[hudhaifahz]

Applications planning a channel rebalance need to choose the outgoing and returning channels, including when several channels connect to the same peer. This adds Bolt11Payment::quote_circular_route and Rust/UniFFI quote records exposing the estimated paths and fees for that exact pair. Quoting creates no invoice, probe, HTLC, or payment-store entry.

Approach

The router receives only the selected outgoing channel as a first hop and a route hint for the selected incoming channel. Since ordinary LDK pathfinding rejects payer == payee, the search targets a fresh synthetic recipient behind the incoming peer. Before returning a quote, every path is checked against both endpoint channel IDs and peer identities; only the terminal synthetic identity is then replaced with the local node. Synthetic payment parameters are discarded.

The search uses the local node's BOLT11 feature support, current in-flight HTLCs, channel capacities, incoming forwarding policy, and routing configuration. This permits multipart estimates while keeping the same selected first and last channel on every path. An unset fee cap preserves LDK's default; an explicit zero cap stays zero. Ambiguous user channel IDs are rejected. A quote is an estimate, not a liquidity reservation or a guarantee of payment success.

Scope and provenance

This extracts the route-quotation work from getAlby/ldk-node#115, source commit 70965f2e58936c0893c48ee0cc1788b84164a4e2, and adapts it to upstream main 79e4d92ae73c753dd64ee68ed8cb599cda8af8c7.

There are no payment-storage changes, new dependencies, swaps, UI changes, or automatic rebalancing in this PR. It does not enable circular sending. The older fork's durable preparation, exact receiving-channel enforcement, and restart recovery are deferred for a separate design review. Opaque serialized routes are deliberately absent from the public quote API so this PR does not establish an execution or durable serialization contract.

Validation

• Six new routing tests cover real LDK pathfinding with parallel channels, private aliases, multipart paths, explicit fee limits, invalid/ambiguous selections, and endpoint mismatch rejection. ChannelManager serialization is unchanged by a successful quote in the functional test.
• The existing unstarted-node API regression test now covers quotation.
• cargo +1.91.1 test --lib: 383 passed.
• cargo +1.91.1 test --no-default-features --features uniffi-default --lib: 382 passed.
• Stable formatting check and the repository's production Clippy policy passed in both configurations.
• Documentation built with RUSTDOCFLAGS=-D warnings.

Checks ran on macOS ARM64. Formatting used the repository-pinned nightly-2026-09-22.

The new pathfinding tests use LDK's in-process functional network. They do not send real payments. External Bitcoin Core/electrs integration and the full cross-platform CI matrix remain for upstream CI.

Review focus

Is Bolt11Payment the preferred location for this quote-only API, and is the synthetic-destination route-hint approach acceptable as an initial circular-routing primitive? The intent is to contribute this functionality to shared upstream ownership before proposing the larger payment lifecycle.

AI disclosure: implementation, adaptation, tests, and this description were prepared with OpenAI Codex. CodeRabbit CLI was used for automated review; its findings are assessed rather than treated as a security audit.

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.