diff --git a/src/launch.test.ts b/src/launch.test.ts
index 571d16e..4eba497 100644
--- a/src/launch.test.ts
+++ b/src/launch.test.ts
@@ -7,6 +7,7 @@ import { bootPrompt, dingCommand, discoverSmalltalkDir, harnessCommand, regenera
 import type { AgentSpec } from "./agent-spec.ts";
 import { stRootOf } from "./paths.ts";
 import { writeNetworkConfig } from "./network-config.ts";
+import { parse as tomlParse } from "smol-toml";
 
 describe("native launch command builders (cold-start boot-prompt)", () => {
   it("harnessCommand claude: exec claude with the mode + boot prompt, NO poker, NO --resume", () => {
@@ -152,6 +153,34 @@ describe("writePtyToml (pinned hostname-prefixed ids, cold start)", () => {
     }
   });
 
+  it("network env is merged into BOTH sessions, UNDER the derived wiring; per-agent env overrides it", () => {
+    const net = mkdtempSync(join(tmpdir(), "convoy-netenv-"));
+    const dir = mkdtempSync(join(tmpdir(), "convoy-ptytoml-netenv-"));
+    try {
+      // a fleet-wide knob (PTY_REAP_ON_EXIT) + a HIJACK attempt on ST_AGENT + a key the agent overrides.
+      writeNetworkConfig(net, { name: "ournet", env: { PTY_REAP_ON_EXIT: "false", ST_AGENT: "hijacked", SHARED: "net" } });
+      writePtyToml(dir, spec({ networkRoot: net, env: { SHARED: "agent-wins" } }));
+      const doc = tomlParse(readFileSync(join(dir, ".convoy", "pty.toml"), "utf8")) as {
+        sessions: { claude: { env: Record<string, string> }; ding: { env: Record<string, string> } };
+      };
+      const harness = doc.sessions.claude.env;
+      const ding = doc.sessions.ding.env;
+
+      // the fleet knob lands on BOTH sessions — the ding daemon reads PTY_REAP_ON_EXIT from its OWN env.
+      expect(harness["PTY_REAP_ON_EXIT"]).toBe("false");
+      expect(ding["PTY_REAP_ON_EXIT"]).toBe("false");
+      // the derived wiring ALWAYS wins — a network env can't repoint ST_AGENT.
+      expect(harness["ST_AGENT"]).toBe("silber.convoy-claude");
+      expect(ding["ST_AGENT"]).toBe("silber.convoy-claude");
+      // a per-agent env key overrides the network default (harness); the ding has no per-agent env → network value.
+      expect(harness["SHARED"]).toBe("agent-wins");
+      expect(ding["SHARED"]).toBe("net");
+    } finally {
+      rmSync(net, { recursive: true, force: true });
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   it("--config-dir sets CLAUDE_CONFIG_DIR on the HARNESS session env only, not the ding sidecar", () => {
     const dir = mkdtempSync(join(tmpdir(), "convoy-ptytoml-cfg-"));
     try {
diff --git a/src/launch.ts b/src/launch.ts
index 08d0234..822abe4 100644
--- a/src/launch.ts
+++ b/src/launch.ts
@@ -186,9 +186,12 @@ export function provisionContext(memberDir: string, identity: string): string |
 export function writePtyToml(dir: string, spec: AgentSpec, opts?: { spawner?: string | null }): void {
   const busId = busAgentId(spec); // the host-prefixed bus identity, e.g. silber.convoy-claude
   const root = spec.networkRoot; // the network DIR; ST_ROOT is <root>/smalltalk (the bus), PTY_ROOT is <root>/pty
-  // The ding SERVICE is a per-network choice, recorded in <net>/convoy.toml (unset → node `st ding`). Read
-  // it here so the sidecar command baked into the pty.toml is the network's chosen ding (node or rust).
-  const dingService = root ? readNetworkConfig(root)?.ding : undefined;
+  // Per-network config from <net>/convoy.toml (unset fields → defaults): the ding SERVICE (node `st ding`
+  // vs rust `ding`) baked into the sidecar command, and a network-wide agent ENV merged into every session
+  // below — the fleet-wide knob seam (e.g. PTY_REAP_ON_EXIT), applied UNDER the derived wiring.
+  const netCfg = root ? readNetworkConfig(root) : null;
+  const dingService = netCfg?.ding;
+  const networkEnv = netCfg?.env ?? {};
   const harnessId = sessionId(spec); // e.g. silber.convoy (agentShort strips the -claude/-codex suffix)
   const dingId = `${harnessId}.ding`; // e.g. silber.convoy.ding
   const permanent = specPermanent(spec);
@@ -210,11 +213,13 @@ export function writePtyToml(dir: string, spec: AgentSpec, opts?: { spawner?: st
   // from the harness table: CLAUDE_CONFIG_DIR for claude, CODEX_HOME for codex. Before, this was
   // hardcoded to CLAUDE_CONFIG_DIR, so `--config-dir` on a codex session set a variable codex does not
   // read — the flag reported success and selected nothing.
-  // Spec `env` first, derived wiring LAST: ST_AGENT/ST_ROOT/PTY_ROOT are correct-by-construction (AC-1)
-  // and a hand-written env key must never be able to repoint the agent at another bus.
+  // Precedence, lowest first: NETWORK env (fleet default) < spec `env` (per-agent) < derived wiring LAST.
+  // ST_AGENT/ST_ROOT/PTY_ROOT are correct-by-construction (AC-1) and always win — neither a network nor a
+  // hand-written per-agent key can repoint the agent at another bus; per-agent env still overrides a network
+  // default.
   const specEnv = spec.env ?? {};
   const configEnv = harnessDescriptor(spec.harness).configEnv;
-  const harnessEnv = { ...specEnv, ...env, ...(spec.configDir && configEnv ? { [configEnv]: spec.configDir } : {}) };
+  const harnessEnv = { ...networkEnv, ...specEnv, ...env, ...(spec.configDir && configEnv ? { [configEnv]: spec.configDir } : {}) };
   const doc: Record<string, unknown> = {
     prefix: harnessId,
     sessions: {
@@ -230,7 +235,9 @@ export function writePtyToml(dir: string, spec: AgentSpec, opts?: { spawner?: st
               id: dingId,
               command: dingCommand(busId, harnessId, root ? stRootOf(root) : null, dingService),
               tags: { role: "ding", ...(permanent ? { strategy: "permanent" } : {}), ...stTag },
-              env,
+              // The ding daemon reads PTY_REAP_ON_EXIT from its OWN env, so the network env goes on the ding
+              // session too (under the derived wiring) — else a preserved-fleet setting would skip the sidecar.
+              env: { ...networkEnv, ...env },
             },
           }
         : {}),
diff --git a/src/network-config.test.ts b/src/network-config.test.ts
index 7fa23b6..fa143a3 100644
--- a/src/network-config.test.ts
+++ b/src/network-config.test.ts
@@ -49,6 +49,23 @@ describe("network-config (<net>/convoy.toml)", () => {
     expect(readNetworkConfig(d)).toEqual({ name: "x" });
   });
 
