Hedera Session Intent for HTTP Payment Authentication

tom@xeno.money

Hedera / Hashgraph

lindsay.w@swirldslabs.com

This document defines the "session" intent for the "hedera" payment method in the Payment HTTP Authentication Scheme. It specifies unidirectional streaming payment channels for incremental, voucher-based payments suitable for low-cost metered services on the Hedera network.

Introduction This document is published as Informational but contains normative requirements using BCP 14 keywords to ensure interoperability between implementations. Payment method specifications that reference this document inherit these requirements. The session intent establishes a unidirectional streaming payment channel using on-chain escrow and off-chain vouchers. This enables high-frequency, low-cost payments by batching many off-chain voucher signatures into periodic on-chain settlements. Unlike the charge intent which requires the full payment amount upfront, the session intent allows clients to pay incrementally as they consume services, paying exactly for resources received. The escrow contract (HederaStreamChannel.sol) is deployed on Hedera's EVM layer and uses standard ERC-20 token transfers. Hedera Token Service (HTS) tokens are exposed as ERC-20 via , enabling payment channels with native HTS tokens such as Circle USDC .

Use Case: LLM Token Streaming Consider an LLM inference API that charges per output token: Client requests a streaming completion (SSE response) Server returns 402 with a session challenge Client opens a payment channel on-chain, depositing funds into the HederaStreamChannel escrow Server begins streaming response As response streams, or over incremental requests, client signs vouchers with increasing amounts Server settles periodically or at stream completion The client pays exactly for tokens received, with no worst-case reservation.

Session Flow The following diagram illustrates the Hedera session flow:

Concurrency Model A channel supports one active session at a time. The cumulative voucher semantics ensure correctness -- each voucher advances a single monotonic counter. The channel is the unit of concurrency; no additional session locking is required. When a client sends a new streaming request on a channel that already has an active session, servers SHOULD terminate the previous session and start a new one. Voucher updates MAY arrive on separate HTTP connections (including HTTP/2 streams) and MUST be processed atomically with respect to balance updates. Servers MUST ensure that voucher acceptance and balance deduction are serialized per channel to prevent race conditions.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology

Streaming Payment Channel: A unidirectional off-chain payment mechanism where the payer deposits funds into an escrow contract and signs cumulative vouchers authorizing increasing payment amounts.
Voucher: An signed message authorizing a cumulative payment amount for a specific channel. Vouchers are monotonically increasing in amount.
Channel: A payment relationship between a payer and payee, identified by a unique channelId. The channel holds deposited funds and tracks cumulative settlements.
Settlement: The on-chain ERC-20 transfer that converts off-chain voucher authorizations into actual token movement. HTS tokens are transferred via standard ERC-20 interfaces exposed through .
Authorized Signer: An address delegated to sign vouchers on behalf of the payer. Defaults to the payer if not specified.
Base Units: The smallest indivisible unit of an HTS token. For example, Circle USDC on Hedera uses 6 decimal places; one million base units equals 1.00 USDC.
Hashio JSON-RPC: Hedera's EVM-compatible JSON-RPC relay that enables standard Ethereum tooling (e.g., viem, ethers.js) to interact with smart contracts deployed on Hedera's EVM layer.

Encoding Conventions This section defines normative encoding rules for interoperability.

Hexadecimal Values All byte arrays (addresses, hashes, signatures, channelId) use: Lowercase hexadecimal encoding 0x prefix No padding or truncation Type Length Example address 42 chars (0x + 40 hex) 0x742d...f8fe00 bytes32 66 chars (0x + 64 hex) 0x6d0f...8e9f signature 130-132 chars 65-byte r s v Implementations MUST use lowercase hex. Implementations SHOULD accept mixed-case input but normalize to lowercase before comparison. Note: Hedera "long-zero" EVM addresses (e.g., 0x0000000000000000000000000000000000001549 for HTS token 0.0.5449) are valid 20-byte addresses and MUST be handled correctly. Implementations MUST use case-insensitive comparison for all address fields.

Numeric Values Integer values (amounts, timestamps) are encoded as decimal strings in JSON to avoid precision loss with large numbers: Field Encoding Example cumulativeAmount Decimal string "250000" requestedAt Decimal string "1736165100" chainId JSON number 296 The chainId uses JSON number encoding as values are small enough to avoid precision issues.

Timestamp Format HTTP headers and receipt fields use formatted timestamps: 2026-04-12T12:05:00Z. Timestamps in EIP-712 signed data use Unix seconds as decimal strings.

