fields are accepted
+entry: hello
+color: purple
+nodes:
+  hello:
+    kind: greeting
+    prompt: Hi.
+    exits: { next: bye }
+    volume: 11
+  bye:
+    kind: hangup
+    prompt: Bye.
diff --git a/crates/compat-old-readers/Cargo.toml b/crates/compat-old-readers/Cargo.toml
new file mode 100644
index 0000000..1050ad2
--- /dev/null
+++ b/crates/compat-old-readers/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "compat-old-readers"
+version = "0.0.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+repository.workspace = true
+authors.workspace = true
+description = "Test-only: checks that documents of the current format still parse under the oldest wavekat-flow release still in the field."
+# Never published, and not in release-please-config.json.
+publish = false
+
+[dev-dependencies]
+# The oldest crate release still running in installed voice daemons. See
+# OLDEST_READER in tests/old_readers.rs before changing this pin.
+old-flow = { package = "wavekat-flow", version = "=0.0.6" }
+# The current format, from this checkout.
+wavekat-flow = { path = "../wavekat-flow" }
diff --git a/crates/compat-old-readers/src/lib.rs b/crates/compat-old-readers/src/lib.rs
new file mode 100644
index 0000000..4686e0b
--- /dev/null
+++ b/crates/compat-old-readers/src/lib.rs
@@ -0,0 +1 @@
+//! Test-only crate; see tests/old_readers.rs.
diff --git a/crates/compat-old-readers/tests/old_readers.rs b/crates/compat-old-readers/tests/old_readers.rs
new file mode 100644
index 0000000..310cfd3
--- /dev/null
+++ b/crates/compat-old-readers/tests/old_readers.rs
@@ -0,0 +1,151 @@
+//! Older readers must keep reading newer documents.
+//!
+//! Voice daemons keep running whatever `wavekat-flow` release they were built
+//! with, while the platform writes the current format. This crate pins the
+//! oldest release still in the field (`old-flow` in Cargo.toml) next to the
+//! current crate and checks that everything the current format produces still
+//! parses, and is judged the same way, by the old one.
+//!
+//! OLDEST_READER tracks the oldest crate release still running in installed
+//! daemons. To retire it once no daemon that old remains: bump the `=x.y.z`
+//! pin in Cargo.toml to the new oldest release and update the constant below.
+//! Never bump it just to make a failure go away: a failure here means a
+//! document we write today would break a daemon that is already installed.
+//!
+//! Corpus cases are frozen, so this also covers every document the format has
+//! ever accepted that the old reader's schema versions can read.
+
+use std::{fs, path::Path, path::PathBuf};
+
+use old_flow::validate::validate as old_validate;
+use wavekat_flow::validate::validate as new_validate;
+
+/// The oldest `wavekat-flow` release still running in installed daemons.
+/// Must match the `old-flow` pin in Cargo.toml (asserted below).
+const OLDEST_READER: &str = "0.0.6";
+
+fn corpus() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../../conformance")
+}
+
+fn yaml_files(dir: PathBuf) -> Vec<PathBuf> {
+    let Ok(entries) = fs::read_dir(&dir) else {
+        return Vec::new();
+    };
+    let mut out: Vec<PathBuf> = entries
+        .flatten()
+        .map(|e| e.path())
+        .filter(|p| p.extension().and_then(|e| e.to_str()) == Some("yaml"))
+        .collect();
+    out.sort();
+    out
+}
+
+/// Every document the current format is expected to produce or accept: the
+/// valid and outbound-valid corpus of every version, and the canonical
+/// outputs of the serializer.
+fn current_documents() -> Vec<PathBuf> {
+    let mut out = Vec::new();
+    for version in fs::read_dir(corpus()).unwrap().flatten() {
+        let name = version.file_name().to_string_lossy().to_string();
+        if name.starts_with('v') && name[1..].chars().all(|c| c.is_ascii_digit()) {
+            out.extend(yaml_files(version.path().join("valid")));
+            out.extend(yaml_files(version.path().join("outbound").join("valid")));
+        }
+    }
+    out.extend(
+        yaml_files(corpus().join("canonical"))
+            .into_iter()
+            .filter(|p| p.to_string_lossy().ends_with(".canonical.yaml")),
+    );
+    out
+}
+
+/// Read the version straight off the text, without going through either
+/// crate's model.
