diff --git a/src/services/metrics.ts b/src/services/metrics.ts
index 0211b88..16eac35 100644
--- a/src/services/metrics.ts
+++ b/src/services/metrics.ts
@@ -135,6 +135,48 @@ export class Timer {
   }
 }
 
+/**
+ * GIT-73: session rows still being created, by session id.
+ *
+ * session_start creates its session row fire-and-forget and records its own
+ * metrics at the same moment, and gitmem_query_metrics.session_id references
+ * gitmem_sessions(id). Whichever request reached the store first decided
+ * whether the metrics insert 409'd (FK violation). recall right after
+ * session_start raced the same way.
+ *
+ * A metrics write for a session registered here waits for its row. If the row
+ * did not land (or takes longer than the wait budget), the metrics row is
+ * written without the FK and keeps the id in metadata.session_id — where
+ * session_close already looks for it (GIT-109) — instead of being lost to a 409.
+ * Entries stay after they settle so later writes know the answer too.
+ */
+const pendingSessionRows = new Map<string, Promise<boolean>>();
+const SESSION_ROW_WAIT_MS = 15_000;
+
+export function registerPendingSessionRow(sessionId: string, created: Promise<boolean>): void {
+  pendingSessionRows.set(sessionId, created.catch(() => false));
+}
+
+/** For tests. */
+export function clearPendingSessionRows(): void {
+  pendingSessionRows.clear();
+}
+
+async function sessionRowLanded(sessionId: string): Promise<boolean | undefined> {
+  const pending = pendingSessionRows.get(sessionId);
+  if (!pending) return undefined; // not created by this process — nothing to wait for
+  let timer: ReturnType<typeof setTimeout> | undefined;
+  const budget = new Promise<boolean>((resolve) => {
+    timer = setTimeout(() => resolve(false), SESSION_ROW_WAIT_MS);
+    timer.unref?.();
+  });
+  try {
+    return await Promise.race([pending, budget]);
+  } finally {
+    if (timer) clearTimeout(timer);
+  }
+}
+
 /**
  * Track a query and record metrics.
  *
@@ -146,9 +188,17 @@ export async function recordMetrics(metrics: QueryMetrics): Promise<void> {
   // Auto-detect agent if not provided by caller
   const agent = metrics.agent || getAgentIdentity() || null;
 
+  // GIT-73: never reference a session row that is not there yet.
+  let sessionId = metrics.session_id || null;
+  let metadata = metrics.metadata || {};
+  if (sessionId && (await sessionRowLanded(sessionId)) === false) {
+    metadata = { ...metadata, session_id: sessionId };
+    sessionId = null;
+  }
+
   const record: Record<string, unknown> = {
     id: metrics.id,
-    session_id: metrics.session_id || null,
+    session_id: sessionId,
     agent,
     tool_name: metrics.tool_name,
     query_text: metrics.query_text || null,
@@ -160,7 +210,7 @@ export async function recordMetrics(metrics: QueryMetrics): Promise<void> {
     phase_tag: metrics.phase_tag || null,
     linear_issue: metrics.linear_issue || null,
     memories_surfaced: metrics.memories_surfaced || null,
-    metadata: metrics.metadata || {},
+    metadata,
     created_at: new Date().toISOString(),
   };
 
diff --git a/src/tools/session-start.ts b/src/tools/session-start.ts
index 28ffedd..0e79189 100644
--- a/src/tools/session-start.ts
+++ b/src/tools/session-start.ts
@@ -24,6 +24,7 @@ import { getStorage } from "../services/storage.js";
 import {
   Timer,
   recordMetrics,
+  registerPendingSessionRow,
   calculateContextBytes,
   PERFORMANCE_TARGETS,
   buildPerformanceData,
@@ -426,14 +427,14 @@ async function createSessionRecord(
   project: Project,
   linearIssue?: string,
   preGeneratedId?: string  // Accept pre-generated UUID for fire-and-forget pattern
-): Promise<{ session_id: string; latency_ms: number; network_call: boolean }> {
+): Promise<{ session_id: string; latency_ms: number; network_call: boolean; stored: boolean }> {
   const sessionId = preGeneratedId || uuidv4();
   const today = new Date().toISOString().split("T")[0];
   const timer = new Timer();
 
   if (!hasSupabase()) {
     // Free tier: session tracked locally only
-    return { session_id: sessionId, latency_ms: timer.stop(), network_call: false };
+    return { session_id: sessionId, latency_ms: timer.stop(), network_call: false, stored: false };
   }
 