Channel Escrow Contract Streaming payment channels require an on-chain escrow contract that holds user deposits and enforces voucher-based withdrawals. On Hedera, this contract is deployed on the EVM layer and interacts with HTS tokens via their ERC-20 interface .

Channel State Each channel is identified by a unique channelId and stores: Field Type Description payer address User who deposited funds payee address Server authorized to withdraw token address ERC-20 token address (HTS via HIP-218) authorizedSigner address Authorized signer (0 = payer) deposit uint128 Total amount deposited settled uint128 Cumulative amount withdrawn by payee closeRequestedAt uint64 Timestamp when close was requested (0 if not) finalized bool Whether channel is closed The channelId MUST be computed deterministically using the escrow contract's computeChannelId() function:

Note: The channelId includes address(this) (the escrow contract address) and block.chainid, explicitly binding the channel to a specific contract deployment and chain. Clients MUST use the contract's computeChannelId() function or equivalent logic to ensure interoperability.

Channel Lifecycle Channels have no expiry -- they remain open until explicitly closed.

Contract Functions Compliant escrow contracts MUST implement the following functions. The signatures shown are the reference HederaStreamChannel.sol implementation.

open Opens a new channel with escrowed funds. Parameter Type Description payee address Server's withdrawal address token address ERC-20 token contract address deposit uint128 Amount to deposit in base units salt bytes32 Random value for channelId authorizedSigner address Delegated signer; 0x0 = payer Returns the computed channelId.

The client MUST approve the escrow contract to spend deposit tokens before calling open(). On Hedera, HTS token approvals via the ERC-20 interface require higher gas limits (approximately 1,000,000 gas) due to the HTS precompile overhead.

settle Server withdraws funds using a signed voucher without closing the channel. Parameter Type Description channelId bytes32 Channel identifier cumulativeAmount uint128 Cumulative total authorized signature bytes EIP-712 signature The contract computes delta = cumulativeAmount - channel.settled and transfers delta tokens to the payee.

topUp User adds more funds to an existing channel. If a close request is pending (closeRequestedAt != 0), calling topUp() MUST cancel it by resetting closeRequestedAt to zero and emitting a CloseRequestCancelled event. Parameter Type Description channelId bytes32 Existing channel identifier additionalDeposit uint256 Additional amount in base units

Note: The additionalDeposit parameter is uint256 (not uint128) in HederaStreamChannel.sol; the contract checks for overflow internally.

close Server closes the channel, settling any outstanding voucher and refunding the remainder to the payer. Only callable by the payee. Parameter Type Description channelId bytes32 Channel to close cumulativeAmount uint128 Final cumulative amount signature bytes EIP-712 signature Transfers cumulativeAmount - channel.settled to payee, refunds channel.deposit - cumulativeAmount to payer, and marks channel finalized.

requestClose User requests channel closure, starting a grace period of at least 15 minutes. Parameter Type Description channelId bytes32 Channel to request closure for Sets channel.closeRequestedAt to current block timestamp. The grace period allows the payee time to submit any outstanding vouchers before forced closure.

withdraw User withdraws remaining funds after the grace period expires. Parameter Type Description channelId bytes32 Channel to withdraw from Requires block.timestamp >= channel.closeRequestedAt + CLOSE_GRACE_PERIOD. Refunds all remaining deposit to payer and marks channel finalized.

associateSelf Associates the escrow contract with an HTS token so it can receive transfers. This is a Hedera-specific function with no Tempo equivalent. Parameter Type Description token address HTS token to associate

This function calls the HTS precompile at address 0x167 to perform token association. Anyone can call it. The escrow contract MUST be associated with the payment token before channels using that token can be opened.

Access Control The escrow contract MUST enforce the following access control: Function Caller Description open Anyone Creates channel; caller = payer settle Payee only Withdraws with voucher topUp Payer only Adds funds close Payee only Closes with final voucher requestClose Payer only Initiates forced close withdraw Payer only Withdraws after grace associateSelf Anyone HTS token association

Signature Verification The escrow contract MUST perform the following signature verification for all functions that accept voucher signatures (settle, close): Canonical signatures: The contract MUST reject ECDSA signatures with non-canonical (high-s) values. Signatures MUST have s <= secp256k1_order / 2 where the half-order is 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E73 57A4501DDFE92F46681B20A0. See for rationale. Authorized signer verification: The contract MUST recover the signer address from the EIP-712 signature and verify it matches the expected signer: If channel.authorizedSigner is non-zero, the recovered signer MUST equal channel.authorizedSigner Otherwise, the recovered signer MUST equal channel.payer Domain binding: The contract MUST use its own address as the verifyingContract in the EIP-712 domain separator, ensuring vouchers cannot be replayed across different escrow deployments. Failure to enforce these requirements on-chain would allow attackers to bypass server-side validation by submitting transactions directly to the contract.