+fn schema_version(yaml: &str) -> u32 {
+    yaml.lines()
+        .find_map(|l| l.strip_prefix("schema_version:"))
+        .expect("schema_version line")
+        .trim()
+        .parse()
+        .expect("numeric schema_version")
+}
+
+fn rel(p: &Path) -> String {
+    p.strip_prefix(corpus()).unwrap().display().to_string()
+}
+
+/// The documents the old reader claims to support, as (name, yaml).
+fn readable_by_old() -> Vec<(String, String)> {
+    current_documents()
+        .iter()
+        .map(|p| (rel(p), fs::read_to_string(p).unwrap()))
+        .filter(|(_, yaml)| old_flow::SUPPORTED_SCHEMA_VERSIONS.contains(&schema_version(yaml)))
+        .collect()
+}
+
+#[test]
+fn pin_matches_constant() {
+    let manifest =
+        fs::read_to_string(PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("Cargo.toml")).unwrap();
+    assert!(
+        manifest.contains(&format!("version = \"={OLDEST_READER}\"")),
+        "OLDEST_READER and the old-flow pin in Cargo.toml disagree"
+    );
+}
+
+#[test]
+fn old_reader_parses_every_current_document() {
+    let docs = readable_by_old();
+    assert!(
+        docs.len() > 10,
+        "only {} documents reached the old reader",
+        docs.len()
+    );
+    for (case, yaml) in docs {
+        old_flow::Flow::from_yaml(&yaml).unwrap_or_else(|e| {
+            panic!("{OLDEST_READER} cannot parse {case}, which the current format writes: {e}")
+        });
+    }
+}
+
+#[test]
+fn old_reader_judges_like_the_current_one() {
+    for (case, yaml) in readable_by_old() {
+        let old = old_flow::Flow::from_yaml(&yaml).unwrap();
+        let new = wavekat_flow::Flow::from_yaml(&yaml).unwrap();
+        assert_eq!(
+            old_validate(&old).is_ok(),
+            new_validate(&new).is_ok(),
+            "{OLDEST_READER} and the current crate disagree on {case}"
+        );
+    }
+}
+
+#[test]
+fn unknown_keys_do_not_disturb_the_fields_the_old_reader_knows() {
+    // Not just "does not fail": the keys it skips must not consume or
+    // reorder what it does understand.
+    let yaml = fs::read_to_string(corpus().join("v2/valid/notes.yaml")).unwrap();
+    let old = old_flow::Flow::from_yaml(&yaml).unwrap();
+    let new = wavekat_flow::Flow::from_yaml(&yaml).unwrap();
+    assert_eq!(old.entry, new.entry);
+    // Order is the author's reading order and irrelevant to execution; the
+    // old reader may not keep it, so compare the node sets.
+    let mut old_ids: Vec<_> = old.nodes.keys().map(|k| k.to_string()).collect();
+    let mut new_ids: Vec<_> = new.nodes.keys().map(|k| k.to_string()).collect();
+    old_ids.sort();
+    new_ids.sort();
+    assert_eq!(old_ids, new_ids);
+}
+
+#[test]
+fn old_reader_refuses_versions_it_does_not_know() {
+    // The other half of the contract: a version bump is how a change old
+    // readers cannot handle is announced, and they must say so rather than
+    // guess.
+    let yaml = "schema_version: 99\nid: f\nname: f\nentry: a\nnodes:\n  a:\n    kind: hangup\n";
+    let flow = old_flow::Flow::from_yaml(yaml).expect("parses; the version gate is validate");
+    assert!(old_validate(&flow).is_err());
+}
diff --git a/crates/wavekat-flow/tests/tolerance.rs b/crates/wavekat-flow/tests/tolerance.rs
new file mode 100644
index 0000000..5589568
--- /dev/null
+++ b/crates/wavekat-flow/tests/tolerance.rs
@@ -0,0 +1,49 @@
+//! Older readers must keep reading newer documents. A daemon built from an
+//! older crate parses whatever the platform writes today, so the schema must
+//! never close an object to unknown keys: no `additionalProperties: false`
+//! and no `unevaluatedProperties: false` anywhere. The generated serde types
+//! follow the schema (typify emits `deny_unknown_fields` for exactly that),
+//! so this one check covers the Rust model too. The TS package has the same
+//! test (packages/flow-schema/test/tolerance.test.ts).
