diff --git a/.changeset/ten-cameras-grab.md b/.changeset/ten-cameras-grab.md
new file mode 100644
index 00000000..ac21640f
--- /dev/null
+++ b/.changeset/ten-cameras-grab.md
@@ -0,0 +1,5 @@
+---
+'mppx': patch
+---
+
+Validate the `did:pkh:eip155` source DID on zero-dollar Tempo proof credentials. Servers now reject malformed proof source DIDs and chain ID mismatches between the source DID and the challenge signing domain.
diff --git a/src/tempo/internal/proof.test.ts b/src/tempo/internal/proof.test.ts
index 93b3e0cb..31037717 100644
--- a/src/tempo/internal/proof.test.ts
+++ b/src/tempo/internal/proof.test.ts
@@ -2,6 +2,47 @@ import { describe, expect, test } from 'vp/test'
 
 import * as Proof from './proof.js'
 
+const parseProofSourceCases = [
+  {
+    expected: {
+      address: '0xa5cc3c03994db5b0d9ba5e4f6d2efbd9f213b141',
+      chainId: 42431,
+    },
+    name: 'parses a valid did:pkh:eip155 source',
+    source: 'did:pkh:eip155:42431:0xa5cc3c03994db5b0d9ba5e4f6d2efbd9f213b141',
+  },
+  {
+    expected: null,
+    name: 'rejects non-numeric chain ids',
+    source: 'did:pkh:eip155:not-a-number:0x1234',
+  },
+  {
+    expected: null,
+    name: 'rejects leading-zero chain ids',
+    source: 'did:pkh:eip155:01:0xa5cc3c03994db5b0d9ba5e4f6d2efbd9f213b141',
+  },
+  {
+    expected: null,
+    name: 'rejects unsafe integer chain ids',
+    source: 'did:pkh:eip155:9007199254740992:0xa5cc3c03994db5b0d9ba5e4f6d2efbd9f213b141',
+  },
+  {
+    expected: null,
+    name: 'rejects invalid addresses',
+    source: 'did:pkh:eip155:42431:not-an-address',
+  },
+  {
+    expected: null,
+    name: 'rejects extra path segments',
+    source: 'did:pkh:eip155:42431:0xAbCdEf1234567890AbCdEf1234567890AbCdEf12:extra',
+  },
+  {
+    expected: null,
+    name: 'rejects unsupported namespaces',
+    source: 'did:pkh:solana:42431:0xa5cc3c03994db5b0d9ba5e4f6d2efbd9f213b141',
+  },
+] as const
+
 describe('Proof', () => {
   test('types has Proof with challengeId field', () => {
     expect(Proof.types).toEqual({
@@ -33,4 +74,10 @@ describe('Proof', () => {
     const address = '0xAbCdEf1234567890AbCdEf1234567890AbCdEf12'
     expect(Proof.proofSource({ address, chainId: 1 })).toBe(`did:pkh:eip155:1:${address}`)
   })
+
+  for (const { expected, name, source } of parseProofSourceCases) {
+    test(`parseProofSource ${name}`, () => {
+      expect(Proof.parseProofSource(source)).toEqual(expected)
+    })
+  }
 })
diff --git a/src/tempo/internal/proof.ts b/src/tempo/internal/proof.ts
index 1164f665..3474ad8f 100644
--- a/src/tempo/internal/proof.ts
+++ b/src/tempo/internal/proof.ts
@@ -1,3 +1,5 @@
+import { isAddress, type Address } from 'viem'
+
 /** EIP-712 typed data types for proof credentials. */
 export const types = {
   Proof: [{ name: 'challengeId', type: 'string' }],
@@ -17,3 +19,17 @@ export function message(challengeId: string) {
 export function proofSource(parameters: { address: string; chainId: number }): string {
   return `did:pkh:eip155:${parameters.chainId}:${parameters.address}`
 }
+
+/** Parses a proof credential `did:pkh:eip155` source DID. */
+export function parseProofSource(source: string): { address: Address; chainId: number } | null {
+  const match = /^did:pkh:eip155:(0|[1-9]\d*):([^:]+)$/.exec(source)
+  if (!match) return null
+
+  const chainIdText = match[1]!
+  const address = match[2]!
+  const chainId = Number(chainIdText)
+  if (!Number.isSafeInteger(chainId)) return null
+  if (!isAddress(address)) return null
+
+  return { address: address as Address, chainId }
+}
diff --git a/src/tempo/server/Charge.test.ts b/src/tempo/server/Charge.test.ts
index 1d3cc2ec..62b2fe29 100644
--- a/src/tempo/server/Charge.test.ts
+++ b/src/tempo/server/Charge.test.ts
@@ -2244,6 +2244,42 @@ describe('tempo', () => {
       httpServer.close()
     })
 
+    test('behavior: rejects proof with mismatched source DID chainId', async () => {
+      const httpServer = await Http.createServer(async (req, res) => {
+        const result = await Mppx_server.toNodeListener(
+          server.charge({ amount: '0', decimals: 6 }),
+        )(req, res)
+        if (result.status === 402) return
+        res.end('OK')
+      })
+
+      const response1 = await fetch(httpServer.url)
+      const challenge = Challenge.fromResponse(response1, {
+        methods: [tempo_client.charge()],
+      })
+
+      const signature = await signTypedData(client, {
+        account: accounts[1],
+        domain: Proof.domain(chain.id),
+        types: Proof.types,
+        primaryType: 'Proof',
+        message: Proof.message(challenge.id),
+      })
+
+      const credential = Credential.from({
+        challenge,
+        payload: { signature, type: 'proof' as const },
+        source: `did:pkh:eip155:1:${accounts[1].address}`,
+      })
+
+      const response2 = await fetch(httpServer.url, {
+        headers: { Authorization: Credential.serialize(credential) },
+      })
+      expect(response2.status).toBe(402)
+
+      httpServer.close()
+    })
+
     test('behavior: rejects proof with malformed source DID', async () => {
       const httpServer = await Http.createServer(async (req, res) => {
         const result = await Mppx_server.toNodeListener(
diff --git a/src/tempo/server/Charge.ts b/src/tempo/server/Charge.ts
index ecf44f37..0d2378e4 100644
--- a/src/tempo/server/Charge.ts
+++ b/src/tempo/server/Charge.ts
@@ -159,11 +159,15 @@ export function charge<const parameters extends charge.Parameters>(
           if (!expectedSource)
             throw new MismatchError('Proof credential must include a source.', {})
 
-          const sourceAddress = expectedSource.split(':').pop() as `0x${string}`
           const resolvedChainId = challenge.request.methodDetails?.chainId ?? chainId!
+          const source = Proof.parseProofSource(expectedSource)
+
+          if (!source || source.chainId !== resolvedChainId) {
+            throw new MismatchError('Proof credential source is invalid.', {})
+          }
 
           const valid = await verifyTypedData(client, {
-            address: sourceAddress,
+            address: source.address,
             domain: Proof.domain(resolvedChainId),
             types: Proof.types,
             primaryType: 'Proof',