Request Schema The request parameter in the WWW-Authenticate challenge contains a base64url-encoded JSON object.

Fields Field Type Required Description amount string REQUIRED Price per unit in base units unitType string OPTIONAL Unit being priced (e.g., "llm_token") suggestedDeposit string OPTIONAL Suggested deposit in base units currency string REQUIRED ERC-20 token address (HTS via HIP-218) recipient string REQUIRED Payee address (server's withdrawal address) For the session intent, amount specifies the price per unit of service in base units (e.g., 6 decimals for USDC), not a total charge. When unitType is present, clients can use it together with amount to estimate costs before streaming begins. The total cost depends on consumption: total = amount * units_consumed. The optional suggestedDeposit indicates the server's recommended channel deposit for typical usage. Clients MAY deposit less (if they expect limited usage) or more (for extended sessions). The minimum viable deposit is implementation-defined but SHOULD be at least amount to cover one unit of service. Challenge expiry is specified via the expires auth-param in the WWW-Authenticate header per , using timestamp format. Unlike the charge intent, the session request JSON does not include an expires field -- expiry is conveyed solely via the HTTP header.

Method Details As of version 00, session-specific request fields are placed in methodDetails. A future high-level "session" intent definition may promote common fields to the core schema. Field Type Required Description methodDetails.escrowContract string REQUIRED Escrow contract address methodDetails.channelId string OPTIONAL Channel ID if resuming methodDetails.minVoucherDelta string OPTIONAL Minimum voucher increment methodDetails.chainId number OPTIONAL Hedera chain ID (default: 295) Note: Unlike the Tempo session spec, there is no feePayer field in this version. Hedera supports native fee delegation via feePayerAccountId but this is deferred to a future revision (see ). Channel reuse is OPTIONAL. Servers MAY include channelId to suggest resuming an existing channel: New channel (no channelId): Client generates a random salt locally, computes channelId using the formula in , opens the channel on-chain, and returns the channelId in the credential. Existing channel (channelId provided): Client MUST verify channel.deposit - channel.settled >= amount before resuming. If insufficient, client SHOULD either call topUp() with the difference or open a new channel. Servers MAY cache (payer address, payee address, token) -> channelId mappings to suggest channel reuse, reducing on-chain transactions. Example (new channel):

This requests a price of 0.000025 USDC per LLM token, with a suggested deposit of 10.00 USDC (10000000 base units). The currency is Circle USDC on Hedera mainnet (HTS token 0.0.456858, exposed as ERC-20 via HIP-218). Example (existing channel):

For existing channels, suggestedDeposit is omitted since the channel already has funds. The channelId tells the client to resume this channel.

Fee Payment

Client-Paid Fees (Default) In this version, the client pays all transaction fees for channel operations (open, topUp, ERC-20 approve). The client broadcasts these transactions directly via Hashio JSON-RPC. Hedera's EVM layer has predictable, low transaction fees. However, HTS precompile interactions require higher gas limits than standard ERC-20 operations: Operation Recommended Gas Limit ERC-20 approve (HTS) 1,000,000 open 1,500,000 topUp 1,500,000 settle 1,500,000 close 1,500,000 Clients MUST set gas limits appropriate for HTS precompile operations. The default gas estimates from Hashio JSON-RPC may be insufficient.

Server-Initiated Operations The settle and close contract functions are server-originated on-chain transactions. The server pays transaction fees for these operations: Voucher updates (action="voucher") are off-chain and incur no transaction fees. Settlement (settle()) and channel close (close()) are initiated by the server using the highest valid voucher. The server covers the fees. Servers MAY recover settlement costs through pricing or other business logic.

Fee Delegation (Future) Hedera natively supports fee delegation via the feePayerAccountId field on transactions. A future revision of this specification MAY add feePayer support to methodDetails, enabling the server to pay transaction fees on behalf of the client. This would pair naturally with a pull-mode open flow where the client signs the transaction and the server broadcasts it.

Credential Schema The credential in the Authorization header contains a base64url-encoded JSON object per .

Credential Structure Field Type Required Description challenge object REQUIRED Echo of the challenge parameters payload object REQUIRED Session-specific payload Implementations MUST ignore unknown fields in credential payloads, request objects, and receipts to allow forward-compatible extensions.

