diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index 2537f8949332..f56c8162d308 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -18,6 +18,7 @@ import { iife } from "@/util/iife"
 import { Global } from "../global"
 import path from "path"
 import { Filesystem } from "../util/filesystem"
+import { sanitizeJsonSurrogates } from "../util/sanitize-surrogates"
 
 // Direct imports for bundled providers
 import { createAmazonBedrock, type AmazonBedrockProviderSettings } from "@ai-sdk/amazon-bedrock"
@@ -1167,6 +1168,12 @@ export namespace Provider {
           }
         }
 
+        // Sanitize JSON-escaped lone surrogates that may exist in previously
+        // stored session data. See: https://github.com/anomalyco/opencode/issues/14630
+        if (typeof opts.body === "string") {
+          opts.body = sanitizeJsonSurrogates(opts.body)
+        }
+
         const res = await fetchFn(input, {
           ...opts,
           // @ts-ignore see here: https://github.com/oven-sh/bun/issues/16682
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index 158b83865dc4..dc16ba6580b5 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -186,7 +186,7 @@ export namespace SessionProcessor {
                       state: {
                         status: "completed",
                         input: value.input ?? match.state.input,
-                        output: value.output.output,
+                        output: value.output.output?.toWellFormed?.() ?? value.output.output,
                         metadata: value.output.metadata,
                         title: value.output.title,
                         time: {
@@ -210,7 +210,7 @@ export namespace SessionProcessor {
                       state: {
                         status: "error",
                         input: value.input ?? match.state.input,
-                        error: (value.error as any).toString(),
+                        error: (value.error as any).toString()?.toWellFormed?.() ?? (value.error as any).toString(),
                         time: {
                           start: match.state.time.start,
                           end: Date.now(),
diff --git a/packages/opencode/src/util/sanitize-surrogates.ts b/packages/opencode/src/util/sanitize-surrogates.ts
new file mode 100644
index 000000000000..1e0a757fe522
--- /dev/null
+++ b/packages/opencode/src/util/sanitize-surrogates.ts
@@ -0,0 +1,17 @@
+// Regex that matches a JSON-escaped high surrogate (\uD800–\uDBFF)
+// NOT followed by a JSON-escaped low surrogate (\uDC00–\uDFFF),
+// or a JSON-escaped low surrogate NOT preceded by a high surrogate.
+//
+// This is necessary because JSON.stringify() (per ECMA-262) encodes lone
+// surrogate code units as \uD8xx ASCII escapes, which are valid JavaScript
+// but violate RFC 8259. Strict JSON parsers (Anthropic serde_json, OpenAI)
+// reject these with errors like "no low surrogate in string".
+const LONE_HIGH_SURROGATE = /\\u[dD][89aAbB][0-9a-fA-F]{2}(?!\\u[dD][cCdDeEfF][0-9a-fA-F]{2})/g
+const LONE_LOW_SURROGATE = /(?<!\\u[dD][89aAbB][0-9a-fA-F]{2})\\u[dD][cCdDeEfF][0-9a-fA-F]{2}/g
+
+const REPLACEMENT = "\\uFFFD"
+
+export function sanitizeJsonSurrogates(json: string): string {
+  if (!json.includes("\\uD") && !json.includes("\\ud")) return json
+  return json.replace(LONE_HIGH_SURROGATE, REPLACEMENT).replace(LONE_LOW_SURROGATE, REPLACEMENT)
+}