bridged/src/main/java/dev/ltms/bridged/mcp/BridgeMcp.java

@@ -15,6 +15,7 @@ import dev.ltms.bridged.msg.MessageService;
 import dev.ltms.bridged.msg.Rendezvous;
 import dev.ltms.bridged.peer.PeerUnreachableException;
 import dev.ltms.bridged.placement.BackendQuarantine;
+import dev.ltms.bridged.placement.PlacementException;
 import dev.ltms.bridged.session.SessionManager;
 import dev.ltms.bridged.session.MemberSession;
 import dev.ltms.bridged.session.WorktreeRequest;
@@ -692,6 +693,10 @@ public final class BridgeMcp {
             return text(json(memberView(member)));
         } catch (GuardException e) {
             return error("subscription boundary: " + e.getMessage());
+        } catch (PlacementException e) {
+            // CB-599: no candidate had capacity (maxLoad, quarantine, or all-exhausted) — distinct
+            // from "profile does not exist" below.
+            return error("no capacity: " + e.getMessage());
         } catch (IllegalArgumentException e) {
             return error(e.getMessage()); // unknown / no-default profile, or a refused resumeSessionId
         } catch (PeerUnreachableException e) {

bridged/src/main/java/dev/ltms/bridged/rest/BridgedApp.java

@@ -13,6 +13,7 @@ import dev.ltms.bridged.herdr.HerdrClient;
 import dev.ltms.bridged.herdr.HerdrException;
 import dev.ltms.bridged.inject.MemberPresence;
 import dev.ltms.bridged.peer.PeerUnreachableException;
+import dev.ltms.bridged.placement.PlacementException;
 import dev.ltms.bridged.msg.MessageService;
 import dev.ltms.bridged.session.SessionManager;
 import dev.ltms.bridged.peer.MemberRole;
@@ -292,6 +293,11 @@ public final class BridgedApp {
             ctx.status(201).json(view(member));
         } catch (GuardException e) {
             ctx.status(403).json(Map.of("error", "subscription_boundary", "detail", e.getMessage()));
+        } catch (PlacementException e) {
+            // CB-599: no candidate had capacity (maxLoad, quarantine, or all-exhausted) — a benign,
+            // likely-transient refusal, distinct from "profile does not exist" below. 503: the
+            // request was valid and will likely succeed later.
+            ctx.status(503).json(Map.of("error", "no_capacity", "detail", e.getMessage()));
         } catch (IllegalArgumentException e) {
             ctx.status(400).json(Map.of("error", "unknown_profile", "detail", e.getMessage()));
         } catch (PeerUnreachableException e) {

bridged/src/test/java/dev/ltms/bridged/mcp/BridgeMcpTest.java

@@ -16,12 +16,15 @@ import dev.ltms.bridged.inject.MemberPresence;
 import dev.ltms.bridged.session.MemberSession;
 import dev.ltms.bridged.session.WorktreeRequest;
 import dev.ltms.bridged.member.ClaudeCodeLauncher;
+import dev.ltms.bridged.member.CompositePeerLauncher;
 import dev.ltms.bridged.placement.BackendQuarantine;
+import dev.ltms.bridged.placement.PlacementPolicies;
 import io.modelcontextprotocol.spec.McpSchema;
 import dev.ltms.bridged.msg.InMemoryReplyInbox;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
@@ -423,6 +426,35 @@ class BridgeMcpTest {
         assertTrue(textOf(res).contains("unknown worker profile"), textOf(res));
     }
 
