diff --git a/web/workspace/src/lib/workspace/api/runtime-management.ts b/web/workspace/src/lib/workspace/api/runtime-management.ts
index 944a5238..cefb9785 100644
--- a/web/workspace/src/lib/workspace/api/runtime-management.ts
+++ b/web/workspace/src/lib/workspace/api/runtime-management.ts
@@ -677,6 +677,41 @@ async function finishMutation(
   return detail;
 }
 
+export async function previewRuntimePublicKeyFingerprint(
+  publicKey: string,
+): Promise<string> {
+  const normalized = publicKey.trim();
+  const prefix = "yoi-ed25519-pub:v1:";
+  if (!normalized.startsWith(prefix)) {
+    throw new RuntimeTrustRequestError(
+      `Public key must start with ${prefix}`,
+    );
+  }
+  const encoded = normalized.slice(prefix.length);
+  if (!/^[A-Za-z0-9_-]+$/.test(encoded)) {
+    throw new RuntimeTrustRequestError("Public key encoding is invalid");
+  }
+  const padded = encoded.replaceAll("-", "+").replaceAll("_", "/") +
+    "=".repeat((4 - (encoded.length % 4)) % 4);
+  let decoded: string;
+  try {
+    decoded = atob(padded);
+  } catch {
+    throw new RuntimeTrustRequestError("Public key encoding is invalid");
+  }
+  if (decoded.length !== 32) {
+    throw new RuntimeTrustRequestError("Public key must contain 32 bytes");
+  }
+  const bytes = Uint8Array.from(
+    decoded,
+    (character) => character.charCodeAt(0),
+  );
+  const digest = new Uint8Array(await crypto.subtle.digest("SHA-256", bytes));
+  const hex = Array.from(digest, (byte) => byte.toString(16).padStart(2, "0"))
+    .join("");
+  return `sha256:${hex}`;
+}
+
 export async function putRuntimeTrustKey(
   workspaceId: string,
   runtimeId: string,
diff --git a/web/workspace/src/lib/workspace/styles/settings.css b/web/workspace/src/lib/workspace/styles/settings.css
index e6f363b3..f085acea 100644
--- a/web/workspace/src/lib/workspace/styles/settings.css
+++ b/web/workspace/src/lib/workspace/styles/settings.css
@@ -470,6 +470,30 @@
     color: var(--text-muted);
   }
 
+  .runtime-trust-comparison {
+    display: grid;
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+    gap: var(--space-3);
+    margin: 0;
+    padding: var(--space-3) 0;
+    border-block: 1px solid var(--line);
+  }
+
+  .runtime-trust-comparison div {
+    min-width: 0;
+  }
+
+  .runtime-trust-comparison dt {
+    color: var(--text-muted);
+    font-size: 0.72rem;
+    font-weight: 700;
+  }
+
+  .runtime-trust-comparison dd {
+    margin: var(--space-1) 0 0;
+    overflow-wrap: anywhere;
+  }
+
   .runtime-trust-form .field-error,
   .runtime-detail-page .section-state.error {
     color: var(--danger);
diff --git a/web/workspace/src/routes/w/[workspaceId]/settings/runtimes/[runtimeId]/+page.svelte b/web/workspace/src/routes/w/[workspaceId]/settings/runtimes/[runtimeId]/+page.svelte
index e05b9663..13a36d5e 100644
--- a/web/workspace/src/routes/w/[workspaceId]/settings/runtimes/[runtimeId]/+page.svelte
+++ b/web/workspace/src/routes/w/[workspaceId]/settings/runtimes/[runtimeId]/+page.svelte
@@ -6,6 +6,7 @@
     RuntimeTrustKeyStatus,
   } from '$lib/generated/workspace-api';
   import {
+    previewRuntimePublicKeyFingerprint,
     putRuntimeTrustKey,
     revokeRuntimeTrustKey,
     RuntimeTrustConflictError,
@@ -23,6 +24,29 @@
   let fieldError = $state<string | null>(null);
   let requestError = $state<string | null>(null);
   let successMessage = $state<string | null>(null);
+  let replacementFingerprint = $state<string | null>(null);
+  let replacementFingerprintError = $state<string | null>(null);
+  let fingerprintGeneration = 0;
+
+  $effect(() => {
+    const key = publicKey.trim();
+    const generation = ++fingerprintGeneration;
+    replacementFingerprint = null;
+    replacementFingerprintError = null;
+    if (!key) return;
+    void previewRuntimePublicKeyFingerprint(key).then(
+      (fingerprint) => {
+        if (generation === fingerprintGeneration) replacementFingerprint = fingerprint;
+      },
+      (error) => {
+        if (generation === fingerprintGeneration) {
+          replacementFingerprintError = error instanceof Error
+            ? error.message
+            : String(error);
+        }
+      },
+    );
+  });
 
