diff --git a/packages/core/src/session-manager.ts b/packages/core/src/session-manager.ts
index 479b445b0..6396f50d7 100644
--- a/packages/core/src/session-manager.ts
+++ b/packages/core/src/session-manager.ts
@@ -931,6 +931,22 @@ export function createSessionManager(deps: SessionManagerDeps): SessionManager {
       }
     }
 
+    if (!reserveSessionId(sessionsDir, sessionId)) {
+      const raceSession = await get(sessionId);
+      if (raceSession?.runtimeHandle) {
+        const raceAlive = await plugins.runtime
+          .isAlive(raceSession.runtimeHandle)
+          .catch(() => false);
+        if (raceAlive && orchestratorSessionStrategy === "reuse") {
+          raceSession.metadata["orchestratorSessionReused"] = "true";
+          return raceSession;
+        }
+      }
+      throw new Error(
+        `Failed to reserve orchestrator session ID ${sessionId} (concurrent spawn detected)`,
+      );
+    }
+
     const reusableOpenCodeSessionId =
       plugins.agent.name === "opencode" && orchestratorSessionStrategy === "reuse"
         ? await resolveOpenCodeSessionReuse({
diff --git a/packages/plugins/agent-opencode/src/index.ts b/packages/plugins/agent-opencode/src/index.ts
index 7f5f6f974..feb2394dc 100644
--- a/packages/plugins/agent-opencode/src/index.ts
+++ b/packages/plugins/agent-opencode/src/index.ts
@@ -98,14 +98,17 @@ function createOpenCodeAgent(): Agent {
 
       if (!existingSessionId) {
         const runOptions = ["--title", shellEscape(`AO:${config.sessionId}`), ...sharedOptions];
-        const runCommand = promptValue
-          ? ["opencode", "run", ...runOptions, promptValue].join(" ")
-          : ["opencode", "run", ...runOptions, "--command", "true"].join(" ");
-        const continueSession = `"$(opencode session list --format json | node -e ${shellEscape("let input='';process.stdin.on('data',c=>input+=c).on('end',()=>{const title=process.argv[1];let rows;try{rows=JSON.parse(input)}catch{process.exit(1)};if(!Array.isArray(rows))process.exit(1);const matches=rows.filter((r)=>r&&r.title===title&&typeof r.id==='string');if(matches.length===0)process.exit(1);process.stdout.write(matches[0].id);});")} ${shellEscape(`AO:${config.sessionId}`)})"`;
-        const continueCommand = ["opencode", "--session", continueSession, ...sharedOptions].join(
+
+        const captureSessionId = `"$(node -e ${shellEscape("let buf='';process.stdin.on('data',c=>buf+=c).on('end',()=>{const lines=buf.toString().split('\\n');for(const line of lines){if(!line.trim())continue;try{const evt=JSON.parse(line);if(evt.type==='step_start'&&evt.session_id){process.stdout.write(evt.session_id);process.exit(0);}}}process.exit(1);})")})"`;
+
+        const fallbackSessionId = `"$(opencode session list --format json | node -e ${shellEscape("let input='';process.stdin.on('data',c=>input+=c).on('end',()=>{const title=process.argv[1];let rows;try{rows=JSON.parse(input)}catch{process.exit(1)};if(!Array.isArray(rows))process.exit(1);const matches=rows.filter(r=>r&&r.title===title&&typeof r.id==='string').sort((a,b)=>new Date(b.updated||0)-new Date(a.updated||0));if(matches.length===0)process.exit(1);process.stdout.write(matches[0].id);});")} ${shellEscape(`AO:${config.sessionId}`)})"`;
+
+        const sessionIdCapture = `"$( { ${promptValue ? ["opencode", "run", "--format", "json", ...runOptions, promptValue].join(" ") : ["opencode", "run", "--format", "json", ...runOptions, "--command", "true"].join(" ")} | ${captureSessionId}; } || echo ${fallbackSessionId} )"`;
+
+        const continueCommand = ["opencode", "--session", sessionIdCapture, ...sharedOptions].join(
           " ",
         );
-        return `${runCommand} && exec ${continueCommand}`;
+        return `${continueCommand}`;
       }
 
       if (promptValue) {