+//!
+//! There is no intentional exception: the opaque `ui` block is simply
+//! unconstrained, and the `additionalProperties` that do exist are
+//! sub-schemas (node map, option labels), not `false`.
+
+use serde_json::Value;
+
+fn closed_objects(value: &Value, path: &str, out: &mut Vec<String>) {
+    match value {
+        Value::Object(map) => {
+            for key in ["additionalProperties", "unevaluatedProperties"] {
+                if map.get(key) == Some(&Value::Bool(false)) {
+                    out.push(format!("{path}/{key}"));
+                }
+            }
+            for (k, v) in map {
+                closed_objects(v, &format!("{path}/{k}"), out);
+            }
+        }
+        Value::Array(items) => {
+            for (i, v) in items.iter().enumerate() {
+                closed_objects(v, &format!("{path}/{i}"), out);
+            }
+        }
+        _ => {}
+    }
+}
+
+#[test]
+fn no_schema_version_closes_an_object_to_unknown_keys() {
+    for version in wavekat_flow::SUPPORTED_SCHEMA_VERSIONS {
+        let schema: Value =
+            serde_json::from_str(wavekat_flow::flow_schema(*version).expect("schema")).unwrap();
+        let mut closed = Vec::new();
+        closed_objects(&schema, "", &mut closed);
+        assert!(
+            closed.is_empty(),
+            "schema v{version} closes objects to unknown keys, which would break older readers \
+             of newer documents: {closed:?}"
+        );
+    }
+}
diff --git a/packages/flow-schema/test/tolerance.test.ts b/packages/flow-schema/test/tolerance.test.ts
new file mode 100644
index 0000000..94cf80b
--- /dev/null
+++ b/packages/flow-schema/test/tolerance.test.ts
@@ -0,0 +1,45 @@
+// Older readers must keep reading newer documents. A daemon built from an
+// older release parses whatever the platform writes today, so the schema must
+// never close an object to unknown keys: no `additionalProperties: false` and
+// no `unevaluatedProperties: false` anywhere. The Rust crate has the same test
+// (crates/wavekat-flow/tests/tolerance.rs).
+//
+// There is no intentional exception: the opaque `ui` block is simply
+// unconstrained, and the `additionalProperties` that do exist are sub-schemas
+// (node map, option labels), not `false`.
+
+import { readFileSync, readdirSync } from 'node:fs';
+import { dirname, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { describe, expect, it } from 'vitest';
+
+const schemaDir = resolve(dirname(fileURLToPath(import.meta.url)), '..', '..', '..', 'schema');
+
+function closedObjects(value: unknown, path: string, out: string[]): void {
+  if (Array.isArray(value)) {
+    value.forEach((v, i) => closedObjects(v, `${path}/${i}`, out));
+  } else if (value !== null && typeof value === 'object') {
+    for (const [key, v] of Object.entries(value)) {
+      if ((key === 'additionalProperties' || key === 'unevaluatedProperties') && v === false) {
+        out.push(`${path}/${key}`);
+      }
+      closedObjects(v, `${path}/${key}`, out);
+    }
+  }
+}
+
+const schemas = readdirSync(schemaDir).filter((f) => /^flow\.v\d+\.schema\.json$/.test(f));
+
+describe('schema tolerates unknown keys', () => {
+  it('finds the schema files', () => {
+    expect(schemas.length).toBeGreaterThan(0);
+  });
+
+  it.each(schemas)('%s closes no object to unknown keys', (file) => {
+    const schema = JSON.parse(readFileSync(resolve(schemaDir, file), 'utf8'));
+    const closed: string[] = [];
+    closedObjects(schema, '', closed);
+    expect(closed, 'closing an object would break older readers of newer documents').toEqual([]);
+  });
+});