Fix: GameMaker Async HTTP Event Data Truncated

Here is how to fix GameMaker GMS2 Async HTTP responses that arrive partially or appear truncated. Large JSON payloads or binary downloads do not fit cleanly into the string-based result. Switching to buffer-based receive solves both issues.

The Symptom

HTTP request returns. The string in async_load[? "result"] is shorter than expected. JSON parses fail. Binary downloads end mid-file.

What Causes This

String size limits. Some platforms cap the string size; large responses cut off.

Binary in string. Strings expect UTF-8; binary bytes confuse the conversion and produce corruption.

Multiple requests. Without filtering by request id, a different request’s data overwrites yours.

The Fix

Step 1: Use http_get_buffer for large or binary data.

// Step or button event
buf = buffer_create(1024, buffer_grow, 1);
req_id = http_get_buffer("https://example.com/data.bin", buf);

The buffer grows as needed; binary or large responses fit.

Step 2: Handle in Async Networking event.

// Async Networking event
if (async_load[? "id"] != req_id) exit;
if (async_load[? "status"] != 0) exit;   // 0 = completed

var _http = async_load[? "http_status"];
if (_http != 200) {
    show_debug_message("HTTP error: " + string(_http));
    exit;
}

// Read from buffer
buffer_seek(buf, buffer_seek_start, 0);
var _json = buffer_read(buf, buffer_string);
// or buffer_read other types for binary

Step 3: Handle status codes. async_load[? “status”] is 0 for completed, 1 for in-progress (incremental data), -1 for error. Check before processing.

Step 4: Free the buffer when done.

buffer_delete(buf);

Step 5: For very large downloads, monitor progress.

if (async_load[? "status"] == 1) {
    // in-progress: progress bar
    var _have = async_load[? "sizeDownloaded"];
    var _total = async_load[? "contentLength"];
    if (_total > 0) progress = _have / _total;
}

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In GameMaker Studio, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in GameMaker. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For GameMaker-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within GameMaker, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Strings cap. Buffers grow. http_get_buffer for anything large or binary; check id, status, http_status before reading.”

Related Issues

For buffer write encoding, see Buffer String Encoding. For surface lifecycle, see Surface Lost.

http_get_buffer. Check id and status. Read via buffer_read. Big payloads survive.