Credential Lifecycle A streaming payment session progresses through distinct phases, each corresponding to a payload action: Open: Client deposits funds on-chain (broadcasting the transaction directly) and presents the open action with the transaction hash. The server verifies the on-chain deposit and validates the initial voucher. Streaming: Client submits voucher actions with increasing cumulative amounts as service is consumed. The server may periodically settle vouchers on-chain. Close: Client sends the close action with the final voucher. The server settles on-chain and returns a receipt. Each action carries action-specific fields directly in the payload object, with the action field discriminating between phases.

Payload Actions The payload object uses an action discriminator with action-specific fields at the same level: Field Type Required Description action string REQUIRED One of the actions below Action Description open Confirms channel is open on-chain topUp Adds funds to an existing channel voucher Submits updated cumulative voucher close Requests server to close channel

Open Payload The open action confirms an on-chain channel opening and begins the streaming session. Unlike the Tempo session where the client sends a signed transaction for server broadcast, the Hedera client broadcasts the open() transaction itself and presents the transaction hash. Payload fields (in addition to action): Field Type Required Description channelId string REQUIRED Channel identifier (hex bytes32) txHash string REQUIRED Transaction hash from open() cumulativeAmount string REQUIRED Initial authorized amount (see below) signature string REQUIRED EIP-712 voucher signature The client broadcasts the open() transaction via Hashio JSON-RPC, waits for the transaction receipt, and presents the txHash for server verification. The server uses the txHash to verify the on-chain channel state: deposit amount, payee, token, and that the channel is not finalized. The initial voucher (cumulativeAmount and signature) proves the client controls the signing key and establishes the voucher chain. Implementations MAY set cumulativeAmount to zero or to the first request's cost; both are valid starting points for the cumulative voucher sequence. Example:

Note: cumulativeAmount here is "2500" (the cost of the first request at 25 base units per token for 100 tokens). Implementations MAY also send "0". The challenge object MUST echo the challenge parameters from the server's WWW-Authenticate header per .

TopUp Payload The topUp action adds funds to an existing channel during a streaming session. The client broadcasts the topUp() transaction itself and presents the transaction hash. Clients MUST include a challenge object in the Payment credential for topUp actions. To obtain a challenge for a top-up outside an active streaming response, clients MAY send a HEAD request to the protected resource; the server returns 402 with a WWW-Authenticate challenge (no body). Servers MUST reject topUp actions referencing an unknown or expired challenge id with problem type challenge-not-found. Payload fields (in addition to action): Field Type Required Description channelId string REQUIRED Channel ID txHash string REQUIRED Transaction hash from topUp() additionalDeposit string REQUIRED Additional amount in base units Example:

Upon successful verification, the server updates the channel's available balance. The new deposit is immediately available for voucher authorization.

Voucher Payload The voucher action submits an updated cumulative voucher during streaming. Payload fields (in addition to action): Field Type Required Description channelId string REQUIRED Channel identifier cumulativeAmount string REQUIRED Cumulative amount authorized signature string REQUIRED EIP-712 voucher signature Example:

Close Payload The close action requests the server to close the channel and settle on-chain. Payload fields (in addition to action): Field Type Required Description channelId string REQUIRED Channel identifier cumulativeAmount string REQUIRED Final cumulative amount signature string REQUIRED EIP-712 voucher signature The server uses the voucher fields to call close(channelId, cumulativeAmount, signature) on-chain via Hashio JSON-RPC. Example:

Voucher Signing Format Vouchers use typed structured data signing compatible with . This section normatively defines the signing procedure; is referenced for background only.

Wire Format Voucher fields are placed directly in the credential payload object (alongside action) rather than in a nested structure: Field Type Required Description channelId string REQUIRED Channel ID (hex bytes32) cumulativeAmount string REQUIRED Cumulative amount (decimal) signature string REQUIRED EIP-712 signature (hex) The EIP-712 domain and type definitions are fixed by this specification. Implementations MUST reconstruct the full typed data structure using the domain parameters from the challenge (chainId, escrowContract) before signature verification.

Type Definitions The types object MUST contain exactly:

Note: The EIP712Domain type is implicit per EIP-712 and SHOULD NOT be included in the types object.

