diff --git a/fleetd/src/main/java/dev/ltms/fleet/member/ClaudeCodeLauncher.java b/fleetd/src/main/java/dev/ltms/fleet/member/ClaudeCodeLauncher.java
index d37773e..6dddea8 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/member/ClaudeCodeLauncher.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/member/ClaudeCodeLauncher.java
@@ -324,11 +324,19 @@ public final class ClaudeCodeLauncher extends HerdrPeerLauncher {
      *
      * <p>CB-618: Claude Code refuses to start when BOTH {@code --append-system-prompt} and
      * {@code --append-system-prompt-file} are on the command line ("Cannot use both ... Please use
-     * only one"), so the two charters can never travel on separate flags. When both are present they
-     * are concatenated into the one file, role charter first and reply charter last — last is where
-     * the reply rule must sit, because it is the rule that must survive. When only the reply charter
-     * is present it keeps its proven inline {@code --append-system-prompt} delivery, which is also
-     * the only form that reaches a member with no repo checkout.
+     * only one"), so the two charters can never travel on separate flags. They are concatenated
+     * into the one file, role charter first and reply charter last — last is where the reply rule
+     * must sit, because it is the rule that must survive.
+     *
+     * <p>fleetd #220: a lone reply charter used to ride inline on {@code --append-system-prompt},
+     * which put ~800 bytes of prose on the command line herdr types into the pane. That line is
+     * capped at {@value HerdrPeerLauncher#PANE_COMMAND_BYTE_LIMIT} bytes by the pty itself, and
+     * everything past the cap is dropped with no error from any layer. The charter alone left about
+     * 50 bytes of headroom, so adding one flag ({@code --session-id}, fleetd #214) truncated the
+     * LAST argument instead — {@code --autocompact 250000} arrived as {@code --autocompact 25},
+     * claude rejected it, and every claude-code spawn died as an unexplained readiness timeout.
+     * The charter now always travels as a file, which takes the prose off the command line for
+     * good; {@link HerdrPeerLauncher#checkPaneCommandFits} is the backstop for whatever grows next.
      */
     private List<String> argvWithFleet(FleetConfig.Profile cfg, LaunchSpec spec) {
         String roleCharter = nonBlank(spec.roleCharter());
@@ -345,16 +353,13 @@ public final class ClaudeCodeLauncher extends HerdrPeerLauncher {
             argv.add("--mcp-config");
             argv.add(mcpConfigJson(cfg));
         }
-        // Combine the charters in order role -> reply, dropping any that are absent. When two
-        // or more survive they must ride one --append-system-prompt-file (CB-618 forbids the inline
-        // flag and the file flag together). A lone reply charter keeps its proven inline delivery.
+        // Combine the charters in order role -> reply, dropping any that are absent. They ride one
+        // --append-system-prompt-file (CB-618 forbids the inline flag and the file flag together),
+        // always — fleetd #220: charter prose on the command line overruns the pane's byte cap.
         List<String> charters = new java.util.ArrayList<>(2);
         if (roleCharter != null) charters.add(roleCharter);
         if (replyCharter != null) charters.add(replyCharter);
-        if (charters.size() == 1 && replyCharter != null && roleCharter == null) {
-            argv.add("--append-system-prompt");
-            argv.add(replyCharter);
-        } else if (!charters.isEmpty()) {
+        if (!charters.isEmpty()) {
             argv.add("--append-system-prompt-file");
             argv.add(writeCharterFile(String.join("\n\n", charters)).toString());
         }
diff --git a/fleetd/src/main/java/dev/ltms/fleet/member/HerdrPeerLauncher.java b/fleetd/src/main/java/dev/ltms/fleet/member/HerdrPeerLauncher.java
index 63053a6..88cbcf6 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/member/HerdrPeerLauncher.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/member/HerdrPeerLauncher.java
@@ -682,6 +682,7 @@ public abstract class HerdrPeerLauncher implements PeerLauncher {
         // Protocol 19 resolves the executable from the agent kind (== namePrefix here), so
         // argv[0] — the configured executable — is dropped and only the extra args are passed.
