Track external call support

[angelol]

This issue tracked end-to-end support for external calls across the Canton/Daml stack, as the umbrella issue for TODOs and follow-up PRs while the feature was introduced in small reviewable steps. The feature is now delivered end-to-end (2026-07-07); the agreed post-merge work is tracked in #564 and #565.

Completed upstream PRs

Foundations:

• feat(external-call): Add transaction-side representation for external call results #506 added the transaction-side representation for recorded external call results.
• Add LF builtin for external calls #514 added the LF EXTERNAL_CALL builtin surface: proto/archive decoding, parser support, typing, spec documentation, and LF-focused tests.
• feat(external-call): add engine and Speedy support #518 added Engine and Speedy interpreter support. It wires the builtin into interpretation as a suspend/resume engine question and records successful external call results on the enclosing exercise node.
• feat(external-call): add transaction coding for external call results #522 added transaction protobuf encoding and decoding support for recorded external call results.
• feat(external-call): add Canton protocol support for external call results #526 added Canton protocol serialization support for recorded external call results using dev-only ViewParticipantData protocol data.
• feat(external-call): add extension service client support #537 added participant-side extension-service client support for configured external call services.

Runtime integration (the former #541, superseded by a reviewable split #549–#556, completed by #567):

• feat(external-call): protocol and transaction-hash foundation #549 added transaction hashing scheme V4 and the ViewParticipantData wiring for recorded results.
• feat(external-call): prepared-transaction codec for recorded results #550 added the prepared-transaction codec for external-call results.
• feat(external-call): command-execution errors, rejection and handler SPI #551 added the command-execution errors, rejection routing and the external-call handler SPI.
• feat(external-call): engine execution, model conformance and validation SPI #552 added engine-side execution/replay, model-conformance integration and the validation SPI.
• feat(external-call): extension-service handler, config and command-execution wiring #553 added the extension-service handler, configuration and command-execution wiring.
• feat(external-call): recorded-result consistency checker and extension-service validator #554 added the external-call consistency checker and shared validation fixtures.
• feat(external-call): minimal protocol integration #567 completed the Canton-side runtime integration with the minimal protocol integration agreed with the reviewer: external-call validation runs as a parallel validation suite in transaction processing, disagreements reject the request on behalf of all hosted confirming parties, and results that cannot be re-validated lead to abstentions. feat(external-call): confirmation-response router #555 and feat(external-call): confirmation-responses factory and validation activation #556 (per-party verdict routing) were closed as superseded by feat(external-call): minimal protocol integration #567 — per-party granularity is deliberately dropped in favor of the fail-fast model tracked in External-call: centralize disagreement validation in TransactionView (fail-fast) + review cleanup items #565.

Daml side:

• feat(external-call): add external call support daml#23099 added the SDK support: DA.External.externalCall, compiler/LF tooling support, and Daml Script submit-error reporting — merged against a Canton snapshot containing feat(external-call): minimal protocol integration #567.

Tracked follow-ups (post-merge, agreed during review)

• External-call: end-to-end and negative-path integration tests (post-merge follow-up for #549-#556) #564 — end-to-end and negative-path integration tests, plus the test cleanups from the feat(external-call): minimal protocol integration #567 review.
• External-call: centralize disagreement validation in TransactionView (fail-fast) + review cleanup items #565 — centralize disagreement validation in TransactionView (fail-fast) and the remaining review cleanup items.
• Daml-side follow-ups are collected in the daml#23099 review discussion (error-shape tidy-up, per-error integration tests).

The feature remains ProtocolVersion.dev-gated and is inert unless an extension service is configured.

[angelol]

Update: #506 has merged with the transaction-side representation for external call results.

The next layer is ready in #514: it adds the Daml-LF EXTERNAL_CALL builtin representation, archive decoding/version gating, parser support, typing, spec docs, and LF-focused tests. Runtime engine/speedy execution support remains for a follow-up PR.