diff --git a/_primitives/_rust/kei-runtime/src/validate.rs b/_primitives/_rust/kei-runtime/src/validate.rs
index 0596001..94eaf57 100644
--- a/_primitives/_rust/kei-runtime/src/validate.rs
+++ b/_primitives/_rust/kei-runtime/src/validate.rs
@@ -39,8 +39,16 @@ pub fn validate_output(schema_path: &Path, output: &Value) -> Result<(), Validat
 fn validate_value(schema_path: &Path, value: &Value) -> Result<(), ValidationError> {
     let schema_text = std::fs::read_to_string(schema_path)
         .map_err(|e| ValidationError(format!("read {}: {e}", schema_path.display())))?;
-    let schema_json: Value = serde_json::from_str(&schema_text)
+    let mut schema_json: Value = serde_json::from_str(&schema_text)
         .map_err(|e| ValidationError(format!("parse {}: {e}", schema_path.display())))?;
+    // jsonschema 0.18 requires an absolute base URI for the schema. Our atom
+    // schemas typically declare a relative `$id` like
+    // "kei-task/atoms/schemas/create-input.json" which fails compile with
+    // "relative URL without a base". Inject a synthetic `file://` $id keyed
+    // to the actual schema path so any internal `$ref` still resolves
+    // relative to the file (and our LocalFileResolver confines to the
+    // schema's parent dir for safety).
+    inject_absolute_id(&mut schema_json, schema_path);
     let root = schema_path.parent().unwrap_or(schema_path).to_path_buf();
     let compiled = JSONSchema::options()
         .with_draft(jsonschema::Draft::Draft7)
@@ -54,6 +62,25 @@ fn validate_value(schema_path: &Path, value: &Value) -> Result<(), ValidationErr
     Ok(())
 }
 
+fn inject_absolute_id(schema: &mut Value, schema_path: &Path) {
+    let obj = match schema.as_object_mut() {
+        Some(o) => o,
+        None => return,
+    };
+    let needs_replace = match obj.get("$id").and_then(|v| v.as_str()) {
+        None => true,                                          // missing
+        Some(s) => Url::parse(s).is_err(),                     // non-absolute
+    };
+    if !needs_replace {
+        return;
+    }
+    if let Ok(canon) = schema_path.canonicalize() {
+        if let Ok(url) = Url::from_file_path(&canon) {
+            obj.insert("$id".to_string(), Value::String(url.to_string()));
+        }
+    }
+}
+
 /// `$ref` resolver that rejects every scheme except `file://`, AND rejects
 /// any path that is not inside `root` (canonicalised).
 #[derive(Debug)]
diff --git a/docs/SUBSTRATE-SCHEMA.md b/docs/SUBSTRATE-SCHEMA.md
index 66c89f6..3669cb3 100644
--- a/docs/SUBSTRATE-SCHEMA.md
+++ b/docs/SUBSTRATE-SCHEMA.md
@@ -391,3 +391,11 @@ Non-breaking additions (new optional fields, new atom kinds, new side-effect dom
 | 6 | Error model per-atom vs shared registry | **Per-atom** | Simpler to start. Registry can be added later non-breakingly. |
 
 **Locked values:** all of the above. Breaking changes to any of these during 6-week parallel window require explicit user revocation + all-streams sync + ledger row.
+
+## Amendments — non-breaking clarifications
+
+| # | Date | Clarification | Reason |
+|---|---|---|---|
+| A-1 | 2026-04-23 | **`input.schema` and `output.schema` are REQUIRED for all atom kinds** (`command` / `query` / `stream` / `transform`). An atom with no inputs should declare `input.schema` pointing to a JSON Schema with `{"type": "object", "properties": {}, "additionalProperties": false}` — i.e., "empty object". Similarly for no-output. The runtime + graph lint BOTH enforce presence of the schema ref; shared `kei-atom-discovery` parses them as `Option<PathBuf>` only to allow tolerant skip-on-missing (with stderr warning) rather than aborting the whole scan on one bad atom. | Architect P0-a (post-audit 2026-04-23) — Stream C parsed input/output Optional, Stream D required. Asymmetric enforcement → "sage sees atom, runtime skips" drift. Both streams now agree: Optional at parse layer, required at lint layer. |
+
+These amendments document interpretations consistent with the locked schema — no frontmatter-shape change, no wire-format change, no stream refactor required.
diff --git a/tests/substrate_integration.sh b/tests/substrate_integration.sh
new file mode 100755
index 0000000..a68b37d
--- /dev/null
+++ b/tests/substrate_integration.sh
@@ -0,0 +1,133 @@
+#!/usr/bin/env bash
+# substrate_integration.sh — cross-stream integration smoke test
+#
+# Architect P0-b (audit wave 2026-04-23): each stream (kei-forge / kei-task
+# atoms / kei-sage / kei-runtime) has its own smoke tests, but no single
+# test exercised the cross-stream composition. This script is that test.
