diff --git a/clients/web/src/components/elements/ContentViewer/ContentViewer.test.tsx b/clients/web/src/components/elements/ContentViewer/ContentViewer.test.tsx
index bfd65add..66d5ace3 100644
--- a/clients/web/src/components/elements/ContentViewer/ContentViewer.test.tsx
+++ b/clients/web/src/components/elements/ContentViewer/ContentViewer.test.tsx
@@ -263,6 +263,39 @@ describe("ContentViewer", () => {
     expect(screen.getByRole("button", { name: "Copy" })).toBeInTheDocument();
   });
 
+  // The `resource` block branch used to render every embedded resource in a
+  // plain code block, so a prompt message carrying JSON did not inherit this
+  // migration even though the text-block branch did.
+  it("renders an embedded JSON resource in the JSON editor", () => {
+    const block: ContentBlock = {
+      type: "resource",
+      resource: {
+        uri: "file:///a.json",
+        mimeType: "application/json",
+        text: '{"a":1}',
+      },
+    };
+    renderWithMantine(<ContentViewer block={block} />);
+    expect(getAceText()).toBe('{\n  "a": 1\n}');
+  });
+
+  // Narrowed to JSON on purpose: routing every declared type through would put
+  // an embedded `text/html` resource into the sandboxed frame, which is a
+  // change to what a tool result is.
+  it("leaves a non-JSON embedded resource on the plain renderer", () => {
+    const block: ContentBlock = {
+      type: "resource",
+      resource: {
+        uri: "file:///a.html",
+        mimeType: "text/html",
+        text: "<p>hi</p>",
+      },
+    };
+    renderWithMantine(<ContentViewer block={block} />);
+    expect(screen.getByText("<p>hi</p>")).toBeInTheDocument();
+    expect(document.querySelector("iframe")).toBeNull();
+  });
+
   it("renders nothing when neither block nor contents is provided", () => {
     const { container } = renderWithMantine(<ContentViewer />);
     // MantineProvider injects a <style> node; assert no actual content renders.
diff --git a/clients/web/src/components/elements/ContentViewer/ContentViewer.tsx b/clients/web/src/components/elements/ContentViewer/ContentViewer.tsx
index 59789532..65d149eb 100644
--- a/clients/web/src/components/elements/ContentViewer/ContentViewer.tsx
+++ b/clients/web/src/components/elements/ContentViewer/ContentViewer.tsx
@@ -482,18 +482,40 @@ function BlockContent({
       return <ImageContent data={block.data} mimeType={block.mimeType} />;
     case "audio":
       return <AudioContent data={block.data} mimeType={block.mimeType} />;
-    case "resource":
+    case "resource": {
+      // An embedded *text* resource that declares JSON goes through the same
+      // dispatch a text block does, so this branch inherits the JSON renderer
+      // rather than staying on a plain code block (#2151).
+      //
+      // Narrowed to JSON on purpose, the same way the Network tab's bodies are:
+      // routing every declared type through would also put an embedded
+      // `text/html` resource into the sandboxed frame and a `text/csv` one into
+      // a table. The Resources screen already renders resource contents that
+      // way, so doing it here too is defensible — but it is a change to what a
+      // tool result *is*, and belongs to whoever decides that rather than to
+      // this swap.
+      const embedded = "text" in block.resource ? block.resource : null;
+      if (embedded && getMimeKind(embedded.mimeType ?? "") === "json") {
+        return (
+          <TextualContent
+            text={embedded.text}
+            mimeType={embedded.mimeType}
+            copyable={copyable}
+            wrap={wrap}
+            jsonLabel={jsonLabel}
+          />
+        );
+      }
       return (
         <Stack gap="xs">
           <ContentWrapper>
             <CodeBlock>
-              {"text" in block.resource
-                ? block.resource.text
-                : `[blob: ${block.resource.uri}]`}
+              {embedded ? embedded.text : `[blob: ${block.resource.uri}]`}
             </CodeBlock>
           </ContentWrapper>
         </Stack>
       );
+    }
     case "resource_link":
       // Static metadata only. The interactive, read-on-demand presentation
       // lives in the `groups/ResourceLink` group, rendered by content-block
diff --git a/clients/web/src/utils/jsonObjectDraft.test.ts b/clients/web/src/utils/jsonObjectDraft.test.ts
index d9fb4462..535deb74 100644
--- a/clients/web/src/utils/jsonObjectDraft.test.ts
+++ b/clients/web/src/utils/jsonObjectDraft.test.ts
@@ -78,9 +78,26 @@ describe("findUnsendableNumberLiteral", () => {
     },
   );
 
