diff --git a/src/core/pglite-engine.ts b/src/core/pglite-engine.ts
index faa090d09..79133967c 100644
--- a/src/core/pglite-engine.ts
+++ b/src/core/pglite-engine.ts
@@ -223,6 +223,8 @@ export class PGLiteEngine implements BrainEngine {
    *   - `pages.deleted_at` column (indexed by `pages_deleted_at_purge_idx`) — v0.26.5
    *   - `mcp_request_log.agent_name` + `params` + `error_message` columns
    *     (indexed by `idx_mcp_log_agent_time`) — v0.26.3
+   *   - `subagent_messages.provider_id` column (indexed by
+   *     `idx_subagent_messages_provider`) — v0.27
    *
    * **Maintenance contract:** when a future migration adds a column-with-index
    * or new-table-with-FK referenced by PGLITE_SCHEMA_SQL, extend this method
@@ -256,7 +258,11 @@ export class PGLiteEngine implements BrainEngine {
         EXISTS (SELECT 1 FROM information_schema.tables
                 WHERE table_schema='public' AND table_name='mcp_request_log') AS mcp_log_exists,
         EXISTS (SELECT 1 FROM information_schema.columns
-                WHERE table_schema='public' AND table_name='mcp_request_log' AND column_name='agent_name') AS agent_name_exists
+                WHERE table_schema='public' AND table_name='mcp_request_log' AND column_name='agent_name') AS agent_name_exists,
+        EXISTS (SELECT 1 FROM information_schema.tables
+                WHERE table_schema='public' AND table_name='subagent_messages') AS subagent_messages_exists,
+        EXISTS (SELECT 1 FROM information_schema.columns
+                WHERE table_schema='public' AND table_name='subagent_messages' AND column_name='provider_id') AS subagent_provider_id_exists
     `);
     const probe = rows[0] as {
       pages_exists: boolean;
@@ -271,6 +277,8 @@ export class PGLiteEngine implements BrainEngine {
       search_vector_exists: boolean;
       mcp_log_exists: boolean;
       agent_name_exists: boolean;
+      subagent_messages_exists: boolean;
+      subagent_provider_id_exists: boolean;
     };
 
     const needsPagesBootstrap = probe.pages_exists && !probe.source_id_exists;
@@ -281,9 +289,12 @@ export class PGLiteEngine implements BrainEngine {
     const needsPagesDeletedAt = probe.pages_exists && !probe.deleted_at_exists;
     // v0.26.3 (v33): idx_mcp_log_agent_time in PGLITE_SCHEMA_SQL needs agent_name col.
     const needsMcpLogBootstrap = probe.mcp_log_exists && !probe.agent_name_exists;
+    // v0.27 (v36): idx_subagent_messages_provider in PGLITE_SCHEMA_SQL needs
+    // provider_id (the SECOND column in the composite index `(job_id, provider_id)`).
+    const needsSubagentProviderId = probe.subagent_messages_exists && !probe.subagent_provider_id_exists;
 
     // Fresh installs (no tables yet) and modern brains both no-op.
-    if (!needsPagesBootstrap && !needsLinksBootstrap && !needsChunksBootstrap && !needsPagesDeletedAt && !needsMcpLogBootstrap) return;
+    if (!needsPagesBootstrap && !needsLinksBootstrap && !needsChunksBootstrap && !needsPagesDeletedAt && !needsMcpLogBootstrap && !needsSubagentProviderId) return;
 
     console.log('  Pre-v0.21 brain detected, applying forward-reference bootstrap');
 
@@ -359,6 +370,18 @@ export class PGLiteEngine implements BrainEngine {
         ALTER TABLE mcp_request_log ADD COLUMN IF NOT EXISTS error_message TEXT;
       `);
     }
+
+    if (needsSubagentProviderId) {
+      // v36 (subagent_provider_neutral_persistence_v0_27) adds provider_id +
+      // schema_version on subagent_messages and subagent_tool_executions.
+      // PGLITE_SCHEMA_SQL's `CREATE INDEX idx_subagent_messages_provider ON
+      // subagent_messages (job_id, provider_id)` crashes without provider_id
+      // (composite-index second column). v36 runs later via runMigrations and
+      // is idempotent.
+      await this.db.exec(`
+        ALTER TABLE subagent_messages ADD COLUMN IF NOT EXISTS provider_id TEXT;
+      `);
+    }
   }
 
