docs(ts): surface session channel reuse next to client.fetch

[EfeDurmaz16]

Summary

Surface the session scheme (channel reuse) right where consumers look, prompted by #212. A production consumer paid a full payment-channel open/settle/finalize per call against an upto endpoint, then had to reverse-engineer the client.fetch throw to find createSessionFetch. Nothing was wrong with the code; the reusable-channel path just was not discoverable.

Two small, docs-and-message-only changes:

• typescript/README.md: a "Reusing a channel across calls" section after the Client section. It states that upto is single-settlement by spec (open/settle/finalize per call) and points repeated-call workloads at the MPP session scheme (open once, cumulative voucher per call, close once), with the createSessionFetch import and the one dependency (the server must advertise a session challenge).
• packages/pay-kit/src/client/index.ts: the session-challenge ConfigurationError now names the import path (createSessionFetch from @solana/mpp/client) so it is directly actionable.

Read order

1. typescript/README.md (the Client section) — the new subsection.
2. packages/pay-kit/src/client/index.ts — the one-line error-message change.

Generated vs handwritten

No generated files. ~25 handwritten lines, docs and one string.

Test plan

• pnpm --dir typescript lint and format:check (prettier clean on both files).
• No behavior change; the throw still fires on the same session-intent path, only the message text is longer. No test asserts the message.

Reviewer note

This deliberately does not touch upto or add any channel-management API. Per the canonical x402 upto spec, upto is single-settlement and reuse is out of scope; channel reuse is the session scheme, which already ships (createSessionFetch). The end-to-end gap for #212 is on the gateway (it must advertise a session challenge), tracked separately in solana-foundation/pay#394.

Fable 5 self-review

Verdict: correct and minimal. This is a discoverability fix, not a feature, and it resists the tempting-but-wrong move of adding settle-without-close to upto, which would break x402 spec conformance. What I would flag as a stranger: it is docs plus one string, so its value depends on people reading the README; a follow-up worth considering is a runnable session example under examples/, which I left out to keep this small. The error-message change is safe (no test pins the text) but does lengthen a user-facing string.

Closes part of #212 (client discoverability). The gateway session route is solana-foundation/pay#394.

[greptile-apps]

Greptile Summary

A discoverability fix surfacing the session channel-reuse path right next to client.fetch, prompted by a production consumer paying a full open/settle/finalize per call when a cheaper reusable-channel path already existed.

• Adds a "Reusing a channel across calls" subsection to typescript/README.md with a complete working snippet (createPaymentChannelSessionOpener + createSessionFetch) and prose explaining why upto is single-settlement by spec.
• Updates the ConfigurationError thrown on a session challenge in client/index.ts to name the correct import path, so the error is self-contained and actionable without needing to search the README.

Confidence Score: 5/5

Safe to merge — purely documentation and a one-line error message update with no behavioral change.

Both changed files are documentation or user-facing string updates only. The README snippet is complete and correct (both required imports and instantiation included). The error message change adds the import path without altering any logic or throw conditions. No tests assert the old message text, so there is no regression risk.

No files require special attention.

Important Files Changed

Filename	Overview
typescript/README.md	Adds a "Reusing a channel across calls" subsection with a complete, correct code snippet (both imports + instantiation) and prose explaining the upto vs. session distinction.
typescript/packages/pay-kit/src/client/index.ts	One-line error-message update: the ConfigurationError on a session challenge now names the import path (createSessionFetch from @solana/mpp/client), making it directly actionable. No logic change.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant App
    participant client.fetch
    participant SessionFetch as createSessionFetch
    participant Server

    Note over App,Server: upto path (single-settlement per call)
    App->>client.fetch: fetch(url)
    client.fetch->>Server: GET (402)
    Server-->>client.fetch: WWW-Authenticate: upto
    client.fetch->>Server: open channel + pay
    Server-->>App: 200

    Note over App,Server: session path (channel reused across calls)
    App->>SessionFetch: "createSessionFetch({ opener })"
    SessionFetch->>Server: GET (402)
    Server-->>SessionFetch: WWW-Authenticate: session
    SessionFetch->>Server: open channel (once)
    loop Each call
        App->>SessionFetch: client.fetch(url)
        SessionFetch->>Server: cumulative voucher
        Server-->>App: 200
    end
    SessionFetch->>Server: close channel (once)

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant App
    participant client.fetch
    participant SessionFetch as createSessionFetch
    participant Server

    Note over App,Server: upto path (single-settlement per call)
    App->>client.fetch: fetch(url)
    client.fetch->>Server: GET (402)
    Server-->>client.fetch: WWW-Authenticate: upto
    client.fetch->>Server: open channel + pay
    Server-->>App: 200

    Note over App,Server: session path (channel reused across calls)
    App->>SessionFetch: "createSessionFetch({ opener })"
    SessionFetch->>Server: GET (402)
    Server-->>SessionFetch: WWW-Authenticate: session
    SessionFetch->>Server: open channel (once)
    loop Each call
        App->>SessionFetch: client.fetch(url)
        SessionFetch->>Server: cumulative voucher
        Server-->>App: 200
    end
    SessionFetch->>Server: close channel (once)

Loading

_{Reviews (2): Last reviewed commit: "docs(ts): make the session snippet runna..." | Re-trigger Greptile}

[greptile-apps]

The snippet only imports createSessionFetch, but the working constructor (shown in typescript/docs/snippets/session.client.ts) requires createPaymentChannelSessionOpener as the opener argument — without it, createSessionFetch({}) has no way to know how to open the channel. A reader following only this section will be stuck at the call site. Including the co-import (and ideally the two-liner instantiation) closes the gap the section is meant to close.

Suggested change

	```ts
	import { createSessionFetch } from '@solana/mpp/client'
	```
	```ts
	import { createSessionFetch, createPaymentChannelSessionOpener } from '@solana/mpp/client'
	const client = createSessionFetch({
	opener: createPaymentChannelSessionOpener({ signer, rpcUrl }),
	})

[EfeDurmaz16]

Fixed in caaa7cd: the snippet now constructs the client with createPaymentChannelSessionOpener as the opener, so it is copy-runnable, matching docs/snippets/session.client.ts.