Document GPT Image 2.5 compatibility

docs/synthid.md

@@ -775,6 +775,20 @@ SynthID. Current OpenAI manifests distinguish the watermarked output with the
 signed `c2pa.watermarked.unbound` action. The detector requires that action, so
 old OpenAI C2PA remains an origin signal without becoming a SynthID claim.
 
+**GPT Image 2.5 compatibility, verified 2026-09-15.** One fresh low-quality
+1024 x 1024 API generation from each released model, `gpt-image-2.5-flare` and
+`gpt-image-2.5-sunburst`, carried a valid bound OpenAI C2PA claim generated by
+`OpenAI Media Service API`, with `trainedAlgorithmicMedia` and the action set
+`created, converted, watermarked.unbound`. `identify` reported high-confidence
+OpenAI provenance plus SynthID, `has_invisible_target` returned true, and the
+automatic strength cohort resolved to OpenAI for both files. After C2PA was
+removed without changing decoded pixels, the official OpenAI Content Provenance
+API still returned SynthID `detected` for both sources. The complete shipped
+`qwen-zimage` profile at strength 0.07675 and seed 0 then produced two outputs
+for which the same metadata-independent API returned SynthID `not_detected`.
+This is one controlled source per model and proves the current routing and
+operating point on those two sources; it is not a provider-wide removal rate.
+
 ### 4.3 Other vendors
 
 - **Kakao** (South Korea): SynthID adopter as of May 2026 (Google announcement)