Domain Separator The domain object MUST contain: Field Type Value name string "Hedera Stream Channel" version string "1" chainId number Hedera chain ID (295 or 296) verifyingContract string Escrow contract address

Signing Procedure To sign a voucher, implementations MUST: Construct the domain separator hash:

Construct the struct hash:

Compute the signing hash:

Sign with ECDSA using secp256k1 curve Encode signature as 65-byte r || s || v where v is 27 or 28

Cumulative Semantics Vouchers specify cumulative totals, not incremental deltas: Voucher #1: cumulativeAmount = 100 (100 total) Voucher #2: cumulativeAmount = 250 (250 total) Voucher #3: cumulativeAmount = 400 (400 total) When settling, the contract computes: delta = cumulativeAmount - settled

Verification Procedure

Open Verification On action="open", servers MUST: Transaction verification: Wait for the transaction receipt using txHash. Verify the transaction succeeded (receipt status = success). On-chain state verification: Query the escrow contract's getChannel(channelId) to verify: Channel exists (deposit > 0) channel.payee matches server's address channel.token matches request.currency channel.deposit - channel.settled >= amount Channel is not finalized channel.closeRequestedAt == 0 Voucher verification: If cumulativeAmount and signature are provided, verify the initial voucher: Recover signer from EIP-712 signature Verify canonical low-s values Signer matches channel.payer or channel.authorizedSigner voucher.channelId matches voucher.cumulativeAmount >= channel.settled Initialize server-side channel state

TopUp Verification On action="topUp", servers MUST: Transaction verification: Wait for the transaction receipt using txHash. Verify the transaction succeeded. On-chain state verification: Query the escrow contract to verify: channel.deposit increased Channel is not finalized Update server-side accounting: increase available balance by additionalDeposit.

Voucher Verification On action="voucher", servers MUST: Verify voucher signature using EIP-712 recovery Verify canonical low-s values (see ) Recover signer and MUST verify it matches expected signer from on-chain state Verify channel.closeRequestedAt == 0. Servers MUST reject vouchers on channels with a pending forced close. Verify monotonicity: cumulativeAmount > highestVoucherAmount (cumulativeAmount - highestVoucherAmount) >= minVoucherDelta Verify cumulativeAmount <= channel.deposit Persist voucher to durable storage before providing service Update highestVoucherAmount = cumulativeAmount Servers MUST derive the expected signer from on-chain channel state by querying the escrow contract. The expected signer is channel.authorizedSigner if non-zero, otherwise channel.payer. Servers MUST NOT trust signer claims in HTTP payloads. Servers MUST persist the highest voucher to durable storage before providing the corresponding service. Failure to do so may result in unrecoverable fund loss if the server crashes after service delivery.

Idempotency Servers MUST treat voucher submissions idempotently: Resubmitting a voucher with the same cumulativeAmount as the highest accepted MUST return 200 OK with the current highestAmount Submitting a voucher with lower cumulativeAmount than highest accepted MUST return 200 OK with the current highestAmount (not an error) Clients MAY safely retry voucher submissions after network failures

Rejection and Error Responses If verification fails, servers MUST return an appropriate HTTP status code with a Problem Details response body: Status When 400 Bad Request Malformed payload or missing fields 402 Payment Required Invalid signature or signer mismatch 410 Gone Channel finalized or not found Error responses use Problem Details format:

Problem type URIs: Type URI Description .../session/invalid-signature Voucher signature invalid .../session/signer-mismatch Signer not authorized .../session/amount-exceeds-deposit Exceeds deposit .../session/delta-too-small Below minVoucherDelta .../session/channel-not-found No such channel .../session/channel-finalized Channel closed .../session/challenge-not-found Challenge expired .../session/insufficient-balance Insufficient balance All problem type URIs above are prefixed with https://paymentauth.org/problems. For errors on the Payment Auth protected resource, servers MUST return 402 with a fresh WWW-Authenticate: Payment challenge per .

Server-Side Accounting Servers MUST maintain per-session accounting state to track authorized funds versus consumed service.

Accounting State For each active session identified by (challengeId, channelId), servers MUST maintain: Field Type Description acceptedCumulative uint128 Highest valid voucher accepted spent uint128 Cumulative amount charged settledOnChain uint128 Last settled amount (informational) The available balance is computed as:

Per-Request Processing For each request carrying a Payment credential with intent="session", servers MUST follow this procedure: Voucher acceptance (if provided in credential): Verify signature and monotonicity per If valid, persist the new acceptedCumulative If invalid, return 402 with a fresh challenge Balance check: Compute available = acceptedCumulative - spent Compute cost for this request If available < cost: return 402 with Problem Details including requiredTopUp = cost - available Charge and deliver (if available >= cost): MUST persist spent := spent + cost BEFORE or atomically with delivering service Deliver the response (or next chunk for streaming) Return Payment-Receipt header Receipt generation: Include balance state in receipt

Crash Safety To prevent fund loss from server crashes: Servers MUST persist spent increments BEFORE delivering corresponding service. Servers MUST persist acceptedCumulative BEFORE relying on the new balance for service authorization. Implementations SHOULD use transactional storage or write-ahead logging to ensure atomicity.

