HTTPS Outcalls

What This Is

HTTPS outcalls allow canisters to make HTTP requests to external web services directly from on-chain code. Because the Internet Computer runs on a replicated subnet (multiple nodes execute the same code), all nodes must agree on the response. A transform function strips non-deterministic fields (timestamps, request IDs, ordering) so that every replica sees an identical response and can reach consensus.

Prerequisites

• For Motoko: mo:core 2.0 and ic >= 2.1.0 in mops.toml
• For Rust: ic-cdk >= 0.19, serde_json for JSON parsing

Canister IDs

HTTPS outcalls use the IC management canister:

You do not deploy anything extra. The management canister is built into every subnet.

Mistakes That Break Your Build

1. Forgetting the transform function. Without a transform, the raw HTTP response often differs between replicas (different headers, different ordering in JSON fields, timestamps). Consensus fails and the call is rejected. ALWAYS provide a transform function.

2. Not attaching cycles to the call. On a normal Application subnet, HTTPS outcalls are not free — the calling canister must attach cycles to cover the cost, and attaching zero fails the call. Both Motoko and Rust have wrappers that compute and attach the required cycles automatically: in Motoko, use await Call.httpRequest(args) from the ic mops package (import Call "mo:ic/Call"); in Rust, use ic_cdk::management_canister::http_request (available since ic-cdk 0.18). Under the hood, both use the ic0.cost_http_request system API to calculate the exact cost from request_size and max_response_bytes — the API is cost-schedule aware, so the same wrapper attaches the correct amount on any subnet type. On a cloud engine (CloudEngine subnet), that amount is always 0 by design; do not "fix" a working outcall by attaching a hardcoded non-zero fee there — see the cloud-engine-canisters skill.

3. Using HTTP instead of HTTPS. The IC only supports HTTPS outcalls. Plain HTTP URLs are rejected. The target server must have a valid TLS certificate.

4. Sizing max_response_bytes against the expected body — the limit is not body-only. The spec defines the size of an HTTP request or response as the total number of bytes representing the names and values of HTTP headers and the HTTP body. Response headers count against max_response_bytes, and a real API commonly sends 1–2 KB of response headers (a unique request id, Date, the rate-limit family, CDN headers) before a single byte of body — against a tight cap that is a large share of the budget. Size the cap for headers + body as they arrive from the server, then add margin. The failure mode is total: the call fails every time rather than returning a truncated response. The maximum is 2MB = 2_000_000 bytes (decimal, not 2^21), and the same headers-plus-body definition caps the request you send at 2_000_000 bytes.

5. Expecting the transform function to shrink an oversized response under the cap. The cap is enforced twice: once on the raw response as it arrives from the server (headers first, then the body against what remains), and again on the transform's Candid-encoded output, which includes serialization overhead. Stripping headers in the transform cannot rescue a raw response that already exceeded the cap, because that first check fails before the transform ever runs. It can keep the transform's own output under the cap — worth doing when the transform would otherwise echo the headers back and its encoded output would exceed the limit. Both checks compare against the same max_response_bytes value, which is why a raw response that only just fits can still fail after the transform: the Candid overhead is added on top. So: size max_response_bytes for the raw response, and keep the transform's output small; never set a tight cap on the theory that stripping headers afterwards will make an oversized response fit.

6. Ignoring the header limits. Independent of max_response_bytes, the spec caps HTTP requests and responses at 64 headers, 8 KiB per header name or value, and 48 KiB for all header names and values combined. The URL must not exceed 8192 bytes. On the request side these are enforced when the replica decodes your arguments, so an over-limit request never leaves the subnet and fails with InvalidManagementPayload — e.g. Deserialize error: The number of elements exceeds maximum allowed 64 — rather than with any HTTP-looking error. If you send no user-agent header the IC adds user-agent: ic/1.0, and that added header does not count toward these limits.

7. Omitting max_response_bytes. If you do not set max_response_bytes, the system assumes the maximum (2MB) and charges cycles accordingly — roughly 20.85 billion cycles on a 13-node subnet. Always set this to a reasonable upper bound for your expected response (see pitfall 4 for what counts toward it).

8. Non-idempotent POST requests without caution. Because multiple replicas make the same request, a POST endpoint that is not idempotent (e.g., "create order") will be called N times (once per replica, typically 13 on a 13-node subnet). Use idempotency keys or design endpoints to handle duplicate requests.

9. Not handling outcall failures. External servers can be down, slow, or return errors. Always handle the error case. There are two distinct timeouts, and neither traps — both come back as rejects (in Motoko the await raises a catchable Error; in Rust the wrapper returns Err):

   • The remote server does not respond within 30 seconds: SysFatal, message Timeout expired.
   • The subnet does not produce a response within 60 seconds: SysTransient, message Canister http request timed out. This one is the retryable one.