         List<String> args = argv.isEmpty() ? argv : argv.subList(1, argv.size());
+        checkPaneCommandFits(cfg, argv);
         HerdrException last = null;
         for (int attempt = 0; attempt < NAME_RETRIES; attempt++) {
             long seq = nameSeq.incrementAndGet();
@@ -697,6 +698,59 @@ public abstract class HerdrPeerLauncher implements PeerLauncher {
         throw last;
     }
 
+    /**
+     * fleetd #220: herdr does not exec the launch command — it TYPES it into the pane as one line,
+     * and a pty line buffer holds only {@value #PANE_COMMAND_BYTE_LIMIT} bytes (BSD/macOS {@code
+     * MAX_CANON}). Everything past that byte is dropped. Nothing reports it: herdr answers "agent
+     * started", the backend exits on the mangled argument it was handed, the pane closes, and the
+     * only symptom is {@link #waitUntilInjectableOrThrow} timing out 20 seconds later with no
+     * reason. That is exactly how #214 broke every claude-code spawn — one 50-byte flag pushed a
+     * 978-byte command to 1028, and the tail that got cut was {@code --autocompact 250000}.
+     *
+     * <p>So measure it here and refuse, loudly and immediately, rather than spawn something that
+     * cannot work. The estimate is deliberately conservative: fleetd cannot see herdr's quoting, so
+     * every argument is charged its own bytes plus a separator and a quote pair. An over-estimate
+     * costs a clear error at a length that was already unsafe; an under-estimate would let the
+     * silent truncation back in.
+     *
+     * @throws PeerUnreachableException when the command cannot fit — the same failure the spawn
+     *                                  would have hit anyway, named at the point it is still
+     *                                  explainable
+     */
+    private void checkPaneCommandFits(FleetConfig.Profile cfg, List<String> argv) {
+        int bytes = 0;
+        String longest = null;
+        int longestBytes = 0;
+        for (String arg : argv) {
+            int argBytes = arg == null ? 0 : arg.getBytes(java.nio.charset.StandardCharsets.UTF_8).length;
+            bytes += argBytes + QUOTING_OVERHEAD_PER_ARG;
+            if (argBytes > longestBytes) {
+                longestBytes = argBytes;
+                longest = arg;
+            }
+        }
+        if (bytes <= PANE_COMMAND_BYTE_LIMIT) {
+            return;
+        }
+        String culprit = longest == null ? "<none>"
+                : longest.substring(0, Math.min(longest.length(), 60)) + (longest.length() > 60 ? "…" : "");
+        throw new PeerUnreachableException(
+                "launch command for profile " + cfg.profile() + " is about " + bytes + " bytes, over the "
+                        + PANE_COMMAND_BYTE_LIMIT + "-byte limit of the pane line herdr types it into. "
+                        + "The pty would drop the tail silently and the backend would exit on a mangled "
+                        + "argument. Longest argument is " + longestBytes + " bytes: " + culprit
+                        + " — move it off the command line (a file flag) or shorten it.");
+    }
+
+    /**
+     * The pty line buffer herdr types a launch command into: BSD/macOS {@code MAX_CANON}. Not a
+     * fleetd choice and not configurable — see {@link #checkPaneCommandFits}.