   try {
@@ -458,6 +459,7 @@ async function createSessionRecord(
       session_id: sessionId,
       latency_ms: timer.stop(),
       network_call: true, // Always writes to Supabase
+      stored: true,
     };
   } catch (error) {
     console.error("[session_start] Failed to create session record:", error);
@@ -466,6 +468,7 @@ async function createSessionRecord(
       session_id: sessionId,
       latency_ms: timer.stop(),
       network_call: true, // Network was attempted
+      stored: false,
     };
   }
 }
@@ -1117,8 +1120,12 @@ export async function sessionStart(
     console.error(`[session_start] Resuming session ${sessionId} — skipping record creation`);
   } else {
     sessionId = uuidv4();
-    // Fire-and-forget: don't await the Supabase write
-    createSessionRecord(agent, project, params.linear_issue, sessionId).catch(() => {});
+    // Fire-and-forget: don't await the Supabase write. GIT-73: metrics writes
+    // for this session wait on it instead of racing it to a 409.
+    registerPendingSessionRow(
+      sessionId,
+      createSessionRecord(agent, project, params.linear_issue, sessionId).then((r) => r.stored)
+    );
     // Mark prior in-memory session as superseded (force=true path)
     // Registry displacement in writeSessionFiles handles the registry case,
     // but priorSession may not be in the registry (e.g., after MCP restart)
diff --git a/tests/e2e/blank-supabase.mjs b/tests/e2e/blank-supabase.mjs
index 84b343b..00a8d41 100644
--- a/tests/e2e/blank-supabase.mjs
+++ b/tests/e2e/blank-supabase.mjs
@@ -48,7 +48,7 @@ const DENY_REFS = new Set(["cjptxyezuxdiinufgrrm"]); // production GitMem — ne
  * Known venue failures. Any other 4xx/5xx from /rest/v1/ or /functions/v1/
  * fails the run. Each entry names the ticket that removes it — delete the entry
  * when that ticket merges, so this list only shrinks. An entry allows a failure;
- * it does not require one (e.g. the GIT-73 race is intermittent).
+ * it does not require one (a failure may be intermittent).
  */
 /**
  * Not defects: the store capability probe (src/services/store-columns.ts) is a
@@ -72,7 +72,6 @@ function isCapabilityProbe(r) {
 }
 
 const EXPECTED_FAILURES = [
-  { ticket: "GIT-73", what: "metrics/session_start FK race", method: "POST", path: /^\/rest\/v1\/gitmem_query_metrics$/, status: 409 },
   { ticket: "GIT-105", what: "knowledge-triple thread id into a uuid column", method: "POST", path: /^\/rest\/v1\/knowledge_triples$/, status: 400 },
 ];
 const HERE = dirname(fileURLToPath(import.meta.url));
diff --git a/tests/unit/services/metrics-session-fk.test.ts b/tests/unit/services/metrics-session-fk.test.ts
new file mode 100644
index 0000000..3ff787a
--- /dev/null
+++ b/tests/unit/services/metrics-session-fk.test.ts
@@ -0,0 +1,74 @@
+/**
+ * GIT-73: a metrics row never references a session row that is not there yet.
+ *
+ * session_start created its session row fire-and-forget and wrote its metrics
+ * row at the same moment; gitmem_query_metrics.session_id is an FK to
+ * gitmem_sessions(id), so whichever arrived first decided whether the metrics
+ * insert 409'd.
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const writes = vi.hoisted(() => ({ log: [] as Array<{ table: string; row: Record<string, unknown>; at: number }> }));
+
+vi.mock("../../../src/services/supabase-client.js", () => ({
+  directUpsert: vi.fn(async (table: string, row: Record<string, unknown>) => {
+    writes.log.push({ table, row, at: Date.now() });
+    return row;
+  }),
+}));
+vi.mock("../../../src/services/tier.js", async (orig) => ({
+  ...(await orig<typeof import("../../../src/services/tier.js")>()),
+  hasSupabase: () => true,
+}));
+
+import { recordMetrics, registerPendingSessionRow, clearPendingSessionRows } from "../../../src/services/metrics.js";
+
+const SID = "393adb34-a80c-4c3a-b71a-bc0053b7a7ea";
+const metric = (extra: Record<string, unknown> = {}) => ({
+  id: "m-" + Math.random().toString(16).slice(2), session_id: SID, tool_name: "session_start",
+  latency_ms: 1, result_count: 0, metadata: { project: "p" }, ...extra,
+}) as never;
+
+beforeEach(() => { writes.log = []; clearPendingSessionRows(); });
+
+describe("metrics wait for their session row (GIT-73)", () => {
+  it("waits until the session row has landed, then keeps the FK", async () => {
+    let land!: (v: boolean) => void;
+    registerPendingSessionRow(SID, new Promise<boolean>((r) => (land = r)));
+
+    const pending = recordMetrics(metric());
+    await new Promise((r) => setTimeout(r, 30));
+    expect(writes.log).toHaveLength(0); // held back while the session row is in flight
+
+    land(true);
+    await pending;
+    expect(writes.log).toHaveLength(1);
+    expect(writes.log[0].row.session_id).toBe(SID);
+  });
+
+  it("a session row that did not land: no FK, id kept in metadata.session_id", async () => {
+    registerPendingSessionRow(SID, Promise.resolve(false));
+    await recordMetrics(metric());
+    expect(writes.log[0].row.session_id).toBeNull();
+    expect(writes.log[0].row.metadata).toEqual({ project: "p", session_id: SID });
+  });
+
+  it("a session row whose creation threw is treated as not landed", async () => {
+    registerPendingSessionRow(SID, Promise.reject(new Error("store down")));
+    await recordMetrics(metric());
+    expect(writes.log[0].row.session_id).toBeNull();
+  });
+
+  it("later writes for the same session reuse the settled answer", async () => {
+    registerPendingSessionRow(SID, Promise.resolve(true));
+    await recordMetrics(metric());
+    await recordMetrics(metric({ tool_name: "recall" }));
+    expect(writes.log.map((w) => w.row.session_id)).toEqual([SID, SID]);
+  });
+
+  it("a session this process did not create (resumed, or another process's) is written immediately", async () => {
+    await recordMetrics(metric());
+    expect(writes.log).toHaveLength(1);
+    expect(writes.log[0].row.session_id).toBe(SID);
+  });
+});