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fix: dispatch.zig — surface OOM and spawn failures instead of silent fallback (#430) #435

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fix: dispatch.zig — surface OOM and spawn failures instead of silent fallback (#430) #435

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55 changes: 46 additions & 9 deletions src/runtime/dispatch.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -15,16 +15,22 @@ const ResolvedAgent = types.ResolvedAgent;

/// Dispatch an agent run to the appropriate backend.
/// Writes the agent's text output to `out`.
/// If all backends fail to produce output, writes an error message to `out`
/// so the MCP caller knows the dispatch did not succeed.
pub fn dispatch(
alloc: std.mem.Allocator,
resolved: ResolvedAgent,
prompt: []const u8,
out: *std.ArrayList(u8),
) void {
const before = out.items.len;
switch (resolved.backend) {
.claude => spawnClaude(alloc, resolved, prompt, out),
.codex => spawnCodex(alloc, resolved, prompt, out),
}
if (out.items.len == before) {
out.appendSlice(alloc, "[dispatch] backend produced no output — agent run may have failed") catch {};
}
}

// ── Claude backend ────────────────────────────────────────────────────────
Expand All @@ -50,10 +56,12 @@ fn spawnClaude(
.cwd = resolved.cwd,
};

const full_prompt = if (resolved.system_prompt.len > 0)
std.fmt.allocPrint(alloc, "{s}{s}", .{ resolved.system_prompt, prompt }) catch prompt
else
prompt;
const full_prompt = if (resolved.system_prompt.len > 0) blk: {
break :blk std.fmt.allocPrint(alloc, "{s}{s}", .{ resolved.system_prompt, prompt }) catch {
std.debug.print("[dispatch] OOM: system prompt dropped for claude backend\n", .{});
break :blk prompt;
};
} else prompt;
defer if (full_prompt.ptr != prompt.ptr) alloc.free(full_prompt);

if (sdk.tryClaudeAgent(alloc, full_prompt, opts, out)) return;
Expand All @@ -74,11 +82,12 @@ fn spawnCodex(

const policy: cas.SandboxPolicy = if (resolved.writable) .writable else .read_only;

// Prepend system prompt to the user's prompt
const full_prompt = if (resolved.system_prompt.len > 0)
std.fmt.allocPrint(alloc, "{s}{s}", .{ resolved.system_prompt, prompt }) catch prompt
else
prompt;
const full_prompt = if (resolved.system_prompt.len > 0) blk: {
break :blk std.fmt.allocPrint(alloc, "{s}{s}", .{ resolved.system_prompt, prompt }) catch {
std.debug.print("[dispatch] OOM: system prompt dropped for codex backend\n", .{});
break :blk prompt;
};
} else prompt;
defer if (full_prompt.ptr != prompt.ptr) alloc.free(full_prompt);

cas.runTurnPolicy(alloc, full_prompt, out, policy);
Expand All @@ -93,3 +102,31 @@ test "dispatch: Backend.label is correct" {
try std.testing.expectEqualStrings("claude", Backend.claude.label());
try std.testing.expectEqualStrings("codex", Backend.codex.label());
}

test "dispatch: empty output sentinel is written when backend produces nothing" {
const sentinel = "[dispatch] backend produced no output — agent run may have failed";
var out: std.ArrayList(u8) = .empty;
const alloc = std.testing.allocator;
defer out.deinit(alloc);

const before = out.items.len;
// Simulate the check that dispatch() does after a backend call:
if (out.items.len == before) {
out.appendSlice(alloc, sentinel) catch {};
}
try std.testing.expectEqualStrings(sentinel, out.items);
}

test "dispatch: non-empty output does not get sentinel appended" {
const alloc = std.testing.allocator;
var out: std.ArrayList(u8) = .empty;
defer out.deinit(alloc);

try out.appendSlice(alloc, "agent reply");
const before = out.items.len;
// Simulate: backend already wrote output
if (out.items.len == before) {
out.appendSlice(alloc, "SHOULD NOT APPEAR") catch {};
}
try std.testing.expectEqualStrings("agent reply", out.items);
}
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