Request Idempotency To prevent double-charging on retries: Clients SHOULD include an Idempotency-Key header Servers SHOULD track (challengeId, idempotencyKey) pairs and return cached responses for duplicates Servers MUST NOT increment spent for duplicate idempotent requests Example idempotent request:

Cost Calculation The cost for a request depends on the pricing model declared in the challenge. Servers MUST support at least one of: Fixed cost: A predetermined amount per request Usage-based fees: Pricing proportional to resource consumption For streaming responses (SSE, chunked), servers SHOULD: Reserve an estimated cost before starting delivery Adjust spent as actual consumption is measured Pause delivery if available is exhausted

Insufficient Balance During Streaming When a streaming response exhausts available balance: Server MUST stop delivering additional content Server MAY hold the connection open awaiting a voucher top-up Server MAY close the response; client retries with a higher voucher If client submits a voucher update, server SHOULD resume delivery if the connection is still open For SSE responses, servers MUST emit a payment-need-voucher event when balance is exhausted:

The payment-need-voucher event data MUST be a JSON object containing: Field Type Required Description acceptedCumulative string REQUIRED Current highest voucher channelId string REQUIRED Channel identifier deposit string REQUIRED Current on-chain deposit requiredCumulative string REQUIRED Minimum next voucher The deposit field allows the client to determine the correct recovery action. When requiredCumulative exceeds deposit, the client MUST submit action="topUp" before sending a new voucher. When requiredCumulative is within deposit, the client can submit action="voucher" directly. After emitting payment-need-voucher, the server MUST pause delivery until a valid voucher is accepted. Servers SHOULD close the stream if no voucher is received within a reasonable timeout (e.g., 60 seconds). Servers SHOULD NOT deliver service beyond the authorized balance under any circumstances. See for rate limiting requirements.

Settlement Procedure

Settlement Timing Servers MAY settle at any time using their own criteria: Periodically (e.g., every N seconds or M base units) When action="close" is received When accumulated unsettled amount exceeds a threshold Based on gas cost optimization Settlement frequency is an implementation detail left to servers. The close() function settles any delta between the provided cumulativeAmount and channel.settled. If the server has already settled the highest voucher via settle(), calling close() with the same amount will only refund the payer the remaining deposit.

Cooperative Close When the client sends action="close": Server receives the signed close request Server calls close(channelId, cumulativeAmount, signature) on-chain via Hashio JSON-RPC Contract settles any delta and refunds remainder Server returns receipt with transaction hash Servers SHOULD close promptly when clients request -- the economic incentive is to claim earned funds immediately. The server MUST set a gas limit of at least 1,500,000 for the close() call due to HTS precompile overhead.

Forced Close If the server does not respond to close requests: Client calls requestClose(channelId) on-chain 15-minute grace period begins Server can still settle() or close() during the grace period After grace period, client calls withdraw(channelId) Client receives all remaining (unsettled) funds Clients SHOULD wait at least 16 minutes after requestClose() before calling withdraw() to account for block time variance.

Sequential Sessions A single channel supports sequential sessions. Each session uses the same cumulative voucher counter. When a new session begins on a channel, the previous session's spending state is irrelevant -- the channel's highestVoucherAmount is the source of truth for the next voucher's minimum value.

Voucher Submission Transport Vouchers are submitted via HTTP requests to the same resource URI that requires payment. There is no separate session endpoint. Clients SHOULD use HTTP/2 multiplexing or maintain separate connections for voucher updates and content streaming when topping up during a long-lived response. For voucher-only updates (no response body needed), clients MAY use HEAD requests. Servers SHOULD support voucher credentials on HEAD requests for resources that require session payment.

Receipt Generation Servers MUST return a Payment-Receipt header on every successful paid request. For streaming responses (SSE, chunked transfer), servers MUST include the receipt in the initial response headers AND in the final message of the stream. For SSE responses, the final receipt SHOULD be delivered as an event:

The session intent extends the receipt with balance tracking: Field Type Description method string "hedera" intent string "session" status string "success" timestamp string response time challengeId string Challenge identifier channelId string Channel identifier acceptedCumulative string Highest voucher accepted spent string Total charged so far reference string Transaction or channel ref units number OPTIONAL: Units consumed txHash string OPTIONAL: Transaction hash The reference field satisfies the core MPP receipt reference requirement. It is set to txHash when a transaction was broadcast (open, close), otherwise set to channelId (voucher). The txHash field is OPTIONAL because not every response involves an on-chain settlement -- voucher updates are off-chain. Example receipt (per-request with metering):