+     */
+    static final int PANE_COMMAND_BYTE_LIMIT = 1024;
+
+    /** Per-argument allowance for the separating space and a shell quote pair fleetd cannot see. */
+    private static final int QUOTING_OVERHEAD_PER_ARG = 3;
+
     /** Start the agent into {@code paneId}, waiting out the seed shell's boot with the sleeper. */
     private Agent startAwaitingShellPrompt(String name, List<String> args, String paneId) {
         HerdrException busy = null;
@@ -860,20 +914,51 @@ public abstract class HerdrPeerLauncher implements PeerLauncher {
      */
     private void waitUntilInjectableOrThrow(String paneId) {
         long deadline = nowMillis.getAsLong() + spawnReadyTimeoutMs;
+        Object lastStatus = null;
         while (nowMillis.getAsLong() < deadline) {
-            if (agents.status(paneId).injectable()) {
+            var status = agents.status(paneId);
+            lastStatus = status;
+            if (status.injectable()) {
                 log.debug("peer pane={} reached injectable state", paneId);
                 return;
             }
             sleeper.run();
         }
-        log.warn("peer pane={} did not become injectable within {}ms — closing", paneId, spawnReadyTimeoutMs);
+        // fleetd #220: read the pane BEFORE stop() closes it. Without this the gate says only that
+        // it timed out, which is true of every cause — a backend that never launched, a binary that
+        // rejected an argument and exited, a trust prompt, a login shell that hung. The pane holds
+        // the one copy of that answer and it is destroyed a line later.
+        log.warn("peer pane={} did not become injectable within {}ms (last status {}) — closing. "
+                        + "Pane tail:\n{}",
+                paneId, spawnReadyTimeoutMs, lastStatus, readPaneQuietly(paneId));
         stop(paneId);
         throw new PeerUnreachableException(
                 "worker pane " + paneId + " did not reach injectable state within "
                         + spawnReadyTimeoutMs + "ms");
     }
 
+    /**
+     * fleetd #220: the pane's recent output, clipped, for the readiness-gate timeout log — or a
+     * short note when it cannot be read. Best-effort by construction: this runs on a path that is
+     * already failing, so it must never replace the real error with one of its own.
+     */
+    private String readPaneQuietly(String paneId) {
+        try {
+            String pane = agents.read(paneId, "recent");
+            if (pane == null || pane.isBlank()) {
+                return "<pane read returned nothing>";
+            }
+            return pane.length() <= SPAWN_FAILURE_PANE_CHARS
+                    ? pane
+                    : pane.substring(pane.length() - SPAWN_FAILURE_PANE_CHARS);
+        } catch (RuntimeException e) {
+            return "<pane could not be read: " + e.getMessage() + ">";
+        }
+    }
+
+    /** How much of a failed spawn's pane the timeout log carries. */
+    private static final int SPAWN_FAILURE_PANE_CHARS = 4000;
+
     /**
      * A concrete {@link PeerHandle} wrapping herdr agent coordinates, the profile that spawned it,
      * the session identity the launch resolved (CB-547a): the bridge's logical name and the peer's
diff --git a/fleetd/src/test/java/dev/ltms/fleet/member/ClaudeCodeLauncherTest.java b/fleetd/src/test/java/dev/ltms/fleet/member/ClaudeCodeLauncherTest.java
index 0fdeefd..3d7a4ec 100644
--- a/fleetd/src/test/java/dev/ltms/fleet/member/ClaudeCodeLauncherTest.java
+++ b/fleetd/src/test/java/dev/ltms/fleet/member/ClaudeCodeLauncherTest.java
@@ -61,8 +61,75 @@ class ClaudeCodeLauncherTest {
         assertTrue(args.stream().noneMatch(a -> a.contains("\"bridge\"")),
                 "the mount is named fleet since CB-632 — a member addresses its tools as "
                         + "mcp__fleet__*, and CLAUDE.md's role-detection ladder names that prefix");
-        assertTrue(args.contains("--append-system-prompt"));
-        assertTrue(args.stream().anyMatch(a -> a.contains("fleet_reply")), "reply charter present");
+        // fleetd #220: the charter travels as a FILE, never inline — charter prose on the command
+        // line overruns the byte cap of the pane line herdr types it into.