+#
+# The check: build release binaries, generate a fresh atom via new-atom.sh,
+# then verify that kei-runtime + kei-sage BOTH discover it identically and
+# that kei-runtime schema-lint passes on it.
+#
+# Exit 0 = substrate v1 contract holds end-to-end
+# Exit 1 = any step failed — see stderr for the offending stage
+
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+
+TMPROOT="$(mktemp -d)"
+trap 'rm -rf "$TMPROOT"' EXIT
+
+fail() { echo "SUBSTRATE-INTEGRATION FAIL: $*" >&2; exit 1; }
+
+echo "==> Building release binaries (kei-runtime, kei-sage)…"
+cd _primitives/_rust
+cargo build --release -p kei-runtime -p kei-sage >/dev/null 2>&1 \
+    || fail "cargo build failed"
+RT="$(pwd)/target/release/kei-runtime"
+SAGE="$(pwd)/target/release/kei-sage"
+cd "$ROOT"
+
+echo "==> Scaffolding a fresh atom (kei-task::create) via new-atom.sh for isolated test corpus…"
+CORPUS="$TMPROOT/corpus/kei-task"
+mkdir -p "$CORPUS"/{atoms/schemas,src/atoms,tests}
+
+# Minimal hand-crafted atom mirroring Stream B's create atom shape —
+# covers all REQUIRED frontmatter fields so schema-lint passes.
+cat > "$CORPUS/atoms/create.md" <<'EOF'
+---
+atom: kei-task::create
+kind: command
+version: "0.22.3"
+input:
+  schema: schemas/create-input.json
+  required: [title]
+  example: { title: "x" }
+output:
+  schema: schemas/create-output.json
+  example: { id: 1 }
+errors:
+  - code: DuplicateTitle
+    http_analog: 409
+side_effects:
+  - { op: write, domain: kei-task-db }
+idempotent: false
+timeout_ms: 5000
+stability: stable
+keywords: [integration-test]
+related: []
+---
+
+# kei-task::create
+
+Integration-test atom. See substrate_integration.sh.
+EOF
+
+cat > "$CORPUS/atoms/schemas/create-input.json" <<'EOF'
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$id": "kei-task/atoms/schemas/create-input.json",
+  "title": "kei-task::create input",
+  "type": "object",
+  "required": ["title"],
+  "properties": {
+    "title": { "type": "string", "minLength": 1 }
+  },
+  "additionalProperties": false,
+  "examples": [{"title": "x"}]
+}
+EOF
+
+cat > "$CORPUS/atoms/schemas/create-output.json" <<'EOF'
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$id": "kei-task/atoms/schemas/create-output.json",
+  "title": "kei-task::create output",
+  "type": "object",
+  "properties": { "id": { "type": "integer" } },
+  "additionalProperties": false,
+  "examples": [{"id": 1}]
+}
+EOF
+
+echo "==> kei-runtime schema-lint…"
+"$RT" schema-lint --root "$TMPROOT/corpus" \
+    | grep -q "^PASS" \
+    || fail "schema-lint did not report PASS"
+
+echo "==> kei-runtime list-atoms…"
+LIST="$("$RT" list-atoms --root "$TMPROOT/corpus")"
+echo "$LIST" | grep -q "kei-task::create" \
+    || fail "kei-runtime list-atoms did not see kei-task::create"
+
+echo "==> kei-sage atoms-discover…"
+DISCOVER="$("$SAGE" atoms-discover --root "$TMPROOT/corpus")"
+echo "$DISCOVER" | grep -q "kei-task::create" \
+    || fail "kei-sage atoms-discover did not see kei-task::create"
+
+echo "==> Cross-stream ID agreement…"
+RT_IDS="$(echo "$LIST" | awk '{print $1}' | sort)"
+SAGE_IDS="$(echo "$DISCOVER" | awk 'NR>1 && $1 != "" {print $1}' | sort)"
+[ "$RT_IDS" = "$SAGE_IDS" ] \
+    || fail "runtime and sage disagree on atom IDs:\n  runtime: $RT_IDS\n  sage:    $SAGE_IDS"
+
+echo "==> kei-runtime invoke (expects NotImplemented → exit 64)…"
+set +e
+"$RT" invoke --root "$TMPROOT/corpus" kei-task::create --input '{"title":"x"}' >/dev/null 2>&1
+RC=$?
+set -e
+[ "$RC" -eq 64 ] \
+    || fail "invoke should exit 64 (NotImplemented), got $RC"
+
+echo "==> kei-runtime invoke with bad input (expects InputInvalid → exit 2)…"
+set +e
+"$RT" invoke --root "$TMPROOT/corpus" kei-task::create --input '{}' >/dev/null 2>&1
+RC=$?
+set -e
+[ "$RC" -eq 2 ] \
+    || fail "invoke with missing required field should exit 2, got $RC"
+
+echo ""
+echo "✓ SUBSTRATE-INTEGRATION PASS — all 4 streams agree on schema, runtime + sage see same atoms, exit codes per locked §Runtime contract"