Example receipt (on close with settlement):

Security Considerations

Replay Prevention Vouchers are bound to a specific channel and contract via: channelId in the voucher message verifyingContract in EIP-712 domain chainId in EIP-712 domain Cumulative amount semantics (can only increase) The escrow contract enforces: cumulativeAmount > channel.settled (monotonicity) cumulativeAmount <= channel.deposit (cap)

No Voucher Expiry Vouchers have no validUntil field. This simplifies the protocol: Channels have no expiry -- closed explicitly Vouchers remain valid until the channel closes The close grace period protects against clients disappearing Operational guidance: Servers SHOULD settle and close channels inactive for extended periods (e.g., 30+ days).

Denial of Service To mitigate voucher flooding, servers MUST implement rate limiting: Servers SHOULD limit voucher submissions to 10 per second per session Servers MAY implement additional IP-based rate limiting Servers MUST enforce minVoucherDelta when present Servers SHOULD skip expensive signature verification for vouchers that do not advance state (return 200 OK with current highestAmount per ) Servers SHOULD perform format validation before expensive ECDSA signature recovery. To mitigate channel griefing via dust deposits: Servers SHOULD enforce a minimum deposit (e.g., 1 USDC equivalent) Servers MAY reject channels below this threshold

Front-Running Protection Cumulative voucher semantics prevent front-running attacks. If a client submits a higher voucher while a server's settle() transaction is pending, the settlement will still succeed -- it merely leaves additional unsettled funds.

Cross-Contract Replay Prevention The EIP-712 domain includes verifyingContract, binding vouchers to a specific escrow contract address.

Escrow Guarantees The escrow contract provides: Payer protection: Funds only withdrawn with valid voucher signature Payee protection: Deposited funds guaranteed Forced close: 15-minute grace period protects both parties

Authorized Signer The authorizedSigner field allows delegation of signing authority to a hot wallet while the main wallet only deposits funds. Security considerations for delegated signing: Clients using authorizedSigner delegation SHOULD limit channel deposits to acceptable loss amounts Clients SHOULD rotate authorized signers periodically Clients SHOULD NOT reuse signers across multiple high-value channels If the authorized signer key is compromised, an attacker can drain the entire channel deposit

Signature Malleability ECDSA signatures are malleable: for any valid signature (r, s), the signature (r, -s mod n) is also valid. To prevent signature substitution attacks, implementations MUST enforce canonical signatures: Signatures MUST use "low-s" values with s <= secp256k1_order / 2 The secp256k1 half-order is: 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E73 57A4501DDFE92F46681B20A0 Servers MUST reject signatures with s values exceeding this threshold Accepted signature formats: 65-byte (r, s, v) format where v is 27 or 28 64-byte EIP-2098 compact format The HederaStreamChannel.sol contract uses Solady's SignatureCheckerLib which enforces these requirements.

Voucher Context and User Experience The voucher message contains only channelId and cumulativeAmount. Wallet implementations are encouraged to: Decode channelId components when the derivation formula is known Display the payee address and token in human-readable form Show cumulative vs. incremental amounts clearly

Session Attribution Vouchers are bound to channels but not to specific HTTP sessions or API requests. The challengeId provides correlation across requests. Servers MUST implement challenge-to-voucher mapping for: Dispute resolution Usage accounting Audit trails

Cross-Session Replay Prevention Vouchers use cumulative amount semantics: each voucher authorizes a total payment up to cumulativeAmount, and the on-chain contract enforces strict monotonicity (cumulativeAmount > channel.settled). A voucher can only ever advance the channel state forward. A separate sessionHash binding is unnecessary: Cross-session replay is harmless: If a voucher from session A is presented in session B, it can only authorize funds up to the amount already committed. Cross-resource replay: Vouchers authorize cumulative payment on a channel, not access to specific resources. Resource authorization is handled at the application layer via challengeId.

Chain Finality Hedera achieves asynchronous Byzantine Fault Tolerant (aBFT) consensus with deterministic finality in approximately 3-5 seconds . Once a transaction reaches consensus, it cannot be reversed. For high-value channels, servers SHOULD: Re-verify channel state periodically during long-lived sessions Monitor for ChannelClosed or CloseRequested events Cease service delivery if the channel becomes invalid

HTS Token Association Before an escrow contract can receive HTS tokens, it MUST be associated with the token via the associateSelf() function (see ). This is a one-time operation per token. If the escrow contract is not associated with the payment token, open() will fail with a transfer error. Servers deploying escrow contracts MUST ensure the contract is associated with all supported payment tokens before advertising session challenges.