+    /**
+     * CB-599: a profile at its {@code maxLoad} cap must surface a readable reason on the MCP
+     * surface too, not merely flip {@code isError} with an opaque or absent message.
+     */
+    @Test
+    void spawnAtMaxLoadSurfacesTheCapacityReason() {
+        FakeHerdr h = new FakeHerdr();
+        BridgedConfig.Profile wcfg = new BridgedConfig.Profile(
+                "ltms-local", "http://gx00.gw:8000", "coder", null, "BRIDGED_WORKER_TOKEN", null,
+                "tab", "bridged-workers", "worker: {profile} #{n}", null,
+                null, null, null, null, null, null, null, 0, null, null, null);
+        Map<String, BridgedConfig.Profile> profiles = Map.of(wcfg.profile(), wcfg);
+        ClaudeCodeLauncher delegate = new ClaudeCodeLauncher(
+                new AgentControl(h), new WorkspaceControl(h), new SubscriptionGuard(Set.of("gx00.gw")),
+                profiles, wcfg.profile(), k -> "BRIDGED_WORKER_TOKEN".equals(k) ? "tok" : null);
+        CompositePeerLauncher composite = new CompositePeerLauncher(
+                List.of(delegate), wcfg.profile(), profiles, PlacementPolicies.fixed(), _ -> 0);
+        SessionManager sm = new SessionManager(composite);
+
+        McpSchema.CallToolResult res = BridgeMcp.spawn(sm, "ltms-local");
+
+        assertTrue(res.isError());
+        String text = textOf(res);
+        assertTrue(text.contains("no capacity"), "surfaces a capacity reason, not a bare error: " + text);
+        assertTrue(text.contains("ltms-local"), "names the profile: " + text);
+        assertTrue(text.contains("maxLoad"), "explains the refusal: " + text);
+        assertFalse(h.called("agent.start"), "at cap, the spawn is refused before any herdr call");
+    }
+
     @Test
     void spawnPassesTheRequestedCwdToTheWorker() {
         FakeHerdr h = new FakeHerdr();

bridged/src/test/java/dev/ltms/bridged/rest/BridgedAppTest.java

@@ -18,6 +18,8 @@ import dev.ltms.bridged.session.GitWorktrees;
 import dev.ltms.bridged.session.SessionManager;
 import dev.ltms.bridged.session.Worktrees;
 import dev.ltms.bridged.member.ClaudeCodeLauncher;
+import dev.ltms.bridged.member.CompositePeerLauncher;
+import dev.ltms.bridged.placement.PlacementPolicies;
 import io.javalin.Javalin;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
@@ -240,6 +242,43 @@ class BridgedAppTest {
         assertFalse(herdr.called("agent.start"), "an unknown profile must not spawn anything");
     }
 
+    /**
+     * CB-599: a profile at its {@code maxLoad} cap must not surface as a bare 500 — the caller
+     * needs a structured, readable reason, distinct from "unknown_profile".
+     */
+    @Test
+    void spawnAtMaxLoadIs503WithTheCapacityReasonNotABare500() throws Exception {
+        FakeHerdr herdr = new FakeHerdr();
+        BridgedConfig.Profile wcfg = new BridgedConfig.Profile(
+                "ltms-local", "http://gx00.gw:8000", "coder", null, "BRIDGED_WORKER_TOKEN", null,
+                "tab", "bridged-workers", "worker: {profile} #{n}", null,
+                null, null, null, null, null, null, null, 0, null, null, null);
+        Map<String, BridgedConfig.Profile> profiles = Map.of(wcfg.profile(), wcfg);
+        ClaudeCodeLauncher delegate = new ClaudeCodeLauncher(
+                new AgentControl(herdr), new WorkspaceControl(herdr), new SubscriptionGuard(Set.of("gx00.gw")),
+                profiles, wcfg.profile(), k -> "BRIDGED_WORKER_TOKEN".equals(k) ? "tok-abc" : null);
+        CompositePeerLauncher workers = new CompositePeerLauncher(
+                List.of(delegate), wcfg.profile(), profiles, PlacementPolicies.fixed(), _ -> 0);
+        SessionManager sessions = new SessionManager(workers, new GitWorktrees());
+        this.presence = sessions.asPresence();
+        Injector injector = new Injector(new AgentControl(herdr));
+        InMemoryReplyInbox inbox = new InMemoryReplyInbox();
+        sessions.onAcquire(inbox::own);
+        MessageService messages = new MessageService(new AgentControl(herdr), injector, new Rendezvous(), inbox);
+        app = new BridgedApp(herdr, workers, sessions, messages, this.presence, null).build().start("127.0.0.1", 0);
+        int port = app.port();
+
+        HttpResponse<String> res = req(port, "POST", "/members?profile=ltms-local");
+
+        assertEquals(503, res.statusCode(), res.body());
+        JsonNode body = mapper.readTree(res.body());
+        assertEquals("no_capacity", body.get("error").asText());
+        String detail = body.get("detail").asText();
+        assertTrue(detail.contains("ltms-local"), "detail names the profile: " + detail);
+        assertTrue(detail.contains("maxLoad"), "detail explains the refusal: " + detail);
+        assertFalse(herdr.called("agent.start"), "at cap, the spawn is refused before any herdr call");
+    }
+
     @Test
     void spawnWorkerReusesExistingWorkerSpace() throws Exception {
         // A space labelled "bridged-workers" already exists → no second workspace.create.