diff --git a/lib/ai/claude_cli.js b/lib/ai/claude_cli.js
index 3fa32ad..7919192 100644
--- a/lib/ai/claude_cli.js
+++ b/lib/ai/claude_cli.js
@@ -59,6 +59,7 @@ import { createInterface } from 'readline';
  * @param {function} [opts.onEvent]        Called for each normalized event
  * @param {string}   [opts.claudeExe]      Path or name of claude binary (default: CLAUDE_EXE env or 'claude')
  * @param {string[]} [opts.tools]          Exclusive available-tools allowlist (--tools); removes built-ins
+ * @param {boolean}  [opts.resume]         Continue an existing session (--resume) vs create (--session-id)
  * @param {string}   [opts.home]           If set, overrides HOME in child env (for service-user creds)
  * @param {string}   [opts.cwd]            Working directory for the child process
  * @param {number}   [opts.timeoutMs]      Milliseconds before SIGTERM (default: 600000)
@@ -73,6 +74,7 @@ export async function runClaudeTurn(opts) {
     mcpConfigPath,
     allowedTools = [],
     tools = [],
+    resume = false,
     onEvent,
     claudeExe = process.env.CLAUDE_EXE || 'claude',
     home = process.env.VOID_CLAUDE_HOME,
@@ -89,7 +91,9 @@ export async function runClaudeTurn(opts) {
     '--verbose',
     '--include-partial-messages',
     '--append-system-prompt', systemPrompt,
-    '--session-id', sessionId,
+    // First turn creates the session (--session-id); later turns MUST continue
+    // it with --resume (reusing --session-id on an existing session errors).
+    ...(resume ? ['--resume', sessionId] : ['--session-id', sessionId]),
   ];
 
   if (mcpConfigPath) {
diff --git a/lib/api/routes/companion.js b/lib/api/routes/companion.js
index 47aacb0..be0c372 100644
--- a/lib/api/routes/companion.js
+++ b/lib/api/routes/companion.js
@@ -52,6 +52,11 @@ spacesScopedRouter.post('/turn',
     const { agent, convo } = await resolveConversation(req.params.space_id);
     const { text, view } = req.body;
 
+    // Resume the claude session if this conversation already had turns (the CLI
+    // keys session history by --session-id; first turn creates it, rest --resume).
+    const priorTurns = (await messages.listByConversation(convo.id)).length;
+    const resume = priorTurns > 0;
+
     await messages.append(convo.id, { role: 'user', body: text });
 
     res.writeHead(200, {
@@ -105,6 +110,7 @@ spacesScopedRouter.post('/turn',
     try {
       result = await runClaudeTurn({
         sessionId: convo.id,
+        resume,
         systemPrompt: SYSTEM,
         userText: text,
         mcpConfigPath,