   async withReservedConnection<T>(fn: (conn: ReservedConnection) => Promise<T>): Promise<T> {
diff --git a/src/core/postgres-engine.ts b/src/core/postgres-engine.ts
index 58e25805a..4a25a3f84 100644
--- a/src/core/postgres-engine.ts
+++ b/src/core/postgres-engine.ts
@@ -167,6 +167,8 @@ export class PostgresEngine implements BrainEngine {
    *   - `pages.deleted_at` column (indexed by `pages_deleted_at_purge_idx`) — v0.26.5
    *   - `mcp_request_log.agent_name` + `params` + `error_message` columns
    *     (indexed by `idx_mcp_log_agent_time`) — v0.26.3
+   *   - `subagent_messages.provider_id` column (indexed by
+   *     `idx_subagent_messages_provider`) — v0.27
    *
    * Keep this in sync with the PGLite version; covered by
    * `test/schema-bootstrap-coverage.test.ts` (PGLite side) and
@@ -191,6 +193,8 @@ export class PostgresEngine implements BrainEngine {
       search_vector_exists: boolean;
       mcp_log_exists: boolean;
       agent_name_exists: boolean;
+      subagent_messages_exists: boolean;
+      subagent_provider_id_exists: boolean;
     }[]>`
       SELECT
         EXISTS (SELECT 1 FROM information_schema.tables
@@ -216,7 +220,11 @@ export class PostgresEngine implements BrainEngine {
         EXISTS (SELECT 1 FROM information_schema.tables
                 WHERE table_schema = current_schema() AND table_name = 'mcp_request_log') AS mcp_log_exists,
         EXISTS (SELECT 1 FROM information_schema.columns
-                WHERE table_schema = current_schema() AND table_name = 'mcp_request_log' AND column_name = 'agent_name') AS agent_name_exists
+                WHERE table_schema = current_schema() AND table_name = 'mcp_request_log' AND column_name = 'agent_name') AS agent_name_exists,
+        EXISTS (SELECT 1 FROM information_schema.tables
+                WHERE table_schema = current_schema() AND table_name = 'subagent_messages') AS subagent_messages_exists,
+        EXISTS (SELECT 1 FROM information_schema.columns
+                WHERE table_schema = current_schema() AND table_name = 'subagent_messages' AND column_name = 'provider_id') AS subagent_provider_id_exists
     `;
     const probe = probeRows[0]!;
 
@@ -230,8 +238,11 @@ export class PostgresEngine implements BrainEngine {
     const needsPagesDeletedAt = probe.pages_exists && !probe.deleted_at_exists;
     // v0.26.3 (v33): idx_mcp_log_agent_time in SCHEMA_SQL needs agent_name col.
     const needsMcpLogBootstrap = probe.mcp_log_exists && !probe.agent_name_exists;
+    // v0.27 (v36): idx_subagent_messages_provider in SCHEMA_SQL needs provider_id
+    // (the SECOND column in the composite index `(job_id, provider_id)`).
+    const needsSubagentProviderId = probe.subagent_messages_exists && !probe.subagent_provider_id_exists;
 
-    if (!needsPagesBootstrap && !needsLinksBootstrap && !needsChunksBootstrap && !needsPagesDeletedAt && !needsMcpLogBootstrap) return;
+    if (!needsPagesBootstrap && !needsLinksBootstrap && !needsChunksBootstrap && !needsPagesDeletedAt && !needsMcpLogBootstrap && !needsSubagentProviderId) return;
 
     console.log('  Pre-v0.21 brain detected, applying forward-reference bootstrap');
 
@@ -307,6 +318,18 @@ export class PostgresEngine implements BrainEngine {
         ALTER TABLE mcp_request_log ADD COLUMN IF NOT EXISTS error_message TEXT;
       `);
     }
+
+    if (needsSubagentProviderId) {
+      // v36 (subagent_provider_neutral_persistence_v0_27) adds provider_id +
+      // schema_version on subagent_messages and subagent_tool_executions.
+      // SCHEMA_SQL's `CREATE INDEX idx_subagent_messages_provider ON
+      // subagent_messages (job_id, provider_id)` crashes without provider_id
+      // (composite-index second column). v36 runs later via runMigrations and
+      // is idempotent.
+      await conn.unsafe(`
+        ALTER TABLE subagent_messages ADD COLUMN IF NOT EXISTS provider_id TEXT;
+      `);
+    }
   }
 
   async transaction<T>(fn: (engine: BrainEngine) => Promise<T>): Promise<T> {
diff --git a/test/schema-bootstrap-coverage.test.ts b/test/schema-bootstrap-coverage.test.ts
index 99dd51251..a31250295 100644
--- a/test/schema-bootstrap-coverage.test.ts
+++ b/test/schema-bootstrap-coverage.test.ts
@@ -76,6 +76,12 @@ const REQUIRED_BOOTSTRAP_COVERAGE: ForwardReference[] = [
   // v0.26.3 (v33) — forward-referenced by `CREATE INDEX idx_mcp_log_agent_time
   // ON mcp_request_log(agent_name, created_at DESC)`.
   { kind: 'column', table: 'mcp_request_log', column: 'agent_name' },
+  // v0.27 (v36) — forward-referenced by `CREATE INDEX
+  // idx_subagent_messages_provider ON subagent_messages (job_id, provider_id)`.
+  // Composite-index second column; the array-based test pattern misses these
+  // by default, which is why this fix wave's Step 3 replaces this with a
+  // SQL parser that extracts every column referenced by any DDL.
+  { kind: 'column', table: 'subagent_messages', column: 'provider_id' },
 ];
 
 test('applyForwardReferenceBootstrap covers every forward reference declared in REQUIRED_BOOTSTRAP_COVERAGE', async () => {
@@ -122,6 +128,9 @@ test('applyForwardReferenceBootstrap covers every forward reference declared in
       ALTER TABLE mcp_request_log DROP COLUMN IF EXISTS agent_name;
       ALTER TABLE mcp_request_log DROP COLUMN IF EXISTS params;
       ALTER TABLE mcp_request_log DROP COLUMN IF EXISTS error_message;
+
+      DROP INDEX IF EXISTS idx_subagent_messages_provider;
+      ALTER TABLE subagent_messages DROP COLUMN IF EXISTS provider_id;
     `);
 
     // Run bootstrap in isolation (NOT initSchema). This is what we're testing.