10. Calling localhost or private IPs. HTTPS outcalls can only reach public internet endpoints. Localhost, 10.x.x.x, 192.168.x.x, and other private ranges are blocked.

11. Forgetting the Host header. Some API endpoints require the Host header to be explicitly set. The IC does not automatically set this from the URL.

Implementation

Motoko

The management canister types are imported via import IC "ic:aaaaa-aa" (compiler-provided). The ic mops package (import Call "mo:ic/Call") provides Call.httpRequest which auto-computes and attaches the required cycles.

import Blob "mo:core/Blob";
import Nat "mo:core/Nat";
import Text "mo:core/Text";
import IC "ic:aaaaa-aa";
import Call "mo:ic/Call";

persistent actor {

  // Transform function: strips headers so all replicas see the same response for consensus.
  // MUST be a `shared query` function.
  public query func transform({
    context : Blob;
    response : IC.http_request_result;
  }) : async IC.http_request_result {
    {
      response with headers = []; // Strip headers -- they often contain non-deterministic values
    };
  };

  // GET request: fetch a JSON API
  public func getIcpPriceUsd() : async Text {
    let url = "https://api.coingecko.com/api/v3/simple/price?ids=internet-computer&vs_currencies=usd";

    let request : IC.http_request_args = {
      url = url;
      // Always set — omitting defaults to 2MB and charges accordingly.
      // Budget for response headers + body: the cap covers both, and the
      // transform cannot bring an oversized response back under it.
      max_response_bytes = ?(10_000 : Nat64);
      headers = [
        { name = "User-Agent"; value = "ic-canister" },
      ];
      body = null;
      method = #get;
      transform = ?{
        function = transform;
        context = Blob.fromArray([]);
      };
      is_replicated = null;
    };

    // Call.httpRequest computes and attaches the required cycles automatically
    let response = await Call.httpRequest(request);

    switch (Text.decodeUtf8(response.body)) {
      case (?text) { text };
      case (null) { "Response is not valid UTF-8" };
    };
  };

  // POST transform: also discards the body because httpbin.org includes the
  // sender's IP in the "origin" field, which differs across replicas.
  public query func transformPost({
    context : Blob;
    response : IC.http_request_result;
  }) : async IC.http_request_result {
    {
      response with
      headers = [];
      body = Blob.fromArray([]);
    };
  };

  // POST request: send JSON data
  public func postData(jsonPayload : Text) : async Text {
    let url = "https://httpbin.org/post";

    let request : IC.http_request_args = {
      url = url;
      max_response_bytes = ?(50_000 : Nat64);
      headers = [
        { name = "Content-Type"; value = "application/json" },
        { name = "User-Agent"; value = "ic-canister" },
        // Idempotency key: prevents duplicate processing if multiple replicas hit the endpoint
        { name = "Idempotency-Key"; value = "unique-request-id-12345" },
      ];
      body = ?Text.encodeUtf8(jsonPayload);
      method = #post;
      transform = ?{
        function = transformPost;
        context = Blob.fromArray([]);
      };
      is_replicated = null;
    };

    // Call.httpRequest computes and attaches the required cycles automatically
    let response = await Call.httpRequest(request);

    if (response.status == 200) {
      "POST successful (status 200)";
    } else {
      "POST failed with status " # Nat.toText(response.status);
    };
  };
};

Rust

# Cargo.toml
[package]
name = "https_outcalls_backend"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

[dependencies]
ic-cdk = "0.19"
candid = "0.10"
serde = { version = "1", features = ["derive"] }
serde_json = "1"

use ic_cdk::api::canister_self;
use ic_cdk::management_canister::{
    http_request, HttpHeader, HttpMethod, HttpRequestArgs, HttpRequestResult,
    TransformArgs, TransformContext, TransformFunc,
};
use ic_cdk::{query, update};
use serde::Deserialize;

/// Transform function: strips non-deterministic headers so all replicas agree.
/// MUST be a #[query] function.
#[query(hidden = true)]
fn transform(args: TransformArgs) -> HttpRequestResult {
    HttpRequestResult {
        status: args.response.status,
        body: args.response.body,
        headers: vec![], // Strip all headers for consensus
        // If you need specific headers, filter them here:
        // headers: args.response.headers.into_iter()
        //     .filter(|h| h.name.to_lowercase() == "content-type")
        //     .collect(),
    }
}