+  // A nonzero literal that parses to zero anyway: the digit comparison skips it
+  // for being exponent-form, and it is not negative zero, so nothing else here
+  // would see a document that says one number and sends none of it.
+  it.each(["1e-400", "-1e-400", "0.00001e-400", "9e-999"])(
+    "rejects a literal that underflows to zero: %s",
+    (literal) => {
+      expect(findUnsendableNumberLiteral(`{"n":${literal}}`)).toBe(literal);
+    },
+  );
+
+  // Subnormal but representable — a real value, not an underflow to nothing.
+  it("accepts a subnormal that still parses to something", () => {
+    expect(findUnsendableNumberLiteral('{"n":1e-320}')).toBeNull();
+  });
+
   it("accepts a positive zero", () => {
     expect(findUnsendableNumberLiteral('{"n":0}')).toBeNull();
     expect(findUnsendableNumberLiteral('{"n":0.0}')).toBeNull();
+    // Written as zero, so it is zero — not an underflow.
+    expect(findUnsendableNumberLiteral('{"n":0e-400}')).toBeNull();
   });
 
   it("says nothing about text that is not JSON", () => {
diff --git a/clients/web/src/utils/jsonObjectDraft.ts b/clients/web/src/utils/jsonObjectDraft.ts
index e3c722ed..7dc26fdd 100644
--- a/clients/web/src/utils/jsonObjectDraft.ts
+++ b/clients/web/src/utils/jsonObjectDraft.ts
@@ -41,9 +41,10 @@ export function isJsonObject(value: unknown): value is StrictJsonObject {
  * Only integer literals written out in full are checked for lost digits. An
  * exponent or a fraction is a double by nature and is sent as the double it
  * parsed to, so nothing is lost between typing and sending — and `{"n":1e308}`
- * stays acceptable, which `parseJsonObjectDraft`'s own tests pin. Negative zero
- * is the exception, and is checked in every spelling: there the loss happens on
- * the way *out*, not on the way in.
+ * stays acceptable, which `parseJsonObjectDraft`'s own tests pin. The two
+ * exceptions are checked in every spelling: negative zero, where the loss
+ * happens on the way *out*, and underflow, where a nonzero literal parses to
+ * zero on the way in.
  */
 export function findUnsendableNumberLiteral(text: string): string | null {
   for (const literal of jsonNumberLiterals(text)) {
@@ -51,6 +52,13 @@ export function findUnsendableNumberLiteral(text: string): string | null {
     // exactly, so the digit comparison below has nothing to say about it, but
     // `JSON.stringify` writes it as `0`: the sign is lost on the way out.
     if (Object.is(Number(literal), -0)) return literal;
+    // Underflow: a literal with a nonzero mantissa that parses to zero anyway,
+    // `1e-400` being the shape of it. The digit comparison skips it for being
+    // exponent-form, and it is not negative zero, so nothing else here sees a
+    // document that says one number and sends none of it.
+    if (Number(literal) === 0 && /[1-9]/.test(mantissaOf(literal))) {
+      return literal;
+    }
     if (!/^-?\d+$/.test(literal)) continue;
     // BigInt on both sides, so the comparison is exact: `Number(literal)` is an
     // integer-valued double, and converting it back shows what survived.
@@ -59,6 +67,15 @@ export function findUnsendableNumberLiteral(text: string): string | null {
   return null;
 }
 
+/**
+ * The part of a numeric literal before its exponent — `1` of `1e-400`, `-0.0`
+ * of `-0.0e5`. Whether it holds a nonzero digit is what separates a literal
+ * that underflowed to zero from one that was written as zero.
+ */
+function mantissaOf(literal: string): string {
+  return literal.split(/[eE]/)[0];
+}
+
 /** Matches one JSON number token, anchored where the scan currently stands. */
 const JSON_NUMBER = /-?\d+(?:\.\d+)?(?:[eE][+-]?\d+)?/y;