   function trustAction(status: RuntimeTrustKeyStatus): TrustAction {
     if (status === 'unconfigured') return 'create';
@@ -69,6 +93,14 @@
       fieldError = 'Public key must be at most 16 KiB of UTF-8 text.';
       return;
     }
+    if (replacementFingerprintError) {
+      fieldError = replacementFingerprintError;
+      return;
+    }
+    if (!replacementFingerprint) {
+      fieldError = 'Wait for the replacement fingerprint preview before saving.';
+      return;
+    }
 
     const trust = data.runtimeDetail.trust_key;
     const action = trustAction(trust.status);
@@ -249,9 +281,23 @@
             spellcheck="false"
             aria-describedby={fieldError ? 'runtime-public-key-error' : undefined}
             aria-invalid={fieldError ? 'true' : undefined}
-            placeholder="ssh-ed25519 …"
+            placeholder="yoi-ed25519-pub:v1:…"
           ></textarea>
 
+          <dl class="runtime-trust-comparison">
+            <div>
+              <dt>Current fingerprint</dt>
+              <dd><code>{trust.fingerprint ?? 'Not configured'}</code></dd>
+            </div>
+            <div>
+              <dt>Replacement fingerprint</dt>
+              <dd><code>{replacementFingerprint ?? 'Enter a valid public key'}</code></dd>
+            </div>
+          </dl>
+          {#if replacementFingerprintError}
+            <p class="field-error">{replacementFingerprintError}</p>
+          {/if}
+
           {#if currentAction !== 'create'}
             <label for="runtime-fingerprint-confirmation">Confirm current fingerprint</label>
             <input
diff --git a/web/workspace/tests/runtime-management-source.test.ts b/web/workspace/tests/runtime-management-source.test.ts
index 034b4626..ac08da59 100644
--- a/web/workspace/tests/runtime-management-source.test.ts
+++ b/web/workspace/tests/runtime-management-source.test.ts
@@ -84,6 +84,14 @@ Deno.test("Runtime detail keeps trust controls owner-only and conflict-safe", as
   const reveal = page.indexOf("Reveal public key");
   const mutation = page.indexOf('id="runtime-public-key-input"');
   assert(ownerGate >= 0, "Runtime trust controls should use manage_runtimes");
+  assert(
+    page.includes("Current fingerprint"),
+    "current fingerprint must be explicit",
+  );
+  assert(
+    page.includes("Replacement fingerprint"),
+    "replacement fingerprint must be previewed before confirmation",
+  );
   assert(
     page.includes("!runtime.management.built_in"),
     "Runtime trust controls should be hidden for the built-in Runtime",
diff --git a/web/workspace/tests/runtime-management.test.ts b/web/workspace/tests/runtime-management.test.ts
index cc938db9..daf80b71 100644
--- a/web/workspace/tests/runtime-management.test.ts
+++ b/web/workspace/tests/runtime-management.test.ts
@@ -6,6 +6,7 @@ import {
   parseRuntimeTrustConflict,
   parseWorkspaceRuntimeDetail,
   parseWorkspaceRuntimeList,
+  previewRuntimePublicKeyFingerprint,
   putRuntimeTrustKey,
   RuntimeTrustConflictError,
 } from "../src/lib/workspace/api/runtime-management.ts";
@@ -180,6 +181,17 @@ Deno.test("Runtime detail rejects unbounded strings and incoherent trust state",
   );
 });
 
+Deno.test("Runtime public key preview matches the Server fingerprint contract", async () => {
+  const fingerprint = await previewRuntimePublicKeyFingerprint(
+    "yoi-ed25519-pub:v1:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
+  );
+  assert(
+    fingerprint ===
+      "sha256:66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925",
+    "fingerprint preview drifted from the Server SHA-256 contract",
+  );
+});
+
 Deno.test("typed trust conflict is validated and preserves authoritative revision", async () => {
   let sentBody: unknown = null;
   const fetchImpl = ((_: RequestInfo | URL, init?: RequestInit) => {