/// GET request: Fetch JSON from an external API
#[update]
async fn fetch_price() -> String {
    let url = "https://api.coingecko.com/api/v3/simple/price?ids=internet-computer&vs_currencies=usd";

    let request = HttpRequestArgs {
        url: url.to_string(),
        // Budget for response headers + body: the cap covers both, and the
        // transform cannot bring an oversized response back under it.
        max_response_bytes: Some(10_000),
        method: HttpMethod::GET,
        headers: vec![
            HttpHeader {
                name: "User-Agent".to_string(),
                value: "ic-canister".to_string(),
            },
        ],
        body: None,
        transform: Some(TransformContext {
            function: TransformFunc::new(canister_self(), "transform".to_string()),
            context: vec![],
        }),
        is_replicated: None,
    };

    // http_request calls automatically attaches the required cycles
    match http_request(&request).await {
        Ok(response) => {
            let body = String::from_utf8(response.body)
                .unwrap_or_else(|_| "Invalid UTF-8 in response".to_string());

            if response.status != candid::Nat::from(200u64) {
                return format!("HTTP error: status {}", response.status);
            }

            body
        }
        Err(err) => {
            format!("HTTP outcall failed: {:?}", err)
        }
    }
}

/// Typed response parsing example
#[derive(Deserialize)]
struct PriceResponse {
    #[serde(rename = "internet-computer")]
    internet_computer: PriceData,
}

#[derive(Deserialize)]
struct PriceData {
    usd: f64,
}

#[update]
async fn get_icp_price_usd() -> String {
    let body = fetch_price().await;

    match serde_json::from_str::<PriceResponse>(&body) {
        Ok(parsed) => format!("ICP price: ${:.2}", parsed.internet_computer.usd),
        Err(e) => format!("Failed to parse price response: {}", e),
    }
}

/// POST transform: strips headers AND body because httpbin.org includes the
/// sender's IP in the "origin" field, which differs across replicas.
#[query(hidden = true)]
fn transform_post(args: TransformArgs) -> HttpRequestResult {
    HttpRequestResult {
        status: args.response.status,
        body: vec![],
        headers: vec![],
    }
}

/// POST request: Send JSON data to an external API
#[update]
async fn post_data(json_payload: String) -> String {
    let url = "https://httpbin.org/post";

    let request = HttpRequestArgs {
        url: url.to_string(),
        max_response_bytes: Some(50_000),
        method: HttpMethod::POST,
        headers: vec![
            HttpHeader {
                name: "Content-Type".to_string(),
                value: "application/json".to_string(),
            },
            HttpHeader {
                name: "User-Agent".to_string(),
                value: "ic-canister".to_string(),
            },
            // Idempotency key: prevents duplicate processing across replicas
            HttpHeader {
                name: "Idempotency-Key".to_string(),
                value: "unique-request-id-12345".to_string(),
            },
        ],
        body: Some(json_payload.into_bytes()),
        transform: Some(TransformContext {
            function: TransformFunc::new(canister_self(), "transform_post".to_string()),
            context: vec![],
        }),
        is_replicated: None,
    };

    // http_request automatically attaches the required cycles
    match http_request(&request).await {
        Ok(response) => {
            if response.status == candid::Nat::from(200u64) {
                "POST successful (status 200)".to_string()
            } else {
                format!("POST failed with status {}", response.status)
            }
        }
        Err(err) => {
            format!("HTTP outcall failed: {:?}", err)
        }
    }
}

Cycle Cost Estimation

The ic0.cost_http_request system API computes the exact cycle cost at runtime, so canisters do not need to hard-code the formula. Both Call.httpRequest from the ic mops package (Motoko) and ic_cdk::management_canister::http_request (Rust) call it internally and attach the required cycles automatically. For manual use: in Motoko, Prim.costHttpRequest(requestSize, maxResponseBytes) (via import Prim "mo:⛔"); in Rust, ic_cdk::api::cost_http_request(request_size, max_res_bytes).

request_size is the sum of byte lengths of the URL, all header names and values, the body, the transform function name, and the transform context.

For reference, the underlying formula on a 13-node subnet (n = 13) is:

Base cost:                      49_140_000 cycles  (= (3_000_000 + 60_000*13) * 13)
+ per request byte:              5_200 cycles      (= 400 * 13)
+ per max_response_bytes byte:  10_400 cycles      (= 800 * 13)

IMPORTANT: The charge is against max_response_bytes, NOT actual response size.
Omitting max_response_bytes assumes the 2MB maximum (2_000_000 bytes) and costs
49_140_000 + 10_400 * 2_000_000 = 20_849_140_000 cycles (~20.85B), plus the
per-request-byte term.

Unused cycles are refunded when the call returns, so over-budgeting is safe but not free. Attached cycles leave the canister's spendable balance for the duration of the call, so a hand-attached margin directly caps how many outcalls the canister can have in flight before it runs out of balance. For a canister making one outcall per user action, that margin is a concurrency limit. This is why both wrappers attach the exact computed amount rather than a round number — as the ic mops package puts it: "Only minimal amount of cycles are added to the call. This helps the canister to make more calls in parallel without running out of cycles."