+        assertTrue(args.contains("--append-system-prompt-file"));
+        assertFalse(args.contains("--append-system-prompt"),
+                "the inline flag would put ~800 bytes of prose on the pane command line");
+        String charterFile = args.get(args.indexOf("--append-system-prompt-file") + 1);
+        assertTrue(readFile(charterFile).contains("fleet_reply"), "reply charter present in the file");
+    }
+
+    /** Read a charter file the launcher wrote, failing the test rather than the build on an IO error. */
+    private static String readFile(String path) {
+        try {
+            return java.nio.file.Files.readString(java.nio.file.Path.of(path));
+        } catch (java.io.IOException e) {
+            throw new AssertionError("charter file " + path + " is not readable", e);
+        }
+    }
+
+    /**
+     * fleetd #220 regression. herdr TYPES the launch command into the pane, and the pty line buffer
+     * holds 1024 bytes — past that the tail is dropped with no error anywhere, so the backend exits
+     * on a mangled argument and the spawn dies as an unexplained readiness timeout. That is what
+     * happened when #214 added --session-id to a command already 978 bytes long: --autocompact
+     * 250000 arrived as --autocompact 25. This asserts the whole assembled command still fits, with
+     * the flags a real spawn carries (MCP mount, charter, model, autocompact, session id).
+     */
+    @Test
+    void theAssembledLaunchCommandFitsThePaneLineLimit() {
+        FakeHerdr herdr = new FakeHerdr();
+        // Shaped like the live sonnet profile, because the bug is a SUM: the charter alone fits,
+        // and so does every flag alone. Only model + autocompact + session id on top of the charter
+        // crossed the cap, which is why nothing caught it until a member failed to spawn.
+        FleetConfig.Profile cfg = new FleetConfig.Profile(
+                "sonnet", null, "claude-sonnet-5", null, null,
+                List.of("claude"), "tab", "fleetd-workers", "w #{n}", "http://127.0.0.1:8765/mcp",
+                null, null, null, null, null, null, null, null, true, null, null, null, null, null,
+                250000);
+        new ClaudeCodeLauncher(new AgentControl(herdr), new WorkspaceControl(herdr),
+                new SubscriptionGuard(Set.of()), Map.of(cfg.profile(), cfg), cfg.profile(), _ -> null)
+                .spawn();
+
+        List<String> args = spawnedArgs(herdr);
+        int bytes = "claude".length();
+        for (String arg : args) {
+            bytes += arg.getBytes(java.nio.charset.StandardCharsets.UTF_8).length + 3;
+        }
+        assertTrue(bytes <= HerdrPeerLauncher.PANE_COMMAND_BYTE_LIMIT,
+                "the launch command must fit the pane line: " + bytes + " bytes vs limit "
+                        + HerdrPeerLauncher.PANE_COMMAND_BYTE_LIMIT + " — args " + args);
+    }
+
+    /**
+     * fleetd #220: the guard refuses a command that cannot fit, instead of letting the pty drop the
+     * tail. The refusal must name the size and the argument to blame — a spawn that fails with
+     * "did not reach injectable state" tells the operator nothing, which is the whole reason this
+     * bug took a live pane scrape to find.
+     */
+    @Test
+    void anOverlongLaunchCommandIsRefusedWithTheSizeAndTheCulprit() {
+        FakeHerdr herdr = new FakeHerdr();
+        String huge = "x".repeat(1500);
+        ClaudeCodeLauncher launcher = service(herdr, List.of("claude", huge), null);
+
+        PeerUnreachableException refused = assertThrows(PeerUnreachableException.class, launcher::spawn);
+
+        assertTrue(refused.getMessage().contains("1024"), "names the limit: " + refused.getMessage());
+        assertTrue(refused.getMessage().contains("1500"), "names the culprit's size: " + refused.getMessage());
+        assertFalse(herdr.called("agent.start"),
+                "nothing may be started — a truncated command is worse than no spawn");
     }
 
     // CB-634: a profile with ideMcpUrl set mounts the IDE Index MCP as a second server and pins