+  it("write + read round-trips a network env map; a non-string-map env is dropped whole", () => {
+    const d = tmp();
+    // unset / empty → absent (callers apply no network env).
+    writeNetworkConfig(d, { name: "default" });
+    expect(readNetworkConfig(d)).toEqual({ name: "default" });
+    writeNetworkConfig(d, { name: "default", env: {} });
+    expect(readNetworkConfig(d)).toEqual({ name: "default" });
+
+    // a string→string map round-trips (the fleet-wide knob shape).
+    writeNetworkConfig(d, { name: "ournet", env: { PTY_REAP_ON_EXIT: "false" } });
+    expect(readNetworkConfig(d)).toEqual({ name: "ournet", env: { PTY_REAP_ON_EXIT: "false" } });
+
+    // a non-string value makes the map invalid → the whole env is dropped (never a half-valid launch env).
+    writeFileSync(networkConfigPath(d), 'name = "x"\n[env]\nGOOD = "1"\nBAD = 2\n');
+    expect(readNetworkConfig(d)).toEqual({ name: "x" });
+  });
+
   it("read is null when the file is missing or nameless", () => {
     const d = tmp();
     expect(readNetworkConfig(d)).toBeNull(); // no file yet
diff --git a/src/network-config.ts b/src/network-config.ts
index 42ad810..9b62834 100644
--- a/src/network-config.ts
+++ b/src/network-config.ts
@@ -25,6 +25,12 @@ export interface NetworkConfig {
   /** The ding sidecar this network runs (see `DingService`). A per-NETWORK choice, not per-agent: the
    *  ding binary is a runtime dependency of the box hosting the net. Unset → "node" (`st ding`). */
   ding?: DingService;
+  /** A network-wide agent env map — merged into EVERY agent's derived session env at render (both the
+   *  harness and the ding session), UNDER the derived wiring so it can never repoint ST_AGENT/ST_ROOT/
+   *  PTY_ROOT; a per-agent `env` still overrides it. This is where a FLEET-WIDE runtime knob lives — e.g.
+   *  `PTY_REAP_ON_EXIT = "false"` to preserve finished sessions network-wide (the pty daemon reads it from
+   *  its own env). Optional; unset → no network env. */
+  env?: Record<string, string>;
 }
 
 /** True iff `v` is a valid `DingService` spelling. */
@@ -32,6 +38,12 @@ export function isDingService(v: unknown): v is DingService {
   return v === "node" || v === "rust";
 }
 
+/** True iff `v` is a flat string→string map — the shape a network env must have (a mis-typed value, e.g.
+ *  a number or nested table, drops the whole env rather than launching agents with a half-valid env). */
+export function isEnvMap(v: unknown): v is Record<string, string> {
+  return typeof v === "object" && v !== null && !Array.isArray(v) && Object.values(v).every((x) => typeof x === "string");
+}
+
 /** The config file location for a network dir: `<dir>/convoy.toml`. */
 export function networkConfigPath(dir: string): string {
   return join(dir, "convoy.toml");
@@ -51,6 +63,7 @@ export function readNetworkConfig(dir: string): NetworkConfig | null {
       name: doc.name,
       ...(typeof doc.megarepo === "string" && doc.megarepo ? { megarepo: doc.megarepo } : {}),
       ...(isDingService(doc.ding) ? { ding: doc.ding } : {}),
+      ...(isEnvMap(doc.env) && Object.keys(doc.env).length > 0 ? { env: doc.env } : {}),
     };
   } catch {
     return null;
@@ -62,5 +75,6 @@ export function writeNetworkConfig(dir: string, config: NetworkConfig): void {
   const doc: Record<string, unknown> = { name: config.name };
   if (config.megarepo) doc["megarepo"] = config.megarepo;
   if (config.ding) doc["ding"] = config.ding;
+  if (config.env && Object.keys(config.env).length > 0) doc["env"] = config.env;
   writeFileSync(networkConfigPath(dir), tomlStringify(doc));
 }