Do not hand-attach a buffer "to be safe". Call the wrapper, or compute the exact cost with Prim.costHttpRequest / ic_cdk::api::cost_http_request and attach that. The way to lower the cost of an outcall is a tighter max_response_bytes, never a larger attachment.

This formula applies on a normal Application subnet. It does not apply on a cloud engine (CloudEngine subnet): there, ic0.cost_http_request returns 0 regardless of request_size or max_response_bytes, because engines run under a free cost schedule. Load the cloud-engine-canisters skill for the engine's call rules before writing or debugging outcall code that will run there.

Deploy & Test

Local Deployment

# Start the local replica
icp network start -d

# Deploy your canister
icp deploy backend

Note: HTTPS outcalls work on the local replica. icp-cli proxies the requests through the local HTTP gateway.

Mainnet Deployment

# Ensure your canister has enough cycles (check balance first)
icp canister status backend -e ic

# Deploy
icp deploy -e ic backend

Verify It Works

# 1. Test the GET outcall (fetch price)
icp canister call backend getIcpPriceUsd '()'   # Motoko example above
icp canister call backend fetch_price '()'      # Rust example above
# Expected: Something like '("{\"internet-computer\":{\"usd\":12.34}}")'
# (actual price will vary)

# 2. Test the POST outcall
icp canister call backend postData '("{\"test\": \"hello\"}")'    # Motoko
icp canister call backend post_data '("{\"test\": \"hello\"}")'   # Rust
# Expected: JSON response from httpbin.org echoing back your data

# 3. If using Rust with the typed parser:
icp canister call backend get_icp_price_usd '()'
# Expected: '("ICP price: $12.34")'

# 4. Check canister cycle balance (outcalls consume cycles)
icp canister status backend
# Verify the balance decreased slightly after outcalls

# 5. Test error handling: call with an unreachable URL
# Add a test function that calls a non-existent domain and verify
# it returns an error message rather than trapping

Debugging Outcall Failures

If an outcall fails:

# Check the replica log for detailed error messages
# Local: icp output shows errors inline
# Mainnet: check the canister logs

# Exact reject messages from the replica (match on these, not on paraphrases):
#
# "Timeout expired"                                                     [SysFatal]
#     The remote server did not respond within 30s.
#
# "Canister http request timed out"                                  [SysTransient]
#     The subnet did not produce a response within 60s. Retryable.
#
# "Deadline Exceeded"                                                [SysTransient]
#     The adapter did not answer the replica within its 60s deadline. Rarer
#     than the two above: the adapter's own 30s timeout usually fires first.
#
# "No consensus could be reached. Replicas had different responses.
#  Details: request_id: <id>, hashes: <...>"                         [SysTransient]
#     Transform is missing or not stripping enough non-determinism.
#
# There are THREE distinct size-limit messages, all [SysFatal], because response
# headers are counted against max_response_bytes BEFORE the body is read:
#
# "Header size exceeds specified response size limit <N>"
#     The response headers ALONE exceeded max_response_bytes.
#
# "Http body exceeds size limit of <N> bytes."
#     The body exceeded the allowance REMAINING after header bytes were
#     subtracted. Note the message prints the full cap <N>, not the remainder,
#     so the body that failed can be well under <N>. Do not read this message
#     as "my body is too big" -- it means headers + body are too big.
#
# "Transformed http response exceeds limit: <N>"
#     The Candid-encoded output of your transform exceeded max_response_bytes.
#
#     Raising max_response_bytes fixes all three. Stripping headers in the
#     transform fixes ONLY this last one: the first two are checked before
#     the transform runs.
#
# "http_request request sent with <X> cycles, but <Y> cycles are required."
#                                                              [CanisterReject]
#     Attached less than the computed cost. Use the wrapper rather than a
#     hand-picked number.

Transform Debugging

If you get "no consensus could be reached" errors, your transform function is not making responses identical. Common culprits:

1. Response headers differ -- strip ALL headers in the transform
2. JSON field ordering differs -- parse and re-serialize the JSON in the transform
3. Timestamps in response body -- extract only the fields you need

Advanced transform that normalizes JSON:

#[query]
fn transform_normalize(args: TransformArgs) -> HttpRequestResult {
    // Parse and re-serialize to normalize field ordering
    let body = if let Ok(json) = serde_json::from_slice::<serde_json::Value>(&args.response.body) {
        serde_json::to_vec(&json).unwrap_or(args.response.body)
    } else {
        args.response.body
    };

    HttpRequestResult {
        status: args.response.status,
        body,
        headers: vec![],
    }
}

Additional References

• Load cloud-engine-canisters for canisters running on a cloud engine, including why outcall cost drops to 0 there and why outcalls must never be routed through the engine's console proxy.