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Rust library FAQs

Can I configure trust anchors using a URL?​

The Rust SDK expects trust anchors as a PEM string, not a URL or file path. Your application must fetch or read the PEM data and pass its contents to the SDK.

In contrast, C2PA Tool accepts URLs and paths as command-line arguments because it retrieves the data before passing it to the SDK.

For example:

let pem = reqwest::blocking::get("https://example.com/trust_anchors.pem")?
    .text()?;

c2pa::settings::Settings::from_toml(
    &toml::toml! {
        [trust]
        trust_anchors = pem
    }
    .to_string(),
)?;

Source: Trust anchors: April 7, 2026

How do I mark assertions as created rather than gathered?​

Use the builder.created_assertion_labels setting to identify assertions attributed to your claim generator. For example:

[builder]
created_assertion_labels = [
  "c2pa.actions",
  "c2pa.thumbnail.claim",
  "c2pa.thumbnail.ingredient",
  "c2pa.ingredient"
]

Choose labels appropriate to your implementation. This example is not a universal conformance configuration. The hard-binding assertion is always a created assertion.

See Created versus gathered assertions for more information.

Source: Created assertions config; June 16, 2026

How do I preserve an existing asset's assertions when editing it?​

Include the original asset as a parentOf ingredient. If the asset is signed, its manifest and assertions are carried forward through that ingredient. You generally do not need to copy those assertions into your new manifest as gathered assertions.

For a typical editing workflow, use:

builder.set_intent(BuilderIntent::Edit);

The Edit intent automatically adds the c2pa.opened action and uses the source as the parent if you do not specify one. Do not manually add a duplicate c2pa.opened action.

It is normal for a simple open-and-edit workflow to have no gathered assertions. If you intentionally reuse metadata as a new gathered assertion, verify that it still applies to the edited asset.

Sources:

Use the Edit intent when an existing asset produces assertion.action.malformed: first action must be created or opened or assertion.action.ingredientMismatch. Ensure manually defined actions are nested inside assertions and the workflow includes a referenced parentOf ingredient. Do not assume an "Invalid Manifest" error is a trust problem without inspecting its validation details.

Sources:

How do I add timestamps for existing ingredient manifests?​

Enable automatic timestamp assertions with these settings:

[builder.auto_timestamp_assertion]
enabled = true
fetch_scope = "all"

Choose the builder intent according to the operation:

  • Use update intent when the asset content has not changed and you only need to add a timestamp assertion.
  • Use edit intent when you are also editing or reformatting the asset.

Add the timestamp assertion while the relevant signing certificate is still within its validity period.

Sources:

Why is no thumbnail generated even though thumbnails are enabled?​

The SDK skips automatic thumbnail generation for update manifests, even when thumbnail generation is enabled. The specification does not permit update manifests to contain thumbnail assertions.

Source: Thumbnail auto-generation, April 19, 2026

How do I avoid sidecar filename collisions?​

Assets with the same base name can produce the same default sidecar filename. For example, file.obj and file.mtl can both produce file.c2pa.

Rename each sidecar immediately after creating it to preserve the original extension, for example:

file.obj.c2pa
file.mtl.c2pa

When reading an asset, explicitly provide the corresponding sidecar using --external-manifest.

Source: Renaming sidecars, September 16, 2026

Why does manifest compression barely reduce a long existing chain?​

Compression applies to the current manifest being written, not retroactively to every manifest in an existing chain. To compare compressed and uncompressed chains, enable prefer_compress_manifests while writing each generation.

In a test of 10 manifests containing a repetitive text custom assertion, the final manifest-store size was 154,638 bytes without compression and 52,583 bytes with compression enabled at each generation. Repetitive test data would likely compress better than real-world content. This is not a promised compression ratio or a way to shrink all prior manifests by appending one compressed manifest.

Sources:

Can I use the Rust implementation from a JVM backend?​

The SDK does not provide a supported Java binding. And the Android bindings focus on Android-specific needs: Adapting them for a JVM service would require additional work; they are not an existing, maintained general-purpose Java package.

You could use the Rust implementation's C API to integrate through JNI.

Source: Android scope and C API integration route, September 14, 2026