Gas Limit Considerations Hedera's EVM layer routes HTS token operations through the HTS precompile at address 0x167. This precompile has higher gas requirements than standard ERC-20 operations. Implementations MUST set appropriate gas limits (see ) to avoid transaction failures. The default gas estimates from Hashio JSON-RPC (eth_estimateGas) may underestimate gas for HTS precompile calls. Implementations SHOULD use hardcoded minimum gas limits for escrow operations.

Grace Period Rationale The 15-minute forced close grace period balances competing concerns: Payer protection: Ensures timely fund recovery Payee protection: Provides time to detect close requests and submit final settlements Block time variance: Allows margin for timestamp variations

IANA Considerations

Payment Intent Registration This document registers the following payment intent in the "HTTP Payment Intents" registry established by : Intent Methods Description Reference session hedera Streaming payment channel This document Contact: Tom Rowbotham (tom@xeno.money)

Problem Type Registration This document registers the following problem types in the "HTTP Problem Types" registry established by : Type URI Title Status Ref .../session/invalid-signature Invalid Signature 402 This document .../session/signer-mismatch Signer Mismatch 402 This document .../session/amount-exceeds-deposit Amount Exceeds Deposit 402 This document .../session/delta-too-small Delta Too Small 402 This document .../session/channel-not-found Channel Not Found 410 This document .../session/channel-finalized Channel Finalized 410 This document .../session/challenge-not-found Challenge Not Found 402 This document .../session/insufficient-balance Insufficient Balance 402 This document All type URIs above are prefixed with https://paymentauth.org/problems. Each problem type is defined in .

&RFC2119; &RFC3339; &RFC4648; &RFC8174; &RFC8259; &RFC9110; &RFC9111; &RFC9457; The 'Payment' HTTP Authentication Scheme &RFC8610; Typed structured data hashing and signing Server-Sent Events WHATWG Hedera Documentation Hedera HIP-218: Smart Contract Verification Hedera Circle USDC on Hedera Circle

Example Note: In examples throughout this appendix, hex values shown with ... (e.g., "0x6d0f4fdf...") are abbreviated. Actual values MUST be full-length as specified in .

Challenge

" ]]> The request decodes to:

Note: Challenge expiry is in the header expires auth-param, not in the request JSON. The client generates a random salt locally for new channels. This requests 0.000025 USDC per LLM token, with a suggested deposit of 10.00 USDC (10000000 base units).

Open Credential The client first broadcasts approve() and open() to Hedera EVM via Hashio JSON-RPC, then retries the same resource URI with the open credential:

]]> The credential payload for an open action:

Voucher Top-Up (Same Resource URI) During streaming, clients submit updated vouchers to the same resource URI. HEAD is recommended for pure top-ups when no response body is needed:

]]> Or with a regular request:

]]> The credential payload for a voucher update:

Close Request (Same Resource URI)

]]> The credential payload for a close request:

The voucher fields contain the final cumulative amount for on-chain settlement.

Reference Implementation This appendix provides reference implementation details. These are informative and not normative.

Solidity Interface

Deployed Contracts Network Chain ID Contract Address Hedera Testnet 296 0x8Aaf6690C2a6397d595F97E224fC19759De6fdaE Hedera Mainnet 295 0x8Aaf6690C2a6397d595F97E224fC19759De6fdaE Both deployments are fully verified on Sourcify.

Supported Tokens Token Network HTS Token ID EVM Address USDC Testnet 0.0.5449 0x00...1549 USDC Mainnet 0.0.456858 0x00...06f89a HTS tokens are exposed as ERC-20 via .

Contract Source The reference implementation is available at: contracts/src/HederaStreamChannel.sol in the mppx-hedera repository.

TypeScript Client Library The mppx-hedera npm package provides client and server implementations: mppx-hedera/client -- hederaSession() client mppx-hedera/server -- hedera.session() server mppx-hedera/server/sse -- SSE transport for metered streaming

Schema Definitions (JSON Schema)

Session Request Schema

Session Payload Schema

Session Receipt Schema Servers MUST include Payment-Receipt only on successful processing of a session action (2xx).

Acknowledgements The author thanks the Tempo team for the MPP session payment channel design and the mppx ecosystem architecture that this specification builds upon. HederaStreamChannel.sol is a port of Tempo's TempoStreamChannel.sol adapted for Hedera's EVM layer and HTS token ecosystem.